Steel sheet, member, and method for manufacturing the same

Abstract

The present invention provides a steel sheet having high strength and excellent delayed fracture resistance and a method for manufacturing the same. The steel sheet contains, in mass%, C: 0.15 to 0.45%, Si: 1.5% or less, Mn: more than 1.7%, P: 0.03% or less, S: less than 0.0040%, sol. Al: 0.20% or less, N: 0.005% or less, B: 0.0015 to 0.0100%, one or more of Nb and Ti: total 0.005 to 0.080%, and the balance consisting of Fe and inevitable impurities, the area ratio of the martensite to the whole structure is 95 to 100%, the original γ grain size is less than 11.2 μm, and the number density A of precipitates having an equivalent circle diameter of 500 nm or more satisfies the formula: A (pieces / mm 2 ) ≤ 8.5 × 10 5 × [B].

Description

Technical Field

[0001] This invention relates to steel sheets such as high-strength steel sheets for cold pressing used in automobiles and the like, components using such steel sheets, and methods for manufacturing them. Background Technology

[0002] In recent years, efforts have been made to improve the lightweighting and crash safety of automobiles, with a push for the use of steel plates with a tensile strength (TS) of 1310 MPa or higher in automotive frame components. Additionally, the use of steel plates with a tensile strength (TS) of 1470 MPa or higher is being promoted for components such as bumpers and anti-collision beams.

[0003] When high-strength steel plates with a tensile strength (TS) of 1470 MPa or higher are cold-pressed to form components, delayed fracture may occur due to the increase in residual stress within the component and the deterioration of the steel plate's resistance to delayed fracture.

[0004] Here, delayed fracture refers to the following phenomenon: when a component is placed in a hydrogen-infiltrating environment under high stress, hydrogen penetrates into the steel plate constituting the component, reducing the interatomic bonding force and causing local deformation, thereby generating microcracks, which progress and eventually lead to fracture.

[0005] As a technique to improve such delayed fracture characteristics, for example, by reducing coarse precipitates that become the starting point of delayed fracture failure, the steel sheet with improved delayed fracture resistance is disclosed in Patent Document 1. This steel sheet contains, by mass percent, C: 0.13% or more and 0.40% or less, Si: 1.5% or less, Mn: 1.8% or more and 4% or less, P: 0.02% or less, S: less than 0.0010%, sol.Al: 0.2% or less, N: less than 0.0060%, B: 0.0003% or more and less than 0.0035%, O: less than 0.0020%, and also satisfies [%Ti] + [%Nb] > 0.007, [%Ti] × [%Nb] 2 <7.5×10 -6 The composition contains one or both of Nb (0.002% or more and less than 0.035%) and Ti (0.002% or more and less than 0.040%), with the balance consisting of Fe and unavoidable impurities. It features an area ratio of martensite and bainite relative to the overall microstructure of greater than 90% and less than 100%, an average grain size of the original austenite grains of 6–15 μm, and reduced inclusions meeting specific conditions. The carbides, with Fe as the main component, have an aspect ratio of less than 2.0 and a major axis of more than 0.30 μm and less than 2 μm, and a concentration of 4000 carbides / mm². 2The following steel structures have a plate thickness of 0.5–2.6 mm and a tensile strength of 1320 MPa or higher.

[0006] Furthermore, Patent Document 2 discloses a high-strength cold-rolled steel sheet with excellent resistance to hydrogen embrittlement and processability, characterized by containing, by mass%, C: 0.05% to 0.30%, Si: 2.0% or less (including 0%), Mn: greater than 0.1% and less than 2.8%, P: less than 0.1%, S: less than 0.005%, N: less than 0.01%, Al: 0.01% to 0.50%, and the total content being 0.01% or more, satisfying the condition [%C]-[%Nb]. The composition consists of one or more of Nb, Ti, and Zr, with the balance being iron and unavoidable impurities, in a manner that is / 92.9×12-[%Ti] / 47.9×12-[%Zr] / 91.2×12>0.03. It also has a microstructure containing 50% or more (including 100%) tempered martensite by area ratio, with the balance being ferrite. Regarding the distribution of precipitates in the tempered martensite, precipitates with an equivalent circle diameter of 1–10 nm relative to each μm... 2 Precipitates containing more than 20 tempered martensite particles and an equivalent circle diameter of more than 20 nm, i.e., precipitates containing one or more of Nb, Ti, and Zr relative to 1 μm 2 The tempered martensite has fewer than 10 grains, and the average grain size of the ferrite surrounded by large-angle grain boundaries with a crystal orientation difference of more than 15° is less than 5 μm.

[0007] Existing technical documents

[0008] Patent documents

[0009] Patent Document 1: Japanese Patent No. 6354921

[0010] Patent Document 2: Japanese Patent No. 4712882 Summary of the Invention

[0011] The problem that the invention aims to solve

[0012] However, the existing technology is not sufficient to ensure a tensile strength TS of over 1470 MPa while also possessing excellent resistance to delayed fracture.

[0013] This invention was made to solve such problems, and aims to provide steel plates, components and methods for manufacturing them with tensile strength of 1470 MPa or more (TS≥1470 MPa) and excellent resistance to delayed fracture.

[0014] Excellent resistance to delayed fracture is defined as having excellent resistance to delayed fracture based on the following evaluation criteria.

[0015] (1) First, cut a strip test piece with a rolling right angle of 100 mm and a rolling direction of 30 mm from the end of the width direction of the obtained steel plate (coil) at 1 / 4 of the width of the coil.

[0016] (2) Set the cutting of the end face of the long side with a length of 100mm as a shearing process. Under the shearing process (without performing deburring machining), perform bending process with the burr as the outer peripheral side of the bending process, maintain the shape of the test piece when bending, and fix the test piece with bolts.

[0017] The shearing gap is set to 13%, the front angle is set to 1°, and the bending process is carried out with a front bending radius of 10mm and an inner angle of 90 degrees at the bending apex (V-shaped bending).

[0018] The punch uses a U-shaped punch with the same front-end radius as the aforementioned front-end bending radius R (the front-end R portion is semi-circular and the thickness of the punch body is 2R). The die uses a die with a corner radius of 30mm. Then, the depth of the punch pressing the steel plate is adjusted to form the shape with a front-end bending angle (the angle inside the bending apex) of 90 degrees (V-shape).

[0019] The test piece is clamped and tightened using a hydraulic jack, with the distance between the flange ends of the straight section during bending forming being the same as that during bending forming (to eliminate the opening of the straight section caused by springback). The bolts are then tightened in this state. The bolts are secured through elliptical holes (10mm minor axis, 15mm major axis) pre-set 10mm inside the short edge of the strip test piece.

[0020] (3) The bolt-tightened test piece was immersed in a solution in which 0.1% by mass ammonium thiocyanate aqueous solution and McIlvaine buffer were mixed at a 1:1 ratio and the pH was adjusted to 8.0 to conduct a delayed fracture resistance evaluation test. The solution temperature was set at 20℃, and the test piece was immersed in a solution containing 0.1% by mass ammonium thiocyanate aqueous solution and McIlvaine buffer at a pH of 1:1. 2 The liquid volume for the surface area is set to 20 ml.

[0021] (4) After 24 hours, check whether there are any cracks that can be visually confirmed (length greater than 1 mm). If no cracks are observed, it is judged as having excellent resistance to delayed fracture.

[0022] Methods for solving problems

[0023] In order to solve the above problems, the inventors have conducted in-depth research and found that by satisfying all of the following conditions, the resistance to delayed fracture can be greatly improved.

[0024] i) The area ratio of martensite is over 95%.

[0025] ii) The average grain size of the original austenite grains (original γ grain size) is less than 11.2 μm.

[0026] iii) The number density A of precipitates with an equivalent circle diameter of 500 nm or more satisfies the following condition.

[0027] A (pieces / mm) 2 )≤8.5×10 5 ×[B]

[0028] Here, [B] indicates the content of B (mass%).

[0029] This invention was completed based on the above insights and further research, and its main points are as follows.

[0030] [1] A steel plate having, by mass %, a composition comprising: C: 0.15% or more and 0.45% or less, Si: 1.5% or less, Mn: greater than 1.7%, P: 0.03% or less, S: less than 0.0040%, sol.Al: 0.20% or less, N: 0.005% or less, B: 0.0015% or more and 0.0100% or less, one or more of Nb and Ti: totaling 0.005% or more and 0.080% or less, with the balance being Fe and unavoidable impurities.

[0031] Furthermore, the tissue exhibits a martensite area comprising more than 95% and less than 100% of the total tissue area.

[0032] The average grain size of the original austenite grains is less than 11.2 μm.

[0033] The number density A of precipitates with an equivalent circle diameter of 500 nm or more satisfies the following equation (1).

[0034] A (pieces / mm) 2 )≤8.5×10 5 ×[B]…Formula (1)

[0035] Here, [B] indicates the content of B (mass%).

[0036] [2] The steel plate according to [1], wherein, as a component of the above composition, it further contains, by mass %, one or both selected from Cu: less than 1.0% and Ni: less than 1.0%.

[0037] [3] The steel plate according to [1] or [2], wherein, as part of the above-mentioned composition, it further contains, by mass %, one or more of the following: Cr: less than 1.0%, Mo: less than 0.3%, V: less than 0.5%, Zr: less than 0.2% and W: less than 0.2%.

[0038] [4] The steel plate according to any one of [1] to [3], wherein, as a component of the above composition, it further contains, by mass %, one or more of the following: Ca: less than 0.0030%, Ce: less than 0.0030%, La: less than 0.0030%, REM (excluding Ce and La): less than 0.0030%, and Mg: less than 0.0030%.

[0039] [5] The steel plate according to any one of [1] to [4], wherein, as a component of the above-mentioned composition, it further contains, by mass %, one or both selected from Sb: less than 0.1% and Sn: less than 0.1%.

[0040] [6] The steel plate according to any one of [1] to [5], wherein the steel plate has a coating on its surface.

[0041] [7] A component made of steel plate as described in any one of [1] to [6].

[0042] [8] A method for manufacturing a steel plate, wherein,

[0043] A steel billet having any one of the compositions described in [1] to [5] is heated from 1000°C to a holding temperature of 1250°C or higher using a steel billet surface thermometer at an average heating rate of 10°C / min or less, and held at the holding temperature for at least 30 minutes.

[0044] The dwell time at 900–1000°C is set to be more than 20 seconds and less than 150 seconds, and hot finishing rolling is performed under the condition that the finishing rolling temperature is set to above 850°C.

[0045] The cooling process will be carried out at an average cooling rate of 40°C / second or higher within the range from the aforementioned finishing rolling temperature to 650°C.

[0046] Then, the steel is wound at a winding temperature below 650°C to produce hot-rolled steel sheet.

[0047] The hot-rolled steel sheet is cold-rolled with a reduction rate of 40% or more to produce a cold-rolled steel sheet.

[0048] Perform the following continuous annealing:

[0049] The annealing temperature is set to 800–950°C, and the cold-rolled steel sheet is heated from 400°C to the annealing temperature at an average heating rate of 1.0°C / second or higher.

[0050] Hold at the above annealing temperature for less than 600 seconds.

[0051] Cool from the above annealing temperature to 420°C at a first average cooling rate of 2°C / second or higher.

[0052] Cooling from 420°C to a cooling stop temperature below 260°C at a second average cooling rate of 10°C / second or higher.

[0053] Then, maintain at a holding temperature of 150–260°C for 20–1500 seconds.

[0054] [9] According to the steel plate manufacturing method described in [8], the cold-rolled steel plate is immersed in a plating bath while being cooled at the first average cooling rate in the continuous annealing process described above, and then heated to 480 to 600°C for alloying treatment after immersion in the plating bath.

[0055]

[10] The steel plate manufacturing method according to [8] wherein the surface of the steel plate is plated after the continuous annealing described above.

[0056]

[11] A method for manufacturing a component, comprising a step of forming a component by performing at least one of forming or joining processes on a steel plate as described in any one of [1] to [6].

[0057] Invention Effects

[0058] According to the present invention, steel plates, components and methods thereof with high strength and excellent resistance to delayed fracture can be provided. Detailed Implementation

[0059] The embodiments of the present invention will be described below.

[0060] The steel plate of the present invention comprises, by mass percent, C: 0.15% or more and 0.45% or less, Si: 1.5% or less, Mn: greater than 1.7%, P: 0.03% or less, S: less than 0.0040%, sol.Al: 0.20% or less, N: 0.005% or less, B: 0.0015% or more and 0.0100% or less, one or more of Nb and Ti: totaling 0.005% or more and 0.080% or less, with the balance being Fe and unavoidable impurities, and has a microstructure in which the area ratio of martensite relative to the overall microstructure is 95% or more and 100% or less, the average grain size (original γ grain size) of the original austenite grains (hereinafter also referred to as original γ grains) is less than 11.2 μm, and the number density A of precipitates with an equivalent circle diameter of 500 nm or more satisfies the following formula (1).

[0061] A (pieces / mm) 2 )≤8.5×10 5 ×[B]…Formula (1)

[0062] Here, [B] indicates the content of B (mass%).

[0063] Composition

[0064] The reasons for limiting the range of the composition of the steel plate of the present invention will be explained below. It should be noted that the % of the composition content refers to "mass %".

[0065] C: Above 0.15% and below 0.45%

[0066] Carbon (C) is added to improve hardenability, thereby obtaining a martensitic steel structure, and to increase the strength of martensite. To ensure a tensile strength of 1470 MPa or higher (hereinafter, also referred to as TS≥1470 MPa), the C content is set to 0.15% or higher. From the viewpoint of reducing the weight of automotive frame components due to increased tensile strength, the C content is preferably 0.20% or higher, more preferably 0.27% or higher. On the other hand, excessive C becomes a factor that deteriorates the resistance to delayed fracture due to the formation of iron carbides and segregation towards grain boundaries. From these viewpoints, the C content is limited to a range of 0.45% or lower. The C content is preferably 0.40% or lower, more preferably 0.37% or lower.

[0067] Si: below 1.5%

[0068] Si is included as a strengthening element based on solid solution strengthening and to improve resistance to delayed fracture by suppressing the formation of film-like carbides during tempering at temperatures above 200°C. Additionally, Si is included to suppress MnS formation by reducing Mn segregation in the central part of the plate thickness. Furthermore, Si is included to suppress decarburization and debonding caused by surface oxidation during annealing on a continuous annealing line (CAL). While no lower limit is specified for the Si content, it is preferable to contain 0.02% or more of Si to achieve the aforementioned effects. The Si content is preferably 0.10% or more, and more preferably 0.20% or more. On the other hand, excessive Si content leads to a significant increase in rolling load and a decrease in toughness during hot and cold rolling. From these perspectives, the Si content is set to 1.5% or less (including 0%). The Si content is preferably 1.2% or less, and more preferably 1.0% or less.

[0069] Mn: greater than 1.7%

[0070] To improve the hardenability and achieve the desired strength of steel by ensuring that the martensite area ratio is within a specified range, the steel contains more than 1.7% Mn. Preferably, it contains 1.8% or more. No specific upper limit is set. Mn is an element that particularly promotes the formation and coarsening of MnS in the central part of the plate thickness, and it co-precipitates with inclusions such as Al2O3, (Nb,Ti)(C,N), TiN, and TiS, promoting delayed fracture. Therefore, the Mn content is preferably 4.0% or less, more preferably 3.0% or less.

[0071] P: below 0.03%

[0072] Phosphorus (P) is an element that strengthens steel. When its content is high, it segregates at grain boundaries, reducing grain boundary strength and leading to a significant deterioration in resistance to delayed fracture and weldability. Based on this viewpoint, the P content is set to 0.03% or less. Preferably, the P content is 0.02% or less, more preferably 0.01% or less. While no lower limit for the P content is specified, it is set to 0.002% as a currently industrially feasible lower limit.

[0073] S: Less than 0.0040%

[0074] S forms coarse MnS, which becomes the initiation point for delayed fracture and reduces resistance to delayed fracture. From the viewpoint of improving resistance to delayed fracture, the S content needs to be set to be at least less than 0.0040%. The S content is preferably less than 0.0020%, more preferably less than 0.0010%, and even more preferably less than 0.0007%. No lower limit is specified, but as a lower limit currently feasible in industry, it is set to 0.0002%.

[0075] sol.Al: 0.20% or less

[0076] Al is included to ensure sufficient deoxidation and reduce inclusions in the steel. While no specific lower limit is specified for sol.Al, it is preferable to set the sol.Al content to 0.005% or more for stable deoxidation. More preferably, it is 0.01% or more, and even more preferably 0.02% or more. On the other hand, when the sol.Al content exceeds 0.20%, the cementite generated during coiling is difficult to dissolve during annealing, leading to deterioration of delayed fracture resistance. Therefore, the sol.Al content is set to 0.20% or less. Preferably, it is 0.10% or less, and more preferably 0.05% or less.

[0077] N: less than 0.005%

[0078] Nitrogen (N) forms precipitates such as TiN, (Nb,Ti), and (C,N) in steel. Due to their formation, the amount of NbC, TiC, and (Nb,Ti)C, which are effective in refining the original austenite grain size, is reduced. These precipitates hinder the adjustment to the steel microstructure required by this invention and adversely affect the resistance to delayed fracture. To reduce such adverse effects, the N content is set to 0.005% or less. The N content is preferably 0.0040% or less. While no lower limit is specified, 0.0006% is set as a currently industrially feasible lower limit.

[0079] B: Above 0.0015% and below 0.0100%

[0080] Boron (B) is an element that improves the hardenability of steel, possessing the advantage of generating martensite with a specified area ratio even with low Mn content. Furthermore, B increases grain boundary bonding strength due to its segregation at grain boundaries, and improves resistance to delayed fracture by suppressing the segregation of phosphorus (P), which reduces grain boundary strength. On the other hand, the addition of excessive B results in Fe... 23 (C,B)6. The increase of BN, which becomes the starting point for delayed fracture, actually reduces the resistance to delayed fracture. Therefore, to achieve the effect of improving the resistance to delayed fracture brought about by adding B, it is necessary to balance the increase of B dissolved at grain boundaries and the suppression of B-based precipitates. In steels with an original γ grain size of 10 μm or less, to obtain a sufficient amount of B dissolved at grain boundaries, the B content is set to 0.0015% or more. The B content is preferably 0.0025% or more, and more preferably 0.0040% or more. On the other hand, when the B content is greater than 0.0100%, it is difficult to reduce B-based precipitates even when controlling the hot rolling and annealing conditions. Therefore, the B content is set to 0.0100% or less. The B content is preferably 0.0090% or less, and more preferably 0.0080% or less.

[0081] One or more of Nb and Ti: totaling more than 0.005% and less than 0.080%.

[0082] Nb and Ti contribute to increased strength by refining the internal structure of martensite and improve resistance to delayed fracture by refining the original γ grain size. From this perspective, the steel contains at least one of Nb and Ti with a total content of 0.005%. The total content of Nb and Ti is preferably 0.010% or more, more preferably 0.020% or more. On the other hand, when the total content of one or more of Nb and Ti exceeds 0.080%, Nb and Ti are not completely dissolved during billet reheating, leading to an increase in precipitates with an equivalent circle diameter of 500 nm or more, such as TiN, Ti(C,N), NbN, Nb(C,N), and (Nb,Ti)(C,N), which become the starting point for delayed fracture, thus deteriorating the resistance to delayed fracture. Therefore, the upper limit for the total content of Nb and Ti is 0.080%. The total content of Nb and Ti (Ti+Nb) is preferably 0.07% or less, more preferably 0.06% or less.

[0083] The steel plate of the present invention contains the aforementioned constituent elements as basic components, with the balance being iron (Fe) and unavoidable impurities. Preferably, the steel plate of the present invention has a composition containing the aforementioned basic components, with the balance consisting of iron (Fe) and unavoidable impurities.

[0084] In this invention, as a component, it may contain one or more groups selected from (A) to (D) below.

[0085] (A) Selected by mass% from one or both of Cu: less than 1.0% and Ni: less than 1.0%;

[0086] (B) Selected by mass% from one or more of the following: Cr: less than 1.0%, Mo: less than 0.3%, V: less than 0.5%, Zr: less than 0.2%, and W: less than 0.2%;

[0087] (C) By mass%, it is selected from one or more of the following: Ca: less than 0.0030%, Ce: less than 0.0030%, La: less than 0.0030%, REM (excluding Ce and La): less than 0.0030%, and Mg: less than 0.0030%;

[0088] (D) Selected by mass% from one or both of Sb: less than 0.1% and Sn: less than 0.1%.

[0089] Cu: below 1.0%

[0090] Cu improves the corrosion resistance of automobiles in the operating environment. Furthermore, by containing Cu, corrosion products are coated onto the steel sheet surface, inhibiting hydrogen penetration into the steel sheet. Additionally, Cu is an element incorporated when effectively utilizing waste materials as raw materials; by allowing Cu inclusion, recycled materials can be effectively utilized as raw materials, reducing manufacturing costs. From the above viewpoints, Cu is preferably contained at 0.01% or more, and further, from the viewpoint of improving resistance to delayed fracture, Cu is preferably contained at 0.05% or more. The Cu content is more preferably 0.10% or more. However, excessive Cu content can cause surface defects; therefore, the Cu content is preferably set to 1.0% or less. Based on the above, when Cu is present, the Cu content is set to 1.0% or less. The Cu content is more preferably 0.50% or less, and even more preferably 0.30% or less.

[0091] Ni: below 1.0%

[0092] Ni also improves corrosion resistance. Furthermore, Ni reduces surface defects that are easily formed when Cu is present. Therefore, from the above perspective, it is preferable that Ni contains 0.01% or more. More preferably, Ni content is 0.05% or more, and even more preferably 0.10% or more. However, excessive Ni content leads to uneven oxide scale formation in the furnace, causing surface defects and significantly increasing costs. Therefore, when Ni is present, the Ni content is set to 1.0% or less. More preferably, Ni content is 0.50% or less, and even more preferably 0.30% or less.

[0093] Cr: less than 1.0%

[0094] Cr can be added to improve the hardenability of steel. To achieve this effect, a Cr content of 0.01% or more is preferred. A Cr content of 0.05% or more is more preferred, and 0.10% or more is even more preferred. However, when the Cr content exceeds 1.0%, the solid solution rate of cementite during annealing is delayed, leaving undissolved cementite residue, thereby deteriorating the resistance to delayed fracture at the shear end face. Furthermore, it deteriorates resistance to pitting corrosion. In addition, it deteriorates the chemical conversion treatment properties. Therefore, when Cr is present, the Cr content is set to 1.0% or less. Resistance to delayed fracture, pitting corrosion, and chemical conversion treatment properties all tend to deteriorate when the Cr content exceeds 0.2%. Therefore, from the viewpoint of preventing these issues, a Cr content of 0.2% or less is more preferred.

[0095] Mo: less than 0.3%

[0096] Mo can be added to improve the hardenability of steel, to promote the formation of fine Mo-containing carbides that serve as hydrogen trapping sites, and to improve resistance to delayed fracture due to martensite refinement. When large amounts of Nb and Ti are added, coarse precipitates are formed, which deteriorates the resistance to delayed fracture; however, the solid solution limit of Mo is relatively large compared to Nb and Ti. When added in combination with Nb and Ti, fine precipitates formed by these compounds with Mo are formed, which has the effect of refining the microstructure. Therefore, by adding Mo in combination with small amounts of Nb and Ti, the microstructure can be refined without leaving coarse precipitates, and fine carbides can be dispersed in large quantities, thereby improving resistance to delayed fracture. To achieve this effect, Mo is preferably contained at 0.01% or more. More preferably, the Mo content is 0.03% or more, and even more preferably 0.05% or more. However, when Mo contains 0.3% or more, the chemical conversion treatment properties deteriorate. Therefore, in the case of Mo content, the Mo content is set to be less than 0.3%. The preferred Mo content is below 0.2%.

[0097] V: Below 0.5%

[0098] V can be added to improve the hardenability of steel, to promote the formation of fine V-containing carbides that serve as hydrogen trapping sites, and to improve resistance to delayed fracture due to martensite refinement. To achieve this effect, the V content is preferably set to 0.003% or more. More preferably, the V content is 0.03% or more, and even more preferably 0.05% or more. However, when the V content exceeds 0.5%, castability deteriorates significantly. Therefore, when V is present, the V content is set to 0.5% or less. More preferably, the V content is 0.3% or less, and even more preferably 0.2% or less. The V content is even more preferably 0.1% or less.

[0099] Zr: below 0.2%

[0100] Zr contributes to increased strength and improved resistance to delayed fracture by refining the original γ-grains and the resulting martensite internal structure. Furthermore, it enhances strength and improves resistance to delayed fracture by forming fine Zr-based carbides / carbonitrides that serve as hydrogen trapping sites. Additionally, Zr improves castability. From this perspective, the Zr content is preferably set to 0.005% or more. More preferably, it is 0.010% or more, and even more preferably 0.015% or more. However, when a large amount of Zr is added, the amount of coarse ZrN and ZrS precipitates remaining due to incomplete solution treatment during the hot rolling process increases, deteriorating the resistance to delayed fracture at the shear face. Therefore, when Zr is present, the Zr content is set to 0.2% or less. More preferably, it is 0.1% or less, and even more preferably 0.04% or less.

[0101] W: below 0.2%

[0102] W, by forming fine W-based carbides and carbonitrides that serve as hydrogen trapping sites, contributes to increased strength and improved resistance to delayed fracture. From this perspective, W content is preferably 0.005% or more. More preferably, it is 0.010% or more, and even more preferably 0.030% or more. However, when W content is high, the amount of coarse precipitates remaining due to lack of solid solution during the hot rolling process increases, deteriorating the resistance to delayed fracture at the shear end face. Therefore, when W is present, the W content is set to 0.2% or less. More preferably, it is 0.1% or less.

[0103] Ca: below 0.0030%

[0104] Ca improves the resistance to delayed fracture by fixing S in the form of CaS. To achieve this effect, it is preferable to contain 0.0002% or more of Ca. More preferably, the Ca content is 0.0005% or more, and even more preferably 0.0010% or more. However, adding a large amount of Ca deteriorates surface quality and flexibility; therefore, the Ca content is preferably 0.0030% or less. Based on the above, when Ca is present, the Ca content is set to 0.0030% or less. More preferably, the Ca content is 0.0025% or less, and even more preferably 0.0020% or less.

[0105] Ce: below 0.0030%

[0106] Ce also fixes S, thus improving resistance to delayed fracture. To achieve this effect, it is preferable to contain 0.0002% or more Ce. More preferably, the Ce content is 0.0003% or more, and even more preferably 0.0005% or more. However, adding a large amount of Ce deteriorates surface quality and flexibility; therefore, the Ce content is preferably 0.0030% or less. Based on the above, when Ce is present, the Ce content is set to 0.0030% or less. More preferably, the Ce content is 0.0020% or less, and even more preferably 0.0015% or less.

[0107] La: below 0.0030%

[0108] La also fixes S, thus improving resistance to delayed fracture. To achieve this effect, it is preferable to contain 0.0002% or more of La. More preferably, the La content is 0.0005% or more, and even more preferably 0.0010% or more. However, adding a large amount of La deteriorates surface quality and flexibility; therefore, the La content is preferably 0.0030% or less. Based on the above, when La is present, the La content is set to 0.0030% or less. More preferably, the La content is 0.0020% or less, and even more preferably 0.0015% or less.

[0109] REM: below 0.0030%

[0110] REM also fixes S, thus improving resistance to delayed fracture. To achieve this effect, it is preferable to contain 0.0002% or more REM. A more preferable REM content is 0.0003% or more, and even more preferable is 0.0005% or more. However, adding a large amount of REM deteriorates surface quality and flexibility; therefore, a REM content of 0.0030% or less is preferable. Based on the above, when REM is present, the REM content is set to 0.0030% or less. A more preferable REM content is 0.0020% or less, and even more preferable is 0.0015% or less.

[0111] It should be noted that, in this invention, REM refers to scandium (Sc) atom number 21, yttrium (Y) atom number 39, and lanthanum (La) atom number 57 to lutetium (Lu) atom number 71, excluding Ce and La. The REM concentration in this invention refers to the total content of one or more elements selected from the aforementioned REM elements.

[0112] Mg: less than 0.0030%

[0113] Mg improves resistance to delayed fracture by fixing O in the form of MgO. To achieve this effect, it is preferable to contain 0.0002% or more of Mg. More preferably, the Mg content is 0.0005% or more, and even more preferably 0.0010% or more. However, adding a large amount of Mg deteriorates surface quality and flexibility; therefore, the Mg content is preferably 0.0030% or less. Based on the above, when Mg is present, the Mg content is set to 0.0030% or less. More preferably, the Mg content is 0.0020% or less, and even more preferably 0.0015% or less.

[0114] Sb: below 0.1%

[0115] Sb inhibits surface oxidation and nitriding, thus suppressing the reduction of carbon (C) and boron (B) as a result. By suppressing the reduction of C and B, the formation of ferrite in the surface layer is suppressed, which contributes to increased strength and improved resistance to delayed fracture. From this point of view, the Sb content is preferably set to 0.002% or more. The Sb content is more preferably 0.004% or more, and even more preferably 0.006% or more. However, when the Sb content exceeds 0.1%, castability deteriorates, and Sb segregates at the original γ grain boundaries, thus deteriorating the resistance to delayed fracture at the shear end face. Therefore, the Sb content is preferably 0.1% or less. Based on the above, in the case of containing Sb, the Sb content is set to 0.1% or less. The Sb content is more preferably 0.05% or less, and even more preferably 0.02% or less.

[0116] Sn: less than 0.1%

[0117] Sn inhibits surface oxidation and nitriding, thereby suppressing the resulting decrease in the content of C and B in the surface layer. By suppressing the reduction of C and B, the formation of ferrite in the surface layer is suppressed, which contributes to increased strength and improved resistance to delayed fracture. From this point of view, the Sn content is preferably set to 0.002% or more. The Sn content is preferably 0.003% or more. However, when the Sn content exceeds 0.1%, castability deteriorates, and Sn segregates at the original γ grain boundaries, resulting in a deterioration in the resistance to delayed fracture at the shear end face. Therefore, in the case of Sn content, the Sn content is set to 0.1% or less. The Sn content is more preferably 0.05% or less, and even more preferably 0.01% or less.

[0118] It should be noted that when the optional element is present in a value lower than the preferred lower limit, the optional element is considered to be present as an unavoidable impurity.

[0119] steel structure

[0120] The steel plate of the present invention has the following steel structure.

[0121] (Composition 1) The area ratio of martensite relative to the total tissue is more than 95% and less than 100%.

[0122] (Composition 2) The average grain size of the original austenite grains is less than 11.2 μm.

[0123] (Construction 3) The number density A of precipitates with an equivalent circle diameter of 500 nm or more satisfies the following formula (1).

[0124] A (pieces / mm) 2 )≤8.5×10 5 ×[B]…Formula (1)

[0125] Here, [B] indicates the content of B (mass%).

[0126] The following is an explanation of each component.

[0127] (Composition 1) The area ratio of martensite relative to the total tissue is more than 95% and less than 100%.

[0128] To balance high strength (TS≥1470MPa) and excellent resistance to delayed fracture, the martensite area fraction in the steel microstructure is set to 95% or more. More preferably, it is 99% or more, and even more preferably 100%. It should be noted that, in the case of microstructures other than martensite, the balance may include bainite, ferrite, and retained austenite (retained γ). The remaining microstructure consists of trace amounts of carbides, sulfides, nitrides, and oxides. The balance microstructure is 5% or less, preferably 1% or less. Furthermore, the martensite also includes martensite that has not undergone tempering, including self-tempering during continuous cooling, but has been tempered by holding at approximately 150°C or above for a certain period of time. It should be noted that the martensite area fraction may also be 100% without any balance.

[0129] (Composition 2) The average grain size of the original austenite grains is less than 11.2 μm.

[0130] For steels with a martensite area ratio of 95% or more in their microstructure, delayed fracture surfaces are mostly grain boundary fractures. The initiation point of delayed fracture and the crack propagation path in the early stages of delayed fracture are considered to be at the proto-austenite grain boundaries. To suppress intergranular fracture, refining the proto-austenite grains is effective, significantly improving resistance to delayed fracture. The mechanism is believed to be that due to the refinement of the proto-austenite grains, the area ratio of the proto-austenite grain boundaries increases, and the concentration of impurity elements such as phosphorus (P), which are grain boundary embrittlement elements, decreases at the proto-austenite grain boundaries. Furthermore, the refinement of the proto-austenite grains also contributes to improved tensile strength. From the viewpoint of delayed fracture resistance and strength, the average grain size (proto-γ grain size) of the proto-austenite grains is less than 11.2 μm. This average grain size is preferably 10 μm or less, more preferably 7.0 μm or less, and even more preferably 5.0 μm or less.

[0131] (Construction 3) The number density A of precipitates with an equivalent circle diameter of 500 nm or more satisfies the following formula.

[0132] A (pieces / mm) 2 )≤8.5×10 5 ×[B]…Formula (1)

[0133] Here, [B] indicates the content of B (mass%).

[0134] In high-strength steels with a strength TS ≥ 1470 MPa, in order to suppress intergranular fracture, besides refining the original austenite grains, strengthening the grain boundaries by inducing boron segregation at the grain boundaries is also effective. However, simply increasing the amount of boron added not only increases the amount of boron segregated at the grain boundaries, but also makes Fe, which is the initiator of delayed fracture, more susceptible to segregation. 23 The increase in B-series precipitates, primarily (C,B)6, actually reduces the resistance to delayed fracture. The inventors discovered that by controlling the hot rolling conditions to reduce the number density A of precipitates with an equivalent circle diameter of 500 nm or more, and satisfying the following conditions, it is possible to balance the improvement in resistance to delayed fracture resulting from grain boundary strengthening of B with the suppression of fracture at the precipitate initiation point.

[0135] A (pieces / mm) 2 )≤8.5×10 5 ×[B]

[0136] Preferred A (pieces / mm) 2 )≤5.0×10 5 ×[B], more preferably A (pieces / mm) 2 )≤2.0×10 5 ×[B].

[0137] The methods for determining each component in the above steel structure are explained.

[0138] The area ratios of martensite, bainite, and ferrite were determined as follows: The L-section of the steel plate (a section parallel to the rolling direction and perpendicular to the steel plate surface, hereinafter also referred to as the perpendicular section parallel to the rolling direction) was ground and etched with a nitric acid-ethanol solution. At a position 1 / 4 thickness from the steel plate surface, four fields of view were observed using a SEM at 2000x magnification. Image analysis was performed on the photographs to determine the area ratios of martensite, bainite, and ferrite. Here, martensite and bainite refer to the gray or white structure in the SEM. Ferrite, on the other hand, is the region appearing as black contrast in the SEM. It should be noted that martensite and bainite contain trace amounts of carbides, nitrides, sulfides, and oxides, but these are difficult to exclude; therefore, the area ratio of the region containing these substances is used as the area ratio.

[0139] Here, bainite has the following characteristics: it has an aspect ratio of 2.5 or greater and a plate-like morphology; it is a slightly darker structure compared to martensite. The width of these plates is 0.3–1.7 μm. The distribution density of carbides with a diameter of 10–200 nm within the bainite is 0–3 per μm. 2 .

[0140] The determination of retained austenite (retained γ) is as follows: The surface 200 μm of the steel plate was chemically ground with oxalic acid. Using the plate surface as the object, the retained austenite (retained γ) was determined by X-ray diffraction intensity method. It was calculated based on the integrated intensity of the diffraction peaks of (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ measured by Mo-Kα rays.

[0141] The average grain size of the original austenite grains (original γ grain size) is determined as follows: After grinding the L-section (a perpendicular section parallel to the rolling direction) of the steel plate, it is etched using a reagent that corrodes the original γ grain boundaries (e.g., a saturated picric acid aqueous solution or a solution obtained by adding ferric chloride). At a position 1 / 4 thickness from the surface of the steel plate, four fields of view are observed at 500x magnification using an optical microscope. In the obtained photographs, 15 lines are drawn along the plate thickness direction and the rolling direction at intervals of at least 10 μm (based on the actual length), and the number of intersections between the grain boundaries and the lines is counted. Furthermore, by multiplying the value obtained by dividing the line length by the number of intersections by 1.13, the original γ grain size (average grain size of the original austenite grains) can be determined.

[0142] The number density A of precipitates with an equivalent circular diameter of 500 nm or more was calculated as follows: After grinding the L-section (the perpendicular section parallel to the rolling direction) of the steel plate, continuously photographed 2 mm images using SEM in the region from 1 / 5 to 4 / 5 of the plate thickness, i.e., from the 1 / 5 position relative to the plate thickness to the 4 / 5 position. 2 In this region, the number of such precipitates was determined from the SEM images, and the number density A of precipitates with an equivalent circle diameter of 500 nm or more was calculated. The imaging magnification was 2000x. Furthermore, during the compositional analysis of each inclusion particle, the particles were magnified to 10000x to analyze the aforementioned precipitates. Here, the precipitate with an equivalent circle diameter of 500 nm or more is Fe. 23 Precipitates containing B, such as (C,B)6, were examined for the presence of B peaks using energy-dispersive X-ray spectroscopy (EDS) with an accelerating voltage of 3 kV. The presence of B peaks indicated the presence of the aforementioned precipitates.

[0143] It should be noted that when the steel billet is not reheated sufficiently, the amount of Nb and Ti precipitates increases, which can also have an adverse effect on the delayed fracture characteristics.

[0144] It should be noted that the equivalent circle diameter refers to the diameter of a circle containing the areas of each precipitate calculated from the SEM image.

[0145] Tensile strength (TS): ≥1470MPa

[0146] The deterioration of resistance to delayed fracture becomes significantly more pronounced when the tensile strength of the steel sheet is 1470 MPa or higher. One of the characteristics of this invention is that the resistance to delayed fracture remains good even at a tensile strength of 1470 MPa or higher. Therefore, in this invention, the tensile strength needs to be 1470 MPa or higher. From the viewpoint of lightweighting automotive frame components, 1700 MPa or higher is preferred. The tensile strength of the steel sheet of this invention can be 2100 MPa or lower.

[0147] Tensile strength can be determined as follows: cut a JIS 5 tensile test piece at 1 / 4 of the width of the roll material with the rolling right angle as the length direction, and determine the tensile strength according to the tensile test in JIS Z2241.

[0148] The steel plate of the present invention described above can also be a steel plate with a coating on its surface. The coating can be a Zn coating or a coating of other metals. Alternatively, it can be any of the following: a hot-dip galvanized coating or an electroplated coating.

[0149] Next, the method for manufacturing the steel plate of the present invention will be described.

[0150] The method for manufacturing the steel plate of the present invention is as follows: a steel billet having the above-described composition is heated from 1000°C to a heating holding temperature of 1250°C or higher at an average heating rate of 10°C / min or less using a steel billet surface thermometer, and held at this heating holding temperature for 30 minutes or more; then, the dwell time at 900-1000°C is set to 20 seconds or more and 150 seconds or less; hot finishing rolling is performed under conditions where the finishing rolling temperature is set to 850°C or higher; cooling is performed at an average cooling rate of 40°C / second or higher in the range from the finishing rolling temperature to 650°C; and then, the steel plate is coiled at a coiling temperature of 650°C or lower. The hot-rolled steel sheet is produced by coiling and then cold-rolled at a reduction rate of 40% or more to produce a cold-rolled steel sheet. The cold-rolled steel sheet is then subjected to continuous annealing as follows: the annealing temperature is set to 800-950°C, the cold-rolled steel sheet is heated from 400°C to the annealing temperature at an average heating rate of 1.0°C / second or more, held at the annealing temperature for 600 seconds or less, cooled from the annealing temperature to 420°C at a first average cooling rate of 2°C / second or more, cooled from 420°C to a cooling stop temperature of 260°C or less at a second average cooling rate of 10°C / second or more, and then held at a holding temperature of 150-260°C for 20-1500 seconds.

[0151] Hot rolling

[0152] In the heating of steel billets before hot rolling, by setting the average heating rate at a holding temperature of 1000°C to 1250°C or higher to 10°C / min or less, solid solution of sulfides can be promoted, and the size and number of inclusions can be reduced. Since Nb and Ti have high melting temperatures, by setting the holding temperature at 1250°C or higher and the holding time at 30 minutes or higher using a steel billet surface thermometer, solid solution of Nb and Ti can be promoted, and the size and number of precipitates can be reduced. The aforementioned holding temperature is preferably set to 1300°C or higher. More preferably, it is 1350°C or higher.

[0153] Here, the average heating rate is defined as "(temperature at the end of billet heating (heat holding temperature) (°C) - temperature at the beginning of billet heating (°C) (1000°C)) / heating time from the start of heating to the end of heating (minutes)".

[0154] Then, the billet is held at 900–1000°C for 20 seconds to 150 seconds. Increasing the holding time within the 900–1000°C temperature range causes the formation / coarsening of precipitates primarily composed of boron (BN). Precipitates formed within these temperature ranges are difficult to dissolve by annealing, resulting in a decrease in the amount of dissolved boron after annealing. Therefore, when the holding time exceeds 150 seconds, an amount of dissolved boron effective in suppressing delayed fracture cannot be obtained. Therefore, the holding time is 150 seconds or less, preferably 120 seconds or less, and more preferably 100 seconds or less. On the other hand, when the holding time is less than 20 seconds, the microstructure may become uneven. Therefore, the holding time is 20 seconds or more, preferably 30 seconds or more, and more preferably 40 seconds or more.

[0155] In hot finishing rolling, the finishing temperature (FT) is set to 850°C or higher to suppress the precipitation of Nb, Ti, B, etc. Preferably, the finishing temperature is 930°C or lower.

[0156] Furthermore, during the cooling process after hot finishing rolling, the average cooling rate is set to 40°C / second or higher within the range from the finishing rolling temperature to 650°C. When the average cooling rate is less than 40°C / second, the coarsening of Nb and Ti carbonitrides leads to an increase in carbonitrides with an equivalent circle diameter of 1.0 μm or higher, resulting in unsatisfactory resistance to delayed fracture. Preferably, the average cooling rate is 250°C / second or lower, more preferably 200°C / second or lower.

[0157] It should be noted that the average cooling rate in the hot rolling process is defined as "(temperature at the start of cooling (finishing temperature) (°C) - temperature at the end of cooling (°C) (650°C)) / cooling time from the start of cooling to the end of cooling (seconds)".

[0158] After cooling to 650°C, the material is cooled further and wound up as needed. At this point, if the winding temperature exceeds 650°C, only the Nb and Ti-based precipitates that have formed in the fine austenite region are coarsened. Therefore, the number of coarse precipitates increases, and the delayed fracture characteristics decrease. Therefore, the winding temperature is set below 650°C. Preferably, the winding temperature is above 500°C.

[0159] Cold rolling

[0160] In cold rolling, setting the reduction rate (cold rolling ratio) to 40% or higher can stabilize the recrystallization behavior and texture orientation during subsequent continuous annealing. When it is less than 40%, some of the austenite grains become coarse during annealing, potentially reducing strength. Furthermore, a cold rolling ratio of 80% or lower is preferred.

[0161] Continuous annealing

[0162] For cold-rolled steel sheets, annealing is carried out through a continuous annealing production line (CAL), and tempering and leveling rolling are performed as needed.

[0163] Fe 23 (C,B)6 forms and coarsens in the ferrite region during annealing heating; therefore, in order to make Fe 23 To reduce (C,B)6 and fully obtain the grain boundary strengthening effect brought about by B, it is extremely important to increase the average heating rate at 400°C and above. Furthermore, from the viewpoint of refining the original γ grain size to less than 11.2 μm, it is also necessary to increase the heating rate. From the above viewpoints, the average heating rate at 400°C and above is 1.0°C / second or more. Moreover, it is preferable that the average heating rate at 400°C and above is 1.5°C / second or more, and more preferably 3.0°C / second or more.

[0164] In addition, it is preferable that the average heating rate is 10°C / second or less.

[0165] It should be noted that the average heating rate here refers to "(annealing temperature (°C) - 400 (°C)) / heating time (minutes) from 400°C to the annealing temperature".

[0166] To minimize the residual Fe due to incomplete solution after annealing 23 (C,B)6 and other precipitates require annealing at high temperatures for an extended period. Specifically, the annealing temperature needs to be set above 800°C.

[0167] On the other hand, annealing at temperatures exceeding 950°C results in coarse γ-particles, failing to achieve the desired microstructure; therefore, the annealing temperature is set to 950°C or below. Furthermore, annealing at temperatures exceeding 900°C causes BN to precipitate at grain boundaries, sometimes deteriorating the delayed fracture resistance; therefore, 900°C or below is more preferable. Prolonged soaking time (holding time) at the annealing temperature also leads to excessively coarse γ-particles; therefore, a soaking time of 600 seconds or less is set. Preferably, this soaking time is 10 seconds or more.

[0168] Then, in order to reduce ferrite and retained austenite and achieve a martensite area ratio of 95% or more, it is necessary to cool from the aforementioned annealing temperature to 420°C at a first average cooling rate of 2°C / second or higher. When the first average cooling rate is less than 2°C / second, a large amount of ferrite is formed, and carbon is enriched in γ, causing the martensite to harden, thereby deteriorating the resistance to delayed fracture. There is no particular upper limit to the first average cooling rate, but 100°C / second is preferred.

[0169] When hot-dip galvanizing steel sheets, during the cooling process from the annealing temperature to 420°C, more specifically, during the cooling at the first average cooling rate described above in continuous annealing, it is preferable to immerse the cold-rolled steel sheet in a galvanizing bath for galvanizing treatment. Alloying treatment can be performed by heating to 480°C to 600°C after immersion in the galvanizing bath, as needed.

[0170] Next, in this invention, in order to suppress the formation of bainitic ferrite and lower bainite and to achieve a martensite area ratio of 95% or more, it is necessary to cool from 420°C to a cooling stop temperature of 260°C or below at a second average cooling rate of 10°C / second or more. In a microstructure with a large amount of bainite formation, the strength decreases and the retained austenite increases, thus deteriorating the resistance to delayed fracture. Therefore, the second average cooling rate from 420°C to the cooling stop temperature of 260°C or below is set to 10°C / second or more. The second average cooling rate is preferably 20°C / second or more, more preferably 70°C / second or more. There is no particular upper limit to the second average cooling rate, but 2000°C / second is preferred.

[0171] Here, the first average cooling rate is defined as "(annealing temperature (°C) - 420°C)) / (cooling time (seconds) from annealing temperature to 420°C)".

[0172] In addition, the second average cooling rate is defined as "(420 (°C) - cooling stop temperature (°C)) / (cooling time from 420°C to cooling stop temperature (seconds))".

[0173] When the cooling stop temperature exceeds 260°C, there are issues with the formation of upper / lower bainite, retained austenite, and an increase in fresh martensite. Therefore, the cooling stop temperature is set below 260°C.

[0174] The carbides distributed within the martensite are formed during the low-temperature holding period after quenching. To ensure excellent resistance to delayed fracture and tensile strength above 1470 MPa (TS≥1470 MPa), the formation of these carbides needs to be appropriately controlled.

[0175] Therefore, the holding time needs to be controlled to be 20 to 1500 seconds at a holding temperature of 150 to 260°C.

[0176] If the holding temperature is below the lower limit of 150°C or the holding time is short, the carbide distribution density within the phase transformation phase becomes insufficient, and the resistance to delayed fracture deteriorates. On the other hand, at a high temperature above the upper limit of 260°C, the coarsening of carbides within the grains and at bulk grain boundaries becomes significant, and the resistance to delayed fracture may deteriorate. Furthermore, when the holding time exceeds 1500 seconds, the coarsening of carbides within the grains and at bulk grain boundaries becomes significant, and the resistance to delayed fracture may deteriorate. Therefore, in this invention, the holding temperature is maintained at 150–260°C for 20–1500 seconds during continuous annealing.

[0177] From the viewpoint of stabilizing the compressive formability, such as adjusting surface roughness and flattening the sheet shape, the steel sheet obtained in this way can be surface-rolled. In this case, the surface-rolling elongation is preferably set to 0.1% or more. Furthermore, the surface-rolling elongation is preferably set to 0.6% or less. In this case, the surface-rolling roll is a rough-surface roll, and from the viewpoint of shape flattening, the surface roughness Ra of the steel sheet is preferably adjusted to 0.8 μm or more. Furthermore, the surface roughness Ra of the steel sheet is preferably adjusted to 1.8 μm or less.

[0178] As described above, coated steel sheets can be manufactured by performing hot-dip galvanizing during cooling after homogenization in annealing, or by performing electroplating after continuous annealing. Examples of coating types include Zn-based coatings (Zn-based, Zn-Ni-based, Zn-Fe-based, etc.) and Al coatings. In the case of hot-dip galvanizing, the steel sheet can be immersed in a plating bath while cooling from the annealing temperature to 420°C at a first average cooling rate of 2°C / second or higher. After immersion in the plating bath, it is heated to 480–600°C for alloying treatment. After this alloying treatment, it can be cooled at the aforementioned second average cooling rate and held at a holding temperature of 150–260°C for 20–1500 seconds.

[0179] After hot-dip galvanizing or electroplating, heat treatment can be carried out at a temperature range below 260°C to reduce the intrusion of hydrogen into the steel.

[0180] According to the present invention, the delayed fracture resistance of high-strength cold-rolled steel sheets is significantly improved, which contributes to the increased strength and weight reduction of components resulting from the application of high-strength steel sheets. Preferably, the thickness of the steel sheet of the present invention is 0.5 mm or more. Furthermore, the thickness is preferably 2.0 mm or less.

[0181] Next, the components of the present invention and their manufacturing method will be described.

[0182] The component of the present invention is formed by performing at least one of forming and joining processes on the steel plate of the present invention. Furthermore, the manufacturing method of the component of the present invention includes a step of forming and joining processes on the steel plate of the present invention to form the component.

[0183] The steel sheet of the present invention has a tensile strength of 1470 MPa or higher and exhibits excellent resistance to delayed fracture. Therefore, components obtained using the steel sheet of the present invention are also high-strength, exhibiting superior resistance to delayed fracture compared to conventional high-strength components. Furthermore, using components of the present invention enables weight reduction. Therefore, components of the present invention can be suitable for applications such as vehicle body frame components.

[0184] Forming processes can utilize general processing methods such as pressure processing without limitation. Furthermore, joining processes can utilize general welding methods such as spot welding and arc welding, as well as riveting and rivet joining without limitation.

[0185] Example

[0186] The embodiments of the present invention will be described below.

[0187] The steel with the composition shown in Table 1 was melted and then cast into steel billets.

[0188] The steel billet was subjected to the heat treatment and rolling shown in Table 2 to obtain a steel plate with a thickness of 1.4 mm.

[0189] Specifically, a steel billet with each component composition is heated at an average heating rate of 6°C / min to the heating holding temperature shown in Table 2 (using a surface thermometer for the billet), and held for the heating holding time shown in Table 2. Then, the billet is held at 900–1000°C for the time shown in Table 2, and hot finishing rolling is performed at a finishing rolling temperature of 870°C. Cooling is then performed at an average cooling rate of 50°C / second from the finishing rolling temperature to 650°C.

[0190] Then, the steel is cooled and wound at a coiling temperature of 550°C to produce a hot-rolled steel sheet. The hot-rolled steel sheet is then cold-rolled at a reduction rate of 50% (cold rolling reduction rate) to produce a cold-rolled steel sheet.

[0191] Then, the cold-rolled steel sheet is heated from 400°C to the annealing temperature shown in Table 2 at the average heating rate shown in Table 2, and then homogenized at the annealing temperature for the homogenization time shown in Table 2.

[0192] Then, the annealing temperature (first cooling start temperature) is cooled to 420°C (second cooling start temperature) at the first average cooling rate shown in Table 2, and then cooled to the cooling stop temperature shown in Table 2 at the second average cooling rate shown in Table 2. The annealing is then performed as needed, and then the annealing is performed continuously at the holding temperature shown in Table 2 for the holding time shown in Table 2.

[0193] Furthermore, regarding No. 12, during continuous annealing, the steel sheet is immersed in a hot-dip galvanizing bath at 480°C while cooling to 420°C at a first average cooling rate. Then, it is heated to 540°C and held for 15 seconds to perform alloying treatment, producing an alloyed hot-dip galvanized steel sheet. Next, it is cooled at a second average cooling rate as shown in Table 2, and then held at the holding temperature and holding time shown in Table 2.

[0194] In addition, regarding No.3, after continuous annealing, the obtained steel sheet is electroplated to obtain a steel sheet with a Zn coating.

[0195]

[0196]

[0197] The obtained steel plate is then quantitatively analyzed using the above method, followed by tensile testing and evaluation of its resistance to delayed fracture.

[0198] Specifically, the tissue assay is performed as follows.

[0199] The area ratios of martensite, bainite, and ferrite were determined as follows: The L-section (perpendicular to the rolling direction) of the steel plate was ground and etched with nitric acid-ethanol solution. At a distance of 1 / 4 thickness from the steel plate surface, four fields of view were observed using a SEM at 2000x magnification. Image analysis was performed on the photographs to determine the area ratios of martensite, bainite, and ferrite. Here, martensite and bainite refer to structures that appear gray or white in the SEM. Bainite is characterized by the following: an aspect ratio of 2.5 or greater, a plate-like morphology, and a slightly darker appearance compared to martensite. The width of these plates is 0.3–1.7 μm. The distribution density of carbides with diameters of 10–200 nm within the bainite is 0–3 particles / μm. 2 On the other hand, ferrite appears as a black contrasting region in SEM. It should be noted that martensite and bainite contain trace amounts of carbides, nitrides, sulfides, and oxides, but these are difficult to exclude; therefore, the area ratio of the regions containing them is used as their area ratio.

[0200] The determination of retained austenite (retained γ) is as follows: The surface 200 μm of the steel plate was chemically ground with oxalic acid. Using the plate surface as the object, the retained austenite (retained γ) was determined by X-ray diffraction intensity method. It was calculated based on the integrated intensity of the diffraction peaks of (200)α, (211)α, (220)α, (200)γ, (220)γ, and (311)γ measured by Mo-Kα rays.

[0201] The average grain size of the original austenite grains (original γ grain size) was determined as follows: After grinding the L-section (a perpendicular section parallel to the rolling direction) of the steel plate, it was etched using a reagent that corrodes the original γ grain boundaries (e.g., a saturated picric acid aqueous solution or a solution obtained by adding ferric chloride). At a position 1 / 4 thickness from the steel plate surface, four fields of view were observed using an optical microscope at 500x magnification. In the resulting photographs, 15 lines were drawn along the plate thickness direction and the rolling direction at intervals of at least 10 μm (based on actual length). The number of intersections between the grain boundaries and the lines was counted. The original γ grain size was calculated by multiplying the value obtained by dividing the line length by the number of intersections by 1.13.

[0202] The number density A of precipitates with an equivalent circular diameter of 500 nm or more was calculated as follows: After grinding the L-section (the perpendicular section parallel to the rolling direction) of the steel plate, continuously photographed 2 mm images using SEM in the region from 1 / 5 to 4 / 5 of the plate thickness, i.e., from the 1 / 5 position relative to the plate thickness to the 4 / 5 position. 2 In this region, the number of such precipitates was determined from the SEM images, and the number density A of precipitates with an equivalent circle diameter of 500 nm or more was calculated. The imaging magnification was 2000x. Furthermore, during the compositional analysis of each inclusion particle, the particles were magnified to 10000x to analyze the aforementioned precipitates. Here, the precipitate with an equivalent circle diameter of 500 nm or more is Fe. 23 Precipitates containing B, such as (C,B)6, were examined for the presence of B peaks using energy-dispersive X-ray spectroscopy (EDS) with an accelerating voltage of 3 kV. The presence of B peaks indicated the presence of the aforementioned precipitates.

[0203] The tensile test is as follows: At 1 / 4 of the width of the roll, a JIS 5 tensile test piece is cut with the rolling right angle as the length direction, and a tensile test is carried out (according to JIS Z2241) to evaluate YP, TS, and El.

[0204] The evaluation of the resistance to delayed fracture is carried out as follows.

[0205] A strip test piece with a rolling right angle of 100 mm and a rolling length of 30 mm was cut from 1 / 4 of the width of the obtained steel sheet (coil) in the width direction. The cutting of the 100 mm long side end was set as a shearing process. While shearing (without deburring), bending was performed with the burr forming the outer periphery of the bend. The test piece shape was maintained during bending and secured with bolts. The shearing clearance was set to 13%, and the front angle was set to 1°. During bending, the front bending radius was 10 mm, and the angle inside the bending apex was set to 90 degrees (V-bend). A punch with a front radius the same as the aforementioned front bending radius R and a U-shape (the front R portion is semi-circular and the thickness of the punch body is 2R) was used. A die with a corner radius of 30 mm was used. Adjust the depth of the punch pressing the steel plate, forming it with a 90-degree (V-shaped) bending angle at the front end (the angle inside the bending apex). Secure the test piece using a hydraulic jack, ensuring the distance between the flange ends of the straight section during bending is the same as during bending (to eliminate openings in the straight section caused by springback). Tighten the bolts in this state. The bolts are fixed through elliptical holes (10mm minor axis, 15mm major axis) pre-set 10mm inside the short edge of the strip test piece. Immerse the bolted test piece in a solution of 0.1% ammonium thiocyanate aqueous solution and McIlvaine buffer at a 1:1 ratio, adjusting the pH to 8.0, to perform a delayed fracture resistance evaluation test. The solution temperature is set at 20°C, and the test piece is 1cm thick. 2 The liquid volume for the surface area was set to 20 ml. After 24 hours, it was checked whether there were any cracks that could be visually confirmed (length greater than 1 mm). If no cracks were observed, it was judged to have excellent delayed fracture resistance.

[0206] Table 3 shows the microstructure and properties of the obtained steel plates.

[0207] [Table 3]

[0208]

[0209] (*1) Residual area ratio: The total area ratio of bainite, ferrite and retained austenite

[0210] (*2) Original γ grain size: The average grain size of the original austenite grains

[0211] (*3)A: Number density of precipitates with an equivalent circle diameter of 500 nm or more.

[0212] (*4)[B]: B content (mass%)

[0213] The steel plates within the scope of this invention are high-strength and have excellent resistance to delayed fracture.

[0214] On the other hand, the C content of No.13 (steel M) is less than the lower limit of the value specified in this invention, and the TS is insufficient.

[0215] The C content of No.14 (steel N) exceeds the upper limit specified in this invention, and therefore does not achieve sufficient resistance to delayed fracture.

[0216] The Mn content of No.15 (steel O) is less than the lower limit of the value specified in this invention, the formation of martensite is insufficient, and the delayed fracture resistance is not fully obtained.

[0217] No. 16 (steel P) has a P content exceeding the upper limit specified in this invention, and therefore does not achieve sufficient resistance to delayed fracture.

[0218] No. 17 (steel Q) has an S content exceeding the upper limit specified in this invention, and therefore does not have sufficient resistance to delayed fracture.

[0219] The SOl.Al content of No.18 (steel R) exceeds the upper limit specified in this invention, and therefore does not achieve sufficient resistance to delayed fracture.

[0220] No. 19 (steel S) has an N content exceeding the upper limit specified in this invention, and therefore does not have sufficient resistance to delayed fracture.

[0221] The Nb and Ti contents of No. 20 (steel T) are less than the lower limit of the values ​​specified in this invention, and the original γ grain size is large, so it does not have sufficient resistance to delayed fracture.

[0222] The Nb and Ti content of No. 21 (steel U) exceeds the upper limit specified in this invention, and therefore does not achieve sufficient resistance to delayed fracture.

[0223] The B content of No. 22 (steel V) exceeds the upper limit specified in this invention, resulting in a large amount of coarse precipitates and insufficient resistance to delayed fracture.

[0224] The B content of No. 23 (steel W) is less than the lower limit of the value specified in this invention, and therefore it does not have sufficient resistance to delayed fracture.

[0225] The heating temperature (slab surface temperature (SRT)) of No. 24 (steel A) is lower than the lower limit of the value specified in this invention. The original γ grain size is large, and the delayed fracture resistance characteristics are not fully obtained.

[0226] The billet heating holding time of No. 25 (steel A) is less than the lower limit of the value specified in this invention, the original γ grain size is large, and the delayed fracture resistance characteristics are not fully obtained.

[0227] The residence time of No. 26 (steel A) at 900-1000°C exceeds the upper limit of the value specified in this invention, resulting in an excessive number density A of precipitates and insufficient resistance to delayed fracture.

[0228] The average heating rate during annealing of No. 27 (steel A) is less than the lower limit of the value specified in this invention, the original γ grain size is large, and the delayed fracture resistance characteristics are not fully obtained.

[0229] The annealing time of No. 28 (steel A) exceeded the upper limit of the value specified in this invention, and the original γ grain size was large, so it did not obtain sufficient resistance to delayed fracture characteristics.

[0230] The first average cooling rate during annealing of No. 29 (steel A) was less than the lower limit of the value specified in this invention, resulting in insufficient martensite formation and inadequate resistance to delayed fracture.

[0231] The second average cooling rate during annealing of No. 30 (steel A) was less than the lower limit of the value specified in this invention, resulting in insufficient martensite formation and inadequate resistance to delayed fracture.

[0232] The cooling stop temperature during annealing of No. 31 (steel A) exceeds the upper limit specified in this invention, resulting in insufficient martensite formation and inadequate resistance to delayed fracture.

[0233] It is also known that: since the steel plate of the present invention has high strength and excellent resistance to delayed fracture, the components obtained by forming and joining the steel plate of the present invention also have high strength and excellent resistance to delayed fracture, just like the steel plate of the present invention.

Claims

1. A steel plate comprising, by mass%, C: 0.15% or more and 0.45% or less, Si: 1.5% or less, Mn: greater than 1.7%, P: 0.03% or less, S: less than 0.0040%, sol.Al: 0.20% or less, N: 0.005% or less, B: 0.0015% or more and 0.0100% or less, one or more of Nb and Ti: totaling 0.005% or more and 0.080% or less, with the balance being Fe and unavoidable impurities. Furthermore, the tissue has a martensite area ratio of over 95% and under 100% relative to the total tissue area. The average grain size of the original austenite grains is less than 11.2 μm. The number density A of precipitates with an equivalent circle diameter of 500 nm or more, measured by SEM images taken at measurement locations of 1 / 5 to 4 / 5 of the plate thickness, satisfies the following equation (1). A ≤ 8.5 x 10 5 x [B]... Equation (1) Here, the unit of A is pieces / mm 2 , [B] represents the content of B (mass %).

2. The steel sheet according to claim 1, wherein, As a component, the ingredient further comprises, by mass%, one or more of groups A, B, C, and D selected below. Group A: Selected from one or both of Cu: less than 1.0% and Ni: less than 1.0%; Group B: Selected from one or more of the following: Cr: less than 1.0%, Mo: less than 0.3%, V: less than 0.5%, Zr: less than 0.2%, and W: less than 0.2%; Group C: Selected from one or more of the following: Ca: less than 0.0030%, Ce: less than 0.0030%, La: less than 0.0030%, REM excluding Ce and La: less than 0.0030%, and Mg: less than 0.0030%. Group D: Selected from one or both of Sb: less than 0.1% and Sn: less than 0.1%.

3. The steel sheet according to claim 1 or 2, wherein, The steel plate has a coating.

4. A method for manufacturing a steel plate, wherein, A steel billet having the composition described in claim 1 or 2 is heated from 1000°C to a holding temperature of 1250°C or higher using a steel billet surface thermometer at an average heating rate of less than 10°C / min, and held at said holding temperature for more than 30 minutes. The dwell time at 900–1000°C is set to be more than 20 seconds and less than 150 seconds, and hot finishing rolling is performed under the condition that the finishing rolling temperature is set to above 850°C. The cooling process is performed with an average cooling rate of 40°C / second or higher within the range from the finishing rolling temperature to 650°C. Then, the steel is wound at a winding temperature below 650°C to produce hot-rolled steel sheet. The hot-rolled steel sheet is cold-rolled with a reduction rate of over 40% to produce a cold-rolled steel sheet. Perform the following continuous annealing: The annealing temperature is set to 800–950°C, and the cold-rolled steel sheet is heated from 400°C to the annealing temperature at an average heating rate of 1.0°C / second or higher. Hold at the annealing temperature for less than 600 seconds. Cool from the annealing temperature to 420°C at a first average cooling rate of 2°C / second or higher. Cooling from 420°C to a cooling stop temperature below 260°C at a second average cooling rate of 10°C / second or higher. Then, maintain at a holding temperature of 150–260°C for 20–1500 seconds.

5. The method for manufacturing a steel plate according to claim 4, wherein, The cold-rolled steel sheet is immersed in a plating bath while being cooled at the first average cooling rate during the continuous annealing. After immersion in a plating bath, the metal is heated to 480–600°C for alloying treatment.

6. The method of producing a steel sheet according to claim 4, wherein The steel plate surface is plated after the continuous annealing.

7. A method for manufacturing a component, comprising the steps of performing at least one of forming or joining processes on the steel plate as described in claim 1 or 2 to produce the component.

8. A method for manufacturing a component, comprising the steps of performing at least one of forming and joining processes on the steel plate of claim 3 to produce the component.

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