KR102649505B1 - high strength steel plate - Google Patents

Abstract

The high-strength steel sheet of the present invention contains predetermined chemical components and has a metal structure in area ratio: ferrite: 20% to 70%, retained austenite: 5% to 40%, fresh martensite: 0% to 30%, Tempered martensite and bainite: 20% to 75%, and pearlite and cementite: 0% to 10%, and in the range of 1/8 thickness to 3/8 thickness from the surface, total retained auste. The ratio of the number of retained austenites with an aspect ratio of 2.0 or more to the number of nites is 50% or more, and at a position 1/4 of the sheet thickness of the cross section parallel to the rolling direction and perpendicular to the surface, across 50 mm along the sheet width direction. The standard deviation of the area ratio of ferrite measured at 10 locations is less than 10%, and the tensile strength is 780 MPa or more.

Description

high strength steel plate

The present invention relates to a high-strength steel sheet that has excellent tensile strength, ductility, elongation flangeability, and bendability, and also has excellent material stability.

This application claims priority based on Japanese Patent Application No. 2019-128612, filed in Japan on July 10, 2019, the contents of which are incorporated herein by reference.

In recent years, there has been a demand for further improvement in fuel efficiency of automobiles from the viewpoint of greenhouse gas emission regulations accompanying global warming measures. In addition, in order to reduce the weight of the car body and ensure crash safety, the application of high-strength steel sheets in automobile parts is gradually expanding.

Steel plates used for automobile parts are required not only for strength but also for various workability requirements such as press formability and weldability when forming parts. Specifically, from the viewpoint of press workability, steel sheets are often required to have excellent ductility (total elongation in a tensile test; EL) and elongation flangeability (hole expansion ratio; λ).

Meanwhile, in high-strength steel sheets, technology for obtaining stable materials within the coil is also important. This is because, while low-strength steel sheets have had a relatively simple structure with ferrite as the main structure and strength secured with a small amount of solid solution strengthening elements as needed, high-strength steels have structures such as bainite and martensite. Low-temperature transformation structures and precipitates such as TiC are used to ensure strength, resulting in a complex structure. These phenomena, such as transformation and precipitation, are greatly influenced by temperature history, but temperature fluctuations sometimes inevitably occur during the manufacturing process. For example, in the manufacturing process of hot rolled steel sheets, there is a possibility that fluctuations in temperature history may occur in the width and length directions, such as unevenness in the method of applying coolant in the width direction or uneven cooling rate due to the position within the coil after winding. . Therefore, in the production of high-strength steel sheets, techniques for stabilizing the material are required, such as using a manufacturing method that reduces the temperature history as much as possible or performing a material design that reduces the influence of the temperature history as much as possible.

As a technology for improving the ductility of high-strength steel sheets, there is TRIP steel, which utilizes the TRIP (transformation induced plasticity) effect by leaving an austenite phase remaining in the steel structure (see, for example, Patent Document 1). TRIP steel has higher ductility than DP steel.

Additionally, Non-Patent Document 1 discloses that the elongation and hole expandability of a steel sheet are improved by using a two-time annealing method in which the steel sheet is annealed twice.

On the other hand, regarding material stability, for example, in Patent Document 2, by controlling the addition amount of Ti and V to a certain range for a hot-rolled steel sheet with a tensile strength of 780 MPa or more, fine carbides are uniformly precipitated during hot-rolling, resulting in A technology to stabilize the material of hot rolled steel sheets has been reported.

Japanese Patent Publication No. 2006-274418 Japanese Patent Publication No. 2013-100574

K. Sugimoto et al.: ISIJ International, Effects of Second Phase Morphology on Retained Austenite Morphology and Tensile Properties in a TRIP-aided Dual-phase Steel Sheet (1993), 775.

The present inventors conducted a search to obtain a steel plate that achieved both elongation and hole expandability. In the method described in Non-Patent Document 1, since annealing is performed twice, the problem is that fuel costs, etc. increase compared to a manufacturing method that performs one annealing. Therefore, the present inventors attempted a manufacturing method of forming a TRIP steel sheet by annealing a hot rolled steel sheet in order to form the same plate-like structure (i.e., a structure with a large austenite aspect ratio) without performing two annealing operations. Specifically, the present inventors studied a manufacturing method in which a hot-rolled steel sheet is wound at a low temperature of 450°C or lower and then annealed. By winding at low temperature, the structure of the hot-rolled steel sheet can be made into a structure mainly composed of a low-temperature transformation structure. The present inventors believed that by annealing a hot-rolled steel sheet having a structure mainly composed of a low-temperature transformation structure, a plate-shaped structure could be obtained in one annealing process.

However, material destabilization occurred in the steel sheet obtained by this method. Specifically, the variation in the amount of ferrite measured along the sheet width direction increased, and as a result, the variation in mechanical properties increased.

The object of the present invention is to provide a high-strength hot-rolled steel sheet that has excellent tensile strength, ductility, elongation flangeability, and bendability, and also has excellent material stability. Additionally, material stability refers to little variation in the tensile strength and overall ductility of each part of the steel sheet.

(1) The high-strength steel sheet according to one embodiment of the present invention has chemical components in terms of mass%: C: 0.030 to 0.280%, Si: 0.50 to 2.50%, Mn: 1.00 to 4.00%, sol.Al: 0.001 to 2.000%. , P: 0.100% or less, S: 0.0200% or less, N: 0.01000% or less, O: 0.0100% or less, B: 0 to 0.010%, Ti: 0 to 0.20%, Nb: 0 to 0.20%, V: 0 to 0. 1.000%, Cr: 0 to 1.000%, Mo: 0 to 1.000%, Cu: 0 to 1.000%, Co: 0 to 1.000%, W: 0 to 1.000%, Ni: 0 to 1.000%, Ca: 0 to 0.0100 %, Mg: 0 to 0.0100%, REM: 0 to 0.0100%, Zr: 0 to 0.0100%, and the balance: Fe and impurities, the metal structure is an area ratio, ferrite: 20% to 70%, residual auste. Knight: 5% to 40%, fresh martensite: 0% to 30%, sum of tempered martensite and bainite: 20% to 75%, and sum of pearlite and cementite: 0% to 10%, and the surface In the range of 1/8 thickness to 3/8 thickness, the ratio of the number of retained austenite with an aspect ratio of 2.0 or more to the total number of retained austenite is 50% or more, and parallel to the rolling direction and perpendicular to the surface At a position of 1/4 of the sheet thickness of the cross section, the standard deviation of the area ratio of ferrite measured at 10 locations across 50 mm along the sheet width direction is less than 10%, and the tensile strength is 780 MPa or more.

(2) In the high-strength steel sheet described in (1), the standard deviation of the surface roughness Ra may be 0.5 μm or less at 10 positions at 50 mm intervals in the sheet width direction.

(3) The high-strength steel sheet according to (1) or (2) has, in mass% as the above chemical components, B: 0.001% to 0.010%, Ti: 0.01 to 0.20%, Nb: 0.01 to 0.20%, and V: 0.005%. to 1.000%, Cr: 0.005% to 1.000%, Mo: 0.005% to 1.000%, Cu: 0.005% to 1.000%, Co: 0.005% to 1.000%, W: 0.005% to 1.000%, Ni: 0.005% to 1.000 %, Ca: 0.0003% to 0.0100%, Mg: 0.0003% to 0.0100%, REM: 0.0003% to 0.0100%, and Zr: 0.0003% to 0.0100%.

According to the above aspect, a high-strength steel sheet having excellent tensile strength, ductility, stretch flangeability, and bendability, and also excellent material stability can be obtained.

1 is a conceptual diagram showing an observation surface for evaluating metal structure.
Figure 2 is a conceptual diagram showing an observation surface for evaluating retained austenite.
Figure 3 is a conceptual diagram showing an observation surface for evaluating the standard deviation of the area ratio of ferrite.

The present inventors have conducted intensive studies on the causes of loss of material stability in steel sheets that have been annealed once. And the present inventors found that variations in the surface properties of the hot rolled steel sheet before annealing affect the material stability of the steel sheet after annealing. The variation in surface properties (surface roughness) of hot rolled steel sheets tends to be greater than that of cold rolled steel sheets. If there is unevenness in surface roughness, during the temperature increase process for annealing, the uneven surface roughness causes uneven emissivity, and temperature fluctuations resulting from this occur in the steel sheet. As a result, variation in the amount of ferrite increases in the steel sheet after annealing. The present inventors' findings have revealed for the first time that controlling the surface properties of a hot-rolled steel sheet contributes to material stabilization of a hot-rolled annealed sheet.

In addition, the present inventors also discovered an effective hot rolling method to suppress variations in the surface properties of steel sheets (hot rolled steel sheets) before annealing. The present inventors have found that the phenomenon in which the surface scale is pressed against the steel sheet by the hot rolling roll during hot rolling largely characterizes the surface properties of the steel sheet after hot rolling. In order to control the surface properties of a hot rolled steel sheet, it is important to control the growth of scale during hot rolling, and it was found that this can be achieved by spraying a water film on the surface of the steel sheet under specific conditions during rolling.

Below, a high-strength steel plate according to an embodiment of the present invention will be described in detail. However, the present invention is not limited to the configuration disclosed in this embodiment, and various changes are possible without departing from the spirit of the present invention. In addition, the limited range of the values shown below includes the lower limit and the upper limit. Numerical values indicated as “greater than” or “less than” are not included in the numerical range. “%” regarding the content of each element means “mass%”.

In the high-strength steel sheet 1 according to the present embodiment, the rolling direction (RD), sheet thickness direction (TD), and sheet width direction (WD) shown in FIGS. 1 to 3 are defined as follows. The rolling direction (RD) refers to the direction in which the steel sheet moves by the rolling roll during rolling. The sheet thickness direction (TD) is a direction perpendicular to the rolling surface 11 of the steel sheet. The sheet width direction (WD) is a direction perpendicular to the rolling direction (RD) and the sheet thickness direction (TD). Additionally, the rolling direction (RD) can be easily specified based on the stretching direction of the grains of the steel sheet. Therefore, the rolling direction RD can be specified even in a steel sheet cut from a raw material steel sheet after rolling.

In the high-strength steel sheet according to the present embodiment, the amount of ferrite in the metal structure, etc. is specified. The metal structure is evaluated in a cross section 12 parallel to the rolling direction RD and perpendicular to the rolling surface 11 (see Fig. 1). Hereinafter, the cross section 12 parallel to the rolling direction RD and perpendicular to the rolling surface 11 may simply be described as a cross section parallel to the rolling direction RD. The detailed metal structure evaluation method is described later.

In addition, in the high-strength steel sheet according to the present embodiment, the ratio of the number of retained austenites with an aspect ratio of 2.0 or more to the total number of retained austenites is determined in advance. Retained austenite is evaluated in a cross section parallel to the rolling direction (RD) and the sheet thickness direction (TD) (see Figure 2). The detailed evaluation method of retained austenite is described later.

Additionally, in the high-strength steel sheet according to the present embodiment, the standard deviation of the area ratio of ferrite is specified. The area ratio of ferrite is measured at a position 121 of 1/4 of the sheet thickness of the cross section 12 parallel to the rolling direction RD and perpendicular to the rolling surface 11 (see Fig. 3). Ten cross sections (12) parallel to the rolling direction (RD) and perpendicular to the rolling surface (11) are fabricated across 50 mm along the sheet width direction (WD), and a standard ferrite area ratio of 10 is measured on these surfaces. The deviation is regarded as the standard deviation of the area ratio of ferrite according to the present embodiment.

In addition, the plate thickness 1/4 position is a position at a depth of 1/4 the thickness of the steel plate 1 from the rolling surface 11 of the steel plate 1. 1 and 2, only the position at a depth of 1/4 the thickness of the steel sheet 1 from the upper rolling surface 11 of the steel sheet 1 is shown as the 1/4 sheet thickness position. However, of course, the depth position of 1/4 the thickness of the steel sheet 1 from the lower rolling surface 11 of the steel sheet 1 can also be treated as the 1/4 sheet thickness position. Additionally, in Figure 3, only a portion of the 10 measurement surfaces is shown. In addition, FIG. 3 merely conceptually shows measurement points for the area ratio of ferrite, and as long as predetermined requirements are met, there is no need to form a measurement surface for the number density as shown in FIG. 3. A detailed evaluation method of the standard deviation of the area ratio of ferrite is described later.

[High-strength steel plate]

The high-strength steel sheet according to this embodiment has the following chemical composition in terms of mass%:

C: 0.030 to 0.280%,

Si: 0.50 to 2.50%,

Mn: 1.00 to 4.00%,

sol.Al: 0.001 to 2.000%,

P: 0.100% or less,

S: 0.0200% or less,

N: 0.01000% or less,

O: 0.0100% or less,

B: 0 to 0.010%,

Ti: 0 to 0.20%,

Nb: 0 to 0.20%,

V: 0 to 1.000%,

Cr: 0 to 1.000%,

Mo: 0 to 1.000%,

Cu: 0 to 1.000%,

Co: 0 to 1.000%,

W: 0 to 1.000%,

Ni: 0 to 1.000%,

Ca: 0 to 0.0100%,

Mg: 0 to 0.0100%,

REM: 0 to 0.0100%,

Zr: 0 to 0.0100% or less, and

Residue: Fe and impurities

Including,

Metal structure is an area ratio,

Ferrite: 20% to 70%,

Retained austenite: 5% to 40%,

Fresh martensite: 0% to 30%,

Sum of tempered martensite and bainite: 20% to 75%, and

Total of perlite and cementite: 0% to 10%

Including,

In the range of 1/8 thickness to 3/8 thickness from the surface, the ratio of the number of retained austenites with an aspect ratio of 2.0 or more to the total number of retained austenites is 50% or more,

The standard deviation of the area ratio of ferrite measured at 10 points across 50 mm along the width direction of the sheet at a position 1/4 of the sheet thickness of the cross section parallel to the rolling direction and perpendicular to the surface is less than 10%,

The tensile strength is more than 780MPa.

1. Chemical composition

Hereinafter, the component composition of the high-strength steel sheet according to this embodiment will be described in detail. The high-strength steel sheet according to the present embodiment contains basic elements as chemical components, optional elements as necessary, and the remainder contains Fe and impurities.

(C: 0.030% or more and 0.280% or less)

C is an important element to ensure the strength of steel sheets. If the C content is less than 0.030%, a tensile strength of 780 MPa or more cannot be secured. Therefore, the C content is set to 0.030% or more, and is preferably 0.050% or more, 0.100% or more, 0.120% or more, or 0.140% or more.

On the other hand, when the C content exceeds 0.280%, weldability deteriorates, so the upper limit is set at 0.280%. Preferably, the C content is 0.260% or less or 0.250% or less, more preferably 0.200% or less, 0.180% or less, or 0.160% or less.

(Si: 0.50% or more and 2.50% or less)

Si is an important element in suppressing precipitation of iron-based carbides and stabilizing residual γ. If the Si content is less than 0.50%, it is difficult to obtain residual γ of 5% or more and ductility deteriorates, so the Si content is set to 0.50% or more. The Si content is preferably 0.80% or more, 1.00% or more, or 1.20% or more.

On the other hand, if the Si content exceeds 2.50%, the surface properties deteriorate, so the Si content is set to 2.50% or less. The Si content is preferably 2.00% or less, more preferably 1.80% or less, 1.50% or less, or 1.30% or less.

(Mn: 1.00% or more and 4.00% or less)

Mn is an effective element to increase the mechanical strength of steel sheets. If the Mn content is less than 1.00%, a tensile strength of 780 MPa or more cannot be secured. Therefore, the Mn content is set to 1.00% or more. The Mn content is preferably 1.50% or more, more preferably 1.80% or more, 2.00% or more, or 2.20% or more.

On the other hand, if Mn is added excessively, the structure becomes non-uniform due to Mn segregation, and bending workability deteriorates. Therefore, the Mn content is set to 4.00% or less, preferably 3.00% or less, more preferably 2.80% or less, 2.60% or less, or 2.50% or less.

(sol.Al: 0.001% or more and 2.000% or less)

Al is an element that has the effect of deoxidizing steel and making the steel sheet sounder. If the sol.Al content is less than 0.001%, sufficient deoxidation cannot be achieved, so the sol.Al content is set to 0.001% or more. However, when sufficient deoxidation is required, addition of 0.010% or more is more preferable. More preferably, the sol.Al content is 0.020% or more, 0.030% or more, or 0.050% or more.

On the other hand, if the sol.Al content exceeds 2.000%, weldability decreases significantly and oxide inclusions increase, leading to significant deterioration of surface properties. Therefore, the sol.Al content is set to 2.000% or less, preferably 1.500% or less, more preferably 1.000% or less or 0.700% or less, and most preferably 0.090% or less, 0.080% or less, or 0.070% or less. do. In addition, sol.Al means acid-soluble Al that does not become an oxide such as Al ₂ O ₃ and is soluble in acid.

The high-strength steel sheet according to this embodiment contains impurities as chemical components. Here, “impurities” refer to things that are mixed from ore or scrap as raw materials, or from the manufacturing environment, etc., for example, when steel is industrially manufactured. Impurities mean elements such as P, S, and N, for example. In order to fully exhibit the effect of this embodiment, these impurities are preferably limited as follows. Additionally, since it is preferable that the content of impurities is small, there is no need to limit the lower limit, and the lower limit of impurities may be 0%.

(P: 0.100% or less)

P is generally an impurity contained in steel, but since it has the effect of increasing tensile strength, P may be actively included. However, if the P content exceeds 0.100%, the deterioration of weldability becomes significant. Therefore, the P content is limited to 0.100% or less. The P content is preferably limited to 0.080% or less, 0.070% or less, or 0.050% or less. In order to more reliably obtain the effect due to the above action, the P content may be 0.001% or more, 0.002% or more, or 0.005% or more.

(S: 0.0200% or less)

S is an impurity contained in steel, and from the viewpoint of weldability, the smaller the S, the more preferable it is. If the S content exceeds 0.0200%, the decline in weldability becomes significant, the amount of MnS precipitated increases, and low-temperature toughness decreases. Therefore, the S content is limited to 0.0200% or less. The S content is preferably limited to 0.0100% or less, more preferably 0.0080% or less, 0.0070% or less, or 0.0050% or less. Additionally, from the viewpoint of desulfurization cost, the S content may be 0.0010% or more, 0.0015% or more, or 0.0020% or more.

(N: 0.01000% or less)

N is an impurity contained in steel, and the smaller the N, the more preferable it is from the viewpoint of weldability. If the N content exceeds 0.01000%, the decline in weldability becomes significant. Therefore, the N content is limited to 0.01000% or less, preferably 0.00900% or less, 0.00700% or less, or 0.00500% or less. The lower limit of the N content is not particularly limited, but for example, the N content may be 0.00005% or more, 0.00010% or more, or 0.00020% or more.

(O: 0.0100% or less)

O is an impurity contained in steel, and the smaller the O, the more preferable it is from the viewpoint of weldability. If the O content exceeds 0.0100%, the decline in weldability becomes significant. Therefore, the O content is limited to 0.0100% or less, and is preferably 0.0090% or less, 0.0070% or less, or 0.0050% or less. The lower limit of the O content is not particularly limited, but for example, the O content may be 0.0005% or more, 0.0008% or more, or 0.0010% or more.

The high-strength steel sheet according to the present embodiment may contain selected elements in addition to the basic elements and impurities described above. For example, instead of part of the remaining Fe as described above, B, Ti, Nb, V, Cr, Mo, Cu, Co, W, Ni, Ca, Mg, REM, and Zr may be contained as optional elements. These optional elements may be contained depending on the purpose. Therefore, there is no need to limit the lower limit of these selected elements, and the lower limit may be 0%. Moreover, even if these selected elements are contained as impurities, the above effect is not impaired.

(B: 0% or more and 0.010% or less)

(Ti: 0% or more and 0.20% or less)

(Nb: 0% or more and 0.20% or less)

(V: 0% or more and 1.000% or less)

(Cr: 0% or more and 1.000% or less)

(Mo: 0% or more and 1.000% or less)

(Cu: 0% or more and 1.000% or less)

(Co: 0% or more and 1.000% or less)

(W: 0% or more and 1.000% or less)

(Ni: 0% or more and 1.000% or less)

B, Ti, Nb, V, Cr, Mo, Cu, Co, W, and Ni are all elements that are effective in ensuring stable strength. Therefore, these elements may be contained. However, even if B is contained in excess of 0.010%, Ti and Nb are each contained in excess of 0.20%, and V, Cr, Mo, Cu, Co, W, and Ni are each contained in excess of 1.000%, the effects due to the above actions tend to be saturated and are economically disadvantageous. There are cases where it is canceled.

Therefore, the B content is set to 0.010% or less, the Ti and Nb contents are each set to 0.20% or less, and the V, Cr, Mo, Cu, Co, W, and Ni contents are set to be 1.0% or less or 1.000% or less, respectively. The content of B may be 0.008% or less, 0.007% or less, or 0.005% or less. The upper limit of each content of Ti and Nb may be 0.18%, 0.15%, or 0.10%. The upper limit of each content of V, Cr, Mo, Cu, Co, W, and Ni may be 0.500% or less, 0.300% or less, or 0.100% or less.

In addition, in order to more reliably obtain the effect of the above action,

B: 0.001% or more, 0.002% or more, or 0.004% or more,

Ti: 0.01% or more, 0.02% or more, or 0.05% or more,

Nb: 0.01% or more, 0.02% or more, or 0.05% or more,

V: 0.005% or more, 0.008% or more, or 0.010% or more,

Cr: 0.005% or more, 0.008% or more, or 0.010% or more,

Mo: 0.005% or more, 0.008% or more, or 0.010% or more,

Cu: 0.005% or more, 0.008% or more, or 0.010% or more,

Co: 0.005% or more, 0.008% or more, or 0.010% or more,

W: 0.005% or more, 0.008% or more, or 0.010% or more, and

Ni: 0.005% or more, 0.008% or more, or 0.010% or more

It is preferable that it contains at least one of the following.

(Ca: 0% or more and 0.0100% or less)

(Mg: 0% or more and 0.0100% or less)

(REM: 0% or more and 0.0100% or less)

(Zr: 0% or more and 0.0100% or less)

Ca, Mg, REM, and Zr are all elements that contribute to inclusion control, especially fine dispersion of inclusions, and have the effect of increasing toughness. Therefore, one or two or more types of these elements may be contained. However, if each element is contained in excess of 0.0100%, deterioration of surface properties may become noticeable. Therefore, it is preferable that the contents of Ca, Mg, REM, and Zr are respectively 0.01% or less or 0.0100% or less. The upper limit of each content of Ca, Mg, REM, and Zr may be 0.0080%, 0.0050%, or 0.0030%. In addition, in order to more reliably obtain the effect due to the above action, it is preferable that the content of at least one of these elements is 0.0003% or more, 0.0005% or more, or 0.0010% or more.

Here, REM refers to a total of 17 elements including Sc, Y, and lanthanoid, and is at least one type among them. The REM content means the total content of at least one type of these elements. In the case of lanthanoids, they are added industrially in the form of misch metal.

In addition, in the high-strength steel sheet according to the present embodiment, as chemical components in terms of mass%, Ca: 0.0003% or more and 0.0100% or less, Mg: 0.0003% or more and 0.0100% or less, REM: 0.0003% or more and 0.0100% or less, Zr: 0.0003% or more. It is preferable to contain at least one type of 0.0100% or less.

The above-described steel components can be measured using a general analysis method for steel. For example, steel components can be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Additionally, C and S can be measured using the combustion-infrared absorption method, N can be measured using the inert gas melting-thermal conductivity method, and O can be measured using the inert gas melting-non-dispersive infrared absorption method.

2. Metal structure

In the high-strength steel sheet according to the present embodiment, the metal structure has an area ratio of ferrite: 20% to 70%, retained austenite: 5% to 40%, fresh martensite: 0% to 30%, tempered martensite, and bainite. sum of: 20% to 75%, and sum of perlite and cementite: 0% to 10%.

(Ferrite: 20% to 70%)

Ferrite is a relatively soft structure that contributes to forming. By having ferrite, ductility, hole expandability, and bendability are improved. To obtain this effect, it is necessary to have 20% or more of ferrite. Therefore, the area ratio of ferrite in the metal structure is set to 20% or more. The area ratio of ferrite may be 25% or more, 30% or more, or 35% or more.

If the ferrite content exceeds 70%, it becomes difficult to achieve a tensile strength of 780 MPa or more. Therefore, the area ratio of ferrite in the metal structure is set to 70% or less. The area ratio of ferrite may be 65% or less, 60% or less, or 50% or less.

(Residual austenite: 5% to 40%)

Retained austenite is a structure that contributes to ductility. To obtain this effect, retained austenite of 5% or more is required. Therefore, the area ratio of retained austenite in the metal structure is set to 5% or more, and is preferably 8% or more, 10% or more, or 15% or more.

In the manufacturing method according to the present embodiment, it is practically impossible to allow 40% or more of retained austenite to remain. Therefore, the upper limit of the area ratio of retained austenite in the metal structure is 40%. The area ratio of retained austenite may be 35% or less, 30% or less, or 25% or less.

(Fresh martensite: 0% to 30%)

Fresh martensite is a structure that hinders formability instead of contributing to strength. Therefore, fresh martensite does not need to be included, and its lower limit is set to 0%.

On the other hand, in order to obtain the effect of improving strength due to fresh martensite, it is preferable to have 2% or more, 5% or more, or 8% or more of fresh martensite.

On the other hand, if the fresh martensite content exceeds 30%, ductility and hole expandability deteriorate, so the area ratio of fresh martensite in the metal structure is set to 30% or less. The area ratio of fresh martensite is preferably 20% or less, and more preferably 15% or less or 10% or less.

(Total of tempered martensite and bainite: 20% to 75%)

Tempering martensite and bainite are structures that contribute to strength. In order to obtain a tensile strength of 780 MPa or more, a total of 20% or more of tempered martensite and bainite is required. Therefore, in the metal structure of the high-strength steel sheet according to the present embodiment, the total area ratio of tempered martensite and bainite is 20% or more, and is preferably 30% or more, 40% or more, or 50% or more.

On the other hand, there is no need to specify an upper limit for the sum of tempered martensite and bainite. As described above, the metal structure of the steel sheet according to the present embodiment includes 20% or more of ferrite and 5% or more of retained austenite, but the balance may all be tempered martensite and bainite. In other words, the total area ratio of tempered martensite and bainite can be set to 75% at maximum. The total area ratio of tempered martensite and bainite may be 70% or less, 60% or less, or 55% or less.

(Total of perlite and cementite: 0% to 10%)

Pearlite and cementite are structures that inhibit formability. If the total area ratio of pearlite and cementite is more than 10%, it is not preferable because the formability deteriorates significantly. Therefore, the total area ratio of pearlite and cementite is set to 10% or less in total. The total area ratio of pearlite and cementite may be 8% or less, 5% or less, or 3% or less. Since pearlite and cementite are not required to solve the problems of the present invention, the lower limit of their total area ratio is 0%. However, the total area ratio of pearlite and cementite may be 0.5% or more, 1% or more, or 2% or more.

Method for measuring metal structure

Identification of bainite, tempered martensite, ferrite, pearlite, retained austenite, and martensite, which constitute the metal structure of the high-strength steel sheet according to the present embodiment as described above, confirmation of their presence positions, and measurement of area fractions are carried out using the following methods. carried out by

First, a cross section parallel to the rolling direction (i.e., a cross section parallel to the rolling direction and perpendicular to the surface) is corroded using the Nital reagent and the reagent disclosed in Japanese Patent Application Laid-Open No. 59-219473. Regarding cross-sectional corrosion, specifically, solution A is a solution of 1 to 5 g of picric acid dissolved in 100 ml of ethanol, and solution B is a solution of 1 to 25 g of sodium thiosulfate and 1 to 5 g of citric acid in 100 ml of water. Liquid A and B are mixed in a ratio of 1:1 to make a mixed liquid. Nitric acid in a ratio of 1.5 to 4% is further added to the total amount of the mixed liquid, and the mixed liquid is used as a pretreatment liquid. Additionally, the pretreatment solution is added to the 2% Naital solution in a ratio of 10% based on the total amount of the 2% Natal solution, and the mixed solution is used as the post-treatment solution. A cross section parallel to the rolling direction (i.e., a cross section parallel to the rolling direction and perpendicular to the surface) is immersed in the pre-treatment solution for 3 to 15 seconds, washed with alcohol and dried, and then immersed in the post-treatment solution for 3 to 20 seconds. After washing, the cross section is corroded by washing with water and drying.

Next, as shown in FIG. 1, a scanning electron microscope was used at a depth of 1/4 of the sheet thickness from the surface (rolled surface 11) of the steel sheet 1 and at a position in the center of the sheet width direction (WD). By observing at least three areas of 40 ㎛ x 30 ㎛ at a magnification of 1,000 to 100,000 times, the metal structure is identified, its location is confirmed, and the area fraction is measured. In addition, even when the measurement object is a steel sheet that has not undergone any special machining after manufacturing (in other words, a steel sheet that has not been cut from a coil), the central position in the sheet width direction is the sheet width direction (WD). It is a position that is substantially equidistant from both ends of the steel plate (1) as seen from ).

Additionally, it is difficult to distinguish between lower bainite and tempered martensite using the above-described measurement method. Therefore, there is no need to distinguish between the two in this embodiment. That is, the total area fraction of “bainite and tempered martensite” is obtained by measuring the area fractions of “upper bainite” and “lower bainite or tempered martensite.” The upper bainite is an aggregate of laths and is a structure containing carbides between the laths. Lower bainite is a structure containing iron-based carbides with a long diameter of 5 nm or more and extending in the same direction. Tempered martensite is a collection of lath-like crystal grains, and is a structure containing iron-based carbides with a long diameter of 5 nm or more and extended in different directions.

Ferrite has low luminance and is an area where the underlying structure cannot be observed. The area where the brightness is high and the underlying structure is not revealed by etching is judged to be fresh martensite or retained austenite. Therefore, the area fraction of fresh martensite can be obtained as the difference between the area fraction of the uncorroded region observed with FE-SEM and the area fraction of retained austenite measured with X-rays, which will be described later.

Pearlite refers to an area where plate-shaped cementite and plate-shaped ferrite are arranged alternately. In observation by FE-SEM, pearlite and the above-mentioned structures (ferrite, bainitic ferrite, bainite, tempered martensite) can be clearly distinguished.

Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSP (Electron Back Scattering Diffraction Pattern) analysis, and magnetic measurement. Measured values may vary depending on the measurement method. . In this embodiment, the area fraction of retained austenite is measured by X-ray diffraction. In the measurement of the retained austenite area fraction by (cross section perpendicular to The strength is determined and then calculated using the strength average method to obtain the area fraction of retained austenite.

(In the range of 1/8 thickness to 3/8 thickness, the number ratio of retained austenite with an aspect ratio of 2.0 or more to the total retained austenite is 50% or more)

Forming the structure of retained austenite into a plate shape contributes to improving ductility, pore expandability, and bendability, and is one of the important structure creation points in the present invention. Forming the retained austenite into a plate has the effect of suppressing distorted distribution of austenite during forming and appropriately stabilizing the retained austenite against plastic deformation, thereby improving ductility and hole expandability. The form of retained austenite that has this effect has an aspect ratio of 2.0 or more.

In order to obtain this effect, in the range of 1/8 thickness to 3/8 thickness, the number ratio of retained austenite with an aspect ratio of 2.0 or more must be 50% or more with respect to the total retained austenite. Therefore, the number ratio is set to 50% or more, and 70% or more is preferable. If the number ratio is less than 50%, it is undesirable because it becomes difficult to achieve both excellent ductility, hole expandability, and bendability.

The aspect ratio and long diameter of the retained austenite grains contained in the steel structure inside the steel plate were evaluated by observing the grains using FE-SEM and performing high-resolution crystal orientation analysis using the EBSD method (electron beam backscattering diffraction method). do.

First, as shown in FIG. 2, a sample is collected using a cross section parallel to the rolling direction and thickness direction of the steel sheet as the observation surface 13, and the observation surface is polished to a mirror finish. Next, one to a plurality of observation fields of view in the range 131 of 1/8 thickness to 3/8 thickness centered on the position of 1/4 thickness from the surface (rolled surface) 11 on the observation surface 13. In this case, crystal structure analysis is performed using the EBSD method for an area of 2.0 x 10 ^-9 m ² or more in total (either multiple views or the same view is possible). Next, in order to avoid measurement errors, from the crystal orientations of the retained austenite particles measured by the above method, only austenite with a major axis length of 0.1 μm or more is extracted and a crystal orientation map is drawn. Boundaries that generate a crystal orientation difference of 10° or more are considered grain boundaries of retained austenite grains. The aspect ratio is determined by dividing the major axis length of the retained austenite grain by the minor axis length. The long diameter is taken as the long axis length of the retained austenite grains. “OIM Analysys 6.0” manufactured by TSL is used to analyze the data obtained by the EBSD method during measurement. Additionally, the distance (step) between ratings is set to 0.01 to 0.20 μm. From the observation results, the region judged to be FCC iron is regarded as retained austenite. From this result, the number ratio of retained austenite with an aspect ratio of 2.0 or more to the total retained austenite in the range of 1/8 thickness to 3/8 thickness is determined.

(When the area ratio of ferrite at 1/4 of the sheet thickness in a cross section parallel to the rolling direction and perpendicular to the surface was measured at 10 locations across 50 mm in the width direction of the sheet, the standard deviation of the area ratio of ferrite was 10%. under)

In the present invention, ferrite is important to ensure ductility and hole expandability. On the other hand, the strength, ductility, and pore expandability change depending on the tissue fraction. Therefore, it is important for obtaining material stability that the ferrite structure fraction is uniformly distributed in the hot rolling width direction.

As shown in FIG. 3, the area ratio of ferrite at the 1/4 plate thickness position 121 of the cross section parallel to the rolling direction (i.e., the cross section 12 parallel to the rolling direction and perpendicular to the surface) is, When measuring at 10 points across 50 mm along the sheet width direction (i.e., the direction perpendicular to the rolling direction (RD)) (WD), if the standard deviation of the area ratio of ferrite is 10% or more, the cause of the mechanical characteristics is fluctuation. Therefore, material stability is not obtained. Therefore, the standard deviation of the area ratio of the above-mentioned ferrite is set to less than 10%, and is preferably 8% or less, 5% or less, or 4% or less. Additionally, when the size of the steel sheet to be measured along the width direction is sufficiently large, the measurement point of the standard deviation of the area ratio of ferrite may be placed on a straight line along the sheet width direction. On the other hand, when the size of the steel sheet to be measured in the width direction is less than 450 mm, the measurement points for the standard deviation of the area ratio of ferrite may be placed on two or more straight lines along the width direction of the steel sheet. Even when measuring the standard deviation in the sheet width direction of properties other than ferrite (e.g. surface roughness, etc.), measurement points can be arranged as described above.

3. Standard deviation of surface roughness Ra

(The standard deviation of surface roughness Ra measured at 10 points across 50 mm along the sheet width direction is preferably 0.5 ㎛ or less.)

The steel sheet according to this embodiment is not particularly limited as long as the chemical composition, metal structure, and tensile strength described later are within predetermined ranges. On the other hand, when the surface roughness Ra of the rolled surface 11 is measured at 10 locations across 50 mm along the sheet width direction (i.e., a direction perpendicular to the rolling direction), the standard deviation of the surface roughness Ra may be 0.5 μm or less. By suppressing variation in surface roughness Ra, variation in bending workability can be suppressed, and material stability can be further improved. Therefore, it is desirable to set the standard deviation to 0.5 μm or less. However, the surface roughness of the steel plate can be freely changed through additional processes. For example, after manufacturing a high-strength steel sheet with excellent material stability by a preferred manufacturing method described later, the high-strength steel sheet may be subjected to processing to change the surface roughness, such as hairline processing. From this point of view as well, it is not essential that the standard deviation of the surface roughness Ra be within the above-mentioned range.

In addition, the surface roughness Ra was determined by using a contact-type roughness meter (Surtest SJ-500 manufactured by Mitutoyo) to obtain a roughness curve over a length of 5 mm in the sheet width direction at each measurement position, using the method described in JIS B0601:2001. Find the arithmetic average illuminance Ra. Using the arithmetic average roughness Ra value at each measurement position obtained in this way, the standard deviation of the surface roughness Ra is obtained.

In addition, when a surface treatment film such as plating or painting is disposed on the surface of a steel sheet, “surface roughness Ra of the steel sheet” means the surface roughness measured after removing the surface treatment film from the steel sheet. That is, the surface roughness Ra of the steel plate is the surface roughness of the base iron. The method for removing the surface treatment film can be appropriately selected depending on the type of the surface treatment film within a range that does not affect the surface roughness of the base iron. For example, when the surface treatment film is zinc plating, diluted hydrochloric acid containing an inhibitor may be used to dissolve the zinc plating layer. Thereby, only the galvanized layer can be peeled from the steel sheet. An inhibitor is an additive used to suppress changes in roughness caused by preventing overdissolution of base iron. For example, to hydrochloric acid diluted 10 to 100 times, a corrosion inhibitor for hydrochloric acid pickling "Evit No. 700BK" manufactured by Asahi Chemical Industry Co., Ltd. was added to a concentration of 0.6 g/L, and this was applied to the zinc plating layer. It can be used as a peeling means.

4. Mechanical characteristics

(Tensile strength TS: 780MPa or more)

The high-strength steel sheet according to this embodiment has sufficient strength to contribute to lightweighting of automobiles and has a tensile strength (TS) of 780 MPa or more. The tensile strength of the steel plate may be 800 MPa or more, 900 MPa or more, or 1000 MPa or more. On the other hand, it is estimated that it will be difficult to exceed 1470 MPa in the configuration of this embodiment. Therefore, there is no need to specifically determine the upper limit of the tensile strength, but in this embodiment, the upper limit of the practical tensile strength can be set to 1470 MPa. Additionally, the tensile strength of the steel plate may be 1400 MPa or less, 1300 MPa or less, or 1200 MPa or less.

In addition, the tensile test may be performed according to the following procedures in accordance with JIS Z2241 (2011). JIS 5 test pieces are taken from 10 locations of the high-strength steel plate at intervals of 50 mm in the width direction of the plate. Here, the width direction of the steel plate and the longitudinal direction of the test piece are made to match. In addition, in order to prevent the sampling positions of each test piece from interfering with each other, each test piece is taken from a position offset from the rolling direction of the steel sheet. A tensile test is performed on these test pieces in accordance with the provisions of JIS Z 2241 (2011), the tensile strength TS (MPa) is determined, and the average value is calculated. This average value is regarded as the tensile strength of the high-strength steel plate.

In addition, the high-strength steel sheet according to the present embodiment may have the following characteristics, such as ductility and hole expandability, as indices of formability. These mechanical properties are obtained by various properties of the high-strength steel sheet according to the present embodiment described above.

(Total elongation EL)

The high-strength steel sheet according to the present embodiment may have a total elongation of 14% or more in a tensile test as an index of ductility. On the other hand, in the configuration of this embodiment, it is difficult to make the total elongation exceed 35%. Therefore, the upper limit of the actual total elongation may be 35%.

(hole expandability)

The high-strength steel sheet according to the present embodiment may have a hole expansion ratio of 25% or more as an index of hole expandability. On the other hand, in the configuration of this embodiment, it is difficult to make the hole expansion ratio exceed 80%. Therefore, the upper limit of the actual hole expansion rate may be 80%.

The hole expansion rate can be evaluated by a hole expansion test based on the test method described in the Japan Iron and Steel Federation standard JFS T 1001-1996.

(bendability)

The high-strength steel sheet according to the present embodiment may have an R/t of 2.0 or less when the value R/t divided by the limit bending R (mm) divided by the sheet thickness t (mm) is used as an index of bendability. On the other hand, in the configuration of this embodiment, it is difficult to set the bendability index R/t to 0.1 or less. Therefore, the lower limit of the actual bendability index R/t may be 0.1.

The limit bending R is obtained by repeatedly performing bending tests applying various bending radii. In the bending test, bending is performed in accordance with JIS Z 2248 (V block 90° bending test). The bend radius (more precisely, the inner radius of the bend) varies with a pitch of 0.5 mm. The smaller the bending radius in the bending test, the more likely it is that cracks, scratches, and other defects will occur in the steel sheet. The minimum bending obtained in this test that does not cause cracks, scratches or other defects in the steel sheet is considered the limit bending R. Then, the value obtained by dividing this limit bending R by the thickness t of the steel sheet is used as an index R/t for evaluating bendability.

In the high-strength steel sheet according to the present embodiment, as an indicator that the material is stable, the standard deviation of TS in the tensile test results measured at 10 points across 50 mm along the sheet width direction (i.e., the direction perpendicular to the rolling direction) It may be 50 MPa or less and the standard deviation of EL may be 1% or less. The method for calculating the TS standard deviation and the EL standard deviation is the same as the tensile test method for calculating the average value of the tensile strength described above. By determining the standard deviation of the results of 10 tensile tests by the above-described method, the TS standard deviation and EL standard deviation are obtained.

In addition, in the high-strength steel sheet according to this embodiment, the standard deviation of R/t (limit bending R (mm), sheet thickness t (mm)) measured at 10 points across 50 mm along the sheet width direction may be 0.2 or less. .

5. Manufacturing method

Next, an example of a preferable manufacturing method for the high-strength steel sheet according to the present embodiment will be described. However, please note that the manufacturing method of the high-strength steel sheet according to this embodiment is not particularly limited. All steel plates that satisfy the above-mentioned requirements are considered to be steel plates according to the present embodiment, regardless of their manufacturing method.

The manufacturing process preceding hot rolling is not particularly limited. That is, following melting in a blast furnace or electric furnace, various secondary smelting may be performed, and then casting may be performed by methods such as normal continuous casting, casting using the ingot method, or thin slab casting. In the case of continuous casting, the cast slab may be cooled to a low temperature and then heated again before hot rolling, or the cast slab may be hot rolled as is after casting without cooling to a low temperature. It is okay to use scrap as raw materials.

A heating process is performed on the cast slab. In this heating process, it is preferable to heat the slab to a temperature of 1100°C or higher and 1300°C or lower. Since coarse precipitates (iron-based carbides, carbonitrides of alloy elements, etc.) deposited in the slab may impair material stability, it is preferable to heat the slab to 1100°C or higher to dissolve them. Meanwhile, from the viewpoint of preventing scale loss, the slab heating temperature is preferably 1300°C or lower.

Next, the heated slab is rough-rolled to produce a rough-rolled plate.

Rough rolling can be done by shaping the slab into the desired size and shape, and the conditions are not particularly limited. In addition, since the thickness of the rough rolled sheet affects the amount of temperature decrease from the front end to the rear end of the hot rolled steel sheet from the start of rolling to the completion of rolling in the finish rolling process, it is preferable to take this into consideration when deciding.

Finish rolling is performed on the rough rolled sheet. In this finish rolling process, multi-stage finish rolling is performed. In this embodiment, finish rolling is performed in a temperature range of 850°C to 1200°C under conditions that satisfy the following formula (1).

K'/Si ^* ≥2.5···(1)

Here, when Si≥0.35, Si ^* =140√Si, and when Si<0.35, Si ^* =80. In addition, Si represents the Si content (mass %) of the steel plate.

In addition, K' in the above formula (1) is expressed by the following formula (2).

Ｋ'=D×(DT-930)×1.5+Σ((FT _n -930)×S _n )···(2)

Here, D is the hourly spray amount of hydraulic descaling before the start of finish rolling (m ³ /min), DT is the steel sheet temperature (°C) when performing hydraulic descaling before the start of finish rolling, and FT _n is the nth stage of finish rolling. The steel sheet temperature (°C), S _n , is the spray amount per hour (m ³ /min) when water is sprayed on the steel sheet in the form of a spray between the n-1st stage and the nth stage of finish rolling.

Si ^* is a parameter related to the steel sheet component that indicates the ease of occurrence of unevenness due to scale. If the amount of Si in the steel sheet component is large, the scale generated on the surface layer during hot rolling grows from wustite (FeO), which is relatively easy to descale and difficult to create unevenness on the steel sheet, so that it spreads roots into the steel sheet, making it easy to create unevenness. It changes into iron olivine (Fe ₂ SiO ₄ ). Therefore, the larger the amount of Si, that is, the larger Si ^* , the more likely it is that irregularities in the surface layer will be formed. Here, the ease of forming irregularities in the surface layer by adding Si becomes particularly effective when 0.35% by mass or more of Si is added. Therefore, when 0.35 mass% or more is added, Si ^* becomes a function of Si, but when it is 0.35 mass% or less, it becomes a constant.

K' is a parameter of manufacturing conditions indicating the difficulty in forming irregularities. The first item of the above equation (2) is that in order to suppress the formation of unevenness, when hydraulic descaling is performed before the start of finish rolling, the greater the hourly spray amount of hydraulic descaling and the higher the steel sheet temperature, the more effective it is from the viewpoint of descaling. indicates. When performing multiple descalings before start of finish rolling, the descaling value closest to the finish rolling is used.

The second item in the above equation (2) is a term expressing the effect of descaling scale that has not yet been peeled off by descaling before finishing or scale that was re-formed during finish rolling during finish rolling, and at high temperatures, a large amount It is shown that descaling becomes easier by spraying water on the steel sheet in the form of a spray.

If the ratio of the parameter K' of the manufacturing conditions, which indicates the difficulty of forming unevenness, and Si ^* , the parameter related to the steel sheet component, which indicates the ease of forming scale flaws, is 2.5 or more or 2.50 or more, unevenness can be sufficiently suppressed, and temperature fluctuations during tempering can be reduced. It can be suppressed. Therefore, K'/Si ^* is set to 2.5 or more, preferably 3.0 or more, and more preferably 3.5 or more.

In addition, in order to set the standard deviation of the surface roughness Ra measured at 10 positions at 50 mm intervals in the sheet width direction (i.e., the direction perpendicular to the rolling direction) to 0.5 μm or less, which is a preferred form of the steel sheet according to the present invention, It is desirable for K'/Si ^* to be 3.0 or more (K'/Si ^* ≥3.0).

Following finish rolling, cooling is performed at an average cooling rate of 50°C/s or higher, and coiling is performed at a coiling temperature of 450°C or lower. As described above, this is to control the form of residual γ after annealing by using bainite and martensite, which are low-temperature transformation structures, as the main structures. Here, the average cooling rate is the difference in temperature between the start of cooling and before winding divided by the time. If the average cooling rate is less than 50°C/s, ferrite transformation occurs, which inhibits control of structure form in the subsequent annealing process, and reduces the ratio of the number of retained austenites with an aspect ratio of 2.0 or more to the total number of retained austenites by 50. It cannot be controlled beyond %.

Likewise, if the coiling temperature exceeds 450°C, ferrite transformation occurs, and it becomes difficult to make the total of bainite and tempered martensite more than 20% of the total. Additionally, if the coiling temperature is higher than 450°C, the ratio of the number of retained austenites with an aspect ratio of 2.0 or more to the total number of retained austenites cannot be controlled to 50% or more. From this viewpoint, the coiling temperature is set to 450°C or lower, preferably 400°C or lower, and more preferably 200°C or lower. In addition, setting the coiling temperature to 450°C or lower also has the effect of suppressing the formation of internal oxides on the surface of the steel sheet after coiling and the increase in the roughness of the surface layer.

The high-strength steel sheet manufactured in this way is pickled for the purpose of removing oxides on the surface of the steel sheet. The pickling treatment may be performed, for example, in hydrochloric acid at a concentration of 3 to 10% at a temperature of 85°C to 98°C for 20 to 100 seconds.

Additionally, the manufactured hot-rolled steel sheet may be subjected to light reduction with a reduction ratio of 20% or less for the purpose of shape correction. However, if the reduction ratio under light pressure exceeds 20%, recrystallization occurs during the annealing process, and the shape control effect obtained during annealing from the low-temperature transformation structure is no longer obtained, so even when light pressure reduction is performed, the reduction The rate is 20% or less. It may be carried out before or after the light pressure pickling process. If light pressure is applied after the pickling process, there is an effect of further reducing the roughness of the surface layer. When the surface roughness Ra, which is a preferred form in the present invention, is measured at 10 positions at 50 mm intervals in the sheet width direction (i.e., the direction perpendicular to the rolling direction), the standard deviation of the surface roughness Ra is 0.5 ㎛ or less. In order to do this, it is necessary to perform light pressure reduction after the pickling process.

Annealing treatment is performed on the obtained steel sheet.

In the annealing process, the heating temperature is set to 10°C from the point A _c1 to the point A _c3 calculated by the following equation.

A _c1 =723-10.7×Mn-16.9×Ni+29.1×Si+16.9×Cr

A _c3 =879-346×C+65×Si-18×Mn+54×Al··(9)

When heated, ferrite-austenite transformation occurs from carbides formed in laths of the low-temperature transformation structure, etc., and plate-shaped austenite is generated. The area that has not been transformed into austenite can be thought of as a low-temperature transformed structure (tempered martensite or tempered bainite) tempered at a high temperature, but the dislocation density is greatly reduced by tempering and the underlying structure also becomes unclear, so observation of the structure after annealing This is the area evaluated as ferrite. For that reason, it is also called ferrite here. In addition, in observing the structure after annealing, the region evaluated as tempered martensite or bainite has a structure created by austenite generated by heating undergoing bainite transformation or martensite transformation during holding at 150°C to 550°C, which will be described later. It mainly refers to

The reason for setting the heating temperature to 10°C from point A _c1 to point A _c3 is to set the area ratio of ferrite to 20% to 70% and achieve an appropriate ferrite-austenite transformation fraction. The heating time is 10 to 1000 seconds. If the holding time is less than 1 second, there is a risk that the cementite in the steel remains without fully melting and the properties of the steel sheet deteriorate. Since this effect is saturated beyond 1000 seconds and leads to a decrease in productivity, the upper limit of the holding time is 1000 seconds.

Thereafter, the temperature is maintained between 150°C and 550°C for 10 seconds to 1000 seconds.

In this temperature range, a part of austenite is transformed into bainite or martensite, and dissolved carbon is discharged into austenite following bainite transformation, or dissolved carbon is discharged into austenite following tempering of martensite. , has the effect of stabilizing austenite. Below 150°C, most of the austenite is transformed into martensite, and a sufficient amount of retained austenite cannot be obtained. On the other hand, if the temperature is 550°C or higher, pearlite transformation occurs and retained austenite cannot be sufficiently stabilized. If the holding time is less than 10 seconds, carbon diffusion does not occur sufficiently, and the retained austenite cannot be sufficiently stabilized. If it exceeds 1000 seconds, the effect of stabilizing retained austenite is saturated, and productivity decreases.

Additionally, while maintaining this temperature range, you may heat or cool it within the temperature range. For example, once the temperature is lowered to a temperature range of 250°C or lower to transform a part of the retained austenite into martensite, and then reheated to a temperature range of approximately 400°C, martensite becomes a nucleation site for bainite transformation, forming a bayite. The effect of accelerating knight transformation is obtained.

Additionally, in this temperature range, hot dip galvanizing or alloyed hot dip galvanizing may be performed. As plating conditions such as zinc plating bath temperature and zinc plating bath composition in the hot dip galvanizing process, general conditions can be used and there are no particular restrictions. For example, the plating bath temperature may be 420 to 500°C, the penetration plate temperature of the steel sheet may be 420 to 500°C, and the immersion time may be 5 seconds or less. The plating bath preferably contains 0.08 to 0.2% of Al, but may also contain impurities such as Fe, Si, Mg, Mn, Cr, Ti, and Pb. Additionally, it is preferable to control the weight per unit area of hot-dip galvanizing using a known method such as gas wiping. The weight per unit area is usually 5 g/m ² or more per side, but is preferably 25 to 75 g/m ² , and more preferably 20 to 120 g/m ² .

When performing alloying treatment, it may be performed according to a normal method, but the alloying treatment temperature is preferably set to 460 to 550°C. If the alloying treatment is less than 460°C, the alloying speed is slowed, which not only impairs productivity, but also causes unevenness in the alloying treatment, so it is preferable that the alloying treatment temperature is 460°C or higher. On the other hand, if the alloying treatment temperature exceeds 550°C, pearlite transformation occurs and the retained austenite cannot be sufficiently stabilized.

In addition, the alloying treatment is preferably performed under conditions where the iron concentration in the hot-dip galvanized layer is 6.0% by mass or more.

When hot-dip galvanizing or alloyed hot-dip galvanizing is not performed, an electrogalvanizing layer may be formed on the steel sheet manufactured as above. The electrogalvanized layer can be formed by a conventionally known method.

The high-strength steel sheet according to this embodiment can be manufactured by the above-described manufacturing method.

Example

The high-strength steel sheet according to the present invention will be described in more detail below with reference to examples. However, the following examples are examples of the high-strength steel sheet of the present invention, and the high-strength steel sheet of the present invention is not limited to the following embodiments. The conditions in the examples described below are examples of conditions adopted to confirm the feasibility and effect of the present invention, and the present invention is not limited to these examples of conditions. The present invention can adopt various conditions as long as it achieves the purpose of the present invention without departing from the gist of the present invention.

Steel with the chemical composition shown in Table 1 is cast, and after casting, it is reheated as is or once cooled to room temperature, heated to a temperature range of 1200°C to 1350°C, and then rough-rolled into a slab at a temperature of 1100°C or higher. A rough rolled plate was produced. Additionally, in Table 1, values outside the scope of the invention are underlined.

For the rough rolled sheet, multi-stage finish rolling including 7 stages in total was performed under the conditions shown in Table 2.

Thereafter, cooling and winding after finish rolling were performed under each condition shown in Table 3.

Afterwards, pickling was performed for all conditions, but for some conditions, light pressure was applied before or after pickling. Thereafter, the temperature was raised to the heating temperature shown in Table 3 at a heating rate of 30°C/s to 150°C/s. After heating, the time and heating temperature shown in Table 3 were maintained. Then, under condition A, it was cooled to 250°C at 50 to 100°C/s, reheated to 400°C, and held for 300 seconds. In condition B, the temperature was cooled to 360°C at 50 to 100°C/s and maintained for 50 seconds. In Condition C, which is a comparative example, the temperature was cooled to 100°C at 100°C/s and maintained for 300 seconds.

After that, alloyed hot-dip galvanizing or hot-dip galvanizing was performed under some conditions. In the plating process, the steel sheet was in a temperature range of 400°C to 520°C.

For the obtained high-strength steel sheet, the metal structure was observed by the following method.

First, a cross section parallel to the rolling direction and perpendicular to the surface was etched using the Nital reagent and the reagent disclosed in Japanese Patent Application Laid-Open No. 59-219473. Regarding cross-sectional corrosion, specifically, solution A is a solution of 1 to 5 g of picric acid dissolved in 100 ml of ethanol, and solution B is a solution of 1 to 25 g of sodium thiosulfate and 1 to 5 g of citric acid in 100 ml of water. Liquid A and B are mixed in a ratio of 1:1 to make a mixed liquid. Nitric acid in a ratio of 1.5 to 4% is further added to the total amount of the mixed liquid, and the mixed liquid is used as a pretreatment liquid. Additionally, the pretreatment solution is added to the 2% Naital solution at a rate of 10% based on the total amount of the 2% Natal solution, and the mixed solution is used as the post-treatment solution. The cross section perpendicular to the rolling direction is immersed in the pretreatment liquid for 3 to 15 seconds, washed with alcohol and dried, then dipped in the posttreatment liquid for 3 to 20 seconds, washed with water and dried, thereby corroding the cross section.

Next, at least three areas of 40 ㎛ x 30 ㎛ are observed at a magnification of 1,000 to 100,000 using a scanning electron microscope at a depth of 1/4 of the sheet thickness from the steel sheet surface and at a central position in the sheet width direction, thereby observing the metal. Tissue identification, location of presence, and area fraction were measured.

In addition, the total area fraction of “bainite and tempered martensite” was obtained by measuring the area fractions of “upper bainite” and “lower bainite or tempered martensite.”

The area with low luminance and no underlying structure was judged to be ferrite. The area where the brightness was high and the underlying structure was not revealed by etching was judged to be fresh martensite or retained austenite. The area fraction of fresh martensite was obtained as the difference between the area fraction of the uncorroded region observed by FE-SEM and the area fraction of retained austenite measured by X-ray.

When observing pearlite by FE-SEM, pearlite, ferrite, bainitic ferrite, bainite, and tempered martensite can be clearly distinguished, and therefore the area ratio was obtained by this method.

The area fraction of retained austenite was measured by X-ray diffraction. First, in a cross section parallel to the rolling direction and perpendicular to the surface at a depth of 1/4 of the plate thickness of the steel plate, using Co-Kα lines, α(110), α(200), α(211), The integrated intensity of the six peaks of γ (111), γ (200), and γ (220) was determined and calculated using the intensity average method to obtain the area fraction of retained austenite.

The aspect ratio and long diameter of the retained austenite grains contained in the steel structure inside the steel plate were evaluated by observing the grains using FE-SEM and performing high-resolution crystal orientation analysis using the EBSD method (electron beam backscattering diffraction method). did.

First, samples were collected using a cross section parallel to the rolling direction and thickness direction of the steel sheet as an observation surface, and the observation surface was polished to a mirror finish. Next, in one to a plurality of observation fields ranging from 1/8 thickness to 3/8 thickness centered on the position of 1/4 thickness from the surface on the observation surface, a total of 2.0 × 10 ^-9 m ² or more Crystal structure analysis was performed using the EBSD method for the area (possible in either multiple fields of view or the same field of view). Next, in order to avoid measurement errors, from the crystal orientations of the retained austenite particles measured by the above method, only austenite with a major axis length of 0.1 μm or more was extracted and a crystal orientation map was drawn. Boundaries that generate a crystal orientation difference of 10° or more were considered grain boundaries of retained austenite particles. The aspect ratio was determined by dividing the major axis length of the retained austenite grain by the minor axis length. The long diameter was taken as the length of the long axis of the retained austenite particles. “OIM Analysys 6.0” manufactured by TSL was used to analyze the data obtained by the EBSD method during measurement. Additionally, the distance (step) between ratings was set to 0.01 to 0.20 μm. From the observation results, the area judged to be FCC iron was considered retained austenite. From these results, the number ratio of retained austenite with an aspect ratio of 2.0 or more to the total retained austenite in the range of 1/8 thickness to 3/8 thickness was determined.

The area ratio of ferrite at a position of 1/4 of the plate thickness in a cross section parallel to the rolling direction and perpendicular to the surface was determined according to the method described above. In the same way, the area ratio of ferrite was determined at 10 locations at 50 mm intervals in the width direction of the plate, and the standard deviation of the area ratio was calculated.

The standard deviation of the surface roughness Ra measured at 10 positions at 50 mm intervals in the sheet width direction was obtained by the following procedure. Using a contact-type roughness meter (Surtest SJ-500 manufactured by Mitutoyo), a roughness curve was acquired over a length of 5 mm in the board width direction at each measurement position, and the arithmetic mean roughness Ra was determined by the method described in JIS B0601:2001. did. The standard deviation of the surface roughness Ra was obtained using the arithmetic mean roughness Ra at each measurement position obtained in this way.

The tensile strength was tested in accordance with the provisions of JIS Z 2241 (2011) using a JIS No. 5 test piece taken from a high-strength steel plate so that the width direction of the plate was in the longitudinal direction, and the tensile strength TS (MPa) and total elongation were measured. (Total elongation) EL (%) was determined. Sampling was conducted at 10 locations at 50 mm intervals in the width direction of the steel plate. The average value of the tensile strength of the test pieces of 10 was regarded as the tensile strength TS of the steel sheet, and when TS ≥ 780 MPa was satisfied, it was said to have passed as a high-strength hot-rolled steel sheet.

Additionally, the standard deviations of TS and EL at 10 positions at 50 mm intervals in the sheet width direction were determined. A steel sheet with a standard deviation of TS of 50 MPa or less and a standard deviation of EL of 1% or less was determined to be a steel sheet with excellent material stability.

The hole expansion rate was evaluated by a hole expansion test based on the test method described in the Japan Iron and Steel Federation standard JFS T 1001-1996.

The bending test was performed in accordance with JIS Z2248 (V block 90° bending test), and the bending R (mm) was tested at a pitch of 0.5 mm.

Additionally, R/t was measured at 10 locations at 50 mm intervals in the width direction of the plate, and the standard deviation was obtained.

In Tables 4 and 5, values outside the scope of the invention are underlined. As shown in the table, in the examples satisfying the conditions of the present invention, tensile strength, ductility, hole expandability (elongation flangeability), bendability, variation in tensile strength, and variation in ductility were all excellent. On the other hand, in the comparative examples that do not meet at least one of the conditions of the present invention, at least one characteristic of tensile strength, ductility, hole expandability (elongation flangeability), bendability, variation in tensile strength, and variation in ductility is not sufficient. didn't

Specifically, in Comparative Examples 9 and 10, the standard deviation of the ferrite area ratio became large, and the TS standard deviation and EL standard deviation were rejected. This is presumed to be because hot rolling was performed under conditions where K'/Si ^* was insufficient.

In Comparative Example 11, the ratio of retained austenite with an aspect ratio of 2.0 or more was insufficient, and hole expandability was impaired. This is presumed to be because the average cooling rate after finish rolling was insufficient.

In Comparative Example 12, the ratio of retained austenite with an aspect ratio of 2.0 or more was insufficient, and hole expandability was impaired. This is presumed to be because the coiling temperature after finish rolling was too high.

In Comparative Example 13, the area ratio of ferrite was excessive, the area ratio of other structures was insufficient, and the tensile strength was insufficient. This is presumed to be because the heating temperature in the annealing process was lower than the A _c1 point of steel material A.

In Comparative Example 14, the ratio of retained austenite with an aspect ratio of 2.0 or more was insufficient, and hole expandability was impaired. This is presumed to be because the reduction ratio under light pressure applied to the steel sheet before annealing the steel sheet was excessive.

In Comparative Example 16, the amount of retained austenite was insufficient, and the overall elongation and hole expandability were impaired. This is presumed to be because the holding pattern in the annealing process was inappropriate, that is, the holding temperature was too low.

In Comparative Examples 31 and 32, the amount of Si was insufficient. Therefore, in Comparative Examples 31 and 32, the amount of retained austenite was insufficient, and the overall elongation and hole expandability were impaired.

1: High-strength steel plate (steel plate)
11: Surface (rolled surface)
12: Section parallel to the rolling direction and perpendicular to the surface
121: Position of 1/4 of the plate thickness in the cross section parallel to the rolling direction and perpendicular to the surface
13: Measuring surface of retained austenite
131: Range of 1/8 thickness to 3/8 thickness from the surface (rolled surface) on the measurement surface of retained austenite
RD: Rolling Direction
TD: Thickness Direction
WD: Width Direction

Claims (3)

As a chemical component, in mass%,
C: 0.030 to 0.280%,
Si: 0.50 to 2.50%,
Mn: 1.00 to 4.00%,
sol.Al: 0.001 to 2.000%,
P: 0.100% or less,
S: 0.0200% or less,
N: 0.01000% or less,
O: 0.0100% or less,
B: 0 to 0.010%,
Ti: 0 to 0.20%,
Nb: 0 to 0.20%,
V: 0 to 1.000%,
Cr: 0 to 1.000%,
Mo: 0 to 1.000%,
Cu: 0 to 1.000%,
Co: 0 to 1.000%,
W: 0 to 1.000%,
Ni: 0 to 1.000%,
Ca: 0 to 0.0100%,
Mg: 0 to 0.0100%,
REM: 0 to 0.0100%,
Zr: 0 to 0.0100%, and
Residue: Fe and impurities
It consists of
Metal structure is an area ratio,
Ferrite: 20% to 70%,
Retained austenite: 5% to 40%,
Fresh martensite: 0% to 30%,
Sum of tempered martensite and bainite: 20% to 75%, and
Total of perlite and cementite: 0% to 10%
Including,
In the range of 1/8 thickness to 3/8 thickness from the surface, the ratio of the number of retained austenites with an aspect ratio of 2.0 or more to the total number of retained austenites is 50% or more,
The standard deviation of the area ratio of ferrite measured at 10 points across 50 mm along the width direction of the sheet at a position 1/4 of the sheet thickness of the cross section parallel to the rolling direction and perpendicular to the surface is less than 10%,
Tensile strength greater than 780MPa
A high-strength steel plate characterized by: The high-strength steel sheet according to claim 1, wherein the standard deviation of the surface roughness Ra is 0.5 μm or less at 10 positions at 50 mm intervals in the width direction of the sheet. The chemical component according to claim 1 or 2, in mass%,
B: 0.001% to 0.010%,
Ti: 0.01 to 0.20%,
Nb: 0.01 to 0.20%,
V: 0.005% to 1.000%,
Cr: 0.005% to 1.000%,
Mo: 0.005% to 1.000%,
Cu: 0.005% to 1.000%,
Co: 0.005% to 1.000%,
W: 0.005% to 1.000%,
Ni: 0.005% to 1.000%,
Ca: 0.0003% to 0.0100%,
Mg: 0.0003% to 0.0100%,
REM: 0.0003% to 0.0100%, and
Zr: 0.0003% to 0.0100%
Containing at least one member of the group consisting of
A high-strength steel plate characterized by: