Technical field
[0001]The present invention relates to a steel sheet and a manufacturing method thereof, particularly, automotive parts and infrastructure structural members whose primary purpose is related high-strength steel sheet having excellent bending resistance.
BACKGROUND
[0002]Underbody parts and skeleton parts of automobiles, and the pillar members and the like of the steel structure obtained to ensure the bending resistance. Bending resistance of the parts or members is mainly changes the strength of the material, the thickness of the steel sheet to be applied to parts and members, in particular, as the strength and plate thickness is large, it can ensure a large bending resistance. Incidentally in recent years, in the field of automobiles and steel structure, is being promoted weight parts and members, Accordingly, the application of a small steel plate thickness is increasing. Therefore, there is an increasing demand for steel sheets having a high bending resistance, even if the plate thickness small.
[0003]Previously, many proposed techniques for improving the bending resistance of the steel parts and members have been made (for example, Patent Documents 1-5, reference).
[0004]For example, Patent Document 1, by optimizing the shape of the spike fitting, to have a conventional 1.4 comparable with spikes fitting using ~ 2.0 mm thick steel or more flexural stress and resistance to torsional moments while maintaining the biting at injection nail to the ground during movement preventing plastic deformation and the falling off of the substrate between the rising portion and the through-hole of the nail, spike fitting in baseball shoes disclosed It is.
[0005]Further, Patent Document 2, in mass%, C: 0.10 ~ 0.35%, Si: 0.40 ~ 1.50%, Mn: 0.10 ~ 1.50%, P: 0.030% hereinafter, S: 0.030% or less, Cr: 0.50 ~ 3.0%, Al: 0.020 ~ 0.200%, N: contains 0.01 to 0.03 percent, and the balance Fe and unavoidable impurities consists, the value content of solute Al obtained from the content of content and N is subtracted 27/14 from the mass% of the mass% of N Al of Al satisfies the 0.020% or more, quenching - an incomplete quenched structure of the core of the product after tempering is suppressed, resistance to pitting strength, flexural fatigue strength, resistance to torsional fatigue strength steel excellent is disclosed.
[0006]Further, Patent Document 3, C: 0.50 ~ 0.70%, Si: 1.2 ~ 2.5%, Mn: 0.4 ~ 1.0%, P: 0.02% or less (0% the not including), S: 0.03% or less (not including 0%), Cr: 0.5 ~ 2%, Al: 0.10% or less (not including 0%), N: 0.03% below (exclusive of 0%) respectively contain, steel balance of iron and unavoidable impurities. The steel disclosed in Patent Document 3, even only by quenching and tempering or carburizing quenching and tempering treatment, it is possible to exhibit excellent flexural fatigue resistance, bending resistance and indentation resistance, pinion axis steel optimal gear as the material of the shaft member of the shaft member or the like, and having a gear for shaft bending resistance obtained from such steel.
[0007]Patent Document 4, the surface of the steel sheet, Al: 50 ~ 60 wt%, and the plating layer and the balance substantially Zn, in painted steel sheet having a coating film on an upper layer than the plating layer, the base material section Hardness H M and (HV), cross-sectional hardness H of the plating layer P (HV) has the formula (1) and satisfies the equation (2), preferably a further flexural return of which is adjusted so as to satisfy the equation (3) excellent Zn-Al-based plated painted steel bending resistance is disclosed. H M > H P ... (1), H P ≧ 90 ... (2), H M ≦ 145 ... (3)
[0008]Patent Document 5, C: 0.05 ~ 0.5% ( mass%, hereinafter the same), Si: 3% or less (not including 0%), Mn: 2.5% or less (exclusive of 0% ), Cr: 2.5 using steel containing ~ 15%, as the heat treatment process, carburizing, carbonitriding or after carbonitriding, immediately or once a 1 transformation point again cooled below a 1 by heating to a temperature of 1100 ° C. or less than the transformation point by performing the decarburization, high strength component manufacturing method of an average particle size of carbides in the cross section within 0.1mm and 5μm or less from the surface is disclosed .
[0009]Patent Document 6, located on the surface layer and the backing layer C: 0.05 ~ 0.2 wt%, Mn: a steel A containing 0.5 ~ 3%, C located on the surface and back layers: 0.01 wt% or less, Mn: is disclosed galvanized steel sheet and a steel B containing 0.5 wt% or less. The galvanized steel sheet disclosed in Patent Document 6, by providing a steel A high strength disposed on both sides of the inner layer and the steel B of the inner layer of soft, dent resistance and resistance to surface strain resistance and processing while securing the sex, it is characterized by securing corrosion resistance by galvanizing.
[0010]Patent Document 7 discloses, in weight%, C: 0.15 ~ 0.45%, Mn: a steel containing 0.2 ~ 2.5%, Hv180 or less hardness and spheroidizing annealing after hot working and it was followed by the production method of the steel for soft-nitriding to cold working and hardness Hv250 or more is disclosed. Further, Patent Document 7, or the surface hardness after nitrocarburizing is Hv 600, and is effective hardening depth is disclosed that it is possible to produce a more soft-nitrided parts 0.1 mm.
CITATION
Patent Document
[0011]
Patent Document 1: JP 2013-198708 Patent Publication
Patent Document 2: JP 2012-132077 Patent Publication
Patent Document 3: JP 2010-242130 Patent Publication
Patent Document 4: JP 2008-156729 Patent Publication
Patent Document 5: JP 2000-129347 JP
Patent Document 6: JP-A 6-287686 Patent Publication
Patent Document 7: JP-A 9-256045 JP
Non-patent literature
[0012]
Non-Patent Document 1:. Masayoshi KUMAGAI, et al, ISIJ International, Vol.54 (2014), No.1, PP.206-201
Summary of the Invention
Problems that the Invention is to Solve
[0013]
　Incidentally, both the prior art described above, by devising the shape of the part, or a technique for improving bending resistance by the surface layer of the modification by heat treatment. On the other hand, these techniques, when changing the shape of the part is difficult, or there is a problem in application to the case where unacceptable changes in the steel part of the size and shape during the heat treatment.
[0014]
　Further, any of the above prior art does not adequately meet the demand for steel sheets having a high bending resistance, even if the plate thickness small. In conventional clad plate, a surface layer hardness and bending resistance are in a trade-off relationship, there is a technical problem that when improving the surface hardness bending resistance is deteriorated. The above prior art, such a technical problem not solved thoroughly, average micro Vickers hardness does not disclose a clad steel plate excellent in bending resistance while have a surface layer of more than 400 HV.
Means for Solving the Problems
[0015]
　The present inventors have intensively studied a technique for solving the above problems. As a result, on one or both surfaces of the steel sheet, the average micro Vickers hardness is to form a hard layer of less than and 800 HV 400 HV, less than 50HV than the hardness of the inner layer of an average micro Vickers hardness of the 350HV or more and a hard layer further, the inner layer of the screw dislocation density × 10 2.0 13 m / m 3 by controlling the above was found that to ensure the bending resistance at a high level.
[0016]
　Non-Patent Document 1, and the stress relief heat treatment of 5 hours at 680 ° C. to S45C carbon steel, the carbon steel after the heat treatment by performing a rolling of 40% cold, dislocation density 7.0 × 10 13 m / m 3 ~ 2.0 × 10 15 m / m 3 increases, have reported that strength, hardness and tensile strength were increased.
[0017]
　However, the production method of the steel sheet satisfying the hardness and bending resistance are merely be difficult to manufacture to devise such as hot rolling conditions and annealing conditions in a single, so-called integrated process, such as hot rolling, annealing step that only not be produced to achieve the optimization Te also to findings by stacking various studies, thereby completing the present invention.
[0018]
　The gist of the present invention is as follows.
[0019]
(1) A steel sheet having an inner layer, a hard layer formed on one or both surfaces of the inner layer, a
　content of C in the hard layer is higher than the content of C in the inner layer, and , the content of Mn in the hard layer is higher than the content of Mn in the inner layer,
　the thickness of the hard layer is 20μm or more, the total thickness of the hard layer has a 2/5 or less of Zen'itaAtsu,
　the the average micro Vickers hardness of the hard layer is 400HV or more and less than 800 HV,
　the average micro Vickers hardness of the inner layer 350HV or more, less than 50HV than the hardness of the hard layer,
　the screw dislocation density of the inner layer is 2.0 × 10 13 m / m 3 , characterized in that at least, the steel sheet.
(2) the hard layer and the inner layer is a
　mass%,
　C: 0.10 ~
　0.60%, Si: 0.01 ~ 3.00%, Mn: 1.000 ~ 10.00%, containing ,
　P: 0.0200% or
　less, S: 0.0200% or
　less, N: 0.0200% or
　less, O: 0.0200% or less, the limits,
　the balance being Fe and impurities, steel sheet according to (1).
(3) wherein at least one of the hard layer or the inner layer further contains, by
　mass%, Al: 0.500% or
　less, Cr: 2.000% or
　less, Mo: 1.000% or
　less, Ti: 0. 500% or
　less, B: 0.0100% or
　less, Nb: 0.500% or
　less, V: 0.500% or
　less, Cu: 0.500% or
　less, W: 0.100% or less,
　Ta: 0.100%
　hereinafter, Ni: 0.500% or
　less, Sn: 0.050% or
　less, Sb: 0.050% or
　less, As: 0.050% or
　less, Mg: 0.0500% or
　less, Ca: 0.050% or less,
　Y: 0.050% or
　less, Zr: 0.050% or
　less, La: 0.050% or
　less, Ce: 0.050% or less,
characterized in that it contains one or more, (2 steel sheet excellent in bending resistance as described in).
The invention's effect
[0020]
　Steel sheet of the present invention, despite the very hard hard layer is formed on the surface layer, excellent bending resistance. That is, according to the present invention, one or both surfaces of the steel sheet, the average micro Vickers hardness is to form a hard layer of less than and 800 HV 400 HV, the hardness of the inner layer of an average micro Vickers 350HV than the hardness and the hard layer smaller than 50HV than, yet, the lining of the screw dislocation density × 10 2.0 13 m / m 3 by controlling the above, it is possible to provide a steel sheet and a manufacturing method thereof excellent in bending resistance.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021]
Is a graph showing the relationship between the hardness difference and the screw dislocation density in FIG. 1 the surface layer (hard layer) and the central layer (inner layer).
[Figure 2] is suitable for cold rolling before hot-rolled sheet annealing, a diagram illustrating the annealing pattern of box annealing.
[3] suitable for cold rolling before hot-rolled sheet annealing and cold-rolled sheet annealing after cold rolling is a diagram showing an annealing pattern of continuous annealing.
[Figure 4] is suitable for cold-rolled sheet annealing after cold rolling is a diagram showing an annealing pattern of reheat cold rolled sheet annealing.
By applying skin pass rolling in FIG. 5 invention steel is a diagram showing that the residual compressive stress in the inner layer is produced.
DESCRIPTION OF THE INVENTION
[0022]
　Steel sheet of the present invention comprises a hard layer made of an average micro Vickers low hardness inner layer and than the hard layer is a steel sheet of two-layer or three-layer structure comprising the hard layer on at least one surface.
[0023]
　First, the hard layer and the thickness of the inner layer, the reason for limiting the average micro Vickers hardness is described.
[0024]
[Hard layer and the thickness of the inner layer]
　The thickness of the hard layer present on one or both surfaces of the inner layer and more per side 20 [mu] m, and 2/5 or less of Zen'itaAtsu. When the thickness of the hard layer is less than 20 [mu] m, small thickness of the hard layer, upon the addition of a bending moment in the steel sheet, easily lead to peeling of the hard layer, it is not obtained excellent bending resistance. For example, even when having a hard layer more than 20 [mu] m in one side of the inner layer, the structure having a hard layer of less than 20 [mu] m on the other surface of the inner layer, upon the addition of a bending moment in the steel plate, 20 [mu] m there is a possibility that the hard layer less than peels. For this reason, the thickness of the hard layer, the inner layer of the single-sided per 20μm or more.
[0025]
　Also, if the total of each of the hard layer thickness formed on one or both surfaces of the inner layer is more than 2/5 of Zen'itaAtsu, stress on the hard layer is increased at the time of bending, cracks from the hard layer since bending resistance is significantly degraded Te, it can not be obtained the advantage of bending resistance improvement by multi-layering. Therefore, the total thickness of each hard layer and 2/5 or less of Zen'itaAtsu. More preferably, the thickness of each hard layer 30μm or more, the sum of the thickness is 3/10 or less of Zen'itaAtsu.
[0026]
　Method of measuring the hard layer and the thickness of the inner layer is not particularly limited as long as can be measured accurately, for example, it may be measured by an optical microscope. When measuring the hard layer and the thickness of the inner layer by using an optical microscope, it is preferable to measure in the next steps.
[0027]
　First, the embedded sample as a measurement target in a cylindrical epoxy resin having a diameter of 30 mm, subjected to rough polishing by wet grinding using abrasive paper # 80-1000, subsequently, an average particle size of 3μm and 1μm performing polished to a mirror-like using a diamond abrasive having. The polishing of diamond particles 1μm is a load of 1N ~ 10 N In addition, on the grinding stand which rotates at a speed of 30 ~ 120mpm, performed under the conditions held for 30 to 600 seconds. The hard layer and the inner layer, since there is a difference in hardness, in the polishing of diamond particles above 1 [mu] m, a difference in polishing amount occurs.
[0028]
　Thus, now a small step on the hard layer and the inner layer of the boundary caused by observation using an optical microscope, determining the hard layer and the inner layer of the boundary, the ratio thickness of each layer and the thickness more precisely it is possible. Incidentally, if a step that is provided by the finish polishing minute, observed by differential interference optical microscope is preferred.
[0029]
[Hard layer and an average micro Vickers hardness of the inner layer]
　Average micro Vickers hardness of the hard layer 400HV or more and less than 800 HV. If the average micro Vickers hardness of less than 400 HV, low hardness of the hard layer, is not obtained excellent bending resistance. Therefore, an average micro Vickers hardness of the hard layer is not less than 400 HV. On the other hand, when the average micro Vickers hardness of the hard layer is more than 800 HV, since the hard layer is excessively high strength, since the hard layer is destroyed brittle upon the addition of a bending moment, bending resistance is significantly deteriorated to. Therefore, an average micro Vickers hardness of the hard layer is less than 800 HV. More preferably, more than 450 HV, or less 780HV.
[0030]
　The average micro Vickers hardness of the inner layer 350HV or more, and smaller than 50HV than the hardness of the hard layer. If the average micro Vickers hardness of less than 350 HV, since the difference in hardness of the hard layer is increased, become distorted biased to excessively inner side when adding the bending moment, the inner layer is easily deformed, It leads to a decrease in bending resistance. Therefore, the average micro Vickers hardness of the inner layer be at least 350 HV. Also, when the average micro Vickers hardness exceeds 50HV less than the hardness of the hard layer, since the difference between the hard layer and the inner layer of the hardness is decreased, upon the addition of a bending moment, the elastic strain in the hard layer from the fact that such a large, bending resistance is degraded. Therefore, the average micro Vickers hardness of the inner layer and smaller than 50HV than the hardness of the hard layer. More preferably, more than 400 HV, which is smaller than 100HV than the hard layer.
[0031]
　The average micro Vickers hardness of the steel sheet, a micro-Vickers hardness of the load 0.098N at each thickness 1/4 position from the hard layer and the inner layer of the surface, were measured 12 points, most hard data, and most soft determined from the average value of 10 points excluding the data. Measurement of Micro Vickers hardness, JIS Z 2244: conforms to defined Vickers hardness test performed on 2009. Note that the diagonal length of about 7μm indentations in the case of 400HV the load 0.098 N, the length of the case of 800HV is about 5 [mu] m, can be suitably evaluated hardness of the hard layer having a thickness of about 20μm it is.
[0032]
　The standard deviation of nano-hardness of the hard layer is 2.0 or less. This can be achieved by suppressing the variation in nano-hardness of the hard layer is because the bending resistance is significantly improved. If the standard deviation exceeds 2.0, since the variation in the hardness of the hard layer is increased, for example, cracking in the hard layer may occur when applying the bending moment. From this point of view, the standard deviation is preferably 2.0 or less, more preferably 1.6 or less. The lower limit of the standard deviation is not specified, but be suppressed to 0.2 or less is technically difficult.
[0033]
　Note that variations in the thickness direction of the nano-hardness of the hard layer does not affect the bending resistance, even if had an inclination of hardness in the sheet thickness direction, without impairing the effects of the present invention. In effect, if the variation of the nano-hardness in the width direction of the steel sheet is large, i.e., when the variation in nano-hardness in a line perpendicular to the thickness direction and the rolling direction is large, bending resistance decreases . Therefore, the standard deviation of nano-hardness, it is necessary to obtain the plate thickness direction and the rolling direction of a sheet thickness cross-section from the hardness at a line perpendicular. However, the initial position of the measurement of nano-hardness can be in the right and either direction of the left starting from the center position in the width direction of the steel sheet.
[0034]
　In the present invention, the "standard deviation of nano-hardness of the hard layer", 1/4 position in the thickness from the surface side of the hard layer, on a line perpendicular to the thickness direction and the rolling direction of the plate thickness cross section nano hardness of 100 points at intervals of 3 [mu] m, measured under the conditions of the indentation depth 80 nm, a histogram from the obtained hardness data, the standard deviation of the fitting curve when approximating the histogram in a normal distribution it is intended to say.
[0035]
[Lining of screw dislocation density]
　screw dislocation density of the inner layer is Cu or, using a small portion X-ray diffractometer equipped with Co tube, measured by modified-Williamson-Hall / Warren- Averbach method. Samples subjected to X-ray diffraction may be used samples used for thickness measurement by the above-mentioned optical microscope. Before subjected to X-ray diffraction, the mirror-polished surface, chemical etching or subjected to electrolytic polishing to remove the distortion introduced by mechanical polishing. Measurement surface is a plane parallel to the rolling direction and thickness direction, irradiation range of X-rays to adjust the X diameter to fit within the thickness of the inner layer by a collimator or the like. In the case of using the X-ray source of Cu tube was placed a monochromator or the like, to suppress as much as possible detection of the fluorescent X-ray. Procedure according to modified-Williamson-Hall / Warren- Averbach method, for example, T. Ungar and A. Borbely: Appl. Phys. Lett. , 69 (1996), 3173. Alternatively, Masayoshi Kumagai, Muneyuki Imafuku, Shin -ichi Ohya: ISIJ International, Vol. 54 (2014) No. 1, p. 206-211 a may be helpful.
[0036]
　Screw dislocation density of the inner layer is × 10 2.0 13 m / m 3 and more. Screw dislocation cause cross slip, it increases the yield strength of the steel from its entanglement. That is, an increase in the screw dislocation density in the inner layer, because it increases the load required to the occurrence of plastic deformation called "bending", is operational effect of increasing the bending resistance. Screw dislocation density of × 10 2.0 13 m / m 3 by weight, because the effect of improving the bending resistance is not obtained, the lower limit × 10 2.0 13 m / m 3 and more. Incidentally, although the screw dislocation density is preferably as high as possible, 1 × 10 17 m / m 3 when it exceeds voids are generated in the site of the entanglement of the dislocations leads to brittle fracture of the steel sheet. Therefore, the upper limit × 10 1 17 m / m 3 is preferred. Therefore, the lower limit of the inner layer of the screw dislocation density × 10 2.0 13 m / m 3 and more. More preferably × 10 5 13 m / m 3 at least.
[0037]
[C, C in the content and the inner layer of Mn, the relationship between the content of Mn in the hard layer]
　and the above-mentioned hard layer and an inner layer of an average micro Vickers hardness requirements, in addition to the requirements of the inner layer of the screw dislocation density, rigid C C in the layer, the content of Mn in the inner layer, to be more than Mn is essential in order to obtain the effect of the present invention. C, Mn are both affecting element micro yield of steel, has the effect of suppressing the occurrence of micro yield with increasing content. The micro-yield, for example, strain macroscopically during the tensile test are considered elastically deformed - in the area of stress, a breakdown phenomenon that occurs in the crystal grains in internal material. Micro yield is essentially dislocation generated from the crystal grain inside or grain boundaries, resulting in propagating to adjacent crystal grains. Yield that occurs in the crystal grain units, that is, micro breakdown phenomenon to propagate throughout the thickness plate is a macroscopic breakdown phenomenon observed in a tensile test. C is by segregating at grain boundaries, dislocations into adjacent particle propagation, i.e., the effect of suppressing the occurrence of micro yield. Further, Mn is considered to be effective to promote cross slip dislocations moving in grain, by suppressing the deposition of dislocations into the crystal grain boundaries at the time of giving an external force, dislocation of the adjacent grains propagation, i.e. , has the effect of suppressing the generation of micro surrender. To ensure the bending resistance, it is necessary to suppress the occurrence of micro breakdown in the hard layer, for this purpose, the strain relaxation when an external force is applied to the multilayered steel, the inner layer of the micro yield there is a need to supplement with. Therefore, to suppress the occurrence of micro breakdown of the hard layer, to facilitate the generation of the inner layer of the micro-yield, C in the hard layer, the content of each of Mn, increasing C, than the respective content of Mn in the inner layer There is a need.
[0038]
　It will be described optimum chemical components in the hard layer and the inner layer of the steel sheet. Hereinafter, "%" for component means mass%.
[0039]
(C: 0.10 ~ 0.60%)
　C is an effective element for strengthening steel. To ensure the bending resistance of the part is required C amount of 0.10% or more, is less than 0.10% is insufficient hardness of the material, it is impossible to obtain excellent wear bendability. Therefore, the lower limit of 0.10% or more. On the other hand, when it exceeds 0.60% encourage the formation of carbides in the material, the upper limit is 0.40% or less because it causes brittle cracks starting from the carbides upon the addition of a bending moment. More preferably 0.15% or more, or less 0.59%.
[0040]
(Si: 0.01 ~
　3.00%) Si acts as a deoxidizer that affect element intensities increased carbide morphology control and steel. Is less than 0.01%, generation of carbides is promoted, a large amount of carbide become present in the steel, bending resistance is deteriorated. Incidentally, suppressing the Si on the content of less than 0.01%, in the current refining processes also result in increased costs. Therefore, the lower limit of Si is 0.01% or more. On the other hand, when the content of Si exceeds 3.0%, lead to embrittlement of the steel components, to reduce the bending resistance, and 3.0% the upper limit. Preferably 0.01% or more, 2.5% or less. More preferably, 0.2% or more, 2.0% or less.
[0041]
(Mn: 1.00 ~
　10.00%) Mn acts as a deoxidizer and also is an element effective in suppressing the steel pearlite transformation. If it is less than 1.00%, in the course of cooling from the austenite region, it becomes difficult to suppress pearlite transformation, with this, because the tissue ratio of martensite is reduced, causing a reduction and bending resistance of degradation of the intensity . Therefore, the lower limit of 1.0% or more. On the other hand, when it exceeds 10.00%, coarse Mn oxide becomes to be present in the steel, to become starting points of fracture at the time of bending, bending resistance is deteriorated. Therefore, to 10.00% the upper limit. Preferably 2.0% or more, or less 9.0%.
[0042]
(P: 0.0001 ～ 0.0200 Pasento)
　P is an element to promote the embrittlement of strongly segregated grain boundaries in the ferrite grain boundaries. Preferably as small, in order to highly purified to less than 0.0001% in the refining step, the number of time required for the refining, leads to a significant increase in cost. Therefore, the lower limit may be 0.0001% or more. On the other hand, if it exceeds 0.0200%, the lowering the bending resistance by grain boundary embrittlement, the upper limit or less 0.0200%. Preferably 0.0010% or more, or less 0.0190%.
[0043]
(S: 0.0001 ~ 0.0200%)
　S generates a non-metallic inclusions of MnS, etc. in steel, it is an element deteriorating the ductility of the steel sheet part. Preferably as small, in order to highly purified to less than 0.0001% in the refining step, the number of time required for the refining, leads to a significant increase in cost. Therefore, the lower limit may be 0.0001% or more. On the other hand, if it exceeds 0.0200% leads to cracking of occurrence starting from the non-metallic inclusions upon the addition of a bending moment, since the bending resistance is lowered, the upper limit or less 0.0200%. Preferably 0.0010% or more, or less 0.0190%.
[0044]
(N: 0.0200% or less)
　N is an element effective in strengthening steel as with C. From the viewpoint of ensuring the bending resistance, preferably as less content may be 0%. However, reducing to less than 0.0001% is because it causes an increase in the refining costs, and 0.0001% or more of the lower limit if it contains a N. On the other hand, in the content of more than 0.0200 percent, from causing embrittlement of steel, bending resistance is significantly reduced. Therefore, to 0.0200% the upper limit. Preferably 0.0010% or more, or less 0.0150%.
[0045]
(O: 0.0200% or less)
　O is an element conducive to the formation of oxides in the steel, oxides present in the ferrite grains is preferably lesser for the voiding site, a 0% it may be. However, it is reduced to less than 0.0001%, because it causes an increase in the refining costs, when O is contained in the lower limit of 0.0001% or more. On the other hand, in the content of more than 0.0200% to reduce the bending resistance, the upper limit or less 0.0200%. Preferably 0.0005% or more, or less 0.0170%.
[0046]
(Al: 0.500% or less)
　Al is an element to stabilize the ferrite acts as a deoxidizer of steel, it is added as required. When adding Al, it is less than 0.001%, because the addition effect is not sufficiently obtained, the lower limit is 0.001% or more. On the other hand, if it exceeds 0.500% produces coarse Al oxides, cause a reduction in bending resistance. Therefore, the upper limit 0.500% or less. Preferably less 0.450% 0.010% or more.
[0047]
(Ti: 0.500% or less)
　Ti is an important element to form the control of carbides is an element to promote increased strength of ferrite by the inclusion of a large amount. From the viewpoint of ensuring the bending resistance, preferably as less content may be 0%. However, reducing to less than 0.001% is because it causes an increase in the refining costs, and less than 0.001% the lower limit if it contains a Ti. On the other hand, in the content of more than 0.500% coarse Ti oxide or TiN is to be present in the steel, reduce the bending resistance. Therefore, the upper limit 0.500% or less. Preferably 0.005% or more, or less 0.450%.
[0048]
(B: 0.0100% or less)
　B is suppressed the formation of ferrite and pearlite in the course of cooling from the austenite, which is an element to promote the formation of low temperature transformation structure such as bainite or martensite. Further, B is an informative element increasing the strength of steel, is added as required. When adding B, and less than 0.0001%, the effect of improving the high strength or bending resistance can not be sufficiently obtained by the addition. Furthermore, along with the identification of less than 0.0001% is necessary to pay close attention to the analysis, it leads to the limit of detection by the analyzer. Therefore, the lower limit of 0.0001% or more. On the other hand, in the content of more than 0.0100% leads to formation of coarse B oxides in the steel, bending resistance is deteriorated. Therefore, the upper limit to 0.0100% or less. More preferably 0.0005% or more and 0.0050% or less.
[0049]
(Cr: 2.000% or
　less) Cr is suppressed pearlite transformation as with Mn, is an element effective for high strength steel, it is added as required. When adding Cr, is less than 0.001%, because it does not obtain the effect of the addition, the lower limit is 0.001% or more. On the other hand, the addition of more than 2.000%, the to form a coarse Cr carbide in the center segregation area, to reduce the bending resistance, the upper limit is 2.000% or less. Preferably 0.010% or more, or less 1.500%.
[0050]
(Mo: 1.000% or
　less) Mo is, Mn, is an element effective to strengthen Like the Cr steel, is added as required. When adding Mo, it is less than 0.001%, the effect is not obtained, the lower limit is 0.001% or more. On the other hand, if it exceeds 1.000% forms coarse Mo carbides, and 1.000% or less the upper limit for lowering the bending resistance. More preferably 0.010% or more, or less 0.700%.
[0051]
(Nb: 0.500% or less)
　Nb is an element effective for morphology control of carbide as well as Ti, is an effective element in improving the toughness for refining the tissue by its addition, needs It is added in response. When adding Nb, it is less than 0.001%, the effect is not obtained, the lower limit is 0.001% or more. On the other hand, if it exceeds 0.500%, the precipitation many fine and hard Nb carbide, lead to significant degradation in ductility with increasing strength of the steel sheet, reducing the bending resistance. Therefore, the upper limit 0.500% or less. Preferably 0.002% or more, or less 0.200%.
[0052]
(V: 0.500% or less)
　V, like the Nb, an element effective to form the control of carbides is effective element in improving the toughness for refining the tissue by the addition, necessary It is added in accordance with the. When adding V, it is less than 0.001%, the effect is not obtained, the lower limit is 0.001% or more. On the other hand, if it exceeds 0.500%, the precipitation many fine V carbide, leading to increase in strength and reduction in the ductility of the steel sheet, reducing the bending resistance. Therefore, the upper limit 0.500% or less. More preferably 0.002% or more, or less 0.400%.
[0053]
(Cu: 0.500% or
　less) Cu is an effective element to increase the strength of the steel sheet, it is added as required. When adding Cu, the content of 0.001% or more in order to effectively exhibit the effect of increasing the strength is preferable. On the other hand, if it exceeds 0.500% upper limit for lowering the productivity of the invitation hot rolling the red shortness is 0.500% or less. More preferably 0.002% or more, or less 0.400%.
[0054]
(W: 0.100% or less)
　W also, Nb, similarly to V, an effective element to increase the strength of the carbide in the form control and steel, are added as needed. When adding W, or less than 0.001%, the effect is not obtained, the lower limit is 0.001% or more. On the other hand, if it exceeds 0.100%, the precipitation many fine W carbides, cause a decrease in strength increase and ductility of the steel sheet, the upper limit to reduce the bending resistance is 0.100% or less. More preferably 0.002% or more and 0.100% or less.
[0055]
(Ta: 0.100% or
　less) Ta also, Nb, V, similarly to the W, an effective element to increase the form control and intensity of carbide, is added as required. When the addition of Ta, is less than 0.001%, the effect is not obtained, the lower limit is 0.001% or more. On the other hand, if it exceeds 0.100%, the precipitation many fine Ta carbides leads to increase in strength and reduction in the ductility of the steel sheet, reducing the bending resistance. Therefore, the upper limit 0.100% or less. More preferably 0.002% or more and 0.100% or less.
[0056]
(Ni: 0.500% or
　less) Ni is an element effective in improving the bending resistance of the part, it is added as needed. When adding Ni, in order to effectively exhibit its effect preferably contains a least 0.001%. On the other hand, if it exceeds 0.500% ductility is lowered, the upper limit is 0.500% or less for lowering the bending resistance. More preferably 0.002% or more, or less 0.400%.
[0057]
(Sn: 0.050% or
　less) Sn is an element contained in the steel when using scrap as a raw material, preferably as small, may be 0%. However, because it causes an increase in the refining cost in reducing to below 0.001%, 0.001% or lower limit if it contains a Sn. Further, in the content of more than 0.050% to cause a reduction in bending resistance due to embrittlement of ferrite, the upper limit 0.050% or less. More preferably 0.001% or more and 0.040% or less.
[0058]
(Sb: 0.050% or
　less) Sb is an element contained in the case of using scrap as a steel material similarly to Sn. Sb is because it causes embrittlement and decreased ductility strongly segregated grain boundaries in a grain boundary, preferably as small, may be 0%. However, because it causes an increase in the refining cost in reducing to below 0.001%, 0.001% or lower limit if it contains a Sb. Further, in the content of more than 0.050% to cause a reduction in bending resistance, the upper limit 0.050% or less. More preferably 0.001% or more and 0.040% or less.
[0059]
(As: 0.050% or
　less) As is, Sn, is contained in the case of using scrap as well as the steel material and Sb, an element which strongly segregate at grain boundaries, preferably as small, even 0% good. However, because it causes an increase in the refining cost in reducing to below 0.001%, 0.001% or lower limit if it contains a As. Further, in the content of more than 0.050% because it causes a decrease in bending resistance, the upper limit 0.050% or less. More preferably 0.001% or more and 0.040% or less.
[0060]
(Mg: 0.0500% or
　less) Mg is an element capable of controlling the form of sulfides dopants are added as needed. Case of adding Mg, the less than 0.0001%, the effect of lower order can not be obtained and 0.0001% or more. On the other hand, excessive addition, to cause a reduction in bending resistance due to the formation of coarse inclusions, and 0.0500% to the upper limit. More preferably 0.0005% or more, or less 0.0400%.
[0061]
(Ca: 0.050% or
　less) Ca is an element capable of controlling the form of sulfides Similarly dopants and Mg, is added as required. When adding Ca, the less than 0.001%, the effect of lower order can not be obtained is 0.001% or more. On the other hand, excessive addition generates coarse Ca oxides, for lowering the bending resistance, the upper limit 0.050%. More preferably 0.001% or more and 0.040% or less.
[0062]
(Y: 0.050% or less)
　Y is, Mg, an element capable of controlling the form of sulfides Similarly dopants and Ca, are added as needed. When adding Y, the less than 0.001%, the effect of lower order can not be obtained is 0.001% or more. On the other hand, excessive addition generates coarse Y oxides, since bending resistance is lowered, the upper limit 0.050%. Preferably 0.001% or more, 0.040% or less.
[0063]
(Zr: 0.050% or
　less) Zr is, Mg, Ca, an element capable of controlling the form of sulfides Similarly dopants and Y, it is added as needed. When adding Zr, the less than 0.001%, the effect of lower order can not be obtained is 0.001% or more. On the other hand, excessive addition generates coarse Zr oxides, since bending resistance is lowered, the upper limit 0.050%. Preferably 0.001% or more, 0.040% or less.
[0064]
(La: 0.050% or
　less) La is an effective element to form the control of sulfides dopants are added as needed. When adding La, the less than 0.001%, the effect of lower order can not be obtained is 0.001% or more. On the other hand, if adding over 0.050%, La oxides are produced, 0.050% of the upper limit for lowering the bending resistance. More preferably 0.001% or more and 0.040% or less.
[0065]
(Ce: 0.050% or
　less) Ce is an element capable of controlling the form of sulfides Similarly dopants and La, it is added as required. When adding Ce, in less than 0.001%, the effect of lower order can not be obtained is 0.001% or more. On the other hand, if adding over 0.050%, Ce oxide is generated, 0.050% of the upper limit for lowering the bending resistance. More preferably 0.001% or more, or less 0.046%.
[0066]
　In the hard layer and the inner layer of the steel sheet of the present invention, the balance other than the above components is Fe and inevitable impurities, other elements within the range not impairing the effects of the present invention may be contained in trace amounts.
[0067]
　Subsequently, describe the observation and measurement method of the steel sheet structure.
[0068]
　Structure observation is conducted with a scanning electron microscope. Prior to observation, etching the sample for microstructure observation was polished by diamond abrasive grains having an average particle size of wet grinding and 1μm by emery paper, after finishing the observation plane mirror, a tissue with 3% nitric acid alcohol solution keep. Observation magnification was 3000 times, at random shooting ten viewing of 30 [mu] m × 40 [mu] m in the hard layer and the inner layer each thickness 1/4 layer. The ratio of the tissue is determined by the point counting method. The obtained tissue image, defines lattice points arranged at intervals in the longitudinal 3μm and horizontal 4μm total 100 points, to determine the tissue underlying the grid points, tissue ratios included in steel from ten average the seek.
[0069]
　The volume percentage of carbide is performed based on the captured tissue image with a scanning electron microscope. Prior to observation, the sample for microstructure observation was polished by diamond abrasive grains having an average particle size of wet grinding and 1μm by emery paper, after finishing the observation plane mirror, etching the tissue with saturated picric acid alcohol solution keep. Observation magnification was 3000 times, randomly to eight photographs a view of 30 [mu] m × 40 [mu] m in the sheet thickness 1/4 layers. The resulting image analysis software represented by Mitani Corp. (Win ROOF) to the tissue image, measures the area of each carbide contained in that region in detail. It obtains the ratio of the total area of the carbide to the total area of the observation field, which is the volume percentage of carbide. Incidentally, in order to suppress the influence of a measurement error due to noise, the area is 0.01 [mu] m 2 is less carbide excluded from the evaluation.
[0070]
　In the steel sheet of the present invention, the volume percentage of carbide in the hard layer and the inner layer is preferably 2.0% or less. Carbide is the hardest tissue in the steel, even if the stress applied to the steel small 0.5 times the tensile strength, due to the difference in hardness relative to other tissues, the steel in contact with the carbide strain is concentrated in the matrix interface. Concentration of strain leads to plastic deformation, causing a reduction in bending resistance and fatigue resistance. Therefore, the more carbides are less preferably, 2.0 or less is preferable by volume. Still more preferably not more than 1.6%.
[0071]
　As described above, the steel sheet of the present invention, and the inner layer, the improved tensile strength of the steel by providing a hard layer, since yield point in improving the wake of the tensile strength is increased, excellent bending resistance having. Bending resistance can be confirmed by the following evaluation methods. That is, to produce a Schenk-type specimens from each sample, after sticking the strain gauge on the test piece surface in the state of stress unloaded, 10 in 0.5 times the applied stress of the tensile strength 3 bending repetitions plane giving, to measure the residual plastic strain after removing the test piece from the tester. According to the present invention, the residual plastic strain can be less than 0.1%.
[0072]
　Further, according to the present invention, it is possible to produce high strength steel sheet excellent in fatigue fracture characteristics. Such high-strength steel sheet, because bending resistance is more excellent, excellent in fatigue strength. Incidentally, fatigue resistance can be confirmed by the following evaluation methods. That is, to produce a Schenk-type specimens from each sample, after stuck a strain gauge on the test piece surface in the state of stress unloaded, 10 in 0.5 times the applied stress of the tensile strength 4 bending repetitions plane give. According to the present invention, 10 4 after repeated bending plane of times even does not occur fatigue failure (fracture) can make a material excellent in fatigue fracture properties. Steel sheet excellent in fatigue strength according to the present invention, at least, none of the hard layer and the inner layer of the steel plate constituting the steel sheet, it is necessary to satisfy the composition of the optimal chemical composition described above.
[0073]
　Such bending resistance excellent steel sheet, not only performs typical carburizing and nitriding, nitrocarburizing, high frequency surface hardening, etc. obtained as a surface modification treatment before. This is because, in the heat treatment process such as carburizing and nitriding, nitrocarburizing, remains until the formation of a hard layer on the surface layer, the inner layer of the screw dislocation density 2.0 × 10 13 m / m 3 , it is difficult to control, the This is because lowering the bending resistance.
[0074]
Manufacturing method of the steel sheet of the present invention]
　Next, a method for manufacturing the steel sheet of the present invention. The following description, there is intended to be illustrative of the characteristic methods for producing the steel sheet of the present invention, the steel sheet of the present invention obtained by pasting inner layer and a hard layer as described below It was not intended to be limited to multi-layer steel sheet. For example during the manufacture of steel sheets, surface treated steel sheet of a single layer, by curing the surface layer portion, it is also possible to produce a steel sheet having an inner layer and a hard layer according to the present invention. By curing the surface layer portion by surface treatment in this way during the production of steel sheet, by heat treatment distortion that occurs when subjected to a surface treatment after part molding can solve the problem of dimensional accuracy decreases.
[0075]
　Production method of the present invention, characterized in that on one or both surfaces of the inner layer, paste 50HV or harder rigid layer than the hardness of the inner layer, and to control the morphology and composition and organization of the inner layer of the dislocations in the optimal state It is set to. The heat as such a manufacturing method is not particularly limited, for example, the diffusion heat treatment process using a blank plate of sheet steel, each steel kind pasted slab of hard layer and the inner layer by electron beam welding using rolled, cold-rolled annealed, and processing of the plating and the like. Characteristic of a specific production method of the present invention is as follows.
[0076]
　Method for producing a multi-layer slab is not particularly limited, for example, may be either a continuous casting process or slabs welding. In the continuous casting method, a casting machine with two tundish, to produce initially an inner side of the cast body in the center, subsequently, to cover the inner side of the casting, the second Tan corresponding to the hard layer from the dish, injecting molten steel of a different component from the inner side, continuously solidified to obtain a slab of double layer state. Alternatively, the slab welding process, the slab was cast into a predetermined composition or the crude rolling stock produced from the slab, the bonding surface is polished by machine scarf, etc., followed by acid and alcohol cleaning, oxides or contamination that is obtained by removing the objects are stacked. Further joined by the stacking slab electron beam welding, to obtain a laminated slab for hot rolling.
[0077]
　More using slab made by process according to example, by producing a multilayered steel by the hot rolling step described below, it is possible to manufacture the steel sheet of the present invention.
[0078]
　Hot rolling characteristics: a multilayer slab described above, it is or once performed hot rolling after heating after cooling, 650 ° C. or higher, and ends the finish hot rolling at a temperature range of 950 ° C. or less. After cooling the steel sheet after finish rolling on a run out table (ROT) and Maki up hot-rolled coil at a temperature less than 700 ℃. Moreover, the hot rolled coil after state or pickled non pickling, providing an elongation of 0.1% or more temper rolling (also referred to as "skin pass rolling."). Hereinafter, the production method of the present invention will be described in detail.
[0079]
　The multilayer heating temperature during the hot rolling of the slab 1100 ° C. or higher, and 1300 ° C. or less, it is preferable to set the heating time than 300 minutes or more 15 minutes in the heating temperature range. Or heating temperature exceeds 1300 ° C., or when the heating time exceeds 300 minutes progressed significantly oxidized between the hard layer and the inner layer, since the hard layer and the inner layer is easily peeled off, a reduction in bending resistance there is a cause. As a more preferable upper limit, the heating temperature is 1250 ° C. or less, the heating time at this temperature range is less than 270 minutes. On the other hand, if the heating temperature during the hot rolling of the multilayered slab is less than 1100 ° C., or the heating time is shorter than 15 minutes, eliminating the hard layer and the inner layer of the solidification segregation becomes insufficient, and the hard layer there is a case in which the inner layer of the tensile strength becomes insufficient. Therefore, as a more preferable lower limit, the heating temperature is 1150 ° C. or higher, the heating time at this temperature range is 30 minutes or more.
[0080]
　Finish hot rolling 650 ° C. or higher, and be terminated at 950 ° C. or less. When finishing hot-rolling temperature is lower than 650 ° C., from the increase in deformation resistance of the steel sheet, it increased significantly the rolling load, further causes an increase of the roll abrasion loss, which may cause a reduction in productivity. For this reason the lower limit to 650 ° C. or higher. Further, when the finishing hot-rolling temperature exceeds 950 ° C., since the flaw due to thick scales of generating ROT during Tsuban occurs on the surface of the steel sheet, surface cracks due to the scale is generated, the bending resistance cause a decrease. Therefore, the upper limit is 950 ° C. or less. More preferably 800 ° C. or more and 930 ° C. or less. In order to make the standard deviation of nano-hardness of the hard layer to 2.0 or less, in addition to the temperature range of the finish rolling, it is more preferable that the optimum chemical components described above a steel rigid layer .
[0081]
　The cooling rate of the steel sheet in the ROT after the finish hot rolling is 10 ° C. / sec or more, it is preferable to 100 ° C. / sec or less. In cooling rate is less than 10 ° C. / sec is not possible to prevent the occurrence of flaws due to its generation of thick scale in the middle cooling, lowering the surface appearance. It is preferable that the order lower and 10 ° C. / sec or more. Further, when cooling the steel plate at a cooling rate exceeding 100 ° C. / sec over the inside from the surface layer of the steel sheet, the outermost layer causes a low-temperature transformation structure, such as over-cooled bainite or martensite. When paying out the coil which is cooled to room temperature after winding the microcracks are generated in the above-mentioned low temperature transformation structure, it is difficult to remove the cracks in the subsequent pickling step. These cracks become starting points of fracture, causing the fracture and reduction in productivity of the steel sheet on production. It is preferred that this order limit the 100 ° C. / sec or less. Point The cooling rate prescribed above is steel sheet after the finish hot rolling is from the time (injection starting point) for receiving the water cooled in the water injection section after passing through the non-water injection section, which is cooled on ROT to the target temperature of the take-up points to a cooling power received from the cooling equipment of the injection interval in, does not indicate the mean cooling rate to a temperature to be wound by a winding machine from injection start point. More preferably 20 ° C. / sec or more and 90 ° C. / sec or less.
[0082]
　The winding temperature is set to 700 ℃ or less. If the coiling temperature exceeds 700 ° C., in addition to the inability to secure a predetermined strength to the hard layer and the inner layer after hot rolling, leads to generation of a large amount of carbides, causing a reduction in bending resistance. Therefore, the upper limit of the coiling temperature is set to 700 ° C. or less. Further, for strength increase in the hard layer, but the coiling temperature is preferably as low, if subjected to low temperature cryogenic treatment than room temperature (sub-zero treatment), there are cases where hot rolled sheet is considerably brittle, take-up the lower limit of the temperature is preferably at least 0 ° C.. More preferably 10 ° C. or more and 680 ° C. or less.
[0083]
　To obtain the embodiment of the present invention, control in the skin pass rolling is very important. When giving plastic deformation by applying a load to the multilayer steel, as shown in FIG. 5, along the hard layer and the inner layer of stress-strain curve, the distribution of stress and strain occurs in the hard layer and the inner layer. For example, when adding a plastic strain corresponding to the coordinates 1 and 2 in FIG. 5, for the inner layer of the deformation is easy, many plastic strain in the inner layer as shown by the coordinates 3 is introduced. In this state, when the load is removed, the stress of the coordinates 1 and 3 - shrinkage from the distortion state by elastic deformation occurs. In the case the stress of the coordinates 1 is higher than the coordinates 3, the direction of the coordinates 1 than contraction amount coordinates 3 by elastic deformation increases. As a result, the stress state after removal of the load, as indicated by the coordinate 4 and 5, the tensile in the hard layer, so that the compression stress field is created in the inner layer.
[0084]
　Thus, after the addition of plastic deformation in the inner layer, by leaving the compressive stress, the lining of the screw dislocation density × 10 2.0 13 m / m 3 can be controlled above. That is, for dislocations grown by plastic deformation, deformation and reverse (in this case, the compression from tension) is Granting stress, dislocations introduced by pulling dislocation originated by, applied compressive stress attempts to return toward the source. Further, the skin pass rolling, when a large amount of dislocations are introduced, dislocations each other entanglement, the movement of dislocations is suppressed by the entanglement, not be able to go back to the dislocation source. In order to try to alleviate the compressive stress generated in the inner layer is different from the original primary slip system is slip system, dislocation moves to another slip system, called cross slip occurs. The cross slip are the phenomenon which occurs only screw dislocation, by the application of skin pass rolling to the multilayered steel, it is possible to increase the screw dislocation density preferentially.
[0085]
　Skin pass giving the multilayer steel sheet and elongation of 0.1% or more. Elongation, that is, if the amount of plastic deformation given to the multilayered steel is less than 0.1%, the plastic deformation amount of the inner layer is small, the screw dislocation density × 10 2.0 13 m / m 3 to control more than can not. Therefore, the lower limit of the skin pass elongation of 0.1% or more. Although the skin pass rolling rate is preferably as high as possible, be granted beyond 5.0%, it becomes possible to place a large load on the rolling mill, because the productivity is reduced, the upper limit is 5.0% or less preferable. More preferably 0.2% or more and 4.0% or less.
[0086]
　Further, the skin pass rolling already, or pickled hot rolled steel strip of non-skin pass rolled, cold rolled, or further cold-rolled sheet annealing, or after hot rolled sheet annealing by applying cold-rolled and cold-rolled sheet annealing , it may be produced cold rolled steel strip having both a bending resistance.
[0087]
　Pickling: The type of acid used in the pickling step is not particularly specified. The purpose of pickling is the removal of oxide scale formed on the surface of the steel strip after hot rolling, it may be a hydrochloric acid pickling or sulfuric pickling. Furthermore, in order to accelerate the pickling, administration of chemical pickling accelerator pickling solution, or vibration-tension increase or decrease, even the addition of mechanical action, the underlying technology of the present invention no effect.
[0088]
　Cold rolling: reduction ratio of cold rolling is 20% or more, preferably not more than 80%. The rolling reduction is less than 20%, the load imparting at each stand of the tandem rolling mill is reduced, it becomes difficult to grasp and control of the plate-shaped, the productivity is reduced. Further, when the rolling reduction exceeds 80% load applied in each stand is significantly increased, due to this, since the Hertz stress generated in the roll increases excessively, leading to deterioration of the roll life, a decrease in productivity cause. Therefore, the rolling reduction is 20% or less is preferably 80%. More preferably 25% or more, 70% or less.
[0089]
　Hot-rolled sheet annealing step: a hot rolled strip prior to subjecting the cold rolling may be annealed. The purpose of the hot-rolled sheet annealing, before cold rolling by softening of the steel strip, ensuring productivity in cold rolling, and by the control of a tissue ratio in hot rolling plate stage after annealing, after cold-rolled sheet annealing it is an excellent demonstration of bending resistance of the steel strip. As a step of hot-rolled sheet annealing is box annealing (BAF) or conventional continuous annealing process (C-CAL) of either good. Box annealing (BAF) and ordinary continuous annealing method outline of steps (C-CAL) are shown in FIGS.
[0090]
　Heating and cooling rates in box annealing is, 5 ° C. / hr or higher, preferably 80 ° C. / hr. The heating rate is less than 5 ° C. / hr, the time required for hot-rolled sheet annealing step is increased, the productivity is reduced. On the other hand, when it exceeds heating rate 80 ° C. / hr, increased temperature difference between the inner and outer peripheral sides of the steel strip is wound into a coil, by the steel strip caused by thermal expansion difference between the difference by, resulting in scratches on the surface of the steel strip. This flaw, in addition to a decrease in the surface appearance of the product, lowering the bending resistance. Therefore, heating rate 5 ° C. / hr or higher, preferably not more than 80 ° C. / hr. More preferably 10 ° C. / sec or more and 60 ° C. / sec or less.
[0091]
　Annealing temperature is 400 ° C. or higher in the box annealing, 720 ° C. or less, the holding time is 1hr above, the following are preferred 150Hr. Annealing temperature is 400 ° C. or less than the retention time is less than 1hr, is not sufficient softening of the steel strip, there is no effect in improving the productivity in cold rolling. Further, if the annealing temperature exceeds 720 ° C., it becomes austenite formed during annealing, causing the generation of scratches on the steel strip due to thermal expansion changes. Furthermore, the retention time is more than 150Hr, the surface of the steel strip and adhesion, to produce a seizure, surface appearance is deteriorated. Therefore, the annealing temperature is 400 ° C. or higher in the box annealing, 720 ° C. or less, the holding time is 1hr above, the following are preferred 150Hr. More preferred annealing temperature is 420 ° C. or higher, 700 ° C. or less, holding time 3hr more, or less 100 hr.
[0092]
　Heating and cooling rates in the continuous annealing is preferably not less than 5 ° C. / sec. Is less than the heating rate of 5 ° C. / sec, the productivity is reduced. Meanwhile, no upper limit on the heating and cooling rates may exceed 80 ° C. / sec. More preferably 10 ° C. / sec or more.
[0093]
　Continuous annealing temperature in the annealing 650 ° C. or higher, 900 ° C. or less, the holding time is 20 seconds or more, preferably 300 seconds or less. In less than 20 seconds annealing temperature is 650 ° C. or less than the retention time, not enough is softened in the strip in a continuous annealing process, there is no effect in improving the productivity in cold rolling. Further, if the annealing temperature exceeds 900 ° C., the strength of the steel strip is significantly reduced, lead to strip breakage in the furnace, causing a reduction in productivity. Further, when the holding time exceeds 300 seconds, the impurities in the furnace become attached to the surface of the steel strip, the surface appearance is deteriorated. Therefore, the annealing temperature is 650 ° C. or higher in the continuous annealing, 900 ° C. or less, the holding time is 20 seconds or more, preferably 300 seconds or less. More preferred annealing temperature is, 680 ° C. or higher, 850 ° C. or less, the holding time is 30 seconds or more, or less 240 seconds.
[0094]
　Temperature at the overaging in the continuous annealing, 200 ° C. or higher, 500 ° C. or less, the holding time is 50 seconds or more, preferably 500 seconds or less. The overaging zone, the austenite produced in the stage of heating is transformed into bainite or martensite, by appropriately controlling the amount and form of residual austenite, so that excellent bending resistance can be obtained. The aging temperature is 200 ° C. and less than the holding time is less than 50 seconds, the transformation of bainite are insufficient, also, the aging temperature is 500 ° C. or higher, and held at the time is less than 500 seconds, a remarkable amount of residual austenite to lower, bending resistance is not compatible. Therefore, the temperature of the overaging zone in the normal continuous annealing, 200 ° C. or higher, 500 ° C. or less, the holding time is 50 seconds or more, preferably 500 seconds or less. More preferred temperatures 250 ° C. or higher, 450 ° C. or less, the holding time is 60 seconds or more, or less 400 seconds.
[0095]
　Cold-rolled sheet annealing step: The purpose of the cold rolled sheet annealing is a bending resistance of the recovery of the steel strip lost by cold rolling, the optimal addition ferrite, pearlite, bainite, the percentage of each tissue of martensite and retained austenite by reduction is to obtain an excellent bending resistance. The process of cold-rolled sheet annealing, conventional continuous annealing process (C-CAL) or reheat type continuous annealing process may be any of (R-CAL).
[0096]
　The cold-rolled sheet annealing, heating and holding and cooling step in the conventional continuous annealing process (C-CAL) can be carried out in continuous annealing and the same conditions described above for the hot-rolled sheet annealing step.
[0097]
　The outline of the reheat type continuous annealing step shown in FIG. Heating and cooling rates in the reheat type continuous annealing is preferably more than 5 ° C. / sec. Is less than the heating rate of 5 ° C. / sec, the productivity is reduced. Meanwhile, no upper limit on the heating and cooling rates may exceed 80 ° C. / sec. More preferably 10 ° C. / sec or more.
[0098]
　Annealing temperature in the continuous annealing of the reheat type 700 ° C. or higher, 900 ° C. or less, the holding time is 20 seconds or more, preferably 300 seconds or less. The annealing temperature or the holding time of less than 700 ° C. of less than 20 seconds, the amount of austenite transformation during continuous annealing is not sufficient, in the subsequent quenching distribution processing, can not be controlled to a desired tissue ratio. Further, if the annealing temperature exceeds 900 ° C., the strength of the steel strip is significantly reduced, lead to strip breakage in the furnace, causing a reduction in productivity. Further, when the holding time exceeds 300 seconds, the impurities in the furnace become attached to the surface of the steel strip, the surface appearance is deteriorated. Therefore, the annealing temperature is 700 ° C. or higher in the continuous annealing, 900 ° C. or less, the holding time is 20 seconds or more, preferably 300 seconds or less. More preferred annealing temperature is, 720 ° C. or higher, 850 ° C. or less, the holding time is 30 seconds or more, or less 240 seconds.
[0099]
　Cooling stop temperature in the continuous annealing of the reheat type 100 ° C. or higher, 340 ° C. or less, the holding time is 5 seconds or more, preferably 60 seconds or less. In the course of this cooling, to transform the part of the austenite into martensite, it increases the strength of the steel. If the cooling stop temperature is lower than 100 ° C., transformation of the martensite becomes excessive, impairing the ductility and bending resistance of the steel. Therefore, the lower limit of the cooling stop temperature is preferably at least 100 ° C.. Cooling stop temperature exceeds the 340 ° C., or if the holding time is less than 5 seconds is not only obtained a small amount of martensite, it is difficult to increase the strength of steel. Therefore, the upper limit of the cooling stop temperature is 340 ° C. or less, the lower limit of the retention time is preferably 5 seconds or more. Further, even if held for more than 60 seconds, since a large organizational change does not occur, the upper limit of the retention time is preferably 60 seconds. More preferred temperatures, 0.99 ° C. or higher, 320 ° C. or less, the holding time is at least six seconds, 50 seconds or less.
[0100]
　Temperature at the overaging in reheat type continuous annealing, 350 ° C. or higher, 480 ° C. or less, the holding time is 50 seconds or more, preferably 500 seconds or less. The overaging zone, a part of the martensite produced during cooling is stopped as nuclei, to promote transformation to the rest austenite bainite, by appropriately controlling the amount and form of residual austenite, excellent bending resistance obtained as to become. The aging temperature is the 350 ° C. and less than the holding time of less than 50 ° C., the transformation of bainite are insufficient, also, the aging temperature is 480 ° C. or higher, and held at the time is less than 500 seconds, a remarkable amount of residual austenite to lower, bending resistance is not compatible. Therefore, the temperature of the overaging in the continuous annealing of reheat type, 350 ° C. or higher, 480 ° C. or less, the holding time is 50 seconds or more, preferably 500 seconds or less. More preferred temperatures, 380 ° C. or higher, 460 ° C. or less, the holding time is 60 seconds or more, or less 400 seconds.
[0101]
　According to the manufacturing method of the present invention described above, one or both surfaces of the steel sheet, the average micro Vickers hardness is to form a hard layer of less than and 800 HV 400 HV, an inner layer of an average micro Vickers hardness of the 350HV or more and hard a smaller value than 50HV than the hardness of the layer, the inner layer of the screw dislocation density 2.0 × 10 13 m / m 3 is controlled to above, it is possible to manufacture a steel sheet having excellent bending resistance. According to the production method of the present invention, residual plastic strain residual plastic strain test described above it is possible to manufacture a steel sheet 0.1% or less.
Example
[0102]
　The following Examples will further illustrate the effects of the present invention.
[0103]
　Levels embodiment is an example of employing the execution conditions for confirming the workability and effects of the present invention, the present invention is not limited to this single example of conditions. The present invention does not depart from the gist of the present invention, as long as to reach the present invention aim is to allow adoption of various conditions.
[0104]
(Example No.1 ~ 113)
　the slab having the composition of Composition No. A ~ BA in Table 1-1 and Table 1-2 were produced by continuous casting. Using slabs produced above composition, Table 2-1 "surface layer" of the "layer structure" in the through Table 2-6, "center", by laminating in the order of "backing layer", a two-layer structure or 3 to produce a laminated slab layer structure. Respectively obtained hot rolling step is heated under the following conditions of the stacked slabs were subjected finish rolling step, ROT cooling step, the winding step. Perform skin pass rolling under the following conditions with respect to multi-layer steel sheet after winding process, to produce a multi-layer steel sheet as a final product of Example No.1 ~ 113. Table 2-1 items to Table 2-6 "surface" and "back layer" is one which is formed respectively on the front and rear surfaces of the inner layer (items in Table 2-1 through Table 2-6 "center") .
[Production conditions]
　hot-rolling process when the heating conditions of the stacked slabs: 1200 ° C. at 50 minutes retention
　stacked slabs of finish rolling temperature: 920 ° C.
　ROT cooling rate: 55 ° C. / sec
　coiling temperature: 220 ° C.
　during skin pass rolling process elongation of the laminated steel: 0.3%
　thickness of the laminated steel sheet as the final product: 2.4 mm
　thick each in the surface layer and the backing layer: 480 .mu.m (1/5 of thickness)
[0105]
　For each of the laminated steel sheets of Examples No.1 ~ 113, the hardness difference between the surface layer and the center layer, the hardness difference between the back layer and the central layer, the tensile strength, screw dislocation density, the flexural properties and fatigue resistance It was measured.
[0106]
　Fatigue resistance, as described above, to produce a Schenk-type test piece from each laminated steel, 10 at 0.5 times the applied stress of the tensile strength to the test piece 4 after giving bending or more repetitions plane, was performed by determining by strain gauge whether the fatigue fracture (fracture). Number of repetitions to failure is 10 4 If more than once ○ (pass), 10 4 in the following cases times was × (fail).
[0107]
　Incidentally, flexural properties, from each example in Table 2-4 to Table 2-6 were prepared Schenk-type specimens, after sticking the strain gauge on the test piece surface in the state of stress unloaded, the tensile strength 10 0.5 times the applied stress 3 gives bending repetitions plane, and measuring the residual plastic strain after removing the test piece from the tester. If residual plastic strain of 0.1% or less, plastic deformation is suppressed in repeated bending, it is determined that excellent bending resistance, gave an indication of "○". Also, if the residual plastic strain exceeds 0.1%, it is determined that it could not suppress the plastic deformation in bending repeatedly, it gave an indication of "×".
[0108]
　Table 2-4 of the embodiment of the through Table 2-6, and high embodiments than the content of C C content in the surface layer or the back layer in the layer of the center (inner layer), the content of Mn in the inner layer higher example than the content of Mn in the surface layer or the back layer can be seen to both flexural characteristics is "×."
[0109]
　Invention example are both standard deviation of nano-hardness of the hard layer it was 2.0 or less.
[0110]
　Incidentally, multilayered steel No.15,26 and No.99 is a comparative example are all the hardness of the inner layer and the hard layer satisfies the requirements of the present invention, the inner layer of the screw dislocation density in satisfying the requirements of the present invention Nevertheless, resistance to bending characteristics is "×". Multilayered steel No.15,26 and No.99, because the C in the hard layer, the content of Mn is lower C, than the content of Mn in the inner layer, to suppress the occurrence of micro breakdown in the hard layer it has become hard is considered to be the cause. Thus, the requirements of an average micro Vickers hardness of the hard layer and the inner layer mentioned above, in addition to the requirements of the inner layer of the screw dislocation density, C C in the hard layer, the content of Mn in the inner layer, to be more than Mn is essential in order to obtain the effect of the present invention.
[0111]
　Backing layer of the multi-layer steel No.17,32 a comparative example, a steel composition No. I. Since the carbon content is higher in the steel sheet No. I, the hardness of the backing layer of the multilayer steel No.17,32 are both exceeded 800 HV. Therefore, multilayered steel No.17,32 Any likely to occur embrittlement cracking, bending resistance is lowered.
[0112]
　Of Examples No.1 ~ 113, were examined as "hardness difference" a "screw dislocation density" for all examples except example No.15,26 and 99. The results are shown in Figure 1. From the position of the comparative example of FIG. 1 "×" and invention examples "○", in order to improve the bending resistance, the screw dislocation density of the inner layer is × 10 2.0 13 m / m 3 or more, and a surface layer it can be seen that the hardness difference between the center layer needs to be more than 50 HV.
[0113]
(Example No.114 ~ 126)
　To examine the effect on the flexural properties and fatigue resistance due to the thickness of the front and back layers (hard layer), the flexural properties and fatigue strength in Tables 2-4 to Table 2-6 both "○" was produced laminated slab by changing the thickness of the front and back layers of the layer structure of the embodiment where (pass). Using each of the laminated slab, under the same production conditions as in Example No.1 ~ 113, to produce a multi-layer steel sheet of Examples No.114 ~ 126. Incidentally, any thickness of the laminated steel plates of Examples No.114 ~ 126 is 2.4 mm. Multilayered steel of Examples No.114 ~ 126 has a layer structure shown in Table 3, respectively, of the composition No. A ~ BA in Table 1-1 and Table 1-2, the optimum chemistry described above It manufactured using the slabs developed steel having a steel composition components.
[0114]
　Column of the item "front and back layers aim Thickness" in Table 3 is a target value of the ratio for the thickness of the respective layer thickness of the surface layer and the backing layer, which is set during manufacturing. Moreover, the Table 3 of the item "substantial thickness" is the actual thickness of the backing layer and the surface layer of the laminated steel plates of Examples No.114 ~ 126. As seen from the examples No.114 ~ 116 of Table 3, the thickness of each hard layer is less than 20 [mu] m, that the total thickness of the hard layer is less than 1/100 of Zen'itaAtsu are flexural properties not It is enough. Moreover, even the thickness of each hard layer is 20μm or more, which total thickness of the hard layer is more than 2/5 of Zen'itaAtsu It can be seen that flexural properties becomes insufficient.
[0115]
(Example No.127 ~ 169)
　To examine the effect on the flexural properties and fatigue properties by hot rolling conditions, among the composition No. A ~ BA in Table 1-1 and Table 1-2, the developed steel with slab, by hot rolling conditions shown in Table 4-1 to Table 4-2 were prepared laminated steel plates of examples No.127 ~ 169. Laminated steel plate of Examples No.127 ~ 169 has a layer structure shown in Table 4-1 and Table 4-2, respectively, except for Example No.147 of laminated steel the hard layer is formed only one surface, respectively surface layer and the backing layer are constituted with the same kind of slab. Any of Examples No.127 ~ 169 a plate thickness of 2.4 mm, each thickness of the surface layer and the backing layer is 240 .mu.m (1/10 of thickness).
[0116]
　As seen from the examples No.132, C in the hard layer, C content of Mn in the inner layer, even if higher than the content of Mn, the coiling temperature exceeds 700 ° C., and the hard layer the difference of the inner layer of the hardness becomes less than 50 Hv, screw dislocation density of × 10 2.0 13 m / m 3 because less than it can be seen that the flexural properties becomes insufficient. As can be seen from Examples No.138 and 157, C in the hard layer, C content of Mn in the inner layer, even if higher than the content of Mn, the finish hot rolling temperature is less than 650 ° C. If, alternatively 950 ° C. greater than the difference between the hard layer and the inner layer of the hardness becomes less than 50 Hv, screw dislocation density of × 10 2.0 13 m / m 3 because less than, flexural properties are insufficient it can be seen. Moreover, as can be seen from Example No.154, C in the hard layer, the Mn content C in the inner layer, even if higher than the content of Mn, the skin pass rolling elongation of less than 0.1% If, screw dislocation density of × 10 2.0 13 m / m 3 because less than it can be seen that the flexural properties becomes insufficient.
[0117]
(Example No.170 ~ 192)
　Tables 1-1 and of the composition No. A ~ BA in Table 1-2, using a slab of developed steel, embodiments having a layer structure shown in Table 5-1 No. to produce a laminated steel sheet from 170 to 192. Except for Example No.179 of laminated steel the hard layer is formed on one side only, the laminated steel sheets shown in Table 5-1, the surface layer and the back layer, respectively using the same type of slab is constructed. Both laminated steel plate of Examples No.170 ~ 192 are plate thickness during the winding process is 2.4 mm, each thickness of the surface layer and the backing layer was 240 .mu.m. Pickled each laminated steel sheet after coiling step, then, hot-rolled sheet annealing under conditions shown in Table 5-2, was subjected to cold rolling and cold-rolled sheet annealing, elongation of 0.3% skin pass rolling by performing, to produce a laminated steel plates of examples No.170 ~ 192.
[0118]
　Patterns in Table 5-2 "BAF" indicates that was box annealing at annealing pattern shown in FIG. 2, (1) heating rate (° C. / hr), (2) annealing temperature (° C.), (3 ) retention time (hr), indicating that it was the size of (4) condition cooling rate (° C. / hr) corresponding to the column of the pattern "BAF" of Table 5-2 (1) to (4). Also, patterns in Table 5-2 "C-CAL" indicates that was continuously annealed at an annealing pattern shown in FIG. 3, (1) heating rate (℃ / hr), (2) annealing temperature (℃ ), (3) retention time (hr), (4) cooling rate (° C. / hr), (5) overaging temperature (° C.), (6) retention time (s), (7) cooling rate (° C. / s ) indicates that was the size of the conditions of Table 5-2 that corresponds to the column of the pattern "C-CAL" of Table 5-2 (1) to (7). Also, patterns in Table 5-2, "R-CAL" indicates that was continuously annealed at an annealing pattern shown in FIG. 4, (1) heating rate (℃ / hr), (2) annealing temperature (℃ ), (3) retention time (hr), (4) cooling rate (° C. / hr), (5) cooling stop temperature (° C.), (6) retention time (s), (7) heating rate (° C. / s ), (8) overaging temperature (° C.), the table 5 corresponding to the column (9) retention time (s), (10) the pattern of the cooling rate (° C. / s) is Table 5-2, "R-CAL" -2 condition (1) indicates that it was the size of ~ (10).
[0119]
　By the manufacturing conditions shown in Table 5-1 and Table 5-2, the laminated steel sheets of Examples No.170 ~ 192 is a metal structure shown in Table 5-3 were formed. Both of these examples are manufactured in suitable conditions, as shown in Table 5-4, flexural properties and fatigue resistance was good.
[0120]
(Example No.193 ~ 215)
　each of Table 5-1 of Example No.170 to produce a laminated slabs having the same layer structure as the laminated steel sheet to 192, obtained multilayer slab, Example No.1 hot rolling step under the same manufacturing conditions as to 113, finishing rolling process, ROT cooling step, carried out the winding process, further, after performing bright annealing and plating under the conditions shown in Table 6-1, elongation There performed 0.3% skin pass rolling to produce a laminated steel plates of examples No.193 ~ 215. Table 6-1 Item "plating species" refers to the composition of the plating layer formed on the surface of the Examples No.193 ~ 215. Each plating species has a thickness of about 12μm. Also, item "alloying presence" in Table 6-1 shows the presence of alloying of the plating layer and the surface layer and / or the backing layer. Except for Example No.202 of laminated steel the hard layer is formed on one side only, the laminated steel sheets shown in Table 6-1 and Table 6-2 is constructed the surface layer and the backing layer by using each of the same type of slab It is (items listed in Table 6-1, "front and back layers"). Further, the thickness of laminated steel plate of Examples No.193 ~ 215 are 2.4 mm, the thickness of the surface layer and the backing layer is 240 .mu.m (1/10 of thickness).
[0121]
　Table 6-2 on the hardness of the hard layer of the laminated steel plates of Examples No.193 ~ 215, shown hardness of the center layer, screw dislocation density, the measurement results of the bending resistance or the like. In any of the laminated steel sheets of Examples No.193 ~ 215, the average micro Vickers hardness of the hard layer is more than 400 HV, less than 800 HV, the average micro Vickers hardness of the inner layer 350HV or more, the average micro-hard layer smaller than 50HV than Vickers hardness, screw dislocation density of the inner layer is × 10 2.0 13 m / m 3 was more. Laminated steel plate of these embodiments, both bending resistance and fatigue resistance was good.
[0122]
[Table 1-1]

[0123]
[Table 1-2]

[0124]
[table 2-1]

[0125]
[Table 2-2]

[0126]
[Table 2-3]

[0127]
[Table 2-4]

[0128]
[Table 2-5]

[0129]
[Table 2-6]

[0130]
[table 3]

[0131]
[Table 4-1]

[0132]
[Table 4-2]

[0133]
[Table 4-3]

[0134]
[Table 4-4]

[0135]
[Table 4-5]

[0136]
[Table 4-6]

[0137]
[Table 5-1]

[0138]
[Table 5-2]

[0139]
[Table 5-3]

[0140]
[Table 5-4]

[0141]
[Table 6-1]

[0142]
[Table 6-2]

Industrial Applicability
[0143]
　Steel sheet of the present invention is excellent in bending resistance, it can be suitably used for automobile parts and infrastructure structural member.

The scope of the claims
[Requested item 1]
　An inner layer, a steel sheet having a one or both hard layer formed on the surface of the inner layer, a
　content of C in the hard layer is higher than the content of C in the inner layer, and the hard the content of Mn in the layer is higher than the content of Mn in the inner layer,
　the thickness of the hard layer is 20μm or more, the total thickness of the hard layer has a 2/5 or less of Zen'itaAtsu,
　the hard layer the average micro Vickers hardness of 400HV or more and less than 800 HV,
　the average micro Vickers hardness of the inner layer 350HV or more, less than 50HV than the hardness of the hard layer,
　the inner layer of the screw dislocation density of 2.0 × 10 13 m / m 3 , characterized in that or more, the steel sheet.
[Requested item 2]
　Said hard layer and inner layer, by
　mass%,
　C: 0.10 ~
　0.60%, Si: 0.01 ~ 3.00%, Mn: 1.000 ~ 10.00%,
　containing, P: 0.0200% or
　less, S: 0.0200% or
　less, N: 0.0200% or
　less, O: 0.0200% or less, the limits,
　the balance being Fe and impurities, claim 1 steel sheet according to.
[Requested item 3]
　At least one of the hard layer or the inner layer further contains, by
　mass%, Al: 0.500% or
　less, Cr: 2.000% or
　less, Mo: 1.000% or
　less, Ti: 0.500% or less ,
　B: 0.0100% or
　less, Nb: 0.500% or
　less, V: 0.500% or
　less, Cu: 0.500% or
　less, W: 0.100% or less,
　Ta: 0.100% or
　less, Ni : 0.500% or
　less, Sn: 0.050% or
　less, Sb: 0.050% or
　less, As: 0.050% or
　less, Mg: 0.0500% or
　less, Ca: 0.050% or
　less, Y: 0 .050% or
　less, Zr: 0.050% or
　less, La: 0.050% or
　less, Ce: 0.050% or less,
characterized in that it contains one or more, according to claim 2 steel plate.