[0001]The present invention relates to a steel sheet, in particular, automotive parts and infrastructure structural members whose primary purpose relates excellent cold-forming a steel sheet for wear resistance.
BACKGROUND
[0002]Automobile seat rails and torque converter housing, and to ensure the wear resistance is required for the member used in a sliding environment as handrail guide, etc. of the escalator. Although the cause of the wear in each of the products are not identical, the wear resistance is ensured by the high strength of the surface in contact with the sliding portion. For example, in the seat rail proceeds wear by the steel members to each other, abrasion due to the metal powder contained in the oil occurs in the housing of the torque converter. Further, the handrail guide of the escalator wear is accelerated by fine particles such as sand biting into the gap of the resin handrail portion and the metal guide. However, by increasing the strength of the surface of the steel member subjected to abrasion, wear resistance is remarkably increased.
[0003]
　Previously, a number of proposals for surface modification technique for improving the wear resistance of the steel member have been made (for example, Patent Documents 1-5, reference).
[0004]
　For example, Patent Document 1, chemical composition, in mass%, C: 0.10 ~ 0.28%, Si: 0.15 ~ 0.35%, Mn: 0.30 ~ 1.50%, P 0.035% or less, S: 0.035% or less, Cr: 1.45 ~ 3.00%, Mo: 0.80% or less (including 0%), Al: 0.020 ~ 0.060% , N: 0.0080 contains ~ 0.0250%, the balance being Fe and unavoidable impurities, the sliding surface has no carburization anomaly layer, range surface carbon concentration is from 0.70 to 0.90 wt% located within, carburized member and a manufacturing method thereof troostite area ratio and excellent wear resistance which can improve the wear resistance is 0.70% of the tissue at depth 50μm from the outermost surface It has been disclosed.
[0005]
　Further, Patent Document 2, C: 0.001 ~ 0.020%, Si (.% By weight of the meanings hereinafter the same.): 0.1% or less (not including 0%), Mn: 0.1 ~ 0.5%, P: 0.02% or less (not including 0%), S: 0.02% or less (not including 0%), Cu: 0.1% or less (not including 0%), Ni: 0.1% or less (not including 0%), Al: 0.040% or less (not including 0%), N: 0.0040% or less (not including 0% of), further, nb: 0.015 ~ 0.08% and / or Ti: a from 0.01 to 0.1% contained so as to satisfy a predetermined formula (1), a balance of iron and inevitable impurities, is steel structure it is excellent in abrasion resistance, which is a ferrite single phase, soft magnetic steel excellent in magnetic properties after nitriding treatment is disclosed.
[0006]
　Further, Patent Document 3, and characterized in that it comprises a step for molding the required cutting a carbon steel material, and performing induction hardening, and a final polishing step carried out immediately after the induction hardening step to wear resistance (or high surface pressure of wear), fatigue strength and a columnar or cylindrical sliding component and a manufacturing method made of carbon steel and to enhance the rigidity is disclosed.
[0007]
　Patent Document 4, by mass%, C: 0.001 ~ 0.005% , Si: 0.03 ~ 0.5%, Mn: 0.1 ~ 1.0%, Al: 0.015% ~ 0.1%, Ti: 0.03 ~ 0.1 %, Cr: contains 0.4 ~ 1.4%, P: is limited to 0.035% or less, the balance being Fe and unavoidable impurities further, the number of the depth of 0.25mm from the steel outermost surface of the ferrite crystal {001} tabular direction size 5 ~ 10 nm of Cr nitrides plate deposited on the surface, the Cr nitrides density there × 10 1 17 cm -3 or more, preferably, the ratio of the N content and Cr content of the Cr nitrides (N / Cr) is within the range of 0.5-0.8, primarily for a motor vehicle parts and applications, and excellent press formability such as total elongation properties and hole expandability before soft-nitriding, surface layer hardness of sufficient thickness after nitrocarburizing treatment Soft nitriding steel excellent in wear resistance having a layer is disclosed.
[0008]
　Patent Document 5, C: 0.15 ~ 0.40%, Si: 0.15 ~ 0.40%, Mn: 0.5 ~ 1.5%, S: 0.003 ~ 0.050%, cr: 0.7 ~ 1.5%, Cu: 0.30 ~ 0.80%, Ni: 0.15 ~ 1.0%, N: 0.003 ~ 0.020%, and Al: 0.005 to 0.050% contained, the balance being Fe and impurities, each P and O in the impurities, P: 0.025% or less, O: it is 0.0020% or less, "rolling contact" "sliding contact" and - carburizing or carbonitriding parts steel material by "rolling-sliding contact" to prevent premature failure of the component is disclosed.
CITATION
Patent Document
[0009]
Patent Document 1: JP 2016-98432 JP
Patent Document 2: JP 2014-74234 JP
Patent Document 3: JP-A-11-44203
Patent Document 4: JP 2005-264318 Patent Publication
Patent Document 5: JP 2014-19920 JP
Summary of the Invention
Problems that the Invention is to Solve
[0010]
　In any of the prior art described above, there is a problem that the size and shape of the steel part changes during the heat treatment. This change is dependent on slight cooling rate unevenness in cooling in the heat treatment in the component. Therefore, accurate prediction of the size and shape of the part has become practically difficult. Also, when corrected by the shape of the member after the above heat treatment, for example bending, it is or member easily cracks occur member to break. Therefore, usually, subjected to cutting or pressing tempering the part after the heat treatment, adjust the required value of parts size and shape. Incidentally, the application of the cutting or pressing tempering, reduced productivity and leads to increased cost of part manufacturing, also, the manufacture of large and elongated member is also faced problems difficult.
Means for Solving the Problems
[0011]
　The present inventors have intensively studied a technique for solving the above problems. As a result, the sliding portion of one or both surfaces of the steel sheet, the average micro Vickers hardness is to form a hard layer of less than and 700 HV 400 HV, further with appropriately controlling the composition of the hard layer and the inner layer, the inner layer the average micro Vickers hardness not less than and less than 400 HV 80HV, the volume percentage of carbide contained in the inner layer is controlled to less than 2.00%, and to control the standard deviation of nano-hardness of the hard layer to 2.00 or less of it was thus found that to achieve both wear resistance 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,
　wherein the hard layer is a
　mass%, C: 0.08
　~ 0.40%, Si: 0.01
　3.00%

　10.00%,~, P: 0.0001 ~ 0.0200%, S: 0.0001 ~ 0.0200%
containing the balance being Fe and impurities,
　the inner layer, by
　mass%,
　C:
　0.001 ~ 0.200%, Si: 0.01 ~
　3.00%, Mn: 0.20 ~ 3.00%, P: 0.0001 ~ 0.0200
　%, S: 0.0001 ~ 0.0200%
containing the balance being Fe and impurities,
　the thickness of each hard layer is 20μm or more and 2/5 or less of Zen'itaAtsu,
　the average micro the hard layer Vickers hardness above 400 HV, less than 700 HV,
　N amount of the hard layer is 0.02 Or less,
　the 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%,
　Wherein the standard deviation of nano-hardness of the hard layer is 2.00 or less, the steel sheet.
　(2) the hard layer further contains, by
　mass%, Al: 0.500% or
　less, N: 0.0200% or
　less, Cr: 2.000% or
　less, Mo: 1.000% or
　less, O: 0. 0200% or
　less, Ti: 0.500% or
　less, B: 0.0100% or less
and containing one or more of,
　the inner layer further contains, by
　mass%, Al: 0.500% or
　less, N: 0.0200% or
　less, Cr: 2.000% or
　less, Mo: 1.000% or
　less, O: 0.0200% or
　less, Ti: 0.500% or
　less, B: 0.0100% or less
of one or characterized by containing the above species, the steel sheet according to (1).
　(3) the hard layer further contains, by
　mass%, 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,
and contain one or more of,
　the the inner layer further contains, by
　mass%, 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, the steel sheet according to (1) or (2).
The invention's effect
[0015]
　According to the present invention, the sliding portion of one or both surfaces of the steel sheet, the average micro Vickers hardness is to form a hard layer of less than and 700 HV 400 HV, further appropriately controlling the composition of the hard layer and the inner layer 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%, and the standard deviation of nano-hardness of the hard layer 2.00 by controlling the, possible to provide a steel sheet excellent in wear resistance and cold workability.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016]
FIG. 1 is a diagram showing the wear resistance of the relationship between the thickness of the hard 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.
DESCRIPTION OF THE INVENTION
[0017]
　The present invention will be described in detail.
[0018]
　First, the thickness of the hard layer and the inner layer, component, the reason for limiting the standard deviation of nano-hardness of the average micro Vickers hardness and the hard layer will be described. Here, "%" for component means mass%.
[0019]
　The thickness of the hard layer present on one or both surfaces of the inner layer, or 20μm per side, 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, in an environment of wear or sliding, lead to peeling of the hard layer, it is not obtained excellent wear resistance. Therefore, the thickness of the hard layer, a 20μm or more. Also, if the thickness of each hard layer is greater than 2/5 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 and 2/5 or less of Zen'itaAtsu. More preferably, the thickness of each hard layer, 30 [mu] m or more and 3/10 or less.
[0020]
　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. 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.
[0021]
　Average micro-Vickers hardness of the hard layer 400HV or more and less than 700 HV. If the average micro Vickers hardness of less than 400 HV, low hardness of the hard layer, has excellent wear resistance under the environment of wear or sliding it can not be obtained. Therefore, an average micro Vickers hardness of the hard layer is not less than 400 HV. On the other hand, the average micro Vickers hardness of the hard layer becomes more than 700 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 700 HV. More preferably, more than 450 HV, or less 680HV.
[0022]
　The N amount contained in the hard layer to 0.02% or less. N is 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 N content 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, the N amount contained in the hard layer to 0.02% or less. More preferably, the N content is 0.018% or less.
[0023]
　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.
[0024]
　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).
[0025]
　The standard deviation of nano-hardness of the hard layer is 2.00 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 2.00, for example, there are cases where cracking occurs during cold roll forming. From this point of view, the standard deviation is preferably 2.00 or less, more preferably 1.60 or less. The lower limit of the standard deviation is not specified, but be kept to 0.20 or less is technically difficult. Note that 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 . In effect, if 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.
[0026]
　Next, reasons for limiting the optimal chemical composition of the steel sheet of the hard layer will be described. Here, "%" for component means mass%.
[0027]
(C: 0.08 ~ 0.40%)
　C is an effective element for strengthening steel. To ensure the wear resistance of parts by heat treatment such as quenching and tempering is required C amount of 0.08% or more, insufficient hardness after quenching is less than 0.08%, excellent wear resistance can not be obtained. Therefore, the lower limit is 0.08% or more. C is also a significant 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 C content exceeds 0.40% 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, the upper limit 0.40% or less. Preferably 0.10% or more, or less 0.39%. More preferably, 0.10% or more, or less 0.38%.
[0028]
(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 the wear resistance 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. Therefore, it is essential content of 0.01% or more Si. Is less than 0.01%, generation of carbides is not suppressed, a large amount of carbide become present in the steel, cold formability deteriorates. 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 is more than 3.00% leads to embrittlement of the steel components, to reduce the cold formability, the upper limit or less 3.00%. Preferably 0.01% or more, 2.5% or less. More preferably, 0.2% or more, 2.0% or less.
[0029]
(Mn: 1.000 ~
　10.00%) Mn acts as a deoxidizer and also is an element effective in suppressing the steel pearlite transformation. If it is less than 1.000%, 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 wear resistance of the deterioration of strength . Therefore, the lower limit 1.000% 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 cold forming, cold formability deteriorates. Therefore, the upper limit or less 10.00%. Preferably 2.0% or more, or less 9.0%.
[0030]
(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 is 0.0001% or more. On the other hand, if it exceeds 0.0200%, the lowering the cold formability by grain boundary embrittlement, the upper limit or less 0.0200%. Preferably 0.0010% or more, or less 0.0190%.
[0031]
(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 parts. 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 is 0.0001% or more. On the other hand, if it exceeds 0.0200% leads to generation of cracks starting from the non-metallic inclusions during cold forming, since cold formability is lowered, the upper limit or less 0.0200%. Preferably 0.0010% or more, or less 0.0190%.
[0032]
　In the present invention, the hard layer may further comprise the following components optionally.
[0033]
(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 cold formability. Therefore, the upper limit 0.500% or less. Preferably less 0.450% 0.010% or more.
[0034]
(N: 0.0200% or less)
　N is an element effective for strengthening steel as well and C, and also, as described above, is influencing element in the development of cross slip dislocation during the cold-forming . From the viewpoint of securing the cold formability, as the low content preferably it 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% as described above, it can not be suppressed concentration of the strain during the cold molding, from causing generation of voids, cold formability is markedly reduced. Therefore, to 0.0200% the upper limit. Preferably 0.0010% or more, or less 0.0150%.
[0035]
(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. Meanwhile, the content exceeding 0.0200% to reduce the cold formability, the upper limit 0.0200% or less. Preferably 0.0005% or more, or less 0.0170%.
[0036]
(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 securing the cold formability, as the low content preferably it 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 cold formability. Therefore, the upper limit 0.500% or less. Preferably 0.005% or more, or less 0.450%.
[0037]
(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 increasing the strength or wear resistance improve it 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 becomes a starting points of voids during cold forming, cold formability deteriorates. Therefore, the upper limit to 0.0100% or less. More preferably 0.0005% or more and 0.0050% or less.
[0038]
(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 cold formability, the upper limit is 2.000% or less. Preferably 0.01% or more, or less 1.500%.
[0039]
(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 cold workability. More preferably 0.010% or more, or less 0.700%.
[0040]
(Nb: 0.500% or less)
　Nb is an element effective for carbide forms controlled similarly to Ti, is an effective element in improving the toughness for refining the tissue by the addition. 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, to lower the cold workability. Therefore, the upper limit 0.500% or less. Preferably 0.002% or more, or less 0.200%.
[0041]
(V: 0.500% or less)
　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. 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 fine V carbide a number leads to increase in strength and reduction in the ductility of the steel material, reduce the cold formability. Therefore, the upper limit 0.500% or less. More preferably 0.002% or more, or less 0.400%.
[0042]
(Cu: 0.500% or
　less) Cu is an effective element to increase the strength of the steel, 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%.
[0043]
(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 material, the upper limit to reduce the cold workability is 0.100% or less. More preferably 0.002% or more, or less 0.080%.
[0044]
(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 increased strength and reduced ductility of the steel material, reduce the cold workability. Therefore, the upper limit 0.100% or less. More preferably 0.002% or more, or less 0.080%.
[0045]
(Ni: 0.500% or
　less) Ni is an element effective in improving the wear resistance of components, it is added as required. 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 cold formability. More preferably 0.002% or more, or less 0.400%.
[0046]
(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 decrease in the cold formability by embrittlement of ferrite, the upper limit 0.050% or less. More preferably 0.001% or more and 0.040% or less.
[0047]
(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 decrease in the cold formability, the upper limit 0.050% or less. More preferably 0.001% or more and 0.040% or less.
[0048]
(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%, the lowering the cold formability, the upper limit 0.050% or less. More preferably 0.001% or more and 0.040% or less.
[0049]
(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 decrease in the cold formability due to the formation of coarse inclusions, and 0.0500% to the upper limit. More preferably 0.0005% or more, or less 0.0400%.
[0050]
(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, to become a starting point of cracking during cold forming, the upper limit 0.050%. More preferably 0.001% or more and 0.040% or less.
[0051]
(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, for cold formability decreases, the upper limit 0.050%. Preferably 0.001% or more, 0.040% or less.
[0052]
(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, for cold formability decreases, the upper limit 0.050%. Preferably 0.001% or more, 0.040% or less.
[0053]
(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 cold formability. More preferably 0.001% or more and 0.040% or less.
[0054]
(Ce: 0.050% or less)
　Ce is an element capable of controlling the form of sulfides Similarly dopants and La, and an element causing a decrease in the number ratio of grain boundary carbides and strongly segregate at the grain boundaries, They are added as needed. 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%, the deteriorated workability due to a decrease in the number ratio of the grain boundary carbides, so the upper limit 0.050%. More preferably 0.001% or more, or less 0.046%.
[0055]
　Furthermore, a description will be given reasons for limiting the optimum chemical component in the inner layer of the steel sheet. Here, "%" for component means mass%.
[0056]
(C: 0.001 ～ 0.200 Pasento)
　C is an effective element to strengthen steel. In order to control the average micro Vickers hardness of the inner layer above 80HV is, it requires the addition of more than 0.001%, the lower limit of 0.001%. On the other hand, if it exceeds 0.200% causes an increase or production of carbides of excessive strength of the inner layer, cold formability is lowered. Therefore, the upper limit 0.200% or less. Preferably 0.002% or more, or less 0.19%.
[0057]
(Mn: 0.20 ~
　3.00%) Mn acts as a deoxidizer and also is an element effective to control the strength of the steel. If it is less than 0.20%, 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 is caused, because the cold formability is lowered, the lower limit is 0.20% or more. On the other hand, when it exceeds 3.00%, lead to an excessive increase in strength, cold formability deteriorates. Therefore, the upper limit or less 3.00%. Preferably 0.30% or more, 2.6% or less.
[0058]
　The inner layer, in addition to the C and Mn, Si, P, contain S, further Al optionally, 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.
[0059]
　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.
[0060]
　Steel sheet of the present invention comprises an inner layer, and a hard layer on one or both surfaces of the inner layer, the hard layer and the inner layer has a predetermined composition, the thickness of each hard layer is 20μm or more, 2 Zen'itaAtsu / 5 or less, an average micro Vickers hardness of the hard layer is more than 400 HV, less than 700 HV, N of the hard layer is not more than 0.02%, the average micro Vickers hardness of the inner layer is more than 80HV, less than 400 HV and the volume percentage of carbide contained in the inner layer is less than 2.00%, by the standard deviation of nano-hardness of the hard layer is 2.00 or less, an excellent cold formability and abrasion resistance it is a novel finding by the present inventors have found that both.
[0061]
　Subsequently, it describes the organization method observations and measurements.
[0062]
　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.
[0063]
　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 at each thickness 1/4 position from the hard layer and the inner layer of the surface side. The ratio of the organization is determined at the point count how. 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.
[0064]
　The average micro Vickers hardness of the steel sheet, each thickness 1/4 0.098 N micro Vickers hardness of at the position of the hard layer and the inner layer of the surface, were measured 12 points, the hardest data, and most soft data determined from the average value of 10 points excluding the. 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 700HV is about 5 [mu] m, can be suitably evaluated hardness of the hard layer having a thickness of about 20μm it is.
[0065]
　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 term "nano hardness standard deviation of the hard layer", 1/4 position in the thickness of the hard layer from the surface side of the hard layer, nano hardness of 100 points at intervals of 3μm to rolling direction and using a tribo-900 of Hysitron Inc., measured under the conditions of an indentation depth 80nm by a diamond indenter Berkovich shape, creating a histogram from the obtained hardness data, fitting when approximating the histogram in a normal distribution it is intended to refer to the standard deviation of the curve.
[0066]
　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, if the length of the recognized cracking and cracks of less than 5 [mu] m, determined that excellent cold workability, " ○ gave an indication of the ". Also, if the length of the recognized cracking and cracks than 5 [mu] m, cold formability gave an indication of the determined and inferior, "×".
[0067]
　Furthermore, we evaluated how the wear resistance. The wear resistance of the samples was evaluated by block-on-ring test. Processed SUJ2 in the shape of a ring test piece, after 50min maintained at 840 ° C. in a furnace was controlled to 95% nitrogen atmosphere, quenching was in 60 ° C. oil. To prepare a ring sample subjected to tempering air cooling after 60min held at 180 ° C. to quenching the sample. Each evaluation sample as a block test piece, sliding speed 0.6 m / s, oil temperature 140 ° C., and tested under a load 5000N, the slippage distance 10000m conditions, by measuring the weight change before and after the test of the block sample, weight loss There is determined that the case within 3.0mg excellent wear resistance, reduction was judged to be inferior in wear resistance if it exceeds 3.0mg.
[0068]
　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 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.
[0069]
　The method for producing the 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 abrasion resistance, and these hard layers and the inner layer of the strength and composition - is characterized by controlling the optimum state of tissue. 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. Characteristic of a specific manufacturing method is as follows.
[0070]
　First, describe the method for producing the steel by diffusion heat treatment method using the blank plate of sheet steel.
[0071]
　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.
[0072]
　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.
[0073]
　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.
[0074]
　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.
[0075]
　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.
[0076]
　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.
[0077]
　Subsequently, by using a slab obtained by laminating a hard layer on the inner layer, through hot rolling or the steps of hot rolling, pickling, cold rolling, annealing, it describes a method for producing a steel strip satisfying the requirements of the present invention.
[0078]
　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.
[0079]
　More using slab made by process according to embodiment to produce a multi-layer strip by conventional hot rolling process.
[0080]
　Hot rolling characteristics: After continuous casting the aforementioned slab heating it, or once after cooling, when rolled between hot, 1100 ° C. or higher, 1300 ° C. or less for at least 15 minutes, and heated between: 300 minutes after, 650 ° C. or higher, and ends the finish hot rolling at a temperature range of 950 ° C. or less. The steel strip after finish rolling and Maki up hot-rolled coil at a temperature less than 700 ℃ After cooling on ROT. The following specifically describes a method for producing the steel sheet of the present invention.
[0081]
　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 the wear resistance and cause a decrease in cold formability. 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.
[0082]
　Also, if the slab heating temperature exceeds 1300 ° C., or the heating time exceeds 300 minutes, the austenite grains in the slab become mixed grain, the particle size of the dispersion is increased. Variation in particle size of the austenite is also taken over in the subsequent hot rolling and cold rolling-annealing. Further, since the particle size which affects the phase transformation, so produce variations in structure and hardness in the interior of the steel sheet. Therefore, when either the slab heating temperature exceeds 1300 ° C., or the heating time exceeds 300 minutes, the standard deviation of nano-hardness is more than 2.00. On the other hand, if the slab heating temperature is less than 1100 ° C., or the heating time is shorter than 15 minutes, it is difficult to eliminate the element segregation occurring during casting. The elemental segregation is also taken over in the subsequent hot rolling and cold rolling-annealing. Since the element segregation affecting phase transformation behavior, so produce variations in structure and hardness in the interior of the steel sheet. Therefore, when either the slab heating temperature is less than 1100 ° C., or the heating time is less than 15 minutes, the standard deviation of nano-hardness is more than 2.00.
[0083]
　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 a decrease in wear resistance. Therefore, the upper limit is 950 ° C. or less. More preferably 800 ° C. or more and 930 ° C. or less.
[0084]
　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 layer 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 from the time when the steel strip after the finish hot rolling is subjected to water cooling in 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 coiling 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.
[0085]
　Maki-up temperature is set to 700 ℃ or less. When Maki-up 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 Maki-up 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 (subzero treatment) than room temperature, for hot-rolled sheet is considerably brittle, the lower limit of the coiling temperature preferably more than 0 ℃. More preferably 10 ° C. or more and 680 ° C. or less.
[0086]
　Furthermore, pickling the hot rolled strip of the above, by performing the cold-rolled and cold-rolled sheet annealing after cold rolling and cold-rolled sheet annealing or hot rolled sheet annealing, cold rolling having both cold formability and abrasion resistance it may be manufactured the steel strip.
[0087]
　Pickling: The type of acid used in the pickling step is not particularly defined. 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 an excellent wear resistance and cold formability of exertion 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.
[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 wear 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, holding time 3hr more, less preferred 150Hr. Annealing temperature is 400 ° 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 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, holding time 3hr more, less preferred 150Hr. More preferred annealing temperature is 420 ° C. or higher, 700 ° C. or less, retention time 5hr 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]
　Cold-rolled sheet annealing step: The purpose 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 an excellent wear resistance and cold formability. 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).
[0095]
　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.
[0096]
　Temperature at the overaging in normal 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, as excellent abrasion resistance and cold formability is obtained Become. 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 decrease wear resistance and cold formability 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.
[0097]
　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 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.
[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, and excellent wear resistance so cold formability is 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 wear resistance 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.
[0101]
　Further, 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, from allowing the entry of excessive carbon and nitrogen in the surface layer, cold formability is because deteriorated. In the technique, such as high frequency surface hardening, since leaving a carbide thickness center, because the cold formability is lowered.
[0102]
　Incidentally, the steel produced by the 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, by hot molding method such as a hot stamp be manufactured parts, the effect of the present invention it may be noted that similarly obtained.
[0103]
　According to the manufacturing method of the above steel sheet, the sliding portion of one or both surfaces of the steel sheet, the average micro Vickers hardness is to form a hard layer of less than and 700 HV 400 HV, further, the hard layer and the composition of the inner layer with properly controlled, the 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%, and the standard deviation of nano-hardness of the hard layer by controlling the 2.00 or less, it is possible to obtain a steel sheet having both the wear resistance and cold workability at a high level.
[0104]
　From the description herein, a preferred embodiment of the method for producing the steel sheet of the present invention, mention may be made of embodiments 1 to 5 below.
　(Aspect 1)
　inner layer and a method for manufacturing steel plate and a hard layer on one or both surfaces of the inner layer,
　the steel comprising an inner layer, and one or both rigid layer disposed on a surface of the inner layer into pieces, forming the steel plate subjected to hot rolling comprising rolling finishing heat at 650 ° C. or higher 950 ° C. or less of the temperature range, and
　the steel sheet to take seeded at 700 ° C. or less
include, the hard layer , by
　mass%,
　C:
　0.08 ~ 0.40%, Si: 0.01 ~
　3.00%, Mn: 1.000 ~ 10.00%, P: 0.0001 ~ 0.0200%,
　S : .0001 to .0200 percent
contained, the balance being Fe and impurities,
　wherein the inner layer, by
　mass%, C:
　0.001 ~ 0.200%, Si: 0.01 ~ 3.00% ,
　Mn:
　0.20 ～ 3.00 Pasento, P: 0.0001 ～ 0.0200
　Pasento, S: 0. 0001 to 0.0200%
containing the balance being Fe and impurities,
　Thickness 20μm or more for each hard layer, and at 2/5 or less of Zen'itaAtsu,
　the average micro Vickers hardness of the hard layer is more than 400 HV, less than 700 HV,
　N amount of the hard layer is 0.02% or less , and the
　said 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%,
　the standard deviation of nano-hardness of the hard layer is 2 and wherein the at .00 or less, the production method of the steel sheet.
　(Embodiment 2)
　to pickling said Maki taken steel sheet, and
　to pickling steel sheet, cold rolling and annealing, or annealing, is subjected to cold rolling and annealing
, characterized in that it further comprises a mode 1 the method according to.
　(Embodiment 3)
　of zinc on both sides of the annealed steel sheet, aluminum, characterized in that it further includes forming a coating layer selected from the group consisting of magnesium and alloys thereof, The process according to embodiment 2 .
　(Aspect 4)
　The hard layer further contains, by
　mass%, Al: 0.500% or
　less, N: 0.0200% or
　less, Cr: 2.000% or
　less, Mo: 1.000% or
　less, O: 0 .0200% or less,
　Ti: 0.500% or
　less, B: 0.0100% or less
and containing one or more of,
　the inner layer further contains, by
　mass%, Al: 0.500% or
　less, N: 0.0200%
　hereinafter, Cr: 2.000% or
　less, Mo: 1.000% or
　less, O: 0.0200% or
　less, Ti: 0.500% or less,
　B: 0.0100% or less
contain one or more of the process according to any one of the features, aspects 1-3 to.
　(Aspect 5)
　The hard layer further contains, by
　mass%, 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 %Less than,
　Y: 0.050% or
　less, Zr: 0.050% or
　less, La: 0.050% or
　less, Ce: 0.050% or less,
and contain one or more of,
　the inner layer further mass in
　Pasento, Nb: 0.500 Pasento
　less, V: 0.500 Pasento
　less, Cu: 0.500 Pasento
　less, W: 0.100 Pasento less,
　Ta: 0.100 Pasento
　less, Ni: 0.500 Pasento below ,
　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 any one of embodiments 1-4 the method of production.
Example
[0105]
　The following Examples will further illustrate the effects of the present invention.
[0106]
　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.
[0107]
[Table 1-1]

[0108]
[Table 1-2]

[0109]
[Table 1-3]

[0110]
[Table 1-4]

[0111]
　Pickling 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, 0.4 mm plate thickness of the front and back layer by cutting the steel sheet pasting a hard layer on the front and back layers, the inner layer thickness of the after finished 1.2 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 0.4mm plate thickness of the surface layer by cutting, the plate thickness of the inner layer to 1.6 mm, was subjected to polishing and cleaning described above. Followed by Table 2. Example No. In the combination shown in 1-24, 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. Example No. 1 to 7 is quenched in 60 ℃ oil, No. 8 to 14 are water-cooled, No. 15-21 was subjected to a mist cooling. In addition, in order to clarify the difference between the previous heat treatment conditions, Example No. In 22-24, put high frequency surface baked, nitride, carburizing: subjected to the heat treatment (carbon potential 0.37%) was prepared comparison samples increased surface hardness. Subsequently, the procedure described above, each sample hardness, tissue, abrasion resistance was evaluated cold formability.
[0112]
[Table 2]

[0113]
　The evaluation results of each sample shown in Table 2. No. of Example 4,5,6,13,19 are all average micro Vickers hardness of the hard layer is more than 400 HV, less than 700 HV, 0.08 ~ 0.40% of C of the hard layer, the amount of N 0. together is 02% or less, 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%, is shown to have excellent wear resistance and cold workability.
[0114]
　On the other hand, No. of Comparative Example 1,8,11,18 The hardness of the surface layer is less than 400 HV, abrasion resistance is lowered. No. 1 to 3, 7 to 10, 14 ~ 17, 21 hardness of the inner layer is greater than 400 HV, cold formability is lowered. In addition, No. 12 has a high C content, cold formability is lowered. In addition, No. The high frequency surface hardening heat treatment of 22, the volume percentage of carbide remaining in the inner layer is more than 2%, cold formability is lowered. No. The nitriding heat treatment of 23, since the N content of the surface layer is more than 0.02%, cold formability is lowered. In addition, No. The carburizing heat treatment 24, in addition to the high content and C, the inner layer of the softening proceeds, cold formability is lowered.
[0115]
　Subsequently, in order to examine the effect of the thickness of the hard layer, Example No. The combination of 5-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. To prepare a 25-43.
[0116]
[table 3]

[0117]
　The evaluation results of each sample in Table 3. No. of Example 28-42 are each thickness of the hard layer is 20μm or more and 2/5 or less of Zen'itaAtsu is shown to be excellent in wear resistance and cold workability.
[0118]
　On the other hand, No. of Comparative Example 25-27, the thickness of the hard layer is less than 20 [mu] m, the wear resistance is lowered. In addition, No. of Comparative Example 43, the thickness of the hard layer is greater than 2/5, cold formability is lowered. For ease of understanding, the wear amount and the thickness of the hard layer for each Example and Comparative Example of Table 3 a relation of (wear resistance) in FIG.
[0119]
　Subsequently, in order to examine the influence of the component, Table 1 D, E, and fixed samples of thickness 1.2mm having a composition of F in the inner layer, the sample having a thickness of 0.4mm having a composition of H ~ BR in combination with a hard layer, it was investigated the effect of component (examples No.44 ~ 229). Further, in Table 1 A, B, a sample having a thickness of 0.4mm having a composition of C is fixed to the hard layer by combining a sample of thickness 1.2mm having a composition of H ~ BR in the inner layer of the component effect was evaluated (example No.230 ~ 418). 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.
[0120]
[Table 4-1]

[0121]
[Table 4-2]

[0122]
[Table 4-3]

[0123]
[Table 4-4]

[0124]
[Table 4-5]

[0125]
[Table 4-6]

[0126]
[Table 5-1]

[0127]
[Table 5-2]

[0128]
[Table 5-3]

[0129]
[Table 5-4]

[0130]
[Table 5-5]

[0131]
[Table 5-6]

[0132]
　The evaluation results of each sample in Table 4 and Table 5. No. of Example 51 ~ 73,113 ~ 135,175 ~ 197,230 ~ 246,267,268,293 ~ 309,330,331,356 ~ 372,393,394, the average micro Vickers hardness of either the hard layer is higher 400HV less than 700 HV, C of the hard layer is from 0.08 to 0.40 percent, with the amount of N is 0.02% or less, the average Vickers hardness of the inner layer 80HV or more and 400HV or less, contained in the inner layer the volume percentage of carbide is less than 2%, is shown to have excellent wear resistance and cold workability.
[0133]
　To further investigate the effect of manufacturing conditions by hot rolling in Table 1 superior properties in the hard layer was observed A, B, C, D, E, O, P, Q, R, S, T, U, V, W, X, Y, Z, AA, AB, AC, AD, AE, AF, AG, AH, AI, AJ, arranged AK as front and back layers, in Table 1 superior properties in the inner layer was found D, E, F, H, I, J, K, L, M, N, O, P, Q, R, S, T, U, V, W, X, AS, of the ingot in combination an AT in the inner layer to prepare a laminate, and held for 120 minutes in a furnace heated to 1000 ° C. over 0.1MPa surface pressure, subjected to diffusion heat treatment. Incidentally, the thickness ratio of the front and back layers in the laminate of the ingot 2/5 was adjusted inner layer thickness ratio to 1/5. Subsequently, to prepare each sample having a thickness of 2.4mm by hot rolling conditions shown in Table 6, after applying pickling, was subjected to various evaluation tests.
[0134]
[Table 6-1]

[0135]
[Table 6-2]

[0136]
　The evaluation results of each sample in Table 6. No. of Example 419,420,422 ~ 425,426,427,429 ~ 431,434 ~ 439,441 ~ 446,448,449,451 ~ 454,457,459,460 are average micro Vickers hardness of either the hard layer 400HV or more and less than 700 HV, C of the hard layer is 0.08 to 0.40%, with the amount of N is 0.02% or less, the inner layer of the average Vickers hardness is 80HV or more and 400HV or less, the inner layer the volume percentage of carbide that contains less than 2% indicated to be excellent in wear resistance and cold workability. Incidentally, amount of wear and nano hardness standard deviation in Table 6 are shown as the mean value of the front and back layers, a wear amount 3.0mg or less and nano hardness standard deviation 2.00 following requirements as an average examples satisfying was not meet these requirements in both all the front and back surfaces.
[0137]
　On the other hand, No. of Comparative Example 428,440 has a high heating temperature of hot rolling, also, No. 433,456 has a long heating time, the between the hard layer and the inner layer oxide layer is formed in the abrasion test and cold formability evaluation, the hard layer and the inner layer of interfacial peeling occurred. No. of Comparative Example 450, 455 has a low heating temperature, but also, No. Comparative Example Since 432,447 is the short heating time, not particularly solved segregation of the hard layer side, cold formability is lowered. No. of Example 420,445 low finishing temperature of hot rolling, the rolling load is increased, the productivity was lowered. No. of Comparative Example 421,458 has a high coiling temperature of hot rolling, reduces the hardness of the hard layer, reduction in the inner layer of the hardness and, it resulted in an increase of the inner layer of the carbide volume fraction.
[0138]
　"Cold - annealing" or to examine the effect of production conditions by "annealing - - cold-rolled annealed", No. of Example 419,422 ~ the hot-rolled sheet of 425,426,427,429,430,435 ~ 439,441 ~ 444,446,448,449,451 ~ 454,457,459,460, Table 7 "cold - annealing "or" annealing - cold rolling - annealed ", was subjected to various tests.
[0139]
[Table 7-1]

[0140]
[Table 7-2]

[0141]
[Table 7-3]

[0142]
[Table 7-4]

[0143]
　The evaluation results of each sample shown in Table 7. "Cold - annealing" or not depending on the "annealing - - cold-rolled annealed", examples satisfying the requirements of the present invention No. 461-488 are all average micro Vickers hardness of the hard layer is more than 400 HV, less than 700 HV, C amount is from 0.08 to 0.40% of the hard layer, together with the N amount is not more than 0.02% the average Vickers hardness of the inner layer 80HV or more and 400HV or less, the volume percentage of carbide contained in the inner layer is less than 2%, is shown to have excellent wear resistance and cold workability.
[0144]
　Following the cold-rolled annealed conditions described above, No. of Example To 463,465,466,470,472,475,476, subjected to melt or molten alloying plating shown in Table 8, were subjected to various tests to produce the plated steel sheet.
[0145]
[Table 8-1]

[0146]
[Table 8-2]

[0147]
[Table 8-3]

[0148]
　It shows the evaluation results of each sample in Table 8. By performing molten plating or molten alloying plating, although strength and abrasion resistance tend to be somewhat degraded, it is shown capable of producing cold-rolled coated steel sheet having excellent wear resistance and cold formability. Note that the pattern and the conditions of hot-rolled sheet annealing and cold-rolled sheet annealing in Table 8. See Figure 2-4.

WE CLAIM

An inner layer, a steel plate and a hard layer on one or both surfaces of the inner layer,
　wherein the hard layer is a
　mass%, C: 0.08
　~ 0.40%, Si: 0.01 ~ 3.
　% 00,
　Mn: 1.000 ~
　10.00%, P: 0.0001 ~ 0.0200%, S: 0.0001 ~ 0.0200%
containing the balance being Fe and impurities,
　wherein the inner layer , by
　mass%,
　C:
　0.001 ~ 0.200%, Si: 0.01 ~
　3.00%, Mn: 0.20 ~ 3.00%, P: 0.0001 ~
　0.0200%, S : 0.0001 to 0.0200 percent
contained, the balance being Fe and impurities,
　the thickness of each hard layer is 20μm or more and 2/5 or less of Zen'itaAtsu,
　the average micro Vickers hardness of the hard layer There 400HV or more and less than 700 HV,
　N amount of the hard layer is 0.02% or less There,
　the 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%,
　Wherein the standard deviation of nano-hardness of the hard layer is 2.00 or less, the steel sheet.
[Requested item 2]
　The hard layer further contains, by
　mass%, Al: 0.500% or
　less, N: 0.0200% or
　less, Cr: 2.000% or
　less, Mo: 1.000% or
　less, O: 0.0200% or less ,
　Ti: 0.500% or
　less, B: 0.0100% or less
and containing one or more of,
　the inner layer further contains, by mass%,
　Al: 0.500% or
　less, N: 0.0200 % or
　less, Cr: 2.000% or
　less, Mo: 1.000% or
　less, O: 0.0200% or
　less, Ti: 0.500% or
　less, B: 0.0100% or less
of one or more characterized in that it contains, steel sheet according to claim 1.
[Requested item 3]
　The hard layer further contains, by
　mass%, 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,
and contain one or more of,
　the inner layer further mass%
　in, Nb: 0.500 Pasento
　less, V: 0.500 Pasento
　less, Cu: 0.500 Pasento
　less, W: 0.100 Pasento
　less, Ta: 0.100 Pasento
　less, Ni: 0.500 Pasento below,
　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, the steel sheet according to claim 1 or 2.