TW201942393A - Steel plate capable of obtaining excellent collision safety and formability - Google Patents

Abstract

The invention discloses a steel plate provided with a predetermined chemical composition and a metallic structure including, in area fraction, 40% or less of polygonal ferrites, 20% or less of martensite, 50%-95% of toughened ferrites and 5%-50% of retained austenite. In area fraction, 80% or more of the toughened ferrites is composed of toughened ferrite crystal grains of which the dislocation density is 8*10<SP>2</SP>(cm/cm3) in a region encircled by a grain boundary of which the length to width ratio is 0.1 to 1.0 and the azimuth difference angle is 15 DEG or more. In addition, in area fraction, 80% or more of the retained austenite is composed of retained austenite crystal grains of which the length to width ratio is 0.1 to 1.0, the major axis length is 1.0 [mu]m to 28.0 [mu]m and the minor axis length is 0.1 [mu]m to 2.8 [mu]m.

Description

Steel plate

The present invention relates to a steel plate suitable for automobile parts.

BACKGROUND ART In order to suppress the amount of carbon dioxide gas emitted from automobiles, automobile bodies using high-strength steel sheets continue to be reduced in weight. For example, in order to ensure the safety of passengers, high-strength steel sheets are increasingly used for skeleton-based parts of vehicle bodies. Examples of mechanical properties that have a significant impact on impact safety include tensile strength, ductility, ductility-brittleness transition temperature, and 0.2% off-site drop strength. For example, excellent ductility may be required for a steel sheet used for a front side member.

On the other hand, the shape of the skeleton parts is complicated, and high-strength steel sheets for skeleton parts are required to have excellent hole expandability and bendability. For example, an excellent hole expandability is required for a steel plate used for a side beam.

However, it is difficult to balance the improvement of impact safety and the improvement of formability. Conventionally, there have been proposed technologies concerning improvement of impact safety or improvement of formability (Patent Documents 1 and 2). However, it is still difficult to achieve both improvement of impact safety and improvement of formability in accordance with these.

Prior Art Literature Patent Literature Patent Literature 1: Japanese Patent Laid-Open No. 5589893 Patent Literature 2: Japanese Patent Laid-Open No. 2013-185196 Patent Literature 3: Japanese Patent Laid-Open No. 2005-171319 Patent Literature 4: International Patent Publication No. 2012/133563

SUMMARY OF THE INVENTION Problems to be Solved by the Invention An object of the present invention is to provide a steel sheet capable of obtaining excellent impact safety and formability.

Means for Solving the Problems The present inventors have conducted intensive studies to solve the above problems. As a result, it was found that in a steel sheet having a tensile strength of 980 MPa or more, an excellent elongation can be exhibited by making the area fraction and morphology of the residual Vostian iron and the toughened ferrous iron into predetermined ones. In addition, it has been found that when the area fraction of polygonal ferrous iron is low, the difference in hardness in the steel sheet is small, not only excellent elongation can be obtained, but also excellent hole expandability and bendability can be obtained, and sufficient Its resistance to embrittlement at low temperature and 0.2% off-position fall-off strength.

Based on the foregoing knowledge and insights, the inventors of the present case conducted intensive studies repeatedly, and came up with various aspects of the invention shown below.

(1) A steel plate characterized in that it has the following chemical composition: as mass%, C: 0.1% ~ 0.5%, Si: 0.5% ~ 4.0%, Mn: 1.0% ~ 4.0%, P: 0.015 % Or less, S: 0.050% or less, N: 0.01% or less, Al: 2.0% or less, Si and Al: 0.5% to 6.0% in total, Ti: 0.00% to 0.20%, Nb: 0.00% to 0.20%, B : 0.0000% to 0.0030%, Mo: 0.00% to 0.50%, Cr: 0.0% to 2.0%, V: 0.00% to 0.50%, Mg: 0.000% to 0.040%, REM: 0.000% to 0.040%, Ca: 0.000 % ~ 0.040%, and the remainder: Fe and impurities; and has the following metal structure: in terms of area fraction, polygonal ferrous iron: 40% or less, Asada loose iron: 20% or less, toughened ferrous iron: 50% ~ 95%, and residual Vostian iron: 5% ~ 50%; in terms of area fraction, more than 80% of the aforementioned toughened fertile iron is caused by the aspect ratio of 0.1 ~ 1.0 and the azimuth difference angle is between In the area surrounded by the grain boundary above 15 °, it consists of toughened fat iron grains with a differential row density of 8 × 10 ² (cm / cm ³ ) or less, and in terms of area fraction, 80% or more are caused by an aspect ratio of 0.1 to 1.0, a long axis length of 1.0 μm to 28.0 μm, and It is composed of residual Vostian iron grains with a minor axis length of 0.1 μm to 2.8 μm.

(2) The steel plate as in (1), in which the aforementioned metal structure is expressed in terms of area fraction: polygonal ferrous iron: 5% -20%, Asada loose iron: 20% or less, toughened ferrous iron: 75% ~ 90% and residual Vostian iron: 5% ~ 20%.

(3) The steel plate according to (1), in which the aforementioned metal structure is expressed in terms of area fraction: polygonal ferrous iron: more than 20% and less than 40%, Asada loose iron: 20% or less, and toughened ferrous iron: 50% ~ 75%, and residual Vostian iron: 5% ~ 30%.

(4) The steel sheet according to any one of (1) to (3), wherein the following chemical composition is established: in terms of mass%, Ti: 0.01% to 0.20%, Nb: 0.005% to 0.20%, and B: 0.0001% ~ 0.0030%, Mo: 0.01% ~ 0.50%, Cr: 0.01% ~ 2.0%, V: 0.01% ~ 0.50%, Mg: 0.0005% ~ 0.040%, REM: 0.0005% ~ 0.040%, or Ca: 0.0005 % ~ 0.040%, or any combination of these.

(5) The steel sheet according to any one of (1) to (4), which has a plating layer formed on the surface.

ADVANTAGE OF THE INVENTION According to this invention, since the area fraction and morphology of the residual Vostian iron and the toughened ferrite iron are appropriate, excellent impact safety and formability can be obtained.

Embodiments of the Invention Embodiments of the present invention will be described below.

First, the metal structure of the steel sheet according to the embodiment of the present invention will be described. The steel plate of this embodiment has the following metal structure: in terms of area fraction, polygonal ferrous iron: 40% or less, Asada loose iron: 20% or less, toughened ferrous iron: 50% to 95%, and residual fertile Staine: 5% ~ 50%. In terms of area fraction, more than 80% of the toughened ferrous iron is in a region surrounded by grain boundaries with an aspect ratio of 0.1 to 1.0 and an azimuth difference angle of 15 ° or more. The differential row density is 8 × 10 ² ( cm / cm ³ ) or less. Moreover, in terms of area fraction, more than 80% of the residual Vosstian iron is composed of residual Vosstian iron crystals with an aspect ratio of 0.1 to 1.0, a major axis length of 1.0 μm to 28.0 μm, and a minor axis length of 0.1 μm to 2.8 μm Made of grains.

(Area fraction of polygonal ferrous iron: 40% or less) Polygonal ferrous iron is a soft tissue. Therefore, the hardness difference between the polygonal ferrous iron and the hard-grained Asada iron is large, and cracks are likely to occur at the interface between them during forming. Cracks sometimes also extend along this interface. When the area fraction of the polygonal ferrous iron is greater than 40%, the occurrence of the cracks and the extension are likely to occur, and it is difficult to obtain sufficient hole expandability, bendability, embrittlement resistance at low temperature, and 0.2% off-position yielding strength . Therefore, the area fraction of the polygonal ferrous iron is set to 40% or less.

The lower the area fraction of polygonal ferrous iron, the more difficult it is for C to be concentrated in the residual Vosstian iron, and the hole expandability will be improved but the ductility will be reduced. Therefore, when the hole expansion is more important than ductility, the area fraction of polygonal ferrous iron should be set to less than 20%. When the ductility is more important than hole expansion, the area fraction of polygonal ferrous iron should be set to More than 20% and less than 40%. In order to ensure ductility when the hole expansion is more important than ductility, the area fraction of polygonal ferrous iron should be set to 5% or more.

(Area percentage of toughened ferrous iron: 50% ~ 95%) Toughened ferrous iron contains differential rows at a higher density than polygonal ferrous iron, which contributes to the improvement of tensile strength. Since the hardness of the toughened ferrous iron is higher than that of the polygonal ferrous iron, and it is lower than that of the Masada loose iron, the hardness difference between the toughened fertilizer iron and the Asada loose iron will be higher than that of the polygonal fertilizer iron and Asada The hardness difference between scattered iron is still small. Therefore, toughening the ferrous iron also contributes to the improvement of hole expandability and bendability. If the area fraction of the toughened ferrous iron is less than 50%, sufficient tensile strength cannot be obtained. Therefore, the area fraction of the toughened ferritic iron is set to 50% or more. When the pore expandability is more important than ductility, the area fraction of the toughened fertilizer iron should be set to more than 75%. On the other hand, if the area fraction of the toughened ferritic iron is more than 95%, the residual Vosted iron is insufficient, and sufficient formability cannot be obtained. Therefore, the area fraction of the toughened ferritic iron is set to 95% or less.

(The area fraction of Asada loose iron: less than 20%) Asada loose iron includes fresh Asada loose iron (untempered Asada loose iron) and tempered Asada loose iron. As described above, the hardness difference between the polygonal ferrous iron and the Asada loose iron is large, and cracks are likely to occur at the interface between these when forming. Cracks sometimes also extend along this interface. When the area fraction of the loose iron in Mata is more than 20%, the occurrence of the cracks and extension described above are likely to occur, and it is difficult to obtain sufficient hole expandability, bendability, embrittlement resistance at low temperature, and 0.2% off-position falloff strength. Therefore, the area fraction of Asada scattered iron is set to 20% or less.

(Area Fraction of Residual Vastfield Iron: 5% ~ 50%) Residual Vastfield iron contributes to the improvement of formability. If the area fraction of the residual Voss iron is less than 5%, sufficient formability cannot be obtained. On the other hand, if the area fraction of the residual Vossian iron is more than 50%, the toughened fertile iron is insufficient, and sufficient tensile strength cannot be obtained. Therefore, the area fraction of the residual Vosstian iron is set to 50% or less.

The identification of polygonal ferrous iron, toughened ferrous iron, residual Vostian iron, and Asada loose iron and the identification of the area fraction can be observed using, for example, a scanning electron microscope (SEM) or a transmission electron microscope ( transmission electron microscope: TEM) observation. When SEM or TEM is used, the specimen is etched with a solution of Nital and Lepera, and the cross section parallel to the rolling direction and thickness direction (vertical) is observed at a magnification of 1000 to 100,000 times. A cross section in the width direction) and / or a cross section perpendicular to the rolling direction.

Polygonal ferritic iron, toughened ferrous iron, residual Vostian iron, and Asada loose iron can also be discriminated by the hardness measurement in a small area such as the analysis of crystal orientation or micro Vickers hardness measurement. The crystal orientation diffraction (FE-SEM-EBSD) function using an electron back scattering diffraction (EBSD) function attached to a field emission scanning electron microscope (FE-SEM) ongoing.

For example, in the area ratio specification of polygonal fertilized iron and toughened ferrous iron, a cross section parallel to the rolling direction and thickness direction of the steel plate (cross section perpendicular to the width direction) is ground, and Etching. Then, the area from the surface of the steel plate to a depth from 1/8 to 3/8 of the thickness of the steel plate was observed by FE-SEM to measure the area fraction. The above observations were performed in 10 fields of view at a magnification of 5000 times, and the area fractions of the polygonal fertilized iron and the toughened ferrous iron can be obtained from the average of the 10 fields of view.

The area fraction of the residual Vosstian iron can be specified by, for example, X-ray measurement. In this method, for example, a portion from the surface of the steel sheet to a quarter of the thickness of the steel sheet is removed by mechanical polishing and chemical polishing, and MoKα rays are used as characteristic X-rays. Then, using the following equations, diffraction from (200) and (211) of the body-centered cubic lattice (bcc) phase and (200), (220), and (311) of the face-centered cubic lattice (fcc) phase The integral intensity ratio of the peaks is used to calculate the area fraction of the residual Vosstian iron. The above observations were performed in 10 fields of view, and the area fraction of the residual Vostian iron was obtained from the average value of the 10 fields of view. Sγ = (I _200f + I _220f + I _311f ) / (I _200b + I _211b ) × 100 (Sγ represents the area fraction of the residual Vostian iron, I _200f , I _220f , and I _311f represent (200 for each fcc phase) ), (220), and (311) diffraction peak intensities, and I _200b and I _211b represent the diffraction peak intensities of (200) and (211) for each bcc phase.)

The area fraction of the Mata loose iron can be specified by, for example, field emission-scanning electron microscope (FE-SEM) observation and X-ray measurement. In this method, for example, a region from the surface of the steel plate having a depth from 1/8 to 3/8 of the thickness of the steel plate is used as an observation object, and a Lepera liquid is used for corrosion. Since the tissues that will not be corroded by Lepera liquid are Asada loose iron and residual Vostian iron, the area fraction of the area that has not been corroded by Lepera liquid can be subtracted by X-ray measurement. The area fraction Sγ of the specified residual Vostian iron is used to specify the area fraction of the Asada loose iron. The area fraction of the Mata loose iron can also be specified using, for example, an electron channeling contrast image obtained by SEM observation. In the contrast images of the electron tunneling effect, the area with higher differential density and lower structures such as crystal blocks and inclusions in the grains is Asada loose iron. The above observations were performed in 10 fields of view, and the area fraction of the Asada scattered iron was obtained from the average value of the 10 fields of view.

(Area fraction of the toughened ferrous iron grains in a predetermined form: 80% or more relative to the whole of the toughened ferrous iron) The toughened ferrous iron grains with high differential density are not the same as the polygonal ferrous iron It helps to improve the elongation, so the higher the area fraction of the toughened fertilizer iron grains with high differential density, the easier the elongation is to decrease. Then, if the aspect ratio is 0.1 to 1.0 and the azimuth difference angle is 15 ° or more, the area surrounded by grain boundaries has a difference in row density of 8 × 10 ² (cm / cm ³ ) or less. If the area fraction is less than 80%, it is difficult to obtain a sufficient elongation. Therefore, the area fraction of the toughened ferritic iron particles in this form is 80% or more, and more preferably 85% or more, with respect to the entire toughened ferrous iron.

The differential row density of the toughened ferrous iron can be specified by observation of the structure using a transmission electron microscope (TEM). The differential row density of the toughened ferrous iron can be specified, for example, by dividing the number of differential rows existing in crystal grains surrounded by grain boundaries with an azimuth difference angle of 15 ° by the area of the crystal grains.

(The area fraction of the residual Vosstian iron grains in a predetermined form: 80% or more relative to the total amount of the residual Vosstian iron.) The residual Vosstian iron is deformed into Asada loose iron due to processing-induced metamorphosis during forming. If the residual Vostian iron is deformed to the loose iron in Asada, in the case that the loose Asta iron is adjacent to the polygonal fertilized iron or the unconverted residual Vostian iron, there will be a large hardness difference between them. As mentioned above, large hardness differences can cause cracks. Such cracks are particularly likely to occur in locations where stress is concentrated, and the stress is likely to be concentrated in the vicinity of Asada loose iron, which is a metamorphosis of residual Vostian iron with an aspect ratio of less than 0.1. Then, if the area fraction of the residual Vostian iron grains with an aspect ratio of 0.1 to 1.0, a major axis length of 1.0 μm to 28.0 μm, and a minor axis length of 0.1 μm to 2.8 μm is less than 80%, accompanying stress is liable to occur. Concentrated cracks make it difficult to obtain sufficient elongation. Therefore, the area fraction of the residual Vosstian iron grains in this form is 80% or more, and more preferably 85% or more, with respect to the entire residual Vosstian iron. Here, the aspect ratio of the residual Vosstian iron crystal grains is a value obtained by dividing the minor axis length of the equivalent ellipse of the residual Vosstian iron crystal grains by the long axis length. An example of an equivalent ellipse is shown in FIG. 1. Even if the residual Vosstian iron grains 1 have a complicated shape, the aspect ratio (L2 / L1) of the Vosstian iron grains can be obtained from the long axis length L1 and the short axis length L2 of the equivalent ellipse 2.

Next, the chemical composition of the steel sheet according to the embodiment of the present invention and the steel slab used for manufacturing the same will be described. As described above, the steel sheet according to the embodiment of the present invention is produced through hot rolling, pickling, cold rolling, first annealing, second annealing, and the like. Therefore, the chemical composition of the steel plate and the steel blank takes into consideration not only the characteristics of the steel plate but also these treatments. In the following description, the unit "%" of the content of each element contained in the steel plate and the steel blank means "mass%" unless otherwise specified. The steel sheet of this embodiment and the steel blank used for manufacturing the same have the following chemical compositions: C: 0.1% to 0.5%, Si: 0.5% to 4.0%, Mn: 1.0% to 4.0%, P : 0.015% or less, S: 0.050% or less, N: 0.01% or less, Al: 2.0% or less, Si and Al: 0.5% to 6.0% in total, Ti: 0.00% to 0.20%, Nb: 0.00% to 0.20% , B: 0.0000% ~ 0.0030%, Mo: 0.00% ~ 0.50%, Cr: 0.0% ~ 2.0%, V: 0.00% ~ 0.50%, Mg: 0.000% ~ 0.040%, REM (rare earth metal: rare earth metal ): 0.000% ~ 0.040%, Ca: 0.000% ~ 0.040%, and the remainder: Fe and impurities.

(C: 0.10% ~ 0.5%) Carbon (C) contributes to the improvement of the strength of the steel sheet, and contributes to the improvement of the elongation through the improvement of the stability of the residual Vosted iron. If the C content is less than 0.10%, it is difficult to obtain sufficient strength, for example, a tensile strength of 980 MPa or more, and the stability of the residual Vosted iron becomes insufficient to obtain a sufficient elongation. Therefore, the C content is set to be 0.10% or more, and preferably 0.15% or more. On the other hand, if the C content is greater than 0.5%, the transformation from Vostian iron to the toughened ferrous iron will be delayed, so the toughened ferrous iron particles of a predetermined form will be insufficient, and sufficient elongation cannot be obtained. Therefore, the C content is set to 0.5% or less, and preferably 0.25% or less.

(Si: 0.5% ~ 4.0%) Silicon (Si) contributes to the improvement of the strength of the steel, and contributes to the improvement of the elongation through the improvement of the stability of the residual Vosted iron. If the Si content is less than 0.5%, these effects cannot be sufficiently obtained. Therefore, the Si content is set to 0.5% or more, and preferably set to 1.0% or more. On the other hand, if the Si content is more than 4.0%, the strength of the steel becomes too high and the elongation decreases. Therefore, the Si content is set to 4.0% or less, and preferably set to 2.0% or less.

(Mn: 1.0% to 4.0%) Manganese (Mn) contributes to the improvement of the strength of the steel, and suppresses the deformation of the polygonal fertilizer grains generated during the cooling of the first annealing or the second annealing. When the hot-dip galvanizing treatment is performed, deformation of polygonal ferrous iron that occurs during cooling of the treatment is also suppressed. If the Mn content is less than 1.0%, these effects cannot be obtained sufficiently, and polygonal fertilized iron is excessively generated to deteriorate the hole expandability. Therefore, the Mn content is set to 1.0% or more, and preferably set to 2.0% or more. On the other hand, if the Mn content is more than 4.0%, the strength of the steel billet and the hot-rolled steel sheet becomes too high. Therefore, it should be set to 4.0% or less, and preferably set to 3.0% or less.

(P: 0.015% or less) Phosphorus (P) is not an essential element, but is contained in steel as an impurity, for example. P segregates at the central portion in the thickness direction of the steel sheet to reduce toughness, and embrittles the welded portion. Therefore, the lower the P content, the better. In particular, when the P content is more than 0.015%, the decrease in toughness and the brittleness of the weldability are significant. Therefore, the P content is set to 0.015% or less, and preferably set to 0.010% or less. It takes cost to reduce the P content. If it is to be reduced to less than 0.0001%, the cost will rise significantly. Therefore, the P content may be set to 0.0001% or more.

(S: 0.050% or less) Sulfur (S) is not an essential element, but is contained in steel as an impurity, for example. S reduces the manufacturability of casting and hot rolling, and forms coarse MnS, which decreases the hole expandability. Therefore, the lower the S content, the better. In particular, when the S content is more than 0.050%, the reduction in weldability, reduction in manufacturability, and reduction in hole expandability are significant. Therefore, the S content is set to 0.050% or less, and preferably set to 0.0050% or less. It is costly to reduce the S content. If it is to be reduced to less than 0.0001%, the cost will increase significantly. Therefore, the S content may be set to 0.0001% or more.

(N: 0.01% or less) Nitrogen (N) is not an essential element, but is contained in steel as an impurity, for example. N forms coarse nitrides, degrades bendability and hole expandability, and causes porosity during welding. Therefore, the lower the N content, the better. In particular, if the N content is more than 0.01%, reduction in bendability and hole expandability, and generation of pores are significant. Therefore, the N content is set to 0.01% or less. It is costly to reduce the N content. If it is to be reduced to less than 0.0005%, the cost will rise significantly. Therefore, the N content may be set to 0.0005% or more.

(Al: 2.0% or less) Although aluminum (Al) can function as a deoxidizing material and can suppress the precipitation of iron-based carbides in Vostian iron, it is not an essential element. If the Al content is greater than 2.0%, the transformation from Vostian iron to polygonal fertilized iron will be promoted, and polygonal ferrous iron will be excessively generated, which will cause the hole expandability to deteriorate. Therefore, the Al content is set to 2.0% or less, and preferably set to 1.0% or less. It is costly to reduce the Al content, and if it is lowered to less than 0.001%, the cost will increase significantly. Therefore, the Al content may be set to 0.001% or more.

(Si and Al: 0.5% ~ 6.0% in total) Both Si and Al will contribute to the improvement of elongation through the improvement of the stability of the residual Vosted iron. If the total content of Si and Al is less than 0.5%, this effect cannot be obtained sufficiently. Therefore, the total content of Si and Al is set to be 0.5% or more, and preferably 1.2% or more. Moreover, it may contain only either Si or Al, and may contain both Si and Al.

Ti, Nb, B, Mo, Cr, V, Mg, REM, and Ca are not essential elements, but any element that can be appropriately contained in a steel plate and a steel billet within a predetermined amount limit.

(Ti: 0.00% ~ 0.20%) Titanium (Ti) contributes to the improvement of the strength of the steel through differential row strengthening due to precipitation strengthening and fine particle strengthening. Therefore, Ti may be contained. In order to fully obtain this effect, the Ti content should be set to 0.01% or more, and more preferably 0.025% or more. On the other hand, if the Ti content is more than 0.20%, the carbonitrides of Ti are excessively precipitated and the formability of the steel sheet is reduced. Therefore, the Ti content is set to 0.20% or less, and preferably 0.08% or less.

(Nb: 0.00% to 0.20%) Niobium (Nb) contributes to the improvement of the strength of the steel by differential strengthening due to precipitation strengthening and fine particle strengthening. Therefore, Nb may be contained. In order to fully obtain this effect, the Nb content should preferably be 0.005% or more, and more preferably 0.010% or more. On the other hand, if the Nb content is more than 0.20%, carbonitrides of Nb are excessively precipitated, and the formability of the steel sheet is reduced. Therefore, the Nb content is set to 0.20% or less, and preferably 0.08% or less.

(B: 0.0000% ~ 0.0030%) Boron (B) strengthens the grain boundaries and suppresses the deformation of polygonal ferrous iron that occurs during the cooling of the first annealing or the second annealing. When the hot-dip galvanizing treatment is performed, deformation of polygonal ferrous iron that occurs during cooling of the treatment is also suppressed. Therefore, B may be contained. In order to fully obtain this effect, the B content should preferably be set to 0.0001% or more, and more preferably 0.0010% or more. On the other hand, if the B content is more than 0.0030%, the effect of addition is saturated, and the manufacturability of hot rolling is reduced. Therefore, the B content is set to 0.0030% or less, and preferably set to 0.0025% or less.

(Mo: 0.00% ~ 0.50%) Molybdenum (Mo) contributes to the strengthening of the steel, and suppresses the deformation of polygonal ferrous iron that occurs during the cooling of the first annealing or the second annealing. When the hot-dip galvanizing treatment is performed, deformation of polygonal ferrous iron that occurs during cooling of the treatment is also suppressed. Therefore, Mo may be contained. In order to fully obtain this effect, the Mo content should preferably be 0.01% or more, and more preferably 0.02% or more. On the other hand, if the Mo content is more than 0.50%, the manufacturability of hot rolling is reduced. Therefore, the Mo content is set to 0.50% or less, and preferably 0.20% or less.

(Cr: 0.0% ~ 2.0%) Chromium (Cr) contributes to the strengthening of the steel, and suppresses the deformation of polygonal ferrous iron that occurs during the cooling of the first annealing or the second annealing. When the hot-dip galvanizing treatment is performed, deformation of polygonal ferrous iron that occurs during cooling of the treatment is also suppressed. Therefore, Cr may be contained. In order to fully obtain this effect, the Cr content should preferably be 0.01% or more, and more preferably 0.02% or more. On the other hand, when the Cr content is more than 2.0%, the manufacturability of hot rolling is reduced. Therefore, the Cr content is set to 2.0% or less, and preferably set to 0.10% or less.

(V: 0.00% ~ 0.50%) Vanadium (V) contributes to the improvement of the strength of the steel by differential strengthening due to precipitation strengthening and fine particle strengthening. Therefore, V may be contained. In order to fully obtain this effect, the V content should preferably be 0.01% or more, and more preferably 0.02% or more. On the other hand, if the V content is more than 0.50%, carbonitrides of V are excessively precipitated and the formability of the steel sheet is reduced. Therefore, the Nb content is set to 0.50% or less, and preferably set to 0.10% or less.

(Mg: 0.000% to 0.040%, REM: 0.000% to 0.040%, Ca: 0.000% to 0.040%) Magnesium (Mg), rare earth metals (REM) and calcium (Ca) exist as oxides or sulfides In steel, it helps to improve the hole expandability. Therefore, it may contain Mg, REM, or Ca, or any combination thereof. In order to fully obtain this effect, the Mg content, the REM content, and the Ca content should be set to 0.0005% or more, and more preferably set to 0.0010% or more. On the other hand, if the Mg content, REM content, or Ca content is more than 0.040%, coarse oxides are formed and the hole expandability is reduced. Therefore, the Mg content, the REM content, and the Ca content are all set to 0.040% or less, and preferably set to 0.010% or less.

REM (rare-earth metals) refers to 17 types of elements in total of Sc, Y, and lanthanides, and "REM content" means the total content of these 17 elements. REM is added in the form of, for example, a rare earth metal alloy (misch metal), and the rare earth metal alloy contains lanthanoid elements in addition to La and Ce. For the addition of REM, metal simple materials such as metal La and metal Ce may also be used.

Examples of the impurities include those contained in raw materials such as ore and waste, and those contained in the manufacturing steps. Specifically, P, S, O, Sb, Sn, W, Co, As, Pb, Bi, and H can be exemplified as impurities. O content should be set to 0.010% or less, Sb content, Sn content, W content, Co content, and As content should be set to 0.1% or less, Pb content and Bi content should be set to 0.005% or less, and H content should be set to 0.0005%. the following.

According to this embodiment, excellent impact safety and formability can be obtained. For example, the following mechanical characteristics can be obtained: the hole expandability is 30% or more, the ratio of the minimum bending radius (R (mm)) to the plate thickness (t (mm)) (R / t) is 0.5 or less, and the total elongation is 21% Above, the 0.2% off-position falloff strength is 680 MPa or more, the tensile strength is 980 MPa or more, and the ductility-brittle transition temperature is -60 ° C or lower. In particular, when the area fraction of polygonal fertilizer iron is 5% to 20%, and the area fraction of toughened fertilizer iron is 75% or more, the pore expandability of more than 50% can be obtained. When the area fraction is more than 20% and less than 40%, a total elongation of more than 26% can be obtained.

Next, a method for manufacturing a steel sheet according to an embodiment of the present invention will be described. In the method for manufacturing a steel sheet according to the embodiment of the present invention, hot rolling, pickling, cold rolling, first annealing, and second annealing of a steel billet having the above-mentioned chemical composition are sequentially performed.

(Hot rolling) In hot rolling, rough rolling, finishing rolling, and coiling of a steel billet are performed. For the steel billet, for example, a steel billet obtained by continuous casting and a steel billet made by a thin steel billet continuous casting machine can be used. The steel billet can be supplied to the hot rolling equipment while being maintained at a temperature of 1000 ° C or higher after casting, or it can be heated and supplied to the hot rolling equipment after cooling to a temperature below 1000 ° C.

The rolling temperature of the final pass of the rough rolling is set to 1000 ° C. to 1150 ° C., and the rolling reduction rate of the final pass is set to 40% or more. If the rolling temperature of the final pass is lower than 1000 ° C, the particle size of Vosstian iron after finishing rolling becomes excessively small. At this time, the transformation from Vostian iron to polygonal ferrous iron is excessively promoted, and the uniformity of the metal structure is reduced, so that sufficient formability cannot be obtained. Therefore, the rolling temperature in the final pass is set to 1000 ° C or higher. On the other hand, if the rolling temperature of the final pass is higher than 1150 ° C, the particle size of the Vosstian iron after the finishing rolling becomes excessively large. In this case, the uniformity of the metal structure is also reduced, and sufficient formability cannot be obtained. Therefore, the rolling temperature in the final pass is set to 1150 ° C or lower. If the final rolling reduction is less than 40%, the particle size of Vosstian iron after finishing rolling will be excessively large, and the uniformity of the metal structure will be reduced, and sufficient formability cannot be obtained. Therefore, the rolling reduction of the final pass is set to 40% or more.

The rolling temperature of the finishing rolling is set to Ar ₃ or more. If the rolling temperature is lower than the Ar ₃ point, it will become the case that the metal structure of hot-rolled steel sheet contains Vosstian iron and ferrous iron, and the mechanical properties are different between Vostian iron and ferrous iron. Therefore, sufficient moldability cannot be obtained. Therefore, this rolling temperature is set to Ar ₃ or more. When the rolling temperature is set to Ar ₃ or more, the rolling load in the finishing rolling can be reduced relatively. In the finishing rolling, a material obtained by joining a plurality of rough rolled sheets obtained by rough rolling may be continuously rolled. After the rough-rolled rolled sheet is temporarily coiled, it may also be rolled while finishing and rolled.

The winding temperature is set to 750 ° C or lower. If the coiling temperature is higher than 750 ° C, coarse ferrous iron or boron iron is generated in the structure of the hot-rolled steel sheet, and the uniformity of the metal structure is reduced, and sufficient formability cannot be obtained. In addition, the oxide may be formed thickly on the surface to reduce the pickling property. Therefore, the winding temperature is set to 750 ° C or lower. The lower limit of the winding temperature is not particularly limited, but it may be difficult to perform winding at a lower temperature than room temperature. The coil of the hot-rolled steel sheet can be obtained by hot-rolling the steel billet.

(Pickling) After hot rolling, the coil of the hot-rolled steel sheet is rolled back and pickled. Pickling is performed once or more. Pickling can remove oxides on the surface of the hot-rolled steel sheet and improve chemical conversion treatment and plating properties.

(Cold rolling) After pickling, cold rolling is performed. The cold rolling reduction ratio is set to 40% to 80%. If the reduction ratio is less than 40%, it may be difficult to keep the shape of the cold-rolled steel sheet flat, and sufficient ductility may not be obtained. Therefore, the reduction ratio is set to 40% or more, and preferably 50% or more. On the other hand, if the rolling reduction ratio is greater than 80%, the rolling load will become too large, and the recrystallization of the ferrous iron will be promoted excessively, forming a coarse polygonal ferrous iron, resulting in an area of the polygonal ferrous iron. The fraction is greater than 40%. Therefore, the reduction ratio is set to 80% or less, and preferably 70% or less. The number of rolling passes and the rolling reduction per pass are not particularly limited. A cold-rolled steel sheet can be produced by cold-rolling a hot-rolled steel sheet.

(First annealing) After the cold rolling, the first annealing is performed. In the first annealing, the first heating, the first cooling, the second cooling, and the first holding of the cold-rolled steel sheet are performed. The first annealing can be performed, for example, on a continuous annealing line.

The annealing temperature of the first annealing is set to 750 ° C to 900 ° C. If the annealing temperature is lower than 750 ° C, the area fraction of polygonal ferrous iron will be too much, and the area fraction of toughened ferrous iron will be too small. Therefore, the annealing temperature is set to 750 ° C or higher, and preferably 780 ° C or higher. On the other hand, if the annealing temperature is higher than 900 ° C, the grain size of Vosstian iron will be coarsened, and the transformation from Vosstian iron to toughened ferrous iron or tempered Asada iron will be delayed. Then, the area fraction of the toughened ferritic iron is too small due to the delay of the metamorphosis. Therefore, the annealing temperature is preferably 900 ° C or lower, and preferably 870 ° C or lower. The annealing time is not particularly limited, and it is set to, for example, 1 second or more and 1000 seconds or less.

The cooling stop temperature of the first cooling is set to 600 ° C to 720 ° C, and the cooling rate up to the cooling stop temperature is set to 1 ° C / sec or more and less than 10 ° C / sec. If the cooling stop temperature of the first cooling is lower than 600 ° C, the area fraction of the polygonal ferrous iron will be excessive. Therefore, the cooling stop temperature is set to 600 ° C or higher, and is preferably set to 620 ° C or higher. On the other hand, if the cooling stop temperature is higher than 720 ° C, the area fraction of the residual Vosstian iron will be insufficient. Therefore, the cooling stop temperature is 720 ° C or lower, and preferably 700 ° C or lower. If the cooling rate of the first cooling is lower than 1.0 ° C / second, the area fraction of the polygonal ferrous iron will be excessive. Therefore, the cooling rate is set to 1.0 ° C / second or more, and preferably 3 ° C / second or more. On the other hand, if the cooling rate is 10 ° C./second or more, the area fraction of the residual Vosstian iron will be insufficient. Therefore, the cooling rate is set to be lower than 10 ° C / second, and is preferably set to 8 ° C / second or less.

The cooling stop temperature of the second cooling is set to 150 ° C to 500 ° C, and the cooling rate up to the cooling stop temperature is set to 10 ° C / sec to 60 ° C / sec. If the cooling stop temperature of the second cooling is lower than 150 ° C, the width of the slats of the toughened ferrous iron or tempered Asada loose iron will become fine, and the residual Vostian iron remaining between the slats will become a fine film. shape. As a result, the area fraction of the residual Vosted iron grains in a predetermined form is too small. Therefore, the cooling stop temperature is set to 150 ° C or higher, and preferably set to 200 ° C or higher. On the other hand, if the cooling stop temperature is higher than 500 ° C., the production of polygonal ferrous iron is promoted and the area fraction of polygonal ferrous iron is excessive. Therefore, the cooling stop temperature is set to 500 ° C. or lower, preferably set to 450 ° C. or lower, and more preferably set to about room temperature. The cooling stop temperature is preferably set to be equal to or lower than the Ms point according to the composition. If the cooling rate of the second cooling is lower than 10 ° C / sec, the production of polygonal ferrous iron is promoted and the area fraction of polygonal ferrous iron is excessive. Therefore, the cooling rate is set to 10 ° C / sec or more, and preferably 20 ° C / sec or more. On the other hand, if the cooling rate is more than 60 ° C./second, the area fraction of the residual Vosted iron will be lower than the lower limit. Therefore, the cooling rate is set to 60 ° C / sec or less, and preferably 50 ° C / sec or less.

The cooling method of the first cooling and the second cooling is not limited, and for example, roll cooling, air cooling, water cooling, or any combination thereof may be performed.

After the second cooling, the cold-rolled steel sheet is held at a temperature of 150 ° C. to 500 ° C. for only a time from t1 seconds to 1000 seconds determined by the following formula (1). This holding (first holding) is performed directly, for example, without cooling down to a temperature lower than 150 ° C. after the second cooling. In the formula (1), T0 is a holding temperature (° C), and T1 is a cooling stop temperature (° C) of the second cooling. t1 = 20 × [C] + 40 × [Mn] -0.1 × T0 + T1-0.1 (1)

During the first holding period, the diffusion of C into the residual Vosstian iron is promoted. As a result, the stability of the residual Vastian iron will be improved, and it can be ensured that the residual Vastian iron is more than 5% in terms of area fraction. If the holding time is less than t1, C will not be sufficiently concentrated in the residual Vosstian iron, and in the subsequent cooling, the residual Vosstian iron will be distorted to Asa, and the area fraction of the residual Vosstian iron will be too small. Therefore, the holding time is set to t1 seconds or more. If the holding time is more than 1000 seconds, the decomposition of the residual Vosstian iron will be promoted, and the area fraction of the residual Vosstian iron will be too small. Therefore, the holding time is set to 1000 seconds or less. An intermediate steel sheet can be obtained by subjecting a cold-rolled steel sheet to the first annealing.

The first holding may be performed, for example, after the temperature is lowered to a temperature lower than 150 ° C and then heated to a temperature of 150 ° C to 500 ° C. If the reheating temperature is lower than 150 ° C, the width of the planks of the toughened ferrous iron or the tempered Asada loose iron will become fine, and the residual Vostian iron remaining between the planks will become a thin film. As a result, the area fraction of the residual Vosted iron grains in a predetermined form is too small. Therefore, the reheating temperature is set to 150 ° C or higher, and preferably set to 200 ° C or higher. On the other hand, if the reheating temperature is higher than 500 ° C., the production of polygonal ferrous iron is promoted and the area fraction of polygonal ferrous iron is excessive. Therefore, the reheating temperature is preferably 500 ° C or lower, and preferably 450 ° C or lower.

The intermediate steel plate has, for example, a metal structure as follows: polygonal ferrous iron in terms of area fraction: 40% or less, toughened ferrous iron or tempered Asada loose iron or both: 40% to 95% in total, and remaining Vostian Iron: 5% ~ 60%. In addition, for example, in terms of area fraction, 80% or more of the residual Vosstian iron is composed of residual Vosstian iron crystal grains having an aspect ratio of 0.03 to 1.00.

(Second Annealing) After the first annealing, the second annealing is performed. In the second annealing, the second heating, the third cooling, and the second holding of the intermediate steel sheet are performed. The second annealing can be performed, for example, on a continuous annealing line. By performing the second annealing under the following conditions, the differential row density of the toughened ferrous iron can be reduced, and the toughened ferritic iron crystals in a predetermined form having a differential row density of 8 × 10 ² (cm / cm ³ ) or less can be increased. Grain area fraction.

The annealing temperature for the second annealing is set to 760 ° C to 800 ° C. If the annealing temperature is lower than 760 ° C., the area fraction of polygonal ferrous iron will be too much, and the area fraction of toughened ferrous iron grains or the area fraction of residual Vostian iron will be too small. Therefore, the annealing temperature is set to 760 ° C or higher, and is preferably set to 770 ° C or higher. On the other hand, if the annealing temperature is higher than 800 ° C, the area fraction of Vosstian iron will increase with the transformation of Vosstian iron, and the area fraction of toughened fertile iron will be too small. Therefore, the annealing temperature is set to 800 ° C or lower, and preferably 790 ° C or lower.

The cooling stop temperature of the third cooling is set to 600 ° C to 750 ° C, and the cooling rate up to the cooling stop temperature is set to 1 ° C / sec to 10 ° C / sec. If the cooling stop temperature is lower than 600 ° C, the area fraction of polygonal ferrous iron will be excessive. Therefore, the cooling stop temperature is set to 600 ° C or higher, and is preferably set to 630 ° C or higher. On the other hand, if the cooling stop temperature is higher than 750 ° C, the area fraction of the loose iron in Asada is excessive. Therefore, the cooling stop temperature is 750 ° C or lower, and preferably 730 ° C or lower. If the cooling rate of the third cooling is lower than 1.0 ° C / sec, the area fraction of the polygonal ferrous iron will be excessive. Therefore, the cooling rate is set to 1.0 ° C / second or more, and preferably 3 ° C / second or more. On the other hand, if the cooling rate is higher than 10 ° C./second, the area fraction of the toughened ferrous iron will be too small. Therefore, the cooling rate is set to 10 ° C / sec or less, and preferably 8 ° C / sec or less.

When hole expansion is more important than ductility, the cooling stop temperature should be set to 710 ° C or higher, and more preferably 720 ° C or higher. This is because it is easier to make the area fraction of polygonal ferrous iron less than 20%. When the ductility is more important than the hole expansion, the cooling stop temperature should be set to less than 710 ° C, and more preferably set to 690 ° C or less. Because it is easier to make the area fraction of polygonal fertilized iron larger than 20% and less than 40%.

After the third cooling, the steel sheet is cooled to a temperature of 150 ° C. to 550 ° C. and maintained at the temperature for 1 second or more. During this holding (second holding), the diffusion of C into the residual Vosstian iron is promoted. If the holding time is less than 1 second, C will not be sufficiently concentrated in the residual Vastfield iron, and the stability of the residual Vastfield iron will be reduced, and the area fraction of the residual Vastfield iron will be too small. Therefore, the holding time is set to 1 second or more, and preferably set to 2 seconds or more. If the holding temperature is lower than 150 ° C, C will not be sufficiently concentrated in the residual Vosstian iron, and the stability of the residual Vosstian iron will be reduced, and the area fraction of the residual Vosstian iron will be too small. Therefore, the holding temperature is set to 150 ° C or higher, and preferably set to 200 ° C or higher. On the other hand, if the temperature is maintained higher than 550 ° C, the transformation from Vosstian iron to the toughened ferrous iron will be delayed, so the diffusion of C into the residual Vosstian iron will not progress, and the stability of the residual Vosstian iron It will be reduced, and the area fraction of the residual Vostian iron will be too small. Therefore, the holding temperature is preferably 550 ° C or lower, and preferably 500 ° C or lower.

In this way, the steel sheet according to the embodiment of the present invention can be manufactured.

In the embodiment of the present invention described so far, a part of the Vostian iron is deformed by controlling the primary cooling rate of the first annealing to be 1 ° C./s or more and less than 10 ° C./s. Granulated iron. With the formation of ferrous iron, Mn diffuses and becomes concentrated in undistorted Vosstian iron. By concentrating Mn in Vosstian iron, during the second holding in the second annealing, the undulating stress of Vosstian iron will increase, and it is beneficial to alleviate the abnormal stress that occurs with the transformation of toughened ferrous iron. The crystalline orientation of is preferentially formed. Therefore, the strain introduced into the toughened ferrous iron can be reduced, and the differential discharge density can be controlled to 8 × 10 ² (cm / cm ³ ) or less. By controlling the differential row density of the toughened fertilizer grain iron to 8 × 10 ² (cm / cm ³ ) or less, the processing effect ability at the time of plastic deformation can be improved, and excellent ductility can be obtained. The mechanism for improving ductility by reducing the differential density of iron in the toughened fertilizer grains is as follows. If TRIP steel produces Asada loose iron from residual Vostian iron due to work-induced metamorphosis, the differential discharge will be introduced into the adjoining toughened ferritic iron and work hardened. As long as the differential row density of the toughened ferrous iron is low, the work hardening rate can be maintained high in areas with large strains, so the uniform elongation will be improved.

The steel sheet may be subjected to plating treatments such as plating treatment and vapor deposition treatment, or may be further alloyed after the plating treatment. In addition, the steel sheet may be subjected to surface treatments such as formation of an organic film, thin film lamination, organic salt / inorganic salt treatment, and chromium-free treatment.

When the steel sheet is subjected to hot-dip galvanizing as a plating treatment, for example, the temperature of the steel sheet is heated or cooled to a temperature that is 40 ° C or lower than the temperature of the zinc plating bath and 50 ° C higher than the temperature of the zinc plating bath. After the following temperature, the steel sheet was passed through a zinc plating bath. By the hot-dip galvanizing treatment, a hot-dip galvanized steel sheet, which is a steel sheet having a hot-dip galvanized layer on the surface, can be produced. The hot-dip galvanized layer has, for example, the following chemical composition: Fe: 7 mass% or more and 15 mass% or less, and the remainder: Zn, Al, and impurities.

When the alloying treatment is performed after the hot-dip galvanizing treatment, for example, the hot-dip galvanized steel sheet is heated to a temperature of 460 ° C or higher and 600 ° C or lower. If the temperature is lower than 460 ° C, the alloying may be insufficient. If the temperature is higher than 600 ° C, there is a case where the alloying is excessive and the corrosion resistance may be deteriorated. By the alloying treatment, a steel sheet having an alloyed hot-dip galvanized layer on the surface, that is, an alloyed hot-dip galvanized steel sheet can be obtained.

It should be noted that the above-mentioned embodiments are merely examples for realizing the implementation of the present invention, and are not intended to limit the interpretation of the technical scope of the present invention. That is, the present invention can be implemented in various forms as long as it does not depart from its technical idea or its main features.

Examples Next, examples of the present invention will be described. The condition in the example is an example of the condition adopted for confirming the feasibility and effect of the present invention, and the present invention is not limited to this example of the condition. As long as the purpose of the present invention can be achieved without departing from the gist of the present invention, the present invention can adopt various conditions.

(First Test) In the first test, steel billets having chemical compositions shown in Tables 1 to 3 were manufactured. The empty columns in Tables 1 to 3 indicate that the content of the element is below the detection limit, and the remaining part is Fe and impurities. The bottom line in Tables 1 to 3 indicates that the value is outside the range of the present invention.

[Table 1]

[Table 2]

[table 3]

Next, after temporarily cooling or without cooling, the steel billet is directly heated to 1100 ° C to 1300 ° C, and hot rolled under the conditions shown in Tables 4 to 7 to obtain a hot rolled steel sheet. Thereafter, pickling was performed, and cold rolling was performed under the conditions shown in Tables 4 to 7 to obtain a cold rolled steel sheet. The bottom line in Tables 4 to 7 indicates that the value is outside the range suitable for manufacturing the steel sheet of the present invention.

[Table 4]

[table 5]

[TABLE 6]

[TABLE 7]

Next, the first annealing of the cold-rolled steel sheet was performed under the conditions shown in Tables 8 to 11 to obtain an intermediate steel sheet. The bottom line in Tables 8 to 11 indicates that the value is outside the range suitable for manufacturing the steel sheet of the present invention.

[TABLE 8]

[TABLE 9]

[TABLE 10]

[TABLE 11]

Next, the metal structure of the intermediate steel plate was observed. In this observation, the area fraction (PF) of polygonal ferrous iron, the area fraction of toughened ferrous iron or tempered Asada loose iron (BF-tM), and the area fraction of residual Vostian iron (residual γ), and further calculate the area fraction of the residual Vosstian iron crystal grains in a predetermined form from the shape of the residual Vosstian iron. The results are shown in Tables 12 to 15. The bottom line in Tables 12 to 15 indicates that the value is outside the range suitable for manufacturing the steel sheet of the present invention.

[TABLE 12]

[TABLE 13]

[TABLE 14]

[Table 15]

Thereafter, a second annealing of the intermediate steel sheet was performed under the conditions shown in Table 16 to Table 19 to obtain a steel sheet sample. In Production No. 150 and No. 151, a plating treatment was performed after the second annealing, and in Production No. 151, an alloying treatment was performed after the plating treatment. As the plating treatment, a hot-dip galvanizing treatment was performed, and the temperature of the alloying treatment was set to 500 ° C. The bottom line in Tables 16 to 19 indicates that the value is outside the range suitable for manufacturing the steel sheet of the present invention.

[TABLE 16]

[TABLE 17]

[TABLE 18]

[TABLE 19]

Next, the metal structure of the steel plate sample was observed. In this observation, the area fraction (PF) of polygonal ferrous iron, the area fraction (BF) of toughened ferrous iron, the area fraction of residual Vostian iron (residual γ), and the area of loose iron in Asada were measured. The fraction (M), and the area fraction of the residual Vostian iron grains in a predetermined form and the area fraction of the toughened ferrous iron grains in a predetermined form are further calculated from the shapes of the residual Vosstian iron and the toughened fat iron . The results are shown in Tables 20 to 23. The underline in Tables 20 to 23 indicates that the value is outside the range of the present invention.

[TABLE 20]

[TABLE 21]

[TABLE 22]

[TABLE 23]

Next, the mechanical properties of the steel plate sample (total elongation, 0.2% off-site drop strength, tensile strength (maximum tensile strength), hole expansion value, ratio of bending radius to plate thickness R / t, and ductility-brittleness transition were measured. temperature). The total elongation, 0.2% off-site yield strength and tensile strength were measured by taking a JIS No. 5 test piece from a steel plate sample with the direction perpendicular to the rolling direction (plate width direction) as the long side direction, and performing JIS Z 2242 is the standard tensile test. The hole expansion value was measured by performing a hole expansion test according to JIS Z 2256. The measurement of the ratio R / t was performed by the test of JIS Z 2248. The measurement of the ductility-brittleness transition temperature was performed by the test of JIS Z 2242. The results are shown in Tables 24 to 27. The bottom line in Table 24 to Table 27 indicates that the value is outside the ideal range.

[TABLE 24]

[TABLE 25]

[TABLE 26]

[TABLE 27]

As shown in Tables 24 to 27, inventive examples such as Test No. 1 and No. 4 within the scope of the present invention, obtained excellent elongation, 0.2% off-position yield strength, tensile strength, hole expansion value, ratio R / t and ductility-brittle transition temperature.

On the other hand, in Comparative Examples such as Manufacturing No. 2 and No. 3, the elongation, hole expansion value, and ratio R / t are low. The manufacturing No. 2 and No. 3 described above are the areas of polygonal ferrous iron. Excessive rate, insufficient area fraction of toughened ferrous iron, insufficient area fraction of residual Vosstian iron, and insufficient ratio of residual Vostian iron grains in a predetermined form, and ratio of toughened ferrous iron grains in a predetermined form insufficient. In Comparative Examples such as Manufacturing No. 5 and No. 6, the elongation, hole expansion value, and ratio R / t are low. The manufacturing No. 5 and No. 6 and the like are due to the insufficient area fraction of the toughened ferrous iron, The area fraction of loose iron in Mata is too large, and the ratio of residual Vostian iron crystal grains in a predetermined form is insufficient, and the ratio of toughened fertilizer iron grains in a predetermined form is insufficient. In Comparative Examples such as Manufacturing No. 30 and No. 37, the elongation was low, and the ratio of residual Vostian iron crystal grains in a predetermined form in the aforementioned Manufacturing No. 30 and No. 37 was insufficient. In Comparative Examples such as Manufacturing No. 70 and No. 85, the elongation, hole expansion value, and ratio R / t are low. The manufacturing No. 70 and No. 85, among others, are due to the insufficient area fraction of the toughened fertilizer iron, The area fraction of loose iron in Mata is too large, and the ratio of residual Vostian iron crystal grains in a predetermined form is insufficient, and the ratio of toughened fertilizer iron grains in a predetermined form is insufficient.

Industrial Applicability The present invention is applicable to industries related to, for example, steel sheets suitable for automobile parts.

1‧‧‧ Residual Vostian Iron Grains

2‧‧‧ Equivalent Ellipse

L1‧‧‧Long axis length

L2‧‧‧ short axis length

FIG. 1 is a diagram showing an example of an equivalent ellipse of the residual Vosted iron grains.

Claims (6)

A steel plate characterized by the following chemical composition: in mass%, C: 0.10% to 0.5%, Si: 0.5% to 4.0%, Mn: 1.0% to 4.0%, P: 0.015% or less, S: 0.050% or less, N: 0.01% or less, Al: 2.0% or less, Si and Al: 0.5% to 6.0% in total, Ti: 0.00% to 0.20%, Nb: 0.00% to 0.20%, B: 0.0000% ~ 0.0030%, Mo: 0.00% ~ 0.50%, Cr: 0.0% ~ 2.0%, V: 0.00% ~ 0.50%, Mg: 0.000% ~ 0.040%, REM: 0.000% ~ 0.040%, Ca: 0.000% ~ 0.040 %, And the remainder: Fe and impurities; and has the following metal structure: in terms of area fraction, polygonal ferrous iron: 40% or less, Asada loose iron: 20% or less, toughened ferrous iron: 50% ~ 95%, and residual Vostian iron: 5% ~ 50%; in terms of area fraction, more than 80% of the aforementioned toughened fertile iron is composed of an aspect ratio of 0.1 to 1.0 and an azimuth difference angle of 15 ° or more In the area surrounded by the grain boundaries, the toughened ferritic iron grains with a differential density of 8 × 10 ² (cm / cm ³ ) or less are formed, and in terms of area fraction, more than 80% of the above-mentioned residual Vostian iron It is composed of an aspect ratio of 0.1 to 1.0, a long axis length of 1.0 μm to 28.0 μm, and a short length. It is composed of residual Vosstian iron grains with a shaft length of 0.1 μm to 2.8 μm. For example, the steel sheet of claim 1, wherein the aforementioned metal structure is expressed in terms of area fraction: polygonal ferrous iron: 5% ~ 20%, Asada loose iron: 20% or less, toughened ferrous iron: 75% ~ 90%, And residual Vostian iron: 5% ~ 20%. For example, the steel sheet of claim 1, wherein the foregoing metal structure is expressed in terms of area fraction: polygonal ferrous iron: more than 20% and less than 40%, Asada loose iron: 20% or less, toughened ferrous iron: 50% ~ 75%, and residual Vostian iron: 5% ~ 30%. The steel sheet according to any one of claims 1 to 3, wherein the following chemical composition is established in terms of mass: Ti: 0.01% ~ 0.20%, Nb: 0.005% ~ 0.20%, B: 0.0001% ~ 0.0030% , Mo: 0.01% ~ 0.50%, Cr: 0.01% ~ 2.0%, V: 0.01% ~ 0.50%, Mg: 0.0005% ~ 0.040%, REM: 0.0005% ~ 0.040%, or Ca: 0.0005% ~ 0.040%, Or any combination of these. The steel sheet according to any one of claims 1 to 3, which has a plating layer formed on the surface. The steel sheet as claimed in claim 4, which has a plating layer formed on the surface.