CN118510925A - High-strength steel sheet and method for producing same - Google Patents

Abstract

The present invention provides a high-strength steel sheet having a tensile strength of 1320 MPa or more and excellent ductility, hole expandability and delayed fracture resistance. The high-strength steel sheet has a specific component composition containing Ti and the like, wherein the amount of diffusible hydrogen in the steel is 0.50 mass ppm or less, tempered martensite and bainite are 70.0 to 95.0%, fresh martensite is 15.0% or less, retained austenite is 5.0 to 15.0%, the average particle size of precipitates A, which are carbides, nitrides or carbonitrides containing at least one element selected from the group consisting of Ti, Nb and V, is 0.001 to 0.050 μm, the number density _NS of precipitates A having a major diameter of 0.050 μm or less is 10 pieces/ ^μm2 or more, and the ratio of the number density _NS to the number density _NL of precipitates A having a major diameter of more than 0.050 μm is 10.0 or more.

Description

High strength steel plate and method for manufacturing the same

Technical Field

The present invention relates to a high-strength steel plate and a method for manufacturing the same.

Background Art

In recent years, from the viewpoint of protecting the global environment, there has been an increasing demand for reducing the weight of automobile bodies in order to improve the fuel efficiency of automobiles.

At this time, the vehicle body is lightened while maintaining its strength. For example, it is desirable to use a high-strength steel sheet having a tensile strength (TS) of 1320 MPa or more for the frame members around the cabin of the vehicle body.

However, as the strength of steel sheets increases, the ductility of the steel sheets tends to decrease. In this case, the formability of the steel sheets is insufficient, and it is difficult to press the steel sheets into complex shapes.

Therefore, for example, Patent Documents 1 and 2 disclose technologies for achieving both strength and ductility of steel sheets.

Prior art literature

Patent Literature

Patent Document 1: Japanese Patent Application Publication No. 2019-2078

Patent Document 2: Japanese Patent Application Publication No. 2011-184756

Summary of the invention

Problem to be solved by the invention

Conventionally, hot pressing has been applied when processing high-strength steel sheets into parts and the like, but recently, the application of cold pressing has been studied in consideration of productivity.

However, delayed fracture may occur in parts obtained by cold-pressing a high-strength steel sheet having a tensile strength of 1320 MPa or more.

Delayed fracture refers to a phenomenon in which, when a stressed component is placed in a hydrogen-penetrated environment, hydrogen penetrates into the component, reducing the interatomic bonding force or causing local deformation, thereby causing microcracks, which then propagate and cause the component to fracture.

Therefore, high-strength steel sheets are required to have not only sufficient formability (ductility and hole expandability) but also good delayed fracture resistance.

The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a high-strength steel sheet having a tensile strength of 1320 MPa or more, excellent formability (ductility and hole expandability), and delayed fracture resistance.

Methods used to solve problems

The present inventors have conducted intensive studies and, as a result, have found that the above-mentioned object can be achieved by adopting the following configuration, thereby completing the present invention.

That is, the present invention provides the following [1] to [8].

[1] A high-strength steel sheet having a composition containing, in mass%, 0.130 to 0.350% C, 0.50 to 2.50% Si, 2.00 to 4.00% Mn, 0.100% or less P, 0.0500% or less S, 0.010 to 2.000% Al, 0.0100% or less N, and at least one element selected from the group consisting of 0.001 to 0.100% Ti, 0.001 to 0.100% Nb, and 0.001 to 0.500% V, with the balance being Fe and unavoidable impurities. The steel has a composition and a microstructure, wherein the amount of diffusible hydrogen in the steel is 0.50 mass ppm or less, and in the above microstructure, the total area ratio of tempered martensite and bainite is 70.0 to 95.0%, the area ratio of fresh martensite is 15.0% or less, the area ratio of retained austenite is 5.0 to 15.0%, the average particle size of precipitates A which are carbides, nitrides or carbonitrides containing at least one element selected from the group consisting of Ti, Nb and V is 0.001 to 0.050 μm, the number density _NS of precipitates _AS which are precipitates A having a major diameter of 0.050 μm or less is 10 pieces/ ^μm2 or more, and the ratio _NS / _NL of the number density _NS of precipitates AS to the _number density _NL of precipitates _A which are precipitates A having a major diameter of more than 0.050 μm is 10.0 or more.

[2] The high-strength steel sheet according to [1], wherein the chemical composition further contains, in mass %, at least one element selected from the group consisting of W: 0.500% or less, B: 0.0100% or less, Ni: 2.000% or less, Co: 2.000% or less, Cr: 1.000% or less, Mo: 1.000% or less, Cu: 1.000% or less, Sn: 0.500% or less, Sb: 0.500% or less, Ta: 0.100% or less, Zr: 0.200% or less, Hf: 0.020% or less, Ca: 0.0100% or less, Mg: 0.0100% or less, and REM: 0.0100% or less.

[3] The high-strength steel sheet according to [1] or [2] above, which has a plating layer on the surface.

[4] The high-strength steel sheet according to [3] above, wherein the plating layer is an alloy plating layer.

[5] A method for producing a high-strength steel sheet, which is a method for producing the high-strength steel sheet described in [1] or [2] above, wherein a steel billet having the composition described in [1] or [2] above is heated to 1100°C or higher, hot rolled at a finish rolling end temperature of 850 to 950°C to obtain a hot-rolled steel sheet, the hot-rolled steel sheet is coiled at a coiling temperature T of 400 to 700°C, held, and then cold rolled to obtain a cold-rolled steel sheet, the cold-rolled steel sheet is heat treated, the total time during the holding time during which the temperature of the hot-rolled steel sheet after coiling is at least the coiling temperature T-50°C is t, and the following formula 1 is satisfied; in the heat treatment, the cold-rolled steel sheet is held in a temperature range T1 of 800 to 950°C for 30 seconds or longer, then cooled to a cooling stop temperature T2 of 150 to 250°C, and then held in a temperature range T3 of 250 to 400°C for 30 seconds or longer.

Formula 1: 0.001＜[1.17×10 ^-6 ×{t/(T+273.15)}] ^1/3 ＜0.050

[6] A method for manufacturing a high-strength steel plate according to the above [5], wherein, before the above hot rolling, the above steel billet is cast and then cooled, and during the above cooling of the above steel billet, the average cooling rate v1 at 700-600°C is greater than 5.0°C/hour, and the average cooling rate v2 at 600-500°C is greater than 2.5°C/hour.

[7] The method for producing a high-strength steel sheet according to [5] or [6] above, wherein after the heat treatment, the cold-rolled steel sheet is subjected to a plating treatment to form a plating layer.

[8] The method for producing a high-strength steel sheet according to [7] above, wherein the plating treatment includes an alloying plating treatment for alloying the plating layer.

Effects of the Invention

According to the present invention, it is possible to provide a high-strength steel sheet having a tensile strength of 1320 MPa or more and excellent in formability (ductility and hole expandability) and delayed fracture resistance.

DETAILED DESCRIPTION

[High-strength steel plate]

The high-strength steel sheet of the present invention has the component composition and microstructure described below, and satisfies the amount of diffusible hydrogen in steel described below.

Hereinafter, the "high-strength steel plate" is also simply referred to as a "steel plate".

The thickness of the steel plate is not particularly limited, but is, for example, 0.3 to 3.0 mm, and preferably 0.5 to 2.8 mm.

High strength means that the tensile strength (TS) is 1320 MPa or more.

The high-strength steel sheet of the present invention has a tensile strength of 1320 MPa or more, and is also excellent in formability (ductility and hole expansion) and delayed fracture resistance. Therefore, the high-strength steel sheet of the present invention has great value in the industrial fields of automobiles, electrical equipment, etc., and is particularly useful for lightweighting the frame parts of automobile bodies.

<Ingredients>

The chemical composition of the high-strength steel sheet of the present invention (hereinafter, also referred to as “the chemical composition of the present invention” for convenience) will be described.

Unless otherwise specified, "%" in the component composition of the present invention means "mass %".

《C: 0.130～0.350％》

C increases the strength of tempered martensite, bainite and fresh martensite.

In addition, C improves the stability of retained austenite and improves the ductility of the steel sheet.

Furthermore, C precipitates fine precipitates (precipitates _AS described later) serving as hydrogen trapping sites inside tempered martensite and bainite, thereby improving delayed fracture resistance.

In order to fully obtain these effects, the amount of C is 0.130% or more, preferably 0.150% or more, more preferably 0.160% or more, and further preferably 0.170% or more.

On the other hand, when the amount of C is too much, the C distribution to austenite is advanced during reheating in the heat treatment described later, and the martensitic transformation and bainitic transformation during cooling after the heat treatment are suppressed, and the area ratio of retained austenite becomes excessive. In addition, when the amount of C is too much, the strength of the steel sheet becomes excessively high, thereby increasing the hydrogen embrittlement sensitivity of the steel and failing to obtain sufficient delayed fracture resistance. In addition, the weldability, which is important when joining automotive parts, deteriorates.

Therefore, the C content is 0.350% or less, preferably 0.330% or less, and more preferably 0.310% or less.

《Si: 0.50～2.50%》

Si suppresses the formation of carbides, thereby suppressing the reduction in hole expandability caused by the hardness difference between carbides and each structure. In addition, Si can obtain stable retained austenite and ensure good ductility.

From the viewpoint of obtaining these effects, the amount of Si is 0.50% or more, preferably 0.55% or more, and more preferably 0.60% or more.

On the other hand, when Si is excessively contained, the hole expandability deteriorates due to embrittlement of the steel sheet, and thus it becomes difficult to obtain desired formability.

Therefore, the Si content is 2.50% or less, preferably 2.30% or less, and more preferably 2.00% or less.

《Mn: 2.00～4.00％》

Mn forms a microstructure mainly composed of tempered martensite and bainite, thereby suppressing the hardness difference between the structures and improving the hole expandability.

Furthermore, Mn is an element that contributes to stabilization of retained austenite and is effective in ensuring good ductility.

From the viewpoint of obtaining these effects, the amount of Mn is 2.00% or more, preferably 2.20% or more, and more preferably 2.50% or more.

On the other hand, when the amount of Mn is too large, the steel sheet becomes brittle, the hole expandability deteriorates, and it becomes difficult to obtain desired formability.

Therefore, the amount of Mn is 4.00% or less, preferably 3.70% or less, and more preferably 3.50% or less.

《P: 0.100% or less》

P makes the steel sheet embrittled by grain boundary segregation, and has adverse effects on delayed fracture resistance and weldability. Therefore, the P content is 0.100% or less, preferably 0.070% or less, more preferably 0.050% or less, further preferably 0.030% or less, and particularly preferably 0.010% or less.

《S: 0.0500% or less》

S segregates at grain boundaries and embrittles the steel sheet during hot working. In addition, S forms sulfides, which adversely affect delayed fracture resistance. Therefore, the S content is 0.0500% or less, preferably 0.0100% or less, and more preferably 0.0050% or less.

《Al: 0.010～2.000％》

Al functions as a deoxidizer to reduce inclusions in the steel sheet. Therefore, the amount of Al is 0.010% or more, preferably 0.015% or more, and more preferably 0.020% or more.

On the other hand, when Al is too much, the risk of cracks in the steel billet during casting increases, which reduces the manufacturability. Therefore, the Al content is 2.000% or less, preferably 1.500% or less, more preferably 1.000% or less, further preferably 0.500% or less, and particularly preferably 0.100% or less.

《N: 0.0100% or less》

If there are coarse nitrides in the steel sheet, voids are formed when the steel sheet is sheared, and delayed fractures are likely to occur from these voids, which deteriorates the delayed fracture resistance of the steel sheet. Therefore, the smaller the N content, the better. Specifically, the N content is 0.0100% or less, preferably 0.0090% or less, and more preferably 0.0080% or less.

《At least one element selected from the group consisting of Ti: 0.001 to 0.100%, Nb: 0.001 to 0.100%, and V: 0.001 to 0.500%》

Ti, Nb, and V contribute to precipitation strengthening and are therefore effective in increasing the strength of the steel sheet. In addition, Ti, Nb, and V refine the prior austenite grain size, or concomitantly refine the tempered martensite and bainite, or form fine precipitates (precipitates _AS described later) that serve as hydrogen traps, thereby further improving the delayed fracture resistance.

From the viewpoint of obtaining these effects, the amount of Ti, the amount of Nb, and the amount of V are each 0.001% or more, preferably 0.003% or more, and more preferably 0.005% or more.

On the other hand, when Ti, Nb and V are too much, Ti, Nb and V may not be dissolved but remain when the slab is heated during hot rolling, and coarse precipitates (precipitates _AL described later) may increase, thereby deteriorating delayed fracture properties.

Therefore, the amount of Ti and the amount of Nb are each 0.100% or less, preferably 0.080% or less, and more preferably 0.050% or less.

The V content is 0.500% or less, preferably 0.450% or less, and more preferably 0.400% or less.

Other Elements

The component composition of the present invention may further contain, in mass %, at least one element selected from the group consisting of the following element compositions.

(W: 0.500% or less)

W improves the hardenability of the steel sheet. In addition, W forms fine carbides containing W that serve as hydrogen trapping sites, or refines tempered martensite and bainite, thereby further improving delayed fracture resistance.

However, when W is too much, coarse precipitates such as WN and WS that remain without solid solution increase when the steel slab is heated during hot rolling, and delayed fracture resistance deteriorates. Therefore, the W content is preferably 0.500% or less, more preferably 0.300% or less, and even more preferably 0.150% or less.

The lower limit of the amount of W is not particularly limited, but from the viewpoint of obtaining the effect of addition of W, it is, for example, 0.010%, and preferably 0.050%.

(B: 0.0100% or less)

B is effective for improving hardenability. In addition, B forms a microstructure mainly composed of tempered martensite and bainite, thereby preventing a decrease in hole expandability.

However, when the amount of B is too large, the formability may be reduced. Therefore, the amount of B is preferably 0.0100% or less, more preferably 0.0070% or less, and further preferably 0.0050% or less.

The lower limit of the amount of B is not particularly limited, but from the viewpoint of obtaining the effect of adding B, it is, for example, 0.0005%, and preferably 0.0010%.

(Ni: 2.000% or less)

Ni is an element that stabilizes retained austenite and is effective in ensuring good ductility. In addition, Ni increases the strength of steel by solid solution strengthening.

However, when the Ni content is too high, the area ratio of fresh martensite becomes too large, and the hole expandability decreases. Therefore, the Ni content is preferably 2.000% or less, more preferably 1.000% or less, and further preferably 0.500% or less.

The lower limit of the amount of Ni is not particularly limited, but from the viewpoint of obtaining the effect of Ni addition, it is, for example, 0.010%, and preferably 0.050%.

(Cο: 2.000% or less)

Co is an element effective in improving hardenability and is effective in strengthening the steel sheet.

However, when the amount of Co is too large, formability deteriorates. Therefore, the amount of Co is preferably 2.000% or less, more preferably 1.000% or less, and further preferably 0.500% or less.

The lower limit of the amount of Co is not particularly limited, but from the viewpoint of obtaining the effect of adding Co, it is, for example, 0.010%, and preferably 0.050%.

(Cr: 1.000% or less)

Cr improves the balance between strength and ductility.

However, when Cr is too much, the cementite solid solution rate is delayed during reheating in the heat treatment described later, and relatively coarse carbides such as cementite with Fe as the main component remain in an unsolvated state, and the delayed fracture resistance is deteriorated. Therefore, the Cr amount is preferably 1.000% or less, more preferably 0.800% or less, and further preferably 0.500% or less.

The lower limit of the amount of Cr is not particularly limited, but from the viewpoint of obtaining the effect of adding Cr, it is, for example, 0.030%, and preferably 0.050%.

(Mo: 1.000% or less)

Mo improves the balance between strength and ductility. Mo also forms fine carbides containing Mo that serve as hydrogen trapping sites, or refines tempered martensite and bainite, thereby further improving delayed fracture resistance.

However, when the amount of Mo is too large, the chemical conversion treatability is significantly deteriorated. Therefore, the amount of Mo is preferably 1.000% or less, more preferably 0.800% or less, and further preferably 0.500% or less.

The lower limit of the amount of Mo is not particularly limited, but from the viewpoint of obtaining the effect of addition of Mo, it is, for example, 0.010%, and preferably 0.050%.

(Cu: 1.000% or less)

Cu is an element effective in strengthening steel. Cu also suppresses hydrogen penetration into the steel sheet, thereby further improving delayed fracture resistance.

However, when Cu is too much, the area ratio of fresh martensite becomes too large, and the hole expandability deteriorates. Therefore, the amount of Cu is preferably 1.000% or less, more preferably 0.500% or less, and further preferably 0.200% or less.

The lower limit of the amount of Cu is not particularly limited, but from the viewpoint of obtaining the effect of addition of Cu, it is, for example, 0.010%, and preferably 0.050%.

(Sn: 0.500% or less, Sb: 0.500% or less)

Sn and Sb suppress decarburization of the surface region of the steel sheet (region about several tens of micrometers deep from the surface of the steel sheet) caused by nitridation or oxidation of the surface of the steel sheet, and prevent a decrease in the area ratio of tempered martensite on the surface of the steel sheet.

However, when these elements are excessively present, the toughness decreases. Therefore, the amount of Sn and the amount of Sb are each preferably 0.500% or less, more preferably 0.100% or less, and further preferably 0.050% or less.

The lower limits of the Sn amount and the Sb amount are not particularly limited, but from the viewpoint of obtaining the effects of addition of Sn and Sb, each is, for example, 0.001%, or preferably 0.003%.

(Ta: 0.100% or less)

Ta forms alloy carbides or alloy carbonitrides, contributing to high strength. In addition, Ta partially dissolves in Nb carbides or Nb carbonitrides to form composite precipitates such as (Nb, Ta)(C, N), thereby significantly inhibiting the coarsening of precipitates and stabilizing the contribution of precipitation strengthening to strength.

However, even if Ta is added excessively, these effects are saturated and the cost increases. Therefore, the amount of Ta is preferably 0.100% or less, more preferably 0.080% or less, and further preferably 0.070% or less.

The lower limit of the amount of Ta is not particularly limited, but from the viewpoint of obtaining the effect of adding Ta, it is, for example, 0.005%, and preferably 0.010%.

(Zr: 0.200% or less)

Zr improves the hardenability of the steel sheet. In addition, Zr forms fine carbides containing Zr that serve as hydrogen trapping sites, or refines tempered martensite and bainite, thereby further improving delayed fracture resistance.

However, when Zr is too much, inclusions increase, defects occur on the surface and inside of the steel sheet, and delayed fracture resistance deteriorates. Therefore, the Zr content is preferably 0.200% or less, more preferably 0.150% or less, and even more preferably 0.100% or less.

The lower limit of the amount of Zr is not particularly limited, but from the viewpoint of obtaining the effect of adding Zr, it is, for example, 0.005%, and preferably 0.010%.

(Hf: 0.020% or less)

Hf affects the distribution state of oxides, thereby improving the delayed fracture resistance.

However, when Hf is too much, the formability of the steel sheet deteriorates. Therefore, the amount of Hf is preferably 0.020% or less, more preferably 0.015% or less, and even more preferably 0.010% or less.

The lower limit of the amount of Hf is not particularly limited, but from the viewpoint of obtaining the effect of adding Hf, it is, for example, 0.001%, and preferably 0.003%.

(Ca: 0.0100% or less, Mg: 0.0100% or less, REM: 0.0100% or less)

Ca, Mg, and REM (rare earth metals) spheroidize the shape of sulfides, thereby improving the adverse effects of sulfides on hole expandability.

However, when these elements are excessive, inclusions increase, defects are caused on the surface and inside of the steel sheet, and delayed fracture resistance deteriorates. Therefore, the amount of Ca, the amount of Mg, and the amount of REM are preferably 0.0090% or less, more preferably 0.0080% or less, and further preferably 0.0070% or less.

The lower limits of the amount of Ca, the amount of Mg, and the amount of REM are not particularly limited, but from the viewpoint of obtaining the effects of addition of Ca, Mg, and REM, each is, for example, 0.0005%, and preferably 0.0010%.

《Breakdown: Fe and unavoidable impurities》

The balance in the component composition of the present invention is composed of Fe and inevitable impurities.

<Microstructure>

Next, the microstructure of the high-strength steel sheet of the present invention (hereinafter, also referred to as “the microstructure of the present invention” for convenience) will be described.

In order to obtain the effects of the present invention, it is not sufficient to satisfy only the component composition of the present invention described above, and it is necessary to satisfy the microstructure of the present invention described below.

Hereinafter, the area ratio is the area ratio relative to the entire microstructure. The area ratio of each structure is obtained by the method described in [Examples] described later.

<Total area ratio of tempered martensite and bainite: 70.0 to 95.0%>

Tempered martensite and bainite contribute to tensile strength.

In addition, tempered martensite and bainite as the main components are effective in improving hole expandability while maintaining high strength.

In order to fully obtain these effects, the total area ratio of bainite and tempered martensite is 70.0% or more, preferably 72.0% or more, and more preferably 74.0% or more.

On the other hand, when there are too many tempered martensite and bainite, there is too little retained austenite.

Therefore, the total area ratio of bainite and tempered martensite is 95.0% or less, preferably 93.0% or less, and more preferably 90.0% or less.

《Area ratio of fresh martensite: less than 15.0%》

There is a large hardness difference between fresh martensite and tempered martensite and bainite, and therefore, the hole expandability is reduced due to the hardness difference during punching. Therefore, it is necessary to avoid excessive fresh martensite in the steel sheet.

Specifically, from the viewpoint of obtaining good hole expandability, the area ratio of fresh martensite is 15.0% or less, preferably 14.0% or less, and more preferably 13.0% or less.

On the other hand, the lower limit is not particularly limited, but from the viewpoint of tensile strength, the area ratio of fresh martensite is preferably 1.0% or more, more preferably 3.0% or more, and further preferably 5.0% or more.

《Area ratio of retained austenite: 5.0-15.0%》

During processing, the retained austenite undergoes a martensitic transformation due to the TRIP (Transformation Induced Plasticity) effect, which increases the strength while improving the strain dispersion capability, thereby improving the ductility.

Therefore, the area ratio of retained austenite is 5.0% or more, preferably 6.0% or more, and more preferably 7.0% or more.

On the other hand, when the retained austenite is too much, voids are likely to be generated at the interface between the retained austenite and the tempered martensite during processing, and hydrogen embrittlement occurs starting from the voids, thereby deteriorating the delayed fracture resistance of the steel sheet.

In addition, the retained austenite undergoes stress-induced martensitic transformation during press forming, thereby reducing the hole expandability.

Therefore, the area ratio of retained austenite is 15.0% or less, preferably 14.0% or less, more preferably 13.0% or less, and further preferably 12.0% or less.

Surplus Organization

Examples of the structure (remaining structure) other than tempered martensite, bainite, fresh martensite and retained austenite include ferrite and pearlite. The area ratio of the remaining structure in the microstructure of the present invention is preferably 5.0% or less because it does not inhibit the effects of the present invention.

《Precipitate A》

Next, the precipitate A will be described.

The precipitate A is a carbide, a nitride or a carbonitride containing at least one element selected from the group consisting of Ti, Nb and V.

(Average particle size of precipitate A: 0.001 to 0.050 μm)

If the amount of precipitate A is too small, the effect of precipitation strengthening cannot be obtained, resulting in insufficient strength.

Therefore, the average particle size of the precipitate A is 0.001 μm or more, preferably 0.005 μm or more, and more preferably 0.010 μm or more.

On the other hand, when the precipitate A is too large, it has an adverse effect on the delayed fracture resistance of the shear edge surface.

Therefore, the average particle size of the precipitate A is 0.050 μm or less, preferably 0.040 μm or less, more preferably less than 0.040 μm, further preferably 0.035 μm or less, particularly preferably 0.030 μm or less, and most preferably 0.020 μm or less.

The average particle size of the precipitate A is determined by the method described in [Examples] described later.

(N _S : 10/μm ² or more)

The precipitate _AS is a precipitate A having a major axis of 0.050 μm or less.

The number density (number per unit area) N _S of the precipitates _AS is 10 pieces/μm ² or more.

Thus, the strength of the steel sheet is increased by precipitation strengthening. In addition, the fine precipitates _AS function as hydrogen trapping sites, thereby improving delayed fracture resistance.

From the viewpoint of obtaining a more excellent delayed fracture resistance, N _S is preferably greater than 125 pieces/µm ² , more preferably 200 pieces/µm ² or more, and still more preferably 310 pieces/µm ² or more.

The upper limit of _NS is not particularly limited. However, when the absolute amount of fine precipitates _AS increases, the rolling load may increase, and the production of steel sheets may become difficult. Therefore, _NS is preferably 1000 pieces/ ^μm2 or less, and more preferably 800 pieces/ ^μm2 or less.

( _NS / _NL : 10.0 or more)

Precipitate A _L is a precipitate A having a major diameter greater than 0.050 μm.

The ratio ( _NS / _NL ) of the number density _NS (unit: pieces/ ^μm2 ) of the precipitates _AS to the number density _NL (unit: pieces/ ^μm2 ) of the precipitates _AL is 10.0 or more. This can provide good delayed fracture resistance. The reason is presumably as follows.

It is believed that the fine precipitates _AS are less likely to accumulate strain and stress due to their small particle size. In addition, it is believed that the fine precipitates _AS are round, so their surface can be understood as a curved surface, and strain and stress are likely to escape along the curved surface.

On the other hand, it is considered that the coarse precipitates _AL have a larger movement distance of strain and stress than the fine precipitates _AS , so strain and stress are easily accumulated.

In particular, it is believed that the coarse precipitate _AL contains a quadrilateral precipitate A, and its surface can be understood as a plane, so strain and stress are more likely to accumulate. It is speculated that in this case, the local strain and residual stress inside the shear end face become higher. When the local strain and residual stress inside the shear end face become higher, initial cracks are likely to occur on the shear end face, and the delayed fracture resistance of the shear end face deteriorates.

Therefore, by reducing the abundance ratio of coarse precipitates _AL , the delayed fracture resistance of the steel sheet can be improved.

From the viewpoint of obtaining a more excellent delayed fracture resistance, N _S /N _L is preferably 11.0 or more, more preferably 12.0 or more, further preferably greater than 12.1, particularly preferably 12.2 or more, and most preferably 13.0 or more.

The upper limit of N _S /N _L is not particularly limited, but is preferably 100.0 or less, more preferably 80.0 or less, further preferably 50.0 or less, and particularly preferably 30.0 or less.

The upper limit of _NL is not particularly limited. However, it is considered that when the absolute amount of coarse precipitates _AL increases, the local strain and residual stress inside the shear end surface increase, and initial cracks are likely to occur on the shear end surface. Therefore, _NL is preferably 50 pieces/ ^μm2 or less, and more preferably 35 pieces/ ^μm2 or less.

_NS and _NL are determined by the method described in [Examples] to be described later.

<Diffusible hydrogen content in steel: 0.50 mass ppm or less>

From the viewpoint of ensuring good delayed fracture resistance, the amount of diffusible hydrogen in the steel is 0.50 mass ppm or less, preferably 0.40 mass ppm or less, more preferably 0.30 mass ppm or less, and further preferably 0.25 mass ppm or less.

The lower limit of the amount of diffusible hydrogen in steel is not particularly limited, but is, for example, 0.01 mass ppm due to constraints in production technology.

The amount of diffusible hydrogen in the steel is determined by the method described in [Examples] described later.

<Plating>

The high-strength steel sheet of the present invention may include a plating layer on its surface. The plating layer is formed by a plating treatment described below.

Examples of the plating layer include a zinc plating layer (Zn plating layer) and an Al plating layer, among which a zinc plating layer is preferred. The zinc plating layer may contain elements such as Al and Mg. The plating layer may also be an alloyed plating layer (alloyed plating layer).

From the viewpoint of controlling the coating weight and corrosion resistance, the coating weight (coating weight per single side) is preferably 20 g/m ² or more, more preferably 25 g/m ² or more, and further preferably 30 g/m ² or more.

On the other hand, from the viewpoint of adhesion, the coating weight of the plating layer is preferably 120 g/m ² or less, more preferably 100 g/m ² or less, and further preferably 70 g/m ² or less.

[Method for producing high-strength steel sheet]

Next, a method for producing a high-strength steel sheet according to the present invention (hereinafter, also referred to as “the production method of the present invention” for convenience) will be described.

<Steel Billet>

In the production method of the present invention, first, a steel slab (steel material) having the above-mentioned component composition of the present invention is prepared.

The steel billet is cast from molten steel by a known method such as continuous casting.

The method for producing molten steel is not particularly limited, and a known method using a converter, an electric furnace, or the like can be adopted.

《Average cooling rate v1: 5.0℃/hour or more and average cooling rate v2: 2.5℃/hour or more》

After the casting, the steel slab may be cooled, for example, by being left standing before being subjected to hot rolling described later.

In this cooling, the average cooling rate v1 at 700 to 600° C. is preferably 5.0° C./hour or more, more preferably 10.0° C./hour or more, and even more preferably 15.0° C./hour or more.

The average cooling rate v2 at 600 to 500° C. is preferably 2.5° C./hour or more, more preferably 5.0° C./hour or more, and even more preferably 10.0° C./hour or more.

During casting, coarse precipitates _AL may be precipitated in the steel slab. When the average cooling rate v1 and the average cooling rate v2 satisfy the above ranges, the distribution of the precipitates in the steel slab becomes uniform, and the coarse precipitates _AL precipitated during casting are easily re-melted when the steel slab is heated in the hot rolling described later. That is, the value of N _S / _NL tends to become large.

The upper limit of the average cooling rate v1 is not particularly limited, but is, for example, 150.0° C./hour, and preferably 100.0° C./hour.

The upper limit of the average cooling rate v2 is not particularly limited, but is, for example, 200.0° C./hour or less, and preferably 150.0° C./hour.

<Hot rolling>

In the production method of the present invention, the prepared steel slab is hot rolled under the conditions (heating temperature and finish rolling end temperature) described below to obtain a hot-rolled steel sheet.

《Heating temperature: above 1100℃》

During hot rolling, the steel slab is heated.

When the heating temperature of the steel slab is too low, it is difficult to sufficiently dissolve at least one element selected from the group consisting of Ti, Nb and V. In addition, excessive growth of the precipitates A occurs, so the number density _NS of fine precipitates _AS becomes too small, or the number density _NL of coarse precipitates _AL becomes too large. That is, the value of _NS / _NL tends to become too small. Therefore, the heating temperature of the steel slab is 1100°C or higher, preferably 1150°C or higher.

The heating temperature of the slab is preferably within the above range from the viewpoint of reducing the rolling load and removing surface defects (bubbles, segregation, etc.) of the slab to smooth the surface of the obtained steel sheet.

The upper limit of the heating temperature of the steel slab is not particularly limited, but when it is too high, the scale loss increases with the increase in the amount of oxidation. Therefore, the heating temperature of the steel slab is preferably 1400°C or lower, more preferably 1350°C or lower.

《Finish rolling end temperature: 850～950℃》

The steel slab heated to the above-mentioned heating temperature is subjected to hot rolling including finish rolling to obtain a hot-rolled steel sheet.

When the finishing temperature of the finishing rolling is too low, the rolling load increases and the rolling load becomes large. In addition, the average particle size of the precipitate A becomes too large, or the value of N _S / _NL becomes too small. In addition, sometimes the various structures of the obtained hot-rolled steel sheet become coarse, and the various structures in the subsequent heat treatment also become coarse. In this case, for example, when the cooling is stopped, it is difficult to stably obtain mechanically stable fine retained austenite that is not prone to martensitic transformation, and the retained austenite cannot be fully obtained, and the ductility is reduced.

Therefore, the finish rolling end temperature is 850°C or higher, preferably 855°C or higher, and more preferably 860°C or higher.

On the other hand, when the finishing temperature of the finishing rolling is too high, the austenite of the hot-rolled steel sheet coarsens, and as a result, the austenite grains of the cold-rolled steel sheet become coarse. In this case, the diffusion distance of C is lengthened, so sufficient C enrichment for obtaining stable austenite does not occur in the heat treatment described later. As a result, retained austenite cannot be sufficiently obtained in the final microstructure, and ductility is reduced.

In addition, the amount of oxide (scale) generated increases rapidly, and the surface quality tends to deteriorate after pickling and cold rolling described later.

Furthermore, when the scale cannot be sufficiently removed by pickling, ductility and hole expandability are adversely affected.

Furthermore, the crystal grain size may become too large, resulting in surface roughness during press working.

Therefore, the finish rolling end temperature is 950°C or lower, preferably 940°C or lower, and more preferably 930°C or lower.

<Coiling>

The hot-rolled steel sheet obtained by hot rolling is coiled under the conditions (coiling temperature T) described below.

《Coiling temperature T: 400～700℃》

When the coiling temperature T is too low, the precipitates A are not sufficiently formed, the number density _NS of fine precipitates _AS becomes too small, or the value of _NS / _NL becomes too small.

In addition, the strength of the hot-rolled steel sheet increases, the rolling load in cold rolling increases, or the cold-rolled steel sheet obtained by cold rolling has shape defects, so the productivity decreases.

Therefore, the coiling temperature T is 400°C or higher, preferably 420°C or higher, and more preferably 430°C or higher.

On the other hand, when the coiling temperature T is too high, the growth of the precipitates A proceeds, the average grain size of the precipitates A becomes too large, or the value of N _S /N _L becomes too small.

Therefore, the coiling temperature T is 700°C or lower, preferably 680°C or lower, and more preferably 670°C or lower.

The coiling temperature T is the end surface temperature of the hot-rolled steel sheet (ie, coil) after coiling.

<Stop>

The coiled hot-rolled steel sheet (coil) is stopped before being subjected to cold rolling described below.

During this residence, when the total time (unit: s) during which the temperature of the hot-rolled steel sheet after coiling is at least the coiling temperature T-50° C. is t (also referred to as “residence time t”), the following formula 1 is satisfied.

Formula 1: 0.001＜[1.17×10 ^-6 ×{t/(T+273.15)}] ^1/3 ＜0.050

Hereinafter, for convenience, “[1.17×10 ^-6 ×{t/(T+273.15)}] ^1/3 ” in the above-mentioned formula 1 is expressed as “X”.

When X in the above formula 1 is below the lower limit, the coil stops staying in a state where sufficient nucleation has not occurred or the nucleus growth is insufficient, the average particle size of the precipitates A becomes too small, or the value of N _S / _NL becomes too small.

On the other hand, when X in the above formula 1 exceeds the upper limit, the precipitates A excessively undergo Ostwald growth due to the diffusion of Ti, Nb or V and C or N, and the average grain size of the precipitates A becomes too large, or the value of N _S / _NL becomes too small.

The method of controlling the thermal history of the hot-rolled steel sheet (coil) after coiling is not particularly limited, and examples thereof include a method of applying a protective cover to the coil and a method of applying hot air and/or cold air to the coil.

The temperature of the hot-rolled steel sheet (coil) after coiling was the temperature of the coil surface measured by a radiation thermometer when there was no protective cover, and was the temperature inside the protective cover measured by a thermocouple when there was a protective cover.

The coil that has been left standing under the conditions satisfying the above formula 1 may be pickled as needed before the cold rolling described below. The pickling method may be conventional. Skin pass rolling may be performed to correct the shape and improve the pickling resistance.

<Cold rolling>

The coiled hot-rolled steel sheet is left to stand under the conditions satisfying the above-mentioned formula 1, pickled as necessary, and then cold-rolled to obtain a cold-rolled steel sheet.

The reduction ratio in cold rolling is preferably 25% or more, more preferably 30% or more.

On the other hand, excessive rolling reduction causes the rolling load to become too large, resulting in an increase in the load on the rolling mill used in cold rolling. Therefore, the rolling reduction rate is preferably 75% or less, and more preferably 70% or less.

<Heat Treatment>

The cold-rolled steel sheet obtained by cold rolling is subjected to heat treatment under the conditions described below.

In short, the cold-rolled steel sheet is maintained in the temperature range T1 (heated), then cooled to the cooling stop temperature T2, and then maintained in the temperature range T3 (reheated).

《Temperature range T1: 800～950℃》

When the temperature in the temperature range T1 is too low, the two-phase region is retained, and therefore, in the microstructure finally obtained, the total area ratio of tempered martensite and bainite becomes too small.

Therefore, the temperature in the temperature range T1 is 800°C or higher, preferably 830°C or higher, and more preferably 850°C or higher.

On the other hand, when the temperature in the temperature range T1 is too high, the precipitates A formed during hot rolling become coarse, the average grain size of the precipitates A becomes too large, or the value of N _S /N _L becomes too small.

Therefore, the temperature in the temperature range T1 is 950°C or lower, preferably 940°C or lower, and more preferably 930°C or lower.

《Maintaining time within temperature range T1: more than 30 seconds》

If the holding time in the temperature range T1 is too short, sufficient recrystallization cannot be performed. In addition, the formation of austenite is insufficient, and the area ratio of retained austenite becomes too small.

Therefore, the holding time in the temperature range T1 is 30 seconds or longer, preferably 65 seconds or longer, and more preferably 100 seconds or longer.

The upper limit of the retention time in the temperature range T1 is not particularly limited, and is, for example, 800 seconds, preferably 500 seconds, and more preferably 200 seconds.

《Cooling stop temperature T2: 150～250℃》

When the cooling stop temperature T2 is too low, the amount of untransformed austenite remaining when cooling is stopped becomes small, and finally the area ratio of retained austenite becomes too small.

Therefore, the cooling stop temperature T2 is 150°C or higher, preferably 160°C or higher, and more preferably 170°C or higher.

On the other hand, when the cooling stop temperature T2 is too high, a large amount of austenite remains when cooling is stopped, and finally the area ratio of retained austenite becomes too large.

Therefore, the cooling stop temperature T2 is 250°C or lower, preferably 240°C or lower, and more preferably 230°C or lower.

《Temperature range T3: 250～400℃》

When the temperature in the temperature range T3 is too low, C is not sufficiently enriched in the untransformed austenite, and the area ratio of retained austenite becomes too small.

Therefore, the temperature in the temperature range T3 is 250°C or higher, preferably 260°C or higher, and more preferably 270°C or higher.

On the other hand, when the temperature in the temperature range T3 is too high, the decomposition of the untransformed austenite proceeds excessively, and the area ratio of the retained austenite becomes too small, so that the ductility deteriorates.

Therefore, the temperature in the temperature range T3 is 400°C or lower, preferably 380°C or lower, and more preferably 360°C or lower.

《Maintaining time within temperature range T3: more than 30 seconds》

When the holding time in the temperature range T3 is too short, in the finally obtained microstructure, the area ratio of fresh martensite becomes too large, or the C enrichment in retained austenite is insufficient and the area ratio of retained austenite becomes too small.

Therefore, the holding time in the temperature range T3 is 30 seconds or longer, preferably 100 seconds or longer, and more preferably 180 seconds or longer.

The upper limit of the retention time in the temperature range T3 is not particularly limited, and is, for example, 800 seconds, preferably 500 seconds, and more preferably 300 seconds.

<Plating treatment>

The cold-rolled steel sheet subjected to the above heat treatment may be subjected to a plating treatment for forming a plating layer. Examples of the plating treatment include hot-dip galvanizing. In this case, a galvanized layer is formed as the plating layer.

When hot-dip galvanizing is performed, for example, the cold-rolled steel sheet subjected to the above heat treatment is immersed in a hot-dip galvanizing bath at 440 to 500° C. After the immersion, the coating weight is adjusted by gas wiping or the like.

The hot-dip galvanizing bath may contain elements such as Al, Mg, and Si, and may further contain elements such as Pb, Sb, Fe, Mg, Mn, Ni, Ca, Ti, V, Cr, Co, and Sn. The amount of Al in the hot-dip galvanizing bath is preferably 0.08 to 0.30%.

The plating treatment may include an alloying treatment for alloying the formed plating layer.

When alloying treatment is performed after hot-dip galvanizing, the galvanized layer is alloyed at a temperature (alloying temperature) of 450 to 600° C. If the alloying temperature is too high, untransformed austenite transforms into pearlite, and the area ratio of retained austenite becomes too small.

The Fe concentration of the zinc-plated layer after alloying is preferably 8 to 17% by mass.

When the plating treatment is performed, the cold-rolled steel sheet subjected to the heat treatment and the plating treatment corresponds to the high-strength steel sheet of the present invention.

On the other hand, when the plating treatment is not performed, the cold-rolled steel sheet subjected to the heat treatment corresponds to the high-strength steel sheet of the present invention.

Example

Hereinafter, the present invention will be described in detail with reference to examples, but the present invention is not limited to the examples described below.

<Manufacturing of Steel Plate>

Molten steel having the component composition shown in the following Table 1, with the balance consisting of Fe and inevitable impurities, was produced in a converter, and steel slabs were obtained by continuous casting.

The obtained steel slab was cooled under the conditions shown in Table 2 below.

Next, the cooled steel slab was subjected to hot rolling, coiling, retention, cold rolling and heat treatment under the conditions shown in Table 2 below to obtain a cold-rolled steel sheet (CR) having a thickness of 1.4 mm. The cold rolling reduction was 50%.

Some cold-rolled steel sheets were subjected to hot-dip galvanizing to form galvanized layers on both sides, thereby obtaining hot-dip galvanized steel sheets (GI). The amount of galvanized layer applied (the amount applied per side) was set to 45 g/m ² .

Furthermore, some galvanized steel sheets (GI) were subjected to alloying treatment to alloy the formed galvanized layer, thereby obtaining alloyed galvanized steel sheets (GA). In the alloying treatment, the Fe concentration of the alloyed galvanized layer was adjusted to be in the range of 9 to 12 mass %.

For the hot-dip galvanized steel sheet (GI), a hot-dip galvanizing bath with an Al content of 0.19 mass % was used. For the alloyed hot-dip galvanized steel sheet (GA), a hot-dip galvanizing bath with an Al content of 0.14 mass % was used. The bath temperature was set to 465°C.

Hereinafter, for convenience, the cold rolled steel sheet (CR), the hot dip galvanized steel sheet (GI) and the alloyed hot dip galvannealed steel sheet (GA) are all referred to as "steel sheets".

In the column of "Type" in Table 2 below, any one of "CR", "GI" and "GA" is described according to the obtained steel plate.

<Observation of Microstructure>

The obtained steel plate was polished so that the cross section (L cross section) at the 1/4 plate thickness position (a position corresponding to 1/4 plate thickness in the depth direction from the surface of the steel plate) parallel to the rolling direction was used as the observation surface to prepare an observation sample.

Using the prepared observation samples, the microstructures were observed as described below, and the area ratio of each structure, etc. were determined. The results are shown in Table 3 below.

In the following Table 3, tempered martensite is represented as "TM", bainite is represented as "B", fresh martensite is represented as "FM", and retained austenite is represented as "γR".

《Area ratio of tempered martensite, bainite and fresh martensite》

After etching the observation surface of the observation sample with nital solution, 10 fields of view were observed with a scanning electron microscope (SEM) at a magnification of 2000 times to obtain a SEM image.

The area ratio (unit: %) of each structure was determined from the obtained SEM image, and the average area ratio of 10 fields of view was defined as the area ratio of each structure.

In the SEM image, the light gray area is determined to be fresh martensite, and the dark gray area where carbides are precipitated is determined to be tempered martensite and bainite.

Since fresh martensite and retained austenite cannot be clearly distinguished in the SEM image, the area ratio of fresh martensite is a value obtained by subtracting the area ratio of retained austenite obtained by the method described later from the area ratio of the light gray region.

《Area ratio of retained austenite》

The area ratio (unit: %) of retained austenite was determined by X-ray diffraction.

Specifically, first, the observation surface of the observation sample was ground by 0.1 mm in the plate thickness direction, and further ground by 0.1 mm by chemical polishing to obtain a polished surface.

The integrated intensity of the diffraction peaks of each of the {200}, {220} and {311} planes of fcc iron and each of the {200}, {211} and {220} planes of bcc iron was measured on the polished surface using CoKα radiation.

On this basis, the ratio (integrated intensity) of the integrated intensity of each of the {200}, {220} and {311} planes of fcc iron to the integrated intensity of each of the {200}, {211} and {220} planes of bcc iron was calculated.

The value obtained by averaging the obtained nine integrated intensity ratios was defined as the volume ratio of retained austenite, and the volume ratio was defined as the area ratio (unit: %) of retained austenite.

《Average particle size, _NS and _NL of precipitate A》

A replica sample is cut from the observation surface of the observation sample by the replica method.

The cut replica sample was observed using a transmission electron microscope (TEM) at an accelerating voltage of 200 kV and a magnification of 20,000 times to obtain a TEM image. The size of one field was 0.5 μm×0.5 μm.

The presence of precipitates was confirmed by observing the obtained TEM image.

Furthermore, energy dispersive X-ray spectroscopy (EDS) was performed in the same field of view as the TEM image to confirm the elements contained in the precipitate.

Among the precipitates confirmed in the TEM image, the precipitate containing at least one selected from the group consisting of Ti, Nb, and V was identified as precipitate A.

The equivalent circle diameter of each precipitate identified as precipitate A was determined, and the average value of 10 fields of view was taken as the average particle size of precipitate A (unit: μm).

Furthermore, the major axis of the precipitate A was measured.

Specifically, for each particle of the precipitate identified as the precipitate A, the longest length passing through the particle was measured, and the length was defined as the major axis of the precipitate A.

On this basis, the number of precipitates A having a major axis of 0.050 μm or less (i.e., precipitates _AS ) was measured, and the measured number was divided by the area of 10 viewing fields to determine the number density N _S (unit: pieces/μm ² ) of precipitates _AS .

Similarly, the number of precipitates A having a major axis larger than 0.050 µm (i.e., precipitates _AL ) was counted, and the number density _NL (unit: precipitates/ ^µm2 ) of precipitates _AL was determined by dividing the measured number by the area of 10 viewing fields.

Furthermore, the ratio of _NS to _NL ( _NS / _NL ) is calculated.

<Determination of Diffusible Hydrogen Amount in Steel>

A test piece having a size of 5 mm×30 mm was cut out from the obtained steel plate. When a plating layer (a zinc plating layer or an alloyed zinc plating layer) was formed, the plating layer was removed using a router (precision grinder).

The test piece was placed in a quartz tube, and the inside of the quartz tube was replaced with argon (Ar). Then, the temperature inside the quartz tube was raised to 400°C at a rate of 200°C/hour, and the amount of hydrogen generated from the quartz tube during the temperature rise was measured by temperature rise analysis using gas chromatography.

The cumulative value of the amount of hydrogen detected in the temperature range of room temperature (25° C.) or higher and lower than 250° C. was determined as the amount of diffusible hydrogen in the steel (unit: mass %). The results are shown in Table 3 below.

<Evaluation>

The obtained steel sheets were evaluated by the following tests. The results are shown in Table 3 below.

Tensile test

JIS No. 5 test pieces were cut out from the obtained steel sheets, with the direction perpendicular to the rolling direction being the tensile direction. Using the cut test pieces, a tensile test was carried out in accordance with JIS Z 2241 (2011) to measure tensile strength (TS) and total elongation (EL).

When TS was 1320 MPa or more, it was evaluated as high strength.

When EL is 10.0% or more, the ductility is evaluated to be excellent.

Hole expansion test

The obtained steel plates were subjected to a hole expansion test in accordance with JIS Z 2256 (2010).

Specifically, the obtained steel plate was cut to obtain a test piece of 100 mm × 100 mm in size. A hole of 10 mm in diameter was punched out in the cut test piece with a gap of 12 ± 1%. Then, a die with an inner diameter of 75 mm was used to press a conical punch with a vertex angle of 60° into the hole under a state of being suppressed by a wrinkle-proof force of 9 tons, and the hole diameter D _f (unit: mm) at the limit of crack generation was measured. The initial hole diameter was set to D ₀ (unit: mm), and the hole expansion rate λ (unit: %) was calculated by the following formula. When λ was 25% or more, it was evaluated as having excellent hole expansion.

λ＝{(D _f −D ₀ )/D ₀ }×100

《Delayed fracture resistance evaluation test》

A test piece was cut out from the obtained steel sheet. If a plating layer was formed, it was dissolved out using diluted hydrochloric acid, and after being stored at room temperature for 1 day (dehydrogenation treatment), a test piece was cut out.

The dimensions of the test piece were such that the length of the long side (the length in the direction perpendicular to rolling) was 100 mm, and the length of the short side (the length in the rolling direction) was 30 mm.

In the test piece, the end surface on the long side was used as the evaluation end surface, and the end surface on the short side was used as the non-evaluation end surface.

The evaluation end face was cut as shearing. The gap of the shearing was set to 10%, and the rake angle was set to 0.5 degrees. The evaluation end face was set as shearing. That is, no mechanical processing to remove burrs was performed. On the other hand, mechanical processing to remove burrs was performed on the non-evaluation end face.

The test piece was subjected to bending under the conditions that the ratio of the bending radius R to the plate thickness t of the test piece (R/t) was 4.0 and the bending angle was 90 degrees (V-shaped bending).

For example, when the plate thickness t is 2.0 mm, a punch with a tip radius of 8.0 mm is used. More specifically, a punch with the above tip radius and a U shape (a semicircular tip portion and a body thickness of 2R) is used.

Furthermore, a die having a corner bending radius of 30 mm was used for the bending process.

During the bending process, the depth of the punch being pressed into the test piece was adjusted to form a bent portion having a bending angle of 90 degrees on the test piece.

The test piece having the bent portion formed thereon was clamped and tightened with a hydraulic jack, and bolts were tightened in a state where the following residual stress S1, S2, or S3 was applied to the outermost layer of the bent portion.

Residual stress S1: Residual stress of 1300MPa or more and 1500MPa or less

Residual stress S2: Residual stress greater than 1500MPa and less than 1700MPa

Residual stress S3: Residual stress greater than 1700MPa and less than 1900MPa

For the residual stresses S1, S2, and S3 of each load, the number of test pieces was set to two.

The required screwing amount is calculated by CAE (Computer Aided Engineering) analysis.

Bolt tightening was performed by passing a bolt through an elliptical hole (minor axis: 10 mm, major axis: 15 mm) previously provided 10 mm inside the non-evaluation end surface of the test piece.

The bolt-fastened test piece was immersed in hydrochloric acid (hydrogen chloride aqueous solution) having a pH of 4, and the pH was controlled to be constant at 25° C. The amount of hydrochloric acid per test piece was set to 1 L or more.

After 48 hours from the immersion, the test piece in the hydrochloric acid was checked for the presence or absence of visually observable micro cracks (having a length of about 1 mm). The micro cracks indicate the initial state of delayed fracture.

The results corresponding to the presence or absence of microcracks ("×", "△", "○" or "◎" shown below) are shown in Table 3 below.

×: One or more micro cracks were observed in the test in which the residual stress S1 was loaded.

Δ: No microcracks were observed in the test piece loaded with the residual stress S1, but one or more microcracks were observed in the test piece loaded with the residual stress S2.

○: No microcracks were observed in the test pieces loaded with the residual stress S1 and the residual stress S2, but one or more microcracks were observed in the test piece loaded with the residual stress S3.

⊚: Micro cracks were not confirmed in any of the test pieces.

If it is "△", "○" or "◎", it is evaluated that the delayed fracture resistance is excellent.

From the reason that the delayed fracture resistance is better, "○" or "◎" is preferred, and from the reason that the delayed fracture resistance is further better, "◎" is more preferred.

The underlined elements in the following Tables 1 to 3 are outside the scope of the present invention.

[Table 2]

Table 2

[Table 3]

Table 3

<Summary of evaluation results>

As shown in Table 3, the steel plates of Nos. 1, 3 to 5, 8 to 9, 11 to 13, 15 to 17, 21 to 22, 26, 30 to 31 and 39 were insufficient in at least one of tensile strength, ductility, hole expandability and delayed fracture resistance.

On the other hand, it can be seen that the steel plates of Nos. 2, 6 to 7, 10, 14, 18 to 20, 23 to 25, 27 to 29, 32 to 38 and 40 to 45 all have a tensile strength of 1320 MPa or more and are excellent in ductility, hole expandability and delayed fracture resistance.

Among these steel sheets, the evaluation result of the delayed fracture resistance of the steel sheets satisfying all of the following (i) to (v) was "◎".

(i) Area ratio of retained austenite: 12.0% or less

(ii) Average particle size of precipitate A: 0.020 μm or less

(iii) N _S : 310/μm ² or more

(iv) N _S /N _L : 13.0 or more

(v) Diffusible hydrogen content in steel: 0.25 mass ppm or less

The evaluation result of the delayed fracture resistance of the steel sheet that does not satisfy at least any one of the above (i) to (v) is "○".

The steel sheet No. 44 (steel symbol: T) satisfied all of the above (i) to (v), but the evaluation result of delayed fracture resistance was "○" presumably because the amount of C was slightly small.

The evaluation result of the delayed fracture resistance of the steel sheet having N _S /N _L of 12.1 or less or having an average grain size of precipitates A of 0.040 μm or more was "Δ".

Claims (8)

1. A high-strength steel sheet comprising, in mass%, C:0.130 to 0.350 percent of Si:0.50 to 2.50 percent of Mn: 2.00-4.00%, P:0.100% or less, S: less than 0.0500%, al:0.010 to 2.000 percent, N:0.0100% or less selected from the group consisting of Ti:0.001 to 0.100 percent of Nb:0.001 to 0.100% and V:0.001 to 0.500%, a composition of at least one element selected from the group consisting of Fe and unavoidable impurities, and a microstructure,

The amount of diffusible hydrogen in the steel is 0.50 mass ppm or less,

In the case of the microstructure of the present invention,

The total area ratio of tempered martensite and bainite is 70.0 to 95.0%,

The area ratio of the fresh martensite is below 15.0%,

The area ratio of the residual austenite is 5.0-15.0%,

The average particle diameter of the precipitate A as carbide, nitride or carbonitride containing at least one element selected from the group consisting of Ti, nb and V is 0.001-0.050 mu m,

The number density N _S of the precipitates A _S as the precipitates A having a major axis of 0.050 μm or less is 10/μm ² or more,

The ratio N _S/N_L of the number density N _S of the precipitate A _S to the number density N _L of the precipitate A _L, which is the precipitate A having a major axis of more than 0.050 μm, is 10.0 or more.

2. The high-strength steel sheet according to claim 1, wherein the composition of the components further contains, in mass%, a composition selected from the group consisting of W:0.500% or less, B: less than 0.0100%, ni: less than 2.000%, C omicrongroup: less than 2.000%, cr: less than 1.000%, mo: less than 1.000%, cu: less than 1.000%, sn:0.500% or less, sb: less than 0.500%, ta: less than 0.100%, zr:0.200% or less, hf: less than 0.020%, ca: less than 0.0100%, mg: below 0.0100% and REM:0.0100% or less of at least one element of the group consisting of.

3. The high-strength steel sheet according to claim 1 or 2, wherein a plating layer is provided on the surface.

4. A high strength steel sheet according to claim 3, wherein the plating layer is an alloyed plating layer.

5. A method for producing a high-strength steel sheet according to claim 1 or 2, wherein,

A hot rolled steel sheet obtained by heating a steel slab having the composition according to claim 1 or 2 to 1100 ℃ or higher and hot rolling the steel slab at a finish rolling temperature of 850 to 950 ℃,

Coiling the hot-rolled steel plate at a coiling temperature T of 400-700 ℃, staying, then cold-rolling to obtain a cold-rolled steel plate,

The cold-rolled steel sheet is subjected to a heat treatment,

In the stay, when T is the total of times when the temperature of the hot rolled steel sheet after coiling is equal to or higher than the coiling temperature T-50 ℃, the following formula 1 is satisfied in units of s,

In the heat treatment, the cold-rolled steel sheet is kept at a temperature range T1 of 800 to 950 ℃ for 30 seconds or more, then cooled to a cooling stop temperature T2 of 150 to 250 ℃, then kept at a temperature range T3 of 250 to 400 ℃ for 30 seconds or more,

Formula 1:0.001 < [1.17 x 10 ^-6×{t/(T+273.15)}]^1/3 < 0.050.

6. The method for producing a high-strength steel sheet according to claim 5, wherein,

Prior to said hot rolling, cooling after casting said billet,

In the cooling of the billet, an average cooling rate v1 at 700 to 600 ℃ is 5.0 ℃/hour or more, and an average cooling rate v2 at 600 to 500 ℃ is 2.5 ℃/hour or more.

7. The method for producing a high-strength steel sheet according to claim 5 or 6, wherein the cold-rolled steel sheet is subjected to a plating treatment for forming a plating layer after the heat treatment.

8. The method for producing a high-strength steel sheet according to claim 7, wherein the plating treatment comprises an alloying plating treatment for alloying the plating layer.