Cladding steel plates and components and their manufacturing methods

Abstract

This invention provides a clad steel sheet with a tensile strength (TS) of 780 MPa or higher and excellent bending properties, impact resistance, and LME resistance. The clad steel sheet is manufactured by forming a base material and cladding materials on the front and back sides of the base material. The composition and microstructure of the base material and cladding materials are appropriately controlled. The average Vickers hardness (HVL) of the cladding material is set to 260 or lower, the value obtained by dividing the average Vickers hardness (HVL) of the cladding material by the average Vickers hardness (HVB) of the base material is set to 0.80 or lower, the boundary roughness between the base material and the cladding material, measured by the maximum height Ry, is set to 50 μm or lower, and the number of voids at the boundary between the base material and the cladding material is controlled to 20 or lower per 10 mm boundary length.

Description

Technical Field

[0001] This invention relates to a cladding steel sheet and component, and a method for manufacturing the same. In particular, it relates to a cladding steel sheet and component with a tensile strength (TS) of 780 MPa or higher, exhibiting excellent bending properties, impact resistance, and LME resistance, and a method for manufacturing the same. The cladding steel sheet of this invention is suitable for the frame components of automobile bodies, and is particularly suitable for impact energy absorption components. Background Technology

[0002] In recent years, improving the fuel efficiency of automobiles has become a crucial issue from the perspective of protecting the Earth's environment. Therefore, efforts are actively underway to reduce the weight of vehicle bodies by increasing the strength of body materials and achieving thinner wall thicknesses. On the other hand, societal demands for improved collision safety in automobiles are also increasing, with a growing expectation for the development of steel sheets and components that not only possess superior strength but also exhibit excellent crash resistance in the event of a collision.

[0003] As such high-strength steel sheets, Patent Document 1 discloses, for example, a single-layer steel sheet with a tensile strength of 900 MPa or more and excellent resistance to molten metal embrittlement cracking, a hot-dip galvanized steel sheet, and an alloyed hot-dip galvanized single-layer steel sheet, which has an internal oxide layer with at least a portion of the grain boundaries covered by oxides from the surface of the base material to a depth of 5.0 μm or more, and the grain boundary coverage of the oxides in the region from the surface of the base material to a depth of 5.0 μm is 60% or more, and a decarburized layer from the surface of the base material to a depth of 50 μm or more.

[0004] Patent Document 2 discloses a thermoforming material comprising: a core layer, which is a cured steel having a tensile strength > 1900 MPa and / or a hardness > 575 HV10 in the pressed and cured state of the thermoforming material; and two outer layers bonded to the core layer material, which are made of steel that is softer than the core layer and has a tensile strength > 750 MPa and / or a hardness > 235 HV10 in the pressed and cured state of the thermoforming material.

[0005] Patent document 3 discloses a steel composite material comprising a core layer of steel with higher strength or high strength and an outer layer of chemically resistant ferritic steel integrally bonded to one or both sides of the core layer. The chemically resistant ferritic steel contains ≤0.07 wt% carbon, ≤1 wt% manganese, 12-30 wt% chromium, ≤7 wt% molybdenum, ≤0.05 wt% phosphorus and sulfur, ≤0.5 wt% aluminum, ≤0.5 wt% silicon, and ≤1 wt% titanium, niobium, vanadium and zirconium. The total proportion of titanium, niobium, vanadium and zirconium is >0.1 wt%, and the remainder is iron and unavoidable impurities.

[0006] Patent document 4 discloses a cladding steel sheet with excellent strength and formability as a martensitic carbon steel. It comprises a base material and cladding materials on both sides of the base material. The base material is an austenitic high-manganese steel containing C: 0.3 to 1.4% by weight, Mn: 12 to 25% by weight, with the remainder being Fe and unavoidable impurities. The cladding material is a martensitic carbon steel containing C: 0.09 to 0.4% by weight, Mn: 0.3 to 4.5% by weight, with the remainder being Fe and unavoidable impurities.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent No. 6388099

[0010] Patent Document 2: Japanese Patent Publication No. 2020-519765

[0011] Patent Document 3: Japanese Patent Publication No. 2020-509223

[0012] Patent Document 4: Japanese Patent Publication No. 2019-524986 Summary of the Invention

[0013] However, impact energy absorbing components, represented by the front and rear longitudinal beams, are only used in steel plates with a tensile strength (TS) of 590MPa to 780MPa. This is because, with the increasing strength, cracking occurs in bending and axial crush tests during simulated impact tests, making it impossible to fully absorb and utilize impact energy.

[0014] Furthermore, in recent years, during the assembly of automobile bodies and components, spot welding of high-strength hot-dip galvanized steel sheets and high-strength alloyed hot-dip galvanized steel sheets, or spot welding of high-strength cold-rolled steel sheets and galvanized steel sheets, has revealed molten metal embrittlement cracking (LMEC: Liquid Metal Embrittlement Cracking, hereinafter also referred to as LME cracking) at the weld joint. LME cracking occurs when the zinc in the galvanized layer melts during spot welding, infiltrating the grain boundaries of the steel structure at the weld joint, and cracking due to the stress generated when the welding electrode is released. Even with ungalvanized high-strength cold-rolled steel sheets, LME cracking can still occur when spot welding with galvanized steel sheets because the molten zinc from the galvanized steel sheet comes into contact with the high-strength cold-rolled steel sheet. High-strength steel sheets with a strength (TS) of 780 MPa or higher are at risk of LME cracking due to their high C, Si, and Mn content.

[0015] However, in Patent Document 1, it is a single-layer steel plate, and therefore the bending and impact resistance characteristics are not studied.

[0016] Patent document 2 describes a hot-working material (cladding steel sheet for hot pressing), not a cladding steel sheet for cold pressing. Furthermore, it exhibits minimal variation in the properties of each layer and high strength and ductility in the area near the surface, but its resistance to LME (Low Metal Melting) has not been studied.

[0017] While Patent Document 3 features low susceptibility to hydrogen-induced cracking and favorable corrosion resistance relative to ductility, it does not address flexural properties, impact resistance, or resistance to LME.

[0018] In Patent Document 4, the base material is a high alloy composition, and there is no study on bending properties, impact resistance, and LME resistance.

[0019] Thus, it cannot be said that a steel plate that can comprehensively meet the requirements of tensile strength (TS), bending properties, impact resistance, and LME resistance has been developed; the current situation is that such a steel plate is expected to be developed.

[0020] The present invention was developed in view of the above-mentioned situation, and aims to provide a cladding steel sheet with a tensile strength (TS) of 780 MPa or more and excellent bending properties, impact resistance and LME resistance, and an advantageous manufacturing method thereof.

[0021] In addition, the present invention aims to provide a component using the above-mentioned cladding steel plate as a blank and a method for manufacturing the same.

[0022] In order to achieve the above-mentioned problem, the inventors have repeatedly conducted in-depth research and obtained the following insights.

[0023] That is, the inventors have obtained the following insight: a cladding steel sheet having a tensile strength (TS) of 780 MPa or higher and exhibiting excellent bending properties, impact resistance, and LME resistance can be obtained by meeting the following requirements:

[0024] (a) Not a so-called single-layer steel sheet, but a cladding steel sheet made of a base material and a cladding material on the front and back of the base material.

[0025] (b) Appropriately control the composition and microstructure of the base material and cladding material.

[0026] (c) Adjust the average Vickers hardness (HVL) of the coating material to below 260, and adjust the value obtained by dividing the average Vickers hardness (HVL) of the coating material by the average Vickers hardness (HVB) of the base material to below 0.80.

[0027] (d) Set the boundary roughness between the base material and the coating material to be less than 50 μm, measured by the maximum height Ry.

[0028] (e) The number of gaps at the boundary between the base material and the cladding material shall be controlled to less than 20 per 10 mm boundary length.

[0029] This invention was completed based on the above insights and further research.

[0030] That is, the main structure of the present invention is as follows.

[0031] 1. A cladding steel plate, comprising a base material and a cladding material on the surface and back of the base material,

[0032] The aforementioned base material has the following composition and steel structure:

[0033] The composition, by mass%, is as follows: C: 0.050%–0.350%, Si: 0.02%–2.00%, Mn: ≥1.80% and <3.50%, P: 0.001%–0.100%, S: <0.0200%, Al: 0.010%–2.000%, and N: <0.0100%, with the remainder being Fe and unavoidable impurities.

[0034] In the steel microstructure, the area fraction of ferrite is less than 55%, the combined area fraction of martensite and tempered martensite is more than 30%, and the volume fraction of retained austenite is less than 5%.

[0035] The aforementioned coating material has the following composition and steel structure:

[0036] The composition, by mass%, is as follows: C: less than 0.100%, Si: less than 0.60%, Mn: 0.05% to 2.50%, P: 0.001% to 0.100%, S: less than 0.0200%, Al: 0.010% to 0.100%, and N: less than 0.0100%, with the remainder being Fe and unavoidable impurities.

[0037] In the steel microstructure, the ferrite area fraction is above 80%.

[0038] The average Vickers hardness (HVL) of the above-mentioned coating materials is below 260.

[0039] The value obtained by dividing the average Vickers hardness (HVL) of the above-mentioned coating material by the average Vickers hardness (HVB) of the above-mentioned base material is 0.80 or less.

[0040] The boundary roughness between the aforementioned base material and the aforementioned coating material, measured in terms of maximum height Ry, is less than 50 μm.

[0041] The number of gaps at the boundary between the base material and the coating material is less than 20 per 10 mm boundary length.

[0042] 2. The cladding steel sheet according to claim 1, wherein at least one of the composition of the base material and the composition of the cladding material further contains, by mass percent, an amount selected from Sb: 0.200% or less, Sn: 0.200% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.00% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.50% or less, Ta: 0.100% or less, W: 0.500% or less, and Mg: 0.020%. At least one of the following: less than 0%, Zn: less than 0.020%, Co: less than 0.020%, Zr: less than 0.020%, Ca: less than 0.0200%, Ce: less than 0.0200%, Se: less than 0.0200%, Te: less than 0.0200%, Ge: less than 0.0200%, As: less than 0.0200%, Sr: less than 0.0200%, Cs: less than 0.0200%, Hf: less than 0.0200%, Pb: less than 0.0200%, Bi: less than 0.0200%, and REM: less than 0.0200%.

[0043] 3. The cladding steel sheet according to 1 or 2 above, wherein the value obtained by dividing the thickness of the base material by the total thickness of the cladding materials is 1 or more.

[0044] 4. The cladding steel sheet according to any one of 1 to 3 above, wherein the surface has a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electro-galvanized layer.

[0045] 5. The cladding steel sheet according to any one of 1 to 4 above, wherein the total diffusible hydrogen content contained in the base material and the cladding material is less than 0.50 ppm by mass.

[0046] 6. A component made of cladding steel sheet as described in any one of 1 to 5 above.

[0047] 7. A method for manufacturing a cladding steel plate, comprising the following steps:

[0048] The first preparation step is to prepare a base steel billet having the composition of the base material described in 1 or 2 above.

[0049] The second preparation step involves preparing a steel billet for a coating material having the composition of the coating material described in 1 or 2 above.

[0050] The surface treatment process involves performing surface treatment to ensure that the surface roughness of at least one of the two surfaces of the base steel billet and the two surfaces of the cladding material steel billet is 30 μm or less in terms of Ra.

[0051] In the lamination process, the surface-treated surface of the base steel billet is in contact with the surface-treated surface of the cladding material steel billet, and the base steel billet and the cladding material steel billet are laminated in the order of cladding material steel billet - base steel billet - cladding material steel billet to obtain a laminated slab.

[0052] In the joining process, the aforementioned cladding material steel billet and the aforementioned base material steel billet are joined together, and a vacuum is drawn so that the vacuum degree between the aforementioned cladding material steel billet and the aforementioned base material steel billet is 1×10⁻⁶. －2 Below Torr, a laminated slab blank is obtained;

[0053] In the hot rolling process, the above-mentioned laminated slab is heated to a temperature range of 1050℃~1350℃ and then hot rolled at a final rolling temperature of 820℃ or above to obtain a hot-rolled steel plate.

[0054] The cold rolling process involves cold rolling the aforementioned hot-rolled steel sheet under a reduction rate of 30% to 80% to obtain a cold-rolled steel sheet; and

[0055] The annealing process involves annealing the above-mentioned cold-rolled steel sheet at an annealing temperature of 750℃~950℃ and a holding time of more than 20 seconds.

[0056] 8. The method for manufacturing cladding steel sheet according to 7 above, further comprising a first reheating step: after the annealing step, the cold-rolled steel sheet is cooled to a cooling stop temperature below 250°C, and then reheated to a temperature range exceeding 250°C and below 450°C, and held for more than 10 seconds.

[0057] 9. The method for manufacturing clad steel sheet according to 7 or 8 above, further comprising a plating process: after the annealing process or after the first reheating process, the cold-rolled steel sheet is subjected to plating to obtain a plating steel sheet.

[0058] 10. The method for manufacturing the cladding steel plate according to 9 above, wherein the coating treatment is hot-dip galvanizing, alloyed hot-dip galvanizing, or electro-galvanizing.

[0059] 11. The method for manufacturing the cladding steel sheet according to 9 or 10 above, further comprising a dehydrogenation treatment step: after the above-mentioned plating treatment step, the above-mentioned plating steel sheet is held at a temperature range of 50°C to 300°C for 0.5 hours to 72.0 hours.

[0060] 12. The method for manufacturing the cladding steel plate according to 7 above, further comprising the following steps:

[0061] The coating process involves hot-dip galvanizing or alloying hot-dip galvanizing of the cold-rolled steel sheet after the annealing process to obtain a coated steel sheet.

[0062] In the second reheating process, after cooling the above-mentioned coated steel sheet to a cooling stop temperature below 250°C, it is reheated to a temperature range of 80°C to 450°C that exceeds the cooling stop temperature and is held for more than 10 seconds.

[0063] 13. The method for manufacturing the cladding steel sheet according to 12 above, further comprising a dehydrogenation treatment step: after the second reheating step above, the cladding steel sheet is held at a temperature range of 50°C to 300°C for 0.5 hours to 72.0 hours.

[0064] 14. A method for manufacturing a component, comprising a step of forming or joining a cladding steel plate as described in any one of 1 to 5 above to produce the component.

[0065] According to the present invention, a cladding steel plate and component having a tensile strength (TS) of 780 MPa or higher, excellent bending properties, impact resistance and LME resistance, and a method thereof are provided for manufacturing the same. Attached Figure Description

[0066] Figure 1 An example of a SEM image showing the boundary location between the base material and the cladding material at section L: (a) is Example No. 4, and (b) is Example No. 44.

[0067] Figure 2 These are the F-S curves of the VDA bending tests in Examples No. 44 and 45.

[0068] Figure 3 This is the F-S curve of the V-bending-orthogonal VDA bending test of Example No.4. Detailed Implementation

[0069] The present invention is described based on the following embodiments.

[0070] [1] Cladding steel plate

[0071] [1-1] Composition of the base material

[0072] First, the composition of the base material of the cladding steel plate according to one embodiment of the present invention will be described. It should be noted that the unit of composition is "mass %", and unless otherwise specified, it will be expressed as "%" below.

[0073] C: 0.050%~0.350%

[0074] C is an effective element for generating the desired amount of martensite, tempered martensite, and retained austenite, and ensuring a strength tolerance (TS) of 780 MPa or higher. When the C content is less than 0.050%, the area fraction of ferrite increases, making it difficult to achieve a TS of 780 MPa or higher. On the other hand, if the C content exceeds 0.350%, the volume fraction of retained austenite increases excessively, and the hardness of the martensite formed from retained austenite during bending deformation increases significantly. As a result, bending and impact resistance properties decrease. Therefore, the C content is set to be 0.050% to 0.350%. The C content is preferably 0.080% or more, more preferably 0.100% or more. Furthermore, the C content is preferably 0.330% or less, more preferably 0.320% or less.

[0075] Si: 0.02%~2.00%

[0076] Si is an effective element for ensuring a strength tolerance (TS) of 780 MPa or higher through solid solution strengthening. When the Si content is less than 0.02%, it is difficult to achieve a TS of 780 MPa or higher. On the other hand, if the Si content exceeds 2.00%, the oxide scale on the base steel billet increases, and the surface roughness Ra of the base steel billet increases. Furthermore, the maximum height Ry of the boundary roughness between the base material and the cladding material, and the number of voids at the boundary between the base material and the cladding material increase, resulting in decreased bending and impact resistance. Therefore, the Si content is between 0.02% and 2.00%. The Si content is preferably 0.20% or more, more preferably 0.40% or more. Additionally, the Si content is preferably 1.80% or less, more preferably 1.60% or less.

[0077] Mn: ≥1.80% and <3.50%

[0078] Mn is an important element for adjusting the area fraction of martensite, tempered martensite, and retained austenite. When the Mn content is less than 1.80%, the area fraction of ferrite increases, making it difficult to achieve a strength (TS) of 780 MPa or higher. On the other hand, if the Mn content is 3.50% or more, the volume fraction of retained austenite increases excessively, leading to a significant increase in the hardness of the martensite formed from retained austenite during bending deformation. Consequently, bending performance and impact resistance decrease. Therefore, the Mn content is 1.80% or more and less than 3.50%. The Mn content is preferably 2.00% or more, more preferably 2.20% or more. Furthermore, the Mn content is preferably 3.30% or less, more preferably 3.20% or less.

[0079] P: 0.001%~0.100%

[0080] Phosphorus (P) is an element that provides solid solution strengthening and increases the strength of steel plates. To achieve this effect, the P content is set to 0.001% or more. On the other hand, if the P content exceeds 0.100%, P segregates at the original austenite grain boundaries, causing grain boundary embrittlement. As a result, the amount of voids generated during bending deformation increases, and bending and impact resistance decrease. Therefore, the P content is 0.001% to 0.100%. More preferably, the P content is 0.030% or less.

[0081] S: below 0.0200%

[0082] Sulfide (S) exists in steel as a sulfide. If its content exceeds 0.0200%, it may reduce the steel sheet's ultimate deformation capacity. As a result, the amount of voids generated during bending deformation increases, and bending and impact resistance decrease. Therefore, the S content is 0.0200% or less, preferably 0.0080% or less. It should be noted that there is no specific lower limit for the S content, but due to limitations in production technology, the S content is mostly 0.0001% or more.

[0083] Al: 0.010%~2.000%

[0084] Al acts as a deoxidizer. To obtain the effect brought about by the addition of Al, the Al content is set to 0.010% or more. On the other hand, if the Al content exceeds 2.000%, the ferrite surface fraction increases, making it difficult to set the TS to 780 MPa or more. Therefore, the Al content is between 0.010% and 2.000%.

[0085] N: below 0.0100%

[0086] Nitrogen (N) exists in steel as a nitride. If its content exceeds 0.0100%, it reduces the steel sheet's ultimate deformation capacity. As a result, the amount of voids formed during bending deformation increases, reducing bending and impact resistance. Therefore, the N content is 0.0100% or less. Furthermore, the N content is preferably 0.0050% or less. It should be noted that there is no specific lower limit for the N content; due to limitations in production technology, the N content is mostly 0.0005% or more.

[0087] The basic composition of the base material of the cladding steel sheet according to one embodiment of the present invention has been described above. However, the base material of the cladding steel sheet according to one embodiment of the present invention has a composition containing the above-described basic components, and the remaining portion other than the above-described basic components contains Fe (iron) and unavoidable impurities. Here, the base material of the cladding steel sheet according to one embodiment of the present invention preferably has a composition containing the above-described basic components, and the remaining portion consists of Fe and unavoidable impurities. In addition, in the base material of the cladding steel sheet according to one embodiment of the present invention, in addition to the above-described basic components, at least one component selected from any of the components shown below may be contained. It should be noted that if any of the components shown below are contained in amounts below the upper limit indicated below, the desired effect can be obtained, therefore a lower limit is not specifically set. It should be noted that when any of the following elements are contained in amounts below the preferred lower limit value described later, the element is contained as an unavoidable impurity.

[0088] Sb: below 0.200%

[0089] Sb is an effective element for suppressing carbon diffusion near the surface of the steel sheet during annealing and controlling the formation of a decarburized layer near the surface of the steel sheet. If the Sb content exceeds 0.200%, there is a possibility that a soft layer will not form on the surface of the steel sheet, resulting in reduced bending and impact resistance. Therefore, the Sb content is preferably 0.200% or less. The Sb content is more preferably 0.020% or less. On the other hand, from the viewpoint of keeping TS (steel sulfide) within a more preferred range, the Sb content is preferably 0.002% or more. The Sb content is more preferably 0.005% or more.

[0090] Sn: below 0.200%

[0091] Sn is an effective element for suppressing carbon diffusion near the surface of the steel sheet during annealing and controlling the formation of a decarburized layer near the surface of the steel sheet. If the Sn content exceeds 0.200%, there is a possibility that a soft layer will not form on the surface of the steel sheet, resulting in reduced bending and impact resistance. Therefore, the Sn content is preferably 0.200% or less. The Sn content is more preferably 0.020% or less. On the other hand, from the viewpoint of keeping TS (carbon dioxide) within a more preferred range, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.005% or more.

[0092] Ti: below 0.200%, Nb: below 0.200%, V: below 0.100%

[0093] Ti, Nb, and V improve steel strength (TS) by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. To achieve this effect, when at least one of Ti, Nb, and V is added, it is preferable to set the content of each of Ti, Nb, and V to 0.001% or more. More preferably, these contents are 0.005% or more. On the other hand, when the Ti content exceeds 0.200%, the Nb content exceeds 0.200%, or the V content exceeds 0.100%, there is a possibility of generating a large number of coarse precipitates and inclusions. In such cases, if diffusible hydrogen is present in the steel sheet, these coarse precipitates and inclusions may become crack initiation points during bending deformation, reducing bending and impact resistance. Therefore, when at least one of Ti, Nb, and V is added, the Ti content is preferably 0.200% or less, the Nb content is preferably 0.200% or less, and the V content is preferably 0.100% or less. Furthermore, the contents of Ti, Nb, and V are preferably 0.060% or less.

[0094] B: Below 0.0100%

[0095] Boron (B) is an element that can improve hardenability through segregation at austenite grain boundaries. By adding B to steel, it is possible to suppress the formation of ferrite and grain growth during annealing cooling. To achieve this effect, it is preferable to set the B content to 0.0001% or more. The B content is more preferably 0.0002% or more. On the other hand, if the B content exceeds 0.0100%, there is a possibility of cracking occurring inside the steel sheet during hot rolling, reducing the ultimate deformation capacity of the steel sheet. As a result, there is a possibility of increased void formation during bending deformation and decreased bending and impact resistance. Therefore, when adding B, its content is preferably 0.0100% or less. Furthermore, the B content is more preferably 0.0050% or less.

[0096] Cu: below 1.00%

[0097] Cu is an element that improves hardenability and is effective in setting the area fraction of the hard phase within a more preferred range, and in setting the total hardness (TS) within a more preferred range. To achieve this effect, it is preferable to set the Cu content to 0.005% or more. The Cu content is more preferably 0.02% or more. On the other hand, if the Cu content exceeds 1.00%, the area fraction of the hard phase increases, and the TS becomes too high. In addition, with the increase of coarse precipitates and inclusions, and in the case of diffusible hydrogen in the steel plate, these precipitates and inclusions may become the initiation point of cracks during bending deformation, and the bending and impact resistance characteristics may decrease. Therefore, when Cu is added, its content is preferably 1.00% or less. In addition, the Cu content is more preferably 0.20% or less.

[0098] Cr: less than 1.000%

[0099] Cr is an element that improves hardenability and is effective in forming hard phases. If the Cr content exceeds 1.000%, there is a possibility of an increase in the area ratio of hard martensite and a decrease in bending and impact resistance. Therefore, when Cr is added, the Cr content is preferably 1.000% or less. Furthermore, the Cr content is more preferably 0.250% or less, and even more preferably 0.100% or less. It should be noted that the Cr content can be 0.0000%, but from the viewpoint of improving hardenability and keeping TS (hardness-to-strength) within a more preferred range, the Cr content is preferably 0.010% or more.

[0100] Ni: below 1.000%

[0101] Ni is an element that improves hardenability and is effective in setting the area ratio of the hard phase within a more preferred range, thus making the hardened steel toughness (TS) within a more preferred range. To achieve this effect, it is preferable to set the Ni content to 0.005% or more. The Ni content is more preferably 0.020% or more. On the other hand, if the Ni content exceeds 1.000%, there is an increase in coarse precipitates and inclusions. In such cases, if diffusible hydrogen is present in the steel sheet, these precipitates and inclusions may become crack initiation points during bending deformation, and the bending and impact resistance properties may decrease. Therefore, when adding Ni, its content is preferably 1.000% or less. Furthermore, the Ni content is more preferably 0.800% or less.

[0102] Mo: 0.50% or less

[0103] Mo is an element that improves hardenability and is effective in forming hard phases. If the Mo content exceeds 0.50%, there is a possibility of an increase in the area ratio of hard martensite and a decrease in bending and impact resistance. Therefore, when adding Mo, the Mo content is preferably 0.50% or less. The Mo content is more preferably 0.45% or less, and even more preferably 0.40% or less. It should be noted that, regarding the lower limit of the Mo content, from the viewpoint of improving hardenability and keeping the hardened steel content (TS) within a more preferred range, the Mo content is preferably 0.01% or more. Furthermore, the Mo content is more preferably 0.03% or more.

[0104] Ta: below 0.100%

[0105] Like Ti, Nb, and V, Ta (Ta) enhances the formation of fine carbides, nitrides, or carbonitrides during hot rolling or annealing, thereby increasing the precipitation strengthening (TS). Furthermore, Ta also has the effect of partially dissolving in Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb,Ta)(C,N), significantly suppressing precipitate coarsening and stabilizing precipitation strengthening to improve the TS of the steel sheet. To achieve this effect, the Ta content is preferably 0.001% or more. On the other hand, if the Ta content exceeds 0.100%, a large number of coarse precipitates and inclusions may form. In such cases, if diffusible hydrogen is present in the steel sheet, these precipitates and inclusions may become crack initiation points during bending deformation, reducing bending and impact resistance. Therefore, when adding Ta, its content is preferably 0.100% or less.

[0106] W: below 0.500%

[0107] W is an element effective in ensuring strength. If the W content exceeds 0.500%, there is a possibility of an increase in the area ratio of hard martensite, and a decrease in bending and impact resistance. Therefore, when adding W, the W content is preferably 0.500% or less. The W content is more preferably 0.450% or less, and even more preferably 0.400% or less. It should be noted that, from the viewpoint of improving hardenability and keeping TS (strength and toughness) within a more preferred range, the W content is preferably set at 0.001% or more. Furthermore, the W content is more preferably 0.030% or more.

[0108] Mg: below 0.0200%

[0109] Mg is an effective element for shaping inclusions such as sulfides and oxides into spherical shapes, improving the ultimate deformation capacity of steel plates, and enhancing bending and impact resistance. To achieve these effects, it is preferable to set the Mg content at 0.0001% or more. On the other hand, if the Mg content exceeds 0.0200%, there is a possibility of generating a large amount of coarse precipitates and inclusions. In such cases, if diffusible hydrogen is present in the steel plate, these precipitates and inclusions may become crack initiation points during bending deformation, reducing bending and impact resistance. Therefore, when adding Mg, its content is preferably 0.0200% or less.

[0110] Zn: less than 0.020%, Co: less than 0.020%, Zr: less than 0.020%

[0111] Zn, Co, and Zr all contribute to the spherical shape of inclusions, thus being effective elements for improving the ultimate deformation capacity, bending properties, and impact resistance of steel plates. To achieve this effect, the contents of Zn, Co, and Zr are preferably 0.001% or more. On the other hand, if the contents of Zn, Co, and Zr exceed 0.020%, there is a possibility of generating a large number of coarse precipitates and inclusions. In such cases, if diffusible hydrogen is present in the steel plate, these precipitates and inclusions may become crack initiation points during bending deformation, potentially reducing bending and impact resistance. Therefore, when adding one or more of Zn, Co, and Zr, the contents of each are preferably 0.020% or less.

[0112] Ca: below 0.0200%

[0113] Ca exists as inclusions in steel. If the Ca content exceeds 0.0200%, and the steel plate contains diffusible hydrogen, these inclusions may become crack initiation points during bending deformation, and bending and impact resistance may decrease. Therefore, when Ca is added, the Ca content is preferably 0.0200% or less. Furthermore, the Ca content is more preferably 0.0020% or less. It should be noted that the lower limit of the Ca content can be 0.0000%, but due to limitations in production technology, the Ca content is preferably 0.0001% or more.

[0114] Ce: less than 0.0200%, Se: less than 0.0200%, Te: less than 0.0200%, Ge: less than 0.0200%, As: less than 0.0200%, Sr: less than 0.0200%, Cs: less than 0.0200%, Hf: less than 0.0200%, Pb: less than 0.0200%, Bi: less than 0.0200%, REM: less than 0.0200%

[0115] Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are effective elements for improving the bending and impact resistance properties of steel sheets by enhancing their ultimate deformation capacity. To achieve this effect, the contents of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are preferably 0.0001% or more. On the other hand, if the contents of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM exceed 0.0200%, there is a possibility of the formation of large coarse precipitates and inclusions. In such cases, if diffusible hydrogen is present in the steel sheet, these precipitates and inclusions may become crack initiation points during bending deformation, reducing bending and impact resistance properties. Therefore, when any one of Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM is added, its content is preferably 0.0200% or less.

[0116] [1-2] Composition of coating materials

[0117] Next, the composition of the cladding material of the cladding steel sheet according to one embodiment of the present invention will be described. It should be noted that the unit of composition is "mass %", and hereafter, unless otherwise specified, it will be expressed as "%".

[0118] C: Below 0.100%

[0119] Carbon (C) reduces LME resistance. Furthermore, C reduces flexural and impact resistance through the formation of martensite and retained austenite. Therefore, C is preferably present in the lowest possible amount. If the C content exceeds 0.100%, excessive increases in martensite and retained austenite lead to decreased flexural and impact resistance. Therefore, the C content is 0.100% or less. The C content is preferably 0.090% or less, more preferably 0.070% or less. It should be noted that there is no specific lower limit for the C content; due to limitations in production technology, the C content is mostly 0.001% or more.

[0120] Si: below 0.60%

[0121] Si reduces LME resistance and thus hinders galvanization, therefore it is preferred to contain as little as possible. If the Si content exceeds 0.60%, both LME resistance and galvanization performance decrease. Therefore, the Si content is 0.60% or less. The Si content is preferably 0.40% or less, more preferably 0.30% or less. It should be noted that there is no specific lower limit for the Si content; due to limitations in production technology, the Si content is mostly 0.01% or more.

[0122] Mn: 0.05%~2.50%

[0123] Mn is an important element for adjusting the area fraction of martensite, tempered martensite, and retained austenite. If the Mn content is less than 0.05%, the area fraction of ferrite increases, making it difficult to achieve a strength (TS) of 780 MPa or higher. On the other hand, if the Mn content exceeds 2.50%, the volume fraction of retained austenite increases excessively, leading to a significant increase in the hardness of the martensite formed from retained austenite during bending deformation. This results in reduced bending and impact resistance. This effect is particularly significant in cladding materials for high-strength cladding steel sheets. Therefore, the Mn content is between 0.05% and 2.50%. The Mn content is preferably 0.15% or more, more preferably 0.20% or more. The Mn content is preferably 2.30% or less, more preferably 2.20% or less.

[0124] P: 0.001%~0.100%

[0125] Phosphorus (P) is an element that provides solid solution strengthening, thereby increasing the strength of steel plates. To achieve this effect, the P content is set at 0.001% or more. On the other hand, if the P content exceeds 0.100%, it deteriorates the galvanization properties and surface appearance. Therefore, the P content is 0.001% to 0.100%. More preferably, the P content is 0.030% or less.

[0126] S: below 0.0200%

[0127] If the sulfur (S) content exceeds 0.0200%, the dissolved S will segregate extensively at the austenite grain boundaries, causing surface cracking during hot rolling. Furthermore, the extensive segregation of S at the oxide scale interface worsens the oxide scale's peelability. Therefore, the S content is preferably 0.0200% or less, and more preferably 0.0080% or less. It should be noted that while there is no specific lower limit for the S content, due to limitations in production technology, the S content is mostly 0.0001% or more.

[0128] Al: 0.010%~0.100%

[0129] Al acts as a deoxidizer. To obtain the desired effect from the addition of Al, the Al content is set at 0.010% or higher. On the other hand, if the Al content exceeds 0.100%, a large number of unrecrystallized grains remain due to the pinning effect of nitrides, which easily leads to surface defects. Therefore, the Al content is set between 0.010% and 0.100%.

[0130] N: below 0.0100%

[0131] Nitrogen (N) exists in steel as nitrides. If its content exceeds 0.0100%, a large number of unrecrystallized grains remain due to the pinning effect of nitrides, easily leading to surface defects. Therefore, the N content is 0.0100% or less. Furthermore, the N content is more preferably 0.0050% or less. It should be noted that there is no specific lower limit for the N content; due to limitations in production technology, the N content is mostly 0.0005% or more.

[0132] The basic composition of the cladding material of the cladding steel sheet according to one embodiment of the present invention has been described above. However, the cladding material of the cladding steel sheet according to one embodiment of the present invention has a composition containing the above-described basic components, and the remaining portion other than the above-described basic components contains Fe (iron) and unavoidable impurities. Here, the cladding material of the cladding steel sheet according to one embodiment of the present invention preferably has a composition containing the above-described basic components, and the remaining portion consists of Fe and unavoidable impurities. In addition, at least one of the above-described basic components selected from any of the components shown below may be added to the cladding material of the cladding steel sheet according to one embodiment of the present invention. It should be noted that if any of the components shown below are contained in amounts below the upper limit indicated below, the desired effect can be obtained, therefore a lower limit is not specifically set. It should be noted that when the preferred lower limit value described below includes any of the following elements, the element is included as an unavoidable impurity.

[0133] Sb: ≤0.200%, Sn: ≤0.200%, Ti: ≤0.200%, Nb: ≤0.200%, V: ≤0.100%, B: ≤0.0100%, Cu: ≤1.00%, Cr: ≤1.000%, Ni: ≤1.000%, Mo: ≤0.50%, Ta: ≤0.100%, W: ≤0.500%, Mg: ≤0.0200%, Zn: ≤0.020%, Co: ≤0.020%. The following concentrations are specified: Zr: less than 0.020%, Ca: less than 0.0200%, Ce: less than 0.0200%, Se: less than 0.0200%, Te: less than 0.0200%, Ge: less than 0.0200%, As: less than 0.0200%, Sr: less than 0.0200%, Cs: less than 0.0200%, Hf: less than 0.0200%, Pb: less than 0.0200%, Bi: less than 0.0200%, and REM: less than 0.0200%.

[0134] It should be noted that the reasons for limiting the above-mentioned arbitrary added components are substantially the same as the reasons for limiting the arbitrary added components of the base material of the cladding steel plate according to one embodiment of the present invention, and therefore are omitted here.

[0135] [1-3] Steel structure of the base material

[0136] Next, the steel structure of the base material of the cladding steel plate according to one embodiment of the present invention will be described.

[0137] According to one embodiment of the present invention, the steel microstructure of the base material of the cladding steel plate is a steel microstructure with an area ratio of ferrite of 55% or less, an area ratio of martensite and tempered martensite of 30% or more, and a volume ratio of retained austenite of 5% or less.

[0138] Ferrite area fraction: below 55%

[0139] To ensure good bending and impact resistance, the ferrite area fraction needs to be set to 55% or less. Furthermore, the ferrite area fraction is preferably 50% or less. There is no particular limitation on the lower limit of the ferrite area fraction, and it can be 0%. The ferrite area fraction is preferably 1% or more.

[0140] The combined area ratio of martensite and tempered martensite is over 30%.

[0141] To ensure a total strength (TS) of 780 MPa or higher, the combined area ratio of martensite and tempered martensite needs to be 30% or higher. Preferably, the combined area ratio of martensite and tempered martensite is 35% or higher. The upper limit of the combined area ratio of martensite and tempered martensite is not particularly limited and can be 100%. Preferably, the combined area ratio of martensite and tempered martensite is 99% or lower.

[0142] It should be noted that if the combined area ratio of martensite and tempered martensite is 30% or more, then the area ratios of martensite and tempered martensite can be 0% and 0% respectively.

[0143] Here, the method for determining the area ratio of ferrite, martensite, and tempered martensite is as follows. It should be noted that the area ratio is measured at 1 / 4 of the thickness of the base material.

[0144] The specimen was cut with the cross-section of the cladding steel plate parallel to the rolling direction as the observation surface. Next, the observation surface was mirror-polished using diamond polishing paste, followed by fine polishing with colloidal silica, and further etched using 3 vol.% nitric acid alcohol to reveal the microstructure. Then, under an accelerating voltage of 15 kV, the observation surface of the specimen was observed using a SEM (Scanning Electron Microscope) at 5000x magnification within a 17 μm × 23 μm field of view in three fields of view. In the obtained microstructure images, Adobe Photoshop (Adobe Systems) was used to calculate the area ratio of each constituent microstructure (ferrite, martensite, tempered martensite) divided by the measured area in the three fields of view. The average of these values ​​was then used to determine the area ratio of each microstructure.

[0145] Volume fraction of retained austenite: less than 5%

[0146] If the volume fraction of retained austenite exceeds 5%, the hardness of the martensite formed from the retained austenite increases, while the bending and impact resistance decreases during bending deformation. Therefore, the volume fraction of retained austenite is 5% or less. Furthermore, the volume fraction of retained austenite is preferably 4% or less. There is no particular limitation on the lower limit of the volume fraction of retained austenite, and it can be 0%. The volume fraction of retained austenite is preferably 1% or more.

[0147] The method for determining the volume fraction of retained austenite is as follows: The cladding steel plate is mechanically ground in the thickness direction (depth direction) to 1 / 4 of the base material thickness, followed by chemical grinding with oxalic acid to create an observation surface. This observation surface is then observed using X-ray diffraction. Using a Co Kα ray source as the incident X-ray, the ratio of the diffraction intensities of the {200}, {220}, and {311} planes of fcc iron (austenite) to those of bcc iron (austenite) is calculated. Based on this ratio, the volume fraction of retained austenite is calculated.

[0148] Furthermore, the area fraction of the remaining microstructure other than ferrite, martensite, tempered martensite, and retained austenite is preferably 30% or less. More preferably, the area fraction of the remaining microstructure is 20% or less. Examples of the remaining microstructure include known microstructures such as bainite, pearlite, and cementite, which are carbides. It should be noted that the presence or absence of the remaining microstructure can be determined, for example, by SEM observation. The area fraction of the remaining microstructure can be 0%. The area fraction of the remaining microstructure is calculated as follows.

[0149] [Area fraction of remaining microstructure (%)] = 100 - [Area fraction of ferrite (%)] - [Area fraction of martensite and tempered martensite combined (%)] - [Volume fraction of retained austenite (%)]

[0150] It should be noted that here, the retained austenite is considered to be three-dimensionally homogeneous, that is, the volume ratio of the retained austenite is considered to be equivalent to the area ratio of the retained austenite, and the area ratio of the remaining part of the structure is calculated.

[0151] [1-4] Steel structure of cladding material

[0152] Next, the steel structure of the cladding material of the cladding steel plate according to one embodiment of the present invention will be described.

[0153] According to one embodiment of the present invention, the cladding material of the cladding steel plate has a steel microstructure with an area ratio of ferrite of 80% or more.

[0154] Ferrite area ratio: over 80%

[0155] To ensure good bending and impact resistance, the ferrite area ratio needs to be set to 80% or more. Furthermore, the ferrite area ratio is preferably 90% or more. There is no particular upper limit to the ferrite area ratio; it can be 100%.

[0156] Here, the method for determining the area ratio of ferrite is as follows. It should be noted that the area ratio is measured at 1 / 4 of the thickness of the cladding material.

[0157] The specimen was cut with the cross-section of the cladding steel plate parallel to the rolling direction serving as the observation surface. Next, the observation surface was mirror-polished using diamond polishing paste, followed by fine polishing with colloidal silica, and then etched using 3 vol.% nitric acid alcohol to reveal the microstructure. Then, under an accelerating voltage of 15 kV, the observation surface of the specimen was observed using a SEM (Scanning Electron Microscope) at 5000x magnification within a 17 μm × 23 μm field of view in three fields of view. In the obtained microstructure images, Adobe Photoshop (Adobe Systems) was used to calculate the area ratio of each constituent microstructure (ferrite) divided by the measured area in the three fields of view. These values ​​were then averaged to obtain the area ratio of each microstructure.

[0158] Furthermore, the area fraction of the remaining microstructure other than ferrite is preferably 20% or less. More preferably, the area fraction of the remaining microstructure is 10% or less. Examples of the remaining microstructure include known microstructures such as unrecrystallized ferrite, martensite, tempered martensite, retained austenite, bainite, pearlite, cementite, and other carbides. It should be noted that the presence or absence of the remaining microstructure can be determined, for example, by SEM observation. The area fraction of the remaining microstructure can be 0%. Alternatively, the area fraction of the remaining microstructure can be calculated as follows.

[0159] [Area fraction of remaining tissue (%)] = 100 - [Area fraction of ferrite (%)]

[0160] [1-5] The average Vickers hardness (HVL) of the cladding material, the value obtained by dividing the average Vickers hardness (HVL) of the cladding material by the average Vickers hardness (HVB) of the base material, the boundary roughness between the base material and the cladding material, and the number of voids present at the boundary between the base material and the cladding material.

[0161] Next, in a cladding steel sheet based on an embodiment of the present invention, the average Vickers hardness (HVL) of the cladding material, which is an important component, the value obtained by dividing the average Vickers hardness (HVL) of the cladding material by the average Vickers hardness (HVB) of the base material, the boundary roughness between the base material and the cladding material, and the number of voids present at the boundary between the base material and the cladding material will be explained.

[0162] Average Vickers hardness (HVL) of the coating material: below 260

[0163] To ensure good flexural properties, impact resistance, and LME resistance, the average Vickers hardness (HVL) of the coating material needs to be set to 260 or less. Furthermore, the average Vickers hardness (HVL) of the coating material is preferably 250 or less. There is no particular limitation on the lower limit of the average Vickers hardness (HVL) of the coating material, but it is preferably 85 or more. It should be noted that both the surface and back of the coating material bonded to the base material need to meet this requirement. The same applies to the value obtained by dividing the average Vickers hardness (HVL) of the coating material by the average Vickers hardness (HVB) of the base material, as described later.

[0164] The value obtained by dividing the average Vickers hardness (HVL) of the coating material by the average Vickers hardness (HVB) of the base material (hereinafter also referred to as the hardness ratio of the coating material to the base material): below 0.80

[0165] To ensure good flexural properties and impact resistance, the hardness ratio of the coating material to the base material needs to be below 0.80. Preferably, the hardness ratio is below 0.75. There is no particular lower limit to the hardness ratio of the coating material to the base material, but it is preferably above 0.07.

[0166] Here, the average Vickers hardness (HVB) of the base material is measured as follows.

[0167] The Vickers hardness is measured at the center of the base material's thickness using an indentation load of 1 kg. Then, starting from this measurement point, the Vickers hardness is measured at 10 points along a line parallel to the rolling direction using an indentation load of 1 kg. The Vickers hardness can be calculated from these average values.

[0168] In addition, the average Vickers hardness (HVL) of the coating material was measured as follows.

[0169] The Vickers hardness of the cladding material is measured at the center of the sheet thickness with an indentation load of 100g. Then, starting from this measurement point, the Vickers hardness is measured at 10 points along a line parallel to the rolling direction with an indentation load of 100g. The average value of these measurements is taken as the average Vickers hardness (HVL) of the cladding material.

[0170] It should be noted that the interval between the measurement points for the average Vickers hardness (HVB) of the base material and the average Vickers hardness (HVL) of the coating material is preferably at least three times the distance of the indentation, if possible. It should be noted that "at least three times the distance of the indentation" means at least three times the length of the diagonal of the rectangular opening of the indentation produced by the diamond indenter during the Vickers hardness measurement.

[0171] Boundary roughness between the base material and the coating material: less than 50 μm based on the maximum height Ry.

[0172] To ensure good bending resistance and impact resistance, the boundary roughness between the base material and the cladding material should be less than 50 μm (Ry). When the boundary roughness exceeds 50 μm (maximum height Ry), stress concentration easily occurs at the boundary between the base material and the cladding material during bending tests (during compression molding) and crush tests (during vehicle collisions), becoming the initiation point for cracking. Preferably, the boundary roughness between the base material and the cladding material is less than 30 μm (maximum height Ry). It should be noted that this requirement must be met at the boundary between the surface and back of the base material. There is no particular lower limit to the boundary roughness between the base material and the cladding material, but it is preferably greater than 5 μm (maximum height Ry).

[0173] It should be noted that the maximum height (Ry) is calculated in accordance with JIS B 0601 (1994) and JIS B 0031 (1994).

[0174] Specifically, the specimen was cut with the thickness section of the cladding steel plate parallel to the rolling direction as the observation surface. Next, the observation surface was mirror-polished using diamond polishing paste, followed by fine polishing with colloidal silica, and then etched using 3 vol.% nitric acid alcohol to reveal the microstructure. Then, under an accelerating voltage of 15 kV, the boundary between the base material and the cladding material was observed using SEM at 150x magnification in five fields of view. Adobe Photoshop was used to define the boundary between the base material and the cladding material by contrast difference, and the maximum height (Ry) was calculated according to the formulas in JIS B 0601 (1994) and JIS B0031 (1994).

[0175] Number of voids at the boundary between the base material and the coating material: less than 20 per 10 mm boundary length.

[0176] To ensure good bending resistance and impact resistance, the number of voids at the boundary between the base material and the cladding material should be set to 20 or less per 10 mm of boundary length. If the number of voids at the boundary between the base material and the cladding material exceeds 20 per 10 mm of boundary length, these voids will become the starting point for cracking during bending tests (during compression molding) and crush tests (during vehicle collisions). Furthermore, the increase in the number of voids, accompanied by the connection of these voids, promotes crack progression. Preferably, the number of voids at the boundary between the base material and the cladding material should be 15 or less per 10 mm of boundary length. There is no particular limitation on the lower limit of the number of voids per 10 mm of boundary length at the boundary between the base material and the cladding material; it can be 0. It should be noted that this requirement must be met at the boundaries on both the surface and back sides of the base material.

[0177] Here, the number of voids present at the boundary between the base material and the cladding material is determined as follows.

[0178] The specimen was cut with the thickness section (L section) of the cladding steel plate parallel to the rolling direction as the observation surface. Next, the observation surface was mirror-polished using diamond polishing paste, followed by fine polishing with colloidal silica, and then etched with 3 vol.% nitric acid alcohol to reveal the microstructure. Then, under an accelerating voltage of 15 kV, the boundary between the base material and the cladding material was observed using SEM at 3000x magnification in 30 fields of view. The total number of voids observed in all 30 fields of view was then counted. The total number of voids observed was divided by the total length (in mm) of the observation area of ​​the 30 fields of view, and then multiplied by 10. This value was taken as the number of voids present at the boundary between the base material and the cladding material for every 10 mm of boundary length.

[0179] It should be noted that, for reference only, Figure 1 The image shows an example of a SEM photograph of the boundary between the base material and the cladding material at section L. Figure 1 (a) is Example No. 4, and (b) is Example No. 44. In the figures, "high magnification" refers to a SEM image with a magnification of 3000x.

[0180] [1-6] Thickness

[0181] The thickness of the cladding steel sheet according to one embodiment of the present invention is not particularly limited, but is preferably 0.5 mm to 3.0 mm. Furthermore, the thickness of the base material is preferably 0.2 mm to 2.8 mm. The total thickness of the cladding material is preferably 0.2 mm to 2.8 mm. The thickness of each sheet of cladding material is preferably 0.1 mm to 1.4 mm.

[0182] In addition, it is preferable to set the value obtained by dividing the thickness of the base material by the total thickness of the coating material to 1 or more.

[0183] The value obtained by dividing the thickness of the base material by the total thickness of the coating materials: 1 or more

[0184] When the value obtained by dividing the thickness of the base material by the total thickness of the cladding materials is 1 or higher, a higher maximum load (F) for VDA bending and V-bending-orthogonal VDA bending can be obtained. Therefore, the value obtained by dividing the thickness of the base material by the total thickness of the cladding materials is preferably 1 or higher. There is no particular upper limit to the value obtained by dividing the thickness of the base material by the total thickness of the cladding materials; for example, the value obtained by dividing the thickness of the base material by the total thickness of the cladding materials is preferably 30 or lower.

[0185] [1-7] Coating

[0186] In one embodiment of the present invention, the cladding steel sheet may have a coating on its surface, such as a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electroplated galvanized layer.

[0187] The composition of hot-dip galvanized layers, alloyed hot-dip galvanized layers, and electroplated galvanized layers is not particularly limited as long as Zn is the main component. For example, it may have the following composition: containing Fe: less than 20% by mass, Al: 0.001% to 1.0% by mass, and further containing a total of 0% to 3.5% by mass of one or more of Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM, with the remainder consisting of Zn and unavoidable impurities.

[0188] The Fe content in the hot-dip galvanized layer is preferably less than 7% by mass. Furthermore, the Fe content in the alloyed hot-dip galvanized layer is preferably 7-15% by mass, more preferably 8-12% by mass.

[0189] In addition, there is no particular limitation on the amount of coating applied, but the preferred coating amount per single side is 20 to 80 g / m². 2 .

[0190] [1-8] Diffusive hydrogen content

[0191] In a cladding steel sheet according to one embodiment of the present invention, it is preferable that the total amount of diffusible hydrogen contained in the base material and the cladding material is 0.50 ppm by mass or less.

[0192] Total diffusible hydrogen content in the base material and coating materials: less than 0.50 ppm by mass.

[0193] From the viewpoint of obtaining superior flexibility, in one embodiment of the present invention, the cladding steel sheet preferably has a total diffusible hydrogen content of 0.50 ppm by mass or less in the base material and the cladding material. Furthermore, a diffusible hydrogen content of 0.35 ppm by mass or less is more preferable. It should be noted that there is no particular lower limit for the diffusible hydrogen content of steel; due to limitations in production technology, the diffusible hydrogen content in steel sheets is mostly 0.01 ppm by mass or more.

[0194] Here, the total amount of diffusible hydrogen contained in the base material and the coating material is measured as follows.

[0195] A test piece measuring 30 mm in length and 5 mm in width was taken from the cladding steel sheet. If a coating was present on the surface, the coating was removed using alkali. Then, the amount of hydrogen released from the test piece was determined using a temperature-dependent hydrogen release analysis method. Specifically, the test piece was continuously heated from room temperature to 300°C at a heating rate of 200°C / h, and then cooled to room temperature. The amount of hydrogen released from the test piece during this continuous heating temperature range from room temperature to 210°C (cumulative hydrogen content) was measured. The measured hydrogen content was divided by the mass of the test piece (or, in the case of a removed coating, the test piece before continuous heating) and converted to ppm (parts per second) to obtain the total diffusible hydrogen content contained in the base material and the cladding material.

[0196] It should be noted that for products (parts) after the cladding steel sheet has undergone forming and joining processes, test pieces are cut from the product placed in a normal usage environment, and the diffusible hydrogen content of the base material and cladding material is measured using the same method as described above. Furthermore, if this value is 0.50 ppm by mass or less, it is considered that the total diffusible hydrogen content contained in the base material and cladding material of the cladding steel sheet in the material stage before forming and joining processes is also 0.50 ppm by mass or less.

[0197] [2] Components

[0198] Next, the components of one embodiment of the present invention will be described.

[0199] One embodiment of the present invention is a component (referred to as a material) made using the aforementioned cladding steel sheet. For example, the component is made by performing at least one of forming or joining processes on the cladding steel sheet, which serves as a blank.

[0200] Here, the tensile strength (TS) of the aforementioned cladding steel sheet is 780 MPa or higher, exhibiting excellent bending resistance, impact resistance, and LME resistance. Therefore, the component of one embodiment of the present invention not only possesses high strength but also exhibits excellent impact resistance in the event of a collision during driving. Furthermore, not only is it strong (780 MPa or higher), but it is also less prone to LME cracking when the steel sheet is plated. Therefore, the component of one embodiment of the present invention is suitable for use as an impact energy absorption component in the automotive field.

[0201] [3] Manufacturing method of cladding steel plate

[0202] Next, a method for manufacturing a cladding steel sheet according to one embodiment of the present invention will be described. It should be noted that, unless otherwise specified, the temperatures at which the various slabs and steel sheets shown below are heated or cooled refer to the surface temperatures of the various slabs and steel sheets.

[0203] A method for manufacturing a cladding steel plate according to one embodiment of the present invention includes the following steps:

[0204] The first preparation step involves preparing a base steel billet with the composition of the base material described above.

[0205] The second preparation step involves preparing a steel billet for the coating material, which has the composition of the coating material described above.

[0206] The surface treatment process involves performing surface treatment to ensure that the surface roughness, in Ra terms, is 30 μm or less on both the surface and back sides of the base steel billet and at least one of the surface and back sides of the cladding material steel billet.

[0207] In the lamination process, the surface-treated surface of the base steel billet is in contact with the surface-treated surface of the cladding material steel billet, and the base steel billet and the cladding material steel billet are laminated in the order of cladding material steel billet – base steel billet – cladding material steel billet to obtain a laminated slab.

[0208] In the joining process, the aforementioned cladding material steel billet and the aforementioned base material steel billet are joined together, and a vacuum is drawn so that the vacuum degree between the aforementioned cladding material steel billet and the aforementioned base material steel billet is 1×10⁻⁶. －2 Below Torr, a laminated slab blank is obtained.

[0209] In the hot rolling process, the above-mentioned laminated slab is heated to a temperature range of 1050℃~1350℃, and then hot-rolled at a final rolling temperature of 820℃ or higher to obtain a hot-rolled steel sheet.

[0210] The cold rolling process involves cold rolling the aforementioned hot-rolled steel sheet under a reduction rate of 30% to 80% to obtain a cold-rolled steel sheet, and...

[0211] The annealing process involves annealing the above-mentioned cold-rolled steel sheet at an annealing temperature of 750℃~950℃ and a holding time of more than 20 seconds.

[0212] First preparation process

[0213] Prepare a steel billet with the composition of the aforementioned base material. For example, melt the steel billet to produce molten steel with the composition of the aforementioned base material. The melting method is not particularly limited; known melting methods such as converter melting and electric furnace melting are suitable. Solidify the resulting molten steel and manufacture steel billets (slabs). The method for manufacturing steel billets from molten steel is not particularly limited; continuous casting, ingot casting, or thin slab casting can be used. To prevent macroscopic segregation, continuous casting is preferred for manufacturing steel billets.

[0214] Second preparation process

[0215] Here, a steel billet with the composition of the aforementioned coating material is prepared. For example, the steel billet is melted to produce molten steel with the composition of the aforementioned coating material. The melting method is not particularly limited; any known melting method such as converter melting or electric furnace melting is suitable. The resulting molten steel is solidified and used to manufacture steel billets (slabs). The method for manufacturing steel billets from molten steel is not particularly limited; continuous casting, ingot casting, or thin slab casting can be used. To prevent macroscopic segregation, continuous casting is preferred for manufacturing the steel billets.

[0216] Surface treatment process

[0217] Surface treatment is performed to ensure that the surface roughness, in terms of Ra, is 30 μm or less on both the front and back surfaces of the base steel billet prepared as described above, and on at least one of the front and back surfaces of the cladding material billet (the lower limit of surface roughness is not particularly limited, but from the viewpoint of productivity, it is preferable to have a surface roughness of 1 μm or more in terms of Ra). This allows the boundary roughness of the base material and cladding material of the final cladding steel sheet to be set to 50 μm or less at its maximum height Ry. Furthermore, it enables good bonding between the base material and the cladding material during the hot rolling process. It should be noted that the surface treatment method is not particularly limited; for example, finishing can be performed by mechanical grinding.

[0218] It should be noted that the surface roughness Ra was measured in accordance with JIS B 0601 (1994) and JIS B 0031 (1994).

[0219] •Lamination process

[0220] Next, the base steel billet and the aforementioned cladding material billet are stacked in the order of cladding material billet – base steel billet – cladding material billet, with the surface-treated surface of the base steel billet in contact with the surface-treated surface of the cladding material billet, to obtain a laminated slab (forming a sandwich structure with the base steel billet sandwiched between the cladding material billets). It should be noted that the surfaces of the base steel billet and the cladding material billet can be cleaned before stacking.

[0221] • Joining process

[0222] Next, the cladding material billet and the base steel billet are joined together, and a vacuum is drawn to ensure that the vacuum degree between the cladding material billet and the base steel billet is 1×10⁻⁶. －2 Below Torr, a laminated slab is obtained (becoming a sandwich structure in which the base material slab is sandwiched between the cladding material slabs).

[0223] There are no particular limitations on the joining method. For example, a base steel billet is placed between two cladding material steel billets to obtain a laminated slab. Electron beam welding (EBW), arc welding, or laser beam welding are performed around the ends of the laminated slab (between the cladding material steel billet and the base steel billet) to join the cladding material steel billet and the base steel billet.

[0224] In addition, the vacuum degree between the steel billet of each cladding material and the base steel billet is reduced to 1×10. －2 Below Torr (the lower limit of vacuum is not particularly limited, but from the point of view of productivity, it is preferably 1×10⁻⁶). －7 The high vacuum level (above Torr) further improves the bonding strength of the interface between the cladding billet and the base billet. Therefore, even when cold rolling is performed after hot rolling, the integrity of the bonding interface can be maintained without creating voids between the cladding material and the base material, enabling the manufacture of a complete cladding sheet (thin sheet).

[0225] There is no particular limitation on the method of vacuuming between each cladding material billet and the base material billet. For example, when laser beam welding is performed around the ends of the laminated slab, a vacuum tube for vacuuming is set between the cladding material billet and the base material billet at the end (before the ends are fully joined). By connecting it to a vacuum pump, a vacuum is thus created between each cladding material billet and the base material billet.

[0226] Hot rolling process

[0227] Next, the obtained laminated slab is subjected to hot rolling consisting of rough rolling and finish rolling to obtain hot-rolled steel plate.

[0228] In one example, the laminated slab manufactured as described above is temporarily cooled to room temperature, and then rolled after the slab is heated.

[0229] Slab heating temperature: 1050℃~1350℃

[0230] Considering the diffusion bonding between the cladding material and the base material, the dissolution of carbides, and the reduction of rolling load, the slab heating temperature is set to 1050°C or higher. Furthermore, to prevent increased oxide scale loss, the slab heating temperature is set to 1350°C or lower. It should be noted that the slab heating temperature is based on the temperature of the surface of the bonded laminated slab during heating.

[0231] In addition, energy-saving processes can also be applied to hot rolling. Examples of energy-saving processes include direct rolling, in which the manufactured steel billet is loaded into the heating furnace as a hot billet without cooling it to room temperature; or direct rolling, in which the manufactured steel billet is rolled immediately after being slightly heated.

[0232] Next, the laminated slab is rough-rolled using conventional methods to obtain a thin slab. This thin slab is then finish-rolled to produce a hot-rolled steel sheet. It should be noted that, from the viewpoint of preventing malfunctions during finish rolling, it is preferable to heat the thin slab using a bar heater or similar device before finish rolling, while reducing the slab heating temperature.

[0233] Final rolling temperature: above 820℃

[0234] To reduce rolling load, and because a higher reduction rate in the non-recrystallized austenite state can lead to the development of abnormally elongated microstructure in the rolling direction, potentially reducing the workability of the annealed sheet, the final rolling temperature is preferably set to 820°C or higher. There is no particular upper limit to the final rolling temperature, but for example, a final rolling temperature of 1100°C or lower is preferred.

[0235] Alternatively, rough-rolled plates can be joined together during hot rolling for continuous finishing rolling. Alternatively, rough-rolled plates (thin slabs) can be temporarily wound before finishing rolling. Furthermore, to reduce the rolling load during hot rolling, part or all of the finishing rolling can be lubricated. Lubricated rolling is also effective from the viewpoint of homogenizing the shape and material of the steel plate. It should be noted that the coefficient of friction during lubricated rolling is preferably in the range of 0.10 to 0.25.

[0236] It should be noted that there is no particular limitation on the winding temperature after hot rolling, but it is preferably 450℃~750℃.

[0237] Next, the hot-rolled steel sheet is subjected to pickling. Pickling removes oxides from the steel sheet surface, which is crucial for ensuring good chemical treatment properties and coating quality of the final product. It should be noted that pickling can be performed once or in multiple stages.

[0238] Cold rolling process

[0239] Next, the hot-rolled steel sheet is cold-rolled to produce a cold-rolled steel sheet. For example, cold rolling is carried out by multi-pass rolling, which requires two or more passes, such as tandem multi-stand rolling or reversible rolling.

[0240] Cold rolling reduction rate: 30%–80%

[0241] By setting the cold rolling reduction rate to 30% or more, residual cracks at the boundary between the base material and the cladding material in the final cladding steel sheet can be suppressed. Furthermore, recrystallization during the heating process in the subsequent annealing step results in good flexibility. Therefore, the cold rolling reduction rate is 30% or more, preferably 35% or more. On the other hand, if the cold rolling reduction rate exceeds 80%, the integrity of the aforementioned joint interface cannot be ensured; therefore, the upper limit for the cold rolling reduction rate is 80% or less.

[0242] It should be noted that there are no special restrictions on the number of rolling passes in cold rolling.

[0243] Annealing process

[0244] Next, the cold-rolled steel sheet is annealed at an annealing temperature of 750℃~950℃ and a holding time of more than 20 seconds.

[0245] Annealing temperature: 750℃~950℃, holding time: 20 seconds or more

[0246] During bending deformation, the hardness of martensite formed from retained austenite increases significantly, reducing bending and impact resistance. When the annealing temperature is less than 750°C or the holding time is less than 20 seconds, unrecrystallized ferrite remains in the coating material, further reducing bending and impact resistance. Furthermore, the proportion of austenite formed during annealing becomes insufficient in the base material. Therefore, the ferrite area ratio increases, making it difficult to achieve a TS of 780 MPa or higher. On the other hand, if the annealing temperature exceeds 950°C, defects may sometimes occur on the surface of the coating material. Therefore, the annealing temperature is between 750°C and 950°C. The annealing temperature is preferably 760°C or higher. Additionally, the annealing temperature is preferably 920°C or lower. It should be noted that the holding time is not particularly limited, but is preferably 600 seconds or less. It should be noted that the annealing temperature is the highest temperature reached during the annealing process. Furthermore, the holding time includes not only the holding time at the annealing temperature but also the residence time within the temperature range of (annealing temperature - 40°C) to the annealing temperature during heating and cooling before and after reaching the annealing temperature.

[0247] There are no particular limitations on the cooling after holding; a reheating process can be performed arbitrarily after the annealing process using conventional methods, under the following conditions. This situation will be described below as a first embodiment of any process after the annealing process.

[0248] [First Implementation Method]

[0249] First reheating process

[0250] Here, after the annealing process, the cold-rolled steel sheet is cooled to a cooling stop temperature below 250°C, and then heated to a temperature range above 250°C but below 450°C and held for more than 10 seconds.

[0251] Cooling stop temperature: below 250℃

[0252] By setting the cooling stop temperature to below 250°C, which is below the martensitic transformation initiation temperature, the area ratio of tempered martensite generated during reheating (described later) can be increased. Furthermore, by causing a portion of the austenite to undergo martensitic transformation at the cooling stop point, the amount of diffusible hydrogen in the steel sheet is reduced. As a result, the amount of voids generated during bending deformation is reduced, thus further improving bending resistance and impact resistance. Therefore, the cooling stop temperature is preferably below 250°C. More preferably, it is below 200°C. The lower limit of the cooling stop temperature is not particularly limited; for example, it is preferably above -30°C.

[0253] It should be noted that the average cooling rate from the end of the annealing process to the aforementioned cooling stop temperature is not particularly limited, but is preferably 1°C / second to 50°C / second.

[0254] Reheating temperature: above 250℃ but below 450℃

[0255] After the aforementioned cooling is stopped, the cold-rolled steel sheet is reheated to a temperature range exceeding 250°C but below 450°C, and held at this temperature range for at least 10 seconds. By reheating the temperature above 250°C, the tempering of the martensite present at the time of cooling cessation is further promoted. However, when the reheating temperature exceeds 450°C, the diffusible hydrogen content in the steel sheet may increase along with the increase in the area ratio of quenched martensite. Therefore, there is a possibility of a decrease in bending properties and impact resistance. Therefore, the reheating temperature is preferably above 250°C but below 450°C. More preferably, the reheating temperature is 300°C or higher. Furthermore, more preferably, the reheating temperature is 400°C or lower. It should be noted that the reheating temperature is the highest temperature reached during the reheating process.

[0256] Duration: 10 seconds or more

[0257] When the holding time within the reheating temperature range (above 250°C and below 450°C) is less than 10 seconds, the diffusible hydrogen content in the steel sheet may increase along with the increase in the area ratio of quenched martensite. Therefore, there is a possibility of reduced flexibility and impact resistance. Therefore, the holding time within the reheating temperature range is preferably 10 seconds or more. It should be noted that there is no particular upper limit to the holding time within the reheating temperature range; however, due to limitations in production technology, it is preferably set to 1000 seconds or less. The holding time within the reheating temperature range is more preferably 10 to 300 seconds. It should be noted that the holding time within the reheating temperature range includes not only the holding time at the reheating temperature but also the residence time within the reheating temperature range (above 250°C and below) during heating and cooling before and after reaching the reheating temperature.

[0258] The average cooling rate, cooling stop temperature, and cooling method after holding at the reheating temperature are not particularly limited. Cooling methods can include gas jet cooling, water mist cooling, roller cooling, water cooling, and air cooling. Furthermore, from the viewpoint of preventing oxidation of the steel plate surface, it is preferable to cool to below 50°C after holding at the reheating temperature, and more preferably to around room temperature. The average cooling rate is typically 1°C / second to 50°C / second.

[0259] Alternatively, the cold-rolled steel sheet that has undergone the above-described processes can also be subjected to quenching and tempering rolling. When the reduction rate of quenching and tempering rolling exceeds 1.50%, the yield stress of the steel increases, and the dimensional accuracy during forming decreases; therefore, it is preferable to be 1.50% or less. It should be noted that there is no particular limitation on the lower limit of the reduction rate in quenching and tempering rolling; from a productivity point of view, it is preferable to be 0.05% or more. Furthermore, quenching and tempering rolling can be performed continuously on an annealing apparatus used for the above-described annealing process (online) or discontinuously on an annealing apparatus used for the annealing process (offline). In addition, the target reduction rate can be achieved in a single rolling pass, or multiple rolling passes can be performed to achieve a total reduction rate of 0.05% to 1.50%. It should be noted that the rolling described here generally refers to quenching and tempering rolling, but rolling using a straightening machine or the like can also be used as long as it can impart the same elongation as quenching and tempering rolling.

[0260] Plating process

[0261] Alternatively, the cold-rolled steel sheet can be plated after the annealing process or the first reheating process described above. There are no particular limitations on the plating method; examples include hot-dip galvanizing, alloyed hot-dip galvanizing, or electro-galvanizing. The conditions for these plating processes are not particularly limited and can be performed using conventional methods.

[0262] When performing hot-dip galvanizing, it is preferable to immerse cold-rolled steel sheets in a galvanizing bath at 440°C to 500°C and perform hot-dip galvanizing, then adjust the coating adhesion by means of gas wiping or the like. As for hot-dip galvanizing, it is preferable to use a galvanizing bath with an Al content of 0.10% to 0.23% by mass and the remainder consisting of Zn and unavoidable impurities.

[0263] It should be noted that hot-dip galvanizing can be performed using an apparatus configured to continuously perform annealing and hot-dip galvanizing.

[0264] When performing alloyed hot-dip galvanizing, it is preferable to perform the alloying treatment of galvanizing within a temperature range of 450°C to 600°C after performing the aforementioned hot-dip galvanizing treatment on the cold-rolled steel sheet. At alloying temperatures below 450°C, the Zn-Fe alloying rate becomes too slow, and alloying may become extremely difficult. On the other hand, if the alloying temperature exceeds 600°C, the untransformed austenite phase transforms into pearlite, and the total ductility (TS) and ductility sometimes decrease. Therefore, when performing the alloying treatment for galvanizing, it is preferable to perform the alloying treatment within a temperature range of 450°C to 600°C. More preferably, the alloying temperature is 470°C or higher. Furthermore, more preferably, the alloying temperature is 550°C or lower, and even more preferably, 530°C or lower.

[0265] When performing electroplating zinc treatment, it is preferable to use a plating bath at room temperature to 100°C, and the plating adhesion per single side is preferably 20 to 80 g / m². 2 .

[0266] In addition, the coating adhesion of preferred hot-dip galvanized steel sheet (GI) and alloyed hot-dip galvanized steel sheet (GA) is 20-80 g / m² per single side. 2 (Double-sided plating). The amount of plating can be adjusted by performing gas wiping or other methods after galvanizing.

[0267] As described above, the coated steel sheet obtained by the coating process can be cooled to below 50°C and then rolled with an elongation of 0.05% to 1.00%. Furthermore, the elongation after cooling to below 50°C is more preferably 0.10% or more. Moreover, the elongation after cooling to below 50°C is more preferably 0.70% or less.

[0268] Rolling after cooling to below 50°C can be performed online on a continuous unit with the galvanizing unit used for the galvanizing process described above, or offline on a separate unit. Furthermore, the target elongation can be achieved in a single rolling pass, or a total elongation of 0.05% to 1.00% can be achieved through multiple rolling passes. It should be noted that the rolling described here generally refers to quenching and tempering rolling; however, if the desired elongation is the same as quenching and tempering rolling, rolling can be performed using methods such as straightening.

[0269] • Dehydrogenation process

[0270] Preferably, the above-mentioned coated steel sheet is further subjected to a dehydrogenation treatment at a temperature range of 50°C to 300°C for 0.5 hours to 72.0 hours. This dehydrogenation treatment further reduces the amount of diffusible hydrogen in the coated steel sheet. As a result, the amount of voids formed after punching is reduced, further improving the tensile flange properties (pore-expanding properties). When the temperature range exceeds 300°C or the holding time exceeds 72.0 hours, it may be difficult to ensure the desired TS (transformation temperature) through tempering. Furthermore, when the temperature is below 50°C or the holding time is less than 0.5 hours, it may be impossible to sufficiently reduce the amount of diffusible hydrogen in the coated steel sheet. Therefore, in the dehydrogenation treatment process, it is preferable to hold the coated steel sheet at a temperature range of 50°C to 300°C for 0.5 hours to 72.0 hours. Furthermore, in the dehydrogenation treatment process, it is more preferable to hold the coated steel sheet at a temperature range of 70°C to 200°C for 1 hour to 36.0 hours.

[0271] It should be noted that the above-mentioned dehydrogenation treatment can be performed on cold-rolled steel sheets after the annealing process or after the reheating process.

[0272] Alternatively, as another embodiment, a plating process can be performed after holding the annealing process, where the cold-rolled steel sheet is cooled to a temperature range of 350°C to 600°C, followed by a hot-dip galvanizing or alloying hot-dip galvanizing treatment, and then a second reheating process. Hereinafter, this will be described as a second embodiment, an arbitrary process following the annealing process.

[0273] [Second Implementation]

[0274] Plating process

[0275] After the annealing process, the cold-rolled steel sheet is cooled to a temperature range of 350°C to 600°C, and then hot-dip galvanizing or alloying hot-dip galvanizing is performed on the cold-rolled steel sheet.

[0276] It should be noted that the conditions for hot-dip galvanizing and alloyed hot-dip galvanizing are the same as those in the first embodiment described above, and therefore are omitted here.

[0277] Second reheating process

[0278] After the above-mentioned plating process, the plated steel sheet is cooled to a cooling stop temperature below 250°C, and then heated to a temperature range of 80°C to 450°C above the cooling stop temperature and held for more than 10 seconds.

[0279] Cooling stop temperature: below 250℃

[0280] By setting the cooling stop temperature to 250°C or below the martensitic transformation initiation temperature, the area ratio of tempered martensite generated during reheating (described later) can be increased. Furthermore, at the cooling stop point, a portion of the austenite undergoes a martensitic transformation, thereby reducing the amount of diffusible hydrogen in the steel sheet. As a result, the amount of voids generated during bending deformation is reduced, thus further improving bending resistance and impact resistance. Therefore, the cooling stop temperature is preferably 250°C or below. More preferably, it is 200°C or below. The lower limit of the cooling stop temperature is not particularly limited; for example, it is preferably -30°C or above.

[0281] Reheating temperature: 80℃ to 450℃ above the cooling stop temperature.

[0282] After the aforementioned cooling is stopped, the cold-rolled steel sheet is reheated to a temperature range of 80°C to 450°C, exceeding the cooling stop temperature, and held within this temperature range for at least 10 seconds. By setting the reheating temperature to be 80°C or higher than the cooling stop temperature, diffusible hydrogen in the steel sheet is released, thus promoting dehydrogenation. However, if the reheating temperature exceeds 450°C, the amount of diffusible hydrogen in the steel sheet may increase along with the increase in the area ratio of quenched martensite. Therefore, there is a possibility of a decrease in bending properties and impact resistance. Therefore, the reheating temperature is preferably 80°C to 450°C above the cooling stop temperature. More preferably, the reheating temperature is 100°C or higher above the cooling stop temperature. More preferably, the reheating temperature is 400°C or lower. It should be noted that the reheating temperature is the highest temperature reached during the reheating process.

[0283] Duration: 10 seconds or more

[0284] When the holding time within the reheating temperature range (above the cooling stop temperature and between 80°C and 450°C) is less than 10 seconds, the diffusible hydrogen content in the steel sheet may increase along with the increase in the area ratio of quenched martensite. Therefore, there is a possibility of reduced flexibility and impact resistance. Therefore, the holding time within the reheating temperature range is preferably 10 seconds or more. It should be noted that there is no particular upper limit to the holding time within the reheating temperature range; however, due to limitations in production technology, it is preferably less than 1000 seconds. The holding time within the reheating temperature range is more preferably 10 to 300 seconds. It should be noted that the holding time within the reheating temperature range includes not only the holding time at the reheating temperature but also the residence time within the reheating temperature range (80°C to 450°C) during heating and cooling before and after reaching the reheating temperature. This excludes the residence time within this temperature range before reaching the aforementioned cooling stop temperature.

[0285] The average cooling rate, cooling stop temperature, and cooling method after holding at the reheating temperature are not particularly limited. Cooling methods can include gas jet cooling, water mist cooling, roller cooling, water cooling, and air cooling. Furthermore, from the viewpoint of preventing oxidation of the steel plate surface, it is preferable to cool to below 50°C after holding at the reheating temperature, and more preferably to room temperature. The average cooling rate is typically 1°C / second to 50°C / second.

[0286] • Dehydrogenation process

[0287] Preferably, the above-mentioned coated steel sheet is further subjected to a dehydrogenation treatment at a temperature range of 50°C to 300°C for 0.5 hours to 72.0 hours. It should be noted that the conditions for the dehydrogenation treatment are the same as those in the first embodiment described above, and therefore are omitted here.

[0288] Conditions other than those described above can be performed using conventional methods. Furthermore, in the series of heat treatments of the cladding steel sheet manufacturing method according to one embodiment of the present invention described above, if the temperature is within the aforementioned range, maintaining a constant temperature is not necessary, and even if the cooling rate varies during cooling, there are no particular problems as long as it remains within the prescribed range. Additionally, as long as the thermal process is satisfied, the steel sheet can be heat-treated using any equipment.

[0289] [4] Manufacturing method of components

[0290] Next, a method for manufacturing a component according to one embodiment of the present invention will be described.

[0291] A component manufacturing method according to one embodiment of the present invention includes a step of forming or joining a cladding steel plate (e.g., a cladding steel plate manufactured by the above-described cladding steel plate manufacturing method) to produce a component.

[0292] Here, there are no particular limitations on the forming process; for example, general processing methods such as stamping can be used. Similarly, there are no particular limitations on the joining process; for example, general welding methods such as spot welding, laser welding, and arc welding, as well as riveting and rivet joining, can be used. It should be noted that there are no particular limitations on the forming and joining conditions; conventional methods can be followed.

[0293] Example

[0294] The present invention will be described in detail with reference to the embodiments. The scope of the present invention is not limited to the following embodiments.

[0295] A steel billet with the composition shown in Table 1-1, the remainder consisting of Fe and unavoidable impurities, is melted in a converter, and a base steel billet is prepared using continuous casting. Separately, a steel billet with the composition shown in Table 1-2, the remainder consisting of Fe and unavoidable impurities, is melted in a converter, and a cladding material billet is prepared using continuous casting. Next, the surface roughness of both the front and back surfaces of the base steel billet, and one of the front and back surfaces of the cladding material billet, is adjusted by surface treatment. It should be noted that "Surface Roughness Ra" in Table 2 records the maximum value of the surface roughness Ra of the surface-treated surface (the surface in contact with the base steel billet and the cladding material billet). Then, with the surface-treated surfaces of the base steel billet and the cladding material billet in contact, the base steel billet and the cladding material billet are stacked in the order of cladding material billet – base steel billet – cladding material billet to obtain a laminated slab. Next, the cladding material billet and the base material billet are joined to obtain a laminated slab. At this time, a vacuum is drawn between the cladding material billet and the base material billet under the conditions shown in Table 2. It should be noted that in No. 45, the cladding material billets are not laminated, and a billet consisting only of the base material billet is used.

[0296] The resulting laminated slab is heated to the slab heating temperatures shown in Table 2 for rough rolling. Then, it is finished rolled at the final rolling temperatures shown in Table 2 to obtain a hot-rolled steel sheet. Next, under the conditions shown in Table 2, a cold rolling and annealing process is performed to obtain a cold-rolled steel sheet (CR).

[0297] Next, for a portion of the cold-rolled steel sheets, coated steel sheets are obtained through the manufacturing process of the first embodiment (first reheating process and plating process) and the manufacturing process of the second embodiment (plating process and second reheating process) under the conditions shown in Table 2, or through plating process after cooling to room temperature. It should be noted that, for convenience, the cooling stop temperature column for the first reheating process records the cooling stop temperature after the annealing process. It should also be noted that the "-" in the cooling stop temperature column for the first reheating process indicates cooling to room temperature after the annealing process.

[0298] It should be noted that in the coating process, cold-rolled steel sheets are coated to obtain hot-dip galvanized steel sheet (GI), alloyed hot-dip galvanized steel sheet (GA), or electro-galvanized steel sheet (EG). As the hot-dip galvanizing bath, in the case of manufacturing GI, a zinc bath containing 0.20% by mass of Al, with the remainder consisting of Zn and unavoidable impurities, is used. In the case of manufacturing GA, a zinc bath containing 0.14% by mass of Al, with the remainder consisting of Zn and unavoidable impurities, is used. The bath temperature is 470°C in the manufacturing of both GI and GA. The coating adhesion is 45–72 g / m² per single side in the case of manufacturing GI. 2(Double-sided plating) Approximately, with each side weighing 45g / m during GA manufacturing. 2 (Platinum on both sides) Approximately.

[0299] The alloying treatment during the manufacture of GA was carried out at the temperatures shown in Table 2. Furthermore, the GI coating composition contained Fe: 0.1–1.0 wt%, Al: 0.2–1.0 wt%, with the remainder consisting of Zn and unavoidable impurities. The GA coating composition contained Fe: 7–15 wt%, Al: 0.1–1.0 wt%, with the remainder consisting of Zn and unavoidable impurities.

[0300] During EG manufacturing, a 30°C plating bath is used, and the plating adhesion per single side is 20–50 g / m². 2 about.

[0301] In addition, some of the coated steel sheets underwent further dehydrogenation treatment under the conditions shown in Table 2.

[0302] The cold-rolled steel sheet and galvanized steel sheet obtained above were used as test steels, and their tensile properties, bending properties, impact resistance, and LME resistance were evaluated according to the following test methods. It should be noted that bending properties were evaluated using the V-bending test. Furthermore, impact resistance was evaluated using the ultimate deformation capacity, VDA bending test, and V-bending-orthogonal VDA bending test. The results are shown in Table 3. It should be noted that the rolling direction of the steel sheet is referred to as the L-direction, and the width direction of the steel sheet is referred to as the C-direction.

[0303] In addition, the steel structure was identified and the average Vickers hardness was measured using the methods described above. The results are shown in Table 3. It should be noted that the results for the steel structure of the cladding material, the boundary roughness between the base material and the cladding material, and the number of voids at the boundary between the base material and the cladding material were approximately the same for (1) cladding material (surface) and (3) cladding material (inner side). Therefore, only (1) cladding material (surface) is recorded as a representative.

[0304] <Tension Properties>

[0305] Tensile testing was performed according to JIS Z 2241. JIS No. 5 test pieces were taken from the obtained steel sheet with the length direction aligned with the C-direction of the steel sheet. Using these test pieces, tensile tests were conducted at a crosshead speed of 10 mm / min, and total elongation (TS) and total elongation (E1) were determined. A TS of 780 MPa or higher was considered acceptable.

[0306] <Extreme Deformation Capability>

[0307] The ultimate deformation capacity is determined by the plate width strain (ε) obtained from the tensile test described above. w ) and plate thickness strain (ε t) Calculate the tensile strain (ε) l The method shown in the report of the RIKEN Institute of Physical and Chemical Research, 45-4 (1969), 79, is used to calculate the result.

[0308] ε l =-(ε w +ε t )

[0309] ε w =ln(w / w0),ε t =ln(t / t0)

[0310] w0: Plate width before tensile test; w: Plate width at fracture after tensile test.

[0311] t0: Plate thickness before tensile test; t: Plate thickness at fracture after tensile test.

[0312] It should be noted that, according to the reports of Nakagawa et al., Plasticity and Processing, 11-29 (1970), 142, and Matsufuji et al., Plasticity and Processing, 14-146 (1973), 201, it is known that the limiting deformation capacity is related to the hole-expanding property (tensile flange property).

[0313] It should be noted that the ultimate deformation capacity ε l When the TS is above 780MPa and below 1180MPa, a value of 0.8 or above is considered good; when the TS is above 1180MPa, it is considered to be above 0.4.

[0314] <V-bending test>

[0315] The V (90-degree) bending test was conducted according to JIS Z 2248. Test pieces with end-face machining dimensions of 1.2 mm thickness × 100 mm width (C direction) × 35 mm length (L direction) and 1.4 mm thickness × 100 mm width (C direction) × 35 mm length (L direction) were used. The bending radius R was varied under conditions of a 10-ton load, a stroke speed of 30 mm / min, and a holding time of 5 s. An N3 evaluation was performed, and the minimum bending radius R without cracking was calculated by dividing R / t by the plate thickness t. Furthermore, using a Leica stereomicroscope at 25x magnification, cracks longer than 200 μm were considered as open cracks.

[0316] It should be noted that when TS is above 780MPa but below 1180MPa, R / t≤3.0; when TS is above 1180MPa, R / t≤4.0 is considered very good.

[0317] <VDA Bending Test>

[0318] The VDA bending test was conducted according to VDA238-100. Test pieces with end-face finishing (1.2mm thickness × 65mm width (C direction) × 60mm length (L direction) or 1.4mm thickness × 70mm width (C direction) × 60mm length (L direction) were used. A bending test machine with a roller spacing of 2 × plate thickness + 0.5mm and a punch tip curvature radius of R = 0.4mm was used to conduct a VDA bending test in the C direction (L-axis bending) at a stroke speed of 20mm / min. α was measured. VDA Maximum load F (N), stroke S (mm) up to maximum load, and F×S. It should be noted that α obtained through the VDA bending test is known. VDA F×S (N·mm) is related to the fracture characteristics of the longitudinal wall and the bending crush characteristics during axial crush.

[0319] It should be noted that when TS is above 780MPa but less than 1180MPa, F≥8000N, S≥11mm, α VDA ≥90°, F×S≥88000N·mm,

[0320] When TS is above 1180MPa, F≥10000N, S≥10mm, α VDA ≥85° and F×S≥100000N·mm are judged as good.

[0321] Additionally, for reference, Figure 2 The F-S curves of the VDA bending test for Examples No. 44 and 45 are shown in the figure.

[0322] <V-bending - Orthogonal VDA bending test>

[0323] The V-bending-orthogonal VDA bending test was performed using the calculation method described in Sato et al. Patent No. 6748382. Using test pieces with end-face finishing, measuring 1.2 mm thick × 65 mm wide (C-direction) × 60 mm long (L-direction) or 1.4 mm thick × 65 mm wide (C-direction) × 60 mm long (L-direction), a V (90°) bending process based on an L-direction bend (C-axis bend) with a bending radius R = 5 mm was performed under conditions of a 10-ton load, a stroke speed of 30 mm / min, and a holding time of 5 s. The V-bending sample was rotated 90° horizontally. Then, using a bending testing machine with a roller spacing of 2 × plate thickness + 0.5 mm and a punch tip curvature radius of R = 0.4 mm, a VDA bending test was performed with the V-bending bending portion's mountain side becoming the punch side, based on bending in the C direction (L-axis bending). The maximum load F, the stroke S up to the maximum load, and F × S (N·mm) were measured. It should be noted that the fracture characteristics of the bending ridge section during shaft crushing are known to be related.

[0324] It should be noted that when TS is less than 780MPa and less than 1180MPa, F≥6000N, S≥28mm, and F×S≥168000N·mm are judged as good. When TS is 1180MPa or more, F≥6500N, S≥27mm, and F×S≥175500N·mm are judged as good.

[0325] Additionally, for reference, Figure 3 The figure shows the F-S curve of the V-bending-orthogonal VDA bending test of Example No.4.

[0326] <LME Resistance>

[0327] Resistance to LME (Low Metal Mechanism) cracking was assessed using a resistance welding cracking test. A 30mm x 100mm test piece, cut with its long side perpendicular to the rolling direction of the cladding steel sheet, and a 980MPa grade hot-dip galvanized steel sheet were used. These were resistively welded (spot welded) to create a component. A single-phase AC (50Hz) resistance welding machine with a servo motor mounted on the welding torch was used. The two overlapping steel sheets were resistively spot welded at a 5° angle. The welding conditions were: a pressure of 3.8 kN and a holding time of 0.2 seconds. The welding current was 5.7–6.2 kA, with 21 cycles of current application and 5 cycles of holding time. The welded component was cut in half, and the cross-section was observed using an optical microscope. The absence of cracks larger than 0.1 mm was considered excellent LME cracking resistance (○), while the presence of cracks larger than 0.1 mm was considered poor LME cracking resistance (×).

[0328] [Table 1-1]

[0329] Table 1-1

[0330]

[0331] "-" indicates the level of unavoidable impurities.

[0332] [Table 1-2]

[0333] Table 1-2

[0334]

[0335] "-" indicates the level of unavoidable impurities.

[0336] [Table 2]

[0337] Table 2

[0338] Table 2 (continued)

[0339]

[0340] *CR: Cold-rolled steel sheet (uncoated), GI: Hot-dip galvanized steel sheet, GA: Alloyed hot-dip galvanized steel sheet, EG: Electro-galvanized steel sheet

[0341] Table 2 (continued)

[0342]

[0343] Table 2 (continued)

[0344]

[0345] *CR: Cold-rolled steel sheet (uncoated), GI: Hot-dip galvanized steel sheet, GA: Alloyed hot-dip galvanized steel sheet, EG: Electro-galvanized steel sheet

[0346] [Table 3]

[0347] Table 3

[0348]

[0349] F: Ferrite, F: Unrecrystallized ferrite, M: Martensite, TM: Tempered martensite, RA: Retained austenite, B: Bainite, P: Pearlite, θ: Cementite and other carbides

[0350] Table 3 (continued)

[0351]

[0352] Table 3 (continued)

[0353]

[0354] *cR: Cold-rolled steel sheet; GI: Hot-dip galvanized steel sheet; GA: Alloyed hot-dip galvanized steel sheet; EG: Electro-galvanized steel sheet

[0355] Table 3 (continued)

[0356]

[0357] F: Ferrite, F: Unrecrystallized ferrite, M: Martensite, TM: Tempered martensite, RA: Retained austenite, B: Bainite, P: Pearlite, θ: Cementite and other carbides

[0358] Table 3 (continued)

[0359]

[0360] Table 3 (continued)

[0361]

[0362] *CR: Cold-rolled steel sheet, GI: Hot-dip galvanized steel sheet, GA: Alloyed hot-dip galvanized steel sheet, EG: Electro-galvanized steel sheet

[0363] As shown in Table 3, the tensile strength (TS) of the example of the present invention is 780 MPa or higher, and it exhibits excellent bending properties, impact resistance, and LME resistance. On the other hand, at least one of these properties of the steel sheet of the comparative example is worse than that of the example of the present invention.

[0364] Furthermore, it is known that the tensile strength (TS) of a component obtained by using the cladding steel plate of the present invention and performing forming processing or by performing joining processing is 780 MPa or higher, and it has excellent bending properties, impact resistance and LME resistance.

Claims

1. A method for manufacturing a cladding steel plate, comprising the following steps: The first preparation step involves preparing a base steel billet with the following composition, expressed as a percentage by mass: C: 0.050%–0.350%, Si: 0.02%–2.00%, Mn: 1.80% or more and less than 3.50%, P: 0.001%–0.100%, S: less than 0.0200%, Al: 0.010%–2.000%, and N: less than 0.0100%, with the remainder being Fe and unavoidable impurities. The second preparation step involves preparing a cladding material billet with the following composition, expressed in mass percent: C: less than 0.100%, Si: less than 0.60%, Mn: 0.05% to 2.50%, P: 0.001% to 0.100%, S: less than 0.0200%, Al: 0.010% to 0.100%, and N: less than 0.0100%, with the remainder being Fe and unavoidable impurities. The surface treatment process involves performing surface treatment to ensure that the surface roughness of at least one of the two surfaces of the base steel billet and the two surfaces of the cladding material steel billet is less than 30 μm in terms of Ra. In the lamination process, the surface-treated surface of the base steel billet is in contact with the surface-treated surface of the cladding material steel billet, and the base steel billet and the cladding material steel billet are laminated in the order of cladding material steel billet - base steel billet - cladding material steel billet to obtain a laminated slab. In the joining process, the cladding material steel billet and the base steel billet are joined together, and a vacuum is drawn so that the vacuum degree between the cladding material steel billet and the base steel billet is 1×10⁻⁶. －2 Below Torr, a laminated slab blank is obtained; In the hot rolling process, the laminated slab of the bonding layer is heated to a temperature range of 1050℃~1350℃ and then hot rolled at a final rolling temperature of 820℃ or above to obtain a hot rolled steel plate. The cold rolling process involves cold rolling the hot-rolled steel sheet under a reduction rate of 30% to 80% to obtain a cold-rolled steel sheet. as well as The annealing process involves annealing the cold-rolled steel sheet at an annealing temperature of 750℃~950℃ and a holding time of more than 20 seconds. The process further includes a first reheating step: after the annealing step, the cold-rolled steel sheet is cooled to a cooling stop temperature below 250°C, and then reheated to a temperature range above 250°C but below 450°C, and held for more than 10 seconds.

2. The method for manufacturing the cladding steel plate according to claim 1, wherein, At least one of the composition of the base steel billet and the composition of the cladding material steel billet further contains, by mass percent, an element selected from Sb: 0.200% or less, Sn: 0.200% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.00% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.50% or less, Ta: 0.100% or less, W: 0.500% or less, and Mg: 0.020%. At least one of the following: less than 0%, Zn: less than 0.020%, Co: less than 0.020%, Zr: less than 0.020%, Ca: less than 0.0200%, Se: less than 0.0200%, Te: less than 0.0200%, Ge: less than 0.0200%, As: less than 0.0200%, Sr: less than 0.0200%, Cs: less than 0.0200%, Hf: less than 0.0200%, Pb: less than 0.0200%, Bi: less than 0.0200%, and REM: less than 0.0200%.

3. The method for manufacturing the cladding steel plate according to claim 1 or 2, wherein, The process further includes a plating process: after the annealing process or after the first reheating process, the cold-rolled steel sheet is subjected to a plating process to obtain a plating steel sheet.

4. The method for manufacturing the cladding steel plate according to claim 3, wherein, The plating treatment is hot-dip galvanizing, alloyed hot-dip galvanizing, or electro-galvanizing.

5. The method for manufacturing the cladding steel plate according to claim 3, wherein, The process further includes a dehydrogenation treatment step: after the plating treatment step, the plating steel sheet is kept at a temperature range of 50°C to 300°C for 0.5 hours to 72.0 hours.

6. The method for manufacturing the cladding steel plate according to claim 4, wherein, The process further includes a dehydrogenation treatment step: after the plating treatment step, the plating steel sheet is kept at a temperature range of 50°C to 300°C for 0.5 hours to 72.0 hours.

7. A method for manufacturing a cladding steel plate, comprising the following steps: The first preparation step involves preparing a base steel billet with the following composition, expressed as a percentage by mass: C: 0.050%–0.350%, Si: 0.02%–2.00%, Mn: 1.80% or more and less than 3.50%, P: 0.001%–0.100%, S: less than 0.0200%, Al: 0.010%–2.000%, and N: less than 0.0100%, with the remainder being Fe and unavoidable impurities. The second preparation step involves preparing a cladding material billet with the following composition, expressed in mass percent: C: less than 0.100%, Si: less than 0.60%, Mn: 0.05% to 2.50%, P: 0.001% to 0.100%, S: less than 0.0200%, Al: 0.010% to 0.100%, and N: less than 0.0100%, with the remainder being Fe and unavoidable impurities. The surface treatment process involves performing surface treatment to ensure that the surface roughness of at least one of the two surfaces of the base steel billet and the two surfaces of the cladding material steel billet is less than 30 μm in terms of Ra. In the lamination process, the surface-treated surface of the base steel billet is in contact with the surface-treated surface of the cladding material steel billet, and the base steel billet and the cladding material steel billet are laminated in the order of cladding material steel billet - base steel billet - cladding material steel billet to obtain a laminated slab. In the joining process, the cladding material steel billet and the base steel billet are joined together, and a vacuum is drawn so that the vacuum degree between the cladding material steel billet and the base steel billet is 1×10⁻⁶. －2 Below Torr, a laminated slab blank is obtained; In the hot rolling process, the laminated slab of the bonding layer is heated to a temperature range of 1050℃~1350℃ and then hot rolled at a final rolling temperature of 820℃ or above to obtain a hot rolled steel plate. The cold rolling process involves cold rolling the hot-rolled steel sheet under a reduction rate of 30% to 80% to obtain a cold-rolled steel sheet. as well as The annealing process involves annealing the cold-rolled steel sheet at an annealing temperature of 750℃~950℃ and a holding time of more than 20 seconds. It further includes the following processes: The coating process involves hot-dip galvanizing or alloying hot-dip galvanizing of the cold-rolled steel sheet after the annealing process to obtain a coated steel sheet. In the second reheating process, after cooling the plated steel sheet to a cooling stop temperature below 250°C, it is reheated to a temperature range of 80°C to 450°C, which exceeds the cooling stop temperature, and held for more than 10 seconds.

8. The method for manufacturing the cladding steel plate according to claim 7, wherein, At least one of the composition of the base steel billet and the composition of the cladding material steel billet further contains, by mass percent, an element selected from Sb: 0.200% or less, Sn: 0.200% or less, Ti: 0.200% or less, Nb: 0.200% or less, V: 0.100% or less, B: 0.0100% or less, Cu: 1.00% or less, Cr: 1.000% or less, Ni: 1.000% or less, Mo: 0.50% or less, Ta: 0.100% or less, W: 0.500% or less, and Mg: 0.020%. At least one of the following: less than 0%, Zn: less than 0.020%, Co: less than 0.020%, Zr: less than 0.020%, Ca: less than 0.0200%, Se: less than 0.0200%, Te: less than 0.0200%, Ge: less than 0.0200%, As: less than 0.0200%, Sr: less than 0.0200%, Cs: less than 0.0200%, Hf: less than 0.0200%, Pb: less than 0.0200%, Bi: less than 0.0200%, and REM: less than 0.0200%.

9. The method for manufacturing the cladding steel plate according to claim 7 or 8, wherein, The process further includes a dehydrogenation treatment step: after the second reheating step, the coated steel sheet is kept at a temperature range of 50°C to 300°C for 0.5 hours to 72.0 hours.

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