JP2023177132A - Production method for steel sheet - Google Patents

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

The present disclosure relates to a method of manufacturing a steel plate, and particularly to a method of manufacturing a steel plate that can be used for various uses including automobile parts.

Steel plates used in the manufacture of automobile parts are required to be thinner in order to improve fuel efficiency through weight reduction, and to achieve both thinner walls and component strength, higher strength is required. ing. In addition, steel plates used in the manufacture of automobile parts are required to have high energy absorption capacity in the event of a collision, in consideration of collision safety, and are required to have high ductility. In general, improving strength reduces ductility, making it difficult to ensure energy absorption in the event of a collision. Therefore, in order to achieve high strength and high ductility, in addition to high strength by improving tensile strength (TS), it is necessary to increase ductility by improving TS×EL (elongation).

Furthermore, steel sheets used in the manufacture of automobile parts must have excellent formability in order to be processed into parts with complex shapes, and in particular, they must have excellent hole expansion ratio (λ), which is an index of local deformability. In other words, it is required to have excellent stretch flangeability.

For example, in Patent Document 1, in hot rolling, a first hot rolling including one or more reduction passes of 40% or more in a temperature range of 1000°C or higher and 1200°C or lower is performed, and the temperature is T1 + 30°C or higher and T1 + 200°C or lower. By performing _a large reduction at a temperature of A steel plate controlled within a range is disclosed. The steel plate is shown to have TS×EL>14000.

Patent Document 2 includes tempered martensite, bainite, and austenite, and after limiting ferrite to 10% or less, in bainite, 80% or more of the bainite grains have grain boundaries that are both tempered martensite and austenite. Steel plates brought into contact are disclosed. It has been shown that the steel plate has a strength of 1300 MPa or more and excellent formability.

Patent Document 3 describes that after hot rolling, the first annealing is held in a temperature range of 300°C to ₃ Ac points for 30 minutes or more, and after cold rolling, the temperature is heated to 150°C to 600°C after being heated to ₁ Ac point to 950°C. After cooling, hot-dip galvanizing is applied, followed by cooling to below 300°C and tempering in a temperature range of 100 to 600°C to reduce the retained austenite to 10% or more and the carbon content in the retained austenite to 0. A steel sheet is disclosed in which the ratio of the Mn amount in retained austenite to the average Mn amount is controlled to be 85% or more and 1.1 or more. It has been shown that the steel plate has a strength of 1470 MPa or more and excellent deformability.

Patent Document 4 discloses that the carbon content is 0.35 to 0.60% by mass or more, the Si content is 2.1 to 2.8% by mass, the Mn content is 1.2 to 1.8% by mass, and bainite and rice are used. The main phase is tick ferrite and martensite, retained austenite and martensite/austenite mixed structure, and 3 to 5% of tick ferrite/martensite is contained in ferrite and rice, which has no cementite, resulting in a tensile strength of 1470 MPa or more and EL14. % or more, and it has been shown that it is possible to secure λ: 25% or more.

International Publication No. 2012/133563 pamphlet Japanese Patent Application Publication No. 2015-151576 Japanese Patent Application Publication No. 2017-053001 Japanese Patent Application Publication No. 2020-132929

A wide range of studies have been conducted to improve the properties of steel sheets, including the techniques described above, but it is currently difficult to manufacture steel sheets that achieve high strength and ductility and have excellent stretch flangeability. be. Note that total elongation is usually used as an indicator of ductility, but since elongation is affected by geometric factors such as the thickness of the test piece, it should be evaluated using "uniform elongation," which indicates the more essential ductility of the material. is preferred.

The present disclosure has been made in view of these circumstances, and its purpose is to provide high strength, excellent strength-ductility balance when ductility is evaluated by uniform elongation, and stretch-flangeability. The purpose of the present invention is to provide an excellent method for manufacturing steel sheets.

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% 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,
In the rolled material where the balance is Fe and unavoidable impurities,
This is a method for manufacturing a steel sheet, which includes performing 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 of Ac3 or more determined from the following formula (1) (b) After the heating,
Cooling from a cooling start temperature of not more than the heating temperature and not less than (the heating temperature -300°C) to a cooling stop temperature 1 of not less than 150°C and not more than 230°C, or
From the cooling start temperature below the above heating temperature and above (the above heating temperature -300°C), it was cooled to a staying temperature range of 350°C or above and 440°C or below, and staying in the staying temperature range for 10 seconds or more and 200 seconds or less. After that, a step of cooling from the residence temperature range to a cooling stop temperature 2 of 150 ° C. or more and 250 ° C. or less,
In either case, the average cooling rate for cooling from the cooling start temperature to 440°C is 10°C/s or more; (c) from the cooling stop temperature 1 or cooling stop temperature 2 to 380°C or more; Step (d) of reheating to a reheating temperature range of 480°C or lower and staying in the reheating temperature range for 5 seconds or more and 1800 seconds or less (d) Step of cooling to room temperature after reheating in (c) Ac 3 points (°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 elements not contained are set to zero.

According to the present disclosure, a steel plate is provided that has high strength, exhibits an excellent strength-ductility balance when ductility is evaluated by uniform elongation, and has excellent stretch flangeability.

As a result of extensive studies, the inventors of the present invention found that by heat-treating a rolled material with a specific chemical composition under predetermined conditions, it is possible to achieve high strength, a good balance between strength and uniform elongation, and excellent stretch flangeability. It was discovered that an excellent steel plate could be obtained.

In order to ensure the balance between strength and ductility of a steel plate, it is necessary to maintain ductility while maintaining a structure mainly composed of one or more of bainite, bainitic ferrite, martensite, and their tempered structures. It is effective to have a structure containing a certain amount or more of retained austenite, martensite/austenite mixed structure.

As a means of achieving these, from the viewpoint of chemical composition, in order to ensure retained austenite, the content of austenite stabilizing elements such as C and Mn is increased, while avoiding C from being consumed as cementite. It is effective to contain elements such as Si and Al that suppress the formation of cementite.

In addition, from the viewpoint of the manufacturing method, in order to avoid the formation of ferrite and pearlite and ensure the above-mentioned structure, hot-rolled sheets or cold-rolled sheets (hereinafter sometimes referred to as "rolled sheets") are After heating to the austenite single phase region, in order to avoid ferrite and pearlite transformation, it is rapidly cooled in a specific temperature range to form bainite, bainitic ferrite, and martensite, while some become fine untransformed austenite. In order to achieve this, it is effective to cool to below the Ms point and above the Mf point, and then reheat to promote the transformation of untransformed austenite and the concentration of C in untransformed austenite, thereby ensuring fine residual austenite. It is.

In the manufacturing method of this embodiment, by further containing a predetermined amount of Ti in the chemical composition, it is possible to achieve a refinement of the prior austenite grain size, and as a result, it is possible to secure retained austenite and to refine the retained austenite. It was possible to achieve both improvement in elongation by ensuring the amount of retained austenite and ensuring excellent stretch flangeability by making the retained austenite fine.

Hereinafter, the method for manufacturing a high-strength steel plate of this embodiment will be explained in order. The chemical composition of the rolled material to be subjected to heat treatment will be explained below. In addition, 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, as an austenite stabilizing element, is an essential element for ensuring the amount of retained austenite, and in order to ensure desired properties including tensile strength, the amount of C is set to 0.32% by mass or more. The amount of C is preferably 0.35% by mass or more, more preferably 0.37% by mass or more, still more preferably 0.38% by mass or more. On the other hand, if the C content is too high, the size of retained austenite becomes coarse, making it impossible to ensure excellent stretch flangeability. Therefore, the amount of C is 0.48% by mass or less, preferably 0.45% by mass or less, more preferably 0.43% by mass or less, still more preferably 0.42% by mass or less.

[Si: 2.1% by mass or more, 3.0% by mass or less]
In addition to increasing the strength through solid solution strengthening, Si suppresses the formation of cementite and suppresses the decomposition of untransformed austenite in the process of heating after cooling to below the Ms point and above the Mf point. It is an element necessary to promote C concentration in transformed austenite and to ensure sufficient residual austenite during heat treatment. Therefore, the amount of Si is 2.1% by mass or more, preferably 2.2% by mass or more, more preferably 2.3% by mass or more, even more preferably 2.4% by mass or more. On the other hand, if Si is contained excessively, the effect of solid solution strengthening becomes excessive and the matrix becomes brittle, resulting in a decrease in elongation and hole expansion rate. Therefore, the amount of Si is 3.0% by mass or less, preferably 2.9% by mass or less, more preferably 2.8% by mass or less, still more preferably 2.7% by mass or less.

[Mn: 1.6% by mass or more, 3.0% by mass or less]
Mn is an effective element as an austenite stabilizing element for suppressing the formation of ferrite and pearlite and ensuring the amount of retained austenite. Therefore, the amount of Mn is set to 1.6% by mass or more. The amount of Mn is preferably 1.7% by mass or more, more preferably 1.8% by mass or more. On the other hand, if the Mn content becomes too large, the size of retained austenite becomes too large, making it impossible to ensure excellent stretch flangeability. Therefore, the amount of Mn is 3.0% by mass or less, preferably 2.8% by mass or less, more preferably 2.6% by mass or less, even more preferably 2.4% by mass or less, even more preferably 2.2% by mass. The following shall apply.

[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. The presence of fine TiC suppresses the growth of austenite grains due to the pinning effect of fine TiC when heated to the austenite single-phase region by heat treatment, and the structure formed thereafter can be refined. Stretch flangeability improves as retained austenite becomes finer. Furthermore, as the structure size becomes finer, carbon concentration in retained austenite tends to occur, and as a result, the amount of retained austenite can be secured, contributing to improved elongation. In order to exhibit these effects, the amount of Ti is set to 0.010% by mass or more. The amount of Ti is preferably 0.012% by mass or more, more preferably 0.015% by mass or more, even more preferably 0.020% by mass or more, even more preferably 0.025% by mass or more, and even more preferably 0.020% by mass or more, and even more preferably 0.025% by mass or more. It may be 0.030% by mass or more, and even more preferably 0.040% by mass or more. On the other hand, if Ti is contained excessively, hard TiN, which is a compound with N, becomes coarse and becomes a starting point of fracture, deteriorating stretch flangeability. Therefore, the amount of Ti is set to 0.15% by mass or less. The amount of Ti is preferably 0.13% by mass or less, more preferably 0.12% by mass or less.

In addition to Ti, Nb and V can be cited as carbide-forming elements, but Nb has a stronger bonding force with C than Ti, and at the C content level of the rolled material of this embodiment, the heating stage of hot rolling Most of the Nb remains coarsely as alloy carbides, 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. Further, V has a weaker bonding force with C than Ti, and a part of V becomes a solid solution when heated to the austenite single phase region during heat treatment, making it difficult to obtain a pinning effect. In addition, V dissolved in solid solution during the heating step to form the austenite single phase region may be finely precipitated during the subsequent cooling and reheating steps, and in that case, there is a concern that the parent phase will be strengthened by precipitation and the ductility will be reduced. I don't like it because of this.

[P: more than 0 mass%, 0.05 mass% or less]
P inevitably exists as an impurity element. When the P content exceeds 0.05% by mass, the elongation (EL) and hole expansion rate 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 smaller the P content is, the more preferable it is, and the most preferable is 0% by mass. However, due to constraints in the manufacturing process, more than 0% by mass, for example, about 0.001% by mass may remain.

[S: more than 0 mass%, 0.01 mass% or less]
S is inevitably present as an impurity element. When the S content exceeds 0.01% by mass, sulfide-based inclusions such as MnS are formed, and the inclusions serve as starting points for cracks, resulting in a decrease in hole expansion rate. 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 smaller the S content is, the more preferable it is, and the most preferable is 0% by mass. However, due to constraints in the manufacturing process, more than 0% by mass, for example, about 0.001% by mass may remain.

[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 material. In order to effectively exhibit such an effect, the Al content is set to 0.01% by mass or more. The Al content is preferably 0.015% by mass or more, more preferably 0.020% by mass or more. On the other hand, if the Al content is excessive, the formation of ferrite will be promoted, making it impossible to obtain the desired structure. Therefore, the Al content is set to 0.1% by mass or less. The Al content is preferably 0.08% by mass or less, more preferably 0.06% by 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 the present embodiment, N combines with Ti to form hard TiN, which becomes a starting point for fracture during deformation and particularly reduces the hole expansion rate. Therefore, although it is most preferable that the N content be 0% by mass, it inevitably remains in the manufacturing process. It is preferable to reduce the N content from the above viewpoint, 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 remainder is Fe and inevitable impurities]
The remainder is Fe and unavoidable impurities (eg, As, Sb, Sn, etc.). Unavoidable impurities are elements that are brought in depending on the conditions of raw materials, materials, manufacturing equipment, etc. Further, elements such as O may be inevitably mixed. For example, if the amount is 100 ppm or less, mixing as an impurity element can be allowed. Note that, for example, there are elements such as P, S, and N, which are preferably contained in smaller amounts and are therefore unavoidable impurities, but whose composition ranges are separately specified as described above. Therefore, in this specification, the term "inevitable impurities" constituting the remainder is a concept excluding elements whose composition range is separately defined.

The method for manufacturing a steel plate according to the present embodiment includes rolling material satisfying the above chemical composition,
It includes performing 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 of Ac3 or more determined from the following formula (1) (b) After the heating,
Cooling from a cooling start temperature of not more than the heating temperature and not less than (the heating temperature -300°C) to a cooling stop temperature 1 of not less than 150°C and not more than 230°C, or
From the cooling start temperature below the above heating temperature and above (the above heating temperature -300°C), it was cooled to a staying temperature range of 350°C or above and 440°C or below, and staying in the staying temperature range for 10 seconds or more and 200 seconds or less. After that, a step of cooling from the residence temperature range to a cooling stop temperature 2 of 150 ° C. or more and 250 ° C. or less,
In either case, the average cooling rate for cooling from the cooling start temperature to 440°C is 10°C/s or more; (c) from the cooling stop temperature 1 or cooling stop temperature 2 to 380°C or more; Step (d) of reheating to a reheating temperature range of 480°C or lower and staying in the reheating temperature range for 5 seconds or more and 1800 seconds or less (d) Step of cooling to room temperature after reheating in (c) Ac 3 points (°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 elements not contained are set to zero.

Each step in the heat treatment will be explained below.

[(a) Step of heating the rolled material for 10 seconds (s) or more at a heating temperature of Ac3 points or more determined from the following formula (1) Ac3 points (°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 elements not contained are set to zero. ]

In order to temporarily dissolve the ferrite and cementite contained in the structure of the rolled material, the rolled material is heated to an Ac level of 3 or higher. If the heating temperature is lower than the Ac3 point, soft ferrite remains and becomes a starting point for destruction 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, the austenite grains will become coarse even if Ti is included, making it difficult to ensure stretch flangeability, so the upper limit of the heating temperature should be 950°C. preferable. The heating time is set to 10 seconds or more in order to sufficiently form a solid solution. The heating time is preferably 20 seconds or more, more preferably 30 seconds or more, even more preferably 100 seconds or more, and can be, for example, 1800 seconds or less, and even 1000 seconds or less from the viewpoint of manufacturability.

[(b) After the heating,
Cooling from a cooling start temperature of not more than the heating temperature and not less than (the heating temperature -300°C) to a cooling stop temperature 1 of not less than 150°C and not more than 230°C, or
From the cooling start temperature below the above heating temperature and above (the above heating temperature -300°C), it was cooled to a staying temperature range of 350°C or above and 440°C or below, and staying in the staying temperature range for 10 seconds or more and 200 seconds or less. After that, a step of cooling from the residence temperature range to a cooling stop temperature 2 of 150 ° C. or more and 250 ° C. or less,
In either case, the average cooling rate from the cooling start temperature to 440°C shall be 10°C/s or more]

After the heating in (a) above, it is cooled to a temperature range in which at least one of bainite, bainitic ferrite, and martensite is formed and a portion remains as untransformed austenite. One or more of bainite, bainitic ferrite, and martensite formed at this time becomes the matrix, ensuring strength and further allowing untransformed austenite to remain between these structures. This untransformed austenite is stabilized by C concentration during heating in the step (c) below, and becomes residual austenite that constitutes the final structure.

In the step (b), the average cooling rate of cooling from the cooling start temperature which is below the heating temperature and above (the heating temperature -300°C) to 440°C is 10°C/s or more. 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 and the like are formed is rapidly cooled. The average cooling rate is preferably 15°C/s or more, more preferably 20°C/s or more. In addition, if the performance of equipment etc. is considered, the upper limit of the said average cooling rate is about 500 degreeC/s.

The average cooling rate in step (b) may be such that the average cooling rate from the cooling start temperature to 440°C is 10°C/s or more, and from 440°C to cooling stop temperature 1 or cooling stop temperature 2, for example. The average cooling rate for cooling up to

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

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

In the step (b), a second aspect includes cooling from the cooling start temperature to 440°C at the average cooling rate, cooling from the cooling start temperature to a staying temperature range of 350°C or more and 440°C or less. and, after staying in the staying temperature range for 10 seconds or more and 200 seconds or less, cooling from the staying temperature range to a cooling stop temperature 2 of 150 °C or more and 250 °C or less, the cooling start temperature An example of this is to set the average cooling rate for cooling from 10° C. to 440° C. to 10° C./s or more.

In this second aspect, during cooling from the cooling start temperature to the cooling stop temperature 2, bainite or bainitic ferrite is produced by staying in a staying temperature range of 350°C or more and 440°C or less where bainite transformation progresses. is formed. By actively forming bainite or bainitic ferrite at least partially, austenite is divided and the martensite structure can be refined during subsequent martensite transformation. As a result, a steel plate with high strength, excellent strength-ductility balance and excellent stretch flangeability can be obtained. Furthermore, the MA, which is the origin of fracture during impact deformation, can be made finer, thereby reducing the brittle fracture rate at -40°C and providing excellent low-temperature toughness.

As mentioned 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 in 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 440°C or less, preferably 435°C or less, more preferably 430°C or less. shall be.

The residence time in the residence 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 residence temperature range is too long, bainite and bainitic ferrite will be formed excessively, making it impossible to ensure strength. More preferably, it is 160 seconds or less.

After staying in the staying temperature range, it 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 value of the cooling stop temperature 2 is the same as the cooling stop temperature 1 mentioned above. If the cooling stop temperature 2 is too low, it will not be possible to secure a sufficient amount of retained austenite, and it will be difficult to maintain an excellent strength-ductility balance. Cannot be secured. Therefore, the cooling stop temperature 2 is set to 150°C or higher. The cooling stop temperature 2 is preferably 160°C or higher, more preferably 170°C or higher. On the other hand, if the cooling stop temperature 2 is too high, it will be difficult to ensure strength. In addition, when providing a stay in the above-mentioned stay temperature range as in the second aspect, the upper limit of the cooling stop temperature 2 can be up to 250° C. because bainite transformation has already partially progressed due to the stay. The cooling stop temperature 2 may further be 230°C or lower, and even 220°C or lower. In this specification, "stay" includes not only a constant temperature but also a temperature fluctuation within a certain temperature range. The same applies to the stay in the reheating temperature range below.

[(c) Step of 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 staying in the reheating temperature range for 5 seconds or more and 1800 seconds or less]

After cooling to cooling stop temperature 1 or cooling stop temperature 2 in the step (b), reheating from the cooling stop temperature 1 or cooling stop temperature 2 to a reheating temperature range of 380 ° C. or higher and 480 ° C. or lower, and Stay 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 matrix can be improved. Furthermore, in this step, the distribution of C from these structures to untransformed austenite is promoted, and by increasing the C concentration in untransformed austenite, it can ultimately remain as residual austenite even when cooled to room temperature. As a result, the strength-ductility balance can be improved. In order to achieve these, the reheating temperature range is set to 380°C or higher and 480°C or lower. The reheating temperature range is preferably 385°C or higher, more preferably 390°C or higher, and preferably 470°C or lower, more preferably 460°C or lower. The heating time in the reheating temperature range is set to 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, even more preferably 300 seconds or more, and from the viewpoint of manufacturability, 1800 seconds or less, preferably 1000 seconds or less.

[(d) Step of cooling to room temperature after reheating in (c) above]
After the reheating in (c) above, it may be cooled to room temperature, and any cooling method may be used, such as water cooling, air cooling, and natural cooling.

In the method for manufacturing a steel plate according to the present embodiment, there are no particular limitations on the heat treatment steps other than the above (a) to (d). After the heat treatment, it may include, for example, performing shape correction or electroplating.

Further, the method for manufacturing a rolled material used in the method for manufacturing a steel plate according to this embodiment is also not limited. The rolled material is not only a hot-rolled sheet obtained by hot rolling, but also a cold-rolled sheet obtained by subjecting a hot-rolled sheet to appropriate pickling, etc. as necessary, and then cold rolling. Good too. In the hot rolling, it is preferable that the heating temperature of the final hot rolling is 1150° C. or higher. In addition, the thickness of the hot-rolled sheet is not particularly limited, but when obtaining a cold-rolled sheet as a final product, the hot-rolled sheet is All you have to do is select the thickness.

The steel plate obtained by the above manufacturing method has high strength, excellent strength-ductility balance, and excellent stretch flangeability. The steel plate has a tensile strength (TS) of 1,470 MPa or more, TS x uEL, which indicates the balance between tensile strength and uniform elongation (uEL), of 17,500 MPa% or more, and a hole expansion rate (λ) of 20% or more. Shows excellent stretch flangeability. Furthermore, according to the manufacturing method of this embodiment, it is possible to have excellent low-temperature toughness, with the brittle fracture ratio being preferably suppressed to 50% or less when performing a Charpy test at -40°C. It is.

Hereinafter, this embodiment will be described in more detail with reference to Examples. The present disclosure is not limited by the following examples, and can be implemented with appropriate changes within the scope that can meet the spirit described above and below, and all of these are within the technical scope of the present disclosure. included in

1. Sample Preparation Steel (ingot) having the chemical composition shown in Table 1 was melted by vacuum melting. In Table 1, the elements marked with a line (-) mean that the component was not detected. Further, in Tables 1 to 3, underlined values indicate that the values are outside the range of the present embodiment or that characteristics above a certain level are not obtained. However, it should be noted that "-" is not underlined even if it is outside the scope 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, and then hot rolled in multiple passes to form a 3.2 mm hot rolled plate. was created. At this time, the target temperature during final rolling was 900°C. In addition, in order to simulate winding during hot rolling, a hot rolled sheet was obtained by holding in a holding furnace at 600° C. for 30 minutes or more and then cooling in the furnace.

The hot rolled plate was pickled to remove surface scale, and then cold rolled to a thickness of 1.6 mm to obtain a rolled plate. This was heat-treated in a salt bath under the conditions shown in Table 2. In Table 2, No. Nos. 1 to 9 were heated to Ac3 point or higher and then cooled to the cooling stop temperature shown in Table 2. For samples Nos. 10 to 14, after heating to Ac3 point or higher, they were allowed to stay at the temperature shown in Table 2, and then cooled to the cooling stop temperature shown in Table 2. Thereafter, each sample was reheated at the temperature and time shown in Table 2.

Regarding the average cooling rate, since it is difficult to measure the cooling rate of the sample itself in a salt bath, we used a dummy material to measure the average cooling rate under conditions with the same heating temperature, staying temperature, and cooling temperature. It was applied. For cases where there were no conditions consistent with the use of the above dummy material, the average cooling rate was estimated from other experimental results.

2. Evaluation of properties (tensile strength, uniform elongation)
Using the steel plate obtained above, after processing it into a JIS No. 5 test piece, a tensile test was conducted according to the method for tensile testing of metal materials of JIS Z2241, and the tensile strength (TS) and uniform elongation (plastic elongation at maximum test force) were measured. I asked for it. If the tensile strength (TS) is 1,470 MPa or more, and TS x uEL, which indicates the balance between tensile strength and uniform elongation (uEL), is 17,500 MPa% or more, it is considered to be high strength and has a strength-ductility balance. Rated as excellent.

(hole expansion rate)
Using the steel plate obtained above, the hole expansion rate (λ) was measured according to JIS Z2256 hole expansion test method for metal materials. A case where the hole expansion ratio (λ) was 20% or more was evaluated as having excellent stretch flangeability. In addition, No. in Tables 1 to 3. Regarding No. 14, since the tensile strength was less than 1470 MPa, the hole expansion rate was not evaluated.

(Brittle fracture rate)
For some steel plates, a rectangular cross section with a length of 55 mm and a cross section of 10 mm x 1.6 mm (plate thickness) is cut out from the steel plate obtained above, with reference to JIS Z2242, and then a V-notch is cut out. A Charpy impact test piece was prepared. Using the Charpy impact test piece, a Charpy impact test was conducted at -40°C to measure the brittle fracture ratio. Table 3 shows these evaluation results.

The following can be seen from Tables 1 to 3. No. 3, 4, 6-8, and No. In all of Nos. 11 to 13, steel sheets were manufactured using the manufacturing method according to the present embodiment, so that steel sheets with high strength, excellent strength-ductility balance, and excellent stretch flangeability were obtained. Also, No. 8 and no. Comparing Nos. 11 to 13, in the steel sheet manufacturing method, after heating at a temperature of Ac 3 or higher, by staying in a staying temperature range of 350 ° C or more and 440 ° C or less from the cooling start temperature to the cooling stop temperature. It can be seen that this material also has excellent low-temperature toughness.

On the other hand, No. No. 1 had a poor strength-ductility balance due to insufficient Si content. Also, No. Since sample No. 2 did not contain Ti, it had poor stretch flangeability.

No. Sample No. 5 had a poor strength-ductility balance due to the low reheating temperature.

No. In No. 9, high strength could not be obtained because the cooling stop temperature was high.

No. In No. 10, the average cooling rate during cooling after heating at a temperature of Ac 3 or higher was slow, and the temperature at which it stayed was high during cooling to a temperature range of 150 to 250°C, resulting in poor stretch flangeability.

No. In No. 14, high strength could not be obtained because the amount of C was insufficient.

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% 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,
In the rolled material where the balance is Fe and unavoidable impurities,
A method for manufacturing a steel plate, which includes 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 of Ac3 or more determined from the following formula (1) (b) After the heating,
Cooling from a cooling start temperature of not more than the heating temperature and not less than (the heating temperature -300°C) to a cooling stop temperature 1 of not less than 150°C and not more than 230°C, or
From the cooling start temperature below the heating temperature and above (the above heating temperature -300°C), it was cooled to a staying temperature range of 350°C or above and 440°C or below, and was allowed to stay in the staying temperature range for 10 seconds or more and 200 seconds or less. After that, a step of cooling from the residence temperature range to a cooling stop temperature 2 of 150 ° C. or more and 250 ° C. or less,
In either case, the average cooling rate for cooling from the cooling start temperature to 440°C is 10°C/s or more; (c) from the cooling stop temperature 1 or cooling stop temperature 2 to 380°C or more; Step (d) of reheating to a reheating temperature range of 480°C or lower and staying in the reheating temperature range for 5 seconds or more and 1800 seconds or less (d) Step of cooling to room temperature after reheating in (c) Ac 3 points (°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 elements not contained are set to zero.