WO2021200169A1

WO2021200169A1 - Steel sheet

Info

Publication number: WO2021200169A1
Application number: PCT/JP2021/010890
Authority: WO
Inventors: 林　宏太郎; 孝彦神武
Original assignee: 日本製鉄株式会社
Priority date: 2020-04-02
Filing date: 2021-03-17
Publication date: 2021-10-07

Abstract

This steel sheet has a chemical composition in which, expressed in mass percentage, C is more than 0.18% and less than 0.30%; Si is at least 0.001% and less than 2.00%; Mn is more than 2.50% and less than 4.20%; sol. Al is at least 0.001% and less than 1.00%; P is 0.030% or less; S is 0.005% or less; N is less than 0.050%; O is less than 0.020%; Cr is 0-0.50%; Mo is 0-0.50%; W is 0-0.30%; Cu is 0-0.30%; Ni is 0-0.50%; Ti is 0-0.100%; Nb is 0-0.100%; V is 0-0.100%; B is 0-0.010%; Ca is 0-0.010%; Mg is 0-0.010%; Zr is 0-0.010%; REM is 0-0.010%; Sb is 0-0.050%; Sn is 0-0.050%; Bi is 0-0.050%; and the balance being Fe and impurities, wherein, in a cross section of the steel sheet, the cross section being parallel to the rolling direction and the sheet thickness direction, the metallographic structure at a position 1/4 sheet thickness from the surface includes, in terms of area percentage,10% or more of residual austenite, 60-80% of tempered martensite, less than 20% of martensite and wherein the low angle boundary density is 0.20-1.0 µm-1, the high angle boundary density is 0.30-0.60 µm-1, and AL/AN is 0.80-1.0.

Description

Steel plate

The present invention relates to a steel sheet.

In general, when the strength of a steel sheet is increased, the elongation is reduced and the formability of the steel sheet can be reduced. Therefore, in order to use a high-strength steel plate as a part for an automobile body, it is necessary to enhance both strength and formability, which are contradictory characteristics, that is, a better strength-ductility balance is required. Further, high-strength steel sheets used for vehicle body parts are required to have excellent bendability and impact characteristics. Therefore, as the mechanical properties of the steel sheet, it is required to have high strength and excellent moldability, and further have excellent bendability and impact characteristics.

So far, in order to improve elongation, that is, formability, Mn has been positively added and about 5% by mass of Mn has been contained in the steel sheet to generate retained austenite in the steel, and its transformation-induced plasticity has been increased. The so-called medium Mn steel used has been proposed (for example, Non-Patent Document 1).

Further, Patent Document 1 proposes a steel containing Mn of 2.6% or more and 4.2% or less. Since the above steel also contains a larger amount of Mn than general high-strength steel, retained austenite is easily generated, has high elongation, and exhibits excellent formability and bendability.

International Publication No. 2017/183348

However, since the steel sheet disclosed in Non-Patent Document 1 has a high Mn content, weldability may become a problem when it is used for parts for automobile bodies. Therefore, considering the utility as automobile parts and the like, it is desired to improve both the strength and formability of the steel sheet with a smaller Mn content. Further, in the steel sheet disclosed in Patent Document 1, the retained austenite is not isotropic and is unevenly distributed, so that the Charpy impact characteristic at room temperature is deteriorated. As described above, when the impact characteristics are lowered, the steel material used cannot be thinned, and the weight reduction of the automobile body cannot be sufficiently achieved.

An object of the present invention is to solve the above problems and to provide a steel sheet having high strength and excellent strength-ductility balance, bendability and impact characteristics.

The present invention has been made to solve the above problems, and the following steel materials are the gist of the present invention.

(1) The chemical composition of the steel sheet is mass%.
C: More than 0.18% and less than 0.30%,
Si: 0.001% or more and less than 2.00%,
Mn: More than 2.50% and less than 4.20%,
sol. Al: 0.001% or more and less than 1.00%,
P: 0.030% or less,
S: 0.005% or less,
N: Less than 0.050%,
O: Less than 0.020%,
Cr: 0 to 0.50%,
Mo: 0 to 0.50%,
W: 0 to 0.30%,
Cu: 0 to 0.30%,
Ni: 0 to 0.50%,
Ti: 0 to 0.100%,
Nb: 0 to 0.100%,
V: 0 to 0.100%,
B: 0 to 0.010%,
Ca: 0 to 0.010%,
Mg: 0 to 0.010%,
Zr: 0 to 0.010%,
REM: 0-0.010%,
Sb: 0 to 0.050%,
Sn: 0 to 0.050%,
Bi: 0 to 0.050%,
Remaining: Fe and impurities,
In the cross section parallel to the rolling direction and the plate thickness direction of the steel plate, the metal structure at a depth of 1/4 of the plate thickness from the surface is, in% area.
Residual austenite: 10% or more,
Tempering martensite: 60-80%,
Martensite: less than 20%,
In tempered martensite and martensite, the grain boundary density of small angles with a crystal orientation difference of 2 degrees or more and less than 20 degrees is 0.20 to 1.0 μm -1, and the grain boundary density of large angles with a crystal orientation difference of 20 to 50 degrees ^{is 0.20 to 1.0 μm -1.} It is 0.30 to 0.60 μm ^-1 .
The ratio _A L / _{A N} of the particle density _{A N} in the particle density _{A L} and the plate thickness direction in the rolling direction of the residual austenite is 0.80 to 1.0
Steel plate.

(2) The chemical composition is mass%.
Cr: 0.01-0.50%,
Mo: 0.01-0.50%,
W: 0.01-0.30%,
Cu: 0.01 to 0.30%, and Ni: 0.01 to 0.50%,
Contains one or more selected from,
The steel sheet according to (1) above.

(3) The chemical composition is mass%.
Ti: 0.005 to 0.100%,
Nb: 0.005 to 0.100%, and V: 0.005 to 0.100%,
Contains one or more selected from,
The steel sheet according to (1) or (2) above.

(4) The chemical composition is mass%.
B: 0.0001 to 0.010%,
Ca: 0.0001 to 0.010%,
Mg: 0.0001 to 0.010%,
Zr: 0.0001 to 0.010%, and REM: 0.0001 to 0.010%,
Contains one or more selected from,
The steel sheet according to any one of (1) to (3) above.

(5) The chemical composition is mass%.
Sb: 0.0005 to 0.050%,
Sn: 0.0005 to 0.050%, and Bi: 0.0005 to 0.050%,
Contains one or more selected from,
The steel sheet according to any one of (1) to (4) above.

(6) A hot-dip galvanized layer is provided on the surface of the steel sheet.
The steel sheet according to any one of (1) to (5) above.

(7) An alloyed hot-dip galvanized layer is provided on the surface of the steel sheet.
The steel sheet according to any one of (1) to (5) above.

According to the present invention, it is possible to obtain a steel sheet having high strength and excellent strength-ductility balance, bendability and impact characteristics.

How to measure the particle density A _N in the particle density A _L and the plate thickness direction in the rolling direction of the residual austenite is a diagram for explaining the.

Hereinafter, each requirement of the present invention will be described in detail.

(A) Chemical composition The reasons for limiting each element are as follows. In the following description, "%" for the content means "mass%".

C: More than 0.18% and less than 0.30% C is an element necessary to increase the strength of steel and secure retained austenite. On the other hand, when C is excessively contained, it becomes difficult to maintain the weldability of the steel sheet. Therefore, the C content is set to more than 0.18% and less than 0.30%. The C content is preferably 0.20% or more. The C content is preferably 0.28% or less, more preferably 0.25% or less.

Si: 0.001% or more and less than 2.00% Si is an element effective for strengthening tempered martensite and further improving moldability. Si also has the effect of suppressing the precipitation of cementite and promoting the formation of retained austenite. On the other hand, when Si is excessively contained, it becomes difficult to maintain the plating property and the chemical conversion treatment property of the steel sheet. Therefore, the Si content is set to 0.001% or more and less than 2.00%. In order to further improve the strength-ductility balance of the steel sheet, the Si content is preferably 0.020% or more, preferably 0.10% or more, 0.30% or more, or 0.50% or more. More preferred. The Si content is preferably 1.80% or less, more preferably 1.60% or less.

Mn: More than 2.50% and less than 4.20% Mn is an element that stabilizes austenite. More than 2.50% Mn is required to stabilize austenite at room temperature. On the other hand, when Mn is excessively contained, the weldability is lowered. Further, by setting the Mn content to less than 4.20%, the non-uniformity of the Mn content due to solidification segregation can be reduced, and the bendability can be improved. Therefore, the Mn content is set to more than 2.50% and less than 4.20%. The Mn content is preferably more than 3.00%, more preferably more than 3.20%. The Mn content is preferably less than 3.80%, more preferably less than 3.50%.

sol. Al: 0.001% or more and less than 1.00% Al is an antacid and needs to be contained in an amount of 0.001% or more. In addition, Al also has an effect of improving material stability because it widens the temperature range of the two-phase region at the time of annealing. The higher the Al content, the greater the effect, but if the Al content is excessive, it becomes difficult to maintain the surface texture, paintability, and weldability. Therefore, sol. The Al content is 0.001% or more and less than 1.00%. sol. The Al content is preferably 0.005% or more, more preferably 0.010% or more, and further preferably 0.020% or more. In addition, sol. The Al content is preferably 0.80% or less, and more preferably 0.60% or less. As used herein, "sol.Al" means "acid-soluble Al".

P: 0.030% or less P is an impurity, and if the steel sheet contains P in excess, the weldability is impaired. Therefore, the P content is 0.030% or less. The P content is preferably 0.025% or less, more preferably 0.020% or less, and even more preferably 0.015% or less. Since the steel sheet according to the present invention does not require P, the P content may be more than 0%, 0.001% or more, or 0.003% or more, but the smaller the P content, the more preferable.

S: 0.005% or less S is an impurity, and if the steel sheet contains S in excess, the weldability is impaired. Therefore, the S content is set to 0.005% or less. The S content is preferably 0.003% or less, and more preferably 0.002% or less. Since the steel sheet according to the present invention does not require S, the S content may be more than 0% or 0.0003% or more, but the smaller the S content, the more preferable.

N: Less than 0.050% N is an impurity, and if the steel sheet contains 0.050% or more of N, the low temperature toughness is reduced. Therefore, the N content is set to less than 0.050%. The N content is preferably 0.030% or less, more preferably 0.010% or less, and even more preferably 0.006% or less. Since the steel sheet according to the present invention does not require N, the N content may be more than 0%, 0.001% or more, or 0.002% or more, but the smaller the N content, the more preferable.

O: Less than 0.020% O is an impurity, and if the steel sheet contains 0.020% or more of O, the low temperature toughness is reduced. Therefore, the O content is set to less than 0.020%. The O content is preferably 0.010% or less, more preferably 0.005% or less, and even more preferably 0.003% or less. Since the steel sheet according to the present invention does not require O, the O content may be more than 0% or 0.001% or more, but the smaller the O content, the more preferable.

In addition to the above elements, the steel sheet of the present invention includes the following amounts of Cr, Mo, W, Cu, Ni, Ti, Nb, V, B, Ca, Mg, Zr, REM, Sb, Sn and It may contain one or more elements selected from Bi.

Cr: 0 to 0.50%
Mo: 0 to 0.50%
W: 0 to 0.30%
Cu: 0 to 0.30%
Ni: 0 to 0.50%
Cr, Mo, W, Cu, and Ni are elements that improve the strength of the steel sheet. Therefore, one or more selected from these elements may be contained. However, if these elements are excessively contained, surface defects are likely to occur during hot rolling, and the strength of the hot rolled steel sheet may become too high, resulting in a decrease in cold rollability. Therefore, the Cr content is 0.50% or less, the Mo content is 0.50% or less, the W content is 0.30% or less, the Cu content is 0.30% or less, and the Ni content is 0.50%. It is as follows. In order to more reliably obtain the effect of the above-mentioned actions of these elements, it is preferable to contain at least 0.01% or more of the above-mentioned elements. The contents of Cr, Mo, W, Cu, and Ni are more preferably 0.05% or more, respectively.

Ti: 0 to 0.100%
Nb: 0 to 0.100%
V: 0 to 0.100%
Since Ti, Nb, and V are elements that produce fine carbides, nitrides, or carbonitrides, they are effective in improving the strength of the steel sheet. Further, when the content of Nb is 0.100% or less, it has an effect of promoting the miniaturization of the structure during hot rolling. Therefore, one or more selected from these elements may be contained. However, if these elements are excessively contained, the strength of the hot-rolled steel sheet may be excessively increased, and the cold rollability may be lowered. Therefore, the Ti content is 0.100% or less, the Nb content is 0.100% or less, and the V content is 0.100% or less. The contents of Ti, Nb, and V are all preferably 0.050% or less, and more preferably 0.030% or less. In order to more reliably obtain the effect of the above-mentioned actions of these elements, it is preferable to contain at least one of the above-mentioned elements in an amount of 0.005% or more. The contents of Ti, Nb, and V are more preferably 0.007% or more, and further preferably 0.010% or more, respectively.

B: 0 to 0.010%
Ca: 0 to 0.010%
Mg: 0 to 0.010%
Zr: 0 to 0.010%
REM: 0-0.010%
B, Ca, Mg, Zr, and REM (rare earth metals) improve the local ductility and hole expandability of steel sheets. Therefore, one or more selected from these elements may be contained. However, if these elements are excessively contained, the moldability of the steel sheet may be deteriorated. Therefore, the B content is 0.010% or less, the Ca content is 0.010% or less, the Mg content is 0.010% or less, the Zr content is 0.010% or less, and the REM content is 0.010%. It is as follows. The contents of B, Ca, Mg, Zr, and REM are all preferably 0.006% or less, and more preferably 0.003% or less. The total content of one or more elements selected from B, Ca, Mg, Zr, and REM may be 0.050% or less, but is preferably 0.030% or less. In order to more reliably obtain the effect of the above-mentioned actions of these elements, the content of at least one of the above-mentioned elements is preferably 0.0001% or more, more preferably 0.0005% or more. It is more preferably 0.001% or more.

Here, REM is a general term for 17 elements including 15 elements of lanthanoids and Y and Sc, and one or more of these elements can be contained. The content of REM means the total content of these elements.

Sb: 0 to 0.050%
Sn: 0 to 0.050%
Bi: 0 to 0.050%
Sb, Sn, and Bi suppress that easily oxidizing elements such as Mn, Si, and / or Al in the steel sheet are diffused on the surface of the steel sheet to form an oxide, and improve the surface texture and plating property of the steel sheet. Therefore, one or more selected from these elements may be contained. However, even if these elements are excessively contained, the effect is saturated. Therefore, the Sb content is 0.050% or less, the Sn content is 0.050% or less, and the Bi content is 0.050% or less. The contents of Sb, Sn, and Bi are all preferably 0.030% or less, 0.010% or less, 0.006% or less, or 0.003% or less. In order to more reliably obtain the effect of the above-mentioned actions of these elements, it is preferable to contain at least one of the above-mentioned elements in an amount of 0.0005% or more. The contents of Sb, Sn, and Bi are more preferably 0.001% or more, respectively.

In the chemical composition of the steel sheet of the present invention, the balance is Fe and impurities. Here, the "impurity" is a component mixed with raw materials such as ore and scrap, and various factors in the manufacturing process when steel is industrially manufactured, and is allowed as long as it does not adversely affect the present invention. Means something.

(B) Metal structure The metal structure of the steel sheet according to the present invention will be described below. In the following description, "%" for the area ratio means "area%".

In the cross section parallel to the rolling direction and the plate thickness direction of the steel sheet according to the present invention (also referred to as "L cross section"), the position at a depth of 1/4 of the plate thickness (also referred to as "1/4 position") from the surface. The metallographic structure contains 10% or more retained austenite, 60-80% tempered martensite, and martensite is limited to less than 20%. The fraction of each structure changes depending on the heat treatment conditions and affects the material of the steel sheet such as strength, elongation, bendability, and impact characteristics. The reasons for the limitation of each organization will be explained in detail.

Residual austenite: 10% or more In the steel sheet according to the present invention, it is important that the amount of retained austenite in the metal structure is within a predetermined range. Residual austenite is a structure that enhances the strength-ductility balance of a steel sheet by transformation-induced plasticity. In order to obtain these effects, the steel sheet according to the present invention needs to contain 10% or more of retained austenite in the metal structure.

The area ratio of retained austenite is preferably 13% or more, more preferably 18% or more. When the area ratio of retained austenite is 13% or more, further 18% or more, both strength and elongation are compatible, and TS × El, which will be described later, becomes higher. The upper limit of the area ratio of retained austenite is not particularly specified, but is substantially 30% or less.

Tempering martensite: 60-80%
Tempered martensite is also a hard phase, but it has a structure different from that of martensite, which will be described later, and has the effect of improving impact characteristics and ensuring the strength of the steel sheet. In order to improve the impact characteristics of the steel sheet, the area ratio of tempered martensite shall be 60% or more. On the other hand, in order to secure the strength of the steel sheet, the area ratio of tempered martensite is 80% or less. The area ratio of tempered martensite is preferably 63% or more, and preferably 77% or less.

The steel sheet of the present invention has a metal structure mainly composed of tempered martensite by being manufactured through the steps described later. However, when the steel sheet has a hot-dip galvanized layer or an alloyed hot-dip galvanized layer, a small amount of bainite may inevitably be mixed in the plating process. In the present invention, the tempered martensite contains bainite.

Therefore, in the method for measuring the metal structure described later, bainite is also measured in addition to tempered martensite. That is, it means that the total area ratio of tempered martensite and bainite is 60 to 80%.

Martensite: less than 20% Martensite is a hard phase with many dislocations in its structure and is an effective structure for obtaining the strength of steel sheets. However, the area ratio of martensite is set to less than 20% because the impact characteristics are significantly deteriorated. When it is necessary to further improve the bendability, the area ratio of martensite is preferably 15% or less, more preferably 10% or less, still more preferably 0%. In addition, martensite means martensite which has not been tempered.

In the metallographic structure, the rest other than retained austenite, martensite, and tempered martensite is cementite, and its area ratio is preferably 1% or less. Further, if ferrite and pearlite are mixed in the metal structure even in a small amount, it becomes difficult to increase the strength, so that the metal structure cannot be included. That is, the area ratio of ferrite and pearlite is 0%.

Small grain boundary densities at tempered martensite and martensite: 0.20 to 1.0 μm ^-1
In the metallographic structure of the steel sheet of the present invention, the tempered martensite and the small-angle grain boundary density at the martensite are 0.20 μm ^-1 or more. By setting the small-angle grain boundary density within the above range, the transformation-induced plasticity of austenite occurs under high stress, and an excellent strength-ductility balance can be obtained. The small-angle grain boundary density is ^{preferably 0.30 μm -1} or more. On the other hand, the higher the grain boundary density is, the better, but it is substantially 1.0 μm ^-1 or less. The small-angle grain boundary density ^{means the total length (μm) of the boundaries observed per unit area (μm 2} ) in which the crystal orientation difference between tempered martensite and martensite is 2 degrees or more and less than 20 degrees. do.

Large-angle grain boundary densities at tempered martensite and martensite: 0.30 to 0.60 μm ^-1
In the metallographic structure of the steel sheet of the present invention, the tempered martensite and the large-angle grain boundary density at the martensite are 0.30 μm ^-1 or more. By keeping the large-angle grain boundary density within the above range, the progress of ductile fracture is suppressed and excellent impact characteristics can be obtained. On the other hand, the higher the grain boundary density is, the better, but it is substantially 0.60 μm ^-1 or less. The large-angle grain boundary density means the total length (μm) of the tempered martensite and the boundary where the crystal orientation difference between martensite is 20 to 50 degrees, which is observed per ^{unit area (μm 2).}

The ratio of the particle density _{A N} in the particle density _{A L} and the plate thickness direction in the rolling direction of the residual austenite _A L _/ A N: 0.80 ~ 1.0
In the metal structure of the steel sheet of the present invention, the ratio A _{L /} A _N of the particle density A _N in the particle density A _L and the plate thickness direction in the rolling direction of the residual austenite is 0.80 or more. By the A L _/ A _N in the above range, become retained austenite are dispersed isotropically excellent bendability, further impact properties. On the other _hand, the upper limit of A L / _{A N} is substantially 1.0 or less.

Average crystal grain size of retained austenite: 2 μm or less In the metal structure of the steel sheet of the present invention, the average crystal grain size of retained austenite is not particularly limited, but from the viewpoint of ensuring high strength, it is 2 μm or less. preferable.

In the present invention, the area ratio of each structure, the small-angle grain boundary density and the large-angle grain boundary density in tempered martensite and martensite, the dispersed state of retained austenite, and the average crystal grain size of retained austenite are measured by the following methods. It shall be.

(Measurement method of area ratio of retained austenite)
The area ratio of retained austenite is calculated as follows. First, a test piece 25 mm in the rolling direction and 25 mm in the rolling perpendicular direction (width direction) is cut out from the steel sheet. At this time, the thickness of the test piece is the same as the thickness of the steel plate. Then, the test piece is mechanically polished and then chemically polished to reduce the plate thickness by 1/4 to obtain a chemically polished test piece having a strain-free surface. X-ray diffraction analysis with a measurement range of 2θ of 45 to 105 degrees is performed three times on the surface of the test piece using a Co tube.

For the fcc phase, the integrated intensities of the peaks (111), (200), and (220) are obtained, and for the bcc phase, the integrated intensities of the peaks (110), (200), and (211) are obtained. The integrated intensities are analyzed, the volume fraction of retained austenite is obtained, and the results of three X-ray diffraction analyzes are averaged to obtain the value as the area fraction of retained austenite.

(Tempering martensite and method of measuring the area ratio of martensite)
The area ratio of tempered martensite and martensite is calculated from microstructure observation with a scanning electron microscope (SEM). The L cross section obtained by cutting the steel sheet in parallel with the thickness direction and the rolling direction is mirror-polished, and then the microstructure is revealed by 3% nital. Then, at a magnification of 5000 times, a microstructure in a range of 100 μm in length (length in the plate thickness direction) × 300 μm in width (length in the rolling direction) is observed around the 1/4 position from the surface.

Tempered martensite is identified as a gray underlying structure, and retained austenite and martensite are identified as a white structure. As described above, the area ratio of tempered martensite includes the area ratio of bainite. On the other hand, among the gray parts, those having a polygonal shape and not containing cementite are judged to be ferrite, and those having a lamellar structure are judged to be pearlite, and are distinguished. Further, the area ratio of martensite is calculated by subtracting the area ratio of retained austenite measured by the X-ray diffraction method from the total area ratio of retained austenite and martensite.

(Measuring method of small-angle grain boundary density and large-angle grain boundary density)
Backscattered electron diffraction (EBSP) measurement is performed in the range of 160 μm (length in the plate thickness direction) × 80 μm (length in the rolling direction) centered on the 1/4 position from the surface. Then, the crystal orientation difference between tempered martensite and martensite is measured, and the total length (μm) of the boundary where the crystal orientation difference is 2 degrees or more and less than 20 degrees and the total of the boundaries where the crystal orientation difference is 20 to 50 degrees. Find the length (μm) respectively. By dividing them by the area of the measurement area, the small-angle grain boundary density and the large-angle grain boundary density are calculated, respectively.

(Measuring method of dispersed state of retained austenite and average crystal grain size)
In the same manner as described above, the EBSP measurement is performed in the range of 160 μm in length (length in the plate thickness direction) × 80 μm in width (length in the rolling direction) centered on the 1/4 position from the surface. Then, in the above measurement range, as shown by a dotted line in FIG. 1, two lines are drawn in the horizontal direction and one line is drawn in the vertical direction, and the number of residual austenite grains crossed by each line is counted. Then, the value obtained by dividing the number of retained austenite grains crossed by the two horizontal lines by the length of the lines is the particle density _AL (μm ^-1 ) in the rolling direction, and the residual austenite grains crossed by the vertical lines. The value obtained by dividing the number of lines by the total length of the two lines is defined as the particle density _AN (μm ^-1 ) in the rolling direction. _{Then, AL} / _AN is obtained from those results.

Further, the average crystal grain size of the retained austenite is obtained by calculating the average value of the circle-equivalent diameters of the retained austenite grains specified by the EBSP measurement.

(C) Mechanical Properties Next, the mechanical properties of the steel sheet according to the present invention will be described.

The tensile strength (TS) of the steel sheet according to the present invention is preferably 980 MPa or more, more preferably 1180 MPa or more. This is because when a steel sheet is used as a material for automobiles, the thickness is reduced by increasing the strength, which contributes to weight reduction. Further, in order to use the steel sheet according to the present invention for press molding, the elongation at break (El) is preferably 15% or more, more preferably 17% or more.

Further, from the viewpoint of achieving both strength and elongation, that is, improving the strength-ductility balance, TS × El is preferably 22000 MPa ·% or more, more preferably 24,000 MPa ·% or more, and 26000 MPa ·%. The above is more preferable. The steel sheet according to the present invention is also excellent in bendability and Charpy impact characteristics at room temperature.

(D) Manufacturing Method A method for manufacturing a steel sheet according to the present invention will be described. Through previous studies by the present inventors, it has been confirmed that the steel sheet of the present invention can be manufactured by the following manufacturing process.

(A) Melting step Steel having the above-mentioned chemical composition is melted by a conventional method and cast to prepare a slab or an ingot. As long as the steel sheet according to the present invention has the above-mentioned chemical composition, the molten steel may be melted by a normal blast furnace method, and the raw material is a large amount of scrap like the steel produced by the electric furnace method. It may include. The slab may be manufactured by a normal continuous casting process or may be manufactured by thin slab casting.

(B) Hot-rolling step The above-mentioned slab or steel ingot is heated and hot-rolled to obtain a hot-rolled steel sheet. The heating temperature of the steel material to be subjected to hot rolling is 1100 to 1300 ° C. By setting the heating temperature to 1100 ° C. or higher, the deformation resistance during hot rolling can be further reduced. On the other hand, by setting the heating temperature to 1300 ° C. or lower, it is possible to suppress a decrease in yield due to an increase in scale loss. In the present specification, the above heating temperature means the surface temperature of a slab or a steel ingot.

The time for holding in the temperature range of 1100 to 1300 ° C. before hot rolling is not particularly specified, but in order to improve the material stability, it is preferably 30 minutes or more, and more preferably 1 h or more. Further, in order to suppress excessive scale loss, it is preferably 10 hours or less, and more preferably 5 hours or less. In addition, in the case of direct rolling or direct rolling, hot rolling may be performed as it is without heat treatment.

Hot rolling may be performed on a normal continuous hot rolling line. Further, in hot rolling, finish rolling is performed after rough rolling. The starting temperature in finish rolling shall be 1000 ° C or lower. By setting the finish rolling start temperature to 1000 ° C or lower, coarsening of the structure during finish rolling is prevented, subsequent structure control becomes easy, and deterioration of the surface texture of the steel sheet due to intergranular oxidation is suppressed. .. The finish rolling start temperature is preferably 750 ° C. or higher. When the finish rolling start temperature is 750 ° C. or higher, the deformation resistance during rolling can be reduced and the structure control can be easily performed.

(C) Cooling step After finish rolling, allow to cool for 1.2 to 4.0 s. By allowing to cool after finish rolling, the formation of ferrite is suppressed, the structure becomes uniform, and retained austenite is isotropically dispersed. Further, by setting the cooling time to 4.0 s or less, it is possible to prevent coarsening of retained austenite.

(D) Accelerated cooling step Following the cooling step, accelerated cooling is performed at an average cooling rate of 30 ° C./s or higher. When the average cooling rate is 30 ° C./s or higher, cementite is uniformly formed at the block boundary of the hot-rolled steel sheet and the former austenite grain boundary during the martensitic transformation. The cooling rate is preferably 500 ° C./s or less. When the cooling rate is 500 ° C./s or less, uneven cooling is less likely to occur and cold rollability is improved.

(E) Winding step After the accelerated cooling step, winding is performed at a temperature of less than 300 ° C. By winding at a temperature of less than 300 ° C., the structure of the hot-rolled steel sheet can be made into a bainite single-phase or martensite single-phase, or a composite structure of bainite and martensite, and the retained austenite grains of the steel sheet according to the present invention. Can be isotropically dispersed.

(F) Cold-rolled steel sheet The hot-rolled steel sheet produced through the above steps is pickled by a conventional method and then cold-rolled at a reduction rate of 30 to 70% to obtain a cold-rolled steel sheet. do. When the rolling reduction of cold rolling is 30% or more, recrystallization occurs uniformly, retained austenite is uniformly generated, and the mixture is isotropically dispersed. Further, when the rolling reduction ratio is 70% or less, fracture is less likely to occur during cold rolling. The rolling reduction of cold rolling is preferably 40% or more, and preferably 60% or less. Cold rolling may be carried out on a normal continuous cold rolling line.

Further, before pickling, light rolling with a reduction ratio of more than 0% and 5% or less may be performed. Modifying the shape by light rolling is advantageous in terms of ensuring flatness. In addition, light rolling before pickling improves pickling properties, promotes removal of surface-concentrating elements, and has the effect of improving chemical conversion treatment properties and plating treatment properties.

(G) First Heat Treatment Step The obtained cold-rolled steel sheet is _{held for 1.0 h or more in a temperature range of 600 ° C. or higher and lower than Ac 3} so as to have a two-phase structure, and then cooled to a temperature of 300 ° C. or lower. Perform heat treatment. By keeping the steel sheet in the temperature range of 600 ° C. or higher and lower than Ac ₃ , the structure becomes two phases of tempered martensite and austenite (fcc) of bcc, and Mn is distributed between bcc and austenite. In addition, by setting the holding time to 1.0 h or more, the finally produced retained austenite is isotropically dispersed, and the small-angle grain boundary density becomes high.

The holding time is preferably 2.0 h or more, and more preferably 4.0 h or more. On the other hand, from the viewpoint of productivity, the holding time is preferably 10.0 h or less, and more preferably 8.0 h or less. From the viewpoint of promoting the distribution of Mn to austenite, the average rate of temperature rise from 300 ° C. to the holding temperature is preferably 0.01 ° C./s or more and 5 ° C./s or less. Similarly, the average cooling rate from the holding temperature to 300 ° C. is preferably 0.001 ° C./s or more and 500 ° C./s or less. In the first heat treatment step, the holding time means the time held in the temperature range of _{600 ° C. or higher and lower than Ac 3.}

(H) Second heat treatment step Following the first heat treatment step, a second heat treatment step is performed. In the second heat treatment step, _{heat treatment is performed in which the temperature is maintained at 3} points or more and Ac ₃ + 80 ° C. or lower for 30 to 180 seconds, and then cooled to a temperature of 300 ° C. or lower. By setting the holding temperature to Ac ₃ points or more, the area ratio of the tempered martensite finally produced can be increased. It is also effective for miniaturizing retained austenite. On the other hand, by setting the holding temperature to Ac ₃ + 80 ° C. or lower, the generation of block boundaries and packet boundaries can be promoted, and the large-angle grain boundary density can be increased.

Here, as a result of examining at a heating rate of 0.5 to 50 ° C./s, the following equation was derived as _{three points of Ac.} Therefore, in the present invention, the Ac ₃ points are calculated using the following formula.
Ac ₃ = 910-200√C + 44Si-25Mn + 44Al

Further, by setting the holding time to 30 s or more, the material stability can be ensured. On the other hand, by setting the holding time to 180 s or less, the generation of block boundaries and packet boundaries can be promoted, and the large-angle grain boundary density can be increased. The holding time is preferably 60 s or more, and preferably 120 s or less. Further, it is preferable that the metal structure is once made into a martensite single phase after the second heat treatment step. From this point of view, the average cooling rate from the holding temperature to 300 ° C. is preferably 2 ° C./s or more and 2000 ° C./s or less. In the second heat treatment step, the holding time means the time for holding in the temperature range of ₃ points or more and _{3 + 80 ° C. or less of Ac.}

(I) Third heat treatment step Following the second heat treatment step, a third heat treatment step is performed. In the third heat treatment step, holding 650 ° C. or higher _Ac 3 -20 ° C. point below the temperature range at 5s above. By setting the holding temperature to 650 ° C. or higher, austenite is likely to be produced. The holding temperature by below Ac 3 _-20 ° C. points, to promote the production of tempered martensite, the strength - thereby improving the ductility balance.

The holding time is preferably 30 s or more from the viewpoint of more reliably dissolving cementite and stably ensuring good low temperature toughness. Further, from the viewpoint of productivity, the holding time is preferably 300 s or less. In the third heat treatment step, the retention time is meant a time that is maintained at a temperature range of 3 _-20 ° C. point 650 ° C. or higher Ac.

Further, upon heating to a temperature range of 650 ° C. or higher _Ac 3 -20 or less ° C. point, it is preferable that the average heating rate in the temperature range of 500 ~ 600 ° C. and 2 ~ 10 ℃ / s. Further, it is preferable that the average cooling rate from the holding temperature to 520 ° C. is 0.1 ° C./s or more and 100 ° C./s or less. Thereby, the cementite content in the metal structure can be lowered. In the third heat treatment step, the area ratio of cementite in the metal structure can be reduced to 1% or less, more preferably 0%, particularly by controlling the rate of temperature rise.

The first to third heat treatments may be performed in a batch furnace such as a box annealing furnace (BAF), or may be performed using a continuous annealing line. The atmosphere of the heat treatment is not particularly limited, for example, an inert atmosphere, or may be any of a reducing atmosphere containing H ₂ or the like.

If the steel sheet is not plated after the third heat treatment step, it may be cooled to room temperature as it is. On the other hand, when plating a steel sheet, it is manufactured as follows.

(J) Hot-dip galvanizing step When hot-dip galvanizing the surface of a steel sheet to produce a hot-dip galvanized steel sheet, cooling after the third heat treatment is stopped in the temperature range of 430 to 500 ° C., and then the cold-rolled steel sheet is used. Is immersed in a hot-dip galvanizing bath to perform hot-dip galvanizing. The conditions of the plating bath may be within the normal range. After the plating treatment, it may be cooled to room temperature.

(K) Alloyed hot-dip galvanizing step When the surface of a steel sheet is subjected to alloying hot-dip galvanizing to produce an alloyed hot-dip galvanized steel sheet, the steel sheet is subjected to hot-dip galvanizing treatment and then cooled to room temperature. Before this, the hot dip galvanizing process is performed at a temperature of 450 to 620 ° C. The alloying treatment conditions may be within the usual range.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the present invention is not limited to these Examples.

1. 1. Production of Steel Sheet for Evaluation The steel having the chemical composition shown in Table 1 was melted in a laboratory melting furnace to obtain a steel piece having a thickness of 30 mm.

The obtained 30 mm-thick steel piece was heated at 1250 ° C. for 1 h, and then hot-rolled under the conditions shown in Table 2. Subsequently, the winding was simulated and held at a predetermined temperature corresponding to the winding temperature for 30 minutes, and then slowly cooled to room temperature at 20 ° C./h to prepare a 2.6 mm thick hot-rolled steel sheet. In Table 2, the example in which the winding temperature is shown as “room temperature” indicates that the sample has been cooled to room temperature depending on the conditions of the accelerated cooling step. The obtained hot-rolled steel sheet was pickled and then cold-rolled under the conditions shown in Table 2 to prepare a 1.6 mm-thick cold-rolled steel sheet.

The obtained cold-rolled steel sheet was subjected to the first to third heat treatments under the conditions shown in Table 3. The heat treatment was carried out in a reducing atmosphere of 98% nitrogen and 2% hydrogen. In the first heat treatment, heating is performed under the condition that the average temperature rise rate from 300 ° C. to the holding temperature is 0.5 ° C./s, and the average cooling rate from the holding temperature to 300 ° C. is 0.5 ° C./s. It was cooled to a temperature of 300 ° C. or lower under the above conditions. Further, in the second heat treatment, the temperature was cooled to 300 ° C. or lower under the condition that the average cooling rate from the holding temperature to 300 ° C. was 10 ° C./s.

Also, in the third heat treatment, the test No. 1, 3 to 10, 13 to 15, 17, 19 to 22 and 24 to 29 are heated under the condition that the average heating rate in the temperature range of 500 to 600 ° C. is 5 ° C./s. It was cooled to room temperature under the condition that the average cooling rate up to 520 ° C. was 10 ° C./s. On the other hand, Test No. The hot-dip cold-rolled steel sheets of 2, 12, 16, 18 and 23 were heated under the condition that the average temperature rise rate in the temperature range of 500 to 600 ° C. in the third heat treatment was 5 ° C./s. Cooling at an average cooling rate of 10 ° C./s is stopped at 460 ° C., the cold-rolled steel sheet is held at that temperature for 10 seconds, and immersed in a hot-dip galvanizing bath at 460 ° C. for 2 seconds to perform hot-dip galvanizing treatment. rice field. The conditions of the plating bath are the same as those of the conventional one.

Among them, test No. For No. 16, after the hot-dip galvanizing treatment, the mixture was cooled to room temperature at an average cooling rate of 10 ° C./s. Test No. The annealed cold-rolled steel sheets of 2, 12, 18 and 23 were subjected to hot-dip galvanizing treatment, then heated to 500 ° C. at 10 ° C./s without cooling to room temperature, and held at 500 ° C. for 5 s. The alloying treatment was performed, and then the mixture was cooled to room temperature at an average cooling rate of 10 ° C./s.

The annealed cold-rolled steel sheet thus obtained was subjected to skin pass rolling with a reduction ratio of 0.5%, and the test No. Steel sheets 1 to 29 were produced.

2. Evaluation method X-ray diffraction measurement, microstructure observation, tensile test, bending test, and Charpy impact test were carried out on the obtained steel sheet, and retained austenite (γ), tempered martensite (TM) and martensite ( F.M) area ratio of tempered martensite and small-angle grain boundary density and large angle grain boundary density in the martensite, particle density in the particle density a _L and the plate thickness direction in the rolling direction of the residual austenite a _N and the ratio a _L / _AN , tensile strength (TS), elongation at break (El), TS × El, impact characteristics and bendability were evaluated. The method of each evaluation is as described above.

(Test method for mechanical properties)
A JIS No. 5 tensile test piece was taken in the width direction of the steel sheet, and a tensile test was performed to measure the tensile strength (TS) and the elongation at break (El). The tensile test was carried out by the method specified in JIS Z 2241: 2011 using a JIS No. 5 tensile test piece.

The bendability was evaluated by performing a bending test.
Bending test pieces having a width of 20 mm and a length of 50 mm were collected from each of the annealed steel sheets. The width direction of the bending test piece is parallel to the bending axis. The case where the width direction of the bending test piece is parallel to the rolling direction of the steel sheet is called a rolling direction bending test, and the case where the width direction of the bending test piece is parallel to the width direction of the steel sheet is called a width direction bending test. The test piece was pushed into the mold with a V-shaped punch having a punch radius of 4.8 mm and an apex angle of 90 ° of 3.2 mm. The bending test was performed according to the V block method of JIS Z 2248: 2006. Observe the sample surface after the bending test, and if cracks are not observed at both the punch radius of 4.8 mm and 3.2 mm, the bendability is further improved, and if cracks are not observed only at the punch radius of 4.8 mm, bending is observed. When the property was good and cracks were observed at both the punch radii of 4.8 mm and 3.2 mm, the bendability was considered to be poor.

The impact characteristics were evaluated by performing a Charpy impact test.
A V-notch test piece was taken from each steel sheet after annealing. At this time, the length direction of the V-notch test piece was made to coincide with the rolling direction of the steel sheet. After stacking four of the test pieces and screwing them together, they were subjected to a Charpy impact test according to JIS Z 2242: 2018. As for the impact characteristics, when the impact value at 20 ° C. was 30 J / cm ² or more, the impact characteristics were good, and when it was less than that, the impact characteristics were poor.

3. 3. Evaluation Results Table 4 shows the above evaluation results. In this example, an example in which TS of 980 MPa or more, TS × El of 22000 MPa ·% or more, good impact characteristics, and good bendability are obtained has high strength, strength-ductility balance, and bending. It was evaluated as a steel sheet with excellent properties and impact characteristics.

As can be seen from Table 4, in the comparative example which does not satisfy the provisions of the present invention, one of the mechanical properties was inferior. Specifically, the test No. In No. 3, the holding temperature in the second heat treatment step was low, ferrite was formed, and the area ratio of tempered martensite was low, so that the strength and impact characteristics deteriorated. Test No. In No. 4, the holding temperature in the first heat treatment step was high, the small-angle particle size density was low, and the isotropic property of the retained austenite was lowered, so that TS × El, impact characteristics, and bendability were deteriorated.

Test No. In No. 5, the Mn content was less than the specified value, and Test No. In No. 6, the C content was less than the specified value. As a result, TS × El deteriorated because the area ratio of retained austenite was low in all cases. In addition, the test No. At 6, the strength was also insufficient.

Test No. In No. 8, since the second heat treatment step was not performed, tempered martensite was not generated and mainly became ferrite, and the small-angle grain boundary density and the large-angle grain boundary density decreased. In addition, retained austenite was also coarsened. As a result, the strength, TS × El and impact characteristics deteriorated. Test No. In No. 9, the holding temperature in the third heat treatment step was high, the area ratio of tempered martensite was low, and the area ratio of martensite was high, so that the impact characteristics deteriorated.

Test No. In No. 11, since the third heat treatment step was not performed, the martensite became a single phase, and TS × El, impact characteristics, and bendability deteriorated. Test No. In No. 13, the holding time in the second heat treatment step was long, and the large-angle particle size density was lowered, so that the impact characteristics were deteriorated. Test No. In No. 15, the holding temperature in the second heat treatment step was high, and the large-angle particle size density was lowered, so that the impact characteristics were deteriorated.

Test No. In No. 19, the holding time in the first heat treatment step was short, the small angle particle size density was low, and the isotropic property of the retained austenite was lowered, so that TS × El, impact characteristics, and bendability were deteriorated. Test No. In No. 22, the take-up temperature was high and the isotropic property of retained austenite was lowered, so that the impact characteristics and bendability were deteriorated. Test No. In No. 24, the holding temperature in the first heat treatment step was low, the small angle particle size density was low, and the isotropic property of the retained austenite was lowered, so that TS × El, impact characteristics, and bendability were deteriorated.

Test No. In No. 26, the holding temperature in the third heat treatment step was low, and the area ratio of tempered martensite was high, so that the strength deteriorated. Test No. In No. 29, since the first heat treatment step and the second heat treatment step were not performed, tempered martensite was not generated and was mainly ferrite, and the area ratio of martensite was further increased. In addition, the small-angle grain boundary density and the large-angle grain boundary density decreased, and the isotropic property of retained austenite also decreased. As a result, TS × El, impact characteristics, and bendability deteriorated.

On the other hand, in the example of the present invention, a steel sheet exhibiting TS of 980 MPa or more, TS × El of 22000 MPa ·% or more, “good” impact characteristics, and “good” or “even better” bendability was obtained. .. In particular, it can be seen that the steel sheet of the present invention exhibits excellent results not only in width direction bendability but also in rolling direction bendability because retained austenite is isotropically dispersed.

According to the present invention, it is possible to obtain a steel sheet having high strength and excellent strength-ductility balance, bendability and impact characteristics. Therefore, the steel sheet of the present invention can be used for various purposes, and is particularly preferably used for structural parts of automobiles such as side sills, and contributes to weight reduction of automobiles.

Claims

The chemical composition of the steel sheet is mass%,
C: More than 0.18% and less than 0.30%,
Si: 0.001% or more and less than 2.00%,
Mn: More than 2.50% and less than 4.20%,
sol. Al: 0.001% or more and less than 1.00%,
P: 0.030% or less,
S: 0.005% or less,
N: Less than 0.050%,
O: Less than 0.020%,
Cr: 0 to 0.50%,
Mo: 0 to 0.50%,
W: 0 to 0.30%,
Cu: 0 to 0.30%,
Ni: 0 to 0.50%,
Ti: 0 to 0.100%,
Nb: 0 to 0.100%,
V: 0 to 0.100%,
B: 0 to 0.010%,
Ca: 0 to 0.010%,
Mg: 0 to 0.010%,
Zr: 0 to 0.010%,
REM: 0-0.010%,
Sb: 0 to 0.050%,
Sn: 0 to 0.050%,
Bi: 0 to 0.050%,
Remaining: Fe and impurities,
In the cross section parallel to the rolling direction and the plate thickness direction of the steel plate, the metal structure at a depth of 1/4 of the plate thickness from the surface is, in% area.
Residual austenite: 10% or more,
Tempering martensite: 60-80%,
Martensite: less than 20%,
In tempered martensite and martensite, the grain boundary density of small angles with a crystal orientation difference of 2 degrees or more and less than 20 degrees is 0.20 to 1.0 μm -1, and the grain boundary density of large angles with a crystal orientation difference of 20 to 50 degrees is 0.20 to 1.0 μm -1. It is 0.30 to 0.60 μm -1 .
The ratio A L / A N of the particle density A N in the particle density A L and the plate thickness direction in the rolling direction of the residual austenite is 0.80 to 1.0
Steel plate.
When the chemical composition is mass%,
Cr: 0.01-0.50%,
Mo: 0.01-0.50%,
W: 0.01-0.30%,
Cu: 0.01 to 0.30%, and Ni: 0.01 to 0.50%,
Contains one or more selected from,
The steel plate according to claim 1.
When the chemical composition is mass%,
Ti: 0.005 to 0.100%,
Nb: 0.005 to 0.100%, and V: 0.005 to 0.100%,
Contains one or more selected from,
The steel sheet according to claim 1 or 2.
When the chemical composition is mass%,
B: 0.0001 to 0.010%,
Ca: 0.0001 to 0.010%,
Mg: 0.0001 to 0.010%,
Zr: 0.0001 to 0.010%, and REM: 0.0001 to 0.010%,
Contains one or more selected from,
The steel sheet according to any one of claims 1 to 3.
When the chemical composition is mass%,
Sb: 0.0005 to 0.050%,
Sn: 0.0005 to 0.050%, and Bi: 0.0005 to 0.050%,
Contains one or more selected from,
The steel plate according to any one of claims 1 to 4.
A hot-dip galvanized layer is provided on the surface of the steel sheet.
The steel sheet according to any one of claims 1 to 5.
Having an alloyed hot-dip galvanized layer on the surface of the steel sheet.
The steel sheet according to any one of claims 1 to 5.