Technical field
[0001]
The present invention relates to a steel sheet and a manufacturing method thereof, the chassis parts of a motor vehicle as a main purpose, it relates to a steel sheet and a manufacturing method thereof.
BACKGROUND
[0002]
Recently, fuel efficiency of automobiles has been desired, reducing automobile weight for it is required. In order to reduce the weight of the vehicle, reducing the thickness of the steel sheet for automobile use becomes effective. However, in that case, the challenge is to improve the fatigue strength of the steel sheet. If a reduced thickness of the steel sheet, the stress applied to the steel increases, the fatigue life is deteriorated. Therefore, higher-fatigue life steel is obtained.
[0003]
　On the other hand, if the parts are many automotive, for use after being processed into the component shape by press forming or roll forming or the like, it is required excellent cold formability. Although the improvement of the fatigue property is effective strength of a steel sheet, or higher strength of the steel sheet is generally accompanied by a reduction in the cold formability is how improved fatigue properties without lowering the cold formability It becomes a problem.
[0004]
　The technique for improving the fatigue properties of the steel member, many proposals have been made.
[0005]
　Patent Document 1, without causing cost increase, and without deteriorating the total elongation, relates a high-strength hot-rolled steel sheet having improved fatigue strength, C mass%: 0.03 ~ 0.09%, Si: 0.01 ~ 2.20%, Mn: 0.30 ~ 2.20%, P: 0.100% or less, S: 0.010% or less, Al: 0.005 ~ 0.050%, N: 0 containing .0100% or less, and the balance of the composition of the iron and unavoidable impurities, the microstructure of the region of the thickness corresponding to at least 10% of the sheet thickness from the front and back surfaces of the steel sheet ferrite and bainite composite structure or bainite made of single-phase structure, microstructure region of thickness corresponding to at least 50% of the thickness of the thickness center portion is a composite structure of ferrite and pearlite, corresponding to at least 10% of the plate thickness from the front and rear surfaces thickness the hardness of the regions Discloses a high strength hot-rolled steel sheet excellent in fatigue properties, characterized in that the thickness of the KibanAtsu center at least 50% more than 1.10 times the average hardness of the region of the corresponding thickness .
[0006]
　Patent Document 2 has high dedendum bending fatigue strength, and relates to a preferred carburizing steel material such as high strength gear superior in surface fatigue properties, C: 0.1 ~ 0.35%, Si: 0. 01 ~ 0.22%, Mn: 0.3 ~ 1.5%, Cr: 1.35 ~ 3.0%, P: 0.018% or less, S: 0.02% or less, Al: 0.015 ~ 0.05%, N: 0.008 ~ 0.015% and O: 0.0015% to less, the following equation (1), contained in a range satisfying (2) and (3), the balance being Fe and a composition of the unavoidable impurities, a further 85% or more total tissue fractions of ferrite and pearlite in the steel structure, and the average particle size of the ferrite discloses a carburized steel is 25μm or less.
[0007]
　3.1 ≧ {([% Si] / 2) + [% Mn] + [% Cr]} ≧ 2.2
　(1) [% C] - ([% Si] / 2) + ([% Mn] /5)+2[%Cr]≧3.0
　(2) 2.5 ≧ [% Al] / [% N] ≧ 1.7 (3)
CITATION
Patent Document
[0008]
Patent Document 1: JP 2015-63737 JP
Patent Document 2: JP 2013-82988 JP
Summary of the Invention
Problems that the Invention is to Solve
[0009]
　Technique of changing the material of the steel by heat treatment, there is a problem to be solved that the size and shape of the steel part changes upon cooling. For example, if you try to correct the bent shape of the changed member, since the strain in the surface layer is applied, the fatigue properties decrease. Further, carburizing, nitriding, heat treatment of the high-frequency hardening has a low productivity, since the production cost is increased drastically, the arm such, application to such links such is difficult.
[0010]
　In view of the above circumstances, at a low cost, and to provide an improved steel fatigue properties without lowering the cold formability.
Means for Solving the Problems
[0011]
　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, forming an average micro Vickers hardness of not less than and less than 400 HV 240HV, hard layer variation of hardness at a depth of 10μm from the surface is 2.0 or less standard deviations is allowed, further, it controls the optimum value of C and N content of the hard layer, an average micro Vickers hardness of the inner layer and over and less than 400 HV 80HV, control the volume percentage of carbide contained in the inner layer to less than 2.00% and, by the average micro Vickers hardness of the hard layer and the inner layer of an average micro Vickers hardness of 1.05 times or more, and found that to achieve both fatigue properties and cold workability at a high level.
[0012]
　Further, the method of manufacturing a steel sheet satisfying this is simply to devise such as hot rolling conditions and annealing conditions in a single difficult to manufacture, achieving an optimization in the so-called integrated process, such as hot rolling, annealing step You may not be able to only manufacture by, and findings by stacking various kinds of research, and completed the present invention.
[0013]
　The gist of the present invention is as follows.
[0014]
　(1) an inner layer, a steel plate and a hard layer on one or both surfaces of the inner layer, the thickness of the hard layer is 20μm or more and 40% or less of the plate thickness of the steel sheet, the hard the average layer micro Vickers hardness of more than 240HV, less than 400 HV, C amount is 0.4 mass% of the hard layer less, the N content is 0.02 mass% or less, of 10μm from the surface of the hard layer variation in hardness in the depth is 2.0 or less in standard deviation, average micro Vickers hardness of the inner layer is more than 80HV, less than 400 HV, the volume percentage of carbide contained in the inner layer is less than 2.00% , and the steel sheet average micro Vickers hardness of the hard layer is equal to or is more than 1.05 times the average micro Vickers hardness of the inner layer.
[0015]
　(2) before the hard layer referred ha, で mass%, C: 0.03 ~ 0.35%, Si: 0.01 ~ 3.00%, Mn: 0.70 ~ 10.00%, P: 0.0200 % or less, S: 0.0200% or less, Al: 0.500% or less, N: 0.0200% or less, O: 0.0200% or less, Ti: 0 ~ 0.500%, B: 0 ~ 0. 0100%, Cr: 0 ~ 2.000%, Mo: 0 ~ 1.000%, Nb: 0 ~ 0.500%, V: 0 ~ 0.500%, Cu: 0 ~ 0.500%, W: 0 ~ 0.100%, Ta: 0 ~ 0.100%, Ni: 0 ~ 0.500%, Sn: 0 ~ 0.050%, Sb: 0 ~ 0.050%, As: 0 ~ 0.050 %, Mg: 0 ~ 0.0500%, Ca: 0 ~ 0.050%, Y: 0 ~ 0.050%, Zr: 0 ~ 0.050%, La: 0 ~ 0.050%, and び e: 0 ~ 0.050% containing wo shi, remnants ga Fe and impurities び で thou ri, the inner wa mind before, で mass%, C: 0.001 ~ 0.300%, Si: 0.01 ~ 3.00%, mn: 0.10 ~ 3.00%, P: 0.0200% or less, S: 0.0200% or less, Al: 0.500% or less, N: 0.0200% or less, O: 0.0200% or less , Ti: 0 ~ 0.500%, B: 0 ~ 0.0100%, Cr: 0 ~ 2.000%, Mo: 0 ~ 1.000%, Nb: 0 ~ 0.500%, V: 0 ~ 0.500%, Cu: 0 ~ 0.500%, W: 0 ~ 0.100%, Ta: 0 ~ 0.100%, Ni: 0 ~ 0.500%, Sn: 0 ~ 0.050%, Sb: 0 ~ 0.050%, As: 0 ~ 0.050%, Mg: 0 ~ 0.0500%, Ca: 0 ~ 0.050%, Y: 0 ~ 0.050% Zr: 0 ~ 0.050%, La: 0 ~ 0.050%, and び Ce: 0 ~ 0.050% wo containing shi, remnants ga Fe and び impurities で thou ru ko と wo special Zhi と suru requested item 1 ni described の steel .
Effect of the invention
[0016]
　According to the present invention, one or both surfaces of the steel sheet, an average micro Vickers hardness of not less than and less than 400 HV 240HV, variation in hardness at a depth of 10μm from the surface is 2.0 or less standard deviations hard to form a layer, further, controls the C and N content of the hard layer to an appropriate value, an average micro Vickers hardness of the inner layer and over and less than 400 HV 80HV, the volume percentage of carbide included in the inner layer 2.00% controlled below, by an average micro Vickers hardness of the hard layer and the inner layer of an average micro Vickers hardness of 1.05 times or more, it is possible to provide a steel sheet and a manufacturing method thereof for cold forming which is excellent in fatigue characteristics.
BRIEF DESCRIPTION OF THE DRAWINGS
[0017]
FIG. 1 is a diagram showing a relationship between thickness and fatigue limit ratio of the hard layer.
DESCRIPTION OF THE INVENTION
[0018]
　The present invention will be described in detail.
[0019]
　First, the thickness of the hard layer and the inner layer, component, the average micro Vickers hardness, and the reason for limiting the standard deviation of nano-hardness of the hard layer will be described. Here, "%" for component means mass%.
[0020]
　The thickness of each hard layer present on one or both surfaces of the inner layer, 20 [mu] m or more per side, and 40% or less of Zen'itaAtsu. If the thickness of each hard layer is less than 20 [mu] m, small thickness of the hard layer, when subjected repeatedly stress leads to delamination of the hard layer, it is not obtained excellent fatigue characteristics. Therefore, the thickness of each hard layer, a 20μm or more. Also, if the thickness of each hard layer is greater than 40% of Zen'itaAtsu, that is, stress applied to the hard layer is increased at the time of cold forming, it can not be obtained the benefits of cold formability improvement by multi-layering. Therefore, the thickness of each hard layer is not more than 40% of Zen'itaAtsu. More preferably, the thickness of each hard layer, 30 [mu] m or more and 30% or less.
[0021]
　Hard layer and the thickness of the inner layer is measured by an optical microscope. 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, diamond having an average particle size of 3μm and 1μm performing a polished to a mirror-like by using the abrasive grains. 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.
[0022]
　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. Thus, now a small step on the hard layer and the inner layer of the boundary caused by observation using an optical microscope, the hard layer and the inner layer of the boundary, can determine the percentage of the thickness and thickness of each layer Become. Incidentally, if a step that is provided by the finish polishing minute, observed by differential interference optical microscope is preferred.
[0023]
　Average micro-Vickers hardness of the hard layer 240HV or more and less than 400 HV. If the average micro Vickers hardness is less than 240HV, low hardness of the hard layer, is not obtained excellent fatigue characteristics. Therefore, an average micro Vickers hardness of the hard layer is not less than 240HV. On the other hand, the average micro Vickers hardness of the hard layer is becomes more than 400 HV, since the hard layer is excessively high strength, cold formability is remarkably deteriorated. Therefore, an average micro Vickers hardness of the hard layer is less than 400 HV. More preferably, not less than 255HV.
[0024]
　The amount of C contained in each hard layer more than 0.4%, and N amounts to 0.02% or less. C and N are markedly element for suppressing cross slip in the steel during cold forming. Cross slip has the effect of suppressing storage and accumulation of dislocations introduced in the steel when the given strain. When the amount of C is 0.4% or the amount of N exceeds 0.02%, cross slip is suppressed significantly, leading to excessive accumulation and stacking of dislocations during cold forming. As a result, voids are generated in the region of low distortion, since it reaches macroscopic destruction these voids are connected, cold formability deteriorates. Therefore, 0.4% or less, or N content of C amount contained in each hard layer to 0.02% or less. More preferably, 0.38% or less, or N content and C content to 0.018% or less.
[0025]
　The average micro Vickers hardness of the inner layer 80HV or more and less than 400 HV. If the average micro Vickers hardness of less than 80HV, since the difference in hardness of the hard layer is increased, become distorted biased to excessively inner side when cold forming, cracks in the hard layer / inner layer interface lead generation and, of the product after cold forming a shape defect. Therefore, the average micro Vickers hardness of the inner layer be at least 80HV. Further, if the average micro Vickers hardness of more than 400 HV, since the difference between the hard layer and the inner layer of the hardness is small, there is no effect was relieve the strain concentration to the hard layer during cold-forming, cold formability is degraded. Therefore, the average micro Vickers hardness of the inner layer is less than 400 HV. More preferably, 90HV or more and less than 380 Hv.
[0026]
　The volume ratio of the inner layer of the carbide is less than 2.00%. Carbides becomes a crack or void generation site during cold forming, becomes more than 2.00%, it prompted the connection of cracks or voids, encourage macroscopic destruction. Therefore, the upper limit of the volume ratio of the inner layer of the carbide is less than 2.00%. More preferably less than 1.90%. Incidentally, carbides and cementite (Fe is a compound of iron and carbon 3 in addition to C), a compound obtained by substituting Fe atoms in cementite Mn, with alloy elements such as Cr, alloy carbides (M 23 C 6 , M 6 C , MC. Incidentally, M is added as Fe and other alloy metal element).
[0027]
　The average micro Vickers hardness of the hard layer, the inner layer of an average micro Vickers hardness of 1.05 times or more. Fatigue strength of the steel sheet can be improved by controlling the hardness and the inner layer of the hardness ratio of the hard layer, an average micro Vickers hardness of the hard layer, the inner layer of an average micro Vickers hardness of 1.05 times or more by, fatigue limit ratio (= fatigue strength / tensile strength) can be set to 0.50 or more. More preferably, the at least one side is 1.08 times or more.
[0028]
　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, because the cold formability is remarkably improved. If the standard deviation exceeds 2.0, for example, there are cases where cracking occurs during cold roll forming. From this point of view, the standard deviation is 2.0, 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.
[0029]
　Variations in the thickness direction of the nano-hardness of the hard layer does not affect the cold formability, even if had an inclination of hardness in the sheet thickness direction, without impairing the effects of the present invention. Incidentally, when the variation of the nano-hardness in a line parallel to the rolling direction in the vertical is large relative to the thickness direction, cold formability decreases.
[0030]
　Steel sheet of the present embodiment, if Sonaere the inner and hard layer mentioned above is not particularly ingredients limitation. Hereinafter, an example of a suitable chemical composition as the hard layer and the inner layer of the steel sheet of the present embodiment will be described for each element. Here, "%" for component means mass%.
[0031]
　First the preferred components of the hard layer will be described.
[0032]
　(C: 0.03 ～ 0.35 Pasento)
　C is an effective element to strengthen steel. C content in order to ensure the fatigue properties of the component by heat treatment such as quenching and tempering is preferably to 0.03% or more, more preferably 0.10% or more. When the C content is increased, preferably to 0.35% or less for easily occurs cracking during cold forming. More preferably 0.30 or less.
[0033]
　(Si: 0.01 ~
　3.00%) Si acts as a deoxidizer and, carbides and that affect element in the form of residual austenite after heat treatment. The balance of fatigue properties and cold formability, and reduce the volume fraction of carbides present in the steel components, further utilizing residual austenite, it is effective to achieve high strength. To obtain this effect, it is preferable to contain Si of 0.01% or more. When the content of Si is too high, the steel parts are brittle, since the cold formability may be deteriorated, it is preferably not more than 3.00%. More preferably 0.10% or more and less 2.50%, more preferably, 0.20% or more and 2.00% or less.
[0034]
　(Mn: 0.70 ~
　10.00%) Mn acts as a deoxidizer and also is an element effective in suppressing the steel pearlite transformation. In the course of cooling from the austenite region to suppress pearlite transformation, to raise the tissue ratio of martensite, the strength, in order to ensure the fatigue characteristics, the Mn content is preferable to 0.70% or more, 2. more than 100% is more preferable. If Mn amount is too large, coarse Mn oxide becomes to be present in the steel becomes a starting point of fracture at the time of cold forming, cold formability deteriorates. Therefore, Mn amount is preferably not more than 10.00%. More preferably not more than 8.00%.
[0035]
　(P: 0.0200% or less)
　P is not an essential element, is contained in steel as an impurity, strongly segregated grain boundary ferrite grain boundaries brittle. Therefore, the content is often as small as possible, it may be 0. However, in order to highly purified to less than 0.0001% in the refining step, the number of time required for the refining, the cost is greatly increased, realistic lower limit is 0.0001%, the cost considering the surface may contain 0.0010% or more. When the P content increases, the cold formability is lowered due to grain boundary embrittlement, preferably not more than 0.0200%. More preferably not more than 0.0190%.
[0036]
　(S: 0.0200% or less)
　S is not an essential element, for example, is included in steel as an impurity, to produce a non-metallic inclusions of MnS, etc. in the steel, reduction of increase in hardness and ductility of the steel parts the lead. In addition, by greatly different nonmetallic inclusions difference steel and hardness in the steel is produced, the variation of the hardness of the vicinity of the surface of the hard layer is increased. Therefore, the content is often as small as possible, it may be 0. However, in order to highly purified to less than 0.0001% in the refining step, the number of time required for the refining, the cost is greatly increased, realistic lower limit is 0.0001%, the cost considering the surface may contain 0.0010% or more. When the S content increases, since the crack starting from the non-metallic inclusions are generated during cold forming, increasing further the hardness, since cold formability is lowered, preferably to less 0.0200% . More preferably not more than 0.0190%.
[0037]
　(Al: 0.500% or less)
　Al acts as a deoxidizer in the steel to stabilize the ferrite, an element that increases the hardness, are added as needed rather than essential element. To obtain the effect of addition, a is preferably 0.001% or more content, and more preferably 0.010% or more. When the amount of Al is increased, and generation of coarse Al oxides, causing a deterioration of cold formability with increasing hardness, also, the difference between the steel and the hardness significantly different nonmetallic inclusions are generated in the steel the Rukoto, since the hardness variation in the vicinity of the surface of the hard layer is increased, preferably to 0.500% or less, and more preferably not more than 0.450%.
[0038]
　(N: 0.0200% or less)
　N is an element effective to strengthen the C as well as steel, but also affects the elements in the generation of cross slip dislocation during cold forming. Not an essential element from the viewpoint of securing the cold formability, preferably as less content may be 0. However, since the reduced to less than 0.0001% refining costs increase, realistic lower limit is 0.0001%, and may contain 0.0010% or more. When the N content is increased, it is impossible to suppress the concentration of strain at the time of cold forming, to cause the generation of voids, cold formability is markedly reduced. Therefore, N content is preferably not more than 0.0200%. More preferably not more than 0.0150%.
[0039]
　(O: 0.0200% or less)
　O is to form an oxide in the steel, is an element causing an increase in hardness. In addition, by greatly different nonmetallic inclusions difference steel and hardness in the steel is produced, the variation of the hardness of the vicinity of the surface of the hard layer is increased. Not an essential element, oxide present in ferrite grains becomes void generation site, because the tissue factor leading to further the hardness increases, O amount is preferably lesser, may be zero. However, when reduced to less than 0.0001%, the refining costs increase, realistic lower limit is 0.0001%, may contain more than 0.0005%. If O amount becomes large, since the cold formability is lowered due to an increase in hardness, preferably not more than 0.0200%. More preferably not more than 0.0170%.
[0040]
　Below the element it is also not an essential element, a steel plate and may optionally elements be appropriately contained in the limits of the predetermined amount to the steel.
[0041]
　(Ti: 0 ~
　0.500%) Ti controls the form of carbides, is an element that increases the strength of ferrite by the inclusion of a large amount. However, from the viewpoint of securing the cold formability, preferably as less content may be 0. Since refining costs and reduces the Ti content to less than 0.001% increases, realistic lower limit is 0.001%, and may contain more than 0.005%. When Ti content is increased, it becomes coarse Ti oxide or TiN is present in the steel, since cold formability is lowered, preferably 0.500% or less. More preferably not more than 0.450%.
[0042]
　(B: 0 ~ 0.0100%) 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. To obtain the sufficient effect of improving the high strength or fatigue characteristics by the addition, B amount is preferably to 0.0001% or more, more preferably 0.0005% or more. When the amount of B increases, leading to formation of coarse B oxides in the steel becomes a starting points of voids during cold forming, since cold formability deteriorates, preferably between 0.0100% or less. More preferably 0.0050% or less.
[0043]
　(Cr: 0 ~
　2.000%) Cr is suppressed pearlite transformation as with Mn, is an element effective for high strength steel, it is added as required. To obtain the effect of addition is preferably added 0.001% or more, more preferably 0.010% or more. When the Cr content increases, come to form a coarse Cr carbide in the center segregation area, because the cold formability is lowered, but at most 2.000% preferred. More preferably not more than 1.500%.
[0044]
　(Mo: 0 ~
　1.000%) Mo is, Mn, is an element effective to strengthen Like the Cr steel, it is added as required. Preferably the addition of 0.001% or more in order to obtain the effect of addition, and more preferably 0.010% or more. When Mo content is increased, to form a coarse Mo carbides, preferably not more than 1.000% for cold workability is deteriorated. More preferably not more than 0.700%.
[0045]
　(Nb: 0 ~ 0.500%) 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 the addition. To obtain the effect of addition is preferably added 0.001% or more, more preferably 0.002% or more. When the Nb content increases, precipitates many fine and hard Nb carbides, the strength of the steel material is increased, the ductility is significantly degraded, since cold workability is deteriorated, preferably 0.500% or less . More preferably not more than 0.200%.
[0046]
　(V: 0 ~ 0.500%)
　V, like the Nb, an element effective to form the control of the carbide is an effective element in improving the toughness for refining the tissue by the addition. To obtain the effect, preferably the addition of 0.001% or more, more preferably 0.002% or more. When the amount of V is increased, precipitation many fine V carbide, the strength of the steel material is increased, the ductility is lowered, since cold formability is lowered, preferably 0.500% or less. More preferably not more than 0.400%.
[0047]
　(Cu: 0 ~
　0.500%) Cu is an effective element for increasing the strength of the steel, is added as required. Preferably the content of 0.001% or more in order to effectively exhibit the effect of increasing the strength, and more preferably 0.002% or more. When the Cu content is increased, since the productivity of the invitation hot rolling the red shortness lowered, preferably 0.500% or less. More preferably not more than 0.400%.
[0048]
　(W: 0 ~ 0.100%)
　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. Preferably the addition of 0.001% or more in order to obtain this effect, and more preferably 0.002% or more. When W amount is increased, precipitation many fine W carbides, the strength of the steel material is increased, the ductility is lowered, since cold workability is deteriorated, preferably 0.100% or less. More preferably not more than 0.080%.
[0049]
　(Ta: 0 ~
　0.100%) Ta also, Nb, V, similarly to the W, an effective element to increase the form control and intensity of carbide, is added as required. Preferably the addition of 0.001% or more in order to obtain this effect, and more preferably 0.002% or more. If Ta content is increased, precipitation many fine Ta carbides, the strength of the steel material is increased, the ductility is lowered, since cold workability is deteriorated, preferably 0.100% or less. More preferably not more than 0.080%.
[0050]
　(Ni: 0 ~
　0.500%) Ni is an element effective in improving the fatigue characteristics of the components, are added as needed. Preferably contains a 0.001% or more in order to effectively exhibit its effect, and more preferably 0.002% or more. When the amount of Ni increases, the ductility is lowered, preferably 0.500% or less because the cold formability is lowered. More preferably not more than 0.400%.
[0051]
　(Sn: 0 ~
　0.050%) Sn is an element contained in the steel when using scrap as a raw material, preferably as small, may be zero. However, when reduced to less than 0.001%, the refining costs increase, a realistic lower limit 0.001%, it may contain more than 0.002%. When Sn content is increased, to cause a decrease in the cold formability by embrittlement of ferrite, preferably 0.050% or less. More preferably 0.040% or less.
[0052]
　(Sb: 0 ~
　0.050%) 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 zero. However, since the refining costs and reduces increases to less than 0.001%, realistic lower limit is 0.001%, and may contain more than 0.002%. When the Sb content is increased, since the cold formability is lowered, preferably 0.050% or less. More preferably 0.040% or less.
[0053]
　(As: 0 ~
　0.050%) As is, Sn, is contained in the case of using scrap as Sb as well as the steel material, an element which strongly segregate at grain boundaries, preferably as small, even 0 good. However, since the refining costs and reduces increases to less than 0.001%, realistic lower limit is 0.001%, and may contain more than 0.002%. If As the amount increases, since cold formability is lowered, 0.050% or less. More preferably 0.040% or less.
[0054]
　(Mg: 0 ~
　0.0500%) Mg is an element capable of controlling the form of sulfides dopants are added as needed. The effect is to get is preferably addition of 0.0001% or more, more preferably 0.0005% or more. When Mg content is increased, to cause a decrease in the cold formability due to the formation of coarse inclusions, preferably a 0.0500%. More preferably not more than 0.0400%.
[0055]
　(Ca: 0 ~
　0.050%) Ca is an element capable of controlling the form of sulfides Similarly dopants and Mg, is added as required. Preferably the addition of 0.001% or more in order to obtain the effect, more preferably 0.002% or more. When the Ca content increases, generates coarse Ca oxides, to become a starting point of cracking during cold forming, preferably 0.050% or less. More preferably 0.040% or less.
[0056]
　(Y: 0 ~ 0.050%)
　Y is, Mg, an element capable of controlling the form of sulfides Similarly dopants and Ca, are added as needed. Preferably the addition of 0.001% or more in order to obtain the effect, more preferably 0.002% or more. When Y amount increases, and generates coarse Y oxides, since cold formability is lowered, preferably 0.050% or less. More preferably 0.040% or less.
[0057]
　(Zr: 0 ~ 0.050% or
　less) Zr is, Mg, Ca, an element capable of controlling the form of sulfides Similarly dopants and Y, are added as needed. Preferably the addition of 0.001% or more in order to obtain the effect, more preferably 0.002% or more. When the amount of Zr is increased, and generation of coarse Zr oxides, since cold formability is lowered, preferably 0.050% or less. More preferably 0.040% or less.
[0058]
　(La: 0 ~
　0.050%) La is an effective element to form the control of sulfides dopants are added as needed. Preferably the addition of 0.001% or more in order to obtain the effect, more preferably 0.002% or more. When La amount is increased, La oxides are produced, since cold formability is lowered, preferably 0.050%. More preferably 0.040% or less.
[0059]
　(Ce: 0 ~
　0.050%) Ce is an element capable of controlling the morphology of sulfides Similarly dopants and La. There is also an element that reduces the number ratio of strongly segregated grain boundary carbides in the grain boundaries, it is added as needed. Preferably the addition of 0.001% or more in order to obtain the effect, more preferably 0.002% or more. When Ce content increases, because the workability is lowered due to a decrease in the number ratio of the grain boundary carbides, preferably 0.050% or less. More preferably not more than 0.046%.
[0060]
　Further, an example of a suitable chemical composition to the inner layer of the steel sheet.
[0061]
　(C: 0.001 ～ 0.300 Pasento)
　C is an effective element to strengthen steel. In order to control the average micro Vickers hardness of the inner layer above 80HV it is preferably added 0.001% or more, more preferably 0.002% or more. When the C content is increased, leading to increase or generation of carbides excessive strength of the inner layer, since the cold formability is lowered, preferably 0.300% or less. More preferably not more than 0.200%.
[0062]
　(Mn: 0.10 ~
　3.00%) Mn acts as a deoxidizer and also is an element effective to control the strength of the steel. If Mn amount is small, in the solidification process in continuous casting, become solidified from a high temperature is initiated, along with this, so that segregation of the central portion is promoted. Further, at the time of cold forming, cracks from the center segregation area occurs, cold formability is lowered. Therefore, it is preferable to 0.10% or more, more preferably at least 0.30%. When the Mn content is increased, leading to an excessive increase in strength, since deterioration of the cold formability, preferably not more than 3.00%. More preferably not more than 2.60%.
[0063]
　The inner layer, in addition to the C and Mn, Si, P, S, Al, N, Cr, Mo, O, Ti, B, Nb, V, Cu, W, Ta, Ni, Sn, Sb, As, mg, Ca, Y, Zr, La, may contain Ce, its addition range and effects are the same as the hard layer.
[0064]
　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 impurities.
[0065]
　Steel sheet of the present invention, the inner layer and, a one or formed on both sides hard layer of the inner layer, the thickness of each hard layer is 20μm or more and 40% or less of Zen'itaAtsu, each hard layer the average micro Vickers hardness of at least 240HV, less than 400 HV, C amount is 0.4% of each hard layer or less, the N content is 0.02% or less, an average micro Vickers hardness of the inner layer is more than 80HV, 400 HV less than a and is the volume percentage of carbide contained in the inner layer is less than 2.00%, the average micro Vickers hardness of the hard layer is not less than 1.05 times the average micro Vickers hardness of the inner layer, thereby , to achieve both the excellent cold formability fatigue properties is a novel finding by the present inventors have found.
[0066]
　Subsequently, it describes the organization method observations and measurements.
[0067]
　The volume percentage of carbide is made by electrolytic extraction residue. Is composed of platinum, place the steel on the electrode also serving as a sample support base, it is immersed in the electrolytic solution. The potential only carbides remain dissolved was added, by 0.1μm mesh, a residue of carbide. From residual amount of weight and carbides of the sample subjected to electrowinning to obtain the volume fraction of carbides present in the steel.
[0068]
　The average micro Vickers hardness of the steel sheet is a hard layer and the inner layer of the surface side, a 0.098N micro Vickers hardness of at 1/4 position of the hard layer and the inner layer of the thickness, respectively measured 12 points, the hardest data , and determined from the average value of the most soft 10 points data except. Note that the diagonal length of about 9μm indentations in the case of 240HV the load 0.098 N, the length of the case of 400HV is about 7 [mu] m, can be suitably evaluated hardness of the hard layer having a thickness of about 20μm it is.
[0069]
　The standard deviation of nano-hardness of the hard layer is, with respect to the thickness direction of a sheet thickness cross-section it is necessary to calculate the hardness at a line parallel to the rolling direction in the vertical. In the present invention, the "standard deviation of nano-hardness of the hard layer", with 10μm from the surface side in the thickness direction of the hard layer, the nano-hardness of 100 points at intervals of 3μm to the rolling direction, Heidi Tron Inc. Made in measured under the conditions of a nano indenter (Ti-900) depth 80nm indentation was used to create 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. Here, the standard deviation of nano-hardness of the hard layer, since it is intended to evaluate the difference in hardness between the fine tissue, rather than the Vickers hardness test, a narrow area of ​​the region to evaluate hardness, determined by the hardness test by a nano indenter.
[0070]
　Subsequently, we describe a method for evaluating cold formability. As the inner bend radius 4 mm, after giving bending 90 ° by roll forming in the sample, to confirm the presence or absence of cracks in the bent corner portion by sectional microstructure observation. After the sample embedded in epoxy resin, after rough polishing by emery wet grinding, using 3μm and 1μm in size of the diamond particles, mirror-finished form the sample cross-section by grinding. Subsequently, without performing etching, observing the cross section structure in 1000x magnification by an optical microscope, the length of the recognized cracks and cracks in the case of less than 5 [mu] m, it is determined that excellent cold workability. The length of the accepted cracks and cracks in the case of more than 5 [mu] m, cold formability is judged to inferior.
[0071]
　Fatigue characteristics of steel sheet, according to the method described in JIS Z2275, performed plane bending fatigue test under conditions of stress ratio = -1, was evaluated at 200 million times fatigue limit, fatigue limit / tensile strength as fatigue ratio calculated, to ratings. The steel sheet of the present embodiment, the fatigue limit ratio is equal to 0.45 or higher, the fatigue characteristics determined to be good.
[0072]
　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.
[0073]
　For example when the production of steel strip, 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 in the present invention. By curing the surface layer portion by surface treatment in this way during the production of steel strip, by annealing the strain that occurs when subjected to a surface treatment after part molding can solve the problem of dimensional accuracy decreases.
[0074]
　Method for producing a steel sheet of the present invention, one or both surfaces of the inner layer which is excellent in cold workability soft, paste the hard layer having excellent fatigue characteristics, and these hard layers and an inner layer of strength-component-tissue It is characterized by controlling the optimum state. The heat as such a manufacturing method is not particularly limited, for example, the diffusion heat treatment method using the blank plate of sheet steel, the by electric resistance welding was adhered each steel type hard layer and the inner layer slab using rolled, cold-rolled annealed, and processing of the plating and the like. Specific manufacturing method is as follows.
[0075]
　First, describe the method for producing the steel by diffusion heat treatment method using the blank plate of sheet steel.
[0076]
　Hard layer and corresponding to the inner layer, and adjusted to a predetermined component and shape, using a thin steel sheet produced by electric furnace or a blast furnace method, an oxide of each bonding surface of the hard layer and the inner layer is removed by pickling, bonding the surface of the application surface after polished with grindstone or abrasive paper # 50 ~ # 1000, the dirt is removed by ultrasonic cleaning, overlay, charged into the heating furnace under a load from the top.
[0077]
　At this time, the average as a steel plate corresponding to the hard layer, by using a steel sheet average micro Vickers hardness of not less than 1.05 times the steel plate corresponding to the inner layer, the average micro Vickers hardness of the hard layer is the inner layer it is possible to obtain a multi-layer steel sheet is 1.05 times or more of the micro-Vickers hardness.
[0078]
　Each thickness of the material of the hard layer and the inner layer to be used for the pasting is preferably a 0.1mm or more. If it is less than 0.1 mm, the residual stresses introduced into the material in the polishing step after pickling, warpage and a gap is generated when the charged materials in a heating furnace, if the hard layer and the inner layer is not bonded with sufficient strength is there. Therefore, each of the thickness is preferably at least 0.1 mm. Meanwhile, no upper limit on the thickness of the material, even 100mm or more, material weight increases, except that the growing difficulty of the work, there is no technical problem in terms of bonding.
[0079]
　It is preferable to use a grinding wheel or abrasive paper # 50 ~ # 1000 in the polishing step. # Of less than 50, low surface roughness of the material, from contacting the lack of the hard layer and the inner layer at the time of diffusion heat treatment, there are cases where sufficient bonding strength can not be obtained after the heat treatment. Therefore, the lower limit of the count of the grinding wheel or abrasive paper used in the polishing # 50 are preferred. Also, when polishing the hard layer and the inner layer of material using a count of the grinding wheel or abrasive paper of more than # 1000, the requested skilled techniques in order to finish the polishing surface planar decreases work efficiency. In addition, even when polished with a count of # 1000 or later, since no observed improvement in significant bonding strength, the upper limit of the count to be used for polishing # 1000 are preferred. More preferably # 80 to # 600.
[0080]
　The furnace temperature is preferably 800 ° C. or higher 1300 ° C. or less. The temperature of the furnace is less than 800 ° C., diffusion of elements between the hard layer and the inner layer is insufficient, it is impossible to obtain a sufficient bonding strength, the lower limit of the heating temperature is preferably at least 800 ° C.. On the other hand, if the heating temperature exceeds 1300 ° C., oxygen is excessively penetrate into the hard layer and the inner layer of the gap between the hard layer and the inner layer, the bonding strength is lowered to form a coarse oxide. Therefore, the upper limit of the heating temperature is preferably 1300 ° C. or less. More preferably 850 ° C. or more and 1200 ° C. or less.
[0081]
　Retention time in the heating furnace for more than 15 minutes, preferably not more than 300 minutes. The retention time is less than 15 minutes, it is not possible to obtain sufficient bonding strength between the hard layer and the inner layer, the lower limit is preferably at least 15 minutes. Meanwhile, in the holding of more than 300 minutes, the oxide between the hard layer and the inner layer is produced, because the bonding strength is lowered, the upper limit of the retention time is preferably 300 minutes or less. More preferably 30 minutes or more and 180 minutes or less.
[0082]
　At the stage of insertion and retention of the furnace, the surface pressure applied to the material, than 10MPa is preferred. Since the surface pressure is less than 10MPa, a sufficient bonding strength can not be obtained, the lower limit of the surface pressure over 10MPa is preferred. Incidentally, the surface pressure is preferably higher, for example, may be given a surface pressure of about 200 MPa.
[0083]
　Heating for multilayer material after cooling, after removal of the load, further, may be subjected to heat treatment. Heat treatment, for example, heated to 800 ~ 1000 ° C., after 1 to 60 minutes retention and cooled. The cooling can be used, for example, water cooling, oil cooling, air cooling. As the cooling rate in the heat treatment is fast, hard layer, since inner both hardness is high, according to the hardness of the multilayer steel sheet hard layer, and the hardness of the material of the steel plate used for the inner layer, it is finally obtained Te, it is possible to set the appropriate cooling rate.
[0084]
　Subsequently, by using a multilayer slab formed by laminating a hard layer on the inner layer, through hot rolling or the steps of hot rolling, pickling, cold rolling, annealing, a process for producing a steel strip satisfying the requirements of the present invention described.
[0085]
　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. Furthermore the upper and lower surfaces of the stacked slabs, sandwiched by high temperature slab just finished casting, encourage diffusion bonding by heat transfer from the hot slab.
[0086]
　More using a multilayer slab made by process according to embodiment to produce a multi-layer strip by conventional hot rolling process.
[0087]
　(Hot rolling)
　after continuous casting a multilayer slab described above, it is or once heated after cooling, when rolled between hot, 1100 ° C. or higher, 1300 ° C. or less for 1 minute or more, between: 300 minutes after heating, 650 ° C. or higher, and ends the finish hot rolling at a temperature range of 950 ° C. or less. Finishing strip after rolling on ROT cooling after 100 ° C. or higher, and Maki up hot-rolled coil at a temperature less than 700 ℃. The following specifically describes how to manufacture the steel sheet of the present invention.
[0088]
　The heating temperature of the slab is 1100 ° C. or more and 1300 ° C. or less. 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, lowering of fatigue properties and cold It causes a decrease between moldability. Additionally, to produce an oxide inside the hard layer, variations in the hardness of the front and back layers of the multilayer strip is increased. Therefore, the upper limit of the heating temperature is 1300 ° C. or less, the upper limit of the soaking time is preferably less 300 minutes. Preferably, the heating temperature is 1250 ° C. or less, soaking time is less than 270 minutes.
[0089]
　Heating the slab, it is important to be held 1-300 minute range of 1100 ~ 1300 ° C., the other, for example, there may be retention time in the temperature range lower than that.
[0090]
　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, increased remarkably the rolling load, further causes an increase of the roll abrasion loss, causing 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., flaws due to thick scales that generates RunOutTable in Tsuban occurs on the surface of the steel sheet, causing the deterioration of fatigue characteristics. Furthermore, since the tissue with the high temperature of the finish rolling temperature becomes mixed grain, variation in hardness of the front and back layers of the multilayer strip is increased. Therefore, the upper limit is 950 ° C. or less. More preferably 800 ° C. or more and 930 ° C. or less.
[0091]
　The cooling rate of the steel strip 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 strip at a cooling rate exceeding 100 ° C. / sec over the inside from the surface of the steel sheet, the outermost layer causes a low-temperature transformation structure, such as over-cooled bainite or martensite. Microcracks in the aforementioned low temperature transformation structure when paying out the coil was cooled to room temperature after removal Maki occurs, 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 strip on the production. It is preferred that this order limit the 100 ° C. / sec or less. The cooling rate prescribed above is steel strip after the finish hot rolling is from the time of receiving water cooled with water injection section after passing through the non-water injection section, each injection interval at the time it is cooled on ROT to the target temperature of the take-up points to a cooling capacity to receive from the cooling equipment does not indicate average cooling rate until the temperature is coiling the coiling machine from injection start point. More preferably 20 ° C. / sec or more and 90 ° C. / sec or less.
[0092]
　Coiling temperature is 50 ° C. or more and 700 ° C. or less. When the coiling temperature is lower than 50 ° C., transformation before winding has substantially completed, thus leaving a large residual stress in the thickness direction. The residual stress, cold formability is significantly reduced, and in the case of applying a cold rolling, in order to lead to deterioration of the plate-shaped, the lower limit of the coiling temperature is set to 50 ° C. or higher. Moreover, when 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 in the inner layer portion, causing a reduction in the cold formability. Therefore, the upper limit of the coiling temperature is set to 700 ° C. or less. More preferably 100 ° C. or more and 680 ° C. or less.
[0093]
　Embodiments of the present invention can not be obtained with typical carburizing and nitriding, nitrocarburizing, high frequency surface quenching such as surface modification treatment before. This is because, in the heat treatment process such as carburizing and nitriding, nitrocarburizing, so allowing the excessive carbon and nitrogen in the surface layer intrusion is for cold formability is lowered, and in techniques such as high-frequency surface hardening is since the carbide remains in thickness center, because the cold formability is lowered.
[0094]
　Pickling the hot-rolled steel strip, by applying cold-rolled and cold-rolled sheet annealing after cold rolling and cold-rolled sheet annealing or hot rolled sheet annealing, cold rolled steel strip having both cold formability and fatigue properties it may be produced.
[0095]
　(Pickling)
　type used in the pickling acid is not 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.
[0096]
　(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.
[0097]
　(Hot-rolled sheet annealing)
　before the hot-rolled steel strip subjected to cold rolling, it 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 an excellent fatigue properties and cold formability of exertion of the steel strip. As a step of hot-rolled sheet annealing may be either box annealing (BAF) or conventional continuous annealing process (C-CAL).
[0098]
　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 fatigue characteristics. 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.
[0099]
　Annealing temperature is 450 ° C. or higher in the box annealing, 720 ° C. or less, holding time 3hr more preferably equal to or less than 100 hr. Annealing temperature is 450 ° C. or less than the retention time is less than 3 hr, 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 100 hr, the surface of the steel strip to adhesion, to produce a seizure, surface appearance is deteriorated. Therefore, the annealing temperature is 450 ° C. or higher in the box annealing, 720 ° C. or less, holding time 3hr more preferably equal to or less than 100 hr.
[0100]
　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.
[0101]
　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.
[0102]
　(Cold-rolled sheet annealing)
　object of the cold-rolled sheet annealing is cold formability of recovery of the steel strip lost by cold rolling, the more the proportion of each structure of ferrite-pearlite-bainite, martensite, residual austenite by optimizing is to obtain excellent fatigue characteristics and cold formability. The process of cold-rolled sheet annealing, conventional continuous annealing process (C-CAL) or reheat type continuous annealing method may be either a (R-CAL).
[0103]
　The cold rolled sheet annealing, characterized on preparation of the heating and holding and cooling step in the conventional continuous annealing process are as described for the continuous annealing of the hot-rolled sheet annealing step.
[0104]
　Temperature at the overaging in normal continuous annealing, 300 ° 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 the resulting superior fatigue properties and cold formability . The aging temperature is 300 ° 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 decrease the fatigue characteristics and cold formability is not compatible. Therefore, the temperature of the overaging zone in the normal continuous annealing, 300 ° C. or higher, 500 ° C. or less, the holding time is 50 seconds or more, preferably 500 seconds or less. More preferred temperature is 450 ° C. or less, the holding time is 90 seconds or more, 300 seconds or less.
[0105]
　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.
[0106]
　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.
[0107]
　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 cold formability of the steel material. 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.
[0108]
　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 fatigue characteristics and cold during moldability can be obtained. 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 decrease the fatigue characteristics and cold formability 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.
[0109]
　Further, in the cold sheet annealing step, the zinc on both surfaces of the steel sheet, aluminum, may be formed a coating layer selected from the group consisting of magnesium and alloys thereof.
[0110]
　Embodiments of the present invention can not be obtained with typical carburizing and nitriding, nitrocarburizing, high frequency surface quenching such as surface modification treatment before. This is because, in the heat treatment process such as carburizing and nitriding, nitrocarburizing, because excessive carbon and nitrogen in the surface layer is penetrated unevenly, variation in hardness of the hard layer is increased, which decreases cold formability is there. In the technique, such as high frequency surface hardening, it remains carbide thickness center, since the volume percentage of carbide becomes large to be contained in the inner layer, because the cold formability is lowered.
[0111]
　Steel produced by diffusion heat treatment method using the blank plate of the thin steel sheet or hot rolled strip was produced from a multilayer slab and cold-rolled annealed steel strip to a material, a component by hot molding method such as a hot stamp It is prepared, the effect of the present invention can be obtained as well.
[0112]
　According to the manufacturing method 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 400 HV 240HV, further controls the C and N content of the hard layer to an appropriate value with an average micro Vickers hardness of the inner layer and over and less than 400 HV 80HV, the volume percentage of carbide contained in the inner layer is controlled to less than 2.00%, the hardness average micro Vickers inner average micro Vickers hardness of the hard layer with the 1.05 times or more, it is possible to obtain a steel sheet having both fatigue characteristics and cold workability at a high level.
Example
[0113]
　The following Examples will further illustrate the effects of the present invention.
[0114]
　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.
[0115]
　[Example 1]
　pickled the front and back surfaces of the hot-rolled steel sheet having a thickness of 2mm having the components of A ~ F shown in Table 1-1, a plate of the front and back layer by cutting the steel sheet pasting a hard layer on the front and back layers 0.8mm thick, the thickness of the inner layer after finished 0.4 mm, polishing the bonding surface with abrasive paper of # 180 to remove the dirt by ultrasonic cleaning. Further, the steel sheet pasted only hard layer on the surface layer finishing 0.8mm plate thickness of the surface layer by cutting, the plate thickness of the inner layer to 1.2 mm, was subjected to polishing and cleaning described above.
[0116]
　Followed by Table 2. Example No. In the combination shown in 1-30, overlay, inserted into a heating furnace of 1000 ° C. under a load of 0.2 MPa, were taken out after 120 minutes holding. Once removed the load was applied to the sample after cooling to room temperature, followed by inserted into a heating furnace of 900 ° C., after 20 minutes holding was subjected to the cooling process.
[0117]
　Example No. 1 to 6 is quenched in 60 ℃ oil, No. 7 to 12 are water-cooled, No. 13 through 24 are subjected to air cooling. Further, to clarify the difference between previous processing conditions, Example No. In 27-30, put high frequency surface baked, nitride, carburizing (carbon potential 0.37%), is subjected to the processes of shot peening, to produce comparative samples with increased surface hardness. Subsequently, the procedure described above, each sample hardness, tissue, fatigue characteristics were evaluated cold formability.
[0118]
　The evaluation results of each sample shown in Table 2. In the table, "front" is hard layer on the surface, "back" is the rear surface of the hard layer, "the" inner layer, VCEM volume percentage of carbide, TS denotes the tensile strength (hereinafter the same). No. of invention example 5,8,12,13,15,17-19,21,23,24 are all average micro Vickers hardness of the hard layer is more than 240HV, less than 400 HV, C of the hard layer is 0.4% below, together with the N content is less than 0.02%, the average Vickers hardness of the inner layer at least 80HV and 400HV or less, the volume percentage of carbide is less than 2% contained in the inner layer, the hard layer and the inner layer hardness of the ratio is not less 1.05 times or more, it has been shown to be excellent in fatigue properties and cold workability.
[0119]
　On the other hand, No. of Comparative Example 1,7,16,22 The hardness of the surface layer is less than 240HV, the fatigue characteristics deteriorated.
[0120]
　No. 2,4,9,10 hardness of the inner layer is greater than 400 HV, cold formability is lowered.
[0121]
　No. 1-4,7,9,10,14,16,20,22, the ratio of the hard layer and the inner layer hardness is not appropriate, the fatigue characteristics deteriorated.
[0122]
　No. The high frequency surface hardening heat treatment of 27, the volume percentage of carbide remaining in the inner layer is more than 2%, cold formability is lowered. No. The nitriding heat treatment of 28, since the N content of the surface layer is more than 0.02%, cold formability is lowered. No. The carburizing heat treatment 29, in addition to the high content and C, the inner layer of the softening proceeds, cold formability is lowered. No. Nitriding treatment 28, No. Carburizing heat treatment of 29, No. In shot peening of 30, variation in hardness is large, the fatigue strength deteriorated.
[0123]
　Example 2
　Next, to examine the effect of the thickness of the hard layer, in Table 2 Invention Example No. The combination of 23 based, by controlling the cold rolling the plate thickness of the hard layer and the inner layer in advance, a sample having a thickness of 2mm with different procedures thickness ratio of the rigid layer according to the previously described No. The 31 to 47 were produced.
[0124]
　The evaluation results of each sample in Table 3. No. of invention example 34-46 are all the thickness of the hard layer is 20μm or more and 40% or less of Zen'itaAtsu is shown to be excellent in fatigue properties and cold workability.
[0125]
　On the other hand, No. of Comparative Example 31-33, the thickness of the hard layer is less than 20 [mu] m, the fatigue characteristics deteriorated. In addition, No. of Comparative Example 47, the thickness of the hard layer is greater than 40%, cold formability is lowered. For ease of understanding, the relationship between thickness and fatigue limit ratio of the hard layer for each invention examples and comparative examples in Table 3 in FIG. 1.
[0126]
　[Example 3]
　Next, to examine the influence of the component, D in Table 1-1, E, a sample of thickness 1.2mm having a composition of F is fixed to the inner layer, having the composition j ~ aj plate samples of thickness 0.4mm by combining the hard layer was investigated the effect of component (examples No.48 ~ 92). Furthermore, by combining a table 1-2, b, a sample having a composition of c is fixed to a rigid layer of a thickness 0.4 mm, a sample having a thickness of 1.2mm having the composition G ~ AJ inner layer, We evaluated the effect of component (examples No.93 ~ 141). In the production of each sample to adjust the thickness by cutting along the procedure described above, it was subjected to diffusion heat treatment is subjected to grinding and cleaning, and insert the sample into a furnace heated to 900 ° C., 20 removed after partial held and pressed at flat die and then cooled mold to room temperature.
[0127]
　The evaluation results of each sample in Table 4-1 and Table 4-2. No. of invention example 49,53,55 ~ 57,73 ~ 96,98 ~ 102,104 ~ 107,109 ~ 113,119,132,133,137,138 are all average micro Vickers hardness of the hard layer is more than 240HV, 400 HV less than, less than 0.4% of C of the hard layer, together with the N content is less than 0.02%, the inner layer of the average Vickers hardness is 80HV or more and 400HV or less, the volume fraction of carbides contained in the inner layer less than 2%, the ratio of the hard layer and the inner layer of the hardness is not less than 1.05, was shown to be excellent in fatigue properties and cold workability.
[0128]
　[Example 4]
　To further investigate the effect of manufacturing conditions by hot rolling, a table 1-2 superior properties in the hard layer was observed, b, c, d, g , j, k, l; m , n, o, p, q , and s are arranged as front and back layers, a of Table 1-1 that superior properties in the inner layer was observed, B, C, D, E , F, G, H, I, J , M, N, produced O, P, the laminate of the ingot that combines Q in the inner layer, and held for 120 minutes in a furnace heated to 1000 ° C. over 0.1MPa surface pressure was subjected to diffusion heat treatment . Incidentally, the thickness ratio of the front and back layers in the laminate of the ingot 40%, was adjusted inner layer thickness ratio of 20%.
[0129]
　Subsequently, to prepare each sample having a thickness of 2.4mm by hot rolling conditions shown in Table 5-1 and 5-2, after performing pickling, was subjected to various evaluation tests. Table retention time in the 5-1 and 5-2 indicates the time in which the heating temperature was in the range of 1100 ~ 1300 ° C..
[0130]
　It shows the evaluation results of each sample in Table 5-1 and 5-2. No. of invention example 142, 144, 146 ~ 150, 152 ~ 156,158 ~ 161,163 ~ 174,176 ~ 190 are all average micro Vickers hardness of the hard layer is more than 240HV, less than 400 HV, the C content of the hard layer less than 0.4%, the N content is less than 0.02%, the average Vickers hardness of the inner layer at least 80HV and 400HV or less, the volume percentage of carbide contained in the inner layer is less than 2%, the hard layer and the inner layer hardness ratio of is at least 1.05, has been shown to be excellent in fatigue properties and cold workability.
[0131]
　On the other hand, No. of Comparative Example 151,162 has a long heating time hot rolling, to form oxide layer between the hard layer and the inner layer, in the evaluation of the fatigue test and cold formability, hard layer and the inner layer of interfacial peeling occurred. No. of Comparative Example 157,175 has a high finishing temperature of hot rolling, thick scale is generated on the steel strip surface, the surface roughness is decreased, the fatigue characteristics deteriorated. No. of Comparative Example 143 is low finishing temperature of hot rolling, hard layer becomes hard, the ratio of the hard layer and the inner layer of the hardness was not appropriate. No. of Comparative Example 145 high coiling temperature of hot rolling, reduces the hardness of the hard layer, it led to reduction of the inner layer of hardness.
[0132]
　Example 5
　- To examine the effect of production conditions according to "cold-rolled
[0133]
　The evaluation results of each sample in Table 6. Regardless of the conditions - "cold-rolled annealed", of the invention examples satisfying the requirements of the present invention No. 191-211 are all average micro Vickers hardness of the hard layer is more than 240HV, less than 400 HV, C of the hard layer is less than 0.4%, the N content is less than 0.02%, the inner layer of above the average Vickers hardness is 80HV and 400HV or less, the volume percentage of carbide is less than 2% contained in the inner layer, the ratio of the hard layer and the inner layer of the hardness is not less than 1.05, the fatigue characteristics and cold workability it is excellent to have been shown.
[0134]
[Table 1-1]

[0135]
[Table 1-2]

[0136]
[Table 2]

[0137]
[table 3]

[0138]
[Table 4-1]

[0139]
[Table 4-2]

[0140]
[Table 5-1]

[0141]
[Table 5-2]

[0142]
[Table 6]

The scope of the claims
[Requested item 1]
　An inner layer, a steel plate and a hard layer on one or both surfaces of the inner layer,
　the thickness of each hard layer is 20μm or more and 40% or less of the plate thickness of the steel plate,
　each hard layer the average micro Vickers hardness of at least 240HV of less than 400 HV,
　said C content of the hard layer is 0.4 wt% or less, N amount is not more than 0.02 wt%,
　10 [mu] m from the surface of each hard layer variations in hardness measured by a nano-indenter at a depth of is 2.0 or less in standard deviation,
　average micro Vickers hardness of the inner layer is more than 80HV, less than 400 HV,
　the volume of the carbide contained in the inner layer rate is less than 2.00%,
　the average micro Vickers hardness of each hard layer is more than 1.05 times the average micro Vickers hardness of the inner layer
steel sheet, characterized in that it.
[Requested item 2]
　Note before the hard layer ha,で
　　mass%, C:
　　0.03
　　~ 0.35%, Si: 0.01 ~ 3.00%, Mn: 0.70
　　~ 10.00%, P: 0.0200% or less,
　　S: 0.0200% or
　　less, of Al: 0.500% or
　　less, N: 0.0200% or
　　less, O: 0.0200% or
　　less, of Ti:
　　0 ~ 0.500%, B: 0.0100 ~ 0%, 　　cr:
　　~ 0 a 2.000%, 　　of Mo: 0 ~ 1.000%, of Nb: 0 ~ 　　0.500%, V: 0 ~ 0.500%, a Cu: 0 ~ 　　0.500%, W is: 0 0 ~ 　　% 　　.100, 　　Ta: 0 ~ 0.100%, 　　of Ni: 0 ~ 0.500%, of Sn: 0 ~ 　　0.050%, Sb: 0 ~ 0.050%, of As: 0 ~ 　　0.050%, of Mg : 　　0 　　~ 0.0500%, of Ca: 0 ~ 　　0.050%, the Y: 0 ~ 0.050%, Zr: 0 ~ 　　0.050%, La: 0 ~ .050%, andび 　　Ce: 0 ~ 0.050% wo shi containing, remnants ga Fe and impuritiesびでthou ri, 　front inner mind ha,で 　　mass%, C: from 0.001 　　~ 0.300%, Si: 0.01 ~ 　　% 3.00, Mn: 0.10 　　~ 3.00%, P: 0.0200% or 　　less, S: 0.0200% or 　　less, of Al: 0.500% or 　　less, N: 0.0200% or 　　less, O: 0.0200% or 　　less, 　　of Ti: 　　0 ~ 0.500%, B: 　　0.0100 ~ 0%, of Cr: 0 ~ a 2.000%, of 　　Mo: 0 ~ 1.000%, of Nb: 0 ~ 0.500% , 　　V: 　　0 　　~ 　　0.500%, a Cu: 0 ~ 0.500%, W is: 0 ~ 　　0.100%, Ta: 0 ~ 0.100%, of Ni: 0 ~ 　　0.500%, of Sn: 0 ~ 　　% 0.050, 　　Sb: 0 ~ 　　0.050%, of As: 0 ~ 0.050%, of Mg: 0.0500 ~ 　　0%, of Ca: 　　0.050% ~, 　　the Y: 0 ~ 0.050%, 　　Zr: 0 ~ 0.050%, La: 0 ~ 0.050%, andび 　　Ce: 0 ~ 0.050% wo shi contained, and Fe remnants gaびimpuritiesでthou ru koととspecial Zhi wo suru ni requested item 1 describedのplate.