Steel plate for hot stamping

Abstract

A steel sheet for hot stamping contains, in mass%, 0.25% to 0.4% (inclusive) of C, 1.05% to 1.4% (inclusive) of Si, 0% to 1.4% (inclusive) of Mn, 0.6% to 3.0% (inclusive) of Cr, 0% to 0.03% (inclusive) of P, 0% to 0.02% (inclusive) of S, 0.01% to 1% (inclusive) of Al, 0% to 0.01% (inclusive) of N, 0.0005% to 0.005% (inclusive) of B, and 0.005% to 0.1% (inclusive) of Ti, with the remainder being iron and unavoidable impurities. The steel sheet for hot stamping satisfies the relational expression [C] + 2 / 9 [Si] + 7 / 9 [Mn] + 8 / 9 [Cr]-7 / 4 > 0, thereby having excellent balance between strength and toughness and also having excellent hardness stability.

Description

[0001] This application is a divisional application of application No. 201980021455.2, application date: March 19, 2019, invention title: "Steel sheet for hot stamping" (PCT / JP2019 / 011606). Technical Field

[0002] This invention relates to steel plates for hot stamping. Background Technology

[0003] In recent years, there has been a demand for improved collision safety in automobiles, which in turn has led to a need for higher strength hot-stamped steel sheets used in rigid components of vehicles. However, increasing the strength of the steel sheet can degrade its low-temperature toughness, resulting in an imbalance between strength and toughness. To address this, Non-Patent Literature 1 proposes refining the grain size of the old austenite after hot stamping to improve the balance between strength and toughness in steel sheets.

[0004] Furthermore, in hot stamping, the cooling rate inside the steel sheet can sometimes decrease due to the rise in die temperature and / or the gap between the die and the steel sheet. When the cooling rate of the steel sheet falls below the critical cooling rate, soft phases such as ferrite and bainite precipitate, resulting in a decrease in the hardness of the steel sheet. In particular, the decrease in cooling rate below the Ms point promotes auto-tempering, which is a major reason for the decrease in the hardness of the steel sheet.

[0005] Non-patent literature 2 investigated the change in cooling rate when the gap between the mold and the steel plate was changed, showing that the cooling rate decreased to about 15°C / s when the gap was 0.4 mm.

[0006] As described in Non-Patent Document 1, a common microstructure design technique for hot-stamping steel sheets involves refining the steel grains, thereby obtaining steel sheets with an excellent balance of strength and toughness. One method for refining grains is by adding elements such as Nb, Ni, and Ti; however, this method reduces the economic efficiency of the steel sheet. Furthermore, for steel sheets with refined grains, hardness stability becomes insufficient due to deteriorated hardenability.

[0007] To address this issue, improvements could be made to key factors leading to reduced hardness, such as increased mold temperature and / or the gap between the mold and the steel sheet. However, this would require repeated mold modifications or the preparation of specialized molds, necessitating significant labor and cost. Therefore, with existing hot-stamping steel sheets, it remains difficult to obtain parts (formed products) that achieve an excellent balance between strength and toughness, along with superior hardness stability, without increasing labor and cost.

[0008] Existing technical documents

[0009] Non-patent literature

[0010] Non-Patent Literature 1: Kazuo Hitta et al., “Development of TS1800MPa Grade Hot Stamping Steel Sheet”, Materials (Materia Japan), Vol. 52, No. 2, 2013, pp. 68-70

[0011] Non-Patent Literature 2: Katsuji Nakajima, "Strengthening Technology of Steel Based on Compression Hardening and Its Application in Body Parts," Materials and Processes (CAMP-ISIJ), Vol. 17, 2004, pp. 980-983. Summary of the Invention

[0012] The purpose of this invention is to provide a hot stamping steel sheet that can suppress the increase in labor and cost in the hot stamping process, and can obtain a formed product with a balance of strength and toughness and excellent hardness stability.

[0013] One aspect of the present invention relates to hot-stamped steel sheets, which contain, by mass percent

[0014] C: Above 0.25% and below 0.4%

[0015] Si: 1.05% or more and 1.4% or less

[0016] Mn: 0% or more and 1.4% or less

[0017] Cr: ≥0.6% and ≤3.0%

[0018] P: Above 0% and below 0.03%

[0019] S: 0% or more and less than 0.02%

[0020] Al: 0.01% or more and less than 1%

[0021] N: 0% or more and less than 0.01%

[0022] B: 0.0005% or more and less than 0.005%, and

[0023] Ti: ≥0.005% and ≤0.1%

[0024] The remainder consists of iron and unavoidable impurities. When the C content is represented by [C], the Si content by [Si], the Mn content by [Mn], and the Cr content by [Cr], the hot stamping steel sheet satisfies the following relationship (1), thus achieving an excellent balance between strength and toughness, and excellent hardness stability.

[0025]

[0026] According to the present invention, a hot stamping steel sheet can be provided that can suppress the increase in labor and cost in the hot stamping process, and can obtain a formed product with excellent balance of strength and toughness and excellent hardness stability. Attached Figure Description

[0027] Figure 1 This is a graph showing the relationship between the absorbed energy of a Charpy impact test when a plate is quenched using a mold and the hardness when quenched at a cooling rate of 10°C / s.

[0028] Figure 2 This is a schematic diagram illustrating the hot stamping process.

[0029] Figure 3 This is a schematic diagram showing the dimensions of the test piece used in the Charpy pendulum impact test.

[0030] Figure 4 It is a schematic diagram showing the dimensions of the test piece used in the hardness test. Detailed Implementation

[0031] The following is a detailed description of the hot-stamping steel sheet according to embodiments of the present invention.

[0032] (Hot stamping steel plate)

[0033] The hot-stamping steel sheet of this embodiment contains, by mass%, %

[0034] C: Above 0.25% and below 0.4%

[0035] Si: 1.05% or more and 1.4% or less

[0036] Mn: 0% or more and 1.4% or less

[0037] Cr: ≥0.6% and ≤3.0%

[0038] P: Above 0% and below 0.03%

[0039] S: 0% or more and less than 0.02%

[0040] Al: 0.01% or more and less than 1%

[0041] N: 0% or more and less than 0.01%

[0042] B: Above 0.0005% and below 0.005%, and

[0043] Ti: ≥0.005% and ≤0.1%

[0044] The remainder consists of iron and unavoidable impurities. When the C content is represented by [C], the Si content by [Si], the Mn content by [Mn], and the Cr content by [Cr], the hot-stamping steel sheet satisfies the following relationship (1), thus achieving an excellent balance between strength and toughness, and excellent hardness stability.

[0045]

[0046] In order to obtain a hot-stamping steel sheet with excellent balance of strength and toughness as well as hardness stability, the inventors conducted an in-depth study on the composition of the steel sheet. According to Non-Patent Document 2, it can be predicted that due to the gap between the die and the steel sheet during the hot stamping process, and / or the rise in die temperature, the cooling rate of a typical component will vary in the range of 30°C / s to 10°C / s. Therefore, the inventors, focusing on the balance of strength and toughness, and also on suppressing hardness unevenness even when the cooling rate varies, conducted a detailed study on the composition system of the steel sheet to achieve this. As a result, the inventors discovered that by ensuring that the composition of each component in the steel sheet meets the above-mentioned range, and by adjusting the balance of the contents of C, Si, Mn, and Cr to satisfy the relationship of Equation (1) above, a new insight can be gained that balances strength and toughness as well as hardness stability, thus creating the present invention.

[0047] First, the composition of each component of the hot stamping steel sheet of this embodiment will be described in detail.

[0048] [C (carbon): ≥0.25% by mass and ≤0.4% by mass]

[0049] The carbon content determines the strength of the steel plate after the mold has cooled. To obtain sufficient strength in the steel plate, the carbon content is 0.25% by mass or more, preferably 0.255% by mass or more, and more preferably 0.260% by mass or more.

[0050] However, if the carbon content is excessive, it can sometimes lead to an increase in the strength of the steel sheet after hot rolling, resulting in cracking during cold rolling and / or reduced weldability. Therefore, the carbon content is 0.4% by mass or less, preferably 0.38% by mass or less, and more preferably 0.36% by mass or less.

[0051] [Si (silicon): ≥1.05% by mass and ≤1.4% by mass]

[0052] Si increases the resistance to softening during tempering, thus contributing to the hardness stability of the steel sheet. Furthermore, when no coating is applied to the steel sheet surface, Si helps prevent the peeling of oxide scale after the mold cools. To achieve these effects, the Si content should be 1.05% by mass or higher.

[0053] On the other hand, Si facilitates the formation of retained austenite (γ) and contributes to the decrease in yield strength (YS) and the segregation of Mn. Therefore, the Si content is 1.4% by mass or less, preferably 1.35% by mass or less.

[0054] [Mn (Manganese): ≥0% by mass and ≤1.4% by mass]

[0055] Mn is one of the important elements contained in the hot stamping steel sheet of this embodiment. By improving the hardenability of the steel sheet, it helps to increase the strength of the steel sheet after the mold cools. In order to achieve this effect, the Mn content is 0.5% by mass or more, preferably 0.6% by mass or more, and more preferably 0.8% by mass or more.

[0056] On the other hand, studies on the balance between strength and toughness of steel plates after mold cooling have confirmed that if Mn is excessive, coarse carbides may precipitate during mold cooling, which can become a major cause of brittle fracture under impact stress at low temperatures. Therefore, the Mn content is 1.4% by mass or less, preferably 1.35% by mass or less, and more preferably 1.30% by mass or less.

[0057] It should be noted that Mn is an element that inevitably mixes into steel plates, and its content is difficult to set as 0% by mass.

[0058] [Cr (chromium): ≥0.6% by mass and ≤3.0% by mass]

[0059] Cr is one of the key elements in the hot stamping steel sheet of this embodiment. Studies on the balance of strength and toughness of the steel sheet after die cooling have confirmed that Cr helps ensure hardness at low cooling rates (e.g., 10°C / s) and helps suppress the precipitation of coarse carbides during die cooling, thereby suppressing brittle fracture under impact stress at low temperatures. To achieve these effects, the Cr content is 0.6% by mass or more, preferably 0.8% by mass or more, and more preferably 1.05% by mass or more.

[0060] On the other hand, if the steel sheet contains excessive Cr, the strength of the steel sheet will increase after hot rolling, leading to cracking during cold rolling and / or deterioration of pickling properties after hot rolling. Therefore, the Cr content is 3.0% by mass or less, preferably 2.5% by mass or less.

[0061] [P (phosphorus): ≥0% by mass and ≤0.03% by mass]

[0062] From the viewpoint of weldability, toughness, and prevention of surface defects of the components, it is necessary to specify an upper limit for the phosphorus (P) content. Therefore, the P content is 0.03% by mass or less, preferably 0.025% by mass or less, and more preferably 0.02% by mass or less.

[0063] It should be noted that phosphorus (P) is an element that is unavoidably mixed into steel plates, so its content is unlikely to be zero by mass.

[0064] [S (sulfur): ≥0% by mass and ≤0.02% by mass]

[0065] S reduces the uniformity of Mn concentration distribution by generating MnS, and deteriorates the weldability of the steel plate. Therefore, the S content is 0.02% by mass or less, preferably 0.018% by mass or less, and more preferably 0.015% by mass or less.

[0066] It should be noted that, like P, S is an element that inevitably mixes into the steel plate, making it difficult to achieve a content of 0% by mass.

[0067] [Al (aluminum): ≥0.01% by mass and ≤1% by mass]

[0068] Al is an element that functions as a deoxidizer. To achieve this effect, the Al content is 0.01% by mass or more, preferably 0.015% by mass or more.

[0069] However, if the steel plate contains excessive Al, the hardness of the mold decreases after cooling and excessive Al2O3 is generated, leading to deterioration of low-temperature toughness. Therefore, the Al content is 1% by mass or less, preferably 0.8% by mass or less, and more preferably 0.1% by mass or less. It should be noted that the Al content here refers to the content of Al in the solid solution state (sol.Al).

[0070] [N (nitrogen): ≥0% by mass and ≤0.01% by mass]

[0071] Nitrogen (N) is an element that inevitably mixes into steel plates. If the steel plate contains excessive N, the amount of dissolved boron (B) in the steel plate will decrease due to the formation of boron compounds from N, resulting in deterioration of hardenability. Therefore, the N content is 0.01% by mass or less, preferably 0.008% by mass or less, and more preferably 0.005% by mass or less.

[0072] [B (boron): ≥0.0005% by mass and ≤0.005% by mass]

[0073] Boron (B) is an important element for improving the hardenability of steel plates. By adding an appropriate amount of B to the steel plate, the hardenability can be improved, thereby stably increasing the strength of the steel plate after the mold has cooled. To achieve this effect, the B content is 0.0005% by mass or more, preferably 0.0010% by mass or more, and more preferably 0.0015% by mass or more.

[0074] On the other hand, if the steel plate contains excessive amounts of boron (B), coarse iron-boron compounds will precipitate, leading to a decrease in toughness. Therefore, the B content is 0.0050% by mass or less, preferably 0.0045% by mass or less, and more preferably 0.0030% by mass or less.

[0075] [Ti (titanium): ≥0.005% by mass and ≤0.1% by mass]

[0076] Ti reduces the amount of BN formed in the steel sheet by generating TiN. Consequently, the amount of dissolved B in the steel sheet increases, enhancing the B-based hardenability. To achieve this effect, the Ti content is 0.0050% by mass or more, preferably 0.010% by mass or more, and more preferably 0.015% by mass or more.

[0077] On the other hand, if the steel plate contains excessive Ti, carbides will precipitate at the grain boundaries, deteriorating the hardenability of the steel plate. Therefore, the Ti content is 0.1% by mass or less, preferably 0.08% by mass or less, and more preferably 0.06% by mass or less.

[0078] The hot-stamping steel sheet of this embodiment may contain one or more elements selected from the group consisting of Mo, Nb, and V, or one or more elements selected from the group consisting of Cu and Ni, in addition to the above-described composition. The range of these compositions will be explained below. It should be noted that these elements are not essential elements in the hot-stamping steel sheet of this invention and may be omitted.

[0079] [Mo (molybdenum): ≥0% by mass and ≤1.0% by mass]

[0080] Mo is an element that helps improve the hardenability of steel sheets. To achieve this effect, the Mo content is preferably 0.01% by mass or more. However, if the steel sheet contains excessive Mo, the strength of the steel sheet increases before hot forming. To prevent this, the Mo content is preferably 1.0% by mass or less.

[0081] [Nb (niobium), V (vanadium): ≥0.1% by mass]

[0082] Nb and V form fine carbides, which have the effect of refining the microstructure of steel through a needle-like effect. Furthermore, V precipitates during tempering, thus having a secondary hardening effect. To achieve these effects, the Nb and V contents are preferably 0.0008% by mass or more, respectively.

[0083] However, if the steel plate contains excessive amounts of Nb and V, coarse carbides are formed, which become the starting point for damage and lead to reduced toughness. Therefore, the Nb and V contents are each 0.1% by mass or less, preferably 0.08% by mass or less, and more preferably 0.07% by mass or less.

[0084] [Cu (copper), Ni (nickel): 0% by mass and less than 0.5% by mass]

[0085] When it is necessary to improve the delayed fracture characteristics of components, it is preferable to add Cu and Ni. However, if the steel sheet contains excessive Cu and Ni, it may become the cause of defects on the surface of the steel sheet and ultimately on the surface of the component. Therefore, the individual content of Cu and Ni is preferably 0.5% by mass or less, and the combined content is preferably 0.5% by mass or less.

[0086] The hot-stamping steel sheet of this embodiment achieves a balance of strength and toughness, as well as excellent hardness stability, by adjusting the balance of C, Si, Mn, and Cr contents to satisfy the following relationship (1). In this relationship (1), [C] represents the C content (mass%) of the hot-stamping steel sheet. [Si] represents the Si content (mass%) of the hot-stamping steel sheet. [Mn] represents the Mn content (mass%) of the hot-stamping steel sheet. [Cr] represents the Cr content (mass%) of the hot-stamping steel sheet.

[0087]

[0088] By ensuring that each component composition meets the composition range of the claims and satisfies the above-mentioned relationship (1), the hot stamping steel sheet of this embodiment becomes a steel sheet with excellent balance between strength and low-temperature toughness after being quenched by cooling with a die, and excellent hardness stability. Specifically, when the absorbed energy of the Charpy impact test at -40°C is set as A (J / cm²), the steel sheet is quenched with a die for a flat plate. 2 When the hardness of a hot stamping steel plate heated to the austenitic region and then cooled to room temperature at a cooling rate of 10℃ / s is set as B(Hv), and the hardness of a hot stamping steel plate heated to the austenitic region and then cooled to room temperature at a cooling rate of 30℃ / s is set as C(Hv), then all of the following relationships (2), (3) and (4) are satisfied.

[0089] B>-4.0A+627 (2)

[0090] B≥516 (3)

[0091] |CB|≤35 (4)

[0092] The above-mentioned equation (2) is an index for the balance of strength and toughness of the steel sheet newly designed by the inventors, and is an important concept when considering the balance of strength and toughness of hot stamping steel sheets. When the inventors studied the balance of strength and toughness, they focused on the hardness at a cooling rate of 10°C / s and the toughness after the plate was molded. The mold cooling of the plate was based on the ideal cooling conditions under which no gap was generated between the mold and the steel sheet during the hot stamping process. By using the above-mentioned equation (2), the balance of strength and toughness when the hot stamping steel sheet is processed into parts (formed products) can be evaluated more faithfully.

[0093] Figure 1 The graph shows the relationship between the absorbed energy A (horizontal axis) of a Charpy impact test at -40°C when the plate was quenched using a mold, and the hardness B (vertical axis) of the steel plate when quenched at a cooling rate of 10°C / s. The straight line (1) in the graph corresponds to the relationship (2). Furthermore, the straight line (2) in the graph corresponds to the equation B = 516.

[0094] Figure 1 In the chart, the horizontal axis (A) represents the toughness of the most brittle part of the component after the die has cooled, assuming the die has cooled. That is, when the plate is molded, the die and the steel sheet are in ideal contact, thus the cooling rate is high. Therefore, the strength is high after cooling, but on the other hand, it becomes very brittle. In other words, this horizontal axis represents the toughness of the most brittle part when hot-stamped steel sheet is formed into a component (formed article).

[0095] on the other hand, Figure 1 The vertical axis (B) of the graph represents the hardness of the softest part of the component after the die has cooled, assuming the die has cooled. As mentioned above, during the hot stamping process, a gap sometimes forms between the die and the steel sheet, and sometimes the die temperature rises. Therefore, in the component after the die has cooled, there are parts with low hardness (strength) that cooled at a low cooling rate. According to Non-Patent Document 2, it can be anticipated that the minimum cooling rate during die cooling is approximately 10°C / s. Therefore, this vertical axis represents the hardness (strength) of the softest part of the component (formed article) after the die has cooled. Thus, by using these two axes, it is possible to evaluate the toughness of the weakest part when impact stress is applied to the formed component and the strength of the weakest part when static stress is applied to the component.

[0096] Typically, in the hardness range of B above 516 Hv, the strength and toughness of steel plates are in a trade-off, thus exhibiting a tendency where increasing the strength of the steel plate leads to a deterioration in toughness. In other words, it is difficult to improve both the strength and toughness of a steel plate; generally, within this range... Figure 1 In the figure, there are distributions of A and B in the region below the straight line (1).

[0097] Furthermore, the straight line (2) is an indicator of hardness stability. In the continuous operation of the die in the hot stamping process, sometimes the temperature of the die rises, and sometimes a gap is generated between the die and the steel plate. For these reasons, the cooling rate of the steel plate decreases during quenching, and the hardness of the steel plate decreases as the cooling rate decreases after quenching. Generally, even steel plates that improve the balance between strength and toughness through grain refinement are difficult to meet the range of hardness above 516Hv when quenched in the low cooling rate region (10℃ / s). Therefore, even steel plates that improve the balance between strength and toughness through grain refinement are generally difficult to meet the range of hardness above 516Hv when quenched in the low cooling rate region (10℃ / s). Figure 1 The region below the straight line (2) also contains distributions of A and B.

[0098] The inventors conducted in-depth research on this matter and found that, in hot-stamping steel sheets that satisfy the above relationship (1), the distribution of A and B is located in Figure 1 The region above the straight lines (1) and (2) in the figure. Therefore, the hot-stamping steel sheet of this embodiment has excellent hardness stability while maintaining a balance between strength and toughness. That is, the hot-stamping steel sheet satisfies the above-mentioned relationship (2), has an excellent balance between strength and toughness, and can achieve a hardness of a specified degree even when cooled at the lowest cooling rate of 10°C / s.

[0099] The above relationship (4) is another indicator of the hardness stability of the steel plate. In hot stamping, when the die temperature rises or a gap is generated between the die and the steel plate, the cooling rate of the steel plate may decrease, and thus the hardness of the steel plate may become unstable after quenching. In addition, as mentioned above, if the grain size is refined, the hardness stability will decrease, so it is usually difficult to satisfy the above relationship (4).

[0100] The inventors conducted in-depth research on this matter and found that hot-stamping steel sheets that satisfy the above-mentioned formula (1) and whose components satisfy the composition range of the claims can achieve a hardness of more than 516 Hv after quenching, even in the low cooling rate region of 10°C / s, and can suppress the hardness difference between cooling rates of 30°C / s and 10°C / s to below 35 Hv. 30°C / s is the ideal cooling rate for mold cooling confirmed by experiments, etc., while 10°C / s is the minimum cooling rate expected as described above. That is, the above-mentioned formula (4) is an index that represents the minimum hardness difference (uniformity) after quenching between the upper and lower limits of the desired cooling rate in hot stamping. According to the hot-stamping steel sheet of this embodiment, regardless of whether the temperature of the mold rises or whether a gap is generated between the mold and the steel sheet, the hardness of the quenched steel sheet can be stabilized to the extent that the above-mentioned formula (4) is satisfied.

[0101] It should be noted that the hot stamping steel sheet of the present invention can be a base steel sheet without surface coating treatment, or a steel sheet with surface coating treatment.

[0102] (Manufacturing method of steel sheet for hot stamping)

[0103] Next, the method for manufacturing the hot stamping steel sheet of the present embodiment will be described.

[0104] First, the slab manufacturing process is carried out. In this process, steel is melted according to the usual method, and the molten steel is poured into a mold and continuously cast to obtain a slab. In this process, the composition of the steel during melting is adjusted so that the composition of each component in the slab meets the above-mentioned range, and the contents of C, Si, Mn and Cr meet the above-mentioned relationship (1).

[0105] Next, the hot rolling process is carried out. In this process, the slab obtained in the above process is first placed in a heating furnace and heated to a specified temperature (e.g., 1200°C), and held at this heating temperature for a specified time (e.g., 30 minutes).

[0106] Next, the heated slab is placed upstream of the hot rolling production line. Then, the slab is passed sequentially through the rolls of the roughing and finishing mill stands and moved downstream, rolling the slab into a steel plate of a specified thickness. The hot-rolled steel plate is then cooled to a specified temperature in a cooling device and coiled by a coiler.

[0107] Next, a cold rolling process is performed. In this process, firstly, the oxide scale (iron oxide) generated on the surface of the steel sheet during the hot rolling process is washed away with acid (pickling), and then the hot-rolled steel sheet is further rolled to reduce its thickness. Specifically, the pickled hot-rolled steel sheet is processed between the rolls of a rolling mill stand to further thin the hot-rolled steel sheet. The cold-rolled steel sheet obtained through the above processes is the hot stamping steel sheet of this embodiment.

[0108] (Hot stamping)

[0109] Next, refer to Figure 2 The hot stamping process using the steel sheet manufactured by the above process will be described. First, the hot stamping steel sheet 1 obtained by the above process is heated to above the austenitic phase transformation temperature in a designated heating furnace 2. Then, the heated hot stamping steel sheet 1 is placed between dies 3 and 4, and the hot stamping steel sheet 1 is stamped into the desired shape by the dies 3 and 4. At this time, the hot stamping steel sheet 1 is cooled by contact with the dies 3 and 4, and is quenched during forming. Next, the quenched steel sheet is removed from the dies 3 and 4 as the formed product 5 (formed part).

[0110] The formed article 5 has the same composition as the hot stamping steel sheet 1 of this embodiment described above, and the balance of the contents of C, Si, Mn and Cr is adjusted to satisfy the above-described relationship (1). Therefore, the formed article 5 has excellent hardness stability while maintaining a balance between strength and toughness, and can be used for various applications, primarily automotive parts.

[0111] It should be noted that the above implementation methods are summarized as follows.

[0112] The hot-stamping steel sheet described in the above embodiments contains, by mass%,

[0113] C: Above 0.25% and below 0.4%

[0114] Si: 1.05% or more and 1.4% or less

[0115] Mn: 0% or more and 1.4% or less

[0116] Cr: ≥0.6% and ≤3.0%

[0117] P: Above 0% and below 0.03%

[0118] S: 0% or more and less than 0.02%

[0119] Al: 0.01% or more and less than 1%

[0120] N: 0% or more and less than 0.01%

[0121] B: 0.0005% or more and less than 0.005%, and

[0122] Ti: ≥0.005% and ≤0.1%

[0123] The remainder consists of iron and unavoidable impurities. When the C content is represented by [C], the Si content by [Si], the Mn content by [Mn], and the Cr content by [Cr], the hot-stamping steel sheet satisfies the following relationship (1), thus achieving an excellent balance between strength and toughness and excellent hardness stability.

[0124]

[0125] The above-mentioned hot-stamping steel plates may contain, by mass%, %

[0126] Select Mo: 0% or more and 1.0% or less.

[0127] Nb: 0% or more and 0.1% or less, and

[0128] V: One or more species in a group consisting of 0% or more and 0.1% or less.

[0129] The above-mentioned hot-stamping steel plates may contain, by mass%, %

[0130] Select free Cu: 0% or more and 0.5% or less, and

[0131] Ni: One or more substances in the group consisting of 0% or more and 0.5% or less.

[0132] Example

[0133] The present invention will now be described in more detail with reference to embodiments. However, the present invention is not limited to the embodiments described below, and appropriate modifications may be made to implement it within the scope of the foregoing and following spirit, all of which are included within the technical scope of the present invention.

[0134] <Manufacturing of Steel Sheets for Hot Stamping>

[0135] First, steel with the composition shown in Table 1 (Nos. 1 to 17 below) was melted to produce slabs. These melted slabs were heated to 1200°C and held for 30 minutes, followed by hot rolling. The finishing temperature was 900±20°C, and the finished plate thickness was 2.8 mm. The hot-rolled steel sheet was then cooled to the coiling temperature (CT temperature) at a cooling rate of 20–30°C / s, held at 650°C for 30 minutes, and then furnace cooled. The hot-rolled steel sheet was then pickled and cold-rolled to a thickness of 1.4 mm.

[0136] Charpy Impact Test

[0137] First, the cold-rolled steel sheet produced in the above order was cut and then quenched. The quenching was carried out under the following conditions using a die-pressing quenching method with a flat plate simulating a mold (testing machine: JIS Charpy impact testing machine (300J)).

[0138] [Quenching conditions]

[0139] Steel plate dimensions before quenching: 1.4mm × 70mm × 150mm

[0140] Steel plate temperature: 900℃

[0141] Temperature holding time for steel plate after reaching 900℃: 100 seconds

[0142] Cooling time: approximately 15 seconds

[0143] Molding quenching start temperature: 700℃

[0144] Molding and quenching load: 2000 kgf

[0145] Lower dead center hold time: 30 seconds

[0146] Next, Charpy pendulum impact tests were conducted using the aforementioned quenched cold-rolled steel sheets. Except for the dimensions of the test pieces, the test was performed according to JIS 2242, "Charpy Impact Test Method for Metallic Materials". The dimensions of the test pieces used in this test are as follows. It should be noted that the symbols representing each dimension are different from those used in the previous test. Figure 3 The symbols shown correspond to each other.

[0147] [Test piece size]

[0148] The height of the test piece, h1, is 10 mm ± 0.05 mm.

[0149] Test piece length L: 55mm ± 0.6mm

[0150] Test piece width b: 1.4mm ± 0.05mm

[0151] Notch shape: V-notch

[0152] Notch angle: 45°±2°

[0153] Notch bottom radius: 0.25mm ± 0.025mm

[0154] Height h2 below the notch: 8mm ± 0.05mm

[0155] Angle between the length direction of the test piece and the plane of symmetry of the notch: 90°±2°

[0156] Angle between adjacent surfaces other than the fracture surface: 90°±2°

[0157] The test specimens of the above dimensions were placed in liquid nitrogen at a temperature adjusted to -40℃ ± 1℃ and maintained for at least 10 minutes. Then, the test specimens were removed from the liquid nitrogen and placed on a support platform, where they were subjected to impact. The time from placing the test specimens on the support platform to the start of the impact was less than 5 seconds.

[0158] A JIS Charpy impact testing machine (300J) was used as the testing machine, and an impact blade with a radius of 2 mm was used as the impact blade. Furthermore, two test pieces were used, and the average value of the measurements from the two pieces was used for evaluation.

[0159] <Evaluation of Oxide Coating Adhesion>

[0160] After quenching using the same conditions as the Charpy impact test, based on the die-hardening method, the adhesion of the oxide scale was evaluated by visually confirming the peeling of the oxide scale on the steel plate surface. A surface area with oxide scale peeling of less than 14% was rated as "○", and a surface area exceeding 14% was rated as "×".

[0161] <Hardness Test>

[0162] First, the cold-rolled steel sheets produced in the above order are processed into... Figure 4 The test piece is shown in the shape shown. Figure 4 In the test piece, L1 is 10 mm, L2 is 2 mm, L3 is 1.4 mm, L4 is 0.7 mm, L5 is 3 mm, and L6 is 1 mm. The test piece was quenched under the following conditions.

[0163] [Quenching conditions]

[0164] Heating rate during austenitization: 10℃ / s

[0165] High temperature hold: Maintain at 900℃ for 100 seconds

[0166] Cooling rate: Cool from 900℃ to room temperature at a uniform rate of 10℃ / s or 30℃ / s.

[0167] Using the quenched test pieces described above, a hardness test was conducted based on the "Vickers Hardness Test Method" specified in JIS Z 2244. In this test, five measurements were taken at a point from the surface of the test piece to one-quarter of the plate thickness with a test load of 9.8 N, and their average values ​​were used for evaluation.

[0168] Tables 1 and 2 below show the composition (mass%) of steel plates No. 1 to 17, and the absorbed energy A (J / cm) of the Charpy impact test at -40°C. 2 The Vickers hardness B (Hv) at a cooling rate of 10℃ / s, the Vickers hardness C (Hv) at a cooling rate of 30℃ / s, the hardness difference (Hv) between cooling rates of 10℃ / s and 30℃ / s, the value of the left side of the above relationship (1), the value obtained by subtracting the right side from the left side of the above relationship (2), and the evaluation of the oxide scale adhesion.

[0169] In addition, Figure 1 The chart shows the data for steel plates No. 1 through 17. Data for No. 1 through 9 and No. 14 through 17 are marked with black dots, and data for No. 10 through 13 are marked with white circles.

[0170]

[0171]

[0172] <Inspection>

[0173] Based on Tables 1 and 2 above, the investigation can be conducted as follows.

[0174] For Nos. 1 to 9 and 14 to 17, the contents of C, Si, Mn, Cr, P, S, Al, N, B, and Ti in the steel plates respectively meet the scope of the present invention, and the contents of C, Si, Mn, and Cr satisfy the above-mentioned relationship (1). In this case, since the value of "B+4A-627" is positive, the above-mentioned relationship (2) is satisfied, so these steel plates have an excellent balance between strength and toughness. Furthermore, in Nos. 1 to 9 and 14 to 17, since "B≥516" and "C-B≤35", the above-mentioned relationships (3) and (4) are also satisfied, so the hardness stability of these steel plates is also excellent. This is from Figure 1 From the perspective that the data (black dots) for No. 1 to 9 and 14 to 17 in the chart are located in the area above the lines (1) and (2), it is obvious. In addition, the evaluation of the oxide scale adhesion is also all "○".

[0175] In contrast, for Nos. 10 to 13, which do not meet the essential conditions of the present invention, as described below, steel plates that achieve both excellent balance between strength and toughness and good hardness stability have not been obtained. Figure 1 As shown in the chart, the data for No.10 to No.13 (white circles) are all located in the area below the lines (1) and (2).

[0176] Regarding No. 10, due to the Si content being less than 1.05% by mass and the negative value of "[C]+2 / 9[Si]+7 / 9[Mn]+8 / 9[Cr]-7 / 4", the value of "B+4A-627" is also negative, resulting in a poor balance between strength and toughness. Furthermore, the hardness B at a cooling rate of 10℃ / s is less than 516 Hv, and the hardness difference between cooling rates of 10℃ / s and 30℃ / s exceeds 35 Hv, indicating poor hardness stability. Additionally, the oxide scale adhesion is rated as "×".

[0177] For Nos. 11 to 13, since the Cr content is less than 0.6% by mass and the value of "[C]+2 / 9[Si]+7 / 9[Mn]+8 / 9[Cr]-7 / 4" is negative, the value of "B+4A-627" is also negative, resulting in a poor balance between strength and toughness. Furthermore, the hardness B at a cooling rate of 10℃ / s is less than 516 Hv, and the hardness difference between cooling rates of 10℃ / s and 30℃ / s exceeds 35 Hv, indicating poor hardness stability.

[0178] The embodiments and examples disclosed herein are illustrative in all respects and should be understood as not limiting. The scope of the invention is defined not by the foregoing description but by the scope of the claims, including any equivalents of the claims and any modifications within that scope.

Claims

1. A method for manufacturing a hot-stamped steel plate, characterized in that... include: The hot rolling process involves hot rolling a slab containing, by mass percent, 0.25% to 0.4% C, 1.05% to 1.4% Si, 0% to 1.4% Mn, 0.6% to 3.0% Cr, 0% to 0.03% P, 0% to 0.02% S, 0.01% to 1% Al, 0% to 0.01% N, 0% to 0.01% B, 0.0005% to 0.005% B, and 0.005% to 0.1% Ti, with the remainder being iron and unavoidable impurities, to obtain a hot-rolled steel sheet; and... In the cold rolling process, the hot-rolled steel sheet is cold-rolled to obtain a steel sheet for hot stamping. The slab satisfies the following relationship (1), In formula (1), [C] represents the C content, [Si] represents the Si content, [Mn] represents the Mn content, and [Cr] represents the Cr content.

2. The method for manufacturing hot-stamping steel plates according to claim 1, characterized in that, The slab also contains, by mass%, % Select Mo: 0% or more and 1.0% or less. Nb: 0% or more and 0.1% or less, and V: One or more species in a group consisting of 0% or more and 0.1% or less.

3. The method for manufacturing hot-stamping steel sheet according to claim 1 or 2, characterized in that, The slab also contains, by mass%, % Select free Cu: 0% or more and 0.5% or less, and Ni: One or more substances in the group consisting of 0% or more and 0.5% or less.

4. A steel plate for hot stamping, characterized in that, It is obtained by the manufacturing method of hot stamping steel sheet according to any one of claims 1 to 3.

5. The hot-stamping steel plate according to claim 4, characterized in that, The hot stamping steel plate satisfies the following relationships (2) to (4). B>-4.0A+627 (2) B≥516 (3) |CB|≤35 (4) In equations (2) to (4), A represents the absorbed energy of a Charpy impact test at -40°C when the plate is quenched using a mold; B represents the hardness of a hot-stamping steel plate heated to the austenitic region and then cooled to room temperature at a cooling rate of 10°C / s; and C represents the hardness of a hot-stamping steel plate heated to the austenitic region and then cooled to room temperature at a cooling rate of 30°C / s. The unit of A is J / cm. 2 The units for B and C are Hv.

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