JP2017150052A - High strength steel sheet excellent in toughness and ductility and manufacturing method therefor - Google Patents

Abstract

A high-strength steel plate excellent in toughness and ductility and a method for producing the same are provided. SOLUTION: It has a predetermined component composition, the structure is an area ratio, ferrite is 70% or more, and the average grain size at a position of 50 μm in the thickness direction from the steel sheet surface is 3500 × [tensile strength TS. (MPa)] -0.85μm or less, average grain size at 1/4 part thickness from steel plate surface is 4000 × [Tensile strength TS (MPa)] -0.85μm or less, at steel sheet surface from 1/4 part thickness The ratio of the average particle size in the sheet thickness direction to the average particle size in the rolling direction is 1.5 or less, the amount of C in the precipitate having a particle diameter of less than 20 nm deposited in the steel is 0.020% by mass or more, and the arithmetic average roughness Ra is 3.0 μm or less. [Selection figure] None

Description

  The present invention can be applied to undercarriage members such as lower arms and frames of automobiles, skeleton members such as pillars and members and reinforcing members thereof, door impact beams, seat members, inflator members, vending machines, desks, home appliances / OA devices, building materials, etc. The present invention relates to a high-strength steel sheet excellent in toughness and ductility optimum as a structural member to be used, and a method for producing the same.

In recent years, there has been an increasing demand for reducing CO ₂ emissions in response to growing interest in the global environment. Furthermore, in the automobile field and the like, there is an increasing demand for reducing the amount of exhaust gas while improving fuel efficiency by reducing the body. There is also a high need for collision safety. Thinning the parts used is most effective for reducing the weight of automobiles. In other words, in order to reduce the weight while maintaining the strength of the automobile, it is effective to reduce the thickness of the steel sheet by increasing the strength of the steel sheet used as the material for automobile parts, and there is a great demand for reducing the thickness of the steel sheet. It is getting stronger.

  Generally, toughness and ductility often decrease due to an increase in strength of a steel sheet. The risk of cracking during pressing and brittle cracking of parts is greatly increased by increasing the strength of the steel sheet.

  Conventionally, as a high-strength steel sheet excellent in toughness and ductility, for example, in Patent Document 1, in mass%, C: 0.01% or more and 0.12% or less, Si: 0.02% or more and 1.3% or less, Mn: 0.5% or more and 3.0% P: 0.10% or less, S: 0.010% or less, sol.Al: 0.11% or more and 1.5% or less, N: 0.0010% or more and 0.020% or less, Ti: 0.041% or more and 0.40% or less, and ferrite in volume% There is a manufacturing technology for high-tensile hot-rolled steel sheets that has a metal structure containing 70% or more of the phase and a residual austenite phase of 4.9% or less, and the ratio of the yield strength to the tensile strength in the direction perpendicular to the rolling is 0.81 or more. It is disclosed.

  Patent Document 2 includes mass%, C: 0.02 to 0.10%, Si: 1.50% or less, Mn: 0.5 to 2.0%, P: 0.025% or less, S: 0.010% or less, Al: 0.02 to 0.15%, Ni 1% or less, Cr: 1% or less, Nb: 0.08% or less, Ti: 0.05-0.20%, and the manufacturing technology of high-strength hot-rolled steel sheet whose metal structure is substantially a single phase structure of ferrite is disclosed. Has been.

  Patent Document 3 contains, in mass%, C: 0.01 to 0.20%, Si: 1.00% or less, Mn: 2.00% or less, Al: 0.10% or less, N: 0.0070% or less, and an average particle size of 2 to 3 μm. Has disclosed a technology for producing a hot-rolled high-tensile steel sheet in which the fine ferrite has an area ratio of 70% or more and the structure containing bainite and martensite has an area ratio of 20% or less.

  However, the technique described in Patent Document 1 has a problem of low toughness. Further, the technique described in Patent Document 2 has a problem that the toughness is insufficient and the ductility is low. The technique described in Patent Document 3 has a problem that it is difficult to increase the strength exceeding 700 MPa.

JP 2004-27249 A JP 2006-274317 A JP 63-145746 JP

  An object of this invention is to provide the high strength steel plate excellent in toughness and ductility, and its manufacturing method in view of this situation.

  We conducted intensive research to solve the problem. As a result, the following knowledge was obtained. Ductility is improved by making the structure mainly composed of precipitation strengthened ferrite. By reducing the surface texture of the steel sheet and reducing the surface roughness of the steel sheet, cracks are suppressed, and fine precipitates and fine grain inside the sheet thickness are reduced. By reducing the ratio of the average grain size in the plate thickness direction to the average grain size (hereinafter also referred to as the crystal grain aspect ratio), the propagation of brittle cracks is suppressed and the toughness is improved.

This invention is made | formed based on the above knowledge, and makes the following a summary.
[1] Component composition is mass%, C: 0.04-0.20%, Si: 0.6-1.5%, Mn: 1.0-3.0%, P: 0.10% or less, S: 0.030% or less, Al: 0.10% or less, N: 0.010% or less, Ti, Nb, V, or one or more of 0.01 to 1.0% each, the balance is made of iron and inevitable impurities, the structure is the area ratio, 70% ferrite The average grain size at a position of 50 μm from the surface of the steel sheet to the depth of the sheet thickness is 3500 × [tensile strength TS (MPa)] ^−0.85 μm or less, and the average at the thickness of 1/4 part from the surface of the steel sheet. The grain size is 4000 × [Tensile strength TS (MPa)] ^−0.85 μm or less, and the ratio of the average grain size in the plate thickness direction to the average grain size in the rolling direction from the steel sheet surface to 1/4 part of the plate thickness is 1.5 or less. A high-strength steel sheet with excellent toughness and ductility, characterized in that the amount of C in the precipitate with a particle diameter of less than 20 nm deposited in steel is 0.020% by mass or more and the arithmetic average roughness Ra is 3.0 μm or less. .
[2] In addition to the component composition, the toughness and ductility according to the above [1], containing 0.005 to 0.50% of one or more of Mo, Ta, and W, respectively, by mass% Excellent high strength steel plate.
[3] In the above [1] or [2], in addition to the above component composition, 0.01% to 1.0% of one or more of Cr, Ni, and Cu are contained by mass%, respectively. High strength steel plate with excellent toughness and ductility.
[4] The composition according to any one of the above [1] to [3], wherein, in addition to the component composition, one or two of Ca and REM are contained by 0.0005 to 0.01% by mass%, respectively. High strength steel plate with excellent toughness and ductility.
[5] High strength excellent in toughness and ductility according to any one of the above [1] to [4], wherein, in addition to the component composition, Sb: 0.005 to 0.050% is contained in mass%. steel sheet.
[6] High strength excellent in toughness and ductility according to any one of the above [1] to [5], wherein B: 0.0005 to 0.0030% is contained in mass% in addition to the component composition steel sheet.
[7] The high-strength steel sheet having excellent toughness and ductility according to any one of [1] to [6], wherein the steel sheet surface has a plating layer.
[8] The steel slab having the composition according to any one of [1] to [6] above is directly cast or reheated to 1200 ° C. or higher after casting, and then after rough rolling and before finish rolling. In addition, it performs descaling with a collision pressure of 3 MPa or more, performs hot rolling with a cumulative reduction ratio of 950 ° C. or less of 0.7 or more and a finish rolling outlet temperature of 800 ° C. or more, and then finishes the finish rolling at 750 ° C. Cooling at a maximum impact pressure of 5 kPa or higher and an average cooling rate of 30 ° C / s or higher, then cooling to a winding temperature of 530 ° C or higher and 680 ° C or lower at an average cooling rate of 10 ° C / s or higher. A method for producing a high-strength steel sheet having excellent toughness and ductility, characterized by winding at a temperature of ℃ to 680 ℃.
[9] The method for producing a high-strength steel sheet having excellent toughness and ductility according to the above [8], further comprising pickling after the winding.
[10] Furthermore, after the pickling, annealing at a soaking temperature of 750 ° C. or lower is performed, and then hot dipping treatment is performed. Method.
[11] The high-strength steel sheet having excellent toughness and ductility according to the above [10], wherein the alloying treatment is performed after the hot dipping treatment at an alloying treatment temperature of 460 to 600 ° C. and a holding time of 1 s or longer. Manufacturing method.
[12] The method for producing a high-strength steel sheet having excellent toughness and ductility according to the above [9], further comprising electroplating after the pickling.
[13] The process as described above, wherein after the winding, the pickling, the hot dipping treatment, the alloying treatment, or the electroplating treatment, a sheet thickness reduction rate of 0.1 to 3.0% is applied. [8] A method for producing a high-strength steel sheet having excellent toughness and ductility according to any one of [12].
[14] A method for producing a high-strength steel sheet excellent in toughness and ductility, characterized by plating the high-strength steel sheet according to any one of [1] to [6].

  In the present invention, the high-strength steel sheet is a steel sheet having a tensile strength (TS) of 780 MPa or more, and surface treatment such as hot-rolled steel sheet, hot-dip galvanizing treatment, alloyed hot-dip galvanizing treatment, and electrogalvanizing treatment. Including a steel sheet obtained by applying to a hot-rolled steel sheet. Furthermore, the steel sheet which has a film | membrane by a chemical conversion treatment etc. on the hot-rolled steel plate and the steel plate which performed the surface treatment is also included. In the present invention, it is assumed that the ductility-brittle transition temperature (DBTT) by the Charpy impact test is -40 ° C. or less and the toughness is excellent. In addition, it is assumed that TS × U-El of a tensile test performed using a JIS No. 5 test piece is 7000 (MPa ·%) or more and has excellent ductility.

  According to the present invention, a high-strength steel sheet having excellent toughness and ductility can be obtained. The high-strength steel sheet of the present invention has a tensile strength of 780 MPa or more, and is excellent in toughness and ductility. Therefore, the high-strength steel sheet can be suitably used for the use of automobile structural members and the like, and has an industrially beneficial effect.

FIG. 1 is a graph showing the relationship between the ductile-brittle transition temperature (DBTT) and the amount of precipitated C having a particle diameter of less than 20 nm. Fig. 2 is a diagram showing the relationship between the ductile-brittle transition temperature (DBTT) with respect to the value obtained by dividing the average particle diameter at the surface layer of 50 µm from the steel sheet surface by 3500 x [TS (MPa)] ^-0.85 . . 3 4000 × the average particle size in the sheet thickness 1/4 parts from the surface of the steel sheet [TS (MPa)] ductility for divided by ^-0.85 - is a graph showing a relationship brittle transition temperature (DBTT). Fig. 4 shows the ratio of ductile-brittle transition temperature (DBTT) to the ratio of the average grain size in the thickness direction to the average grain size in the rolling direction at a thickness of 1/4 part from the steel sheet surface (plate thickness 1/4 part aspect ratio). It is a figure which shows a relationship. FIG. 5 is a graph showing the relationship between the ductile-brittle transition temperature (DBTT) and the arithmetic average roughness. FIG. 6 is a diagram showing the relationship between the ferrite area ratio and TS × U-El.

  Hereinafter, the present invention will be described in detail. In addition, the following% shall mean the mass% unless there is particular notice.

  First, the reason for limiting the component composition of the high-strength steel sheet of the present invention will be described.

C: 0.04-0.20%
C forms fine carbides with Ti, Nb, and V, and contributes to increasing the strength of steel sheets and improving the grain size and toughness. In order to obtain such an effect, the C content needs to be 0.04% or more. When more strength is required, it is preferably 0.06% or more, more preferably 0.08% or more. On the other hand, a large amount of C suppresses ferrite transformation. Excessive C lowers weldability and leads to the formation of a large amount of cementite, greatly reducing toughness and formability. Moreover, a bainite transformation will be suppressed and a martensitic transformation will be promoted. Therefore, the C content needs to be 0.20% or less. Preferably it is 0.15% or less, More preferably, it is 0.12% or less.

Si: 0.6-1.5%
Si promotes the formation of fine carbides of Ti, Nb, and V in the cooling process after hot rolling. It also has the effect of promoting ferrite transformation and reducing the crystal grain aspect ratio. Furthermore, it is possible to contribute to increasing the strength of the steel sheet without greatly reducing the formability as a solid solution strengthening element. In order to obtain such an effect, the Si content needs to be 0.6% or more. On the other hand, when Si is contained in a large amount, a surface pattern called a red scale is generated, and the surface roughness of the steel sheet is increased. Further, ferrite transformation in the cooling process after hot rolling is promoted too much, and Ti, Nb, and V carbides are coarsely precipitated. Furthermore, toughness is reduced. Moreover, since it becomes easy to produce | generate the oxide of Si on the surface, it becomes easy to produce defects, such as a chemical conversion treatment defect in a hot-rolled steel plate, and non-plating in a plated steel plate. Therefore, the Si content needs to be 1.5% or less. Accordingly, the Si content is set to 0.6% to 1.5%, preferably 0.8% to 1.2%.

Mn: 1.0-3.0%
Since Mn slows the timing at which ferrite transformation starts in cooling after hot rolling, it is effective in reducing the structure of the steel sheet. Furthermore, Mn can also contribute to increasing the strength of the steel sheet by solid solution strengthening. It also has the effect of detoxifying harmful S in steel as MnS. In order to obtain such an effect, the Mn content needs to be 1.0% or more. Preferably it is 1.3% or more. On the other hand, a large amount of Mn causes slab cracking and suppresses the progress of ferrite transformation, resulting in an increase in the aspect ratio of the crystal grains. Moreover, the formation of fine carbides due to C and Ti, Nb, and V is suppressed. Therefore, the Mn content needs to be 3.0% or less. Preferably it is 2.3% or less, More preferably, it is 1.6% or less.

P: 0.10% or less
P has the effect of reducing weldability and segregates at the grain boundaries to deteriorate the ductility and toughness of the steel sheet. Further, when P is contained in a large amount, ferrite transformation is promoted in the cooling process after hot rolling, and carbides of Ti, Nb, and V are coarsely precipitated. From the above, the P content needs to be 0.10% or less. Preferably it is 0.05% or less, More preferably, it is 0.03% or less, More preferably, it is 0.01% or less. However, since reducing P more than necessary causes an increase in manufacturing cost, the lower limit value of P is preferably 0.001%.

S: 0.030% or less
S has the effect of lowering the weldability and significantly lowers the ductility in hot rolling, so induces hot cracking and significantly deteriorates the surface properties. Moreover, S hardly contributes to the strength improvement of the steel sheet. Furthermore, by forming a coarse sulfide as an impurity element, the ductility, toughness and stretch flangeability of the steel sheet are lowered. Since these problems become significant when the S content exceeds 0.030%, it is desirable to reduce them as much as possible. Therefore, the S content needs to be 0.030% or less. Preferably it is 0.010% or less, More preferably, it is 0.003% or less, More preferably, it is 0.001% or less. However, since reducing S more than necessary causes an increase in manufacturing cost, the lower limit value of S is preferably 0.0001%.

Al: 0.10% or less
When a large amount of Al is contained, the toughness and weldability of the steel sheet are greatly reduced. Furthermore, since it becomes easy to produce | generate the oxide of Al on the surface, it becomes easy to produce defects, such as a chemical conversion treatment defect, in a hot-rolled steel plate, and defects, such as non-plating, in a plated steel plate. Therefore, the Al content needs to be 0.10% or less. Preferably it is 0.06% or less. There is no specific lower limit. Even if it is contained 0.01% or more as Al killed steel, there is no problem.

N: 0.010% or less
N forms coarse nitrides at high temperatures with Ti, Nb, and V. However, coarse nitrides do not contribute much to the strength improvement, so that not only the effect of increasing the strength of the steel sheet by adding Ti, Nb, and V is reduced, but also the toughness is reduced. Further, when N is contained in a large amount, slab cracking may occur during hot rolling, and surface defects may occur. Therefore, the N content needs to be 0.010% or less. Preferably it is 0.005% or less, More preferably, it is 0.003% or less, More preferably, it is 0.002% or less. However, since reducing N more than necessary directly leads to an increase in manufacturing cost, the lower limit value of N is preferably 0.0001%.

Ti, Nb, V: 0.01% to 1.0% for 1 type or 2 types or more
Ti, Nb, and V form fine carbides with C, contributing to high strength of the steel sheet and toughness. In order to obtain such an action, it is necessary to contain at least 0.01% of one or more of Ti, Nb, and V, respectively. On the other hand, even if Ti, Nb, and V are contained in a large amount exceeding 1.0%, the effect of increasing the strength is saturated. Therefore, the contents of Ti, V, and Nb must be 1.0% or less, respectively. is there.

  The balance is iron and inevitable impurities. Inevitable impurities include Sn, Mg, Co, As, Pb, Zn, O, etc., and a total of 0.5% or less is acceptable.

  With the above essential additive elements, the steel sheet of the present invention has the desired characteristics, but in addition to the above essential additive elements, the following elements can be added as necessary.

One or more of Mo, Ta, W or more, 0.005-0.50% each
Mo, Ta, and W contribute to improving the strength and toughness of the steel sheet by forming fine precipitates. In order to obtain such an effect, when Mo, Ta, and W are contained, the content of one or more of Mo, Ta, and W is set to 0.005% or more. On the other hand, since the effect is saturated even if a large amount of Mo, Ta, and W is contained, the content of one or more of Mo, Ta, and W is preferably 0.50% or less.

0.01% to 1.0% of one or more of Cr, Ni and Cu
Cr, Ni, and Cu contribute to an increase in strength and toughness of the steel sheet by making the structure of the steel sheet finer and acting as a solid solution strengthening element. In order to obtain such an effect, when Cr, Ni, or Cu is contained, the content of one or more of Cr, Ni, and Cu is set to 0.01% or more. On the other hand, even if a large amount of Cr, Ni, or Cu is contained in a large amount, the effect is not only saturated but also the manufacturing cost is increased. Therefore, the content of one or more of Cr, Ni, and Cu is increased. Each is preferably 1.0% or less.

One or two of Ca and REM 0.0005-0.01% each
Ca and REM can improve the ductility, toughness, bendability and stretch flangeability of the steel sheet by controlling the form of sulfide. In order to obtain such an effect, when Ca and REM are contained, the content of one or two of Ca and REM is set to 0.0005% or more. On the other hand, when Ca and REM are contained, the content of one or two of Ca and REM is set to 0.01% or less respectively when Ca and REM are contained because the effect is not only saturated but also the cost increases even if it is contained in a large amount. It is preferable.

Sb: 0.005 to 0.050%
Since Sb segregates on the surface during hot rolling, nitrogen can be prevented from entering the slab and formation of coarse nitrides can be suppressed. In order to acquire such an effect, when it contains Sb, it is set as 0.005% or more of content. On the other hand, when a large amount of Sb is contained, the production cost increases. Therefore, when Sb is contained, the content is made 0.050% or less.

B: 0.0005-0.0030%
B can contribute to increasing the strength and toughness of the steel sheet by refining the structure. In order to obtain such an effect, when B is contained, the content is made 0.0005% or more. Preferably it is 0.0010% or more. On the other hand, since a large amount of B may increase the rolling load during hot rolling, when B is contained, the content is made 0.0030% or less. Preferably it is 0.0020% or less.

Next, the structure etc. which are important requirements for the high-strength steel sheet of the present invention will be described.
Ferrite: 70% or more in area ratio Since ferrite is excellent in ductility, in the present invention, a steel sheet having excellent ductility is obtained by making ferrite 70% or more in area ratio. Preferably, the area ratio of ferrite is 80% or more, more preferably 90% or more. The structure other than ferrite may be pearlite, bainite, martensite, retained austenite, or the like. In addition, the area ratio of a ferrite can be measured by the method as described in the Example mentioned later. In addition, the area ratio of the ferrite phase can be set to 70% or more by controlling the manufacturing conditions, particularly the cooling rate at the time of cooling the winding temperature during winding.

Average grain size at a position of 50 μm from the surface of the steel sheet to the thickness direction: 3500 × [Tensile strength TS (MPa)] ^−0.85 μm or less Cracks at the time of brittle fracture by reducing the grain size near the surface of the steel sheet Can be suppressed. Furthermore, since the cracks tend to extend as the strength of the steel plate increases, it is necessary to reduce the particle size. Such a particle size in the vicinity of the steel sheet surface can be more accurately evaluated at a position within 50 μm inside from the surface excluding the scale in the thickness direction of the plate, rather than the evaluation at the outermost surface of the steel sheet. Therefore, in the present invention, the average particle size at a position of 50 μm in the plate thickness depth direction from the steel plate surface is defined. In the present invention, the position of 50 μm in the sheet thickness depth direction from the steel sheet surface is a position entering 50 μm in the sheet thickness direction from the steel sheet surface excluding the scale, and may also be referred to as “surface layer 50 μm position”. is there.
By setting the average particle diameter at the position of the surface layer of 50 μm to 3500 × [tensile strength TS (MPa)] − ^0.85 μm or less, the occurrence of cracks during brittle fracture can be suppressed. Preferably, the average particle size at the position of 50 μm of the surface layer is 3000 × [tensile strength TS (MPa)] ^−0.85 μm or less, more preferably 2500 × [tensile strength TS (MPa)] ^−0.85 μm or less, more preferably 2000 × [ Tensile strength TS (MPa)] ^−0.85 μm or less. The lower limit is not particularly specified, but about 0.5 μm is sufficient. The average particle diameter at the position of the surface layer of 50 μm can be measured by the method described in Examples described later. In addition, the average grain size of ferrite at the surface layer position of 50 μm can be controlled by the production conditions, particularly the cumulative rolling reduction during hot rolling and the finish rolling exit temperature.

Average grain size at 1/4 thickness from the steel sheet surface: 4000 x [Tensile strength TS (MPa)] ^-0.85 μm or less By reducing the grain size inside the sheet thickness, crack extension during brittle fracture is suppressed. can do. Furthermore, the higher the strength, the easier it is for the crack to extend, so it is necessary to make the particle size smaller. In addition, since the particle size inside such a plate thickness can be represented by the position of the plate thickness 1/4 part from the steel plate surface, in the present invention, the average particle size from the steel plate surface to the plate thickness 1/4 part is specified. I will do it. By setting the average particle diameter at a thickness of 1/4 part from the steel sheet surface to 4000 × [tensile strength TS (MPa)] − ^0.85 μm or less, the extension of cracks during brittle fracture can be suppressed. Preferably 3500 × [Tensile strength TS (MPa)] ^−0.85 μm or less, More preferably 3000 × [Tensile strength TS (MPa)] ^−0.85 μm or less, More preferably 2500 × [Tensile strength TS (MPa)] ^−0.85 It is below μm. The lower limit is not particularly specified, but about 0.5 μm is sufficient. In addition, the average particle diameter from the surface of the steel sheet to the thickness of 1/4 part can be measured by the method described in Examples described later. In addition, the average particle diameter from the steel sheet surface to the 1/4 thickness part can be controlled by the production conditions, particularly the cumulative rolling reduction during hot rolling, the finish rolling exit temperature, and the like.

Ratio of the average grain size in the thickness direction to the average grain size in the rolling direction at 1/4 part of the thickness: 1.5 or less If the grains inside the thickness are elongated in the rolling direction, cracks at the time of brittle fracture will easily extend turn into. Therefore, the ratio of the average grain size in the thickness direction to the average grain size in the rolling direction of the 1/4 thickness part (hereinafter referred to as the average grain size in the thickness direction with respect to the average grain size in the rolling direction of 1/4 part thickness). The ratio may be simply referred to as an aspect ratio) to be 1.5 or less. Preferably it is 1.3 or less. The lower limit is not specified, but is about 1.0. In addition, the ratio of the average particle size in the plate thickness direction to the average particle size in the rolling direction at a thickness of 1/4 part can be measured by the method described in the examples described later. In addition, the ratio of the average grain size in the thickness direction to the average grain size in the rolling direction at a thickness of 1/4 part is controlled by the production conditions, in particular, the cumulative reduction ratio during hot rolling, the finish rolling exit temperature, etc. be able to.

Of the precipitates precipitated in steel with a C content of 0.020% or more deposited in steel with a particle diameter of less than 20 nm, precipitates with a particle diameter of less than 20 nm can contribute to the improvement of the strength and toughness of the steel sheet. Such fine precipitates are mainly composed of carbides. Therefore, in order to obtain such an effect, the amount of C in the precipitate having a particle diameter of less than 20 nm (hereinafter sometimes abbreviated as the amount of precipitated C) needs to be 0.020% or more. Preferably it is 0.030% or more. On the other hand, even if precipitates having a particle diameter of less than 20 nm are present in the steel more than necessary, the effect of increasing the strength is saturated, so the amount of precipitated C is preferably 0.15% or less, more preferably 0.12% or less, Preferably it is 0.10% or less. The amount of precipitated C can be measured by the method described in the examples described later. Moreover, the amount of precipitated C can be made 0.020% or more by controlling the production conditions.

Arithmetic average roughness Ra is 3.0 μm or less By reducing the arithmetic average roughness of the surface of the high-strength steel sheet, the occurrence of cracks during brittle fracture can be suppressed. Therefore, the arithmetic average roughness (Ra) needs to be 3.0 μm or less. The thickness is preferably 2.0 μm or less, more preferably 1.5 μm or less, and still more preferably 1.0 μm or less. The lower limit is not particularly specified, but is preferably about 0.5 μm. The arithmetic average roughness Ra can be measured by the method described in Examples described later.

Next, the manufacturing method of the high strength steel plate of this invention is demonstrated.
The high-strength steel sheet of the present invention is a steel slab having the above component composition, after casting, directly rolled or reheated to 1200 ° C or higher, and then after rough rolling and before finish rolling, the impact pressure is 3 MPa or more. Descaling is performed, hot rolling is performed with a cumulative reduction ratio of 950 ° C or lower being 0.7 or higher, and a finish rolling outlet temperature of 800 ° C or higher, and then the maximum impact pressure of 5 kPa or higher is averaged up to 750 ° C after finishing rolling. Cooling is performed at a cooling rate of 30 ° C / s or higher, then cooled to a winding temperature of 530 ° C to 680 ° C at an average cooling rate of 10 ° C / s or higher, and wound at a winding temperature of 530 ° C to 680 ° C. Can be obtained. After winding, pickling can be performed. Furthermore, after pickling, annealing at a soaking temperature of 750 ° C. or lower can be performed, followed by hot dip galvanizing or electroplating at a plating bath temperature of 420 to 500 ° C. After the hot dip galvanizing treatment, the alloying treatment can be performed at an alloying treatment temperature of 460 to 600 ° C. and a holding time of 1 s or longer. Further, the high strength steel plate obtained as described above can be processed with a plate thickness reduction rate of 0.1 to 3.0%.

  Details will be described below.

  In the present invention, the method for melting steel is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, secondary refining may be performed in a vacuum degassing furnace. After that, slab (steel material) is produced by continuous casting due to problems in productivity and quality. It is good also as slab by well-known casting methods, such as the ingot-making-slabbing method and the thin slab continuous casting method.

Post-cast slab: Directly rolled slab after casting, or reheated slab that has become hot or cold to 1200 ℃ or higher
In order to precipitate Ti, Nb, and V finely, it is necessary to dissolve these elements in the steel before the start of hot rolling. Therefore, it is preferable to carry out hot rolling (direct rolling) by conveying the cast slab to the entry side of the hot rolling mill at a high temperature. However, once the slab after casting becomes a hot piece or a cold piece, and Ti, Nb, and V have precipitated as precipitates, the slab must be 1200 ° C or higher to re-dissolve Ti, Nb, and V It is necessary to start rough rolling after reheating. When the slab heating temperature is low, the re-dissolution of Ti, V, and Nb is hindered and the coarse carbide remains, so that the formation of fine carbide is suppressed. The holding time at 1200 ° C. or higher is not particularly limited, but is preferably 10 minutes or longer, more preferably 30 minutes or longer. From the viewpoint of operational load, the upper limit is preferably 180 minutes or less. The reheating temperature is preferably 1220 ° C. or higher, more preferably 1250 ° C. or higher.

Hot rolling: After rough rolling and before finishing rolling, descaling with impact pressure of 3 MPa or more is performed, cumulative rolling reduction of 950 ° C or lower in finish rolling is 0.7 or higher, and finish rolling exit temperature is 800 ° C or higher. In the present invention, after rough rolling and before finish rolling, descaling using high-pressure water is performed on the entrance side of the finish rolling mill. At this time, the collision pressure of the high pressure water is set to 3 MPa or more. When removing the scale, if the impact pressure is low, the scale cannot be removed and remains on the surface. When finish-rolling in that state, the remaining scale is pushed into the steel plate surface, and the surface roughness of the steel plate increases. For this reason, the collision pressure of high-pressure water on the entrance side of the finish rolling mill needs to be 3 MPa or more. Preferably it is 5 MPa or more, More preferably, it is 8 MPa or more, More preferably, it is 10 MPa or more. There is no particular upper limit, but 15 MPa is preferred. The time is not particularly limited, but is preferably 0.1 to 5 s so that the temperature of the steel plate during finish rolling does not become too low. In the above, the collision pressure is a force per unit area at which high-pressure water collides with the steel surface.

Cumulative rolling reduction at 950 ° C. or lower: When the rolling reduction at a temperature lower by 0.7 or more is increased, the ferrite grain size can be reduced. Therefore, the rolling reduction at 950 ° C. or lower is set to 0.7 or higher. Preferably it is 1.0 or more, More preferably, it is 1.3 or more, More preferably, it is 1.6 or more. The upper limit is not particularly specified, but 2.0 is preferable. The cumulative reduction ratio is the sum of the reduction ratios at each rolling mill at 950 ° C or lower when the rolling reduction ratio at each rolling mill is the sheet thickness ratio between the entry side and the exit side in finish rolling. Is the total.

Finishing rolling exit temperature: 800 ° C. or more When the finishing rolling exit temperature is lowered, ferrite transformation is excessively promoted in the cooling process after hot rolling, and Ti, Nb, and V carbides are coarsely precipitated. Furthermore, when the finishing temperature of the finish rolling is in the ferrite region, the ferrite grain size increases and Ti, Nb, and V carbides precipitate coarsely due to strain-induced precipitation. Therefore, the temperature on the finish rolling exit side is set to 800 ° C. or higher. Preferably it is 820 degreeC or more, More preferably, it is 850 degreeC or more. The upper limit of the finish rolling outlet temperature is not particularly defined, but 920 ° C. is preferable.

After the finish rolling, the maximum impact pressure is 5 kPa to 750 ° C and the average cooling rate is 30 ° C / s or more. The maximum impact pressure from the finish of finish finish rolling to 750 ° C is 5 kPa or more. When cooling the steel sheet with water, the ferrite grain size of the steel sheet surface layer can be reduced by increasing the maximum collision pressure of the cooling water. Therefore, the maximum collision pressure of the cooling water from the finish rolling to the start of 750 ° C. is set to 5 kPa or more. The pressure is preferably 10 kPa or more, more preferably 15 kPa or more. The upper limit of the maximum collision pressure is not particularly specified, but 200 kPa is preferable. In the above, the maximum collision pressure is the maximum force per unit area at which high-pressure water collides with the steel surface.
Average cooling rate from finish rolling to 750 ° C: 30 ° C / s or more When cooling rate from finish rolling to 750 ° C is small, ferrite transformation occurs at high temperature, grain size increases, Ti, Nb, V Carbide precipitates coarsely. Accordingly, the average cooling rate from the finish rolling to 750 ° C. is set to 30 ° C./s or more. Preferably it is 50 ° C./s or more, more preferably 70 ° C./s or more. The upper limit is not particularly defined, but 200 ° C./s is preferable from the viewpoint of temperature control.

Cooling from 750 ° C to coiling temperature 530 ° C to 680 ° C with an average cooling rate of 10 ° C / s or higher
When the cooling rate from 750 ° C. to the coiling temperature is slow, the ferrite transformation is promoted, so that carbides of Ti, Nb, and V are coarsened and ferrite crystal grains are coarsened. Therefore, the average cooling rate from 750 ° C. to winding is 10 ° C./s or more. Preferably it is 20 degrees C / s or more. The upper limit is not particularly defined, but 100 ° C./s is preferable from the viewpoint of temperature control.

Winding temperature: 530 ° C. or higher and 680 ° C. or lower When the winding temperature is high, the ferrite transformation is promoted, so that the carbides of Ti, Nb, and V are coarsened and the ferrite crystal grains are coarsened. For this reason, the coiling temperature needs to be 680 ° C. or lower, preferably 650 ° C. or lower. On the other hand, when the coiling temperature is low, ferrite transformation is suppressed, the ferrite fraction is reduced, and the formation of fine carbides of Ti, Nb, and V is also suppressed. Furthermore, an aspect ratio becomes large because grain growth is suppressed too much. Therefore, the winding temperature is set to 530 ° C. or higher. Preferably it is 570 degreeC or more, More preferably, it is 600 degreeC or more.

  As described above, the high-strength steel sheet of the present invention is manufactured. In the above, the finish rolling outlet temperature and the coiling temperature are the temperatures of the steel sheet surface. The average cooling rate from the finish rolling to 750 ° C. and the average cooling rate from 750 ° C. to the coiling temperature are defined based on the surface temperature of the steel sheet.

After winding, pickling (preferred conditions)
Pickling can be performed with respect to the high strength steel plate obtained by the above. The method of pickling is not particularly limited. Examples include hydrochloric acid pickling and sulfuric acid pickling. By pickling, the scale on the surface of the steel sheet is removed, and chemical conversion treatment and paint adhesion are improved. Moreover, the plating adhesiveness in the case of performing subsequent hot dipping treatment or electroplating treatment is improved.

  In addition, since the material of the high-strength steel sheet of the present invention is not affected by the plating treatment or the composition of the plating bath, any of hot dip galvanizing treatment, alloyed hot dip galvanizing treatment, and electroplating treatment should be performed as the plating treatment. Can do.

After pickling, annealing is performed at a soaking temperature of 750 ° C or lower, followed by hot dipping (preferred conditions)
After pickling, annealing is performed at a soaking temperature of 750 ° C or lower. By setting the soaking temperature to 750 ° C. or less, the coarsening of Ti, Nb, and V carbides and the coarsening of crystal grains can be suppressed. Then, it is immersed in a plating bath and a hot dipping process is performed. For example, in the case of hot dip galvanizing treatment, the plating bath is preferably 420 to 500 ° C. If the plating bath is less than 420 ° C, zinc will not melt. On the other hand, when the temperature exceeds 500 ° C., alloying of the plating proceeds excessively.

After hot dipping, alloying is performed at an alloying temperature of 460 to 600 ° C and a holding time of 1 s or longer (preferred conditions).
After the hot dip treatment, the steel sheet is reheated to 460 to 600 ° C., and kept at the reheat temperature for 1 s or longer to obtain an alloyed hot dip galvanized steel sheet. When the reheating temperature is less than 460 ° C., alloying is insufficient. On the other hand, when the temperature exceeds 600 ° C., alloying proceeds excessively. Further, when the holding time is less than 1 s, alloying is insufficient. The reheating temperature is the temperature of the steel sheet surface.

After pickling, electroplating After pickling, electroplating is performed to form zinc plating, zinc-Al composite plating, zinc-Ni composite plating, Al plating, Al-Si composite plating on the steel sheet surface can do.

By adding light processing to a high-strength steel plate obtained by processing at a thickness reduction rate of 0.1 to 3.0% or more, movable dislocations can be increased and toughness can be increased. In order to obtain this effect, it is preferable to perform light processing at a plate thickness reduction rate of 0.1% or more. More preferably, the plate thickness reduction rate is 0.3% or more. On the other hand, when the plate thickness reduction rate is large, dislocations are less likely to move due to the interaction of dislocations, and the toughness is reduced, so when performing light machining, the plate thickness reduction rate is preferably 3.0% or less, More preferably, it is 2.0% or less, More preferably, it is 1.0% or less. Here, as the light processing, rolling by a rolling roll may be applied to the steel plate, or processing by tension that gives tension to the steel plate may be used. Furthermore, a combined process of rolling and tension may be used.

Steel slabs were manufactured by melting and continuously casting molten steel having the composition shown in Table 1 by a generally known method. These slabs were hot-rolled, cooled and wound under the production conditions shown in Table 2 to obtain hot-rolled steel sheets. In addition, some were pickled (hydrochloric acid concentration: 10% by mass%, temperature: 80 ° C.), and plated under the conditions shown in Table 2.
Test pieces were collected from the high-strength steel plates obtained as described above, and the following tests and evaluations were performed. In addition, in the case of the plated steel plate, it tested and evaluated with the steel plate after plating.

Ferrite area ratio Rolling direction-Thickness direction section embedded and polished, after Nital corrosion, 3 photographs of 100 × 100μm area with a magnification of 1000 times centered on 1/4 part of thickness with scanning electron microscope (SEM) The image was obtained by photographing and processing the SEM photograph.

An average grain diameter rolling direction-plate thickness direction cross section at a surface layer of 50 μm was embedded and polished, and after Nital corrosion, EBSD measurement was performed at a measurement step of 0.1 μm, and an orientation difference of 15 ° or more was determined as a grain boundary. The measurement length at the surface layer of 50 μm excluding the scale is 500 μm, and for all the crystal grains at the surface layer of 50 μm, the respective areas are converted into circles to obtain the diameter, and the average value of the diameters is defined as the average particle diameter. .

After embedding and polishing the average grain diameter rolling direction-thickness direction cross section at a thickness of 1/4 part from the steel sheet surface, after Nital corrosion, EBSD measurement was performed at a measurement step of 0.1 μm, and the orientation difference was observed for three fields of 100 × 100 μm area. 15 ° or more was determined as the grain boundary. The diameter of each grain in the field of view was converted into a circle to determine the diameter, and the average value of the diameters was taken as the average particle diameter.

In the same field of view as the average grain size of the average grain size in the thickness direction with respect to the average grain size in the rolling direction at the thickness of 1/4 part from the surface of the steel plate (3 fields in the 100 × 100 μm region), The average particle size in the plate thickness direction was determined and determined from the value of (average particle size in the plate thickness direction) / (average particle size in the rolling direction).

Precipitation C amount First, as shown in Japanese Patent No. 4737278, in a 10% AA-based electrolyte (10% by volume acetylacetone-1% by mass tetramethylammonium chloride-methanol electrolyte) using a test piece taken from a steel plate as an anode After conducting constant current electrolysis and dissolving a certain amount of this test piece, the electrolytic solution was filtered using a filter with a pore diameter of 20 nm, and then the amount of Ti, Nb and V, and further Mo, Ta and The amount of W was determined by analysis by ICP emission spectroscopy. Assuming that Ti, Nb and V, as well as Mo, Ta and W were all carbides, the amount of precipitated C was calculated in terms of the measurement results.

Arithmetic mean roughness Ra
Ra was calculated according to JIS B0601. Ra was obtained by measuring 5 times in the direction perpendicular to the rolling direction. The Ra of the steel plate after plating was determined for the plated plate, and the Ra of the steel plate after pickling was determined for the hot-rolled steel plate.

Mechanical properties JIS No. 5 tensile test piece was cut out in the direction perpendicular to the rolling direction and the tensile test was performed according to JIS Z2241, yield strength (YP), tensile strength (TS), uniform elongation (U-El), total elongation (El). Two tests were performed, and the average value of each was used as the mechanical property value of the steel sheet. Tensile strength (TS) ≧ 780 MPa, TS × U-El of 7000 (MPa ·%) or more was considered acceptable.

A V-notch test piece conforming to JISZ2242 was prepared so that the longitudinal direction was in the direction perpendicular to the rolling direction except that the toughness plate thickness was kept at the original thickness (thickness at the time of specimen collection), and the ductile-brittle transition temperature was measured by Charpy impact test. (DBTT) was determined and evaluated. When the ductile-brittle transition temperature (DBTT) is -40 ° C or lower, the toughness is considered excellent.
The results obtained as described above are shown in Table 3.

  From Table 3, it can be seen that in the present invention example, a high-strength steel sheet having excellent toughness and ductility is obtained.

1 to 6 are arranged based on the results shown in Table 3. FIG. 1 is a diagram showing the relationship between the ductile-brittle transition temperature (DBTT) and the amount of precipitated C having a particle diameter of less than 20 nm, FIG. Fig. 3 shows the relationship between the ductile-brittle transition temperature (DBTT) and the average grain size at a surface layer of 50 µm from the steel sheet surface in the thickness direction divided by 3500 x TS ^-0.85 . Fig. 4 shows the relationship between the ductility-brittle transition temperature (DBTT) and the average grain size at 1/4 part divided by 4000 x [TS (MPa)] ^-0.85 . Fig. 5 is a graph showing the relationship of ductility-brittle transition temperature (DBTT) to the ratio of the average grain size in the sheet thickness direction to the average grain size in the rolling direction (sheet thickness 1/4 part aspect ratio) at the joint, and Fig. 5 shows the arithmetic average roughness FIG. 6 is a diagram showing the relationship between the ferrite area ratio and TS × U-El.

  FIG. 1 shows that the ductile-brittle transition temperature (DBTT) can be reduced to -40 ° C. or lower by setting the amount of precipitated C within the range of the present invention.

2 that the ductile-brittle transition temperature (DBTT) can be reduced to -40 ° C. or less by setting the average particle diameter at the surface layer of 50 μm from the steel sheet surface to the thickness direction within the range of the present invention. From the results, it can be seen that the ductile-brittle transition temperature (DBTT) can be reduced to -40 ° C. or lower by setting the average particle diameter at a thickness of 1/4 part from the steel sheet surface within the range of the present invention.

  From FIG. 4, the ratio of the average grain size in the thickness direction to the average grain size in the rolling direction at the thickness of 1/4 part from the steel sheet surface (the thickness ratio of 1/4 part thickness) is within the scope of the present invention. It can be seen that the ductile-brittle transition temperature (DBTT) can be reduced to -40 ° C or lower.

  FIG. 5 shows that the ductility-brittle transition temperature (DBTT) can be reduced to −40 ° C. or lower by setting the arithmetic average roughness within the range of the present invention.

  FIG. 6 shows that TS × U-El can be increased to 7000 MPa ·% or more by setting the ferrite area ratio within the range of the present invention.

Claims (14)

Component composition is mass%, C: 0.04-0.20%, Si: 0.6-1.5%, Mn: 1.0-3.0%, P: 0.10% or less, S: 0.030% or less, Al: 0.10% or less, N: 0.010 % Or less, containing one or more of Ti, Nb, and V in an amount of 0.01 to 1.0% each, the balance being iron and inevitable impurities,
The structure is the area ratio, ferrite is 70% or more,
The average grain size at the position of 50 μm from the steel sheet surface to the thickness direction is 3500 × [Tensile strength TS (MPa)] ^−0.85 μm or less, and the average grain size from the steel sheet surface to 1/4 part of the thickness is 4000 × [Tensile strength TS (MPa)] ^-0.85 μm or less, the ratio of the average grain size in the plate thickness direction to the average grain size in the rolling direction at a thickness of 1/4 part from the steel sheet surface is 1.5 or less,
The amount of C in the precipitate having a particle diameter of less than 20 nm precipitated in the steel is 0.020% by mass or more,
A high-strength steel sheet with excellent toughness and ductility characterized by an arithmetic average roughness Ra of 3.0 μm or less.   2. High strength with excellent toughness and ductility according to claim 1, characterized by containing 0.005 to 0.50% of one or more of Mo, Ta and W by mass% in addition to the component composition. steel sheet.   3. In addition to the component composition, it contains 0.01 to 1.0% of one or more of Cr, Ni and Cu by mass%, respectively, and has excellent toughness and ductility according to claim 1 or 2 High strength steel plate.   The toughness and ductility according to any one of claims 1 to 3, wherein 0.0005 to 0.01% of one or two of Ca and REM are contained in mass% in addition to the component composition. Excellent high strength steel plate.   The high-strength steel sheet excellent in toughness and ductility according to any one of claims 1 to 4, characterized by containing Sb: 0.005 to 0.050% in mass% in addition to the component composition.   6. The high-strength steel sheet having excellent toughness and ductility according to any one of claims 1 to 5, wherein B: 0.0005 to 0.0030% is contained in mass% in addition to the component composition.   The high-strength steel sheet excellent in toughness and ductility according to any one of claims 1 to 6, wherein the steel sheet surface has a plating layer. For the steel slab having the component composition according to any one of claims 1 to 6, after casting, direct rolling or reheating to 1200 ° C or higher,
Next, after rough rolling and before finish rolling, descaling is performed so that the impact pressure is 3 MPa or more, and hot rolling is performed so that the cumulative reduction ratio of 950 ° C or less is 0.7 or more and the finish rolling exit temperature is 800 ° C or more. ,
Next, after finishing rolling, cooling is performed to 750 ° C. with a maximum collision pressure of 5 kPa or higher and an average cooling rate of 30 ° C./s or higher.
Next, the coil is cooled at an average cooling rate of 10 ° C./s or higher to a winding temperature of 530 ° C. or higher to 680 ° C.
A method for producing a high-strength steel sheet having excellent toughness and ductility, characterized by winding at a coiling temperature of 530 ° C to 680 ° C.   9. The method for producing a high-strength steel sheet having excellent toughness and ductility according to claim 8, wherein pickling is performed after the winding.   10. The method for producing a high-strength steel sheet having excellent toughness and ductility according to claim 9, wherein after the pickling, annealing is performed at a soaking temperature of 750 ° C. or lower, followed by hot dip plating.   11. The method for producing a high-strength steel sheet having excellent toughness and ductility according to claim 10, wherein the alloying treatment is performed after the hot dipping treatment at an alloying treatment temperature of 460 to 600 ° C. and a holding time of 1 s or longer.   10. The method for producing a high-strength steel sheet having excellent toughness and ductility according to claim 9, wherein electroplating is performed after the pickling.   9. The process according to claim 8, wherein after the winding, pickling, hot dipping treatment, alloying treatment, or electroplating treatment, processing with a plate thickness reduction rate of 0.1 to 3.0% is performed. 13. A method for producing a high-strength steel sheet having excellent toughness and ductility according to any one of 12 above.   The manufacturing method of the high strength steel plate excellent in toughness and ductility characterized by performing the plating process with respect to the high strength steel plate as described in any one of Claims 1-6.

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