JP7168087B2 - high strength steel plate - Google Patents

Description

TECHNICAL FIELD The present invention relates to a high-strength steel sheet having excellent tensile strength, elongation, stretch-flangeability and bendability and excellent material stability.
This application claims priority based on Japanese Patent Application No. 2019-128612 filed in Japan on July 10, 2019, the content of which is incorporated herein.

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

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

On the other hand, for high-strength steel sheets, it is also important to have a technique to obtain stable material properties in the coil. This is because, until now, low-strength steel sheets had a relatively simple structure consisting mainly of ferrite, with a small amount of solid-solution strengthening elements as necessary to ensure strength. , low-temperature transformation structures such as bainite and martensite, and precipitates such as TiC are used to ensure strength, resulting in a complex structure. These phenomena such as transformation and precipitation are greatly affected by the temperature history, and temperature variations may inevitably occur in the manufacturing process. For example, in the manufacturing process of hot-rolled steel sheets, there is a possibility that the temperature history varies in the width direction and the longitudinal direction, such as unevenness in the way cooling water is applied in the width direction and unevenness in the cooling rate depending on the position in the coil after winding. be. Therefore, in the production of high-strength steel sheets, it is necessary to use a manufacturing method that reduces these temperature histories as much as possible, or to design materials that minimize the effects of temperature histories, and to stabilize the material properties.

As a technique for improving the ductility of a high-strength steel sheet, there is a TRIP steel that utilizes the TRIP (transformation-induced plasticity) effect by leaving an austenite phase in the steel structure (see, for example, Patent Document 1). TRIP steel has higher ductility than DP steel.
In addition, Non-Patent Document 1 discloses that the elongation and hole expansibility of the steel sheet are improved by using the double 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 in a hot rolled steel sheet having a tensile strength of 780 MPa or more, fine carbides are formed during hot rolling winding. is uniformly precipitated, and as a result, a technique for stabilizing the quality of the hot-rolled steel sheet has been reported.

Japanese Patent Application Laid-Open No. 2006-274418 Japanese Patent Application Laid-Open 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 sheet having both elongation and hole expansibility. In the method described in Non-Patent Document 1, since annealing is performed twice, the problem is that the fuel cost increases compared to the manufacturing method in which annealing is performed only once. Therefore, the present inventors annealed a hot-rolled steel sheet to create a similar plate-like structure (that is, a structure with a large austenite aspect ratio) without performing annealing twice. I tried a manufacturing method to create. Specifically, the present inventors investigated a manufacturing method in which a hot-rolled steel sheet is coiled at a low temperature of 450° C. or less and then annealed. By coiling at a 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 considered that by annealing a hot-rolled steel sheet having a structure mainly composed of a low-temperature transformation structure, a plate-like structure can be obtained in a single annealing.
However, in the steel sheet obtained by this method, material instability occurred. Specifically, variations in the amount of ferrite measured along the sheet width direction increased, and as a result, variations in mechanical properties increased.

An object of the present invention is to provide a high-strength hot-rolled steel sheet having excellent tensile strength, elongation, stretch-flangeability and bendability, and excellent material stability. The material stability means that the tensile strength and the total elongation of each part in the steel sheet have little variation.

(1) The high-strength steel sheet according to one aspect of the present invention has a chemical composition, in mass%, of C: 0.030 to 0.280%, Si: 0.50 to 2.50%, Mn: 1.00. ~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: 0-0.20%, Nb: 0-0.20%, V: 0-1.000%, Cr: 0-1.000%, Mo: 0-1.000%, Cu: 0 ~1.000%, Co: 0-1.000%, W: 0-1.000%, Ni: 0-1.000%, Ca: 0-0.0100%, Mg: 0-0.0100% , REM: 0 to 0.0100%, Zr: 0 to 0.0100%, and the balance: Fe and impurities , and the metal structure has an area ratio of ferrite: 20% to 70%, retained austenite: 5% to 40%, fresh martensite: 0% to 30%, total of tempered martensite and bainite: 20% to 75%, and total of pearlite and cementite: 0% to 10%, 1/8 thickness from the surface to In the range of 3/8 thickness, the number ratio of retained austenite with an aspect ratio of 2.0 or more to the total number of retained austenite is 50% or more, and the plate thickness of the cross section parallel to the rolling direction and perpendicular to the surface is 1/ The standard deviation of the area ratio of ferrite measured at 10 locations at 50 mm intervals along the width direction of the sheet at 4 positions is less than 10%, the tensile strength is 780 MPa or more, and it is a steel sheet for automobiles .
(2) The high-strength steel sheet described in (1) may have a standard deviation of surface roughness Ra of 0.5 μm or less at 10 positions at intervals of 50 mm in the sheet width direction.
(3) The high-strength steel sheet according to (1) or (2) has, as the chemical components, B: 0.001% to 0.010%, Ti: 0.01 to 0.20%, Nb: 0.01-0.20%, V: 0.005%-1.000%, Cr: 0.005%-1.000%, Mo: 0.005%-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% At least one may be contained.

According to the above aspect, it is possible to obtain a high-strength steel sheet having excellent tensile strength, elongation, stretch-flangeability and bendability, and excellent material stability.

It is a conceptual diagram showing an observation surface for evaluating metallographic structure. It is a conceptual diagram showing an observation surface for evaluating retained austenite. FIG. 2 is a conceptual diagram showing an observation surface for evaluating the standard deviation of the area ratio of ferrite;

The present inventors have extensively studied the cause of deterioration of material stability in a steel sheet that has been annealed once. The inventors of the present invention have 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. 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, the unevenness in surface roughness causes unevenness in emissivity in the process of heating for annealing, resulting in temperature variations in the steel sheet. As a result, the variation in the amount of ferrite increases in the steel sheet after annealing. The findings of the present inventors have revealed for the first time that controlling the surface properties of a hot-rolled steel sheet contributes to the stabilization of the quality of the hot-rolled and annealed steel sheet.
The present inventors have also found a hot rolling method that is effective for suppressing variations in the surface properties of steel sheets (hot-rolled steel sheets) before annealing. The present inventors discovered that the phenomenon in which the surface layer scale is pressed against the steel sheet by the hot rolling rolls during hot rolling greatly characterizes the surface properties of the steel sheet after hot rolling. In order to control the surface properties of hot-rolled steel sheets, it is important to control scale growth during hot rolling. It has been found that it can be done.

A high-strength steel sheet according to an embodiment of the present invention will be described in detail below. However, the present invention is not limited to the configuration disclosed in this embodiment, and various modifications can be made without departing from the gist of the present invention. Moreover, the lower limit value and the upper limit value are included in the range of numerical limits described below. Any numerical value indicated as "greater than" or "less than" is not included in the numerical range. "%" regarding the content of each element means "% by mass".

In the high-strength steel sheet 1 according to the present embodiment, the rolling direction RD, thickness direction TD, and width direction WD shown in FIGS. 1 to 3 are defined as follows. The rolling direction RD means the direction in which the steel sheet is moved by the rolling rolls during rolling. The plate thickness direction TD is a direction perpendicular to the rolling surface 11 of the steel plate. The width direction WD is a direction perpendicular to the rolling direction RD and the thickness direction TD. Note that the rolling direction RD can be easily identified based on the orientation direction of the crystal grains of the steel sheet. Therefore, it is possible to identify the rolling direction RD even in the steel sheet cut out from the material steel sheet after rolling.

In the high-strength steel sheet according to the present embodiment, the amount of ferrite in the metal structure and the like are specified. The metallographic structure is evaluated in a section 12 parallel to the rolling direction RD and perpendicular to the rolling plane 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 referred to as a cross section parallel to the rolling direction RD. A detailed metallographic evaluation method will be described later.
In addition, in the high-strength steel sheet according to the present embodiment, the ratio of the number of retained austenite having an aspect ratio of 2.0 or more to the total number of retained austenite is set. Retained austenite is evaluated in a cross section parallel to the rolling direction RD and thickness direction TD (see FIG. 2). A detailed method for evaluating retained austenite will be described later.

Furthermore, in the high-strength steel sheet according to the present embodiment, the standard deviation of the ferrite area ratio is specified. The area ratio of ferrite is measured at the 1/4 plate thickness position 121 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 were prepared at intervals of 50 mm along the sheet width direction WD, and the standard deviation of the area ratio of 10 ferrites measured on these surfaces was It is regarded as the standard deviation of the area ratio of ferrite according to the embodiment.

Note that the position of 1/4 of the plate thickness is a position of a depth of 1/4 of the thickness of the steel plate 1 from the rolled surface 11 of the steel plate 1 . In FIGS. 1 and 2, only the position at a depth of 1/4 of the thickness of the steel plate 1 from the upper rolling surface 11 of the steel plate 1 is shown as the 1/4 plate thickness position. However, as a matter of course, the position at a depth of 1/4 of the thickness of the steel plate 1 from the lower rolled surface 11 of the steel plate 1 can also be treated as the plate thickness 1/4 position. Moreover, in FIG. 3, only a part of the 10 measurement surfaces is illustrated. Furthermore, FIG. 3 only conceptually shows the measurement points of the ferrite area ratio, and as long as predetermined requirements are satisfied, it is not necessary to form the number density measurement surface as shown in FIG. A detailed evaluation method of the standard deviation of the area ratio of ferrite will be described later.

[High-strength steel plate]
The high-strength steel sheet according to the present embodiment has a chemical composition of, 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: 0-0.0100%,
REM: 0 to 0.0100%,
Zr: 0 to 0.0100% or less, and the balance: including Fe and impurities,
The metal structure is the 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 sum of pearlite and cementite: 0% to 10%
consists of
In the range of 1/8 thickness to 3/8 thickness from the surface, the number ratio of retained austenite with an aspect ratio of 2.0 or more to the total number of retained austenite is 50% or more,
The standard deviation of the area ratio of ferrite measured at 10 locations every 50 mm along the width direction at the 1/4 position of the thickness of the cross section parallel to the rolling direction and perpendicular to the surface is less than 10%,
Tensile strength is 780 MPa or more.

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

(C: 0.030% or more and 0.280% or less)
C is an important element for ensuring the steel plate strength. 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 0.030% or more, preferably 0.050% or more, 0.100% or more, 0.120% or more, or 0.140% or more.

On the other hand, if the C content exceeds 0.280%, the weldability deteriorates, so the upper limit is made 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 for suppressing precipitation of iron-based carbides and stabilizing residual γ. If the Si content is less than 0.50%, it is difficult to obtain a residual γ of 5% or more and the elongation deteriorates, so the Si content is made 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 quality is deteriorated, so the Si content is made 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 for increasing the mechanical strength of the steel sheet. If the Mn content is less than 1.00%, a tensile strength of 780 MPa or more cannot be secured. Therefore, the Mn content should be 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, when Mn is excessively added, the structure becomes uneven due to Mn segregation, and the bendability deteriorates. Therefore, the Mn content is 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 sound. sol. If the Al content is less than 0.001%, the sol. Al content shall be 0.001% or more. However, when sufficient deoxidation is required, addition of 0.010% or more is more desirable. More preferably, sol. Al content is 0.020% or more, 0.030% or more, or 0.050% or more.

On the other hand, sol. If the Al content exceeds 2.000%, the weldability is remarkably lowered, and oxide inclusions are increased, resulting in remarkably deteriorated surface properties. Therefore, sol. The Al content is 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, 0.700% or less. 080% or less, or 0.070% or less. In addition, sol. Al means acid-soluble Al that does not form an oxide such as Al ₂ O ₃ and is soluble in acid.

The high-strength steel sheet according to the present embodiment contains impurities as chemical components. The term "impurities" refers to, for example, substances mixed from ores and scraps used as raw materials or from the manufacturing environment or the like when steel is manufactured industrially. Impurities mean elements such as P, S, and N, for example. These impurities are preferably restricted as follows in order to fully exhibit the effects of the present embodiment. Also, 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 an impurity generally contained in steel, but since it has the effect of increasing the tensile strength, P may be positively contained. 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 of 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 amount, the better. If the S content exceeds 0.0200%, the weldability is remarkably deteriorated, and the precipitation amount of MnS is increased, resulting in deterioration of the low temperature toughness. 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. 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 from the viewpoint of weldability, the less the better. If the N content exceeds 0.01000%, the weldability is significantly deteriorated. 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. Although the lower limit of the N content is not particularly limited, 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 from the viewpoint of weldability, the smaller the amount, the better. If the O content exceeds 0.0100%, the weldability is significantly deteriorated. Therefore, the O content is limited to 0.0100% or less, preferably 0.0090% or less, 0.0070% or less, or 0.0050% or less. Although the lower limit of the O content is not particularly limited, the O content may be, for example, 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 selective elements in addition to the basic elements and impurities described above. For example, B, Ti, Nb, V, Cr, Mo, Cu, Co, W, Ni, Ca, Mg, REM, and Zr are contained as selective elements in place of part of Fe, which is the remainder. good too. These selective elements may be contained depending on the purpose. Therefore, it is not necessary to limit the lower limit of these selective elements, and the lower limit may be 0%. Moreover, even if these selective 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 effective elements for ensuring stable strength. Therefore, these elements may be contained. However, even if B exceeds 0.010%, Ti and Nb each exceed 0.20%, and V, Cr, Mo, Cu, Co, W, and Ni each exceed 1.000%, the effect of the above action is easily saturated and may be economically disadvantageous.

Therefore, the content of B is 0.010% or less, the content of Ti and Nb is 0.20% or less, and the content of V, Cr, Mo, Cu, Co, W and Ni is 1.0% each. or less, or 1.000% or less. The B content may be 0.008% or less, 0.007% or less, or 0.005% or less. The upper limit of the content of Ti and Nb may be 0.18%, 0.15%, or 0.10%. The upper limits of the contents 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 obtain the effect of the above action more reliably,
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,
Contains at least one of 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 preferably.

(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, particularly fine dispersion of inclusions, and have the effect of increasing toughness. Therefore, one or more of these elements may be contained. However, if any of the elements is included in an amount exceeding 0.0100%, deterioration of the surface properties may become apparent. Therefore, the contents of Ca, Mg, REM, and Zr are preferably 0.01% or less or 0.0100% or less, respectively. The upper limit of each content of Ca, Mg, REM, and Zr may be 0.0080%, 0.0050%, or 0.0030%. In order to more reliably obtain the effects of the above action, the content of at least one of these elements is preferably 0.0003% or more, 0.0005% or more, or 0.0010% or more.

Here, REM refers to a total of 17 elements of Sc, Y and lanthanides, at least one of which. The above REM content means the total content of at least one of these elements. In the case of lanthanides, they are industrially added in the form of misch metals.

In addition, in the high-strength steel sheet according to the present embodiment, the chemical components, in terms of % by mass, are Ca: 0.0003% or more and 0.0100% or less, Mg: 0.0003% or more and 0.0100% or less, and REM: 0.01% or less. 0003% or more and 0.0100% or less, and Zr: 0.0003% or more and 0.0100% or less.

The steel components described above may be measured by a general analysis method for steel. For example, the steel composition may be measured using ICP-AES (Inductively Coupled Plasma-Atomic Emission Spectrometry). Incidentally, C and S can be measured using a combustion-infrared absorption method, N can be measured using an inert gas fusion-thermal conductivity method, and O can be measured using an inert gas fusion-nondispersive 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%, and tempered marten. Total of site and bainite: 20% to 75%, and total of pearlite and cementite: 0% to 10%.

(Ferrite: 20% to 70%)
Ferrite is a structure that is relatively soft and contributes to forming. Having ferrite improves elongation, hole-expandability, and bendability. In order 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.

(Retained austenite: 5% to 40%)
Retained austenite is a structure that contributes to elongation. In order to obtain this effect, retained austenite must be 5% or more. Therefore, the area ratio of retained austenite in the metal structure is set to 5% or more, preferably 8% or more, 10% or more, or 15% or more.
In the manufacturing method according to the present embodiment, it is substantially impossible to retain 40% or more of retained austenite. 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 inhibits formability instead of contributing to strength. Therefore, fresh martensite may not be contained, and the lower limit is 0%.
On the other hand, in order to obtain the effect of improving strength by fresh martensite, it is preferable to have fresh martensite in an amount of 2% or more, 5% or more, or 8% or more.
On the other hand, if the fresh martensite content exceeds 30%, the elongation and hole expansibility are deteriorated, so the area ratio of fresh martensite in the metal structure is made 30% or less. The area ratio of fresh martensite is preferably 20% or less, more preferably 15% or less, or 10% or less.

(Total of tempered martensite and bainite: 20% to 75%)
Tempered martensite and bainite are structures that contribute to strength. In order to obtain a tensile strength of 780 MPa or more, tempered martensite and bainite must account for 20% or more in total. 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, preferably 30% or more, 40% or more, or 50% or more.
On the other hand, it is not necessary 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 contains 20% or more ferrite and 5% or more retained austenite, and the rest may be tempered martensite and bainite. In other words, the total area fraction of tempered martensite and bainite can be up to 75%. The total area fraction of tempered martensite and bainite may be 70% or less, 60% or less, or 55% or less.

(Total of pearlite and cementite: 0% to 10%)
Pearlite and cementite are structures that inhibit formability. If the total area ratio of pearlite and cementite exceeds 10%, it is not preferable because the deterioration of moldability becomes large. Therefore, the total area ratio of pearlite and cementite is set to 10% or less. 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, confirmation of the existence position, and area fraction is measured by the following method.
First, a cross section parallel to the rolling direction (that is, a cross section parallel to the rolling direction and perpendicular to the surface) is corroded using a nital reagent and a reagent disclosed in JP-A-59-219473. Specifically, regarding the corrosion of the cross section, a solution in which 1 to 5 g of picric acid was dissolved in 100 ml of ethanol was used as liquid A, and 1 to 25 g of sodium thiosulfate and 1 to 5 g of citric acid were dissolved in 100 ml of water. The solution is defined as B liquid, A liquid and B liquid are mixed at a ratio of 1: 1 to obtain a mixed liquid, and nitric acid is further added at a rate of 1.5 to 4% with respect to the total amount of this mixed liquid and mixed. The resulting solution is used as a pretreatment solution. In addition, a post-treatment liquid is prepared by adding 10% of the above pretreatment liquid to the total amount of the 2% nital liquid and mixing the 2% nital liquid. A section parallel to the rolling direction (that is, a section parallel to the rolling direction and perpendicular to the surface) is immersed in the pretreatment liquid for 3 to 15 seconds, washed with alcohol, dried, and then dipped in the posttreatment liquid for 3 to 20 seconds. After being immersed for a second, the cross section is corroded by washing with water and drying.
Next, as shown in FIG. 1, a scanning electron microscope is used at a depth of 1/4 of the plate thickness from the surface (rolled surface 11) of the steel plate 1 and at the center of the plate width direction WD. By observing at least three regions of 40 μm×30 μm at a magnification of 100,000, identification of the metallographic structure, confirmation of the existing position, and measurement of the area fraction are performed. It should be noted that even if the object to be measured is a steel sheet that has not undergone special machining after production (in other words, a steel sheet that has not been cut out from a coil) or a steel sheet that has been cut out from a coil, the center position in the sheet width direction , means a position substantially equidistant from both ends of the steel sheet 1 viewed in the sheet width direction WD.
In addition, it is difficult to distinguish between lower bainite and tempered martensite by the measurement method described above. Therefore, it is not necessary 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". Upper bainite is an aggregate of laths and has a structure containing carbides between laths. The lower bainite is a structure containing iron-based carbides having a major axis of 5 nm or more and extending in the same direction. Tempered martensite is an aggregate of lath-like crystal grains, and is a structure containing iron-based carbides with major diameters of 5 nm or more and elongated in different directions.

Ferrite is a region of low brightness and no substructure. A region with high brightness and in which 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 by FE-SEM and the area fraction of retained austenite measured by X-rays described later. can.

Pearlite means a region in which plate-like cementite and plate-like ferrite are alternately arranged. In observation by FE-SEM, pearlite can be clearly distinguished from the above structures (ferrite, bainitic ferrite, bainite, tempered martensite).

Methods for measuring the area fraction of retained austenite include X-ray diffraction, EBSP (Electron Back Scattering Diffraction Pattern) analysis, magnetic measurement, and the like, and the measured value 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 X-ray diffraction in this embodiment, first, a cross section parallel to the rolling direction (that is, 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) section), using Co-Kα rays, obtain the integrated intensity of a total of six peaks α(110), α(200), α(211), γ(111), γ(200), and γ(220). Then, the area fraction of retained austenite is obtained by calculating using the intensity average method.

(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 in the total retained austenite is 50% or more)
Forming the structure of retained austenite into a plate shape contributes to the improvement of elongation, hole expandability, and bendability, and is one of the important points of structure formation in the present invention. Making the retained austenite plate-like suppresses strain distribution to the austenite during molding, and moderately stabilizes the retained austenite against plastic deformation, thereby improving elongation and hole expandability. The form of retained austenite having this effect has an aspect ratio of 2.0 or more.
In order to obtain this effect, it is necessary that the number ratio of retained austenite having an aspect ratio of 2.0 or more to the total retained austenite is 50% or more in the range of ⅛ thickness to ⅛ thickness. Therefore, the number ratio is set to 50% or more, preferably 70% or more. If the number ratio is less than 50%, it is difficult to achieve both excellent elongation, hole expansibility, and bendability, which is not preferable.

The aspect ratio and major axis of the retained austenite grains contained in the steel structure inside the steel plate are evaluated by observing the crystal grains using FE-SEM and performing high-resolution crystal orientation analysis by the EBSD method (electron beam backscatter diffraction method). do.
First, as shown in FIG. 2, a sample is taken with a cross section parallel to the rolling direction and thickness direction of the steel sheet as an observation plane 13, and the observation plane is polished to a mirror finish. Then, in one or a plurality of observation fields 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, a total of 2.0 A crystal structure analysis is performed by the EBSD method for an area of ×10 ⁻⁹ m ² or more (either from multiple fields of view or from the same field of view). Next, from the crystal orientation of the retained austenite grains measured by the above method, only austenite with a long axis length of 0.1 μm or more is extracted to draw a crystal orientation map in order to avoid measurement errors. A boundary that produces a crystal orientation difference of 10° or more is regarded as a grain boundary of retained austenite grains. The aspect ratio is a value obtained by dividing the length of the major axis of the retained austenite grains by the length of the minor axis. The major axis is the major axis length of the retained austenite grains. "OIM Analysys 6.0" manufactured by TSL is used for analysis of data obtained by the EBSD method for measurement. Also, the distance between scores (step) is set to 0.01 to 0.20 μm. Based on the observation results, the region determined to be FCC iron is defined as retained austenite. From this result, the number ratio of retained austenite having an aspect ratio of 2.0 or more to the total retained austenite in the thickness range of 1/8 to 3/8 is obtained.

(When the area ratio of ferrite at the 1/4 position of the plate thickness of the cross section parallel to the rolling direction and perpendicular to the surface is measured at 10 locations every 50 mm in the width direction, the standard deviation of the area ratio of ferrite is 10%. Less than)
In the present invention, ferrite is important for securing elongation and hole expansibility. On the other hand, the strength, elongation, and expandability change depending on the tissue fraction. Therefore, uniform distribution of the ferrite structure fraction in the hot-rolling width direction is important for obtaining material stability.
As shown in FIG. 3, the area ratio of ferrite at the plate thickness 1/4 position 121 of the cross section parallel to the rolling direction (that is, the cross section 12 parallel to the rolling direction and perpendicular to the surface) is measured in the width direction (that is, the rolling If the standard deviation of the area ratio of ferrite is 10% or more when measuring 10 points along the direction perpendicular to the direction RD (direction perpendicular to the direction RD) WD, it will cause variations in mechanical properties, and material stability will not be obtained. do not have. Therefore, the standard deviation of the area ratio of ferrite is set to less than 10%, preferably less than 8%, less than 5%, or less than 4%. When the size along the width direction of the steel sheet to be measured is sufficiently large, the points for measuring the standard deviation of the area ratio of ferrite should be arranged on a straight line along the width direction. On the other hand, when the size of the steel sheet to be measured along the width direction is less than 450 mm, the measurement points for the standard deviation of the area ratio of ferrite are arranged on two or more straight lines along the width direction. do it. When measuring the standard deviation in the sheet width direction of properties other than ferrite (for example, surface roughness), the measurement points can be arranged as described above.

3. Standard deviation of surface roughness Ra (standard deviation of surface roughness Ra measured at 10 locations every 50 mm along the plate width direction is preferably 0.5 μm or less)
The steel sheet according to the present embodiment is not particularly limited as long as the chemical composition, metallographic structure, and tensile strength, which will be 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 every 50 mm along the plate width direction (that is, the direction perpendicular to the rolling direction), the standard deviation of the surface roughness Ra is 0.5 μm or less. may be By suppressing variations in surface roughness Ra, variations in bending workability can be suppressed, and material stability can be further enhanced. Therefore, it is preferable to set the standard deviation to 0.5 μm or less. However, the surface roughness of the steel plate can be freely changed by additional machining. 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 such as hairline processing to change the surface roughness. From this point of view as well, it is not essential to set the standard deviation of the surface roughness Ra within the above range.

The surface roughness Ra is obtained by using a contact roughness meter (Surftest SJ-500 manufactured by Mitutoyo) to obtain a roughness curve over a length of 5 mm in the plate width direction at each measurement position, and JIS B0601: 2001. Calculate the arithmetic mean roughness Ra by the method described in . Using the values of the arithmetic mean roughness Ra at each measurement position thus obtained, the standard deviation of the surface roughness Ra is obtained.

In addition, when a surface treatment film such as plating and painting is arranged on the surface of the steel sheet, the "surface roughness Ra of the steel sheet" means the surface roughness measured after removing the surface treatment film from the steel sheet. do. 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 according to 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, the zinc plating layer may be dissolved using dilute hydrochloric acid containing an inhibitor. Thereby, only the galvanized layer can be separated from the steel sheet. An inhibitor is an additive used to suppress changes in roughness due to prevention of excessive dissolution of base iron. For example, 10 to 100-fold diluted hydrochloric acid is added with a corrosion inhibitor for hydrochloric acid pickling "Ibit No. 700BK" manufactured by Asahi Chemical Industry Co., Ltd. to a concentration of 0.6 g / L, and zinc plating. It can be used as a layer peeling means.

4. Mechanical properties (tensile strength TS: 780 MPa or more)
The high-strength steel sheet according to this embodiment has a tensile strength (TS) of 780 MPa or more, which is sufficient strength to contribute to weight reduction of automobiles. The steel sheet may have a tensile strength of 800 MPa or more, 900 MPa or more, or 1000 MPa or more. On the other hand, it is estimated that it is difficult to exceed 1470 MPa with the configuration of this embodiment. Therefore, although the upper limit of the tensile strength does not need to be defined in particular, the practical upper limit of the tensile strength can be set to 1470 MPa in this embodiment. Also, the tensile strength of the steel sheet 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 JIS Z2241 (2011) according to the following procedures. JIS No. 5 test pieces are sampled from 10 positions at intervals of 50 mm in the width direction of the high-strength steel sheet. Here, the width direction of the steel plate and the longitudinal direction of the test piece are made to coincide. In addition, each test piece is sampled at a position shifted in the rolling direction of the steel plate so that the sampling positions of the test pieces do not interfere with each other. These test pieces are subjected to a tensile test in accordance with JIS Z 2241 (2011) to obtain tensile strength TS (MPa), and an average value thereof is calculated. This average value is regarded as the tensile strength of the high-strength steel sheet.

In addition, the high-strength steel sheet according to the present embodiment may have elongation and hole expansibility as indicators of formability, and the following properties. These mechanical properties are obtained from the 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 elongation index. On the other hand, it is difficult to achieve a total elongation of over 35% with the configuration of this embodiment. Therefore, the upper limit of the substantial total elongation may be 35%.

(Hole expansibility)
The high-strength steel sheet according to the present embodiment may have a hole expansion rate of 25% or more as an index of hole expandability. On the other hand, it is difficult to achieve a hole expanding ratio of more than 80% with the configuration of this embodiment. Therefore, the upper limit of the substantial hole expansion ratio may be set to 80%.
The hole expansion rate can be evaluated by a hole expansion test according to 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 has an R/t of 2.0 or less when the value R/t obtained by dividing the limit bending R (mm) by the plate thickness t (mm) is used as an index of bendability. You may On the other hand, it is difficult to set the bendability index R/t to 0.1 or less with the configuration of this embodiment. Therefore, the lower limit of the substantial bendability index R/t may be set to 0.1.
The critical bend R is obtained by repeatedly performing bend tests applying various bend radii. In the bending test, bending is performed according to JIS Z 2248 (V block 90° bending test). The bending radius (more precisely, the inner bending radius) is changed at 0.5 mm pitches. The smaller the bend radius in the bend test, the more likely the steel sheet will develop cracks and other defects. The minimum bending that does not cause cracks and other defects in the steel sheet determined in this test is considered the critical bending R. A value obtained by dividing the limit bending R by the thickness t of the steel sheet is used as an index R/t for evaluating bendability.

The high-strength steel sheet according to the present embodiment is a tensile test result measured at 10 locations every 50 mm along the sheet width direction (that is, the direction perpendicular to the rolling direction) as an index of the stability of the material. , the standard deviation of TS may be 50 MPa or less and the standard deviation of EL may be 1% or less. The method for determining the TS standard deviation and the EL standard deviation is the same as the tensile test method for determining the average value of the tensile strength described above. TS standard deviation and EL standard deviation are obtained by determining the standard deviation of the results of 10 tensile tests by the method described above.

Further, in the high-strength steel sheet according to the present embodiment, the standard deviation of R / t (limit bending R (mm), thickness t (mm)) measured at 10 locations every 50 mm along the width direction of the sheet is It 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, it should be noted that the method for manufacturing the high-strength steel sheet according to this embodiment is not particularly limited. All steel sheets that satisfy the above requirements are considered to be steel sheets according to the present embodiment, regardless of their manufacturing method.

The manufacturing process preceding hot rolling is not particularly limited. That is, following smelting by a blast furnace, an electric furnace, etc., various secondary smelting may be performed, and then casting may be performed by a method such as ordinary continuous casting, casting by the ingot method, or thin slab casting. In the case of continuous casting, the cast slab may be cooled to a low temperature once, then heated again and then hot rolled, or the cast slab may be hot rolled directly after casting without cooling to a low temperature. good. Scrap may be used as the raw material.

The cast slab is subjected to a heating process. In this heating step, the slab is preferably heated to a temperature of 1100° C. or higher and 1300° C. or lower. Coarse precipitates (iron-based carbides, carbonitrides of alloying elements, etc.) deposited in the slab may hinder material stability, so the slab must be heated to 1100°C or higher to dissolve them. preferable. On the other hand, from the viewpoint of preventing scale loss, the slab heating temperature is preferably 1300° C. or less.

Next, the heated slab is roughly rolled to obtain a roughly rolled sheet.
Rough rolling is not particularly limited as long as the slab is formed into a desired size and shape. In addition, since the thickness of the rough-rolled sheet affects the amount of temperature decrease from the front end to the tail end of the hot-rolled steel sheet from the start of rolling to the end of rolling in the finish rolling process, it should be determined in consideration of this. is preferred.

Finish rolling is applied to the rough rolled sheet. In this finish rolling step, multistage finish rolling is performed. In this embodiment, finish rolling is performed in a temperature range of 850° C. to 1200° C. under conditions satisfying 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 Si content (mass%) of a steel plate.

Moreover, K' in the above formula (1) is represented by the following formula (2).
K′=D×(DT−930)×1.5+Σ((FT −930)×S _n ) ₍ 2)
Here, D is the spraying amount per time of hydraulic descaling before the start of finish rolling (m ³ /min), DT is the steel plate temperature (° C.) when performing hydraulic descaling before the start of finish rolling, FT _n is the finishing The temperature of the steel sheet at the n-th stage of rolling (° C.), S _n is the spray amount per hour (m ³ /min ).

Si ^* is a parameter related to the composition of the steel sheet that indicates the ease with which scale-induced unevenness occurs. When the amount of Si in the steel sheet is large, the scale formed on the surface layer during hot rolling is relatively easy to descale and is difficult to form unevenness in the steel sheet. It changes to fayalite (Fe ₂ SiO ₄ ), which is easy to make. Therefore, the larger the Si content, that is, the larger the Si ^* , the easier it is to form irregularities on the surface layer. Here, the ease of forming irregularities on the surface layer due to the addition of Si becomes particularly pronounced when Si is added in an amount of 0.35% by mass or more. Therefore, Si ^* becomes a function of Si when added in an amount of 0.35% by mass or more, but becomes a constant when added in an amount of 0.35% by mass or less.

K' is a parameter of manufacturing conditions that indicates the difficulty of forming unevenness. The first item in the above formula (2) is that when performing hydraulic descaling before the start of finish rolling in order to suppress the formation of unevenness, the greater the amount of water pressure descaling sprayed per time and the higher the steel plate temperature, the more We show that it is effective in terms of descaling. When multiple descalings are performed before the start of finish rolling, the descaling value closest to the finish rolling is used.

The second term in the above formula (2) is a term that indicates the effect of descaling during finish rolling the scale that could not be removed by descaling before finishing or the scale that was formed again during finish rolling. Yes, indicating that at higher temperatures, descaling is more likely to occur with more water on the steel plate on the spray.

If the ratio of the manufacturing condition parameter K′ indicating the difficulty of forming unevenness to the parameter Si ^* related to the steel sheet composition indicating the ease of forming scale scratches is 2.5 or more, or 2.50 or more, unevenness is formed. This can be sufficiently suppressed, and the temperature variation during tempering 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, the standard deviation of the surface roughness Ra measured at 10 positions at intervals of 50 mm in the width direction (that is, the direction perpendicular to the rolling direction), which is a preferred form of the steel sheet related to the present invention, is 0.5 μm or less. To achieve this, it is preferable that K'/Si ^* is 3.0 or more (K'/Si ^* ≧3.0).

After finishing rolling, the steel sheet is cooled at an average cooling rate of 50°C/s or more and coiled at a coiling temperature of 450°C or less. This is to control the form of residual γ after annealing by using bainite and martensite, which are low-temperature transformed structures, as main structures, as described above. Here, the average cooling rate is a value obtained by dividing the temperature difference between when cooling is started and before winding by the time. If the average cooling rate is less than 50 ° C./s, ferrite transformation occurs, hindering the control of the structure morphology in the subsequent annealing process, and the number ratio of retained austenite with an aspect ratio of 2.0 or more to the total number of retained austenite is 50%. can't control more.

Similarly, if the coiling temperature exceeds 450° C., ferrite transformation occurs, and similarly, it becomes difficult to make the total of bainite and tempered martensite 20% or more of the whole. Moreover, when the coiling temperature exceeds 450° C., the ratio of the number of retained austenites having an aspect ratio of 2.0 or more to the total number of retained austenites cannot be controlled to 50% or more. From this point of view, the winding temperature is 450° C. or lower, preferably 400° C. or lower, 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 thus produced is subjected to pickling for the purpose of removing oxides on the surface of the steel sheet. The pickling treatment may be performed, for example, with hydrochloric acid having a concentration of 3 to 10% at a temperature of 85° C. to 98° C. for 20 seconds to 100 seconds.

Further, the produced hot-rolled steel sheet may be subjected to a light rolling reduction of 20% or less for the purpose of shape correction. However, if the reduction rate of light reduction exceeds 20%, recrystallization occurs in the annealing process, and the effect of morphology control obtained during annealing from the low-temperature transformed structure cannot be obtained. Also, the rolling reduction is 20% or less. Light reduction may be performed before or after the pickling step. Light reduction after the pickling step has the 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 intervals of 50 mm in the plate width direction (that is, the direction perpendicular to the rolling direction), the standard deviation of the surface roughness Ra is 0.5 μm. In order to satisfy the following, it is necessary to perform light reduction after the pickling process.

Annealing treatment is performed on the obtained steel plate.
In the annealing step, the heating temperature is A _c1 point to A _c3 point −10° C. calculated by the following formula.
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)
During heating, ferrite-austenite transformation occurs from carbides generated between laths of the low-temperature transformed structure, and plate-like austenite is generated. The region that did not undergo austenite transformation can be considered as a low-temperature transformed structure (tempered martensite or tempered bainite) tempered at high temperature, but the dislocation density is greatly reduced by tempering, and the underlying structure is also unclear. Therefore, it is a region evaluated as ferrite in observation of the structure after annealing. Therefore, it is also called ferrite here. Note that the regions evaluated as tempered martensite and bainite in the observation of the structure after annealing were generated by the bainite transformation and martensite transformation of the austenite generated by heating during holding at 150 ° C. to 550 ° C. described later. Mainly refers to organizations.
The reason why the heating temperature is set to Ac1 point to Ac3 point −10° C. is to obtain an appropriate ferrite-austenite transformation fraction in order to set the ferrite area ratio to 20% to 70%. The heating time is 10 seconds to 1000 seconds. If the holding time is less than 1 second, there is concern that the cementite in the steel will remain undissolved and the properties of the steel sheet will deteriorate. This effect saturates beyond 1000 seconds, leading to a decrease in productivity, so the upper limit of the retention time is 1000 seconds.

After that, the temperature is kept between 150° C. and 550° C. for 10 seconds to 1000 seconds.
In this temperature range, a part of austenite undergoes bainite transformation or martensite transformation, and solid solution carbon is expelled into austenite as a result of bainite transformation, and solid solution carbon is expelled into austenite as martensite is tempered. , has the effect of stabilizing austenite. At 150° C. or less, most of the austenite transforms into martensite, and a sufficient amount of retained austenite cannot be obtained. On the other hand, at 550° C. or higher, pearlite transformation occurs and retained austenite cannot be sufficiently stabilized. If the holding time is less than 10 seconds, diffusion of carbon does not occur sufficiently, and retained austenite cannot be sufficiently stabilized. If it exceeds 1000 seconds, the effect of stabilizing retained austenite is saturated, and productivity decreases.

It should be noted that heating or cooling may be performed within the temperature range while the temperature range is maintained. For example, once the temperature is lowered to a temperature range of 250 ° C. or less to transform a part of the retained austenite into martensite, and then reheated to a temperature range of about 400 ° C., the martensite becomes a nucleation site for bainite transformation, resulting in bainite transformation. has the effect of accelerating

Further, in this temperature range, hot-dip galvanizing or alloying hot-dip galvanizing may be applied. As the plating conditions such as the zinc plating bath temperature and the zinc plating bath composition in the hot-dip galvanizing step, general conditions can be used, and there is no particular limitation. For example, the plating bath temperature may be 420 to 500° C., the penetration plate temperature of the steel plate 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% Al, but may contain other impurities such as Fe, Si, Mg, Mn, Cr, Ti, and Pb. Moreover, it is preferable to control the basis weight of the hot-dip galvanizing by a known method such as gas wiping. The basis weight is generally 5 g/m ² or more per side, preferably 25 to 75 g/m ² , more preferably 20 to 120 g/m ² .
When alloying treatment is carried out, it may be carried out according to a conventional method, but the alloying treatment temperature is preferably 460 to 550°C. If the alloying treatment temperature is lower than 460°C, the alloying speed becomes slow, which not only impairs productivity, but also causes irregularities in the alloying treatment. On the other hand, if the alloying treatment temperature exceeds 550° C., pearlite transformation occurs and retained austenite cannot be sufficiently stabilized.
Moreover, the alloying treatment is preferably performed under conditions such that the iron concentration in the hot-dip galvanized layer is 6.0% by mass or more.
If hot-dip galvanizing or alloying hot-dip galvanizing is not applied, the steel sheet manufactured as described above may be provided with an electrogalvanized layer. The electrogalvanized layer can be formed by a conventionally known method.

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

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 aspects. The conditions in the examples described below are examples of conditions adopted for confirming the feasibility and effects of the present invention, and the present invention is not limited to these examples of conditions. Various conditions can be adopted in the present invention as long as the object of the present invention is achieved without departing from the gist of the present invention.

Cast steel with the chemical composition shown in Table 1, after casting, as it is or once cooled to room temperature, reheated, heated to a temperature range of 1200 ° C. to 1350 ° C., and then roughened the slab at a temperature of 1100 ° C. or higher. It was rolled to produce a rough rolled plate. In Table 1, values outside the scope of the invention are underlined.

The rough-rolled sheet was subjected to multistage finish rolling consisting of 7 stages in total under the conditions shown in Table 2.
Thereafter, cooling and coiling after finish rolling were performed under the respective conditions shown in Table 3.
After that, pickling was performed under all conditions, but light reduction was performed before or after pickling for some conditions. After that, 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 heating temperature was maintained for the time shown in Table 3. After that, under condition A, it was cooled to 250° C. at 50 to 100° C./s, reheated to 400° C., and held for 300 seconds. Under condition B, the temperature was lowered to 360° C. at a rate of 50 to 100° C./s and held for 50 seconds. In Condition C, which is a comparative example, the temperature was lowered to 100° C. at 100° C./s and held for 300 seconds.
After that, alloying hot-dip galvanizing and hot-dip galvanizing were applied to some conditions. In the plating process, the steel sheet was in the temperature range of 400°C to 520°C.

The metal structure of the obtained high-strength steel sheet was observed by the following method.
First, a cross section parallel to the rolling direction and perpendicular to the surface was corroded using a nital reagent and a reagent disclosed in JP-A-59-219473. Specifically, regarding the corrosion of the cross section, a solution in which 1 to 5 g of picric acid was dissolved in 100 ml of ethanol was used as liquid A, and 1 to 25 g of sodium thiosulfate and 1 to 5 g of citric acid were dissolved in 100 ml of water. The solution is defined as B liquid, A liquid and B liquid are mixed at a ratio of 1: 1 to obtain a mixed liquid, and nitric acid is further added at a rate of 1.5 to 4% with respect to the total amount of this mixed liquid and mixed. The resulting solution was used as a pretreatment solution. A post-treatment liquid was prepared by adding 10% of the pretreatment liquid to the total amount of the 2% nital liquid and mixing them. A cross section parallel to the rolling direction and perpendicular to the surface is immersed in the pretreatment liquid for 3 to 15 seconds, washed with alcohol and dried, immersed in the posttreatment liquid for 3 to 20 seconds, washed with water, and dried. As a result, the cross section was corroded.

Next, at a depth of 1/4 of the plate thickness from the steel plate surface and at the center position in the plate width direction, using a scanning electron microscope at a magnification of 1000 to 100000 times, by observing at least three areas of 40 μm × 30 μm, Identification of the metal structure, confirmation of the existence position, and measurement of the area fraction were performed.
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".

A region with low brightness and no substructure was determined to be ferrite. A region with high brightness and in which the underlying structure was not exposed 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.

Pearlite can be clearly distinguished from pearlite, ferrite, bainitic ferrite, bainite, and tempered martensite in observation by FE-SEM, so the area ratio was obtained by this method.

The area fraction of retained austenite was measured by X-ray diffraction. First, using a Co—Kα line, α(110), α(200), α(211), α(211), α(200), α(211), The area fraction of retained austenite was obtained by obtaining integrated intensities of a total of six peaks of γ(111), γ(200), and γ(220) and calculating them using the intensity average method.

The aspect ratio and major axis of the retained austenite grains contained in the steel structure inside the steel plate are evaluated by observing the crystal grains using FE-SEM and performing high-resolution crystal orientation analysis by the EBSD method (electron beam backscatter diffraction method). did.
First, a sample was 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. Then, a total of 2.0 × 10 ^-9 m ² or more in one or more observation fields in a range of 1/8 thickness to 3/8 thickness centered at a position 1/4 thickness from the surface on the observation surface A crystal structure analysis was performed by the EBSD method on the area of (both multiple fields of view and the same field of view are acceptable). Next, from the crystal orientation of the retained austenite grains measured by the above method, only austenite with a long axis length of 0.1 μm or more was extracted and a crystal orientation map was drawn in order to avoid measurement errors. A boundary that causes a crystal orientation difference of 10° or more is regarded as a grain boundary of retained austenite grains. The aspect ratio was obtained by dividing the length of the long axis of the retained austenite grains by the length of the short axis. The major axis was the major axis length of the retained austenite grains. "OIM Analysys 6.0" manufactured by TSL was used for the analysis of the data obtained by the EBSD method for the measurement. Further, the distance between scores (step) was set to 0.01 to 0.20 μm. Based on the observation results, the region judged to be FCC iron was defined as retained austenite. From this result, the number ratio of retained austenite having an aspect ratio of 2.0 or more to the total retained austenite in the thickness range of 1/8 to 3/8 was obtained.

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

The standard deviation of the surface roughness Ra measured at 10 positions at intervals of 50 mm in the sheet width direction was obtained by the following procedure. Using a contact roughness meter (Mitutoyo Surftest SJ-500), at each measurement position, obtain a roughness curve over a length of 5 mm in the plate width direction, and JIS B0601: Arithmetic average by the method described in 2001 Roughness Ra was obtained. The standard deviation of the surface roughness Ra was determined using the values of the arithmetic mean roughness Ra at each measurement position thus determined.

Tensile strength is obtained by conducting a tensile test in accordance with JIS Z 2241 (2011) using a JIS No. 5 test piece sampled from a high-strength steel sheet so that the plate width direction is the longitudinal direction. TS (MPa) and butt elongation (total elongation) EL (%) were determined. Samples were taken from 10 positions at intervals of 50 mm in the width direction of the steel plate. The average value of the tensile strength of the ten test pieces was regarded as the tensile strength TS of the steel sheet, and when TS≧780 MPa was satisfied, the steel sheet was judged to be a high-strength hot-rolled steel sheet and passed.
In addition, standard deviations of TS and EL were obtained at 10 positions at intervals of 50 mm in the sheet width direction. A steel sheet having a TS standard deviation of 50 MPa or less and an EL standard deviation of 1% or less was determined to be a steel sheet having excellent material stability.

The hole expansion ratio was evaluated by a hole expansion test in accordance with the test method described in JFS T 1001-1996 of the Japan Iron and Steel Federation Standard.

The bending test was performed in conformity with JIS Z2248 (V block 90° bending test), and the bending R (mm) was tested at a pitch of 0.5 mm.
In addition, R/t was measured at 10 positions at intervals of 50 mm in the sheet width direction, and its standard deviation was obtained.

In Tables 4 and 5, values outside the scope of the invention are underlined. As shown in the table, the examples satisfying the conditions of the present invention were excellent in all of tensile strength, elongation, hole expansibility (stretch flangeability), bendability, variation in tensile strength, and variation in elongation. On the other hand, in comparative examples that do not satisfy at least one of the conditions of the present invention, at least one of tensile strength, elongation, hole expansibility (stretch flangeability), bendability, variation in tensile strength, and variation in elongation characteristics were not sufficient.

Specifically, in Comparative Examples 9 and 10, the standard deviation of the ferrite area ratio was large, and the TS standard deviation and the EL standard deviation were rejected. It is presumed that this is because the hot rolling was carried out under conditions in which K'/Si ^* was insufficient.

In Comparative Example 11, the ratio of retained austenite having an aspect ratio of 2.0 or more was insufficient, and the hole expansibility was impaired. It is presumed that this is because the average cooling rate after finish rolling was insufficient.

In Comparative Example 12, the ratio of retained austenite having an aspect ratio of 2.0 or more was insufficient, and the hole expansibility was impaired. It is presumed that this is because the coiling temperature after finish rolling was too high.

In Comparative Example 13, the ferrite area ratio was excessive, the area ratio of other structures was insufficient, and the tensile strength was insufficient. It is presumed that this is because the heating temperature in the annealing step was lower than the _Ac1 point of the steel material A.

In Comparative Example 14, the ratio of retained austenite having an aspect ratio of 2.0 or more was insufficient, and the hole expansibility was impaired. It is presumed that this is because the reduction rate of the light reduction 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 total elongation and hole expansibility were impaired. It is presumed that this is because the holding pattern in the annealing step was inappropriate, ie the holding temperature was too low.

Comparative Examples 31 and 32 lacked the amount of Si. Therefore, in Comparative Examples 31 and 32, the amount of retained austenite was insufficient, and the total elongation and hole expansibility were impaired.

1 High-strength steel plate (steel plate)
11 surface (rolled surface)
12 Cross section parallel to the rolling direction and perpendicular to the surface 121 Plate thickness 1/4 position of the cross section parallel to the rolling direction and perpendicular to the surface 13 Measurement surface of retained austenite 131 1 from the surface (rolling surface) in the measurement surface of retained austenite /8 thickness to 3/8 thickness range 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: 0-0.0100%,
REM: 0 to 0.0100%,
Zr: 0 to 0.0100%, and balance: Fe and impurities
consists of
The metal structure is the 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 sum of pearlite and cementite: 0% to 10%
consists of
In the range of 1/8 thickness to 3/8 thickness from the surface, the number ratio of retained austenite with an aspect ratio of 2.0 or more to the total number of retained austenite is 50% or more,
The standard deviation of the area ratio of ferrite measured at 10 locations every 50 mm along the width direction at the 1/4 position of the thickness of the cross section parallel to the rolling direction and perpendicular to the surface is less than 10%,
Tensile strength is 780 MPa or more ,
Automotive steel plate
A high-strength steel plate characterized by: 2. 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 intervals of 50 mm in the sheet width direction. As the chemical component, 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%
The high-strength steel sheet according to claim 1 or 2, characterized by containing at least one selected from the group consisting of:

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