Technical field
[0001]
　The present invention relates to a heat treatment for steel plates.
Background technique
[0002]
In the field of automotive steel sheets, against the background of tightening of recent environmental regulations and crash safety standards, in order to achieve both the fuel efficiency and collision safety, the application of high-strength steel sheet has expanded with a high tensile strength. However, since the press formability of the steel sheet with a high strength is lowered, it is possible to manufacture a product having a complicated shape becomes difficult. Specifically, the decrease in ductility of the steel sheet with a high strength, has arisen a problem that breakage of the high machining area. Moreover, springback and wall warpage occurs due to the residual stress after working, it is caused a problem that the dimensional accuracy is deteriorated. Accordingly, high strength, is not easy, especially a steel sheet having a tensile strength of at least 780 MPa, pressed into a product having a complicated shape. Incidentally, according to the roll forming rather than press forming, but easily processed steel plate high strength, its application target is limited to parts having a uniform cross section in the longitudinal direction.
[0003]
　In recent years, for example, as disclosed in Patent Document 1, a technique for press-molding the molding is difficult materials such as high-strength steel sheet, hot stamping techniques are employed. The hot stamp technique, a hot forming technique for molding after heating the material to be subjected to molding. In this technique, for shaping after heating the material, at the time of molding, the steel has a good moldability soft. Thus, even a high-strength steel can be molded accurately into complex shapes. Further, since the simultaneous quenching and molding by the press die steel after molding it has sufficient strength. For example, according to Patent Document 1, by a hot stamping technique, it is possible to impart a tensile strength of at least 1400MPa in steel after molding.
[0004]
　Further, Patent Document 2, the hot-forming member having both a stable strength and toughness, hot-forming process for making the same are disclosed. Patent Document 3, a press, bending and roll forming a moldability such as good formability and hot-rolled steel sheet excellent in hardenability capable of imparting a high tensile strength after quenching, and cold-rolled steel sheet It has been disclosed. Patent Document 4, technology for the purpose of obtaining an ultra-high strength steel sheet having both strength and moldability is disclosed.
[0005]
　Further, Patent Document 5, the high-strength steel material having both a high strength has been high yield ratio and high strength, steel grade and its manufacturing method capable of separately formed various intensity levels of the material be the same steel species It has been disclosed. Patent Document 6, and moldability, aimed at obtaining a thin high-strength welded steel pipe having excellent torsional fatigue characteristics after cross molding method for producing a steel pipe is disclosed. Patent Document 7, which can be in the hot press forming apparatus for forming by heating the metal sheet to obtain a press product with excellent short time strength and dimensional accuracy to facilitate cooling of the mold and molded articles hot press forming apparatus and hot press molding method is disclosed.
CITATION
Patent Literature
[0006]
Patent Document 1: JP 2002-102980 Patent Publication
Patent Document 2: JP 2004-353026 Patent Publication
Patent Document 3: JP 2002-180186 Patent Publication
Patent Document 4: JP 2009-203549 Patent Publication
Patent Document 5: JP 2007-291464 JP
Patent Document 6: JP 2010-242164 Patent Publication
Patent Document 7: JP 2005-169394 JP
Summary of the Invention
Problems that the Invention is to Solve
[0007]
　For hot forming techniques such as the hot stamping is an excellent molding method capable of increasing the strength of the member while securing formability, that need to be heated to a temperature as high as 800 ~ 1000 ° C., there is a problem that the surface of the steel sheet is oxidized. Its scale comprising iron oxide which occurs when drops and productivity to adhere to the mold and dropping during pressing. Further, there is a problem that the appearance when scale product after pressing remains becomes poor.
[0008]
　Moreover, if the scale remains on the surface of the steel sheet, adhesion between the steel sheet and the coating film is deteriorated when the paint in the next step, lowering the corrosion resistance. So after press molding, the scale removal treatment of the shot blasting is required. Accordingly, the characteristics required for generation scale, without causing mold staining and spalling during pressing, is that likely to be easily peeled off at the time of the shot blasting treatment.
[0009]
　Further, as described above, the automobile steel sheet is also required collision safety. Crashworthiness of automobiles is evaluated by crushing strength and absorbed energy at impact test of the vehicle body all or steel members. In particular for crushing strength is largely dependent on the material strength, the demand for ultra-high strength steel sheet has increased dramatically. However, since generally fracture toughness along with high strength is lowered, broken at the site, such as whether to break early collision collapse of the automobile member, or deformation is concentrated, crushing strength commensurate with the material strength is not exerted , the absorption energy decreases. Therefore, in order to improve collision safety, not only material strength, it is important to improve the toughness of an important indicator of the fracture toughness of the automotive member material.
[0010]
　In the conventional techniques described above, for obtaining a proper scale characteristic and excellent crashworthiness, not sufficient studies have been made, room for improvement is left.
[0011]
　The present invention has been made to solve the above problems, and an object thereof is to provide a heat treatment for steel sheet excellent in scale characteristic and toughness after heat treatment at the time of hot molding. In the following description, (including hot forming.) Heat treating the steel sheet after referred to as "heat treatment steel."
Means for Solving the Problems
[0012]
　The present invention has been made to solve the above problems, and the gist of the heat treatment for steel below.
[0013]
　(1) chemical composition of the steel sheet contains, by
　mass%,
　C: 0.05 ~
　0.50%, Si: 0.50 ~ 5.0%, Mn: 1.5 ~
　4.0%, P: 0. 0.05% or
　less, S: 0.05% or
　less, N: 0.01% or
　less,
　Ti: 0.01 ~ 0.10%, B: 0.0005
　~ 0.010%, Cr: 0 ~ 1.0% , 　Ni:
　　0 ~ 2.0%, 　Cu: 0 ~ 1.0%, Mo: 0 ~ 　1.0%, V: 0 ~ 1.0%, Ca: 0 ~ 　0.01%, Al: 0 ~ 　% 1.0, 　Nb: 0 ~ 1.0%, REM: 0 ~ 0.1%, 　the balance is Fe and impurities, 　the maximum height roughness Rz of the surface of the steel sheet is 3.0 ~ 10.0 [mu] m , and the 　equivalent circle diameter present in the steel sheet is the number density of the above carbides 0.1μm is 8.0 × 10 3 cells / mm 2 or less, 　the heat treatment for steel plates.

[0014]
　(2) the chemical composition, in
　mass%,
　Cr: 0.01 ~
　1.0%, Ni: 0.1 ~ 2.0%, Cu: 0.1
　~ 1.0%, Mo: 0.1
　1.0%
　~,
　0.1 ~ 1.0%, Ca: 0.001 ~ 0.01%,
　Al: 0.01 ~ 1.0% Nb: 0.01 ~ 1.0%, and
　REM: 0.001 ~ 0.1%,
　containing one or more selected from,
　for heat treatment steel sheet according to (1).
[0015]
　(3) below (i) Mn segregation ratio of the formula α is 1.6 or less,
　heat-treating a steel sheet according to (1) or (2).
　alpha = [Maximum Mn concentration (mass%) at the center of plate thickness] / [average Mn concentration (mass%) of 1/4 depth position of the sheet thickness from the surface] · · · (i)
[0016]
　(4) the value of the cleanliness of the steel defined by JIS G 0555 (2003) is less than 0.10%,
　the heat treatment for steel sheet according to any one of (1) to (3).
Effect of the invention
[0017]
　According to the present invention, it is possible to obtain a heat-treated steel plate having excellent scale characteristic at the time of hot forming. Then, the heat treatment for the steel sheet of the present invention, by a heat treatment or hot forming process, it becomes possible to obtain a heat-treated steel having excellent toughness and having a tensile strength of at least 1.4 GPa.
DESCRIPTION OF THE INVENTION
[0018]
　The present inventors have made intensive studies on the chemical components and organization relationship for satisfying both the scale characteristic and toughness after heat treatment at the time of hot molding, leading to obtain the following findings.
[0019]
　Component of the heat treatment for steel that is produced in (a) and abroad are almost identical, C: 0.2 ~ 0.3% and Mn: containing about 1-2%, further containing Ti and B. In the heat treatment step, the steel sheet Ac 3 was heated to a temperature above points, ferrite is conveyed quickly not precipitated, by rapidly cooling by the mold press to martensitic transformation start temperature (Ms point), the intensity high martensitic structure to get the organization of members, which account for the majority.
[0020]
　(B) By the Si content in steel to more than conventional heat treatment for steel, further the maximum height roughness Rz of the thermal treatment before the steel sheet 3.0 ~ 10.0 [mu] m, adequate during hot-forming to demonstrate the scale characteristics.
[0021]
　When (c) coarse carbides in the steel sheet during the thermal are present in excess, carbides may be deteriorated much residual tenacity in the grain boundary after the heat treatment. Therefore, it is necessary to below a specified value the number density of carbides present in heat treated steel plate.
[0022]
　(D) quantifying the Mn segregation ratio included in the heat treatment for steel, the toughness of the heat-treated steel material is further improved by reducing it.
[0023]
　(E) inclusions contained in the heat treatment for the steel sheet significantly affecting toughness of ultra high strength steel. For toughness improvement, it is preferable to reduce the value of the cleanliness of the steel defined by JIS G 0555 (2003).
[0024]
　The present invention has been made based on the above findings. It will be described in detail below each requirement of the present invention.
[0025]
　(A) Chemical composition
　reasons for limiting each element are as follows. Incidentally, "%" for the content in the following description means "mass%".
[0026]
　C: 0.05 ~ 0.50%
　C increases the hardenability of steel, and is an element that improves the strength of steel after hardening. However, the C content is less than 0.05%, it is difficult to ensure sufficient strength in the steel after quenching. Therefore, C content is 0.05% or more. On the other hand, when the C content exceeds 0.50%, too high strength of the steel after quenching, degradation of the toughness becomes significant. Therefore, C content is 0.50% or less. C content is preferably at 0.08% or more, and preferably 0.45% or less.
[0027]
　Si: 0.50 ~
　5.0% Si is, Fe on the surface of the steel sheet during the heat treatment 2 SiO 4 to generate, as well as inhibit scale formation, serves to reduce the FeO in the scale. The Fe 2 SiO 4 is a barrier layer, since the supply of Fe into the scale is interrupted, it is possible to reduce the scale thickness. Further scale thickness is thin, and difficult to peel at the time of hot-forming, there is a merit of easy peeling during scale removal treatment after molding. In order to obtain these effects, it is necessary to contain Si 0.50% or more. Incidentally, the Si is 0.50% or more, carbides tend to be less. As described below, the carbide precipitated in the steel sheet before the heat treatment is large, the rest they dissolve during the heat treatment can not secure sufficient hardenability, since the low strength ferrite is precipitated, may become insufficient strength, in this sense Si is 0.50% or more.
[0028]
　However, when the Si content in the steel exceeds 5.0%, during the heat treatment, the heating temperature required for austenite transformation becomes remarkably high. Accordingly, or cause an increase in cost required for heat treatment, which may or inviting quenching shortage due to insufficient heating. Therefore, Si content is 5.0% or less. Si content is preferably 0.75% or more is preferably 4.0% or less.
[0029]
　As described later, Si is to generate the partial roughness is large or other parts of the steel sheet surface as fayalite when heated pressing has the effect of adjusting the iron scale wustite composition. In a preferred range of the effect thereof becomes large.
[0030]
　Mn: 1.5 ~ 4.0% Mn
　enhances the hardenability of steel, and in order to secure the strength after quenching stable, an element with a very effective. Further Ac 3 lowers the point, an element which promotes lowering the quenching temperature. However, Mn content, the effect is not sufficient below 1.5%. On the other hand, the effect of Mn content exceeds 4.0% is saturated, further leading to toughness deterioration of the quenched portion. Therefore, Mn content is from 1.5 to 4.0%. Mn content is preferably 2.0% or more. Further, it is preferable Mn content is less 3.8%, and more preferably not more than 3.5%.
[0031]
　P: 0.05% or less
　P is an element degrading the toughness of the steel after quenching. In particular, when the P content exceeds 0.05%, deterioration of toughness becomes remarkable. Accordingly, P content is 0.05% or less. P content is preferably 0.005% or less.
[0032]
　S: 0.05% or less
　S is an element degrading the toughness of the steel after quenching. In particular, when the S content exceeds 0.05%, deterioration of toughness becomes remarkable. Thus, S content is 0.05% or less. S content is preferably 0.003% or less.
[0033]
　N: 0.01% or less
　N is an element degrading the toughness of the steel after quenching. In particular, when the N content exceeds 0.01%, coarse nitrides in the steel are formed, the local deformability and toughness is remarkably deteriorated. Therefore, N content is 0.01% or less. It is not necessary to particularly limit the lower limit of the N content, since it is economically undesirable for the N content to less than 0.0002%, N content is preferably 0.0002% or more, 0 and more preferably to .0008% or more.
[0034]
　Ti: 0.01 ~ 0.10% Ti
　is a steel sheet Ac 3 suppresses the recrystallization during the heat treatment by heating to a temperature above points, to suppress the grain growth to form fine carbides in an element having an effect of the austenite grains fine. Therefore, by containing Ti, the effect of the toughness of the steel material is significantly improved can be obtained. Moreover, Ti can suppress the consumption of B by precipitation of BN by preferentially bind N in the steel, to promote the effect of improving hardenability by later-described B. The Ti content is less than 0.01%, not sufficiently obtained the above effects. Therefore, Ti content is 0.01% or more. On the other hand, if the Ti content exceeds 0.10%, the precipitation amount of TiC is consumed C increases the strength of the steel after hardening is reduced. Therefore, Ti content is 0.10% or less. Ti content is preferably 0.015% or more is preferably not more than 0.08%.
[0035]
　B: 0.0005 ~ 0.010%
　B, so it has a dramatically enhancing effect the hardenability of steel even in trace amounts, is a very important element in the present invention. Further, B is that segregates at the grain boundaries, increasing the toughness to strengthen grain boundaries. Furthermore, B suppresses grain growth of austenite during heating of the steel sheet. The B content is less than 0.0005%, may not be sufficiently obtained the above effects. Therefore, B content is 0.0005% or more. On the other hand, when the B content exceeds 0.010%, coarse compounds are often deposited, toughness of the steel is deteriorated. Thus the B content is 0.010% or less. B content is preferably at 0.0010% or more is preferably 0.008% or less.
[0036]
　The heat treatment for the steel sheet of the present invention, in addition to the above elements, containing Cr in an amount shown below, Ni, Cu, Mo, V, Ca, Al, one or more elements selected from Nb and REM it may be.
[0037]
　Cr: 0 ~ 1.0% Cr
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel after hardening stable, it may be contained. Also as with Si, FeCr on the surface of the steel sheet during the heat treatment 2 O 4 serves to generate, as well as inhibit scale formation, to reduce the FeO in the scale. The FeCr 2 O 4 becomes a barrier layer, since the supply of Fe into the scale is interrupted, it is possible to reduce the scale thickness. Further scale thickness is thin, and difficult to peel at the time of hot-forming, there is a merit of easy peeling during scale removal treatment after molding. However, the above effects when the Cr content exceeds 1.0% is saturated, unnecessarily cause an increase in cost. Therefore, Cr content of the case of containing the 1.0%. Cr content is preferably at 0.80%. To obtain the above effect, Cr content is preferably at least 0.01%, and more preferably 0.05% or more.
[0038]
　Ni: 0 ~ 2.0% Ni
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel after hardening stable, may be contained. However, when the Ni content exceeds 2.0%, economic efficiency and the above effect is saturated is reduced. Therefore, Ni content in the case of incorporating is 2.0% or less. To obtain the above effect, it is preferable to contain Ni at least 0.1%.
[0039]
　Cu: 0 ~ 1.0% Cu
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel after hardening stable, may be contained. However, when the Cu content exceeds 1.0%, economic efficiency and the above effect is saturated is reduced. Therefore, Cu content in the case of incorporating is 1.0% or less. To obtain the above effect, it is preferable to contain Cu of 0.1% or more.
[0040]
　Mo: 0 ~ 1.0% Mo
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel after hardening stable, it may be contained. However, when the Mo content exceeds 1.0%, economic efficiency and the above effect is saturated is reduced. Therefore, Mo content in the case of incorporating is 1.0% or less. To obtain the above effect, it is preferable to contain Mo of 0.1% or more.
[0041]
　V: 0 ~ 1.0% V
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel after hardening stable, it may be contained. However, when the V content exceeds 1.0%, economic efficiency and the above effect is saturated is reduced. Therefore, V content in the case of incorporating is 1.0% or less. To obtain the above effect, it is preferable to contain V of 0.1% or more.
[0042]
　Ca: 0 ~ 0.01%
　Ca, in order to inclusions in the steel refining, which is an element having an effect of improving the toughness and ductility after quenching, may be contained. However, the effect when the Ca content exceeds 0.01% saturated, unnecessarily cause an increase in cost. Therefore, in the case of containing Ca content thereof is 0.01% or less. Ca content is preferably 0.004% or less. If it is desired to obtain the above effect, it is preferable that the Ca content is 0.001% or more, and more preferably 0.002% or more.
[0043]
　Al: 0 ~ 1.0% Al
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel after hardening stable, may be contained. However, if the Al content exceeds 1.0%, economic efficiency and the above effect is saturated is reduced. Therefore, Al content in the case of incorporating is 1.0% or less. To obtain the above effect, it is preferable to contain Al 0.01% or more.
[0044]
　Nb: 0 ~ 1.0% Nb
　is an element that makes it possible to enhance the hardenability of steel, and to ensure the strength of the steel after hardening stable, may be contained. However, when the Nb content exceeds 1.0%, economic efficiency and the above effect is saturated is reduced. Therefore, Nb content case of containing the 1.0% or less. To obtain the above effect, it is preferable to contain Nb of 0.01% or more.
[0045]
　REM: 0 ~ 0.1%
　REM, since refining inclusions in like the Ca steel, is an element having an effect of improving the toughness and ductility after quenching, it may be contained. However, the effect when the REM content exceeds 0.1% is saturated, unnecessarily cause an increase in cost. Therefore, REM content case of containing is 0.1% or less. REM content is preferably 0.04% or less. If it is desired to obtain the above effect, it is preferable that the REM content is 0.001% or more, and more preferably 0.002% or more.
[0046]
　Here, REM is, Sc, refers to a total of 17 elements of Y and lanthanoids, the content of the REM means the total content of these elements. REM is added to the molten steel by using, for example, Fe-Si-REM alloy, this alloy, for example, Ce, La, Nd, includes Pr.
[0047]
　In the chemical composition of the heat treatment for the steel sheet of the present invention, the balance being Fe and impurities.
[0048]
　Here, the "impurities", in manufacturing the steel sheet industrially, ores, raw material scraps, a component mixed by various factors of the manufacturing process, is allowed to the extent that the present invention does not adversely affect means shall.
[0049]
　(B) a surface roughness
　maximum height roughness Rz: 3.0 ~ 10.0 [mu] m
　for heat treatment steel sheet according to the present invention is a steel sheet surface, the maximum height roughness Rz defined in JIS B 0601 (2013) it is a 3.0 ~ 10.0μm. The maximum height roughness Rz of the steel sheet surface by more than 3.0 [mu] m, scale adhesion during hot-forming is improved by an anchor effect. On the other hand, when the maximum height roughness Rz is more than 10.0 [mu] m, may at the stage of the scale removal treatment shot blasting after press molding, the scale will partially remain, causing indentation flaws .
[0050]
　By the maximum height roughness Rz of 3.0 ~ 10.0 [mu] m at the surface of the steel sheet, it is possible to achieve both the scale peelability for scale adhesion and shot blasting time during pressing. In order to obtain a suitable anchoring effect as described above, of managing an arithmetic mean roughness Ra is insufficient, it is necessary to use the maximum height roughness Rz.
[0051]
　If the maximum height roughness Rz of the steel sheet surface is a more steel 3.0μm and hot forming, shows a tendency to the ratio of wustite iron oxide that forms on the surface increases. Specifically, the percentage of wustite is in area%, by a 30-70%, resulting excellent scale adhesion.
[0052]
　Wustite hematite, excellent plastic deformability at high temperatures than magnetite, scaling is also considered to exhibit the easy features to plastic deformation when plastic deformation of the steel sheet during hot-forming. The reason why the ratio of wustite is increased, but clearly is not known, the area of ​​the scale areas iron surface becomes large when the irregularity is present, outward diffusion of iron ions is accelerated during the oxidation, the ratio of iron it is believed that a high wustite is increased.
[0053]
　Further, the surface of the steel sheet during hot molding by containing Si Fe 2 SiO 4 is generated and suppressing scale formation is as described above. Scale thickness of the whole becomes thinner, and that the wustite ratio in the scale is increased, the scale adhesion during hot molding is thought to be improved. Specifically, by the scale thickness becomes 5μm or less, excellent scale adhesion can be obtained.
[0054]
　(C) carbides: 8.0 × 10 3 cells / mm 2 or less
　when the heat treatment coarse carbides before the steel sheet there are many, undissolved they during the heat treatment can not secure sufficient hardenability, low strength ferrite There is precipitated. Therefore, it is possible carbides in the steel sheet before the heat treatment is the less, improves hardenability and ensuring high strength.
[0055]
　Also, the carbide is deposited on the former γ grain boundaries, thereby embrittling the grain boundaries. In particular, the circle equivalent diameter number density of more carbide 0.1μm is 8.0 × 10 3 cells / mm 2 fear by weight, the carbides after heat treatment remain much in the grain boundary, toughness after heat treatment is deteriorated there is. Thus, the circle equivalent diameter present in heat-treated steel plate is the number density of more carbide 0.1μm is 8.0 × 10 3 cells / mm 2 or less. The above carbide refers to those particulate, specifically directed to what aspect ratio is 3 or less.
[0056]
　(D) Mn segregation ratio
　Mn segregation ratio alpha: 1.6 or less
　alpha = average Mn concentration in the Maximum Mn concentration (mass%) at the center of plate thickness] / [1/4 depth position of the sheet thickness from the surface (wt%)] ··· (i)
　heat-treating a steel sheet according to the present invention, Mn segregation ratio α is preferably not more than 1.6. The thickness sectional center of the steel, Mn is concentrated by center segregation occurs. Accordingly, MnS is concentrated in the center as inclusions, since easily available hard martensite, there is a possibility that a difference occurs in the hardness of the surrounding, the toughness is deteriorated. In particular, when the value of the segregation ratio α of Mn represented by the above formula (i) is more than 1.6, there is a possibility that the toughness is deteriorated. Therefore, in order to improve the toughness, it is preferable to set the value of α of the heat treatment steel 1.6 or less. For further improvement of the toughness, it is preferable that the value of α is 1.2 or less.
[0057]
　Since there is no possibility that the value of α by heat treatment or hot forming is greatly changed, the value of α of heat treatment for steel that within the above range, it is also 1.6 below the value of α of the heat treatment steel It is possible, i.e. it is possible to improve the toughness of the heat treatment steel.
[0058]
　Maximum Mn concentration in the center of plate thickness is determined by the following method. In the center of plate thickness of the steel sheet by an electron probe microanalyzer (EPMA), perform line analysis in the thickness direction perpendicular to the direction, select the three measurements in the descending order from the analysis result, and calculates the average value . The average Mn concentration at 1/4 depth position of the sheet thickness from the surface is determined by the following method. Also analyzed the 10 locations in the 1/4 depth position of the steel sheet by using EPMA, and calculates the average value.
[0059]
　Segregation of Mn in the steel sheet is mainly steel composition, and in particular impurity content is controlled by the continuous casting conditions, does not substantially change before and after the hot rolling and hot forming. Thus, segregation conditions of heat treatment for steel satisfies the requirements of the present invention, then meets the specifications likewise present invention segregation conditions of the heat-treated steel.
[0060]
　(E) cleanliness
　cleanliness: 0.10% or less
　in the heat treatment in the steel material JIS G 0555 (2003) A system according to, the B system and C-type inclusions there are many, the cause of the inclusions toughness deteriorate Become. Since the inclusions increases easily happen crack propagation, there is a possibility that the toughness deteriorates. In particular, when the heat treatment steel material having a tensile strength of at least 1.4 GPa, it is preferable to suppress the existence ratio of the inclusions low. When the value of the cleanliness of the steel defined by JIS G 0555 (2003) is more than 0.10%, the amount of inclusions is large, it is difficult to ensure practically sufficient toughness. Therefore, cleanness of the value of heat treatment for steel sheet is preferably set to 0.10% or less. It is more preferable to further improve the toughness of less 0.06% the value of cleanliness. Incidentally, cleanliness value of the steel is obtained by calculating the area percentage occupied by the above A type, B type and C type inclusions.
[0061]
　Since not the value of cleanliness is greatly changed by heat treatment or hot forming, the value of the cleanliness of the heat treatment for the steel sheet is within the above range, the cleanliness of the value of the heat treatment the steel is also 0.10% It can be below.
[0062]
　In the present invention, the cleanliness of the value of the heat treatment for steel plates or heat treatment steel is obtained by the following method. For the heat treatment for steel plates or a heat treatment steel, cut out test pieces from the five locations. Then, for each position of each sample in the plate thickness 1 / 8t, 1 / 4t, 1 / 2t, 3 / 4t, 7 / 8t, to investigate the cleanliness at point algorithm. The (lowest cleanability) numeric value of cleanliness is greatest with respect to the plate thickness, the value of cleanliness of test materials.
[0063]
　(F) the production method of heat treatment for steel
　no particular limitation for preparation conditions of the heat treatment for steel sheet according to the present invention, by using the manufacturing method described below, it can be produced. In the following manufacturing method, for example, it performs the hot rolling, pickling, cold rolling and annealing.
[0064]
　After melting the steel having the chemical composition described above in a furnace to produce a slab by casting. At this time, in order to suppress concentration of MnS which becomes a starting point of delayed fracture, it is desirable to center segregation reduction process to reduce the center segregation of Mn. The central segregation reduction treatment, in the non-solidified layer before the slab is completely solidified, and a method of discharging the molten steel Mn is enriched.
[0065]
　Specifically, electromagnetic stirring, by a process such unsolidified layer pressure applied, can be completely solidified before Mn is to discharge the molten steel enriched. The electromagnetic stirring process described above may be carried out by giving fluidity to the unsolidified molten steel at 250 to 1000 gauss, the unsolidified layer rolling process, the final solidified portion by reduction with a gradient of about 1 mm / m It can be carried out.
[0066]
　Against a slab obtained by the method described above, soaking (soaking) process may be carried out as necessary. Soaking treatment by performing, it is possible to reduce the segregation ratio to diffuse segregated Mn. Preferred soaking temperature in the case of performing the soaking treatment is 1200 ~ 1300 ° C., soaking time is 20 ~ 50h.
[0067]
　Furthermore, the cleanliness of the steel sheet to 0.10% or less, at the time of continuous casting of molten steel, the heating temperature of the molten steel temperature high 5 ° C. or more above the liquidus temperature of the steel, and, per unit time it is desirable to suppress the molten steel pouring amount below 6t / min.
[0068]
　When the amount of casting per unit of molten steel during continuous casting time exceeds 6t / min, for faster flow of molten steel in the mold, inclusions solidified shell is easily trapped, inclusions in the slab increases. Further, when the molten steel heating temperature is 5 ° C. higher than a temperature above the liquidus temperature, high viscosity of the molten steel, difficult to inclusions floating in a continuous casting machine, as a result, inclusions in the slab is increased cleanliness Te is likely to deteriorate.
[0069]
　On the other hand, the molten steel heating temperature from the molten steel liquidus temperature 5 ° C. or higher, and the molten steel pouring amount per unit time by casting as follows 6t / min, inclusions is hardly brought into the slab. As a result, the slab can effectively reduce the amount of inclusions in a step of manufacturing, it is possible to easily achieve the steel cleanness of 0.10% or less.
[0070]
　When the continuous casting of molten steel, the molten steel heating temperature of the molten steel is preferably set to be 8 ° C. or more above the liquidus temperature higher temperatures, also, it is preferable that the molten steel pouring amount per unit time below 5t / min. The molten steel heating temperature and liquidus temperature 8 ° C. or higher than the high temperature, and, by a molten steel pouring amount per unit time below 5t / min, it is easy to cleanliness and 0.06% or less for desirable.
[0071]
　Then, subjected to a hot rolling on the slab. Hot rolling conditions, from the viewpoint of more uniformly generate a carbide, a hot rolling start temperature to a temperature range of 1000 ~ 1300 ° C., the hot rolling completion temperature is preferably set to 950 ° C. or higher.
[0072]
　In the hot rolling step, after the rough rolling, it performs descaling is performed as necessary finally finish rolling. At this time, when the time until the rough rolling starts finish rolling from the end to the 10s or less, recrystallization austenite is suppressed, not only is consequently suppressed the growth of carbides, scale generated in a high temperature inhibition, it is possible to adjust the suppression of oxidation of the austenite grain boundaries, and the maximum height roughness on the surface of the steel sheet in an appropriate range. In addition, the inhibition of scale formation and grain boundary oxidation, since Si in the surface layer is likely to remain in a state of solid solution, easily generated fayalite when heated pressing, considered wustite be easily generated for the It is.
[0073]
　Coiling temperature after hot rolling is preferably higher from the viewpoint of workability, since too high yield by scale formation is reduced, preferably in the 500 ~ 650 ° C.. Also, better to the coiling temperature in the low temperature, carbides easily finely dispersed, and also reduced the number of carbides.
[0074]
　Form of carbides, in addition to the conditions of hot rolling, it is possible to also control by adjusting the subsequent annealing conditions. That is, the annealing temperature to a high temperature, after once a solid solution carbides in annealing step, cause the transformation at a low temperature is desirable. Since carbide is hard, in the cold rolling is not that the form is changed, the presence form of after the cold rolling also hot rolling is maintained.
[0075]
　Subjected to descaling by pickling or the like hot-rolled steel sheet obtained by hot rolling. To adjust the maximum height roughness on the surface of the steel sheet in an appropriate range, it is preferable to adjust the scarfing amount in pickling. Maximum height roughness Smaller scarfing amount is increased, while the maximum height roughness by increasing the scarfing amount is small. Specifically, it is preferable to scarfing amount 1.0 ~ 15.0 .mu.m by pickling, and more preferably to 2.0 ~ 10.0 [mu] m.
[0076]
　The heat treatment for steel in the present invention, it is possible to use a hot-rolled sheet or hot-rolled annealed steel sheet or cold-rolled steel sheet or cold-rolled annealed steel sheet. Processing step may be appropriately selected depending on the plate thickness accuracy required level of products.
[0077]
　That is, hot-rolled steel sheet descaling has been performed, a hot-rolled annealed steel sheet is subjected to annealing as needed. Further, hot-rolled steel sheet or hot-rolled annealed steel sheets described above, if necessary subjected to a cold rolling and cold-rolled steel sheet, further, cold-rolled steel sheet, a cold-rolled annealed steel sheet is subjected to annealing as needed . Incidentally, if the steel sheet subjected to cold rolling is rigid, it is preferable to increase the workability of the steel sheet subjected to annealing before the cold rolling is subjected to cold rolling.
[0078]
　Cold rolling may be performed using conventional methods. From the viewpoint of ensuring good flatness, reduction ratio in cold rolling is preferably 30% or more. Meanwhile, in order to avoid that the load becomes excessive, the rolling reduction in cold rolling is preferably 80% or less. Incidentally, there is no possibility that the maximum height roughness greatly varies at the surface of the steel sheet by cold rolling.
[0079]
　When producing the annealed hot-rolled steel sheet or annealed cold-rolled steel sheet as the heat-treating a steel sheet, performing annealing with respect to hot-rolled steel sheet or cold-rolled steel sheet. The annealing, for example, holding the hot-rolled steel sheet or cold-rolled steel sheet in a temperature range of 550 ~ 950 ° C..
[0080]
　By the temperature to hold at the annealing and 550 ° C. or higher, even when manufacturing any annealed hot-rolled steel sheet or annealed cold-rolled steel sheet, the difference in characteristics due to the difference in the hot rolling condition is reduced, after quenching can be further assumed that stable and characteristics. Also, when performing annealing cold-rolled steel sheet at 550 ° C. or more, cold-rolled steel sheet by recrystallization to soften, thereby improving the workability. In other words, it is possible to obtain the annealed cold-rolled steel sheet with good workability. Therefore, the temperature holding at the annealing is preferably set to 550 ° C. or higher.
[0081]
　On the other hand, if the temperature holding at the annealing is higher than 950 ° C., it is that the tissue is coarsening. Roughening of tissue may reduce the toughness after quenching. Further, even beyond 950 ° C. temperature to hold at the annealing, not be effective enough to raise the temperature obtained, the cost is increased and the productivity is only lowered. Therefore, the temperature holding at the annealing is preferably set to 950 ° C. or less.
[0082]
　After annealing, it is preferable to cool to 550 ° C. at an average cooling rate of 3 ~ 20 ℃ / s. With the average cooling rate 3 ° C. / s or more, is suppressed formation of coarse pearlite and coarse cementite, thereby improving the characteristics after quenching. Further, by the average cooling rate 20 ° C. / s or less, and suppress the occurrence of such intensity irregularity, it is easy to assume that stable the material of the annealed hot-rolled steel sheet or annealed cold-rolled steel sheet.
[0083]
　(G) the production method of the heat treatment steel
　by subjecting to heat treatment for heat treatment steel sheet according to the present invention, it is possible to obtain a heat-treated steel material excellent in toughness and has a high strength. Although not provided any special restriction on heat treatment conditions, for example, it can be subjected to a heat treatment comprising the following heating and cooling steps in this order.
[0084]
　Heating step
　at 5 ° C. / s or more an average heating rate, Ac 3 point ~ Ac 3 to heat the steel sheet to a temperature range of point + 200 ° C.. This heating step, the tissue of the steel sheet to austenite single phase. When or heating temperature rise speed is too slow in the heating step is too high, and coarsened γ grains, the strength of the steel material after cooling may deteriorate. In contrast, by carrying out the heating step satisfying the above conditions, it is possible to prevent deterioration of the strength of the heat treatment steel.
[0085]
　Cooling process
　was subjected to the heating step the steel sheet, does not occur diffusion transformation (i.e. ferrite does not precipitate) As described above, since the temperature range is cooled in the upper critical cooling rate or higher to Ms point, then, from the Ms point to 100 ° C. 5 ° C. / s cooling below the average cooling rate. The cooling rate from a temperature below 100 ° C. to room temperature, the cooling rate of the order of air cooling is preferred. By carrying out the cooling step satisfying the above conditions, can prevent the formation of ferrite in the cooling process, and in a temperature range of Ms point or lower, the carbon diffusion in the untransformed austenite by automatic tempering, thickened, plastic deformation stable retained austenite is generated for. Thereby, it becomes possible to obtain excellent heat-treated steel in toughness and ductility.
[0086]
　The above heat treatment can be carried out by any method, for example, it may be performed by high-frequency heating quenching. In the heating step, the steel sheet Ac 3 point ~ Ac 3 is time for maintaining a temperature range of point + 200 ° C., from the viewpoint of enhancing the hardenability of the steel by dissolving carbides complete the austenite transformation, be a 10s or preferable. Further, the holding time, from the viewpoint of productivity, it is preferably not more than 600s.
[0087]
　As the steel sheet subjected to heat treatment, it may be used annealed hot-rolled steel sheet or annealed cold-rolled steel sheet was subjected to annealing treatment in hot-rolled steel sheet or cold-rolled steel sheet.
[0088]
　In the heat treatment, Ac 3 point ~ Ac 3 after heating to a temperature range of point + 200 ° C., before cooling to the Ms point, may be subjected to hot forming, such as hot stamping described above. The hot forming, bending, drawing, bulging, hole expansion molding, and the flanging and the like. Further, if provided with means for cooling the steel sheet forming the same time or immediately after, the present invention may be applied to forming methods other than press forming, for example, roll forming.
[0089]
　The following examples illustrate the present invention more specifically, the present invention is not limited to these examples.
Example
[0090]
　The steel having the chemical components shown in Table 1 were melted in a test converter, implemented continuously cast in a continuous casting test machine to produce a width 1000 mm, a thickness of 250mm slab. At this time, in the conditions shown in Table 2 were molten steel pouring amount of the adjustment of the heating temperature and the unit of the molten steel time.
[0091]
[Table 1]

[0092]
　Control of the cooling rate of the slab was carried out by changing the amount of water of the secondary cooling spray zone. The center segregation reduction treatment using a roll in the coagulation end portion, implement soft reduction with a gradient of 1 mm / m, the concentrated molten steel in the final solidified portion was carried out by discharging. For some of the slab, then, 1250 ° C., was carried out soaking treatment under the conditions of 24h.
[0093]
　The obtained slab is subjected to hot rolling by hot rolling tester, it was hot rolled steel plate having a thickness of 3.0 mm. The hot rolling step performs descaling after the rough rolling was carried out last finish rolling. Then pickling the hot-rolled steel sheet at the laboratory. Further subjected to cold rolling at a cold rolling tester, a cold rolled steel sheet having a thickness of 1.4 mm, to obtain a heat-treated steel plate (steel No.1 ~ 19).
[0094]
　The presence or absence of central segregation reduction treatment and soaking treatment in the manufacturing process of the heat treatment for steel, the time from the rough rolling in the hot rolling process is finished to the start of the finish rolling, coiling the hot rolling completion temperature and hot-rolled steel sheet temperature, as well as the scarfing amount of pickling, shown in Table 2.
[0095]
[Table 2]

[0096]
　The resulting heat-treated steel plate, the maximum height roughness, arithmetic average roughness, the number density of the carbides was measured Mn segregation ratio and cleanliness. In the present invention, the maximum time of obtaining the height roughness Rz and the arithmetic average roughness Ra, the maximum height roughness Rz and the rolling direction of the arithmetic mean roughness Ra and the rolling vertical 2mm intervals using a surface roughness meter measured each 10 places in, and the average value was recorded.
[0097]
　In the equivalent circle diameter determined the number density of more carbides 0.1 [mu] m, the surface of the heat-treated steel plate, corrosion using picral solution, magnified 2000 fold with a scanning electron microscope, and observed multiple field-of-view It was. At this time, 1 mm by counting the number of viewing the circle equivalent diameter is present above carbides 0.1 [mu] m 2 was calculated number per.
[0098]
　Measurements of Mn segregation ratio was carried out by the following procedure. In mid-thickness portion of the heat-treated steel plate using EPMA, perform line analysis in the thickness direction perpendicular to the direction, after selecting the three measurement values ​​in the descending order from the analysis results, to calculate the average value, the thickness It was determined maximum Mn concentration in the center. Further, the surface of the heat-treated steel plate at 1/4 depth position of the sheet thickness, analyzed for 10 sites using EPMA, and the average value was calculated from the surface at 1/4 depth position of the sheet thickness to obtain an average Mn concentration. Then, the maximum Mn concentration in the above thickness center portion, by dividing the average Mn concentration in the 1/4 depth position of the sheet thickness from the surface, to determine the Mn segregation ratio alpha.
[0099]
　Cleanliness, for each position of thickness 1 / 8t, 1 / 4t, 1 / 2t, 3 / 4t, 7 / 8t, was measured at a point algorithm. Then, the (lowest cleanliness) Numerical highest value of cleanliness in each plate thickness, and the value of the cleanliness of the steel sheet.
[0100]
　Maximum height roughness Rz of the thermal treatment for the steel sheet, the arithmetic mean roughness Ra, the number density of the carbides, the measurement results of the Mn segregation ratio α and cleanliness shown in Table 3.
[0101]
[table 3]

[0102]
　Then, from the steel plate of the thickness: 1.4 mm, width: 30 mm, and length: samples were taken 200mm two by two. For one of the samples taken, in accordance with the heat treatment conditions shown in Table 4 below simulating the hot forming, after the energization heating and cooling, cut out soaking site of each sample, the tensile test and Charpy impact They were subjected to the test.
[0103]
　Tensile test, in accordance with the provisions of the ASTM standard E8, was carried out in the Instron tensile testing machine. After grinding the heat-treated sample to 1.2mm thick, so that the test direction is parallel to the rolling direction, half-size plate-shaped test piece of ASTM Standard E8 (parallel portion length: 32 mm, parallel portion plate width: 6.25 mm ) were collected. Strain gauge on each specimen (Kyowa Electronic Instruments Ltd. KFG-5, gauge length: 5 mm) Paste was conducted at room temperature tensile test at a strain rate of 3 mm / min. In electrical heating device cooling apparatus used in this embodiment, the soaking region derived from the length 200mm about the samples for limited, we decided to use the half-size plate-shaped test piece of ASTM Standard E8.
[0104]
　The Charpy impact test, the ground to a thickness of the soaking region is 1.2 mm, to prepare a V notched specimen which was laminated three sheets, -80 ° C. performing Charpy impact test of the test piece the impact value was determined. In the present invention, 40 J / cm 2 was to evaluate the superior case having more impact value toughness.
[0105]
　Also, among the samples taken, the other one, after electrically heating with the heat treatment conditions shown in Table 4 below simulating the hot forming, subjected to bending against the soaking region and then cooled. After cooling, cut out parts subjected to bending of each sample was subjected to scale characteristic evaluation test. Note that when subjected to bending is supported at both ends of the sample support, by pressing the jig from above the R10mm near the longitudinal center were bent U-shaped. Spacing support each other was 30 mm.
[0106]
　Scale characteristic evaluation test, and scale adhesion evaluation indicative of whether or not spalling during pressing, the divided on whether the index becomes scale peelability rating and can be easily separated and removed by shot blasting or the like went. First, observe whether peeling occurs due bending after electrical heating, was evaluated the scale adhesion by the following criteria. In the present invention, when the result is "○○" or "○", it was decided to determine that excellent scale adhesion.
　○○: No peeling
　　○: 1 ~ 5 one release strip falling
　　×: 6 ~ 20 pieces of release strip falling
　××: 21 or more release strip falling
[0107]
　Then, except for the sample became "××" in evaluation of the scale adhesion above for further bending point subjected to processing, was subjected to a tape peeling test of attaching and peeling the adhesive tape. Then, the scale is observed whether readily peeled adhered to the tape was evaluated for scale peelability on the following criteria. In the present invention, when the result is "○○" or "○", it was decided to determine that excellent scale peelability. When excellent in both of the scale adhesion and scale peeling resistance was the scale characteristics during hot forming is excellent.
　○○: all peeling
　　○: 1 ~ 5 one release strip remains
　　×: 6 ~ 20 pieces of release strip remains
　××: 21 or more release strip remains
[0108]
　Tensile test, the results of Charpy impact tests and scale characteristic evaluation test shown in Table 4. In Table 4, Ac of the steel plate 3 are also shown a point and Ms point.
[0109]
[Table 4]

[0110]
　Referring to Tables 1-4, fulfill all chemical composition and the tissue as defined in the present invention steel No. Test using a 1 ~ 10 No. In 1-11, with excellent in scale characteristic, 40 J / cm 2 resulted in excellent toughness have more impact value. Of these Mn value and cleanliness than 1.6 of segregation ratio α is less than 0.10% Test No. In ~ 9 1, 3, 50 J / cm 2 have more impact value, particularly resulted in excellent toughness.
[0111]
　On the other hand, the steel does not satisfy the chemical composition of the present invention No. 11 to 13 test using a No. In 12-14, since the maximum value of the height roughness Rz is less than 3.0 [mu] m, scale adhesion was poor. In addition, steel No. Test Using the 14 and 16 No. In 15 and 17, scarfing amount in pickling process after hot rolling is due to insufficient, since the maximum value of the height roughness Rz exceeds 10.0 [mu] m, the scale peelability poor Met. In addition, the steel No. Test using a 15 No. In 16, due to scarfing amount was excessive in pickling process after hot rolling, since the maximum value of the height roughness Rz is less than 3.0 [mu] m, scale adhesion was poor It was.
[0112]
　Steel No. Test using the 17 and 18 No. In 18 and 19, the time from rough rolling in the hot rolling process is completed until the start of finish rolling exceeds 10s. In addition, steel No. Test using a 19 No. In 20, lower than the range Si content defined in the present invention, also the coiling temperature was high. Due to these, test No. 18 In - 20, in addition to the maximum value of the height roughness Rz is less than 3.0 [mu] m, carbides number density of 8.0 × 10 3 cells / mm 2 for exceeded, scale adhesion is bad There, and impact value 40 J / cm 2 becomes less than the desired toughness is not obtained.
Industrial Applicability
[0113]
　According to the present invention, it is possible to obtain a heat-treated steel plate having excellent scale characteristic at the time of hot forming. Then, the heat treatment for the steel sheet of the present invention, by a heat treatment or hot forming process, it becomes possible to obtain a heat-treated steel having excellent toughness and having a tensile strength of at least 1.4 GPa.
The scope of the claims

[Claim 1]
　Chemical composition of the steel sheet contains, by
　mass%,
　C: 0.05 ~
　0.50%, Si: 0.50 ~ 5.0%, Mn: 1.5
　~ 4.0%, P: 0.05% or less ,
　S: 0.05% or
　less, N: 0.01% or
　less,
　Ti: 0.01 ~ 0.10%, B: 0.0005
　~ 0.010%, Cr: 0 ~ 1.0%,
　Ni:
　~
　2.0%
　0,
　~ 1.0%, V: 0 ~ 1.0%, Ca: 0
　~ 0.01%, Al: 0 ~ 1.0
　%, Nb:
　0 ~ 1.0%, REM: 0 ~ 0.1%,
　the balance is Fe and impurities,
　the maximum height roughness Rz of the surface of the steel sheet is 3.0 ~ 10.0 [mu] m,
　circle equivalent diameter number density of more carbide 0.1μm is 8.0 × 10 existing in the steel sheet 3 pieces / mm 2 or less,
　the heat treatment for steel plates.
[Claim 2]
　The chemical composition, in
　mass%,
　Cr: 0.01 ~
　1.0%, Ni: 0.1 ~ 2.0%, Cu: 0.1
　~ 1.0%, Mo: 0.1 ~ 1.
　% 0,
　V:
　0.1 ~ 1.0%, Ca: 0.001 ~ 0.01%,
　Al: 0.01 ~ 1.0% Nb: 0.01 ~ 1.0%, and
　REM: 0 .001 to 0.1 percent,
　contains one or more selected from,
　for heat treatment steel sheet according to claim 1.
[Claim 3]
　Mn segregation ratio α represented by the following formula (i) is 1.6 or less,
　the heat treatment for steel sheet according to claim 1 or claim 2.
　alpha = [Maximum Mn concentration (mass%) at the center of plate thickness] / [average Mn concentration (mass%) of 1/4 depth position of the sheet thickness from the surface] · · · (i)
[Claim 4]
　The value of the cleanliness of the steel defined by JIS G 0555 (2003) is less than 0.10%,
　the heat treatment for steel sheet according to any one of claims 1 to 3.