TW201814068A - Steel plate excellent in tensile strength, malleability, extensibility, hydrogen embrittlement resistance and toughness - Google Patents

Abstract

A steel sheet which is provided with a predetermined chemical composition and the following steel components in terms of volume fraction: tempered martensite and bainite: total amount is 70% or more and less than 92%, residual Austenite: 8% or more and less than 30%, ferrite: less than 10%, martensite: less than 10%, and peralite: less than 10%. The density in terms of number of the iron-based carbide in the tempered martensite and lower bainite is 1.0×10 <6> (piece/mm 2 ) or more, and the effective crystal diameter of the tempered martensite and bainite is 5 micrometers or less.

Description

Steel plate

FIELD OF THE INVENTION The present invention relates to a high-strength steel sheet suitable for automobiles, building materials, and home appliances.

BACKGROUND OF THE INVENTION In order to improve the lightweight and collision safety of automobiles, the use of high-strength steel plates with a tensile strength of 980 MPa or more in automotive components is rapidly expanding. In addition, as a high-strength steel sheet capable of obtaining good ductility, a TRIP steel sheet using transformation induced plasticity (TRIP) is known.

However, in the conventional TRIP steel sheet, in addition to tensile strength and ductility, it is impossible to achieve both hole expandability, hydrogen embrittlement resistance, and toughness.

Prior Art Literature Patent Literature Patent Literature 1: Japanese Patent Laid-Open No. 11-293383 Patent Literature 2: Japanese Patent Laid-Open No. 1-230715 Patent Literature 3: Japanese Patent Laid-Open No. 2-217425 Patent Literature 4: Japanese Patent Japanese Patent Application Publication No. 2010-90475 Patent Document 5: International Patent Publication No. 2013/051238 Patent Document 6: Japanese Patent Publication No. 2013-227653 Patent Document 7: International Patent Publication No. 2012/133563 Patent Document 8: Japanese Patent Japanese Patent Application Laid-Open No. 2014-34716 Patent Document 9: International Patent Publication No. 2012/144567

SUMMARY OF THE INVENTION Problems to be Solved by the Invention An object of the present invention is to provide a steel sheet having a balance between tensile strength, ductility, hole expandability, hydrogen embrittlement resistance, and toughness.

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 the TRIP steel plate, the main phase was made of tempered Asada loose iron or toughened iron, or both, with a predetermined effective crystal grain size, and tempered Asada loose iron and lower toughened iron were used. By containing a predetermined number of iron-based carbides, tensile strength, ductility, hole expansion, hydrogen embrittlement resistance, and toughness can be taken into consideration.

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

(1) A steel sheet characterized by the following chemical composition in terms of mass: C: 0.15% to 0.45%, Si: 1.0% to 2.5%, Mn: 1.2% to 3.5%, and Al: 0.001% ~ 2.0%, P: 0.02% or less, S: 0.02% or less, N: 0.007% or less, O: 0.01% or less, Mo: 0.0% ~ 1.0%, Cr: 0.0% ~ 2.0%, Ni: 0.0% ~ 2.0 %, Cu: 0.0% ~ 2.0%, Nb: 0.0% ~ 0.3%, Ti: 0.0% ~ 0.3%, V: 0.0% ~ 0.3%, B: 0.00% ~ 0.01%, Ca: 0.00% ~ 0.01%, Mg: 0.00% to 0.01%, REM: 0.00% to 0.01%, and the remainder: Fe and impurities; and in terms of volume fraction, it has the following steel structure: tempered Asada loose iron and toughened iron: total 70% or more and less than 92%, Residual Vostian iron: 8% or more and 30% or less, Fertilizer iron: less than 10%, Fresh Asada loose iron: Less than 10%, and Poly iron: Low And the number density of iron-based carbides in tempered Asada loose iron and lower toughened iron is 1.0 × 10 ⁶ (pieces / mm ² ) or more; effective for tempered Asada loose iron and toughened iron The crystal grain size is 5 μm or less.

(2) The steel sheet according to (1), characterized in that the foregoing chemical composition is established: in terms of mass%, Mo: 0.01% to 1.0%, Cr: 0.05% to 2.0%, Ni: 0.05% to 2.0%, Or Cu: 0.05% ~ 2.0%, or any combination of these.

(3) The steel sheet as described in (1) or (2), characterized in that the aforementioned chemical composition is established: in mass%, Nb: 0.005% to 0.3%, Ti: 0.005% to 0.3%, or V: 0.005 % ~ 0.3%, or any combination of these.

(4) The steel sheet according to any one of (1) to (3), characterized in that the aforementioned chemical composition is established: in terms of mass%, B: 0.0001% to 0.01%.

(5) The steel sheet according to any one of (1) to (4), characterized in that the foregoing chemical composition is established: in terms of mass%, Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01%, or REM: 0.0005% ~ 0.01%, or any combination of these.

Advantageous Effects of Invention According to the present invention, since the steel structure and the effective crystal grain size of tempered Asada loose iron and toughened iron are appropriate, both tensile strength, ductility, hole expandability, hydrogen embrittlement resistance, and toughness.

Embodiments for Carrying Out the Invention Embodiments of the present invention will be described below.

First, a steel structure of a steel sheet according to an embodiment of the present invention will be described. The steel plate of this embodiment has the following steel structure in volume fraction: tempered Asada loose iron and toughened iron: total 70% or more and less than 92%, residual Vostian iron: 8% or more and low At 30%, ferrous iron: less than 10%, newborn Asada loose iron: less than 10%, and boron iron: less than 10%.

(Tempered Asada loose iron and toughened iron: above 70% and less than 92% in total) Tempered Asada loose iron and toughened iron is a low-temperature metamorphic structure containing iron-based carbides, and helps to account for pore expansion And hydrogen embrittlement resistance. In the case where the volume fraction of tempered Asada loose iron and toughened iron is less than 70%, it is difficult to take into account both the hole expandability and the hydrogen embrittlement resistance. Therefore, the volume fraction of tempered Asada loose iron and toughened iron should be set to a total of 70% or more. On the other hand, when the volume fraction of tempered Asada loose iron and toughened iron is 92% or more, the residual Vostian iron described later may be insufficient. Therefore, the volume fraction of tempered Asada loose iron and toughened iron should be set to less than 92%.

Tempered Asada loose iron is a collection of lath crystal grains, and contains iron-based carbides with a long diameter of 5 nm or more inside. The iron-based carbides contained in the tempered Asada loose iron have a plurality of deformed bodies, and the iron-based carbides existing in one crystal grain are elongated in a plurality of directions.

The toughened iron contains upper toughened iron and lower toughened iron. A collection of lower-toughened iron-based slab-like crystal grains, and contains iron-based carbides with a long diameter of 5 nm or more inside. However, unlike tempered Asada scattered iron, the iron-based carbide contained in the lower toughened iron has a single deformed body, and the iron-based carbide existing in one crystal grain substantially extends in a single direction. The "substantially in a single direction" as used herein means a direction in which the angular difference is within 5 °. The upper toughened iron is a collection of lath-like crystal grains that do not contain iron-based carbides in the interior.

Tempered Asada loose iron and lower toughened iron can be judged based on whether the direction of iron-based carbide elongation is plural or single. If the volume fraction of tempered Asada loose iron and lower toughened iron is 70% or more in total, the details are not limited. This is because the deformed body of the iron-based carbide does not affect both the hole expandability and the hydrogen embrittlement resistance. The details will be described later. However, since lower toughened iron must be maintained at a temperature of 300 ° C to 500 ° C for a long period of time, from the viewpoint of productivity, a higher proportion of tempered Asada loose iron is appropriate.

(Residual Vastfield Iron: 8% or more and less than 30%) The residual Vastfield Iron is beneficial in that it can improve ductility through transformation induced plasticity (TRIP). When the volume fraction of the residual Vostian iron is less than 8%, sufficient ductility cannot be obtained. Therefore, the volume fraction of the residual Vastian iron is set to 8% or more, and preferably 10% or more. On the other hand, when the volume fraction of the residual Vostian iron is 30% or more, tempered Asada loose iron and toughened iron are insufficient. Therefore, the volume fraction of the residual Vosstian iron is set to less than 30%.

(Fertilized iron: less than 10%) Fertilized iron is a soft tissue that does not contain lower tissues such as slat, and it is easy to produce poor strength at the interface between tempered Asada loose iron and toughened iron. Accompanying damage. In other words, the ferrous iron is liable to deteriorate toughness and hole expandability. In addition, ferrous iron causes deterioration in low-temperature toughness. Therefore, the lower the volume fraction of ferrous iron, the better. Especially when the volume fraction of ferrous iron is more than 10%, toughness and pore expandability are significantly reduced. Therefore, the volume fraction of ferrous iron is set to less than 10%.

(New born Asada loose iron: less than 10%) New Asada loose iron is a kind of Asada loose iron that does not contain iron-based carbides and remains quenched. Although it helps improve strength, it will greatly improve hydrogen embrittlement resistance. Degradation. In addition, the new-born Asada loose iron will cause its low-temperature toughness deterioration accompanied by the hardness difference between tempered Asada loose iron and toughened iron. Therefore, the lower the volume fraction of the fresh Asada loose iron, the better. In particular, when the volume fraction of fresh Mata loose iron is 10% or more, the hydrogen embrittlement resistance is significantly deteriorated. Therefore, the volume fraction of the fresh Asada loose iron is set to less than 10%.

(Polite: less than 10%) Like fertile grain iron, Polai iron deteriorates toughness and hole expandability. Therefore, the lower the volume fraction of Plei iron, the better. Especially when the volume fraction of boron iron is more than 10%, the toughness and hole expandability are significantly reduced. Therefore, the volume fraction of boron iron is set to less than 10%.

Next, the iron-based carbides in tempered Asada loose iron and lower toughened iron will be described. There is an integrated interface between the iron-based carbides in tempered Asada loose iron and lower toughened iron and the parent phase, and there is an integrated strain at the integrated interface. This integrated strain will exert the hydrogen trapping ability, improve the resistance to hydrogen embrittlement, and improve the resistance to delayed failure. When the number density of such iron-based carbides is less than 1.0 × 10 ⁶ (pieces / mm ² ), sufficient hydrogen embrittlement resistance cannot be obtained. Therefore, the number density of iron-based carbides in tempered Asada loose iron and lower toughened iron is set to 1.0 × 10 ⁶ (pieces / mm ² ) or more, and preferably 2.0 × 10 ⁶ (pieces / mm) ² ) or more, more preferably 3.0 × 10 ⁶ (pieces / mm ² ) or more.

The so-called iron-based carbides are mainly a general term for carbides composed of Fe and C. For example, ε carbides, χ carbides, and citronite (θ carbides) with different crystal structures are iron-based carbides. Iron-based carbides exist in the parent phase of Asada loose iron and lower toughened iron in a specific orientation relationship. A part of Fe contained in the iron-based carbide can also be replaced with other elements such as Mn, Si, and Cr. Even in this case, as long as the number density of iron-based carbides having a major axis length of 5 nm or more is 1.0 × 10 ⁶ (pieces / mm ² ) or more, excellent hydrogen embrittlement resistance can be obtained.

The counting target of the number density is set to an iron-based carbide having a major axis size of 5 nm or more. Although the size that can be observed with a scanning electron microscope and a transmission electron microscope is limited, iron-based carbides with a major axis size of 5 nm or more can be observed in general. Tempered Asada loose iron and lower toughened iron may also contain iron-based carbides with a major axis size of less than 5 nm. The finer the iron-based carbide, the better the hydrogen embrittlement resistance can be obtained. Therefore, the iron-based carbide is preferably fine. For example, the average length of the major axis is preferably 350 nm or less, preferably 250 nm or less, and more preferably 200 nm or less.

As of now, there is no opinion on the fact that iron-based carbides can improve hydrogen embrittlement resistance. In my opinion, this is because in order to make use of the residual Vostian iron and the accompanying improvement of the formability, in general, it is particularly important to suppress the precipitation of iron-based carbides, and to suppress the precipitation of iron-based carbides. In other words, I think that as of now, no research has been conducted on steel plates containing residual Vostian iron and fine iron-based carbides, and the effect of improving the hydrogen embrittlement resistance through iron-based carbides in TRIP steel has never been found.

Next, the effective crystal grain sizes of tempered Asada loose iron and toughened iron will be described. The method for measuring the effective crystal grain size of tempered Asada loose iron and toughened iron will be described later, but when the effective crystal grain size of tempered Asada loose iron and toughened iron exceeds 5 μm, sufficient toughness cannot be obtained. Therefore, the effective crystal grain size of tempered Asada loose iron and toughened iron is set to 5 μm or less, and preferably 3 μm or less.

Next, an example of a method for measuring the tissue volume fraction will be described.

In the measurement of the volume fractions of ferrous iron, boron iron, upper toughened iron, lower toughened iron, and tempered Asada loose iron, a section parallel to the rolling direction and parallel to the thickness direction is set as the observation surface. While taking samples from the steel plate. Next, the observation surface was polished, etched with nitric acid, and transmitted through a field emission scanning electron microscope (FE-SEM) at a magnification of 5000 times to observe the depth of the steel sheet surface when the thickness of the steel sheet was t. The range is from t / 8 to a depth of 3t / 8. With this method, it is possible to identify ferrous iron, boron iron, toughened iron, and tempered Asada loose iron. Tempered Asada loose iron, upper toughened iron, and lower toughened iron can be distinguished from each other by the presence or absence of iron-based carbides in the lath-like crystal grains and the direction of elongation. This observation is performed on 10 visual fields, and the area fractions of ferrous iron, boron iron, upper toughened iron, lower toughened iron, and tempered loose field iron can be obtained from the average of the 10 visual fields. Since the area fraction is equivalent to the volume fraction, it can be directly set as the volume fraction. In this observation, it is also possible to specify the number density of iron-based carbides in tempered Asada loose iron and lower toughened iron.

In the measurement of residual Vostian iron, a sample is taken from a steel plate, and a portion from the surface of the steel plate to a depth of t / 4 is chemically polished. Then, the measurement is performed parallel to the rolled surface and the depth from the surface of the steel plate is t / X-ray diffraction intensity in the plane of 4. For example, the volume fraction Vγ of the residual Vosstian iron can be expressed by the following formula. Vγ = (I _200f + I _220f + I _311f ) / (I _200b + I _211b ) × 100 (I _200f , I _220f , I _311f respectively represent (200), (220) of the face-centered cubic lattice (fcc) phase The diffraction peak intensities of (311) and (311); I _200b and I _211b represent the diffraction peak intensities of (200) and (211) of the body-centered cubic lattice (bcc) phase, respectively.)

Since fresh Asada loose iron and residual Vostian iron are not sufficiently corroded by etching with nitrate, it can be distinguished from fertilizer iron, boron iron, toughened iron and tempered Asada loose iron. Therefore, by subtracting the volume fraction Vγ of the residual Vosstian iron from the volume fraction of the remaining portion observed by FE-SEM, the volume fraction of the fresh Asada iron can be specified.

In the measurement of the effective crystal grain size of tempered Asada scattered iron and toughened iron, the crystal orientation analysis is performed using an electron back-scatter diffraction (EBSD) method. With this analysis, the azimuth difference between two adjacent measurement points can be calculated. Although there are various ideas regarding the effective crystal grain size of tempered Asada iron and toughened iron, the inventors have found that the block boundary is an effective crystalline unit for the propagation of cracks that can control toughness. The block boundary can be roughly judged by the area surrounded by the boundary with an azimuth difference of 10 ° or more. Therefore, the boundary with an azimuth difference of 10 ° or more can be shown on the crystal orientation distribution map measured by EBSD. To reflect. A circle-equivalent diameter of a region surrounded by such a boundary having an azimuth difference of 10 ° or more is set as an effective crystal grain size. According to the verification performed by the present inventors, when measurement points with an azimuth difference of 10 ° or more were deemed to have effective crystal grain boundaries, it was confirmed that there is a non-incidental correlation between effective crystal grain boundaries and toughness.

Next, the chemical composition of the steel sheet according to the embodiment of the present invention and the steel billet used in manufacturing the steel sheet will be described. As described above, the steel sheet according to the embodiment of the present invention is manufactured through hot rolling, cold rolling, continuous annealing, and tempering treatment of the billet. Therefore, the chemical composition of the steel sheet and the steel blank not only considers the characteristics of the steel sheet, but also considers 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 according to the embodiment of the present invention has the following chemical composition in mass%: C: 0.15% to 0.45%, Si: 1.0% to 2.5%, Mn: 1.2% to 3.5%, Al: 0.001% to 2.0%, P: 0.02% or less, S: 0.02% or less, N: 0.007% or less, O: 0.01% or less, Mo: 0.0% to 1.0%, Cr: 0.0% to 2.0%, Ni: 0.0% to 2.0%, Cu: 0.0% ~ 2.0%, Nb: 0.0% ~ 0.3%, Ti: 0.0% ~ 0.3%, V: 0.0% ~ 0.3%, B: 0.00% ~ 0.01%, Ca: 0.00% ~ 0.01%, Mg: 0.00% ~ 0.01%, REM: 0.00% ~ 0.01%, and the rest: Fe and impurities. The impurities include, for example, those contained in raw materials such as ore and waste, and those contained in manufacturing steps, for explanation.

(C: 0.15% ~ 0.45%) C helps to improve the strength and also improves the hydrogen embrittlement resistance by generating iron-based carbides. When the C content is less than 0.15%, sufficient tensile strength cannot be obtained, for example, a tensile strength of 980 MPa or more. Therefore, the C content should be set to 0.15% or more, preferably 0.18% or more. On the other hand, when the content of C exceeds 0.45%, the temperature of the start of metamorphosis of Asada loose iron becomes extremely low, and it is impossible to ensure a sufficient volume fraction of Asada loose iron, and the volume of tempered Asada loose iron and toughened iron cannot be obtained. The score is set above 70%. In addition, the strength of the welded portion may be insufficient. Therefore, the C content is set to 0.45% or less, preferably 0.35% or less.

(Si: 1.0% ~ 2.5%) Si helps to improve the strength and also suppresses the precipitation of coarse iron-based carbides in Vosstian iron, and generates residual Vosstian iron that is stable at room temperature. When the Si content is less than 1.0%, precipitation of coarse iron-based carbides cannot be sufficiently suppressed. Therefore, the Si content is preferably 1.0% or more, and more preferably 1.2% or more. On the other hand, when the Si content exceeds 2.5%, the steel sheet becomes brittle and the formability is lowered. Therefore, it is desirable to set the Si content to 2.5% or less, and preferably to 2.0% or less.

(Mn: 1.2% ~ 3.5%) Mn contributes to the improvement of strength, and also suppresses the iron metamorphosis during the cooling period after annealing. When the Mn content is less than 1.2%, excessive iron is produced in the fertilizer particles, and it is difficult to ensure sufficient tensile strength, for example, a tensile strength of 980 MPa or more. Therefore, the Mn content is preferably 1.2% or more, and more preferably 2.2% or more. On the other hand, when the Mn content exceeds 3.5%, the steel billet and the hot-rolled steel sheet are excessively high-strengthened, thereby reducing the manufacturability. Therefore, the Mn content is preferably 3.5% or less, and preferably 2.8% or less. From the viewpoint of manufacturability, it is preferable to set Mn to 3.00% or less.

(Al: 0.001% to 2.0%) Although Al is inevitably contained in steel, it helps to suppress the precipitation of coarse iron-based carbides in Vosstian iron, and generates residual Vastian that is stable at room temperature. iron. Al also functions as a deoxidizer. Therefore, it may contain Al. On the other hand, when the Al content exceeds 2.0%, the manufacturability decreases. Therefore, the Al content should be 2.0% or less, and preferably 1.5% or less. Cost is required to reduce the Al content, and when it is lowered below 0.001%, the cost will increase significantly. Therefore, the Al content is set to 0.001% or more.

(P: 0.02% or less) P is not an essential element and is contained as an impurity in steel, for example. P tends to segregate at the central portion in the thickness direction of the steel sheet, and embrittles the welded portion. Therefore, the lower the P content, the better. In particular, when the P content exceeds 0.02%, the weldability is significantly reduced. Therefore, the P content should be set to 0.02% or less, preferably 0.015% or less. It takes cost to reduce the P content, and when it is lowered to less than 0.0001%, the cost will rise significantly. Therefore, the P content may be set to 0.0001% or more.

(S: 0.02% or less) S is not an essential element and is contained as an impurity in steel, for example. S forms coarse MnS and reduces hole expandability. S also reduces weldability and reduces the manufacturability of casting and hot rolling. Therefore, the lower the S content, the better. In particular, when the S content exceeds 0.02%, the hole expandability is significantly reduced. Therefore, the S content should be set to 0.02% or less, preferably 0.005% or less. It takes cost to reduce the S content, and when it is lowered to less than 0.0001%, the cost will increase significantly. When it is lowered to less than 0.0001%, the cost will increase significantly. Therefore, the S content may be set to 0.0001% or more.

(N: 0.007% or less) N is not an essential element and is contained as an impurity in steel, for example. N forms coarse nitrides and deteriorates bendability and hole expandability. N may also cause pores during welding. Therefore, the lower the N content, the better. In particular, when the N content exceeds 0.007%, the deterioration of bendability and hole expandability is significantly reduced. Therefore, the N content should be set to 0.007% or less, preferably 0.004% or less. It takes cost to reduce the N content, and when it is lowered to less than 0.0005%, the cost will increase significantly. Therefore, the N content may be set to 0.0005% or more.

(O: 0.01% or less) O is not an essential element and is contained as, for example, impurities in steel. O forms oxides and deteriorates formability. Therefore, the lower the O content, the better. In particular, when the O content exceeds 0.01%, the decrease in formability becomes significant. Therefore, the O content is set to 0.01% or less, preferably 0.005% or less. Furthermore, a cost is required to reduce the O content, and when it is desired to reduce it to less than 0.0001%, the cost will increase significantly. Therefore, the O content may be set to 0.0001% or more.

Mo, Cr, Ni, Cu, Nb, Ti, V, B, Ca, Mg, and REM are not essential elements, and are also a steel plate and a steel billet containing a predetermined amount of any element to a limited extent.

(Mo: 0.0% ~ 1.0%, Cr: 0.0% ~ 2.0%, Ni: 0.0% ~ 2.0%, Cu: 0.0% ~ 2.0%) Mo, Cr, Ni, and Cu help increase strength and suppress annealing During the subsequent cooling period, the iron in the fertile grains becomes abnormal. Therefore, it may contain Mo, Cr, Ni, or Cu, or any combination thereof. In order to fully obtain this effect, the Mo content should be above 0.01%, the Cr content should be above 0.05%, the Ni content should be above 0.05%, and the Cu content should be above 0.05%. On the other hand, when the Mo content exceeds 1.0%, or the Cr content exceeds 2.0%, or the Ni content exceeds 2.0%, or the Cu content exceeds 2.0%, the hot rolling manufacturability decreases. Therefore, it is necessary to set the Mo content below 1.0%, the Cr content below 2.0%, the Ni content below 2.0%, and the Cu content below 2.0%. That is, Mo: 0.01% ~ 1.0%, Cr: 0.05% ~ 2.0%, Ni: 0.05% ~ 2.0%, or Cu: 0.05% ~ 2.0%, or any combination of these should be established.

(Nb: 0.0% to 0.3%, Ti: 0.0% to 0.3%, V: 0.0% to 0.3%) Nb, Ti, and V form alloy carbonitrides, which contribute to precipitation strengthening and fine grain strengthening Increase strength. Therefore, it may contain Nb, Ti, or V, or any combination thereof. In order to fully obtain this effect, the Nb content is preferably 0.005% or more, the Ti content is preferably 0.005% or more, and the V content is preferably 0.005% or more. On the other hand, when the Nb content exceeds 0.3%, the Ti content exceeds 0.3%, or the V content exceeds 0.3%, the alloy carbonitrides are excessively precipitated to deteriorate the formability. Therefore, the Nb content is set to 0.3% or less, the Ti content is set to 0.3% or less, and the V content is set to 0.3% or less. That is, Nb: 0.005% to 0.3%, Ti: 0.005% to 0.3%, or V: 0.005% to 0.3%, or any combination of these should be established.

(B: 0.00% to 0.01%) B strengthens the grain boundaries and suppresses the transformation of ferrous iron during the cooling period after annealing. Therefore, B may be contained. In order to fully obtain this effect, the B content should preferably be 0.0001% or more. On the other hand, when the B content exceeds 0.01%, the manufacturability of hot rolling is reduced. Therefore, the B content should be set below 0.01%. That is, it should be established B: 0.0001% ~ 0.01%.

(Ca: 0.00% ~ 0.01%, Mg: 0.00% ~ 0.01%, REM: 0.00% ~ 0.01%) Ca, Mg, and REM help control the morphology of oxides and sulfides, and improve hole expandability. Therefore, it may contain Ca, Mg, or REM, or any combination thereof. In order to fully obtain this effect, the Ca content is preferably 0.0005% or more, the Mg content is preferably 0.0005% or more, and the REM content is preferably 0.0005% or more. On the other hand, when the Ca content exceeds 0.01%, the Mg content exceeds 0.01%, or the REM content exceeds 0.01%, manufacturability such as castability is deteriorated. Therefore, the Ca content is set to 0.01% or less, the Mg content is set to 0.01% or less, and the REM content is set to 0.01% or less. That is, Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01%, or REM: 0.0005% to 0.01%, or any combination of these.

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.

According to this embodiment, high tensile strength can be obtained, for example, tensile strength of 980 MPa or more, preferably 1180 MPa or more, and excellent ductility, hole expandability, hydrogen embrittlement resistance, and toughness can be obtained at the same time.

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, cold rolling, continuous annealing, and tempering treatment of a steel having the above-mentioned chemical composition are sequentially performed.

(Hot rolling) In hot rolling, rough rolling and finish rolling are performed. The method for manufacturing the steel billet for hot rolling is not limited, and a continuous billet billet can be used, or it can be manufactured by a thin billet caster or the like. In addition, hot rolling may be performed immediately after continuous casting. The cast steel billet is not cooled or temporarily cooled after casting, and then heated to above 1150 ° C. When the heating temperature is lower than 1150 ° C., the finished rolling temperature is likely to be lower than 850 ° C. and the rolling load is increased. From a cost standpoint, the heating temperature should preferably be set below 1350 ° C.

In the rough rolling, rolling is performed at least once at a temperature of 1000 ° C. to 1150 ° C. and a reduction ratio of 40% or more. The Vosted iron is finely granulated before the rolling is completed.

Continuous rolling is performed in the finished rolling. The foregoing continuous rolling is performed by using 5 to 7 finished rolling mills arranged at intervals of about 5 m. Then, the last three stages of rolling are performed at 1020 ° C. or lower, the total rolling reduction of the last three stages of rolling is set to 40% or more, and the passage time of the last three stages of rolling is 2.0 seconds or less. In addition, water cooling was started with an elapsed time of 1.5 seconds or less from the rolling of the final stage. Here, the last three rolling stages mean rolling using the last three rolling mills. For example, continuous rolling delay with 6 rolling mills means that during rolling from the 4th to the 6th rolling mill, the thickness of the sheet when entering the 4th rolling mill is set to t4, from When the thickness of the sixth rolling mill is t6, the total rolling reduction of the last three rolling stages is calculated as "(t4-t6) / t4 × 100 (%)". The passing time of the last three stages of rolling refers to the time from when the steel plate comes out of the fourth rolling mill to the sixth rolling machine, and the elapsed time from the rolling of the last stage means that the steel plate passes from the sixth rolling mill. Time between rolling out to start cooling. There may be a section for measuring the properties of the steel sheet such as temperature and thickness between the rolling mill and the water-cooling equipment in the final stage.

For the fine graining of the rolled structure after finishing rolling, the rolling reduction rate, temperature, and time between passes during the finishing rolling are important.

In the last three stages of rolling, once the steel sheet temperature exceeds 1020 ° C, it is impossible to sufficiently fine-grain the Vosstian iron. Therefore, the last three stages of rolling should be performed below 1020 ° C. When 6 rolling mills are used for continuous rolling delay, in order to perform the last 3 stages of rolling below 1020 ° C, the entry side temperature of the 4th rolling mill should be set below 1020 ° C, and the steel plate temperature should be maintained after The heat generated during rolling does not exceed 1020 ° C.

When the total rolling reduction rate of the last three stages of rolling is less than 40%, the cumulative rolling strain becomes insufficient, and the Vostian iron particles cannot be sufficiently fine-grained. Therefore, the total rolling reduction of the last three stages of rolling should be set to more than 40%.

The passing time of the last three stages of rolling is related to the time between passes. The longer the passing time, the longer the time between passes. Between two consecutive rolling mills, the recrystallization of iron particles in Vostian and Grain growth is easier. In addition, when the passage time exceeds 2.0 seconds, recrystallization and grain growth of Vostian iron particles tend to become remarkable. Therefore, the passing time of the last three stages of rolling should be set below 2.0 seconds. From the viewpoint of suppressing the recrystallization and grain growth of Vostian iron grains, the shorter the elapsed time from the rolling of the final stage to the start of water cooling, the better. When the elapsed time exceeds 1.5 seconds, the recrystallization and grain growth of Vostian iron particles tend to become significant. Therefore, the elapsed time from the rolling of the final stage to the start of water cooling must be set to 1.5 seconds or less. Even if there is a section for measuring steel properties such as temperature and thickness between the final rolling mill and the water-cooling equipment, if the water cooling cannot be started immediately, as long as the elapsed time is 1.5 seconds or less, the Recrystallization and grain growth of iron particles.

In the range that does not hinder the rolling capacity of the completion, it can be cooled by water-cooled nozzles and the like immediately after the completion of rolling. Alternatively, after rough rolling, a plurality of rough rolled sheets obtained by rough rolling may be joined, and these may be continuously supplied to the finished rolling. In addition, the rough-rolled rolled sheet can also be temporarily coiled, and then rolled and supplied to the finished rolling.

The finish rolling temperature (end temperature of finish rolling) is set to 850 ° C or higher and 950 ° C or lower. When the finish rolling temperature is a two-phase region of Vosstian iron and ferrous iron, the steel sheet structure becomes non-uniform, and excellent formability cannot be obtained. In addition, when the finish rolling temperature is lower than 850 ° C, the rolling load becomes high. From the viewpoint of fine graining of iron particles in Vostian, the rolling rolling temperature should be set below 930 ° C.

The coiling temperature after hot rolling must be set below 730 ° C. When the coiling temperature exceeds 730 ° C, the effective crystal grain size of tempered Asada iron and toughened iron in the steel sheet cannot be less than 5 µm. In addition, when the coiling temperature exceeds 730 ° C, a thick oxide may be formed on the surface of the steel sheet, which may reduce the pickling property. The effective crystal grain size is made fine to improve the toughness and uniformly disperse the residual Vosted iron to improve the hole expandability. From the foregoing point of view, the coiling temperature should be set below 680 ° C. Although the lower limit of the coiling temperature is not limited, because coiling below the room temperature is technically difficult, the coiling temperature should be set higher than the room temperature.

After the hot rolling, the hot-rolled steel sheet obtained by the hot rolling is pickled once or twice. By acid pickling, surface oxides generated during hot rolling are removed. Pickling also helps to improve the chemical conversion processability of cold-rolled steel plates and the plating properties of plated steel plates.

Between hot rolling and cold rolling, the hot rolled steel sheet may be heated to 300 ° C to 730 ° C. By this heat treatment (tempering treatment), the hot-rolled steel sheet is softened, and cold rolling is easily performed. When the heating temperature exceeds 730 ° C, the microstructure during heating will become two phases of ferrous iron and Vostian iron. Therefore, even after tempering for the purpose of softening, the strength of the hot-rolled steel sheet after cooling is still There is a possibility of improvement. Therefore, the temperature of this heat treatment (tempering treatment) is set to 730 ° C or lower, and preferably 650 ° C or lower. On the other hand, when the heating temperature is lower than 300 ° C, the tempering effect is insufficient and the hot-rolled steel sheet is not sufficiently softened. Therefore, the temperature of this heat treatment (tempering treatment) is set to 300 ° C or higher, and preferably 400 ° C or higher. However, when heat treatment is performed at a temperature of 600 ° C or more for a long time, various alloy carbides are precipitated during the heat treatment, and it becomes difficult to remelt these alloy carbides in subsequent continuous annealing, and it may become impossible to obtain all Desired mechanical properties.

(Cold rolling) After pickling, cold rolling of a hot-rolled steel sheet is performed. The reduction ratio during cold rolling is set to 30% to 90%. When the rolling reduction is less than 30%, the Vosted iron particles will coarsen during annealing, and the effective crystal grain size of tempered Asada loose iron and toughened iron in the steel plate cannot be less than 5 μm. Therefore, the reduction rate should be set to 30% or more, preferably 40% or more. On the other hand, when the reduction ratio exceeds 90%, the rolling load becomes too high, which makes operation difficult. Therefore, the reduction ratio should be set to 90% or less, and preferably set to 70% or less. The number of rolling passes and the rolling reduction per pass are not limited.

(Continuous annealing) After the cold rolling, continuous annealing of the cold-rolled steel sheet obtained by the cold rolling is performed. The continuous annealing is performed by, for example, a continuous annealing line or a continuous hot-dip galvanizing line. The maximum heating temperature in continuous annealing is set to 760 ℃ ~ 900 ℃. When the maximum heating temperature is lower than 760 ° C, the volume fraction of tempered Asada loose iron and toughened iron becomes less than 70% in total, and it is impossible to take into account both hole expandability and hydrogen embrittlement resistance. On the other hand, when the maximum heating temperature exceeds 900 ° C, the Vostian iron particles will coarsen, and it will become impossible to make the effective crystal grain size of tempered Asada iron and toughened iron in the steel plate below 5 μm, but it will not Increase costs.

During continuous annealing, the temperature should be maintained in the temperature range of 760 ° C to 900 ° C for more than 20 seconds. If the holding time is less than 20 seconds, the iron-based carbide cannot be fully melted during continuous annealing, and the volume fraction of tempered Asada iron and toughened iron becomes less than 70% in total, which not only fails to take into account both hole expandability and hydrogen resistance. The embrittlement characteristics and the coarseness of the remaining carbides deteriorate the hole expandability and toughness. From a cost standpoint, the holding time should be set to 1000 seconds or less. It can be maintained isothermally at the maximum heating temperature, or it can be tilted and cooled immediately after reaching the maximum heating temperature.

In continuous annealing, the average heating rate from room temperature to the maximum heating temperature should be set to 2 ° C / sec or more. When the average heating rate is lower than 2 ° C / sec, the strain introduced through cold rolling will be released during heating, and the iron particles in Vostian will be coarsened, and the tempered Asada iron in the steel plate cannot be loosened and toughened. The effective crystal grain size of iron is 5 μm or less.

After holding in a temperature range of 760 ° C to 900 ° C for 20 seconds or more, cooling is performed to 150 ° C to 300 ° C. At this time, the average cooling rate from the holding temperature to 300 ° C is set to 5 ° C / second or more. When the cooling stop temperature at this time exceeds 300 ° C, there may be cases where the cooling stop temperature is higher than the starting temperature of the metamorphosis of Asada, or even if the cooling stop temperature is lower than the starting temperature of Asada Situation of loose iron. As a result, the volume fractions of tempered Asada loose iron and toughened iron became less than 70% in total, and it was impossible to take into account both hole expandability and hydrogen embrittlement resistance. When the cooling stop temperature is lower than 150 ° C, Asada scattered iron is excessively generated, and the volume fraction of the residual Vostian iron becomes lower than 8%. When the average cooling rate from the holding temperature to 300 ° C. is lower than 5 ° C./sec, the ferrous iron is excessively generated during cooling, and sufficient Asada loose iron is not generated. From a cost standpoint, the average cooling rate should preferably be set below 300 ° C / second. The cooling method is not limited, and for example, hydrogen cooling, roll cooling, air cooling, or water cooling may be performed, or any combination thereof. In this cooling, a nucleation site where fine iron-based carbides are precipitated in the subsequent tempering is introduced into Asada loose iron. In this cooling, the cooling stop temperature is important, and the holding time after the stop is not limited. This is because although the volume fraction of tempered Asada loose iron and toughened iron is related to the cooling stop temperature, it is not related to the holding time.

(Tempering treatment) After cooling to 150 ° C to 300 ° C, it is heated to 300 ° C to 500 ° C and maintained in this temperature range for more than 10 seconds. The Asada scattered iron, which is formed during the continuous annealing cooling and remains in a brittle-quenched state, has low hydrogen embrittlement resistance. By reheating to 300 ° C to 500 ° C, Asada scattered iron is tempered, and the number density of iron-based carbides becomes 1.0 × 10 ⁶ (pieces / mm ² ) or more. In addition, during this reheating, since the toughened iron is generated, C diffuses from the Asada iron and the toughened iron to the Vosstian iron, so the Vosstian iron becomes stable.

When the reheating temperature (holding temperature) exceeds 500 ° C., Asada scattered iron is excessively tempered, and sufficient tensile strength cannot be obtained, for example, tensile strength of 980 MPa or more. In addition, the precipitated iron-based carbide may be coarsened, and sufficient hydrogen embrittlement resistance may not be obtained. What's more, even if Si is contained, carbides are formed in the Vosstian iron and the Vosstian iron is decomposed, so the volume fraction of the remaining Vosstian iron becomes less than 8%, and sufficient formability cannot be obtained. With the decrease of the volume fraction of the residual Vosted iron, the volume fraction of the fresh Asada loose iron may become 10% or more. On the other hand, when the reheating temperature is lower than 300 ° C, the number density of iron-based carbides does not become 1.0 × 10 ⁶ (pieces / mm ² ) or more due to insufficient tempering, and sufficient hydrogen embrittlement resistance cannot be obtained. characteristic. If the holding time is less than 10 seconds, the number density of iron-based carbides does not become 1.0 × 10 ⁶ (pieces / mm ² ) or more due to insufficient tempering, and sufficient hydrogen embrittlement resistance cannot be obtained. In addition, there may be insufficient increase in the concentration of C which diffuses to Vosstian iron, causing the volume fraction of residual Vosstian iron to be less than 8%, and there may be cases where sufficient formability cannot be obtained. From a cost standpoint, the holding time should be set to 1000 seconds or less. It can be maintained isothermally in a temperature range of 300 ° C to 500 ° C, and it can also be cooled or heated in this temperature range.

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

After the tempering treatment, a plating treatment of Ni, Cu, Co, or Fe, or any combination thereof may be performed. By performing such a plating treatment, chemical conversion treatment properties and coating properties can be improved. In addition, the steel sheet can also be heated in an ambient gas with a dew point of -50 ° C to 20 ° C to control the form of oxides formed on the surface of the steel sheet, in order to further improve chemical conversion. It is also possible to temporarily increase the dew point in the furnace, to oxidize Si, Mn, etc., which adversely affect the chemical conversion processability, inside the steel sheet, and then perform a reduction treatment to improve the chemical conversion processability. Alternatively, the steel sheet may be subjected to a plating treatment. The tensile strength, ductility, hole expansion, hydrogen embrittlement resistance and toughness of the steel sheet are not affected by the plating treatment. The steel plate of this embodiment is also suitable as a raw material for electroplating.

Alternatively, the steel sheet may be subjected to a hot-dip galvanizing treatment. When performing the hot-dip galvanizing treatment, the continuous annealing and tempering treatments described above are performed in a continuous hot-dip galvanizing line. Next, the steel sheet temperature is set to 400 ° C to 500 ° C and the steel sheet is immersed in a plating bath. If the temperature of the steel sheet is lower than 400 ° C, the heat of the plating bath during immersion invasion is large, and a part of the molten zinc may solidify, which may damage the appearance of the plating. On the other hand, when the temperature of the steel sheet exceeds 500 ° C., there is a fear that an operation problem may occur due to the temperature of the plating bath. If the temperature of the steel sheet after tempering is lower than 400 ° C, it is only necessary to heat it to 400 ° C to 500 ° C before dipping. The plating bath may be a pure zinc plating bath, and may contain Fe, Al, Mg, Mn, Si, or Cr in addition to zinc, or any combination thereof.

In this way, a hot-dip galvanized steel sheet having a plating layer mainly composed of Zn can be obtained. The Fe content of the plating layer of the hot-dip galvanized steel sheet is substantially less than 7%.

The hot-dip galvanized steel sheet may be alloyed. The alloying treatment temperature is set to 450 ° C to 550 ° C. When the alloying temperature is lower than 450 ° C, the alloying progresses slowly and the productivity is low. When the alloying treatment temperature exceeds 550 ° C., the Vostian iron is decomposed, and excellent formability cannot be obtained, and the tempered Mata loose iron is excessively softened, and sufficient tensile strength cannot be obtained.

In this way, an alloyed hot-dip galvanized steel sheet can be obtained. The Fe content of the plating layer of the alloyed hot-dip galvanized steel sheet is approximately 7% or more. Since the melting point of the plating layer of the alloyed hot-dip galvanized steel sheet is higher than the melting point of the plating layer of the hot-dip galvanized steel sheet, the spot-weldability of the alloyed hot-dip galvanized steel sheet is excellent.

In the plating process, any one of the Sendzimir process, the full reduction furnace method, and the fluxing method may be used. In the Sendzimir process, after degreasing and pickling, it is heated in a non-oxidizing ambient gas, annealed in a reducing ambient gas containing H ₂ and N ₂ , and then cooled to near the temperature of the plating bath. Immerse in a plating bath. In the all-reduction furnace method, the ambient gas during annealing is adjusted to oxidize the surface of the steel sheet first, and then clean it before plating by re-reduction, and then immerse it in the plating bath. In the fluxing method, the steel sheet is degreased and pickled, and then fluxed with ammonium chloride or the like before being immersed in the plating bath.

Surface temper rolling may also be performed after tempering treatment, plating treatment, or alloying treatment. The reduction ratio of surface rolling is 1.0% or less. When the rolling reduction exceeds 1.0%, the volume fraction of the remaining Vosstian iron during surface rolling is significantly reduced. When the reduction ratio is less than 0.1%, the effect of surface smooth rolling is small, and it is difficult to control. Surface rolling can be performed on the production line in a continuous annealing line, or it can be performed outside the production line after continuous annealing on the continuous annealing line is completed. The skin pass rolling may be performed once or in a plurality of times so that the total reduction ratio becomes 1.0% or less.

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 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 embodiment 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 object of the present invention can be achieved without departing from the gist of the present invention, the present invention can adopt various conditions.

The steel billet having the chemical composition shown in Table 1 was heated to 1230 ° C, and hot rolled under the conditions shown in Tables 2 and 3 to obtain a hot rolled steel sheet having a thickness of 2.5 mm. In hot rolling, water-cooling is performed after rough rolling and finish rolling using 6 rolling mills, and then hot rolled steel sheets are coiled. The steel types "CR" in Tables 2 and 3 indicate cold-rolled steel sheets, "GI" indicates hot-dip galvanized steel sheets, and "GA" indicates alloyed hot-dip galvanized steel sheets. The "extraction temperature" in Tables 2 and 3 is the temperature of the steel slab when it is taken out from the heating furnace during heating of the steel slab before rough rolling. The "pass count" is the number of rolling passes at a temperature of 1000 ° C to 1150 ° C and a reduction ratio of 40% or more. The "first pass time" is the time from when the steel sheet comes out of the fourth rolling mill to the fifth rolling mill, and the "second pass time" is the time when the steel sheet comes out of the fifth rolling mill. Time to enter the sixth rolling mill. The "elapsed time" is the time from when the steel plate comes out of the sixth rolling mill to the start of water cooling, and the "passing time" is the time from when the steel plate comes out of the fourth rolling mill to the sixth rolling mill. "Total rolling reduction ratio" is set to "t4" when the plate thickness when entering the fourth rolling mill is t4 and "t6" when the plate thickness is exiting from the sixth rolling mill. "(T4-t6) / t4 × 100 (%) ". The balance of the chemical composition shown in Table 1 is Fe and impurities. The bottom line in Table 1 indicates that the value is outside the range of the present invention. The bottom line in Tables 2 and 3 indicates that the value is outside the range suitable for manufacturing the steel sheet of the present invention.

[Table 1]

[Table 2]

[table 3]

Next, the hot-rolled steel sheet was pickled and cold-rolled to obtain a cold-rolled steel sheet having a thickness of 1.2 mm. Thereafter, continuous annealing and tempering treatments were performed on the cold-rolled steel sheet under the conditions shown in Tables 4 and 5, and a surface finish rolling with a reduction ratio of 0.1% was performed. In continuous annealing, the holding temperature in Tables 4 and 5 was set to the maximum heating temperature. The cooling rate is the average cooling rate from the holding temperature to 300 ° C. A part of the samples were subjected to hot-dip galvanizing treatment between tempering treatment and skin pass rolling. The basis weight at this time was set to about 50 g / m ^{2 on} both sides. A part of the samples which have been subjected to the hot-dip galvanizing treatment is subjected to alloying treatment under the conditions shown in Tables 4 and 5 between the hot-dip galvanizing treatment and the surface pass rolling. In the hot-dip galvanizing process, continuous hot-dip galvanizing equipment is used to continuously perform continuous annealing, tempering, and hot-dip galvanizing. The bottom line in Tables 4 and 5 indicates that the value is outside the range suitable for manufacturing the steel sheet of the present invention.

[Table 4]

[table 5]

Then, the steel structure of the steel sheet after the surface rolling was observed, and the volume fraction of each structure and the number density and average size of the iron-based carbides were measured. The results are shown in Tables 6 and 7. The underline in Tables 6 and 7 indicates that the value is outside the range of the present invention. The "average length" in Tables 6 and 7 means the average length of the major axis of the iron-based carbide, and the blank field indicates that the number density of the iron-based carbide cannot be measured.

[TABLE 6]

[TABLE 7]

Furthermore, the strength, ductility, hole expandability, hydrogen embrittlement resistance, and toughness of the steel sheet after skin pass rolling were evaluated.

In the evaluation of strength and ductility, a JIS No. 5 test piece was taken from a steel plate. The JIS No. 5 test piece was set to a longitudinal direction in a direction perpendicular to the rolling direction, and a tensile test was performed according to JIS Z2242 to determine the tensile strength TS. And full stretch El. In the evaluation of the hole expandability, a hole expansion test was performed in accordance with the Japan Iron and Steel Alliance specification JFST1001 to determine the hole expansion rate λ. These results are shown in Tables 8 and 9. The bottom lines in Tables 8 and 9 indicate that the value is outside the desired range. The so-called desired range here is that the tensile strength TS is 980 MPa or more, the ductility index (TS × El) is 15000 MPa% or more, and the hole expandability index (TS ^1.7 × λ) is 5000000 MPa ^1.7 % or more.

In the evaluation of hydrogen embrittlement resistance, a strip test piece of 100 mm × 30 mm was taken from a steel plate. The strip test piece was formed by setting the direction perpendicular to the rolling direction to the long side direction, and formed at both ends thereof. Hole for stress. Next, the test piece was bent with a radius of 10 mm, a strain gauge was installed on the surface of the curved apex of the test piece, a bolt was passed through the holes at both ends, and a nut was installed at the front end of the bolt. Then, the bolt and the nut are locked to apply stress to the test piece. The applied stress is set to 60% and 90% of the maximum tensile strength TS measured by a tensile test. When the stress is applied, the strain read from the strain gauge is converted into a stress by the Young's modulus. . Thereafter, it was immersed in an aqueous solution of ammonium thiocyanate, and electrolytic hydrogen filling was performed at a current density of 0.1 mA / cm ² , and the occurrence of damage after 2 hours was observed. Then, those who did not break and fracture due to a load stress of 60% of the maximum tensile strength TS, and judged as "OK" were those who broke or fractured due to a load stress of 90% of the maximum tensile strength TS. It was judged as "bad", and those who were not broken due to any of the conditions were judged as "good". The results are shown in Tables 8 and 9. In Tables 8 and 9, "○" indicates "good", "△" indicates "may", and "×" indicates "bad". The bottom lines in Tables 8 and 9 indicate that the value is outside the desired range.

In the evaluation of toughness, a sand blast impact test was performed. The test level was to fix the plate thickness to 1.2 mm and perform three times at a test temperature of -40 ° C to measure the absorbed energy at -40 ° C. The results are shown in Tables 8 and 9. The bottom lines in Tables 8 and 9 indicate that the value is outside the desired range. The desired range here is that the absorbed energy is 40 J / cm ² or more.

[TABLE 8]

[TABLE 9]

As shown in Tables 8 and 9, samples A-1, A-6, A-8, B-1, C-1, D-1, E-1, F-1, and G within the scope of the present invention -1, G-3, G-4, G-7, H-1, I-1, J-1, K-1, L-1, M-1, N-1, O-1, P-1 , Q-1, R-1, S-1, S-7, T-1, U-1, V-1, W-1, W-3, X-1 and Y-1, you can get excellent Tensile strength, ductility, hole expansion, hydrogen embrittlement resistance, and toughness.

On the other hand, in sample A-2, the volume fraction of residual Vostian iron was too low, the volume fraction of fresh Asada loose iron was too high, and the total volume fraction of tempered Asada loose iron and toughened iron was too low. , The number density of iron-based carbides is too low, and the ductility, hole expansion, hydrogen embrittlement resistance and toughness are low. In Sample A-3, the volume fraction of the residual Vostian iron was too low, and the total volume fraction of the tempered Asada loose iron and the toughened iron was too high, and the ductility was low. In sample A-4, the volume fraction of residual Vostian iron was too low, the volume fraction of fresh Asada loose iron was too high, the number density of iron-based carbides was too low, and the ductility, hole expandability, and toughness low. In sample A-5, the volume fraction of residual Vostian iron was too low, the effective crystal grain size of tempered Asada loose iron and toughened iron was too large, and the ductility, hole expandability, and toughness were low. In sample A-7, the volume fraction of the residual Vosted iron was too low, and the ductility and toughness were low. In sample A-9, the volume fraction of the residual Vosstian iron was too low, and the ductility, hole expandability, and toughness were low. In sample A-10, the volume fraction of ferrous iron was too high, the volume fraction of residual Vostian iron was too low, and the effective crystal grain size of tempered Asada loose iron and toughened iron was too large. Low toughness. In sample A-11, the volume fraction of residual Vostian iron is too low, the volume fraction of fresh Asada loose iron is too high, the number density of iron-based carbides is too low, and the pore expandability and hydrogen embrittlement resistance Low characteristics and toughness.

In sample G-2, the volume fraction of ferrous iron was too high, the volume fraction of residual Vostian iron was too low, and the total volume fraction of tempered Asada loose iron and toughened iron was too low. The effective crystal grain size of iron and toughened iron is too large, and the hole expandability and toughness are low. In sample G-5, the volume fraction of the residual Vostian iron was too low, the number density of the iron-based carbides was too low, and the ductility, hole expandability, and toughness were low. In sample G-6, the volume fraction of the residual Vostian iron was too low, and the ductility was low. In sample G-8, the volume fraction of ferrous iron was too high, the volume fraction of residual Vostian iron was too low, the volume fraction of fresh Asada loose iron was too high, and tempered Asada loose iron and toughened iron The effective crystal grain size is too large, the number density of iron-based carbides is too low, and the ductility, hole expansion, hydrogen embrittlement resistance, and toughness are low. In sample G-9, the volume fraction of residual Vostian iron was too low, and the total volume fraction of tempered Asada loose iron and toughened iron was too high, and the ductility was low.

In sample S-2, the effective crystal grain size of tempered Asada loose iron and toughened iron was too large, but the hole expandability, hydrogen embrittlement resistance and toughness were low. In sample S-3, the effective crystal grain size of tempered Asada loose iron and toughened iron is too large, and the hole expandability and toughness are low. In sample S-4, the effective crystal grain size of tempered Asada loose iron and toughened iron was too large, but the toughness was low. In sample S-5, the volume fraction of residual Vostian iron was too low, the volume fraction of fresh Asada loose iron was too high, and the total volume fraction of tempered Asada loose iron and toughened iron was too low, tempering The effective crystal grain size of Asada loose iron and toughened iron is too large, the number density of iron-based carbides is too low, and the ductility, hole expansion, hydrogen embrittlement resistance and toughness are low. In sample S-6, the effective crystal grain size of tempered Asada loose iron and toughened iron is too large, and the hole expandability and toughness are low. In sample S-8, the effective crystal grain size of tempered Asada loose iron and toughened iron was too large, but the toughness was low. In the sample S-9, the number density of the iron-based carbides was too low, and the hole expandability, hydrogen embrittlement resistance, and toughness were low. In sample S-10, the volume fraction of ferrous iron was too high, the volume fraction of residual Vostian iron was too low, and the total volume fraction of tempered Asada loose iron and toughened iron was too low. The effective crystal grain size of iron and toughened iron is too large, but the hole expandability, hydrogen embrittlement resistance and toughness are low. In sample S-11, the volume fraction of the residual Vostian iron was too low, the volume fraction of the fresh Matsuda loose iron was too high, and the hole expandability, hydrogen embrittlement resistance, and toughness were low. In sample S-12, the volume fraction of residual Vostian iron is too low, the volume fraction of boron iron is too high, and the effective grain size of tempered Asada loose iron and toughened iron becomes too large, and the pores are enlarged. Low resistance to hydrogen embrittlement and toughness. In sample S-13, the volume fraction of the residual Vostian iron was too low, the volume fraction of the fresh Matsuda loose iron was too high, and the ductility and hydrogen embrittlement resistance were low. In sample S-14, the volume fraction of the residual Vosstian iron was too low, and the hole expandability, hydrogen embrittlement resistance, and toughness were low. In sample W-2, the volume fraction of fresh Asada loose iron was too high, the volume fraction of residual Vostian iron was too low, and the ductility was low.

In sample a-1, the content of C is too low, the volume fraction of ferrous iron is too high, the volume fraction of residual Vostian iron is too low, the volume fraction of fresh Asada loose iron is too high, and tempered Asada loose iron The total volume fraction of the toughened iron is too low, and the ductility, hole expandability, and toughness are low. In sample b-1, the C content was too high, and the volume fraction of the residual Vostian iron was too low, while the ductility, hole expansion, hydrogen embrittlement resistance, and toughness were low. In sample c-1, the content of Si is too low, the volume fraction of ferrous iron is too high, the volume fraction of residual Vostian iron is too low, the volume fraction of fresh Asada loose iron is too high, and tempered Asada loose iron And the total volume fraction of the toughened iron is too low, and the ductility is low. In sample d-1, the Mn content is too low, the volume fraction of ferrous iron is too high, the volume fraction of residual Vostian iron is too low, and the total volume fraction of tempered Asada loose iron and toughened iron is too low. , While the ductility, hole expansion, hydrogen embrittlement resistance and toughness are low. In sample e-1, the P content was too high, but the hole expansion property, hydrogen embrittlement resistance, and toughness were low. In sample f-1, the S content was too high, but the hole expansion property, hydrogen embrittlement resistance, and toughness were low. In sample g-1, the content of Al is too high, the volume fraction of ferrous iron is too high, the volume fraction of residual Vostian iron is too low, the volume fraction of fresh Asada loose iron is too high, and tempered Asada loose iron And the total volume fraction of the toughened iron is too low, and the hole expandability, hydrogen embrittlement resistance and toughness are low. In sample h-1, the effective crystal grain size of tempered Asada loose iron and toughened iron was too large. Therefore, the hole expandability and toughness are low. In sample i-1, the effective crystal grain size of tempered Asada loose iron and toughened iron was too large. Therefore, the toughness is low. In sample j-1, the effective crystal grain size of tempered Asada loose iron and toughened iron was too large. Therefore, the toughness is low. In sample k-1, the effective crystal grain size of tempered Asada loose iron and toughened iron was too large. Therefore, the toughness is low.

When focusing on the manufacturing method, the cooling stop temperature of Sample A-2 in continuous annealing was too high. Therefore, the volume fraction of fresh Asada loose iron becomes too high, the volume fraction of residual Vostian iron becomes too low, and the total volume fraction of tempered Asada loose iron and toughened iron becomes too low, and iron The number density of base carbides becomes too low. The cooling stop temperature of sample A-3 in continuous annealing was too low. Therefore, the volume fraction of the residual Vossite iron becomes too low, and the total volume fraction of the tempered Asada loose iron and the toughened iron becomes too high. The holding temperature of sample A-4 during the tempering process was too low. Therefore, the volume fraction of the fresh Asada loose iron becomes too high, the volume fraction of the residual Vostian iron becomes too low, and the number density of iron-based carbides becomes too low. The holding temperature of Sample A-5 during the tempering treatment was too high. Therefore, the volume fraction of the residual Vostian iron becomes too low, and the effective crystal grain size of the tempered Asada iron and the toughened iron becomes too large. The holding time of sample A-7 in the tempering process was too short. Therefore, the volume fraction of the residual Vosstian iron becomes too low. Sample A-9 was too hot during the alloying process. The volume fraction of the residual Vostian iron has become too low. The holding temperature of sample A-10 in continuous annealing was too low. Therefore, the volume fraction of the ferrous iron becomes too high, the volume fraction of the residual Vostian iron becomes too low, and the effective crystal grain size of the tempered Asada loose iron and the toughened iron becomes too large. The cooling stop temperature of sample A-11 in continuous annealing was too high. Therefore, the volume fraction of the fresh Asada loose iron becomes too high, the volume fraction of the residual Vostian iron becomes too low, and the number density of iron-based carbides becomes too low.

The heating temperature of sample G-2 in continuous annealing was too low. Therefore, the volume fraction of fertilized iron becomes too high, the volume fraction of residual Vostian iron becomes too low, and the total volume fraction of tempered Asada loose iron and toughened iron becomes too low, and tempered Asada The effective crystal grain size of loose iron and toughened iron becomes too large. The holding temperature of sample G-5 during tempering was too low. Therefore, the volume fraction of the residual Vosstian iron becomes too low, and the number density of iron-based carbides becomes too low. The cooling stop temperature of the sample G-6 during continuous annealing was too low, and the holding temperature during the tempering treatment was too high. Therefore, the volume fraction of the residual Vosstian iron becomes too low. The average cooling rate of sample G-8 in continuous annealing was too low, and the cooling stop temperature was too high. Therefore, the volume fraction of ferrous iron becomes too high, the volume fraction of fresh Asada loose iron becomes too high, the volume fraction of residual Vostian iron becomes too low, and tempered Asada loose iron and toughened iron The effective crystalline particle size becomes too large, and the number density of iron-based carbides becomes too low. The cooling stop temperature of sample G-9 during continuous annealing was too low, and the holding time during tempering was too short. Therefore, the volume fraction of the residual Vossite iron becomes too low, and the total volume fraction of the tempered Asada loose iron and the toughened iron becomes too high.

The number of passes of the sample S-2 under the predetermined conditions in the rough rolling was 0, and the temperature on the entry side on the fourth rolling mill in the finished rolling was too high, and the finishing temperature was too high. Therefore, the effective crystal grain size of tempered Asada loose iron and toughened iron becomes too large. The passing time of the last three stages of rolling of sample S-3 in the finished rolling was too long, and the elapsed time from the final stage of rolling to the start of water cooling was too long. Therefore, the effective crystal grain size of tempered Asada loose iron and toughened iron becomes too large. The total rolling reduction of the final three stages of the rolling of Sample S-4 was too low. Therefore, the effective crystal grain size of tempered Asada loose iron and toughened iron becomes too large. The cooling stop temperature of sample S-5 in continuous annealing was too low. Therefore, the volume fraction of fresh Asada loose iron becomes too high, the volume fraction of residual Vostian iron becomes too low, and the total volume fraction of tempered Asada loose iron and toughened iron becomes too low, and tempering The effective crystal grain size of Asada loose iron and toughened iron becomes too large, and the number density of iron-based carbides becomes too low. The heating rate of sample S-6 in continuous annealing was too low. Therefore, the effective crystal grain size of tempered Asada loose iron and toughened iron becomes too large. The holding temperature of sample S-8 during continuous annealing was too high. Therefore, the effective crystal grain size of tempered Asada loose iron and toughened iron becomes too large. The holding time of sample S-9 in continuous annealing was too short. Therefore, the number density of iron-based carbides becomes too low. The cooling stop temperature of sample S-10 in continuous annealing was too low. Therefore, the volume fraction of fertilized iron becomes too high, and the volume fraction of residual Vostian iron becomes too low. The effective crystal grain size of loose iron and toughened iron becomes too large. The holding temperature of sample S-11 during tempering was too high. As a result, the volume fraction of fresh Asada iron has become too high, while the volume fraction of residual Vosda iron has become too low. Sample S-12 was held for too long in the tempering process. Therefore, the volume fraction of the residual Vossite iron becomes too low, the volume fraction of the boron iron becomes too high, and the effective crystal grain size of tempered Asada iron and toughened iron becomes too large. The cooling stop temperature of sample S-13 in continuous annealing was too high. Therefore, the volume fraction of the residual Vostian iron becomes too low, and the volume fraction of the fresh Asada loose iron becomes too high. The cooling stop temperature of the sample S-14 in continuous annealing was too low, and the temperature of the alloying treatment was too high. The volume fraction of the residual Vostian iron has become too low. The holding temperature of sample W-2 during the tempering process was too high. Therefore, the volume fraction of the fresh Asada loose iron becomes too high, and the volume fraction of the residual Vosted iron becomes too low.

The temperature of the entrance side of the sample i-1 and the sample j-1 on the fourth rolling mill in the finished rolling was too high. Therefore, the effective crystal grain size of tempered Asada loose iron and toughened iron becomes too large. The passing time of the last three stages of rolling of sample k-1 in the finished rolling was too long, and the elapsed time from the final stage of rolling to the start of water cooling was too long. Therefore, the effective crystal grain size of tempered Asada loose iron and toughened iron becomes too large. The extraction temperature of sample 1-1 from the heating furnace was too low. Therefore, the temperature before the finish rolling becomes too low, and the finish annealing is not performed.

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

Claims (9)

A steel sheet characterized by having the following chemical composition in terms of mass%, C: 0.15% to 0.45%, Si: 1.0% to 2.5%, Mn: 1.2% to 3.5%, Al: 0.001% to 2.0% , P: 0.02% or less, S: 0.02% or less, N: 0.007% or less, O: 0.01% or less, Mo: 0.0% ~ 1.0%, Cr: 0.0% ~ 2.0%, Ni: 0.0% ~ 2.0%, Cu : 0.0% ~ 2.0%, Nb: 0.0% ~ 0.3%, Ti: 0.0% ~ 0.3%, V: 0.0% ~ 0.3%, B: 0.00% ~ 0.01%, Ca: 0.00% ~ 0.01%, Mg: 0.00 % ~ 0.01%, REM: 0.00% ~ 0.01%, and the rest: Fe and impurities, and has the following steel structure in volume fraction, tempered Asada loose iron and toughened iron: 70% in total Above and below 92%, Residual Vostian Iron: Above 8% and below 30%, Fertilizer Iron: Below 10%, Fresh Asada Iron: Less than 10%, and Poly Iron: Less than 10% In addition, the number density of iron-based carbides in tempered Asada loose iron and lower toughened iron is 1.0 × 10 ⁶ (pieces / mm ² ) or more; effective grain size of tempered Asada loose iron and toughened iron It is 5 μm or less. The steel plate of claim 1, characterized in that the foregoing chemical composition is established: in terms of mass%, Mo: 0.01% to 1.0%, Cr: 0.05% to 2.0%, Ni: 0.05% to 2.0%, or Cu: 0.05% ~ 2.0%, or any combination of these. For example, the steel sheet of claim 1 or 2 is characterized in that the foregoing chemical composition is established: in mass%, Nb: 0.005% to 0.3%, Ti: 0.005% to 0.3%, or V: 0.005% to 0.3%, or Any combination of these. For example, the steel sheet of claim 1 or 2 is characterized in that the foregoing chemical composition is established: in terms of mass%, B: 0.0001% to 0.01%. The steel sheet of claim 3 is characterized in that the foregoing chemical composition is established: in terms of mass%, B: 0.0001% to 0.01%. For example, the steel sheet of claim 1 or 2 is characterized in that the foregoing chemical composition is established: in terms of mass%, Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01%, or REM: 0.0005% to 0.01%, or Any combination of these. The steel plate of claim 3 is characterized in that the foregoing chemical composition is established: in terms of mass%, Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01%, or REM: 0.0005% to 0.01%, or the like Any combination of. The steel sheet of claim 4, characterized in that the foregoing chemical composition is established: in terms of mass%, Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01%, or REM: 0.0005% to 0.01%, or the like Any combination of. The steel sheet of claim 5, characterized in that the foregoing chemical composition is established: in terms of mass%, Ca: 0.0005% to 0.01%, Mg: 0.0005% to 0.01%, or REM: 0.0005% to 0.01%, or the like Any combination of.