Steel sheet manufacturing method

Abstract

To provide a method for producing a steel sheet having high strength, excellent in strength-ductility balance when the ductility is evaluated by uniform elongation, and excellent in stretch flange formability.SOLUTION: A method for producing a steel sheet comprises applying a heat treatment including the steps (a)-(d) below to a rolled material having a predetermined chemical composition. (a) Heating for at least 10 seconds at a heating temperature equal to or higher than the Ac3 point. (b) From the cooling start temperature below the heating temperature and above (the heating temperature-300°C), cooling to a cooling stop temperature 1 of 150-230°C, or from the cooling start temperature below the heating temperature and above (the heating temperature-300°C), cooling to a stay temperature range of 350-440°C, and after staying in the temperature range for 10-200 seconds, cooling to the cooling stop temperature 2 of 150-250°C, both at an average cooling rate of 10°C / s or more from the cooling start temperature to 440°C. (c) Reheating from each cooling stop temperature to a reheating temperature range of 380-480°C and staying at the reheating temperature range for 5-1800 seconds. (d) Cooling to room temperature after the reheating.SELECTED DRAWING: None

Description

[Technical field]

[0001] The present disclosure relates to a method for producing steel sheet, and more particularly to a method for producing steel sheet that can be used for various applications including automotive parts. [Background technology]

[0002] Steel sheets used in the manufacture of automotive parts are required to be thin in order to improve fuel efficiency through weight reduction, and high strength is required to achieve both thinning and ensuring part strength. In addition, steel sheets used in the manufacture of automotive parts are required to have high energy absorption capacity during a collision in consideration of collision safety, and high ductility is required. In general, improving strength reduces ductility, making it difficult to ensure energy absorption during a collision. Therefore, in order to achieve high strength and high ductility, in addition to increasing strength by improving tensile strength (TS), high ductility is required by improving TS x EL (elongation).

[0003] Furthermore, steel sheets used in the manufacture of automotive parts are required to have excellent formability in order to be processed into parts with complex shapes. In particular, they are required to have an excellent hole expansion ratio (λ), which is an index of local deformability, that is, excellent stretch flangeability.

[0004] For example, Patent Document 1 describes a method for hot rolling, which includes performing a first hot rolling including one or more passes of 40% or more reduction in a temperature range of 1000° C. or more and 1200° C. or less, performing a large reduction at a temperature of T1+30° C. or more and T1+200° C. or less, and performing an Ar 3 This paper discloses a steel sheet in which the pole density of a specific crystal orientation in the thickness range of 5 / 8 to 3 / 8 is controlled within a predetermined range by restricting the rolling reduction in the temperature range of 1000° C. or more and less than T1+30° C. The steel sheet is shown to have TS×EL>14000.

[0005] Patent Document 2 discloses a steel plate containing tempered martensite, bainite, and austenite, with ferrite limited to 10% or less, and in 80% or more of the bainite, the grain boundaries are in contact with both tempered martensite and austenite. The steel plate has a strength of 1300 MPa or more and is shown to have excellent formability.

[0006] In Patent Document 3, after hot rolling, 300°C to Ac 3 After first annealing, which is held at the temperature range of the point for more than 30 minutes, and cold rolling, 1 This steel sheet is heated to 300°C or lower and then cooled to 150-600°C, hot-dip galvanized, and then cooled to 300°C or lower, and tempered in the temperature range of 100-600°C, thereby controlling the retained austenite to 10% or more, the carbon content in the retained austenite to 0.85% or more, and the ratio of the Mn content in the retained austenite to the average Mn content to 1.1 or more. This steel sheet has a strength of 1470 MPa or more and is shown to have excellent deformability.

[0007] Patent Document 4 shows that by containing 0.35 to 0.60 mass% or more of carbon, 2.1 to 2.8 mass% of Si, 1.2 to 1.8 mass% of Mn, with bainite and rhombic ferrite and martensite, and retained austenite and a mixed structure of martensite and austenite as the main phases, and with 3 to 5% rhombic ferrite and martensite in ferrite and rhombus where no cementite is present, it is possible to ensure a tensile strength of 1470 MPa or more, an EL of 14% or more, and a λ: 25% or more. [Prior art documents] [Patent documents]

[0008] [Patent Document 1] International Publication No. 2012 / 133563 Brochure [Patent Document 2] JP 2015-151576 A [Patent Document 3] JP 2017-053001 A [Patent Document 4] JP 2020-132929 A Summary of the Invention [Problem to be solved by the invention]

[0009] Although extensive studies, including the above-mentioned techniques, have been conducted with the aim of improving the properties of steel sheets, it is currently difficult to manufacture steel sheets that have high strength and high ductility and excellent stretch flangeability. Although total elongation is usually used as an index of ductility, it is preferable to evaluate it using "uniform elongation", which indicates the more essential ductility of the material, since elongation is affected geometrically by the thickness of the test piece and the like.

[0010] The present disclosure has been made in consideration of the above circumstances, and an object of the present disclosure is to provide a method for producing a steel sheet which has high strength, exhibits an excellent strength-ductility balance when ductility is evaluated by uniform elongation, and has excellent stretch flangeability. [Means for solving the problem]

[0011] Aspect 1 of the present invention is The chemical composition is C: 0.32% by mass or more, 0.48% by mass or less, Si: 2.1% by mass or more, 3.0% by mass or less, Mn: 1.6% by mass or more, 3.0% by mass or less, Ti: 0.010 mass% or more, 0.15 mass% or less, P: more than 0% by mass, less than 0.05% by mass, S: More than 0% by mass, 0.01% by mass or less, Al: 0.01% by mass or more and 0.1% by mass or less, and N: More than 0% by mass and 0.010% by mass or less; The balance is Fe and unavoidable impurities. The method for producing a steel sheet includes carrying out a heat treatment including the following steps (a) to (d). (a) heating the rolled material for 10 seconds or more at a heating temperature equal to or higher than the Ac3 point calculated from the following formula (1) (b) after said heating, Cooling from a cooling start temperature equal to or lower than the heating temperature (the heating temperature -300°C) to a cooling stop temperature 1 of 150°C to 230°C, or a step of cooling from a cooling start temperature equal to or lower than the heating temperature and equal to or higher than (the heating temperature -300°C) to a residence temperature range of 350°C to 440°C, allowing the temperature range to remain in the residence temperature range for 10 seconds to 200 seconds, and then cooling from the residence temperature range to a cooling stop temperature 2 of 150°C to 250°C, In either case, the average cooling rate from the cooling start temperature to 440°C must be 10°C / s or more. (c) Reheating from the cooling stop temperature 1 or the cooling stop temperature 2 to a reheating temperature range of 380°C or more and 480°C or less, and allowing the material to stay in the reheating temperature range for 5 seconds or more and 1800 seconds or less. (d) A step of cooling the mixture to room temperature after the reheating step (c). Ac3 points (℃)=910-203√(C)+44.7Si-30Mn-11Cr+31.5Mo-20Cu-15.2Ni+700P...(1) In formula (1), the element symbols indicate the contents (mass%) of the elements contained in the rolled material, and elements that are not contained are represented as zero. Effect of the Invention

[0012] According to the present disclosure, there is provided a steel sheet which has high strength, exhibits an excellent strength-ductility balance when ductility is evaluated by uniform elongation, and has excellent stretch flangeability. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0013] As a result of extensive research, the inventors have found that by subjecting rolled material of a specific chemical composition to heat treatment under prescribed conditions, a steel sheet can be obtained which has high strength, a good balance between strength and uniform elongation, and excellent stretch flangeability.

[0014] In order to ensure the strength-ductility balance of a steel plate, it is effective to have a structure that is primarily composed of one or more of bainite, bainitic ferrite, martensite, or tempered structures thereof, while also containing a certain amount or more of retained austenite and a mixed structure of martensite / austenite in order to ensure ductility.

[0015] From the viewpoint of chemical composition, a means for achieving these goals is to increase the content of austenite stabilizing elements such as C and Mn in order to ensure retained austenite, while including elements such as Si and Al that inhibit the formation of cementite in order to prevent C from being consumed as cementite.

[0016] From the viewpoint of the manufacturing method, in order to avoid the formation of ferrite or pearlite and to secure the above-mentioned structure, it is effective to first heat a hot-rolled sheet or cold-rolled sheet (hereinafter, these may be referred to as "rolled sheet") to the austenite single-phase region, then rapidly cool a specific temperature range to avoid ferrite or pearlite transformation, form bainite, bainitic ferrite, and martensite, and cool a portion to below the Ms point and above the Mf point to turn it into fine untransformed austenite, and then reheat the sheet to promote the transformation of the untransformed austenite and the concentration of C in the untransformed austenite, thereby securing fine retained austenite.

[0017] In the manufacturing method of this embodiment, by further containing a predetermined amount of Ti in the above chemical composition, it is possible to realize refinement of the prior austenite grain size, and as a result, it is possible to sufficiently secure retained austenite and refine the retained austenite, thereby achieving both improved elongation by securing the amount of retained austenite and excellent stretch flangeability by refining the retained austenite.

[0018] The manufacturing method of the high strength steel sheet according to the present embodiment will be described below in order. The chemical composition of the rolled material to be subjected to the heat treatment will be described below. The chemical composition described below is also the chemical composition of the steel sheet to be finally obtained.

[0019] [Chemical composition] [C: 0.32 mass% or more, 0.48 mass% or less] C is an austenite stabilizing element and is an essential element for ensuring the amount of retained austenite. In order to ensure the desired properties including tensile strength, the C content is set to 0.32 mass% or more. The C content is preferably 0.35 mass% or more, more preferably 0.37 mass% or more, and even more preferably 0.38 mass% or more. On the other hand, if the C content is too high, the size of the retained austenite becomes coarse, and excellent stretch flangeability cannot be ensured. Therefore, the C content is set to 0.48 mass% or less, preferably 0.45 mass% or less, more preferably 0.43 mass% or less, and even more preferably 0.42 mass% or less.

[0020] [Si: 2.1% by mass or more, 3.0% by mass or less] Si is an element necessary for increasing strength by solid solution strengthening, suppressing the formation of cementite in the process of cooling to the Ms point or less and the Mf point or more, and then heating, suppressing the decomposition of untransformed austenite, promoting C concentration in untransformed austenite, and ensuring sufficient retained austenite by heat treatment. Therefore, the amount of Si is 2.1 mass% or more, preferably 2.2 mass% or more, more preferably 2.3 mass% or more, and even more preferably 2.4 mass% or more. On the other hand, if Si is contained in excess, the effect of solid solution strengthening becomes excessive, the parent phase becomes embrittled, and the elongation and hole expansion ratio decrease. Therefore, the amount of Si is 3.0 mass% or less, preferably 2.9 mass% or less, more preferably 2.8 mass% or less, and even more preferably 2.7 mass% or less.

[0021] [Mn: 1.6 mass% or more, 3.0 mass% or less] Mn is an effective austenite stabilizing element that suppresses the formation of ferrite and pearlite and ensures the amount of retained austenite. Therefore, the Mn content is set to 1.6 mass% or more. The Mn content is preferably 1.7 mass% or more, more preferably 1.8 mass% or more. On the other hand, if the Mn content is too high, the size of the retained austenite becomes too coarse, making it impossible to ensure excellent stretch flangeability. Therefore, the Mn content is set to 3.0 mass% or less, preferably 2.8 mass% or less, more preferably 2.6 mass% or less, even more preferably 2.4 mass% or less, and even more preferably 2.2 mass% or less.

[0022] [Ti: 0.010 mass% or more, 0.15 mass% or less] Ti combines with C to form TiC, which is a fine precipitate. When fine TiC is present, the growth of austenite grains is suppressed by the pinning effect of fine TiC when the steel is heated to the austenite single phase region by heat treatment, and the structure formed thereafter can be refined. The refinement of the retained austenite improves the stretch flangeability. In addition, the refinement of the structure size makes it easier for C to concentrate in the retained austenite, and as a result, the amount of retained austenite can be secured, which contributes to improving elongation. In order to achieve these effects, the Ti content is set to 0.010 mass% or more. The Ti content is preferably 0.012 mass% or more, more preferably 0.015 mass% or more, even more preferably 0.020 mass% or more, even more preferably 0.025 mass% or more, and may be even 0.030 mass% or more, and even more preferably 0.040 mass% or more. On the other hand, if Ti is contained in excess, hard TiN, which is a compound with N, becomes coarse and becomes the starting point of fracture, and the stretch flangeability deteriorates. Therefore, the Ti content is set to 0.15 mass % or less, preferably 0.13 mass % or less, and more preferably 0.12 mass % or less.

[0023] In addition to Ti, Nb and V are also listed as carbide forming elements. However, Nb has a stronger bond with C than Ti, and at the C content level of the rolled material of this embodiment, most of Nb remains as coarse alloy carbides at the heating stage of hot rolling, that is, there are few fine alloy carbides, and it is difficult to obtain a pinning effect by heating to the austenite single phase region during heat treatment. In addition, V has a weaker bond with C than Ti, and a part of it is dissolved in solid solution by heating to the austenite single phase region during heat treatment, so it is difficult to obtain a pinning effect. In addition, V that is dissolved in solid solution at the heating stage to the austenite single phase region may precipitate finely in the subsequent cooling and reheating process, and in that case, there is a concern that the parent phase will be precipitation strengthened and the ductility will decrease, which is not preferable.

[0024] [P: more than 0 mass%, 0.05 mass% or less] P is inevitably present as an impurity element. If the P content exceeds 0.05% by mass, the elongation (EL) and hole expansion ratio decrease. Therefore, the P content is set to 0.05% by mass or less. The P content is preferably 0.03% by mass or less. The lower the P content, the more preferable it is, and 0% by mass is the most preferable, but due to constraints in the manufacturing process, etc., there are cases where the P content exceeds 0% by mass, for example, about 0.001% by mass remains.

[0025] [S: More than 0 mass%, 0.01 mass% or less] S is inevitably present as an impurity element. If the S content exceeds 0.01% by mass, sulfide-based inclusions such as MnS are formed, and the inclusions become the starting points of cracks, resulting in a decrease in the hole expansion ratio. Therefore, the S content is set to 0.01% by mass or less. The S content is preferably 0.005% by mass or less. The lower the S content, the more preferable it is, and 0% by mass is the most preferable, but there are cases where the S content exceeds 0% by mass, for example, about 0.001% by mass, remains due to constraints in the manufacturing process.

[0026] [Al: 0.01% by mass or more, 0.1% by mass or less] Al functions as a deoxidizing element and reduces the amount of oxygen in molten steel, thereby reducing the number density of inclusions and improving the basic quality of the steel. In order to effectively exert such an effect, the Al content is set to 0.01 mass% or more. The Al content is preferably 0.015 mass% or more, more preferably 0.020 mass% or more. On the other hand, if the Al content is excessive, the formation of ferrite is promoted, making it impossible to obtain the desired structure. Therefore, the Al content is set to 0.1 mass% or less. The Al content is preferably 0.08 mass% or less, more preferably 0.06 mass% or less.

[0027] [N: More than 0 mass%, 0.010 mass% or less] N is inevitably present as an impurity element. When Ti is included as in the chemical composition according to this embodiment, N bonds with Ti to form hard TiN, which becomes the starting point of fracture during deformation, and in particular reduces the hole expansion ratio. Therefore, it is most preferable that the N content is 0 mass%, but it inevitably remains in the manufacturing process. From the above viewpoint, it is preferable to reduce the N content, and the N content is 0.010 mass% or less, preferably 0.008 mass% or less, and more preferably 0.006 mass% or less.

[0028] [The balance is Fe and unavoidable impurities] The balance is Fe and inevitable impurities (e.g., As, Sb, Sn, etc.). Inevitable impurities are elements that are brought in depending on the conditions of raw materials, materials, manufacturing equipment, etc. Elements such as O can also be unavoidably mixed in. For example, each of these elements can be allowed to be mixed in as an impurity element if it is 100 ppm or less. Note that, for example, there are elements such as P, S, and N, which are usually preferable to have a lower content and are therefore inevitable impurities, but whose composition ranges are separately specified as above. For this reason, in this specification, when referring to the "unavoidable impurities" that make up the balance, this is a concept that excludes elements whose composition ranges are separately specified.

[0029] In the method for producing a steel sheet according to the present embodiment, a rolled material satisfying the above-mentioned chemical composition is The method includes carrying out a heat treatment including the following steps (a) to (d). (a) heating the rolled material for 10 seconds or more at a heating temperature equal to or higher than the Ac3 point calculated from the following formula (1) (b) after said heating, Cooling from a cooling start temperature equal to or lower than the heating temperature (the heating temperature -300°C) to a cooling stop temperature 1 of 150°C to 230°C, or a step of cooling from a cooling start temperature equal to or lower than the heating temperature and equal to or higher than (the heating temperature -300°C) to a residence temperature range of 350°C to 440°C, allowing the temperature range to remain in the residence temperature range for 10 seconds to 200 seconds, and then cooling from the residence temperature range to a cooling stop temperature 2 of 150°C to 250°C, In either case, the average cooling rate from the cooling start temperature to 440°C must be 10°C / s or more. (c) Reheating from the cooling stop temperature 1 or the cooling stop temperature 2 to a reheating temperature range of 380°C or more and 480°C or less, and allowing the material to stay in the reheating temperature range for 5 seconds or more and 1800 seconds or less. (d) A step of cooling the mixture to room temperature after the reheating step (c). Ac3 points (℃)=910-203√(C)+44.7Si-30Mn-11Cr+31.5Mo-20Cu-15.2Ni+700P...(1) In formula (1), the element symbols indicate the contents (mass%) of the elements contained in the rolled material, and elements that are not contained are represented as zero.

[0030] Each step in the heat treatment will now be described.

[0031] [(a) heating the rolled material at a heating temperature equal to or higher than the Ac3 point calculated from the following formula (1) for 10 seconds (s) or more] Ac3 points (℃)=910-203√(C)+44.7Si-30Mn-11Cr+31.5Mo-20Cu-15.2Ni+700P...(1) In formula (1), the element symbol indicates the content (mass%) of the element contained in the rolled material, and elements that are not contained are represented as zero.

[0032] In order to dissolve the ferrite and cementite contained in the structure of the rolled material, the rolled material is heated to the Ac3 point or higher. If the heating temperature is lower than the Ac3 point, soft ferrite remains and becomes the starting point of fracture during deformation, making it impossible to ensure strength and stretch flangeability. There is no particular upper limit for the heating temperature, but if it exceeds 950°C, austenite grains become coarse even if Ti is contained, making it difficult to ensure stretch flangeability, so the heating temperature is preferably set to an upper limit of 950°C. The heating time is 10 seconds or more in order to dissolve sufficiently. The heating time is preferably 20 seconds or more, more preferably 30 seconds or more, and even more preferably 100 seconds or more, and can be, for example, 1800 seconds or less, or even 1000 seconds or less from the viewpoint of manufacturability.

[0033] [(b) after the heating, Cooling from a cooling start temperature equal to or lower than the heating temperature (the heating temperature -300°C) to a cooling stop temperature 1 of 150°C to 230°C, or a step of cooling from a cooling start temperature equal to or lower than the heating temperature and equal to or higher than (the heating temperature -300°C) to a residence temperature range of 350°C to 440°C, allowing the temperature range to remain in the residence temperature range for 10 seconds to 200 seconds, and then cooling from the residence temperature range to a cooling stop temperature 2 of 150°C to 250°C, In either case, the average cooling rate from the cooling start temperature to 440°C is 10°C / s or more.

[0034] After the heating in (a) above, the steel is cooled to a temperature range where one or more of bainite, bainitic ferrite, and martensite are formed and a portion of the structure remains as untransformed austenite. One or more of bainite, bainitic ferrite, and martensite formed at this time become the parent phase, ensuring strength, and allowing untransformed austenite to remain between these structures. This untransformed austenite is stabilized by C enrichment during heating in the following step (c), and becomes the retained austenite that constitutes the final structure.

[0035] In step (b), the average cooling rate from the cooling start temperature, which is equal to or lower than the heating temperature (the heating temperature -300°C), to 440°C is set to 10°C / s or higher. In order to suppress the formation of ferrite, coarse bainite, or coarse bainitic ferrite during cooling from the cooling start temperature, the temperature range in which these ferrites are formed is rapidly cooled. The average cooling rate is preferably 15°C / s or higher, and more preferably 20°C / s or higher. In consideration of the performance of the equipment, etc., the upper limit of the average cooling rate is about 500°C / s.

[0036] The average cooling rate in step (b) may be 10°C / s or more from the cooling start temperature to 440°C, and the average cooling rate from 440°C to, for example, cooling stop temperature 1 or cooling stop temperature 2 is not important.

[0037] As a first aspect including cooling from the cooling start temperature to 440°C at the average cooling rate in step (b), in cooling from the cooling start temperature to a cooling stop temperature 1 of 150°C or more and 230°C or less, the average cooling rate from the cooling start temperature to 440°C is set to 10°C / s or more.

[0038] In the first embodiment, if the cooling stop temperature 1 is too low, a sufficient amount of retained austenite cannot be secured, and an excellent balance between strength and ductility cannot be secured. Therefore, the cooling stop temperature 1 is set to 150°C or higher. The cooling stop temperature 1 is preferably 160°C or higher, and more preferably 170°C or higher. On the other hand, from the viewpoint of sufficiently forming martensite in the cooling stage and suppressing the formation of bainite or bainitic ferrite in the subsequent heating stage and securing strength, the cooling stop temperature 1 is set to 230°C or lower. The cooling stop temperature 1 is preferably 220°C or lower.

[0039] A second aspect including cooling from the cooling start temperature to 440°C at the average cooling rate in step (b) includes cooling from the cooling start temperature to a residence temperature range of 350°C or more and 440°C or less, allowing the temperature to remain in the residence temperature range for 10 seconds or more and 200 seconds or less, and then cooling from the residence temperature range to a cooling stop temperature 2 of 150°C or more and 250°C or less, in which the average cooling rate from the cooling start temperature to 440°C is 10°C / s or more.

[0040] In the second embodiment, during cooling from the cooling start temperature to the cooling stop temperature 2, the temperature is maintained in a temperature range of 350°C or more and 440°C or less where bainite transformation progresses, thereby forming bainite or bainitic ferrite. By actively forming bainite or bainitic ferrite at least partially, austenite is split, and the martensite structure can be refined during the subsequent martensite transformation. As a result, a steel sheet having high strength and exhibiting an excellent strength-ductility balance and excellent stretch flangeability can be obtained. Furthermore, MA, which is the starting point of fracture during impact deformation, can be refined, thereby reducing the brittle fracture rate at -40°C and also providing excellent low-temperature toughness.

[0041] As described above, from the viewpoint of forming bainite or bainitic ferrite, the residence temperature range is set to 350°C or higher. The residence temperature range is preferably 355°C or higher, more preferably 360°C or higher. On the other hand, if the temperature of the residence temperature range is too high, coarse bainite or bainitic ferrite is likely to be formed. Therefore, from the viewpoint of suppressing the formation of these coarse structures and achieving excellent stretch flangeability and low temperature toughness, the residence temperature range is set to 440°C or lower, preferably 435°C or lower, more preferably 430°C or lower.

[0042] From the viewpoint of forming the above-mentioned structure, the residence time in the above-mentioned temperature range is 10 seconds or more, preferably 15 seconds or more, and more preferably 20 seconds or more. On the other hand, if the residence time in the above-mentioned temperature range is too long, bainite and bainitic ferrite are excessively formed, making it impossible to ensure strength, so the residence time is 200 seconds or less, preferably 170 seconds or less, and more preferably 160 seconds or less.

[0043] After staying in the staying temperature range, the steel is cooled to a cooling stop temperature 2 of 150°C or more and 250°C or less. The reason for setting the lower limit of the cooling stop temperature 2 is the same as that of the cooling stop temperature 1 described above. If the cooling stop temperature 2 is too low, a sufficient amount of retained austenite cannot be secured, and an excellent balance between strength and ductility cannot be secured. Therefore, the cooling stop temperature 2 is set to 150°C or more. The cooling stop temperature 2 is preferably 160°C or more, more preferably 170°C or more. On the other hand, if the cooling stop temperature 2 is too high, it becomes difficult to ensure strength. As in the second embodiment, when a stay is provided in the staying temperature range, the bainite transformation has already progressed partially due to the stay, so that the upper limit of the cooling stop temperature 2 can be up to 250°C. The cooling stop temperature 2 may further be 230°C or less, or even 220°C or less. In this specification, "stay" includes not only a constant temperature within a certain temperature range, but also a fluctuating temperature. The same applies to a stay in the reheating temperature range below.

[0044] [(c) a step of reheating from the cooling stop temperature 1 or the cooling stop temperature 2 to a reheating temperature range of 380°C or more and 480°C or less, and allowing the reheating temperature range to remain for 5 seconds or more and 1800 seconds or less]

[0045] After cooling to cooling stop temperature 1 or cooling stop temperature 2 in the step (b), the material is reheated from cooling stop temperature 1 or cooling stop temperature 2 to a reheating temperature range of 380°C or more and 480°C or less, and is allowed to remain in the reheating temperature range for 5 seconds or more and 1800 seconds or less.

[0046] By tempering one or more of the bainite, bainitic ferrite, and martensite formed in the step (b) in this step, the ductility of the parent phase can be improved. Furthermore, by promoting the distribution of C from these structures to untransformed austenite in this step and increasing the C concentration in the untransformed austenite, it is possible to make it remain as retained austenite even when finally cooled to room temperature, and as a result, it is possible to improve the strength-ductility balance. In order to achieve this, the reheating temperature range is 380°C or more and 480°C or less. The reheating temperature range is preferably 385°C or more, more preferably 390°C or more, and preferably 470°C or less, more preferably 460°C or less. The heating time in the reheating temperature range is 5 seconds or more in order to perform sufficient tempering. The heating time is preferably 10 seconds or more, more preferably 100 seconds or more, and even more preferably 300 seconds or more, and from the viewpoint of manufacturability, it is 1800 seconds or less, preferably 1000 seconds or less.

[0047] [(d) Step of cooling to room temperature after the reheating of (c)] After the reheating in (c) above, the mixture may be cooled to room temperature by any method, including water cooling, air cooling, and natural cooling.

[0048] In the method for producing a steel sheet according to the present embodiment, the steps in the heat treatment other than the steps (a) to (d) are not particularly limited, and may include, for example, shape correction and electroplating after the heat treatment.

[0049] In addition, the manufacturing method of the rolled material used in the manufacturing method of the steel sheet according to the present embodiment is not limited. The rolled material may be a hot-rolled sheet obtained by hot rolling, or a cold-rolled sheet obtained by subjecting a hot-rolled sheet to an appropriate pickling or the like as necessary and then subjecting the hot-rolled sheet to cold rolling. In the hot rolling, the heating temperature of the final hot rolling is preferably 1150°C or higher. In addition, the thickness of the hot-rolled sheet is not particularly limited, but when a cold-rolled sheet is obtained as a final product, the thickness of the hot-rolled sheet may be selected so that the cold rolling reduction ratio is 30 to 70% according to the thickness of the cold-rolled sheet.

[0050] The steel sheet obtained by the above manufacturing method has high strength, an excellent balance of strength and ductility, and also has excellent stretch flangeability. The above steel sheet has a tensile strength (TS) of 1470 MPa or more, a TS×uEL, which indicates the balance between tensile strength and uniform elongation (uEL), of 17500 MPa% or more, and further has excellent stretch flangeability with a hole expansion ratio (λ) of 20% or more. Furthermore, according to the manufacturing method of this embodiment, it is possible to simultaneously provide excellent low-temperature toughness, in which the brittle fracture surface ratio when a Charpy test is performed at -40°C is preferably suppressed to 50% or less. EXAMPLES

[0051] The present embodiment will be described in more detail below with reference to examples. The present disclosure is not limited to the following examples, and may be modified as appropriate within the scope of the above and below-described aims, and all such modifications are within the technical scope of the present disclosure.

[0052] 1. Sample Preparation Steel (ingot) having the chemical composition shown in Table 1 was melted by vacuum melting. In Table 1, an element marked with a line (-) means that the component was not detected. In Tables 1 to 3, underlined values ​​indicate that the value is outside the range of this embodiment or that characteristics above a certain level were not obtained. However, it should be noted that "-" is not underlined even if the value is outside the range of this embodiment.

[0053] The obtained ingot was hot rolled twice, and in the final hot rolling, the heating temperature was held at 1200°C for 300 seconds, and then hot rolling was performed in multiple passes to produce a 3.2 mm hot-rolled sheet. The target temperature for the final rolling was 900°C. In order to simulate the coiling process of hot rolling, the sheet was held in a 600°C holding furnace for 30 minutes or more, and then cooled in the furnace to obtain a hot-rolled sheet.

[0054] The hot-rolled sheets were pickled to remove surface scale, and then cold-rolled to 1.6 mm to obtain rolled sheets. These were then heat-treated in a salt bath under the conditions shown in Table 2. In Table 2, Nos. 1 to 9 were heated to the Ac3 point or higher and then cooled to the cooling stop temperature in Table 2, while Nos. 10 to 14 were heated to the Ac3 point or higher, allowed to remain at the temperature shown in Table 2, and then cooled to the cooling stop temperature in Table 2. All of them were then reheated at the temperature and time shown in Table 2.

[0055] As for the average cooling rate, since it is difficult to measure the cooling rate of the sample itself in a salt bath, a dummy material was used to measure the average cooling rate under the same heating temperature, residence time, and cooling temperature conditions. For cases where there were no conditions that matched the above-mentioned dummy material, the average cooling rate was estimated from the results of other experiments.

[0056] 2. Characterization (tensile strength, uniform elongation) The steel sheets obtained above were processed into JIS No. 5 test pieces, and tensile tests were conducted according to the JIS Z2241 metallic material tensile test method to determine the tensile strength (TS) and uniform elongation (plastic elongation at maximum test force). Steels with a tensile strength (TS) of 1470 MPa or more and a TS×uEL, which indicates the balance between tensile strength and uniform elongation (uEL), of 17500 MPa% or more were evaluated as having high strength and excellent strength-ductility balance.

[0057] (Hole expansion ratio) The hole expansion ratio (λ) was measured using the steel sheets obtained above according to the hole expansion test method for metal materials of JIS Z2256. A hole expansion ratio (λ) of 20% or more was evaluated as excellent in stretch flangeability. Note that the hole expansion ratio was not evaluated for No. 14 in Tables 1 to 3 because the tensile strength was less than 1470 MPa.

[0058] (Brittle fracture rate) For some of the steel plates, a rectangular shape having a length of 55 mm and a cross section of 10 mm x 1.6 mm (plate thickness) was cut out from the obtained steel plates with reference to JIS Z2242, and a V-notch was provided to prepare a Charpy impact test specimen. The Charpy impact test specimen was then subjected to a Charpy impact test at -40°C to measure the brittle fracture ratio. The evaluation results are shown in Table 3.

[0059] [Table 1]

[0060] [Table 2]

[0061] [Table 3]

[0062] The following can be seen from Tables 1 to 3. Nos. 3, 4, 6 to 8, and 11 to 13 were all produced by the production method according to this embodiment, and thus the steel sheets had high strength, an excellent balance of strength and ductility, and also had excellent stretch flangeability. In addition, a comparison of No. 8 with Nos. 11 to 13 shows that the steel sheet production method, in which the steel sheet is heated to a temperature of the Ac3 point or higher and then maintained in a temperature range of 350°C or higher and 440°C or lower between the cooling start temperature and the cooling stop temperature, also provides excellent low-temperature toughness.

[0063] In contrast, No. 1 had a poor balance of strength and ductility due to an insufficient amount of Si, and No. 2 had poor stretch flangeability due to the lack of Ti.

[0064] No. 5 had a poor balance of strength and ductility due to the low reheating temperature.

[0065] In No. 9, high strength was not obtained because the cooling stop temperature was high.

[0066] In No. 10, the average cooling rate during cooling after heating to a temperature of the Ac3 point or higher was slow, and the temperature at which the material remained during cooling to a temperature range of 150 to 250° C. was high, resulting in poor stretch flangeability.

[0067] No. 14 did not achieve high strength due to an insufficient C content.

Claims

[Claim 1] The chemical composition is C: 0.32 mass% or more, 0.48 mass% or less, Si: 2.1% by mass or more, 3.0% by mass or less, Mn: 1.6% by mass or more, 3.0% by mass or less, Ti: 0.010% by mass or more and 0.15% by mass or less, P: more than 0 mass%, 0.05 mass% or less, S: more than 0% by mass, 0.01% by mass or less, Al: 0.01% by mass or more and 0.1% by mass or less, and N: more than 0 mass% and 0.010 mass% or less; The balance being Fe and unavoidable impurities, A method for producing a steel sheet, comprising carrying out a heat treatment including the following steps (a) to (d). (a) a step of heating the rolled material for 10 seconds or more at a heating temperature equal to or higher than the Ac3 point calculated from the following formula (1) (b) after said heating, Cooling from a cooling start temperature equal to or lower than the heating temperature and equal to or higher than (the heating temperature - 300 ° C) to a cooling stop temperature 1 of 150 ° C or higher and 230 ° C or lower, or a step of cooling from a cooling start temperature equal to or lower than the heating temperature and equal to or higher than (the heating temperature - 300°C) to a residence temperature range of 350°C to 440°C, allowing the temperature range to remain for 10 seconds to 200 seconds, and then cooling from the residence temperature range to a cooling stop temperature 2 of 150°C to 250°C, In any case, the average cooling rate from the cooling start temperature to 440°C is 10°C / s or more. (c) Reheating from the cooling stop temperature 1 or the cooling stop temperature 2 to a reheating temperature range of 380° C. or more and 480° C. or less, and allowing the material to stay in the reheating temperature range for 5 seconds or more and 1,800 seconds or less. (d) A step of cooling the mixture to room temperature after the reheating step (c). Ac 3 points (℃) = 910-203√(C) + 44.7Si-30Mn-11Cr+31.5Mo-20Cu-15.2Ni+700P...(1) In formula (1), the element symbols indicate the contents (mass%) of the elements contained in the rolled material, and elements that are not contained are represented as zero.

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