Clad steel sheet and component and method for manufacturing the same

Abstract

This invention provides a cladding steel sheet with a tensile strength (TS) of 780 MPa or higher and excellent ductility, bending properties, impact resistance, and LME resistance. The cladding steel sheet is manufactured by appropriately controlling the composition and microstructure of the base material and the cladding material, ensuring that the average Vickers hardness (HVL) of the cladding material is 260 or less, 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 0.80 or less, the boundary roughness between the base material and the cladding material, measured in terms of maximum height Ry, is 50 μm or less, and the number of voids at the boundary between the base material and the cladding material is suppressed to 20 or less per 10 mm boundary length.

Description

Technical Field

[0001] This invention relates to cladding steel sheets and components, and methods for manufacturing the same. Particularly, it relates to cladding steel sheets and components with a tensile strength (TS) of 780 MPa or higher, and excellent ductility, bending properties, impact resistance, and LME resistance, as well as methods for manufacturing the same. The cladding steel sheets of this invention are suitable for the skeletal components of automobile bodies, and particularly for impact energy absorption components. Background Technology

[0002] In recent years, improving fuel efficiency in automobiles has become a crucial issue from the perspective of protecting the Earth's environment. Consequently, there has been a growing trend towards reducing the weight of car bodies by increasing the strength of body materials to achieve thinner walls. On the other hand, societal demands for improved collision safety in automobiles have also increased, with a focus not only on increasing the strength of steel sheets but also on developing steel sheets and components with excellent crash resistance characteristics in real-world collisions. Furthermore, from a processability perspective, there is a desire to develop steel sheets that, in addition to crash resistance, also possess excellent ductility.

[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 composed of three layers of composite 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 or higher strength and an outer layer of chemically resistant ferritic steel integrally bonded to the core layer on 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 respectively, ≤0.5 wt% aluminum, ≤0.5 wt% silicon, and ≤1 wt% titanium, niobium, vanadium and zirconium respectively. 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, comprising a base material and cladding material on both sides of the base material. The base material is an austenitic high-manganese steel containing, by weight %: C: 0.3-1.4%, Mn: 12-25%, balance Fe and unavoidable impurities. The cladding material is a martensitic carbon steel containing, by weight %: C: 0.09-0.4%, Mn: 0.3-4.5%, balance 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, such as the front and rear longitudinal beams, only use steel plates with a tensile strength (TS) of 590MPa to 780MPa. This is because, with the increase in strength, cracking occurs in bending and axial crush tests during simulated collision experiments, making it unable to fully absorb impact energy.

[0014] Furthermore, in recent years, during the assembly of automobile bodies and components, it has been found that molten metal embrittlement cracking (LMEC, also known as LME cracking) occurs at the weld joint when spot welding high-strength hot-dip galvanized steel sheets and high-strength gold-plated hot-dip galvanized steel sheets, or when spot welding high-strength cold-rolled steel sheets to galvanized steel sheets. LME cracking occurs when the zinc in the galvanized layer melts during spot welding, and the molten zinc penetrates into the grain boundaries of the steel structure at the weld joint, resulting in cracking due to the stress generated when the welding electrode is released. Even with ungalvanized high-strength cold-rolled steel sheets, LME cracking can occur when spot welding them to galvanized steel sheets because the molten zinc in 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 particularly 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] In Patent Document 2, the material is a hot-working material (cladding steel sheet for hot pressing), not a cladding steel sheet for cold pressing. Furthermore, although it exhibits minimal variation in the properties of each layer and high strength and ductility in the area near the surface, its resistance to LME (Low Metal Effluent Spectroscopy) has not been investigated.

[0017] While patent document 3 features ductility, low sensitivity to hydrogen-induced cracking, and favorable corrosion resistance, its flexibility, impact resistance, and LME resistance have not been investigated.

[0018] In Patent Document 4, the base material is a high alloy composition, and therefore the bending properties, impact resistance and LME resistance were not studied.

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

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

[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 and thoroughly studied the subject matter and have obtained the following insights.

[0023] That is, the inventors have obtained the following insight: a cladding steel sheet with a tensile strength (TS) of 780 MPa or higher and excellent ductility, 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 steel 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) The boundary roughness between the base material and the coating material is less than 50 μm, measured by the maximum height Ry;

[0028] (e) The number of voids at the boundary between the base material and the cladding material is reduced 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 having 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 of the components, by mass%, is as follows: C: 0.080%–0.350%, Si: 0.50%–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 total area fraction of bainite and tempered martensite is more than 30%, and the volume fraction of retained austenite is more than 5%.

[0035] The aforementioned coating material has the following composition and steel structure: the composition, by mass%, is: 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;

[0036] In the steel microstructure, the area fraction of ferrite is 80% or more;

[0037] The average Vickers hardness (HVL) of the above-mentioned coating materials is below 260.

[0038] 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.

[0039] 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.

[0040] The number of gaps at the boundary between the base material and the coating material is less than 20 per 10 mm of boundary length.

[0041] 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, a component 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: 0.02%. At least one of the following: less than 0.00%, 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%.

[0042] 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 material is 1 or more.

[0043] 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.

[0044] 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.

[0045] 6. A component made of cladding steel sheet as described in any one of 1 to 5 above.

[0046] 7. A method for manufacturing a cladding steel plate, comprising the following steps:

[0047] The first preparation step is to prepare a base steel billet having the composition of the base material described in 1 or 2 above.

[0048] 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.

[0049] The surface treatment process is performed such that the surface roughness of at least one of the two surfaces of the base steel billet and the surface and back of the cladding material steel billet is 30 μm or less in terms of Ra.

[0050] 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.

[0051] 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;

[0052] 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.

[0053] 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

[0054] 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.

[0055] 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.

[0056] 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.

[0057] 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.

[0058] 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.

[0059] 12. The method for manufacturing the cladding steel plate according to 7 above, further comprising the following steps:

[0060] The coating process involves, after the annealing process, subjecting the cold-rolled steel sheet to hot-dip galvanizing or alloyed hot-dip galvanizing to obtain a coated steel sheet; and

[0061] The second reheating process involves cooling the galvanized steel sheet to a cooling stop temperature below 250°C, and then heating it to a temperature range above 250°C but below 450°C and holding it for more than 10 seconds.

[0062] 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.

[0063] 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.

[0064] According to the present invention, cladding steel plates and components with a tensile strength (TS) of 780 MPa or higher and excellent ductility, bending, impact resistance and LME resistance, and methods thereof can be provided. Detailed Implementation

[0065] The present invention is described based on the following embodiments.

[0066] [1] Cladding steel plate

[0067] [1-1] Composition of the base material

[0068] 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 in the composition is "mass %", and unless otherwise specified, it will be expressed as "%".

[0069] C: 0.080%~0.350%

[0070] C is an element effective in generating the required amount of martensite, tempered martensite, and retained austenite, and ensuring a strength tolerance (TS) of 780 MPa or higher. If the C content is less than 0.080%, 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. Consequently, bending and impact resistance properties decrease. Therefore, the C content is 0.080% to 0.350%. The C content is preferably 0.090% or more, more preferably 0.100% or more. Furthermore, the C content is preferably 0.330% or less, more preferably 0.320% or less.

[0071] Si: 0.50%~2.00%

[0072] Si is an effective element for ensuring a strength tolerance (TS) of 780 MPa or higher through solid solution strengthening. If the Si content is less than 0.50%, 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, the number of voids at the boundary between the base material and the cladding material increases, and the bending and impact resistance properties decrease. Therefore, the Si content is 0.50% to 2.00%. The Si content is preferably 0.60% or more, more preferably 0.70% or more. Furthermore, the Si content is preferably 1.80% or less, more preferably 1.60% or less.

[0073] Mn: ≥1.80% and <3.50%

[0074] Mn is an important element for adjusting the area fraction of martensite, tempered martensite, and retained austenite. If the Mn content is less than 1.80%, the area fraction of ferrite increases, making it difficult to achieve a TS (steel strength) 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, and the hardness of the martensite formed from retained austenite during bending deformation increases significantly. As a result, 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.

[0075] P: 0.001%~0.100%

[0076] 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 0.001% or more. However, if the P content exceeds 0.100%, P segregates towards the original austenite grain boundaries, causing grain boundary embrittlement. Consequently, the amount of voids generated during bending deformation increases, reducing bending and impact resistance. Therefore, the P content is 0.001% to 0.100%. More preferably, the P content is 0.030% or less.

[0077] S: below 0.0200%

[0078] Sulfide (S) exists in steel as sulfides. If its content exceeds 0.0200%, it may reduce the ultimate deformation capacity of the steel sheet. 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 often 0.0001% or more.

[0079] Al: 0.010%~2.000%

[0080] Al acts as a deoxidizer. To obtain the desired effect from added Al, the Al content should be 0.010% or higher. On the other hand, if the Al content exceeds 2.000%, the ferrite surface fraction increases, making it difficult to achieve a TS of 780 MPa or higher. Therefore, the Al content is typically between 0.010% and 2.000%.

[0081] N: below 0.0100%

[0082] Nitrogen (N) exists in steel in the form of nitrides. If its content exceeds 0.0100%, it reduces the ultimate deformation capacity of the steel sheet. As a result, the amount of voids generated during bending deformation increases, and bending and impact resistance decrease. 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, but due to limitations in production technology, the N content is often 0.0005% or more.

[0083] 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 included. It should be noted that any of the components shown below can achieve the desired effect as long as they are contained in amounts below the upper limit shown below, therefore no specific lower limit is set. It should be noted that when any of the following elements are contained in amounts below the preferred lower limit value described below, the element is contained as an unavoidable impurity.

[0084] Sb: below 0.200%

[0085] 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.

[0086] Sn: below 0.200%

[0087] Sn is an effective element for suppressing carbon diffusion near the surface of 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 (steel sulfide) within a more preferred range, the Sn content is preferably 0.002% or more. The Sn content is more preferably 0.005% or more.

[0088] Ti: below 0.200%, Nb: below 0.200%, V: below 0.100%

[0089] Ti, Nb, and V improve steel strength (TS) by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. When at least one of Ti, Nb, and V is added, to achieve this effect, it is preferable that the content of each of Ti, Nb, and V is 0.001% or more. More preferably, their contents are 0.005% or more. On the other hand, when the content of Ti exceeds 0.200%, the content of Nb exceeds 0.200%, or the content of V 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.

[0090] B: Below 0.0100%

[0091] Boron (B) is an element that improves hardenability by segregating towards austenite grain boundaries. Adding B to steel can suppress ferrite formation and grain growth during annealing cooling. To achieve this effect, the B content is preferably 0.0001% or more. More preferably, the B content is 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 steel sheet's ultimate deformation capacity. As a result, there is a possibility of increased void formation during bending deformation and decreased bending and impact resistance. Therefore, when B is added, its content is preferably 0.0100% or less. Furthermore, the B content is more preferably 0.0050% or less.

[0092] Cu: below 1.00%

[0093] Cu is an element that improves hardenability and is effective in achieving a more desirable area fraction of the hard phase and a more desirable total hardness (TS). To obtain this effect, it is preferable that the Cu content is 0.005% or more. More preferably, the Cu content is 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 excessively high. Furthermore, with the increase of coarse precipitates and inclusions, and the presence of diffusible hydrogen in the steel plate, these precipitates and inclusions may become crack initiation points during bending deformation, and bending and impact resistance may decrease. Therefore, when Cu is added, its content is preferably 1.00% or less. More preferably, the Cu content is 0.20% or less.

[0094] Cr: less than 1.000%

[0095] 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 also 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.

[0096] Ni: below 1.000%

[0097] Ni is an element that improves hardenability and is effective in achieving a more favorable area ratio of the hard phase and a more favorable total hardness (TS). To obtain this effect, it is preferable that the Ni content is 0.005% or more. More preferably, the Ni content is 0.020% or more. On the other hand, if the Ni content exceeds 1.000%, coarse precipitates and inclusions may increase. 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 bending and impact resistance may decrease. Therefore, when Ni is added, its content is preferably 1.000% or less. Furthermore, the Ni content is more preferably 0.800% or less.

[0098] Mo: 0.50% or less

[0099] Mo is an element that improves hardenability and is effective in forming a hard phase. 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 hardenability test (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.

[0100] Ta: below 0.100%

[0101] Like Ti, Nb, and V, Ta increases the precipitation toughness (TS) by forming fine carbides, nitrides, or carbonitrides during hot rolling or annealing. Additionally, Ta partially dissolves in Nb carbides and Nb carbonitrides to form composite precipitates such as (Nb,Ta)(C,N), significantly suppressing precipitate coarsening and stabilizing precipitation strengthening, thereby increasing 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%, large amounts 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.

[0102] W: below 0.500%

[0103] 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 within a more preferred range, the W content is preferably 0.001% or more. In addition, the W content is more preferably 0.030% or more.

[0104] Mg: below 0.0200%

[0105] 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 this effect, it is preferable that the Mg content is 0.0001% or more. On the other hand, if the Mg content exceeds 0.0200%, there is a possibility of forming 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, reducing bending and impact resistance. Therefore, when Mg is added, its content is preferably 0.0200% or less.

[0106] Zn: less than 0.020%, Co: less than 0.020%, Zr: less than 0.020%

[0107] 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, respectively. On the other hand, if the contents of Zn, Co, and Zr exceed 0.020%, there is a possibility of the formation of large 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 one or more of Zn, Co, and Zr, it is preferable that each is 0.020% or less.

[0108] Ca: below 0.0200%

[0109] Ca exists in steel as inclusions. 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 also be 0.0000%, but due to limitations in production technology, the Ca content is preferably 0.0001% or more.

[0110] 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%

[0111] Ce, Se, Te, Ge, As, Sr, Cs, Hf, Pb, Bi, and REM are effective elements for improving the ultimate deformation capacity, bending properties, and impact resistance of steel sheets. 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, respectively. 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. 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, respectively.

[0112] [1-2] Composition of coating materials

[0113] Next, the composition of the cladding material of the cladding steel plate according to one embodiment of the present invention will be described. It should be noted that the unit in the composition is "mass%", and unless otherwise specified, it will be expressed as "%".

[0114] C: Below 0.100%

[0115] Carbon (C) reduces LME resistance. Furthermore, C reduces bending and impact resistance by forming 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 bending 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 while there is no specific lower limit for the C content, due to limitations in production technology, the C content is often 0.001% or more.

[0116] Si: below 0.60%

[0117] 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, but due to limitations in production technology, the Si content is often 0.01% or more.

[0118] Mn: 0.05%~2.50%

[0119] 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, and the hardness of the martensite formed from retained austenite during bending deformation increases significantly. 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.

[0120] P: 0.001%~0.100%

[0121] 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 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.

[0122] S: below 0.0200%

[0123] If the sulfur (S) content exceeds 0.0200%, a large amount of dissolved S will segregate at the austenite grain boundaries, causing surface cracking during hot rolling. Furthermore, the significant 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 often 0.0001% or more.

[0124] Al: 0.010%~0.100%

[0125] Al acts as a deoxidizer. To achieve the desired effect, the Al content should be 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, easily leading to surface defects. Therefore, the Al content is between 0.010% and 0.100%.

[0126] N: below 0.0100%

[0127] Nitrogen (N) exists in steel in the form of nitrides. If its content exceeds 0.0100%, a large number of unrecrystallized grains remain due to the pinning effect of the 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, but due to limitations in production technology, the N content is often 0.0005% or more.

[0128] 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, in the cladding 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 included. It should be noted that any of the components shown below can achieve the desired effect as long as they are contained in amounts below the upper limit shown below, therefore no specific lower limit is 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.

[0129] 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%. Less than 0.020%, Zr: less than 0.0200%, 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%.

[0130] It should be noted that the reasons for limiting the above-mentioned arbitrary added components are basically the same as the reasons for limiting the arbitrary added components of the base material of the cladding steel plate in one embodiment of the present invention, so they are omitted here.

[0131] [1-3] Steel structure of the base material

[0132] Next, the steel structure of the base material of the cladding steel plate according to one embodiment of the present invention will be described.

[0133] In one embodiment of the present invention, the cladding steel plate has a steel microstructure in which the total area ratio of bainite and tempered martensite is more than 30%, and the volume ratio of retained austenite is more than 5%.

[0134] The combined area ratio of bainite and tempered martensite is over 30%.

[0135] To ensure a total strength (TS) of 780 MPa or higher, the combined area ratio of bainite and tempered martensite needs to be 30% or higher. Preferably, the combined area ratio of bainite and tempered martensite is 35% or higher. There is no particular upper limit to the combined area ratio of bainite and tempered martensite, and it can be 100%. Preferably, the combined area ratio of bainite and tempered martensite is 92% or lower.

[0136] It should be noted that if the combined area ratio of bainite and tempered martensite is 30% or more, then the area ratios of bainite and tempered martensite can be 0% each.

[0137] Here, the method for determining the area ratio of bainite 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.

[0138] Specifically, the specimen was cut with the 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 with 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, using Adobe Photoshop from Adobe Systems, the area ratio of each constituent microstructure (bainite, tempered martensite) was calculated by dividing the area of ​​each component 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.

[0139] Volume fraction of retained austenite: over 5%

[0140] When the volume fraction of retained austenite is 5% or less, there is a possibility that the ductility of the base material and the ductility of the cladding steel sheet will decrease. Therefore, the volume fraction of retained austenite exceeds 5%. Furthermore, the volume fraction of retained austenite is preferably 6% or more. More preferably, it is 7% or more, and even more preferably 8% or more. There is no particular upper limit to the volume fraction of retained austenite, but it is preferably 45% or less.

[0141] Here, the method for determining the volume fraction of retained austenite is as follows. After mechanically grinding the cladding steel sheet to 1 / 4 of the thickness of the base material in the thickness direction (depth direction), chemical polishing with oxalic acid is performed to form an observation surface. Next, this observation surface is observed by 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 the diffraction intensities of the {200}, {211}, and {220} planes of bcc iron is calculated, and the volume fraction of retained austenite is calculated from the ratio of the diffraction intensities of each plane.

[0142] Furthermore, the area fraction of the remaining microstructure, excluding bainite, 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 ferrite, martensite, pearlite, and cementite carbides. It should be noted that the presence or absence of the remaining microstructure can be confirmed and 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.

[0143] [Area fraction of remaining microstructure (%)] = 100 - [Total area fraction of bainite and tempered martensite (%)] - [Volume fraction of retained austenite (%)]

[0144] It should be noted that here, the retained austenite is considered to be three-dimensional 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.

[0145] [1-4] Steel structure of cladding material

[0146] Next, the steel structure of the cladding material of the cladding steel plate according to one embodiment of the present invention will be described.

[0147] In one embodiment of the present invention, the cladding material of the cladding steel plate has a ferrite area ratio of 80% or more.

[0148] Ferrite area ratio: over 80%

[0149] To ensure good bending and impact resistance, the ferrite area ratio needs to be 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%.

[0150] 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.

[0151] Specifically, 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 with 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, using Adobe Photoshop from Adobe Systems, the area ratio of each constituent microstructure (ferrite) was calculated by dividing the area of ​​each component 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.

[0152] Furthermore, the area fraction of the remaining microstructure, excluding ferrite, is preferably 20% or less. More preferably, it 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 confirmed and 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.

[0153] [Area fraction of remaining tissue (%)] = 100 - [Area fraction of ferrite (%)]

[0154] [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.

[0155] Next, the average Vickers hardness (HVL) of the cladding material, which is a particularly important component in the cladding steel sheet according to one embodiment of the present invention, 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.

[0156] Average Vickers hardness (HVL) of the coating material: below 260

[0157] To ensure good flexural properties, impact resistance, and LME resistance, the average Vickers hardness (HVL) of the coating material needs to be 260 or less. Furthermore, the average Vickers hardness (HVL) of the coating material is preferably 250 or less. While there is no particular limitation on the lower limit of the average Vickers hardness (HVL) of the coating material, it is preferably 85 or more. It should be noted that this requirement must be met for coating materials bonded to both the front and back surfaces of the base material. 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.

[0158] 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

[0159] To ensure good flexural properties and impact resistance, the hardness ratio of the cladding material to the base material must be below 0.80. Preferably, the hardness ratio is below 0.75. While there is no particular lower limit to the hardness ratio, it is preferably above 0.07.

[0160] Here, the average Vickers hardness (HVB) of the base material is measured as follows.

[0161] That is, the Vickers hardness at the center of the base material thickness is measured with an indentation load of 1 kg. Then, starting from this measurement point (position), the Vickers hardness is measured at 10 points along a line parallel to the rolling direction with an indentation load of 1 kg, and the average value can be used to calculate the Vickers hardness.

[0162] In addition, the average Vickers hardness (HVL) of the coating material was measured as follows.

[0163] Specifically, the Vickers hardness at the center of the coating material thickness is measured 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 then taken as the average Vickers hardness (HVL) of the coating material.

[0164] It should be noted that, if possible, the interval between the measurement points in the determination of 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. 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.

[0165] Boundary roughness between base material and coating material: less than 50 μm based on maximum height Ry.

[0166] To ensure good bending resistance and impact resistance, the boundary roughness (Ry) between the base material and the cladding material must be less than 50 μm. When the boundary roughness (Ry) exceeds 50 μm, 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 starting point for cracking. The maximum height Ry of the boundary roughness between the base material and the cladding material is preferably less than 30 μm. It should be noted that this requirement must be met on both the surface and back surfaces of the base material. There is no particular limitation on the lower limit of the boundary roughness between the base material and the cladding material, but it is preferably 5 μm or more (Ry) at its maximum height.

[0167] It should be noted that the maximum height (Ry) is calculated in accordance with JIS B 0601 (1994) and JIS B 0031 (1994).

[0168] 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 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 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 B 0031 (1994).

[0169] Number of voids at the boundary between the base material and the coating material: less than 20 per 10 mm boundary length.

[0170] 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 20 or less per 10 mm boundary length. When the number of voids at the boundary exceeds 20 per 10 mm boundary length, these voids become the starting point for cracking during bending tests (during compression molding) and crush tests (during vehicle collisions). Furthermore, the increased number of voids leading to their connection contributes to crack propagation. Preferably, the number of voids at the boundary between the base material and the cladding material is 15 or less per 10 mm boundary length. There is no particular limitation on the lower limit of the number of voids per 10 mm 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 of both the surface and back surfaces of the base material.

[0171] Here, the number of voids at the boundary between the base material and the cladding material is determined as follows.

[0172] Specifically, 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 was then divided by the total length (in mm) of the observation area in the 30 fields of view, and multiplied by 10 to obtain the number of voids present at the boundary between the base material and the cladding material for every 10 mm boundary length.

[0173] [1-6] Thickness

[0174] In one embodiment of the present invention, the thickness of the cladding steel plate 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 materials 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.

[0175] In addition, it is preferable that the value obtained by dividing the thickness of the base material by the total thickness of the coating material is 1 or more.

[0176] The value obtained by dividing the thickness of the base material by the total thickness of the coating materials: 1 or more

[0177] When the value obtained by dividing the thickness of the base material by the total thickness of the cladding materials is 1 or more, 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 more. 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, but 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 less.

[0178] [1-7] Coating

[0179] 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.

[0180] The composition of hot-dip galvanized layers, alloyed hot-dip galvanized layers, and electro-galvanized layers is not particularly limited as long as Zn is the main component. However, for example, they 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.

[0181] 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.

[0182] In addition, there is no particular limitation on the amount of plating, but the preferred amount of plating per single side is 20 to 80 g / m². 2 .

[0183] [1-8] Diffusive hydrogen content

[0184] 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 less than 0.50 ppm by mass.

[0185] Total diffusible hydrogen content in the base material and coating materials: less than 0.50 ppm by mass.

[0186] From the viewpoint of obtaining superior flexibility, the cladding steel sheet of one embodiment of the present invention 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, but due to limitations in production technology, the diffusible hydrogen content in steel sheets is often 0.01 ppm by mass or more.

[0187] Here, the total amount of diffusible hydrogen contained in the base material and the coating material is determined as follows.

[0188] Specifically, a test piece with a length of 30 mm and a width of 5 mm is taken from the cladding steel sheet. If a coating is present on the surface, the coating is removed using alkali. Then, the amount of hydrogen released from the test piece is determined using a temperature-programmed desorption analysis method. Specifically, the test piece is continuously heated from room temperature to 300 °C at a heating rate of 200 °C / h, and then cooled to room temperature. At this point, the amount of hydrogen released from the test piece during this continuous heating, from room temperature to 210 °C (cumulative hydrogen content), is measured. Then, the measured hydrogen content is divided by the mass of the test piece (in the case of coating removal, the test piece after coating removal and before continuous heating) and converted to a value in ppm (parts per second) to obtain the total diffusible hydrogen content contained in the base material and the cladding material.

[0189] 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 use 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 can be considered that the total diffusible hydrogen content contained in the base material and cladding material of the cladding steel sheet in the blank stage before forming and joining processes is also 0.50 ppm by mass or less.

[0190] [2] Components

[0191] Next, the components of one embodiment of the present invention will be described.

[0192] One embodiment of the present invention is a component made using the aforementioned cladding steel sheet (as a blank). For example, the component is made by performing at least one of forming or joining processes on the cladding steel sheet as a blank.

[0193] Here, the tensile strength (TS) of the aforementioned cladding steel sheet is 780 MPa or higher, exhibiting excellent ductility, 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 under in-vehicle collision conditions. Furthermore, in addition to its high strength of 780 MPa or higher, it is also less prone to LME cracking even when the steel sheet undergoes a plating process. Therefore, the component of one embodiment of the present invention is suitable for use as an impact energy absorption component in the automotive field.

[0194] [3] Manufacturing method of cladding steel plate

[0195] 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 used for heating or cooling various slabs and steel sheets as shown below refer to the surface temperatures of the various slabs and steel sheets.

[0196] A method for manufacturing a cladding steel plate according to one embodiment of the present invention includes:

[0197] The first preparation step is to prepare a base steel billet with the composition of the above-mentioned base material.

[0198] The second preparation step involves preparing a steel billet for the coating material having the composition of the coating material described above.

[0199] The surface treatment process involves performing surface treatment to ensure that the surface roughness, in terms of Ra, 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.

[0200] In the lamination process, the surface-treated surface of the base steel billet is connected to 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.

[0201] 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;

[0202] 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.

[0203] 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

[0204] 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.

[0205] First preparation process

[0206] Prepare a steel billet with the composition of the aforementioned base material. For example, the steel billet is smelted to produce molten steel with the composition of the aforementioned base material. The smelting method is not particularly limited; known smelting methods such as converter smelting and electric furnace smelting are suitable. The resulting molten steel is solidified to produce a steel billet (slab). The method for producing the steel billet from the molten steel is not particularly limited; continuous casting, ingot casting, or thin slab casting can be used. To prevent macroscopic segregation, the steel billet is preferably produced by continuous casting.

[0207] Second preparation process

[0208] Here, a steel billet with the composition of the aforementioned coating material is prepared. For example, the steel billet is smelted to produce molten steel with the composition of the aforementioned coating material. The smelting method is not particularly limited; known smelting methods such as converter smelting and electric furnace smelting are suitable. The resulting molten steel is solidified to produce a steel billet (slab). The method for producing the steel billet from the molten steel is not particularly limited; continuous casting, ingot casting, or thin slab casting can be used. To prevent macroscopic segregation, the steel billet is preferably produced by continuous casting.

[0209] Surface treatment process

[0210] 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 50 μm or less in terms of 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.

[0211] It should be noted that the surface roughness Ra was measured according to JIS B 0601 (1994) and JIS B 0031 (1994).

[0212] •Lamination process

[0213] Next, with the surface-treated surfaces of the base steel billet and the cladding material steel billet in contact, the base steel billet and the aforementioned cladding material steel billet are stacked in the order of cladding material steel billet – base steel billet – cladding material steel billet to obtain a laminated slab (forming a sandwich structure with the base steel billet sandwiched between the cladding material steel billets). It should be noted that the surfaces of the base steel billet and the cladding material steel billet can be cleaned before stacking.

[0214] • Joining process

[0215] 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).

[0216] There are no particular limitations on the joining method, but for example, a laminated slab is made by placing a base steel billet between two cladding steel billets, and electron beam welding (EBW), arc welding, or laser beam welding is performed around the ends of the laminated slab (between the cladding steel billet and the base steel billet), thereby joining the cladding steel billet and the base steel billet.

[0217] In addition, the vacuum level between the cladding material billet and the base material billet was 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 preferred to be 1×10). －7 A high vacuum level (above Torr) can further improve 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 steel sheet (thin steel sheet).

[0218] There is no particular limitation on the method of vacuuming between each cladding material billet and the base material billet. However, for example, when laser beam welding is performed around the ends of the laminated slab, a vacuum valve is installed between the cladding material billet and the base material billet at the end (before the ends are fully joined), and a vacuum pump is connected to it, thereby evacuating the vacuum between each cladding material billet and the base material billet.

[0219] Hot rolling process

[0220] Next, the obtained laminated slab is subjected to hot rolling consisting of rough rolling and finish rolling to produce hot-rolled steel sheet.

[0221] In one example, the laminated slab manufactured as described above is temporarily cooled to room temperature, and then heated and rolled.

[0222] Slab heating temperature: 1050℃~1350℃

[0223] From the perspectives of diffusion bonding between the cladding material and the base material, dissolution of carbides, and reduction of rolling load, the slab heating temperature is 1050°C or higher. Furthermore, to prevent increased oxide scale loss, the slab heating temperature is 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.

[0224] 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.

[0225] Next, the laminated slab is rough-rolled using conventional methods to produce 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 problems during finish rolling, it is preferable to heat the thin slab using a bar heater or similar device before finish rolling, while reducing the heating temperature of the slab.

[0226] Final rolling temperature: above 820℃

[0227] To reduce rolling load, and because a higher reduction rate in the non-recrystallized austenite state leads to the development of abnormal structures elongating along the rolling direction, potentially reducing the workability of the annealed sheet, the final rolling temperature is preferably 820°C or higher. There is no particular upper limit to the final rolling temperature, but for example, it is preferably 1100°C or lower.

[0228] Alternatively, rough-rolled plates can be joined together during hot rolling and continuously finished rolled. Alternatively, the 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.

[0229] It should be noted that there is no particular limitation on the winding temperature after hot rolling, but it is preferably 450℃~750℃.

[0230] Next, the hot-rolled steel sheet is subjected to pickling. Pickling removes oxides from the steel sheet surface, which is important for ensuring good chemical conversion properties and coating quality in the final product. It should be noted that pickling can be performed once or in multiple stages.

[0231] Cold rolling process

[0232] 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.

[0233] Cold rolling reduction rate: 30%–80%

[0234] By achieving a cold rolling reduction rate of 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 next annealing step results in good flexibility. Therefore, a cold rolling reduction rate of 30% or more is preferred, 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.

[0235] It should be noted that there are no special restrictions on the number of rolling passes in cold rolling.

[0236] Annealing process

[0237] Next, the cold-rolled steel sheet is annealed at an annealing temperature of 750℃~950℃ and a holding time of more than 20 seconds.

[0238] Annealing temperature: 750℃~950℃, holding time: 20 seconds or more

[0239] During bending deformation, the hardness of martensite formed from retained austenite increases significantly, reducing bending and impact resistance. When the annealing temperature is below 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 (steel strength) of 780 MPa or higher. On the other hand, if the annealing temperature exceeds 950°C, defects 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 lower. It should be noted that the annealing temperature is the highest temperature reached during the annealing process. In addition to the holding time at the annealing temperature, the holding time also includes the residence time within the temperature range (annealing temperature - 40°C) above and below the annealing temperature during heating and cooling before and after reaching the annealing temperature.

[0240] There are no particular limitations on the cooling after holding; any conventional method can be followed. However, after the annealing process, a reheating process can be performed under the following conditions. Hereinafter, this situation will be described as the first embodiment of any process after the annealing process.

[0241] [First Implementation Method]

[0242] First reheating process

[0243] 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.

[0244] Cooling stop temperature: below 250℃

[0245] 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, by allowing 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 250°C or below. A cooling stop temperature of 200°C or below is more preferable. The lower limit of the cooling stop temperature is not particularly limited, but for example, a cooling stop temperature of -30°C or above is preferred.

[0246] It should be noted that the average cooling rate from the end of the annealing process to the above-mentioned cooling stop temperature is not particularly limited, but is preferably 1°C / second to 50°C / second.

[0247] Reheating temperature: above 250℃ but below 450℃

[0248] 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 within 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, if the reheating temperature exceeds 450°C, the diffusible hydrogen content in the steel sheet may increase 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. The reheating temperature is more preferably above 300°C. Furthermore, the reheating temperature is more preferably below 400°C. It should be noted that the reheating temperature is the highest temperature reached in the reheating process.

[0249] Duration: 10 seconds or more

[0250] When the holding time within the reheating temperature range (above 250°C and below 450°C) is less than 10 seconds, there is a possibility that the amount of diffusible hydrogen in the steel sheet may increase with the increase in the area ratio of quenched martensite. Therefore, there is a possibility of decreased 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, but due to limitations in production technology, it is preferably 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.

[0251] 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, spray 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.

[0252] Alternatively, the cold-rolled steel sheet that has undergone the above-mentioned processes can also be subjected to quenching and tempering rolling. If 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, but from the viewpoint of productivity, it is preferable to be 0.05% or more. Furthermore, quenching and tempering rolling can be performed on an apparatus continuous with the annealing apparatus used for the above-mentioned annealing process (online) or on an apparatus discontinuous with the annealing apparatus used for the annealing process (offline). In addition, the target reduction rate can be achieved by rolling in one pass, or by performing multiple passes to achieve a total reduction rate of 0.05% to 1.50%. It should be noted that the rolling described here usually refers to quenching and tempering rolling, but rolling using a straightening machine or the like can also be used as long as the same elongation as quenching and tempering rolling can be achieved.

[0253] Plating process

[0254] Alternatively, the cold-rolled steel sheet can be plated after the annealing process or the first reheating process. 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.

[0255] 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.

[0256] It should be noted that hot-dip galvanizing can also be performed using an apparatus configured to continuously perform annealing and hot-dip galvanizing.

[0257] 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 hot-dip galvanizing treatment as described above on the cold-rolled steel sheet. If the alloying temperature is less than 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 TS (transformed ductility) and ductility sometimes decrease. Therefore, when performing the alloying treatment of galvanizing, it is preferable to perform the alloying treatment within a temperature range of 450°C to 600°C. The alloying temperature is more preferably 470°C or higher. Furthermore, the alloying temperature is more preferably 550°C or lower, and even more preferably 530°C or lower.

[0258] When performing electroplating zinc treatment, it is preferable to use a plating bath at room temperature to 100°C, with a coating thickness of 20 to 80 g / m² per single side. 2 .

[0259] In addition, the coating adhesion of hot-dip galvanized steel sheet (GI) and alloyed hot-dip galvanized steel sheet (GA) is preferably 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.

[0260] As described above, the coated steel sheet obtained by the coating process can also be rolled with an elongation of 0.05% to 1.00% after cooling to below 50°C. Furthermore, the elongation after rolling after cooling to below 50°C is more preferably 0.10% or more. Moreover, the elongation after rolling after cooling to below 50°C is more preferably 0.70% or less.

[0261] Rolling after cooling to below 50°C can be performed on an apparatus continuous with the galvanizing apparatus used for the above-described galvanizing process (online) or on an apparatus discontinuous with the galvanizing apparatus used for the galvanizing process (offline). 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, but rolling using methods such as straightening can also be performed as long as the same elongation as quenching and tempering rolling can be achieved.

[0262] • Dehydrogenation process

[0263] 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). If the temperature range exceeds 300°C or is maintained for more than 72.0 hours, it may be difficult to ensure the desired TS (transfer rate) due to tempering. Furthermore, if the temperature is maintained below 50°C or for less than 0.5 hours, the effect of reducing the amount of diffusible hydrogen in the coated steel sheet may not be sufficiently obtained. Therefore, in the dehydrogenation treatment step, it is preferable to maintain 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 step, it is more preferable to maintain the coated steel sheet at a temperature range of 70°C to 200°C for 1 hour to 36.0 hours.

[0264] It should be noted that the above-mentioned dehydrogenation treatment can also be performed on cold-rolled steel sheets after the annealing process or after the reheating process.

[0265] Alternatively, as another embodiment, a plating process can be performed where, after holding the annealing process, the cold-rolled steel sheet is cooled to a temperature range of, for example, 350°C to 600°C, followed by a hot-dip galvanizing or alloyed hot-dip galvanizing treatment, and a second reheating process. Hereinafter, this will be described as a second embodiment, an arbitrary process following the annealing process.

[0266] [Second Implementation]

[0267] Plating process

[0268] After the annealing process, the cold-rolled steel sheet is cooled to a temperature range of, for example, 350°C to 600°C, and then hot-dip galvanizing or alloying hot-dip galvanizing is performed on the cold-rolled steel sheet.

[0269] 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, so they are omitted here.

[0270] Second reheating process

[0271] 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 above 250°C but below 450°C and held for more than 10 seconds.

[0272] Cooling stop temperature: below 250℃

[0273] 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, by allowing 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 250°C or below. A cooling stop temperature of 200°C or below is more preferable. The lower limit of the cooling stop temperature is not particularly limited, but for example, a cooling stop temperature of -30°C or above is preferred.

[0274] Reheating temperature: above 250℃ but below 450℃

[0275] 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 within this temperature range for at least 10 seconds. By reheating the temperature above 250°C, diffusible hydrogen in the steel sheet is released, thus promoting dehydrogenation. However, if the reheating temperature exceeds 450°C, there is a possibility that the amount of diffusible hydrogen in the steel sheet may increase 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. It should be noted that the reheating temperature is the highest temperature reached in the reheating process.

[0276] Duration: 10 seconds or more

[0277] When the holding time within the reheating temperature range (above 250°C and below 450°C) is less than 10 seconds, there is a possibility that the amount of diffusible hydrogen in the steel sheet may increase with the increase in the area ratio of quenched martensite. Therefore, there is a possibility of decreased 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, but due to limitations in production technology, it is preferably 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.

[0278] The average cooling rate, cooling stop temperature, and cooling method after holding at the reheating temperature are not particularly limited. As cooling methods, gas jet cooling, spray cooling, roller cooling, water cooling, and air cooling can be used. 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.

[0279] • Dehydrogenation process

[0280] 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.

[0281] Other than those described above, conventional methods are sufficient. Furthermore, in the series of heat treatments within the manufacturing method of the cladding steel sheet according to one embodiment of the present invention described above, the temperature need not be kept constant as long as it remains within the aforementioned temperature range. Additionally, even if the cooling rate varies during cooling, there are no particular problems as long as it remains within the prescribed range. Moreover, the steel sheet can be heat-treated using any equipment, provided the thermal process is satisfied.

[0282] [4] Manufacturing method of components

[0283] Next, a method for manufacturing a component according to one embodiment of the present invention will be described.

[0284] One embodiment of the present invention provides a method for manufacturing a component that includes a step of forming or joining a cladding steel sheet (e.g., a cladding steel sheet manufactured by the above-described method for manufacturing cladding steel sheets) to produce a component.

[0285] 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, arc welding, riveting, and press-fitting can be used. It should be noted that there are no particular limitations on the forming and joining conditions; conventional methods are acceptable.

[0286] Example

[0287] The present invention will be specifically described with reference to the embodiments. The scope of the present invention is not limited to the following embodiments.

[0288] A steel billet having the composition shown in Table 1-1, with the remainder consisting of Fe and unavoidable impurities, is melted in a converter to prepare a base steel billet using continuous casting. Separately, a steel billet having the composition shown in Table 1-2, with the remainder consisting of Fe and unavoidable impurities, is melted in a converter to prepare a cladding material steel billet 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 steel 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 where the base steel billet and the cladding material steel billet meet). Then, with the surface-treated surfaces of the base steel billet and the cladding material steel billet in contact, the base steel billet and the cladding material steel billet are stacked in the order of cladding material steel billet – base steel billet – cladding material steel 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, a billet consisting only of the base material billet is used instead of a cladding material billet.

[0289] The obtained laminated slab is heated to the slab heating temperatures shown in Table 2 and subjected to rough rolling. Next, finish rolling is performed at the final rolling temperatures shown in Table 2 to obtain hot-rolled steel sheet. Then, cold rolling and annealing processes are performed under the conditions shown in Table 2 to obtain cold-rolled steel sheet (CR).

[0290] Next, for a portion of the cold-rolled steel sheets, coated steel sheets are obtained by undergoing 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 by plating process after cooling to room temperature. It should be noted that, for convenience, the cooling stop temperature after the annealing process is recorded in the cooling stop temperature column of the first reheating process. It should also be noted that the "-" in the cooling stop temperature column of the first reheating process refers to cooling to room temperature after the annealing process.

[0291] 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, a zinc bath containing 0.20% by mass of Al, with the remainder consisting of Zn and unavoidable impurities, is used in the manufacture of GI. Conversely, a zinc bath containing 0.14% by mass of Al, with the remainder consisting of Zn and unavoidable impurities, is used in the manufacture of GA. The bath temperature is 470°C in the manufacture of both GI and GA. The coating weight is 45–72 g / m² per single side in the manufacture of GI. 2 (Double-sided plating) Approximately 45g / m² per side during GA manufacturing. 2(Double-sided plating) around.

[0292] The alloying treatment during the manufacture of GA was carried out at the temperatures shown in Table 2. Furthermore, the GI coating composition contains 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 contains Fe: 7–15 wt%, Al: 0.1–1.0 wt%, with the remainder consisting of Zn and unavoidable impurities.

[0293] When manufacturing EG, a plating bath at 30°C is used, and the plating adhesion per single side is 20–50 g / m². 2 about.

[0294] In addition, some of the coated steel sheets were further subjected to dehydrogenation treatment under the conditions shown in Table 2.

[0295] Using the cold-rolled and galvanized steel sheets obtained above as test steels, 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.

[0296] 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 side) and (3) cladding material (back side), so only (1) cladding material (surface side) is recorded as representative.

[0297] <Tension Properties>

[0298] Tensile testing was conducted according to JIS Z 2241. JIS No. 5 test pieces were taken from the obtained steel sheet, with the long side as the C-direction. Using these test pieces, tensile tests were performed at a crosshead speed of 10 mm / min, and the total elongation (TS) and total elongation (E1) were measured. A TS of 780 MPa or higher was considered acceptable. Furthermore, for a TS of 780 MPa or higher but less than 1180 MPa, an E1 ≥ 15% was considered good; for a TS of 1180 MPa or higher, an E1 ≥ 12% was considered good.

[0299] <Extreme Deformation Capability>

[0300] Ultimate deformation capacity is achieved by utilizing 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.

[0301] ε l =-(ε w +ε t )

[0302] ε w =ln(w / w0),ε t =ln(t / t0)

[0303] w0: Plate width before tensile test; w: Plate width at fracture after tensile test.

[0304] t0: Plate thickness before tensile test; t: Plate thickness at fracture after tensile test.

[0305] It should be noted that, according to reports in Nakagawa et al., Plasticity and Processing, 11-29 (1970), 142, and Matsutō et al., Plasticity and Processing, 14-146 (1973), 201, the ultimate deformation capacity is known to be related to the pore-expanding property (tensile flange property).

[0306] In addition, regarding 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, a value of 0.4 or above is considered good.

[0307] <V-bending test>

[0308] The V (90°) bending test was conducted according to JIS Z 2248. Test pieces with end-face finishing were used, measuring 1.2 mm thick × 100 mm wide (C direction) × 35 mm long (L direction) and 1.4 mm thick × 100 mm wide (C direction) × 35 mm long (L direction). 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 it by the plate thickness t (R / t). Furthermore, using a Leica stereomicroscope at 25x magnification, cracks longer than 200 μm were considered as open fractures.

[0309] It should be noted that when TS is above 780MPa and below 1180MPa, R / t≤3.0 is judged as good; when TS is above 1180MPa, R / t≤4.0 is judged as good.

[0310] <VDA Bending Test>

[0311] 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. The VDA bending test (L-axis bending) was performed in a bending testing machine with a roller distance of 2 × plate thickness + 0.5mm and a punch tip curvature radius of R = 0.4mm at a stroke speed of 20mm / min. The α value 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.

[0312] It should be noted that when TS is above 780MPa and below 1180MPa, F≥8500N, S≥12mm, and α VDA ≥95° and F×S≥102000N·mm are judged as good.

[0313] When TS is above 1180MPa, F≥10500N, S≥11mm, α VDA ≥90° and F×S≥115500N·mm are judged as good.

[0314] <V-bending - Orthogonal VDA bending test>

[0315] The V-bending-orthogonal VDA bending test was calculated using the method described in Sato et al. Patent No. 6748382. Using test pieces with end-face finishing of 1.2mm thickness × 65mm width (C direction) × 60mm length (L direction) or 1.4mm thickness × 65mm width (C direction) × 60mm length (L direction), a V (90°) bend with a bending radius R = 5mm in the L direction (C-axis bending) was performed under conditions of a load of 10ton, a stroke speed of 30mm / min, and a holding time of 5s. The V-bending sample was rotated 90° horizontally. Then, in a bending testing machine with a roller distance 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 facing the punch side at a stroke speed of 20 mm / min, 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 edge portion are known to be related to shaft crushing.

[0316] It should be noted that when TS is above 780MPa but less than 1180MPa, F≥6500N, S≥29mm, and F×S≥188500N·mm are judged as good.

[0317] When TS is above 1180MPa, F≥7000N, S≥28mm, and F×S≥196000N·mm are judged as good.

[0318] <LME Resistance>

[0319] LME resistance 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 resistance 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 plate assembly consisting of two overlapping steel sheets was resistance 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 energization and 5 cycles of holding time. The welded component was cut in half, and the cross-section was observed using an optical microscope. No cracks larger than 0.1 mm were considered good LME cracking resistance (〇), while cracks larger than 0.1 mm were considered poor LME cracking resistance (×).

[0320] [Table 1-1]

[0321] Table 1-1

[0322]

[0323] "-" indicates the level of unavoidable impurities.

[0324] [Table 1-2]

[0325] Table 1-2

[0326]

[0327] "-" indicates the level of unavoidable impurities.

[0328] [Table 2]

[0329] Table 2

[0330]

[0331] Table 22 (continued)

[0332]

[0333] *CR: Cold-rolled steel sheet (uncoated), GI: Hot-dip galvanized steel sheet, GA: Alloyed hot-dip galvanized steel sheet, EG: Electro-galvanized steel sheet

[0334] Table 2 (continued)

[0335]

[0336] Table 2 (continued)

[0337]

[0338] *CR: Cold-rolled steel sheet (uncoated), GI: Hot-dip galvanized steel sheet, GA: Alloyed hot-dip galvanized steel sheet, EG: Electro-galvanized steel sheet

[0339] [Table 3]

[0340] Table 3

[0341]

[0342] F: Ferrite, F': Unrecrystallized ferrite, M: Martensite, TM: Tempered martensite, RA: Retained austenite

[0343] B: bainite, P: pearlite, θ: cementite and other carbides

[0344] Table 3 (continued)

[0345]

[0346] Table 3 (continued)

[0347]

[0348] *CR: Cold-rolled steel sheet, GI: Hot-dip galvanized steel sheet, GA: Alloyed hot-dip galvanized steel sheet, EG: Electro-galvanized steel sheet

[0349] Table 3 (continued)

[0350]

[0351] F: Ferrite, F': Unrecrystallized ferrite, M: Martensite, TM: Tempered martensite, RA: Retained austenite

[0352] B: bainite, P: pearlite, θ: cementite and other carbides

[0353] Table 3 (continued)

[0354]

[0355] Table 33 (continued)

[0356]

[0357] *CR: Cold-rolled steel sheet, GI: Hot-dip galvanized steel sheet, GA: Alloyed hot-dip galvanized steel sheet, EG: Electro-galvanized steel sheet

[0358] 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 ductility, 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 inferior to that of the example of the present invention.

[0359] Furthermore, it is known that the tensile strength (TS) of the parts obtained by forming or joining the cladding steel sheet of the present invention is 780 MPa or higher, and that the parts have excellent ductility, bending, impact resistance and LME resistance.

Claims

1. A cladding steel sheet having a base material and a cladding material on the surface and back of the base material, The base material has the following composition and steel structure: The composition of the components, by mass%, is as follows: C: 0.080%–0.350%, Si: 0.50%–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. In the steel microstructure, the total area fraction of bainite and tempered martensite is more than 30%, and the volume fraction of retained austenite is more than 5%. The cladding material has the following composition and steel structure: 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. In the steel microstructure, the area fraction of ferrite is 80% or more; The average Vickers hardness (HVL) of the coating material is below 260. The value obtained by dividing the average Vickers hardness HVL of the coating material by the average Vickers hardness HVB of the base material is 0.80 or less. The boundary roughness between the base material and the coating material, measured by the maximum height Ry, is less than 50 μm. The number of gaps at the boundary between the base material and the coating material is less than 20 per 10mm boundary length.

2. The cladding steel plate according to claim 1, wherein, At least one of the composition of the base material and the composition of the coating material 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, Mg: 0.0200% or less, Zn: At least one of the following: 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%.

3. The cladding steel plate according to claim 1, wherein, The value obtained by dividing the thickness of the base material by the total thickness of the coating materials is 1 or more.

4. The cladding steel plate according to claim 2, wherein, The value obtained by dividing the thickness of the base material by the total thickness of the coating materials is 1 or more.

5. The cladding steel plate according to any one of claims 1 to 4, wherein, It has a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electroplated galvanized layer on its surface.

6. The cladding steel plate according to any one of claims 1 to 4, wherein, The total amount of diffusible hydrogen contained in the base material and the coating material is less than 0.50 ppm by mass.

7. The cladding steel plate according to claim 5, wherein, The total amount of diffusible hydrogen contained in the base material and the coating material is less than 0.50 ppm by mass.

8. A component made using the cladding steel sheet according to any one of claims 1 to 7.

9. A method for manufacturing a cladding steel plate, comprising the steps described in any one of claims 1 to 7: The first preparation step involves preparing a base steel billet having the composition of the base material as described in claim 1 or 2. The second preparation step involves preparing a steel billet for a coating material having the composition of the coating material as described in claim 1 or 2. 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 surface and back 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. joining the clad material steel blank and the base material steel blank, and evacuating to make the vacuum degree between the clad material steel blank and the base material steel blank each 1 x 10 －2 Torr or less, to obtain a joined laminated plate blank; 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%. Obtain 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.

10. The method for manufacturing the cladding steel plate according to claim 9, wherein, 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.

11. The method for manufacturing the cladding steel plate according to claim 9 or 10, 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.

12. The method for manufacturing the cladding steel plate according to claim 11, wherein, The plating treatment is hot-dip galvanizing, alloyed hot-dip galvanizing, or electro-galvanizing.

13. The method for manufacturing the cladding steel plate according to claim 11, 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.

14. The method for manufacturing the cladding steel plate according to claim 12, 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.

15. The method for manufacturing the cladding steel plate according to claim 9, wherein, It further includes the following processes: The coating process involves, after the annealing process, subjecting the cold-rolled steel sheet to hot-dip galvanizing or alloyed hot-dip galvanizing to obtain a coated steel sheet; and In the second reheating process, the plated 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.

16. The method for manufacturing the cladding steel plate according to claim 15, 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.

17. A method for manufacturing a component, comprising a step of forming or joining a cladding steel plate according to any one of claims 1 to 7 to produce the component.

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