JP7682128B2 - Steel sheet manufacturing method - Google Patents

Description

This disclosure relates to a method for manufacturing steel sheets, and in particular to a method for manufacturing steel sheets that can be used for a variety of applications, including automotive parts.

Steel plates 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 plates 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. Generally, 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).

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

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

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

Patent Document 3 discloses a steel sheet in which the retained austenite is controlled to 10% or more, the carbon content in _the retained austenite is _0.85 % or more, and the ratio of the Mn content in the retained austenite to the average Mn content is controlled to 1.1 or more by performing a first annealing in which the steel sheet is held for 30 minutes or more in a temperature range of 300° C. to Ac 3 point after hot rolling, heating to Ac 1 point to 950° C. after cold rolling, cooling to 150 to 600° C., hot-dip galvanizing, and then cooling to 300° C. or less, followed by tempering in a temperature range of 100 to 600° C. The steel sheet has a strength of 1470 MPa or more and is shown to have excellent deformability.

Patent Document 4 shows that by including 0.35-0.60 mass% or more of carbon, 2.1-2.8 mass% of Si, 1.2-1.8 mass% of Mn, and with bainite and rice ferrite and martensite, and retained austenite and a mixed structure of martensite and austenite as the main phases, and with ferrite and rice containing 3-5% ferrite and martensite in the absence of cementite, it is possible to ensure a tensile strength of 1470 MPa or more, EL of 14% or more, and λ: 25% or more.

International Publication No. 2012/133563 JP 2015-151576 A JP 2017-053001 A JP 2020-132929 A

A wide range of research has been conducted, including the above-mentioned technologies, with the aim of improving the properties of steel sheets, but it is currently difficult to manufacture steel sheets that have high strength and high ductility as well as excellent stretch flangeability. Total elongation is usually used as an index of ductility, but since elongation is affected geometrically by the thickness of the test piece, etc., it is preferable to evaluate it using "uniform elongation," which indicates the more fundamental ductility of the material.

This disclosure has been made in light of these circumstances, and its purpose is to provide a manufacturing method for a steel sheet that is high in strength, exhibits an excellent balance of strength and ductility when ductility is evaluated by uniform elongation, and has excellent stretch flangeability.

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% 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 mass% or more and 0.1 mass% or less; and N: more than 0 mass% and 0.010 mass% or less;
The balance being 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 the 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 either case, the average cooling rate from the cooling start temperature to 440°C is 10°C/s or more. (c) A process of reheating from the cooling stop temperature 1 or the cooling stop temperature 2 to a reheating temperature range of 380°C to 480°C and allowing the reheating temperature range to remain for 5 seconds to 1800 seconds. (d) A process of cooling to room temperature after the reheating in (c). Ac3 point (°C) = 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.

The present disclosure provides a steel sheet that is high in strength, exhibits an excellent strength-ductility balance when ductility is evaluated by uniform elongation, and has excellent stretch flangeability.

After extensive research, the inventors discovered that by subjecting rolled material of a specific chemical composition to heat treatment under specified conditions, it is possible to obtain a steel sheet that is high in strength, has a good balance between strength and uniform elongation, and has excellent stretch flangeability.

To ensure the strength-ductility balance of steel plate, it is effective to use a structure that is primarily made up of one or more of bainite, bainitic ferrite, martensite, or tempered versions of these structures, while also containing a certain amount or more of retained austenite and a mixed structure of martensite/austenite to ensure ductility.

From the standpoint of chemical composition, one effective way to achieve this is to increase the content of austenite stabilizing elements such as C and Mn to ensure retained austenite, while including elements such as Si and Al that inhibit the formation of cementite to prevent C from being consumed as cementite.

From the viewpoint of manufacturing method, in order to avoid the formation of ferrite or pearlite and 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 it to promote the transformation of the untransformed austenite and the enrichment of C in the untransformed austenite, thereby securing fine retained austenite.

In the manufacturing method of this embodiment, by further including a specified 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 the amount of 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.

The manufacturing method of the high-strength steel plate of this embodiment will be described below. The chemical composition of the rolled material to be subjected to heat treatment will be described below. Note that the chemical composition below is also the chemical composition of the steel plate finally obtained.

[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.

[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 residual 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 and the parent phase becomes embrittled, resulting in a decrease in elongation and hole expansion ratio. 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.

[Mn: 1.6% by mass or more, 3.0% by mass or less]
Mn is an effective element for stabilizing austenite, suppressing the formation of ferrite and pearlite and ensuring 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, and excellent stretch flangeability cannot be ensured. 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.

[Ti: 0.010% by mass or more, 0.15% by 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 amount is set to 0.010 mass% or more. The Ti amount 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 more 0.030 mass% or more, and even more preferably 0.040 mass% or more. On the other hand, if Ti is contained in excess, the 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. The Ti content is preferably 0.13 mass% or less, and more preferably 0.12 mass% or less.

In addition to Ti, Nb and V can be mentioned 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 the Nb remains as coarse alloy carbides at the heating stage of hot rolling, i.e., 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 some of it is dissolved in solid solution when heated to the austenite single phase region during heat treatment, making it 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, which is not preferable because there is a concern that the parent phase will be precipitation strengthened and the ductility will decrease.

[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, it may remain at more than 0% by mass, for example, about 0.001% by mass.

[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 due to constraints in the manufacturing process, there are cases where more than 0% by mass, for example, about 0.001% by mass, remains.

[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, and the desired structure cannot be obtained. 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.

[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% by 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% by mass or less, preferably 0.008% by mass or less, and more preferably 0.006% by mass or less.

[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 due to the conditions of raw materials, materials, manufacturing equipment, etc. Elements such as O can also be inevitably 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, P, S, and N, which are usually preferable to have a lower content and therefore are inevitable impurities, have their composition ranges separately specified as above. For this reason, in this specification, when referring to the "unavoidable impurities" that constitute the balance, it is a concept that excludes elements whose composition ranges are separately specified.

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 the 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 either case, the average cooling rate from the cooling start temperature to 440°C is 10°C/s or more. (c) A process of reheating from the cooling stop temperature 1 or the cooling stop temperature 2 to a reheating temperature range of 380°C to 480°C and allowing the reheating temperature range to remain for 5 seconds to 1800 seconds. (d) A process of cooling to room temperature after the reheating in (c). Ac3 point (°C) = 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.

Each step in the heat treatment is explained below.

[(a) A step of heating the rolled material for 10 seconds (s) or more at a heating temperature equal to or higher than the Ac3 point calculated from the following formula (1): Ac3 point (°C) = 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 the element that is not contained is set to zero.

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 it is preferable to set the heating temperature at an upper limit of 950°C. The heating time is 10 seconds or more to sufficiently dissolve the ferrite and cementite. 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.

[(b) after the 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 either case, the average cooling rate from the cooling start temperature to 440° C. is 10° C./s or more.

After the heating in step (a), the material is cooled to a temperature range where one or more of bainite, bainitic ferrite, and martensite are formed, and some of the material remains as untransformed austenite. One or more of the 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 step (c) below, and becomes the retained austenite that constitutes the final structure.

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) or higher, 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 cooled rapidly. The average cooling rate is preferably 15°C/s or higher, and more preferably 20°C/s or higher. Taking into consideration the performance of the equipment, etc., the upper limit of the average cooling rate is about 500°C/s.

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

In the first embodiment including cooling from the cooling start temperature to 440°C at the average cooling rate in step (b), the average cooling rate from the cooling start temperature to 440°C is set to 10°C/s or more in cooling from the cooling start temperature to a cooling stop temperature 1 that is 150°C or more and 230°C or less.

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 set to 160°C or higher, and more preferably set to 170°C or higher. On the other hand, from the viewpoint of forming sufficient martensite in the cooling stage and suppressing the formation of bainite or bainitic ferrite in the subsequent heating stage, thereby securing strength, the cooling stop temperature 1 is set to 230°C or lower. The cooling stop temperature 1 is preferably set to 220°C or lower.

In the second embodiment, which includes cooling from the cooling start temperature to 440°C at the average cooling rate in step (b), the average cooling rate from the cooling start temperature to 440°C is set to 10°C/s or more in the steps of cooling from the cooling start temperature to a residence temperature range of 350°C to 440°C, staying 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 this second embodiment, during cooling from the cooling start temperature to the cooling stop temperature 2, bainite or bainitic ferrite is formed by staying in the temperature range of 350°C or more and 440°C or less where bainite transformation progresses. 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 is obtained that has high strength, an excellent strength-ductility balance, and excellent stretch flangeability. Furthermore, it is possible to refine the MA, which is the starting point of fracture during impact deformation, and thereby reduce the brittle fracture rate at -40°C and also provide excellent low-temperature toughness.

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 form. 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.

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, from the viewpoint of forming the above-mentioned structure. On the other hand, if the residence time in the above-mentioned temperature range is too long, bainite and bainitic ferrite are formed in excess, 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.

After staying in the stay 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 strength-ductility balance 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 stay temperature range, the bainite transformation has already progressed partially due to the stay, so 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.

[(c) Reheating from the 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 allowing the material to remain in the reheating temperature range for 5 seconds or more and 1,800 seconds or less]

After cooling to Cooling Stop Temperature 1 or Cooling Stop Temperature 2 in step (b), reheat 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 allow to remain in the reheating temperature range for 5 seconds or more and 1800 seconds or less.

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 the 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, the strength-ductility balance can be improved. 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 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.

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

In the method for manufacturing steel sheet according to this embodiment, the steps in the heat treatment other than (a) to (d) are not particularly limited. After the heat treatment, steps such as shape correction and electroplating may be included.

The manufacturing method of the rolled material used in the manufacturing method of the steel plate according to this embodiment is not limited. The rolled material may be a hot-rolled plate obtained by hot rolling, or a cold-rolled plate obtained by cold rolling after subjecting the hot-rolled plate to an appropriate pickling or other treatment as necessary. 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 plate is not particularly limited, but when a cold-rolled plate is obtained as the final product, the thickness of the hot-rolled plate may be selected so that the cold rolling reduction ratio is 30 to 70% depending on the thickness of the cold-rolled plate.

The steel sheet obtained by the above manufacturing method has high strength and an excellent balance of strength and ductility, and also has excellent stretch flangeability. The above steel sheet has a tensile strength (TS) of 1,470 MPa or more, a TS x uEL, which indicates the balance between tensile strength and uniform elongation (uEL), of 17,500 MPa% or more, and further exhibits 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 obtain excellent low-temperature toughness, with the brittle fracture rate suppressed to preferably 50% or less when a Charpy test is performed at -40°C.

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.

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, please note that "-" is not underlined even if the value is outside the range of this embodiment.

The obtained ingot was hot rolled twice, and in the final hot rolling, the heating temperature was held at 1200°C for 300 seconds, after which multiple passes of hot rolling were performed to produce a 3.2 mm hot-rolled sheet. The target temperature for the final rolling was 900°C. To simulate the coiling process of hot rolling, the ingot 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.

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 Ac3 or higher and then cooled to the cooling stop temperature in Table 2, while Nos. 10 to 14 were heated to Ac3 or higher, allowed to remain at the temperature shown in Table 2, and then cooled to the cooling stop temperature in Table 2. All were then reheated at the temperature and time shown in Table 2.

As it is difficult to measure the cooling rate of the sample itself in a salt bath, dummy material was used to measure the average cooling rate under the same heating temperature, duration, and cooling temperature conditions, and this was then applied. For cases where there were no conditions that matched those when using the dummy material, the average cooling rate was inferred from the results of other experiments.

2. Evaluation of properties (tensile strength, uniform elongation)
The steel plate thus obtained was processed into a JIS No. 5 test piece, and a tensile test was carried out in accordance with the JIS Z2241 metallic material tensile test method to determine the tensile strength (TS) and uniform elongation (plastic elongation at maximum test force). A steel having a tensile strength (TS) of 1470 MPa or more and a TS×uEL value indicating the balance between tensile strength and uniform elongation (uEL) of 17500 MPa% or more was evaluated as having high strength and excellent strength-ductility balance.

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

(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.

From Tables 1 to 3, the following can be seen. Nos. 3, 4, 6 to 8, and 11 to 13 were all produced using the manufacturing method according to this embodiment, and as a result, the steel sheets obtained had high strength, an excellent balance of strength and ductility, and also had excellent stretch flangeability. In addition, when comparing No. 8 with Nos. 11 to 13, it can be seen that in the manufacturing method of the steel sheets, after heating to a temperature of the Ac3 point or higher, the steel sheets are allowed to remain in a temperature range of 350°C or higher and 440°C or lower between the cooling start temperature and the cooling stop temperature, and thus excellent low-temperature toughness can also be obtained.

In contrast, No. 1 had a poor balance of strength and ductility due to an insufficient amount of Si. Also, No. 2 did not contain Ti, so it had poor stretch flangeability.

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

No. 9 did not achieve high strength due to the high cooling stop temperature.

No. 10 had poor stretch flangeability because the average cooling rate was slow when cooling after heating to a temperature above the Ac3 point, and the temperature at which it remained was high when cooling to the 150-250°C temperature range.

No. 14 did not achieve high strength due to an insufficient amount of C.

Claims (1)

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% 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 mass% or more and 0.1 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) 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 the 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 either case, the average cooling rate from the cooling start temperature to 440°C is 10°C/s or more. (c) A process of reheating from the cooling stop temperature 1 or the cooling stop temperature 2 to a reheating temperature range of 380°C to 480°C and allowing the reheating temperature range to remain for 5 seconds to 1800 seconds. (d) A process of cooling to room temperature after the reheating in (c). Ac3 point (°C) = 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.