[0001]The present invention relates to a steel sheet and a manufacturing method thereof.
Background technique
[0002]Automobile parts such as gears and clutches, stamping, forging, is produced through a machining process such as press working. In that process step, product quality improvement, stabilization, the measure to reduce the manufacturing cost, improve the processability of the carbon steel which is a material is obtained. Also, these components, after quenching and tempering, for use in high intensity, is required to have excellent hardenability.
[0003]
　In order to ensure the workability of ensuring the hardenability of carbon steel, conventionally, many proposals have been made.
[0004]
　Patent Document 1, by mass%, C: 0.20 ~ 0.45%, Mn: 0.40 ~ 1.50%, P: 0.03% or less, S: 0.02% or less, P + S: 0.010% or more, Cr: 0.01 ~ 0.80%, Ti: 0.005 ~ 0.050%, B: containing from 0.0003 to 0.0050 percent, the balance being Fe and unavoidable impurities further, Sn: 0.05% or less, Te: 0.05% or less, and contains a total of Sn + Te is more than 0.005%, ferrite and pearlite mixed structure, or a mixed ferrite and cementite workability characterized by comprising the tissue, hardenability, toughness superior high carbon steel sheet after the heat treatment is disclosed.
[0005]
　Patent Document 2, in mass%, C: 0.2 ~ 0.7%, Si: 2% or less, Mn: 2% or less, P: 0.03% or less, S: 0.03% or less, sol . Al: 0.08% or less, N: contains 0.01% or less, the steel balance of iron and inevitable impurities, after the hot rolling was performed at a finishing temperature (Ar @ 3 transformation point -20 ° C.) or higher, beyond the cooling rate 120 ° C. / sec, and performs cooling at a cooling end temperature 620 ° C. or less, then coiling at a coiling temperature 600 ° C. or less, after controlling the tissue having a bainite phase exceeds 20% index volume, after pickling, subjected to annealing below Ac1 transformation point annealing temperature 640 ° C. or higher, the method of producing a high hardenability high carbon hot-rolled steel sheet, characterized in that the spheroidized structure is disclosed.
CITATION
Patent Literature
[0006]
Patent Document 1: JP Patent No. 4319940
Patent Document 2: JP Patent No. 3879459
Summary of the Invention
Problems that the Invention is to Solve
[0007]
　However, high carbon steel sheet described in Patent Document 1, the material tissue, hardness and high perlite also used, not necessarily excellent in workability. Patent Document 2, the specific morphology having excellent workability is not described.
[0008]
　In view of the state of the prior art, to improve the moldability and wear resistance, especially thick plates gear by molding, and a suitable steel to obtain components such as a clutch, a process for producing the same for the purpose.
Means for Solving the Problems
[0009]
　Wherein the foregoing problems, in order to obtain a steel sheet suitable for materials such as the drive system components, in a steel sheet containing the necessary C to increase the hardenability, increase the grain size of the ferrite, carbide (mainly cementite ) was spheronized in a suitable grain size, it can be understood that there may be fewer pearlite structure. This is due to the following reasons.
[0010]
　Ferrite phase has a low hardness, high ductility. Accordingly, ferrite mainly as tissue was, by increasing the grain size, it is possible to improve the material formability.
[0011]
　Carbides, by properly dispersed in the metallic structure, while maintaining the material formability, since it is possible to impart excellent wear resistance and rolling fatigue characteristics, must be in the drive system components tissue it is. Also, the carbide in the steel sheet is a strong particles that prevent slipping, the presence of the carbides in the ferrite grain boundaries, to prevent the propagation of slip exceeding a crystal grain boundary, to suppress the formation of shear bands can, to improve the cold forgeability, at the same time, the moldability of the steel sheet improved.
[0012]
　However, cementite is hard and brittle structure, when present in pearlite state a layered structure of ferrite, steel hard, so becomes brittle, should be present in a spherical shape. And cold forgeability, considering the occurrence of cracks during forging, the particle size should be appropriate range.
[0013]
　However, the manufacturing method for realizing the tissue has not been previously disclosed. Accordingly, the present inventors have conducted extensive studies on manufacturing method for realizing the above tissues.
[0014]
　As a result, since the cementite metal structure of the steel sheet after coiling after hot rolling in small fine pearlite or fine ferrite having lamella spacing is the dispersed bainite structure, at a relatively low temperature (400 ~ 550 ℃) winding take. By winding at a relatively low temperature, cementite dispersed in ferrite also tends to spheronization. Subsequently, partially spheroidized cementite with annealing at temperatures just below Ac1 point as annealing in the first stage. Then, in annealing at a temperature between Ac1 point and Ac3 points as annealing in the second stage (two-phase region of the so-called ferrite and austenite), while leaving a portion of the ferrite grains, thereby austenite transformation part. While growing the subsequent slow cooling to leave ferrite grains, by which it to ferrite transformation of austenite in the nucleus, grain boundary precipitation of cementite while obtaining large ferrite phase, it was found to be able to realize the tissue.
[0015]
　That is, the manufacturing method of the steel sheet to satisfy the hardenability formability at the same time, such as hot rolling conditions and annealing conditions are also difficult to implement to devise in a single, in the so-called integrated process, such as hot rolling, annealing step It was found that can be realized by achieving an optimization.
[0016]
　The present invention has been made based on the above findings and has as its gist is as follows.
[0017]
　(1) で mass%, C: 0.10 ~ 0.40%, Si: 0.01 ~ 0.30%, Mn: 1.00 ~ 2.00%, P: 0.020% or less, S: 0.010% or less, Al: 0.001 ~ 0.10%, N: 0.010% or less, O: 0.020% or less, Cr: 0.50% or less, Mo: 0.10% or less, Nb : 0.10% or less, V: 0.10% or less, Cu: 0.10% or less, W: 0.10% or less, Ta: 0.10% or less, Ni: 0.10% or less, Sn: 0 .050% or less, Sb: 0.050% or less, As: 0.050% or less, Mg: 0.050% 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 containing wo shi, remnants ga Fe and unavoidable impurities び で thou ru で thou ~ te steel, the steel referred to Metallic structure ga fu S Factory LITE intragranular の number carbide の ni Dui suru fu S Factory LITE grain boundaries の number carbide の の ratio ga ultrafine, fu S Factory LITE than 5μm 50μm or less diameter ga and pa ー LITE の area ratio び ga 6% wo the Man ta shi, a note sheet の Bldg ッ grades ー su hard sa ga 100HV less than 170HV で thou ru ko と と special Zhi wo suru plate.
[0018]
　(2) instead of a part of the Fe, Ti: 0.10% or less, and B: 0.010% or less, the steel sheet of (1), characterized in that it contains one or two.
[0019]
　(3) (1) or (2) In the method for manufacturing the steel sheet, the (1) or a billet of composition of (2), the finish rolling at a temperature range temperatures higher than 750 ℃ ​​850 ° C. or less a hot rolled steel sheet subjected to a rolling finish, winding the hot-rolled steel sheet at 400 ° C. or higher 550 ° C. or less, subjected to pickling to wound hot rolled steel sheet, the pickled hot-rolled steel sheet 650 ° C. or higher at 720 ° C. below the temperature range, annealed in the first stage to 3 hours or more 60 hours or less, then held in a temperature range of 790 ° C. or less 725 ° C. or higher hot-rolled steel sheet, 50 hours or less than 3 hours 2 annealed th stage, the hot-rolled steel sheet after annealing, manufacturing method of the steel sheet, characterized by cooling to 650 ° C. at 1 ° C. / time or 30 ° C. / time less cooling rate.
Effect of the invention
[0020]
　According to the present invention, it is excellent in formability and abrasion resistance, it is possible to provide a suitable steel and a manufacturing method thereof for obtaining gear, a component such as a clutch, in particular by the thick plate forming.
DESCRIPTION OF THE INVENTION
[0021]
　Hereinafter, the detailed description about the present invention. First, a description will be given reasons for limiting the chemical composition of the steel sheet of the present invention. Less than. "%" For component means "% by mass".
[0022]
　[C: 0.10 ~ 0.40%]
　C is a carbide is formed in the steel is an element effective for refining the strengthening and ferrite grains of the steel. Suppressing the occurrence of satin in the cold working, to ensure the surface appearance of the cold working parts, it is necessary to suppress the coarsening of the ferrite grain size is less than 0.10%, the volume percentage of carbide There was insufficient, it is impossible to suppress the coarsening of carbides in box annealing, C is 0.10% or more. Preferably at least 0.12.
[0023]
　On the other hand, when it exceeds 0.40%, the increase in the volume percentage of carbide, momentarily crack as a starting point of fracture upon by a load is generated in a large amount, since the impact resistance is lowered, C is 0.40% or less. Preferably not more than 0.38%.
[0024]
　[Si: 0.01 ~
　0.30%] Si acts as a deoxidizer and is also affects elements in the form of carbides. To obtain the deoxidizing effect, Si is 0.01% or more. Is preferably 0.05% or more.
[0025]
　On the other hand, when it exceeds 0.30%, the ferrite ductility is reduced, cracking during cold working is likely to occur, since the cold workability is deteriorated, Si is not more than 0.30%. Preferably not more than 0.28%.
[0026]
　[Mn: 1.00 ~
　2.00%] Mn enhances hardenability, an element which contributes to improvement in strength. If it is less than 1.00%, since the securing of residual carbides strength after the quenching after quenching is difficult, Mn is set to 1.00% or more. Preferably not less than 1.09%.
[0027]
　On the other hand, when it exceeds 2.00%, Mn segregation becomes extreme banded, so is significantly decreased workability, Mn is not more than 2.00%. Preferably not more than 1.91%.
[0028]
　[Al: 0.001 ~
　0.10%] Al acts as a deoxidizer of steel, is an element for stabilizing ferrite. If it is less than 0.001%, because the addition effect is not sufficiently obtained, Al is 0.001% or more. Preferably 0.004% or more.
[0029]
　On the other hand, when it exceeds 0.10%, inclusions are produced in large quantities, since cold workability is deteriorated, Al is not more than 0.10%. Preferably not more than 0.08%.
[0030]
　The following elements are impurities, it should be controlled below a certain amount.
[0031]
　[P: 0.0001 ～ 0.020 Pasento]
　P segregates in the ferrite grain boundaries, is a element for suppressing the formation of grain boundary carbides. Is preferably as small in refining process and reducing the P to less than 0.0001%, the refining cost is significantly increased, P is set to 0.0001% or more. Preferably it is greater than or equal to 0.0013%.
[0032]
　On the other hand, if it exceeds 0.020%, the decreased number ratio of the grain boundary carbides, since cold workability is deteriorated, P is the 0.020% or less. Preferably not more than 0.018%.
[0033]
　[S: 0.0001 ~ 0.010%]
　S is an impurity element that forms non-metallic inclusions such as MnS. Non-metallic inclusions during cold working, since the starting point of cracking, although S is preferably as small and reduces the S to less than 0.0001%, the refining cost rises dramatically, S 0. to 0001% or more. Preferably it is greater than or equal to 0.0012%.
[0034]
　On the other hand, if it exceeds 0.010%, the cold workability is deteriorated, S is 0.010% or less. Preferably 0.007% or less.
[0035]
　: [N 0.0001 ~ 0.010%]
　is N, by the inclusion of a large amount, an element that causes embrittlement of ferrite, the less preferred. The content of N may be 0, but when reduced to less than 0.0001%, the refining cost is significantly increased, substantial lower limit is 0.0001 to 0.0006%. On the other hand, if it exceeds 0.010%, ferrite is brittle, since the cold workability is deteriorated, N represents 0.010% or less. Preferably 0.007% or less.
[0036]
　[O: 0.0001 ~ 0.020%]
　O is the inclusion of a large amount, an element that forms coarse oxides in the steel, lesser is preferable. The content of O may be 0, but when reduced to less than 0.0001%, the refining cost is significantly increased, substantial lower limit is 0.0001 to 0.0011%. On the other hand, if it exceeds 0.020%, coarse oxides in the steel is produced, since the starting point of cracking during cold working, O is set to 0.020% or less. Preferably is less than or equal to 0.017%.
[0037]
　[Sn: 0.001 ~
　0.050%] Sn is an element which is mixed from the steel raw material (scraps). It segregates in grain boundaries, since lowering the number ratio of the grain boundary carbides, the less preferred. May be any 0 content of Sn, when reduced to less than 0.001%, the refining costs increase significantly, substantially the lower is from 0.001 to 0.002 percent or more. On the other hand, if it exceeds 0.050% ferrite is brittle, since the cold workability is decreased, Sn is set to 0.050% or less. Preferably 0.040% or less.
[0038]
　[Sb: 0.001 ~
　0.050%] Sb, like Sn, an element which is mixed from the steel raw material (scraps). It segregates in grain boundaries, since lowering the number ratio of the grain boundary carbides, the less preferred. May be any 0 content of Sb, when reduced to less than 0.001%, the refining costs increase significantly, substantially the lower is from 0.001 to 0.002 percent or more. On the other hand, if it exceeds 0.050% ferrite is brittle, since the cold workability is deteriorated, Sb is set to 0.050% or less. Preferably 0.040% or less.
[0039]
　[As: 0.001 ~
　0.050%] As is, Sn, As with Sb, is an element which is mixed from the steel raw material (scraps). It segregates in grain boundaries, since lowering the number ratio of the grain boundary carbides, the less preferred. May be any 0 content of As, when reduced to less than 0.001%, the refining costs increase significantly, substantially the lower is from 0.001 to 0.002 percent or more. On the other hand, if it exceeds 0.050%, the decreased number ratio of the grain boundary carbides, since cold workability is deteriorated, As is the 0.050% or less. Preferably 0.040% or less.
[0040]
　The present invention steel sheet, the element is a fundamental component, further, for the purpose of improving the cold forgeability of the steel sheet may contain the following elements. The following elements, since it is not essential for obtaining the effects of the present invention, the content may be 0.
[0041]
　[Cr: 0.50% or less]
　Cr increases the hardenability, is an element contributing to the improvement of strength, also enriched carbide, is an element which forms a stable carbide in the austenite phase. To obtain the effect of addition, Cr is preferably 0.001% or more. More preferably 0.007% or more. On the other hand, when it exceeds 0.50%, carbide is stabilized dissolution of carbides during quenching is delayed, because it may be impossible to achieve the required temper strength, Cr is 0.50% or less. Preferably not more than 0.45%.
[0042]
　[Mo: 0.10% or
　less] Mo, like Mn, is an element effective to form the control of the carbides. To obtain the effect of addition, Mo is preferably 0.001% or more. More preferably 0.010% or more. On the other hand, when it exceeds 0.10%, worse-plane anisotropy of r value, the cold workability is deteriorated, Mo is not more than 0.10%. Preferably not more than 0.08%.
[0043]
　[Nb: 0.10% or less]
　Nb is an effective element to form the control of carbides, also tissue refining an element which contributes to the improvement of toughness. To obtain the effect of addition, Nb is preferably 0.001% or more. More preferably 0.002% or more. On the other hand, when it exceeds 0.10%, precipitation many fine Nb carbide, strength is excessively increased, also reduces the number ratio of the grain boundary carbides, since cold workability is deteriorated, Nb is 0 and .10% or less. Preferably not more than 0.08%.
[0044]
　: [V 0.10% or less]
　Also V, as with Nb, an element effective to form the control of carbides, also tissue refining an element which contributes to the improvement of toughness. To obtain the effect of addition, V is preferably 0.001% or more. More preferably 0.004% or more. On the other hand, when it exceeds 0.10%, precipitates fine V carbide are many, the strength is excessively increased, also reduces the number ratio of the grain boundary carbides, since cold workability is deteriorated, V 0 and .10% or less. Preferably not more than 0.08%.
[0045]
　[Cu: 0.10% or less]
　Cu is segregated in grain boundaries of ferrite, also to form a fine precipitate, which is an element contributing to the improvement of strength. To obtain the effect of addition, Cu is preferably 0.001% or more. More preferably 0.005% or more. On the other hand, when it exceeds 0.10%, occurs hot shortness, the productivity in the hot-rolled is reduced, Cu is not more than 0.10%. Preferably not more than 0.08%.
[0046]
　[W: 0.10% or less]
　W also, Nb, similarly to V, is an element effective to form the control of the carbides. To obtain the effect of addition, W is preferably set to 0.001% or more. More preferably 0.003% or more. On the other hand, when it exceeds 0.10%, precipitation many fine W carbides, strength is excessively increased, also reduces the number ratio of the grain boundary carbides, since cold workability is deteriorated, W 0 and .10% or less. Preferably not more than 0.08%.
[0047]
　[Ta: 0.10% or
　less] Ta also, Nb, V, similarly to the W, is an element effective to form the control of the carbides. To obtain the effect of addition, Ta is preferably set to 0.001% or more. More preferably 0.005% or more. On the other hand, when it exceeds 0.10%, precipitation many fine W carbides, strength is excessively increased, also reduces the number ratio of the grain boundary carbides, since cold workability is deteriorated, Ta is 0 and .10% or less. Preferably not more than 0.08%.
[0048]
　[Ni: 0.10% or
　less] Ni is an element effective in improving the components of toughness. To obtain the effect of addition, Ni is preferably 0.001% or more. More preferably 0.004% or more. On the other hand, when it exceeds 0.10%, it decreases the number ratio of the grain boundary carbides, since cold workability is deteriorated, Ni is not more than 0.10%. Preferably not more than 0.08%.
[0049]
　[Mg: 0.050% or
　less] Mg is an element capable of controlling the form of sulfide by the addition of small amount. To obtain the effect of addition, Mg is preferably 0.0001% or more. More preferably not less than 0.0008%. On the other hand, if it exceeds 0.050% ferrite is brittle, since the cold workability is deteriorated, Mg is set to 0.050% or less. Preferably 0.040% or less.
[0050]
　[Ca: 0.050% or
　less] Ca, like Mg, an element capable of controlling the form of sulfide by the addition of small amount. To obtain the effect of addition, Ca is preferably 0.001% or more. More preferably 0.003% or more. On the other hand, if it exceeds 0.050% produces coarse Ca oxides, since the starting point of cracking during cold working, Ca is set to 0.050% or less. Preferably 0.040% or less.
[0051]
　[Y: 0.050% or less]
　Y is, Mg, as with Ca, an element that can control the form of sulfide by the addition of small amount. To obtain the effect of addition, Y is preferably 0.001% or more. More preferably 0.003% or more. On the other hand, if it exceeds 0.050% produces coarse Y oxides, since the starting point of cracking during cold working, Y is a 0.050% or less. Preferably not more than 0.035%.
[0052]
　[Zr: 0.050% or
　less] Zr is, Mg, Ca, similar to the Y, is an element capable of controlling the form of sulfide by the addition of small amount. To obtain the effect of addition, Zr is preferably set to 0.001% or more. More preferably 0.004% or more. On the other hand, if it exceeds 0.050% produces coarse Zr oxides, since the starting point of cracking during cold working, Zr is set to 0.050% or less. Preferably not more than 0.045%.
[0053]
　[La: 0.050% or less]
　La is an effective element to form the control of sulfide with the addition of small amount segregates in grain boundaries, is also an element lowering the number ratio of grain boundary carbides. To obtain the effect of addition, La is preferably 0.001% or more. More preferably 0.004% or more. On the other hand, if it exceeds 0.050%, the decreased number ratio of the grain boundary carbides, since cold workability is deteriorated, La is set to 0.050% or less. Preferably not more than 0.045%.
[0054]
　[Ce: 0.050% or less]
　Ce, like La, is an element capable of controlling the form of sulfide by the addition of trace amounts, it segregated in the grain boundary, leading to reduction in the number ratio of the grain boundary carbide element But there is. To obtain the effect of addition, Ce is preferably 0.001% or more. More preferably 0.004% or more. On the other hand, if it exceeds 0.050%, the decreased number ratio of the grain boundary carbides, since cold workability is deteriorated, Ce is the 0.050% or less. Preferably is less than or equal to 0.046%.
[0055]
　Balance of chemical composition of the steel sheet of the present invention is Fe and unavoidable impurities.
[0056]
　Instead of some of the above Fe, it may contain one or two of Ti and B.
[0057]
　[Ti: 0.10% or less]
　Ti is an element effective for morphology control of carbides, also tissue are also elements contributing to the improvement of miniaturization toughness. To obtain the effect of addition, Ti is preferably 0.001% or more. More preferably 0.005% or more. On the other hand, when it exceeds 0.10%, coarse Ti oxide is generated, since the starting point of cracking during cold working, Ti is not more than 0.10%. Preferably not more than 0.08%.
[0058]
　[B: 0.0001 ~ 0.010%]
　B further enhances the hardenability during component heat treatment to homogenize the tissue, an element which contributes to the improvement of toughness. To obtain the effect of addition, B is preferably 0.0001% or more. More preferably not less than 0.0006%. On the other hand, if it exceeds 0.010%, coarse B oxides are produced, since the starting point of cracking during cold working, B is 0.010% or less. Preferably not more than 0.009%.
[0059]
　Next, a description will be given tissue of the steel sheet of the present invention.
[0060]
　Organization of the steel sheet of the present invention are substantially, tissue composed of ferrite and carbides. Carbides, cementite (Fe is a compound of iron and carbon 3 in addition to C), the Fe atoms in cementite, Mn, compounds or substituted with alloy elements such as Cr, alloy carbides (M 23 C 6 , M 6 C , MC, etc. [M: Fe, and other added metal element as an alloy is a).
[0061]
　When forming the steel sheet into a predetermined shape, shear zones are formed in the macrostructure of the steel sheet, in the vicinity of the shear zone, caused by slip deformation is concentrated. Slip deformation is accompanied by proliferation of dislocation, in the vicinity of the shear zone, the region of high dislocation density is formed. With the increase of the strain amount applied to the steel sheet, slip deformation is promoted, the dislocation density increases.
[0062]
　In cold forging, large deformation is applied to more than 1 in equivalent strain. Therefore, in the conventional steel sheet, it is impossible to prevent the occurrence of voids and / or cracks with the increase in dislocation density, in the conventional steel sheet, improvement of cold forgeability was difficult. To solve this problem, it is effective to suppress the formation of shear zones during molding.
[0063]
　In terms of microstructure, the formation of shear zones, slip generated at a certain one crystal grain, overcame grain boundaries, is understood as a phenomenon that continuously propagated to neighboring grains. Therefore, in order to suppress the formation of shear bands, it is necessary to prevent the propagation of slip exceeding a grain boundary.
[0064]
　Carbides in the steel sheet is a strong particles that prevent slipping, the presence of the carbides in the ferrite grain boundaries, can be prevented the propagation of slip exceeding a grain boundary, to suppress the formation of shear zones, it is possible to improve the cold forgeability. At the same time, to improve moldability of the steel sheet.
[0065]
　Formability of the steel sheet has a large extent due to the accumulation of distortion into grains (accumulation of dislocations) at the grain boundaries, if propagation is prevented to distortion of adjacent crystal grains, the strain amount in the crystal grains increased. As a result, work hardening rate increases, to improve moldability.
[0066]
　Based on theory and principles, cold workability, it is considered that strongly influenced by coverage of the ferrite grain boundary carbide, it is necessary to measure the coverage with high accuracy.
[0067]
　In three-dimensional space, in order to measure the coverage of carbides in the ferrite grain boundaries, using a scanning electron microscope in serial sectioning SEM observation performed repeatedly observed with sample cutting with FIB, or, 3D EBSP observation Required next, along with requiring enormous measurement time, the accumulation of technical know-how is indispensable. This, the present inventors have revealed, general analytical techniques concluded to be unsuitable.
[0068]
　Therefore, the result of searching a high rating indicator of simple and precise, if the ratio of the number of ferrite grain boundary carbide to the number of carbide in ferrite grains as an indicator, it is possible to evaluate the cold workability , the ratio of the number of the ferrite grain boundary carbide to the number of carbide in ferrite grains exceeds 1, the present inventors have found that the cold workability is remarkably improved were found.
[0069]
　Incidentally, buckling of the steel sheet occur during cold working, folding, none of the convolution, since those caused by localized distortion due to the formation of the shear zones, by the presence of carbides in the ferrite grain boundaries, shear relieve localized formation of the band and distortion, buckling, it can fold, to effectively suppress the generation of convolution.
[0070]
　When spheroidization ratio of carbides on the crystal grain boundary is less than 80%, locally strain is concentrated on the rod-shaped or plate-shaped carbides, since voids and / or cracks are likely to occur, on the grain boundaries spheroidization ratio of carbides is preferably at least 80%, more preferably 90% or more.
[0071]
　When the average particle diameter of carbide is less than 0.1 [mu] m, and significantly increased the hardness of the steel sheet, so workability is decreased, the average particle size of the carbide is preferably at least 0.1 [mu] m. More preferably not less than 0.17μm. On the other hand, when the average particle diameter of the carbide is more than 2.0 .mu.m, cracks become a starting point occurs and coarse carbides at the time of cold working, since cold workability is deteriorated, the average particle size of the carbides is 2.0 .mu.m or less preferable. More preferably not more than 1.95μm.
[0072]
　Observation of carbide is carried out with a scanning electron microscope. Prior to observation, the sample for microstructure observation was polished by diamond abrasive grains having an average particle size of wet grinding and 1μm by emery paper, after finishing the observation plane mirror 3% nitric acid - the tissue at alcohol solution keep etching. Observation magnification selects the magnification can be determined ferrite and carbide in 3000 times. The selected magnification, randomly to eight photographs a view of 30 [mu] m × 40 [mu] m in the sheet thickness 1/4 layers.
[0073]
　The obtained tissue image, the image analysis software represented by Mitani Corp. (Win ROOF), measuring the area of ​​each carbide contained in that region in detail. Determined circle equivalent diameter (= 2 × √ (area /3.14)) from the area of ​​the carbides, and the average value and the carbide particle size. Further, spheroidization ratio of carbides with carbide, approximated to an ellipse and the moment of inertia is equal areas are equal, the maximum and length was determined by calculating the percentage of what the ratio of the maximum length of a direction perpendicular thereto is less than 3 .
[0074]
　Incidentally, in order to suppress the influence of a measurement error due to noise, the area is 0.01 [mu] m 2 or less of carbides were excluded from the evaluation. Counting the number of cementite present in the ferrite grain boundaries on, to determine the number of carbide in ferrite grains by subtracting the number of carbides on grain boundaries from all the carbides number. Based on the measured number was determined the number ratio of the carbide on the ferrite grain boundary to the carbides in the ferrite grain.
[0075]
　In tissue after annealed cold-rolled steel sheet, a ferrite grain size With more than 5.0 .mu.m, it is possible to improve the cold workability. When the ferrite grain diameter is less than 5 [mu] m, to increase the hardness, during cold working, a crack or cracks are likely to occur, the ferrite grain size is not less than 5 [mu] m. Preferably it is 7μm or more.
[0076]
　On the other hand, when it exceeds 50 [mu] m, the number of carbides is reduced in suppressing the grain boundary propagation of slip, since cold workability is deteriorated, the ferrite grain diameter is set to 50 [mu] m or less. Preferably is less than or equal to 37μm.
[0077]
　Ferrite grain size is in the polishing method described above, after polishing the observation surface of the sample to a mirror, 3% nitric acid - etched in alcoholic solution, to observe the viewing surface the tissue under an optical microscope or a scanning electron microscope and photographed measured by applying a line segment method with respect to the image.
[0078]
　Furthermore, cementite is carbide iron is hard and brittle structure, when present in pearlite state a layered structure of ferrite, hard and steel becomes brittle. Therefore, perlite must be minimized, in the steel sheet of the present invention, 6% or less in area ratio.
[0079]
　Perlite because of its unique lamellar structure, it is possible distinguish SEM, an optical microscope observation. By calculating the area of ​​the lamellar structure in any cross-section, it is possible to determine the area ratio of pearlite.
[0080]
　Further, the Vickers hardness of the steel sheet by a 100HV or more 170HV or less, it is possible to improve the cold workability. When Vickers hardness is less than 100 HV, since buckling is likely to occur during cold working, Vickers hardness is not less than 100 HV. Preferably is equal to or greater than 110HV.
[0081]
　On the other hand, the Vickers hardness exceeds 170 Hv, ductility is lowered and the internal cracking is likely to occur during cold working, Vickers hardness is not more than 170 Hv. Preferably it is less than 168HV.
[0082]
　Next, the present invention will be described manufacturing method.
[0083]
　The present invention production process, using a steel slab of chemical composition as described above, manages to consistently hot rolling conditions and annealing conditions, the basic idea to make the microstructure control of the steel sheet.
[0084]
　First, it provides a slab continuous casting of molten steel of the required component composition to hot rolling. Slab after the continuous casting may be directly subjected to hot rolling, it may be subjected to hot rolling after once heating after cooling.
[0085]
　If heated after once cooling the slab subjected to hot rolling, the heating temperature is preferably 1000 ° C. or higher 1250 ° C. or less, the heating time is preferably 3 hours or less than 0.5 hours. The continuous cast billet, directly when subjected to hot rolling, the temperature of the steel strip subjected to hot rolling is preferably set to 1250 ° C. 1000 ° C. or higher.
[0086]
　Billet temperature or slab heating temperature exceeds 1250 ° C., or, if more than slab heating time is 3 hours, will significantly decarburization from the surface layer billet, during heating before carburizing and quenching, the steel sheet surface layer austenite grains abnormally grows, the impact resistance is lowered. Therefore, the steel strip temperature or billet heating temperature is preferably 1250 ° C. or less, the heating time is preferably 3 hours or less. More preferably 1200 ° C. or less, or less 2.5 hours.
[0087]
　Billet temperature or slab heating temperature is less than 1000 ° C., or the heating time is shorter than 0.5 hours, not eliminated resulting micro-segregation and macro segregation in casting, inside the slab, Si Ya alloy elements such as Mn is remaining locally thickened region, the impact resistance is lowered. Therefore, the steel strip temperature or billet heating temperature is preferably 1000 ° C., the heating time is preferably 0.5 hour or more. More preferably 1050 ° C. or higher, at least 1 hour.
[0088]
　Finish rolling in the hot rolling is completed at a temperature range temperatures higher than 750 ℃ ​​850 ° C. or less. When the finish rolling temperature is lower than 750 ° C., been an increase in the deformation resistance of the steel sheet, rolling load considerably increases, also the roll wear amount increases, with productivity is lowered, improving the plastic anisotropy since recrystallization does not proceed sufficiently necessary to, finish rolling temperature is set to 750 ° C. or higher. In terms of promoting recrystallization, preferably 770 ° C. or higher.
[0089]
　When the finish rolling temperature exceeds 850 ° C., a Run Out Table (ROT) thick scale is formed during Tsuban, due to this scale, scratches occur on the surface of the steel sheet, cold forging and after carburizing quenching and tempering when an impact load is applied, a crack is easily generated flaws starting, the impact resistance of the steel sheet is lowered. Therefore, the finish rolling temperature to 850 ° C. or less. Preferably less than or equal to 830 ℃.
[0090]
　Upon cooling the hot-rolled steel sheet after finish rolling in the ROT, the cooling rate is preferably 10 ° C. / sec or higher 100 ° C. / sec or less. When the cooling rate is less than 10 ° C. / sec, thick scale is generated during cooling, can not suppress the occurrence of caused by flaws in it, since the impact resistance is lowered, the cooling rate is preferably at least 10 ° C. / sec . More preferably 20 ° C. / sec or more.
[0091]
　Over the inside from the surface layer of the steel sheet, and cooled at a cooling rate exceeding 100 ° C. / sec, the outermost layer is excessively cooled, resulting in a low temperature transformation structure such as bainite or martensite. After winding, when paying out a hot-rolled steel coil cooled to 100 ° C. ~ room temperature, fine cracks occur in the low-temperature transformation structure. The micro cracks, it is difficult to remove by pickling and cold rolling.
[0092]
　Then, the steel sheet, an impact load is applied after cold forging and carburizing quenching and tempering, because crack develops microcracks starting from the impact resistance is lowered. Therefore, the outermost layer of the steel sheet, to prevent the low-temperature transformation structure such as bainite or martensite occurs, the cooling rate is preferably 100 ° C. / sec or less. More preferably at most 90 ° C. / sec.
[0093]
　Incidentally, the cooling rate after passing through the hot-rolled steel sheet without irrigation interval after the finish rolling, from the time of receiving water cooled in the water injection section, at the time it is cooled on ROT to the target temperature of the winding, the water injection section cooling points to a cooling power received from the equipment, do not indicate the average cooling rate to a temperature to be wound by the winder from injection start point.
[0094]
　Coiling temperature to 400 ° C. or higher 550 ° C. or less. This is a common temperature lower than the winding temperature, especially if a high content of C is usually not performed conditions. The hot-rolled steel sheets produced in the conditions described above, by winding at this temperature range, the structure of the steel sheet, carbide fine ferrite can be dispersed bainite.
[0095]
　When the coiling temperature is below 400 ° C., transformed into untransformed a an austenitic stiff martensite before winding, during payout of hot-rolled steel coil, cracks occur in the surface layer of the hot-rolled steel sheet, the impact sex is reduced.
[0096]
　Further, when recrystallized from austenite to ferrite, for recrystallization driving force is small, the orientation of recrystallized ferrite grains, becomes strongly affected by orientation of the austenite grains, it is difficult to randomization texture. Therefore, the winding temperature is set to 400 ℃ or more. Preferably at 430 ℃ or more.
[0097]
　When the coiling temperature exceeds 550 ° C., a large pearlite generates a lamellar spacing, high thermal stability, thick acicular carbides are generated. The needle-shaped carbides are also remains after two-stage annealing. During molding of the cold forging of the steel sheet, crack generates the acicular carbides as a starting point.
[0098]
　Further, when recrystallized ferrite from austenite, conversely, too large recrystallization driving force, in this case, it is recrystallized ferrite grains strongly dependent on the orientation of the austenite grains is not made randomized texture. Therefore, the winding temperature is set to 550 ℃ or less. Preferably at 520 ℃ or less.
[0099]
　Dispensing a hot-rolled steel coil, after applying pickling, the 2-step type to keep the two temperature ranges annealing (2-step annealing) performed. By applying a two-stage annealing hot-rolled steel sheet, to control the stability of the carbides and promotes the formation of carbides in the ferrite grain boundaries.
[0100]
　Prior to annealing, the subjected to cold rolling steel sheet after pickling, since the ferrite grain becomes finer, the steel sheet is hardly softened. Therefore, in the present invention, not preferred subjected to a cold rolling prior to annealing, pickling, is subjected to a annealing process without cold rolling preferred.
[0101]
　Annealing the first-stage, 650 ~ 720 ℃, preferably A c1 performed in a temperature range of below points. This annealing, carbide is coarsened and causes partially spheroidized, the alloying element is concentrated in the carbide, enhance the thermal stability of the carbide.
[0102]
　In the annealing of the first stage, the heating rate up to the annealing temperature (hereinafter referred to as "first-stage heating rate") is not more than 30 ° C. / time or 0.99 ° C. / hour. When the first stage the heating rate is less than 30 ° C. / time, since productivity is lowered takes time to raise the temperature, first stage heating rate is set to 3 ° C. / time or more. Preferably at 10 ° C. / time or more.
[0103]
　On the other hand, when the first stage heating rate exceeds 0.99 ° C. / time, temperature difference between the inner and the outer peripheral portion is increased in the hot-rolled steel coil, with scratches or burn due to thermal expansion difference occurs, the surface of the steel sheet unevenness is formed on. At the time of molding such as cold forging, because the irregularities generated cracks becomes a starting point, or reduces the cold forging, impact resistance after molding property and carburizing quenching and tempering is deteriorated, the first stage heating rate 150 ℃ / and time below. Preferably at most 130 ° C. / hour.
[0104]
　1-stage annealing at annealing temperature (hereinafter referred to as "first stage annealing temperature") is set to 650 ° C. or higher 720 ° C. or less. When the first stage annealing temperature is lower than 650 ° C., but the stabilization of carbides is sufficiently, when the second-stage annealing, it is difficult to leave carbides in the austenite. Therefore, the first stage annealing temperature is set to 650 ° C. or higher. Preferably at 670 ℃ or more.
[0105]
　On the other hand, when the first stage annealing temperature exceeds 720 ° C., austenite is produced before the stability of the carbides increases, since the control of the aforementioned tissue changes difficult, the first stage annealing temperature is set to 720 ° C. or less . Preferably 700 ° C. or less.
[0106]
　The first stage of annealing in the annealing time (hereinafter referred to as "first-stage annealing time") is not more than 60 hours or more 3 hours. When the first stage annealing time is less than 3 hours, but the stabilization of carbides is sufficiently, at the time of the second-stage annealing, it is difficult to leave carbides in the austenite. For this reason, the first stage annealing time shall not be less than 3 hours. Preferably it is greater than or equal to 5 hours.
[0107]
　On the other hand, when the first stage annealing time exceeds 60 hours, not be expected even more stabilization of carbides, further, the productivity is lowered, the first stage annealing time is less 60 hours. Preferably not more than 55 hours.
[0108]
　Then, 725 ~ 790 ° C., preferably A c1 or points A 3 raised to a temperature range below point to produce an austenitic in structure. At this time, the carbide in fine ferrite grains is dissolved in the austenite, coarsened carbide by annealing in the first stage is left in the austenite.
[0109]
　When cooled without annealing of the second stage, not ferrite grain size is large, it is impossible to obtain an ideal tissue.
[0110]
　Heating rate up to the annealing temperature of the second-stage annealing (hereinafter referred to as "second-stage heating rate") is set to 80 ° C. / time or less 1 ° C. / time or more. During the annealing of the second stage, austenite is generated to grow from ferrite grain boundaries. At that time, by slowing the heating rate up to the annealing temperature to suppress the nucleation of austenite, in tissue formed by slow cooling after annealing, it is possible to increase the grain boundary coverage carbide.
[0111]
　Therefore, although towards the second stage the heating rate is slow Preferably, 1 is less than ° C. / time, it takes time to increase the temperature, since productivity is lowered, the second stage the heating rate is 1 ° C. / time or more to. Preferably at 10 ° C. / time or more.
[0112]
　When the second stage heating rate exceeds 80 ° C. / time, the hot-rolled steel coil, increasing the temperature difference between the inner and the outer peripheral portion, scratches and seizure due to large thermal expansion difference due to transformation occurs, irregularities are formed on the surface of the steel sheet. During cold forging, cracking is the uneven starting, reduced moldability and cold forgeability, and since also decreases the impact resistance after carburizing quenching and tempering, the second stage the heating rate is 80 ° C. / the time or less. Preferably at most 70 ° C. / hour.
[0113]
　The second stage of the annealing in the annealing temperature (hereinafter referred to as "second-stage annealing temperature") shall be 790 ℃ less than 725 ℃ or more. When the second stage annealing temperature is lower than 725 ° C., the amount of austenite becomes small, after the after the second stage annealing cooling, the number of carbides is reduced in the ferrite grain boundary, also ferrite grain diameter decreases . Therefore, the second stage annealing temperature is set to 725 ° C. or higher. Preferably at 735 ℃ or more.
[0114]
　On the other hand, when the second stage annealing temperature exceeds 790 ° C., it becomes difficult to leave the carbide austenite, since the control of a tissue change is difficult, the second stage annealing temperature is set to 790 ° C. or less. Preferably at 770 ℃ or less.
[0115]
　2-stage annealing at annealing time (second stage annealing time) is less than 3 hours or more 50 hours. When the second stage annealing time is less than 3 hours, the amount of austenite is small, and does not proceed sufficiently dissolution of carbides in the ferrite grains, it is difficult to increase the number of carbides of the ferrite grain boundaries, Furthermore, the ferrite particle diameter decreases. For this reason, the second stage annealing time shall not be less than 3 hours. Preferably it is greater than or equal to 5 hours.
[0116]
　On the other hand, when the second stage annealing time exceeds 50 hours, it is difficult to leave the carbide in austenite, and because also increases manufacturing costs, the second stage annealing time is less than 50 hours. Preferably not more than 40 hours.
[0117]
　After the two-stage annealing, the steel sheet is cooled to 650 ° C. at 1 ° C. / time or 30 ° C. / time less cooling rate.
[0118]
　By slow cooling, by slow cooling the austenite generated in the annealing in the second stage, as well as transformed into ferrite, carbon atoms adsorbed on carbide remaining in the austenite, carbides and austenite covers the ferrite grain boundary, ultimately a, it can be tissue carbides there are many in the ferrite grain boundaries.
[0119]
　For this purpose, the cooling rate is slower preferably is less than 1 ° C. / time, increases the time required for cooling, since productivity is lowered, the cooling rate is set to 1 ° C. / time or more. Preferably at 10 ° C. / time or more.
[0120]
　On the other hand, if the cooling rate exceeds 30 ° C. / time, austenite transforms to pearlite, increases the hardness of the steel sheet, reduces the cold forgeability and impact resistance after carburizing quenching and tempering is deteriorated since the cooling rate is less 30 ° C. / hour. Preferably not more than 20 ° C. / hour.
[0121]
　Furthermore, cooling the steel sheet was cooled to 650 ° C. to room temperature. The cooling rate at this time is not limited.
[0122]
　Atmosphere in the two-stage annealing, particularly, but not limited to a specific atmosphere. For example, an atmosphere of 95% nitrogen, an atmosphere of hydrogen over 95%, may be any atmosphere of air atmosphere.
[0123]
　As described above, the hot rolling conditions and annealing conditions consistently manage present invention, according to the production method for performing tissue control of the steel plate, the diaphragm, cold combining thickened molding formability during forging good, further, it is possible to manufacture a steel sheet having excellent hardenability required for the improvement of impact resistance after carburizing quenching and tempering.
Example
[0124]
　Next, as will be described for Example, levels embodiment is an example of the conditions employed for confirming the workability and effects of the present invention, the present invention is to be limited to this single example of conditions is not. The present invention does not depart from the gist of the present invention, as long as the purposes of this invention, those capable of adopting various conditions.
[0125]
　Evaluation of cold workability from annealed Mom material thickness 3 mm, subjected to a tensile test were taken JIS5 No. Tensile test pieces were evaluated total elongation from 0 ° direction and the rolling direction of the direction of 90 ° from the rolling direction in both directions at least 35%, and the difference of the total elongation in each direction | .DELTA.EL | when is 4% or less, cold workability is to be dominant.
[0126]
　Evaluation of hardenability, annealing Mom material thickness 3 mm, was ground to a thickness 1.5 mm, subjected to 880 ° C. × 10 minutes hold in a vacuum atmosphere, and quenching at a cooling rate of 30 ° C. / sec, if the fraction of martensite is 60% or more, it was to be superior hardenability.
[0127]
　(Example 1)
　Continuous casting of compositions shown in Table 1 the billet (the steel ingot) and heated for 1.8 hours at 1240 ° C., subjected to hot rolling, after completion of the finish hot rolled at 890 ° C., wound at 510 ° C., to produce a hot-rolled coil having a thickness of 3.0 mm. The hot rolled coil was pickled, it was charged with hot rolled coil in a box type annealing furnace, by controlling the atmosphere of 95% hydrogen -5% nitrogen, and heated to 705 ° C. from room temperature, 36 hour hold at 705 ° C. the temperature distribution in the hot-rolled coil after homogenized, heated to two 760 ° C., and held for 10 hours at 760 ° C..
[0128]
　Thereafter, up to 650 ° C. and cooled at a cooling rate of 10 ° C. / time, then furnace cooling to room temperature, to prepare a sample for property evaluation. Incidentally, a sample of tissue was measured by the method described above.
[0129]
[Table 1]

[0130]
　Table 2 shows the Vickers hardness of the samples prepared, the ratio of the number of carbide on the grain boundary of ferrite to the number of carbide in ferrite grains, pearlite area ratio, the cold workability, the result of the hardenability were measured or evaluated .
[0131]
[Table 2]

[0132]
　As shown in Table 2, the inventive steels B-1, E-1, F-1, H-1, J-1, K-1, L-1, M-1, N-1, P-1, R-1, T-1, W-1, X-1, Y-1, Z-1, AB-1 and,, AC-1 are both the ferrite grain boundaries to the number of carbide in ferrite grains the ratio of the number of carbide exceeds 1, Vickers hardness of not more than 170 Hv, is excellent in cold workability and hardenability.
[0133]
　In contrast, the comparative steels G-1 has a high C content, the cold workability is decreased. Comparative Steel O-1 has a high amount of Mo and Cr content, due to high stability carbides, carbide not dissolved during quenching, small austenite formation amount, hardenability is inferior.
[0134]
　Comparative Steel Q-1 and AD-1 are, Si, high amounts of Al, has high point A3, the austenite formation amount is small at the time of quenching, the hardenability is inferior. Comparative Example U-1 has a high S content, coarse MnS is generated in the steel, a low cold workability. Comparative Example AA-1 has a low Mn content, hardenability is inferior.
[0135]
　Comparative Example I-1 is lower finishing temperature of hot rolling, the productivity was lowered. Comparative Example D-1 has a high finishing temperature of hot rolling, scale defects are generated on the surface of the steel sheet. Comparative Example C-1 and S-1 is low coiling temperature of hot rolling, low-temperature transformation structure such as bainite or martensite is often made by embrittlement, cracking during payout of the hot-rolled coil is frequently, productivity reduced.
[0136]
　Comparative Example A-1 and V -1 is coiling temperature of hot rolling is high and thick pearlite and high thermal stability acicular coarse carbides generated the lamellar spacing in the hot-rolled tissue, the carbides 2 remain in the steel sheet after the stage step annealing, cold workability is decreased.
[0137]
　(Example 2)
　To examine the influence of the annealing conditions, was heated for 1.8 hours at a billet of composition shown in Table 1 1240 ° C., subjected to hot rolling, after completion of the finish hot rolled at 820 ° C. , was cooled to 520 ° C. at a cooling rate of 45 ° C. / sec on ROT, wound at 510 ° C., to produce a hot-rolled coil having a thickness of 3.0 mm, a two-step type in the annealing conditions shown in Table 3 the box annealed, to prepare a sample having a thickness of 3.0mm.
[0138]
　Table 3, the samples prepared, carbide size, ferrite grain size, Vickers hardness, the ratio of the number of carbide on the grain boundary of ferrite to the number of carbide in ferrite grains, pearlite area ratio, cold workability, hardenability We are shown the results of measurement or evaluation.
[0139]
[table 3]

[0140]
　As shown in Table 3, invention steels B-2, C-2, E-2, F-2, H-2, I-2, J-2, K-2, M-2, N-2, R-2, S-2, V-2, Z-2 and,, AC-2 are both, the ratio of the number of ferrite grain boundary carbide exceeds 1 to the number of carbides in the ferrite grain, Vickers hardness Saga 170HV or less, is excellent in cold workability and hardenability.
[0141]
　In contrast, the comparative steels G-1 has a high C content, the cold workability is decreased. Comparative Steel O-1 is, Mo amount and Cr amount is high, cold workability is decreased. Moreover, not dissolved carbides during due to the high stability of the carbide hardening, small austenite formation amount, hardenability is inferior.
[0142]
　Comparative Steel Q-1 has a high Si content, due to the high hardness of the ferrite, workability is lowered. Further, since A3 point is high, the austenite formation amount is small at the time of quenching, the hardenability is inferior. Comparative Steel AD-1 has a high Al content, due to high point A3, the austenite formation amount is small at the time of quenching, the hardenability is inferior. Comparative Steel U-1 has a high S content, coarse MnS is generated in the steel, cold workability is decreased. Comparative Steel AA-1 has a low Mn content, hardenability is inferior.
[0143]
　Comparative Steel T-2, the two-stage stepped holding temperature at the first stage of the annealing box annealing is low, and is insufficient coarsening process carbide below Ac1 temperature, the thermal stability of the carbides by insufficient, carbides remaining at the second stage annealing is reduced, the tissue after slow cooling, can not be suppressed pearlite transformation, cold workability is decreased.
[0144]
　Comparative steel A-2, the higher the holding temperature at the first stage of the annealing of the two-stage step-type box annealing, austenite is formed during the annealing, it is impossible to increase the stability of the carbides, the second stage carbides remaining decreases during annealing, cold workability can not be suppressed pearlite transformation in tissue after slow cooling is lowered.
[0145]
　Comparative Steel L-2 has a short 2-step type retaining time at the first stage of the annealing box annealing, is insufficient coarsening process carbide below Ac1 temperature, the thermal stability of the carbides by insufficient, carbides remaining at the second stage annealing is reduced, the tissue after slow cooling, can not be suppressed pearlite transformation, cold workability is decreased.
[0146]
　Comparative Steel W-2 is 2-step holding time at the first stage annealing during annealing is long, the productivity was lowered. Comparative Steel X-2 is a two-stage holding temperature at the second stage annealing time step annealing is low, because it is not possible to increase the ratio of the number of carbides in the amount less grain boundaries of austenite, decrease cold workability did.
[0147]
　Comparative Steel AB-2 is higher the holding temperature at the second stage annealing of the two-stage step-type box annealing is, since the dissolution of carbides is promoted, carbides remaining decreases, the tissue after slow cooling, pearlite transformation can not be suppressed, cold forging workability is lowered.
[0148]
　Comparative Steel P-2, the two-stage stepped holding temperature at the second stage of the annealing box annealing is low, and less the amount of austenite, can not increase the number ratio of carbides in the ferrite grain boundary, cold between workability is decreased. Comparative Steel Y-2 is 2-step type box annealing in the second stage of longer holding time during annealing of, for dissolution of carbides is promoted, carbides remaining decreases, the tissue after slow cooling, pearlite transformation can not be suppressed, cold forging workability is lowered.
[0149]
　Comparative Steel D-2, the two-stage stepped cooling rate from the end of the second-stage annealing box annealing up to 650 ° C. is large, happening pearlite transformation during cooling, cold workability is decreased.
Industrial Applicability
[0150]
　As described above, according to the present invention, it is possible to provide manufactured steel sheet excellent in formability and abrasion resistance. Steel sheet of the present invention, punching, bending, automobile parts produced through a machining process such as press working, cutlery, since other is a suitable steel as a material for machine parts, the present invention provides industrial applicability is it is high.

claims

[Claim 1]でmass%, C:
　　0.10
　　~ 0.40%, Si: 0.01 ~ 0.30%, Mn: 1.00
　　~ 2.00%, P: 0.020% or
　　less, S: 0.010 % or
　　less, of Al: from 0.001
　　~ 0.10%, N: 0.010% or
　　less, O: 0.020% or
　　less, of Cr: 0.50% or less,
　　of Mo: 0.10% or
　　less, of Nb: 0. 10% or
　　less, V: 0.10% or
　　less, a Cu: 0.10% or
　　less, W is: 0.10% or
　　less, Ta: 0.10% or
　　less, of Ni: 0.10% or
　　less, of Sn: 0.050% or
　　less, Sb: 0.050% or
　　less of As: 0.050% or
　　less of Mg: 0.050% or
　　less of Ca: 0.050% or
　　less, the Y: 0.050% or
　　less, Zr: 0.050% or less,
　　la: 0.050% or
　　less, of Ce: 0.050% or less
wo shi containing, remnants ga Fe and unavoidable impuritiesびでthou ru steelで ~ Te,
　the note sheetのmetallic structure ga
　fu S Factory LITE intragranularのnumber carbideのni Dui suru fu S Factory LITE grain boundariesのnumber carbideののratio ga ultrafine,
　fu S Factory LITE diameter ga above 5μm, 50μm or less, andび
　paーLITEのarea ratio ga 6%
wo the Man shi ta,
　the note sheetのBldgッgradesーsu sa ga hard 100HV less than 170HVでthou ru
koととspecial Zhi wo suru plate.
[Claim 2]
　Instead of a part of the
　　Fe, Ti: 0.10% or less, and
　　B: 0.010% or less,
the steel sheet according to claim 1, characterized in that it contains one or two.
[Claim 3]
　A method of manufacturing a steel sheet according to claim 1 or 2,
　according to claim 1 or a steel slab of chemical composition according to 2, between complete hot finish rolling at a temperature range temperatures higher than 750 ℃ 850 ° C. or less a hot-rolled steel sheet subjected to rolling,
　the hot-rolled steel sheet was wound at 400 ° C. or higher 550 ° C. or less,
　wound subjected to pickling hot-rolled steel sheet,
　pickled temperature below the hot-rolled steel sheet 720 ° C. 650 ° C. or more in-band, annealed in the first stage to 3 hours or more 60 hours or less, then,
　in a temperature range of 725 ° C. or higher 790 ° C. or less hot-rolled steel sheet, the annealing in the second stage to hold 3 hours or more 50 hours or less subjecting,
　to cool the hot-rolled steel sheet after annealing, to 650 ° C. at 1 ° C. / time or 30 ° C. / time less cooling rate
method for manufacturing a steel sheet, characterized in that.