JP2017066454A - Cold rolled steel sheet and manufacturing method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a cold rolled steel sheet having high strength and excellent in fatigue resistance.SOLUTION: There is provided a cold rolled steel sheet containing, by mass%, C:0.10% to 0.50% inclusive, Si:2.0% or less, Mn:0.9% or less, P:0.05% or less, S:0.01% or less, Al:0.08% or less, N:0.008% or less, Nb:0.10% to 2.0% inclusive and the balance Fe with inevitable impurities, and having segregation width of Mn generated in a sheet thickness center part of 1 μm or less, the number density of carbide containing Nb of 2.3×10/mor more and total of percentage content of martensite and percentage content of retained austenite of 1.5% or less, yield strength of 1000 MPa or more and fatigue limit of 650 MPa or more.SELECTED DRAWING: None

Description

  The present invention relates to a cold-rolled steel sheet having a high yield strength (YS) of 1000 MPa or more and excellent fatigue resistance, which is useful for the use of a skeleton member for automobiles, and a method for producing the same.

In recent years, from the viewpoint of global environmental conservation, improvement of automobile fuel consumption has been directed to the entire automobile industry for the purpose of regulating CO ₂ emissions. In order to improve the fuel efficiency of automobiles, it is most effective to reduce the weight of automobiles by reducing the thickness of parts used. In recent years, the amount of high-strength steel sheets used as materials for automobile parts is increasing. In general, a high-strength steel sheet is thinner than a low-strength steel sheet, and therefore excellent fatigue resistance is required in addition to high strength.

  As a strengthening technique for obtaining a high-strength steel sheet, particle dispersion strengthening in which fine carbides are dispersed can be mentioned. Conventionally, in an annealing process, which is an indispensable process for producing a cold-rolled steel sheet or a plated steel sheet, it is difficult to obtain high-strength particle dispersion strengthened steel because carbides are coarsened. For this reason, proposals have been made for cold-rolled steel sheets in which fine carbides are dispersed.

  For example, in Patent Document 1, by mass%, C: more than 0.06 to 0.24%, Si ≦ 0.3%, Mn: 0.5 to 2.0%, P ≦ 0.06%, S ≦ 0.005%, Al ≦ 0.06%, N ≦ 0.006%, Mo: 0.05 to 0.5%, Ti: 0.03 to 0.2%, V: more than 0.15 to 1. 2%, ferrite phase is 95% or more in area ratio, and carbide containing Ti, Mo and V having an average particle diameter of less than 10 nm is dispersed and precipitated. Tensile cold-rolled steel sheets have been proposed.

In Patent Document 2, in mass%, C: 0.10 to 0.25%, Si: 1.5% or less, Mn: 1.0 to 3.0%, P: 0.10% or less, S: 0 0.005% or less, Al: 0.01 to 0.5%, N: 0.010% or less and V: 0.10 to 1.0%, and (10Mn + V) / C ≧ 50 is satisfied, and the balance Has a composition of Fe and inevitable impurities, the volume fraction of the tempered martensite phase is 80% or more, and 1000 carbides / μm ³ or more of carbides containing V having a particle size of 20 nm or less are precipitated, and the particle size is 20 nm. There is disclosed a high-strength cold-rolled steel sheet characterized in that the carbides containing V below have an average particle size of 10 nm or less and a tensile strength of 980 MPa or more.

  Patent Document 3 contains, in mass%, C: 0.05 to 0.20%, Mn: 0.50 to 3.00%, Ti: 0.03 to 0.15%, and Si: 2. 50% or less, Al: 1.50% or less, P: 0.15% or less, S: 0.010% or less, N: 0.0060% or less, Nb: 0.03% or less (including 0), Mo : 0.25% or less (including 0), V: 0.25% or less (including 0), and the contents of C, N, Ti, Nb, Mo, V are 0.18 ≦ 6Ti + 25Nb + 3Mo + 3V ≦ 1 0.0 and 20C + 17.1N-5Ti-2.6Nb-2.5Mo-4.7V ≧ 0.6, the balance being Fe and inevitable impurities, and the particle size of Ti-based carbonitride is 1 to 50 nm The area ratio of ferrite is 50% or more, and is either one or both of martensite and bainite. The area ratio of the hard structure is 5 to 50%, the total area ratio of the remaining pearlite, retained austenite and cementite is limited to 5% or less, the average grain size of the ferrite is limited to 20 μm or less, and the ferrite By limiting the proportion of the non-recrystallized ferrite to 25% or less, a precipitation strengthened double phase cold rolled steel sheet is obtained.

  In Patent Document 4, in mass%, C: 0.03 to 0.10%, Si: 0.5% or less, Mn: 0.8 to 2.0%, P: 0.030% or less, S: 0 0.01% or less, Al: 0.005 to 0.1%, N: 0.01% or less, Ti: 0.035 to 0.090%, including a ferrite phase in a fraction of 70% or more, It is said that a high-strength cold-rolled steel sheet can be obtained by defining the amount of Ti present in precipitates having an aspect ratio of ferrite grains of 10.0 or more and a size of less than 20 nm.

  In patent document 5, in mass%, C: 0.07-0.12%, Si + Al: 0.05-0.10%, Mn: 0.15-0.45%, P: 0.01-0. 05%, S: 0.004% or less, N: 0.0045% or less, Ti: 0.09 to 0.14% is contained, C and Ti contents are defined, Volume ratio: 90 to 95% Ferrite phase and volume ratio: 5 to 10% cementite, and the ferrite phase has a recovery structure with a recrystallization rate of 15% or more and 30% or less, and Ti carbide (TiC) is precipitated in the ferrite phase. A cold-rolled steel sheet having a structure in which the average length of the Ti carbide is less than 10 nm, the precipitation rate of Ti is 85% or more, and the tensile strength is 590 MPa or more is disclosed.

JP 2008-174802 A JP 2006-183140 A JP 2010-285657 A JP 2011-21224 A JP 2014-141717 A

  However, in the technique proposed in Patent Document 1, 1.3% by mass or more of Mn is contained as shown in Table 1 of the examples, and the problem that fatigue resistance decreases due to center segregation is inevitable. It was. In addition, since Mo is an element that rapidly increases the recrystallization temperature, there is a problem that the allowable range of the annealing temperature that restores ductility and suppresses the growth of carbide particles is extremely narrow.

  In the technique proposed in Patent Document 2, the number of carbides having a particle size of 20 nm or less is defined, but the matrix structure of the high-strength cold-rolled steel sheet proposed in Patent Document 2 has a large amount of tempered martensite. Contains. In order to obtain this tempered martensite, it is necessary to contain a large amount of Mn which is a hardenable element. Furthermore, since tempered martensite is highly sensitive to tempering temperature and time after annealing, cold-rolled steel sheets and plated steel sheets cannot be produced simultaneously.

  With the technique proposed in Patent Document 3, a high-strength plated steel sheet having a large carbide particle size and a thermal history different from that of a cold-rolled steel sheet cannot be obtained.

  With the technique proposed in Patent Document 4, a technique for suppressing the coarsening of carbide is insufficient, and a steel sheet with good strength and ductility cannot be obtained unless the annealing temperature is very low, 600 ° C.

  Similarly to Patent Document 4, the technique proposed in Patent Document 5 is insufficient in suppressing the coarsening of carbides, and a steel sheet having a yield strength of 1000 MPa or more necessary to satisfy high strength and excellent fatigue resistance is obtained. I can't.

  In view of such circumstances, an object of the present invention is to provide a cold-rolled steel sheet having high strength and excellent fatigue resistance.

  As a result of intensive studies on the requirements of a steel sheet having high strength and excellent fatigue resistance, the present inventors have found that bainite, martensite and retained austenite are not good in fatigue resistance due to a large number of movable dislocations. Therefore, as a result of studying the structure of the structure having as few movable dislocations as possible, it was found that the particle dispersion strengthening that reinforces the steel sheet with fine particles is most effective in improving the fatigue resistance. However, when manufacturing a cold-rolled steel sheet having a yield strength of 1000 MPa or more, which is required in the present invention, two major problems have emerged. That is, in order to obtain many fine carbides, it is necessary to contain a large amount of carbide constituent elements C and Nb, but they remain in the steel material (slab) as coarse carbides in the slab heating process before the hot rolling process. Since it cannot be completely dissolved, the number of carbides finer than expected cannot be obtained, and carbide particles grow by high-temperature heating in the annealing process, and the number of carbides decreases as fine carbides disappear. That is. On the other hand, as a result of further studies by the present inventors, for the problem that the carbide cannot be completely dissolved in the former slab heating process, the hot rolling process is performed in a short time while maintaining the high temperature after continuous casting. It has been found that the hot rolling process can be completed without starting to form coarse carbides. Moreover, about the coarsening suppression of the carbide | carbonized_material in an annealing process, when the austenite transformation was performed from the ferrite during annealing, the solubility product of the carbide | carbonized_material containing Nb changed, and it discovered that particle growth was accelerated | stimulated more in austenite. Therefore, unlike many conventional techniques that heat to the austenite region, the present invention has found that it is important to promote the recovery of dislocations in ferrite while suppressing the occurrence of transformation from ferrite to austenite. . In order to suppress the transformation from ferrite to austenite, it is effective to limit the Mn content, which is an austenite stabilizing element. To promote the recovery of dislocations in ferrite, the recrystallization temperature is increased. It was found that it is effective to suppress Mn to be contained and not to contain Mo. Further, it has been clarified that Mn segregation can be suppressed by reducing the amount of Mn, thereby improving fatigue resistance.

  Also, regarding the formation of coarse carbides by the disappearance of fine carbides during annealing, the standard deviation of the particle size distribution is much larger than that of hot-rolled sheets, so the average particle diameter of carbides and steel sheet strength are not necessarily It was found that there was no strong correlation. The amount of strengthening of particle dispersion varies depending on the distance between particles. Thus, as a result of focusing on the number of particles per unit volume, it was found that a strong correlation was obtained.

The present invention has been completed based on the above findings, and the gist thereof is as follows.
[1] By mass%, C: 0.10% to 0.50%, Si: 2.0% or less, Mn: 0.9% or less, P: 0.05% or less, S: 0.01% Hereinafter, Al: 0.08% or less, N: 0.008% or less, Nb: 0.10% or more and 2.0% or less, with the balance being Fe and unavoidable impurities, and occurring in the center of the plate thickness The segregation width of Mn is 1 μm or less, the number density of carbides containing Nb is 2.3 × 10 ²² pieces / m ³ or more, the total content of martensite and residual austenite is 1.5% or less, yield strength A cold-rolled steel sheet having a thickness of 1000 MPa or more and a fatigue limit of 650 MPa or more.
[2] Further, it is characterized by containing one or two of V: 0.01% to 1.0% and Ti: 0.01% to 2.0% by mass% [1] Cold-rolled steel sheet as described in 1.
[3] Furthermore, by mass%, B, Ca, Mg, Cr, Co, Ni, Cu, Sb, Zr, Y, REM, Hf, Ta, W, or a total of 0.0001% The cold-rolled steel sheet according to [1] or [2], which is contained in an amount of 0.1% or less.
[4] On the surface of the cold-rolled steel sheet according to any one of [1] to [3], Fe: 5.0% to 20.0%, Al: 0.001% to 1.0 %, And further, Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, REM, or a total of one or more selected from A cold-rolled steel sheet comprising a galvanized layer having a composition containing 0% or more and 30% or less, the balance being Zn and inevitable impurities.
[5] A cold-rolled steel sheet, wherein the plated layer according to [4] is any one of a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrogalvanized layer.
[6] The steel material having the composition according to any one of [1] to [3] is cast by a continuous casting machine, and the temperature of the cast steel material is maintained at 1000 ° C. or higher, and the time of slab cutting. Within about 1 hour from the beginning, and after the finish rolling at a finish rolling temperature of 800 ° C. or higher, the hot rolling step of winding at a winding temperature of 720 ° C. or less, and the hot rolled sheet after the hot rolling step, A cold rolling step for cold rolling and a cold-rolled sheet after the cold rolling step are heated at an annealing temperature of 750 ° C. or higher and 850 ° C. or lower, and then cooled to 680 ° C. at an average cooling rate of 5 ° C./s or higher. A method for producing a cold-rolled steel sheet, comprising an annealing step.
[7] A steel material having the composition according to any one of [1] to [3] is cast by a continuous casting machine, and the temperature of the cast steel material is maintained at 1000 ° C. or higher, and the time of slab cutting. Within about 1 hour from the beginning, and after the finish rolling at a finish rolling temperature of 800 ° C. or higher, the hot rolling step of winding at a winding temperature of 720 ° C. or less, and the hot rolled sheet after the hot rolling step, A method for producing a cold-rolled steel sheet, comprising: a cold-rolling step for cold-rolling; and an annealing step for box annealing the cold-rolled sheet after the cold-rolling step at an annealing temperature of 600 ° C. or higher and lower than 680 ° C. .
[8] After the annealing step, in mass%, Fe: 5.0% to 20.0%, Al: 0.001% to 1.0%, and further Pb, Sb, Si, Sn , Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, REM, or a total of 0% or more and 30% or less, selected from Zn and unavoidable impurities. A method for producing a cold-rolled steel sheet according to [6] or [7], further comprising a galvanizing treatment step for applying a plating layer.
[9] The method for producing a cold-rolled steel sheet according to [8], wherein the plated layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer.

  According to the present invention, a cold-rolled steel sheet having high strength and good fatigue resistance suitable for the use of automobile structural members and the like can be obtained, and the effects of reducing the weight of automobile parts and improving the reliability thereof can be obtained. Is remarkable.

  Hereinafter, the present invention will be described in detail.

  First, the reasons for limiting the component composition of the steel sheet of the present invention will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.

C: 0.10% or more and 0.50% or less C is an element that combines with Nb to form fine carbides and contributes substantially to increasing the strength of the steel sheet. In order to obtain the yield strength of 1000 MPa or more required in the present invention, it is necessary to contain at least 0.10% or more. On the other hand, when C is excessively contained, coarse cementite precipitates. Coarse inclusions and precipitates cause a significant concentration of stress in the vicinity thereof, thereby reducing fatigue resistance. Therefore, the C content is 0.10% or more and 0.50% or less. Preferably they are 0.16% or more and 0.45% or less.

Si: 2.0% or less Si is an element that contributes to strengthening by solid solution strengthening and increases the temperature at which austenite transforms from ferrite, and suppresses coarsening of carbides due to austenite phase generation. From the viewpoint of suppressing the coarsening of the carbide, the Si content is preferably 0.1% or more. On the other hand, when Si is contained excessively, the adverse effect of chemical conversion treatment property or plating property deterioration becomes obvious. Therefore, in the present invention, the Si content is set to 2.0% or less. Preferably it is 1.5% or less. There is no particular lower limit. In addition, 0.01% Si is inevitably mixed in the steel.

Mn: 0.9% or less Mn lowers the temperature at which austenite transforms from ferrite, and the austenite phase is likely to cause coarsening of the carbide, and causes Mn segregation. In order to make the segregation width of Mn 1 μm or less, it is necessary to reduce it to 0.9% or less in the present invention, so the Mn content is set to 0.9% or less. Preferably, it is 0.7% or less. On the other hand, since Mn has a role of fixing S, which is a harmful element, as a sulfide, it is preferably contained in an amount of 0.1% or more.

P: 0.05% or less P is preferably reduced as much as possible because it segregates at the grain boundary to cause grain boundary cracking during repeated stress loading and reduces fatigue resistance. In the present invention, the P content is acceptable up to 0.05% or less. Preferably it is 0.03% or less, and it is desirable to reduce it as much as possible. In production, 0.001% is inevitably mixed.

S: 0.01% or less S is present as an inclusion such as MnS in steel. This inclusion becomes an inclusion that extends in a wedge shape by rolling, and a large stress concentration occurs around the inclusion, which adversely affects fatigue resistance. Therefore, in the present invention, it is preferable to reduce the S content as much as possible, and set it to 0.01% or less. Preferably it is 0.005% or less, and it is more desirable to reduce as much as possible. In production, 0.0001% is inevitably mixed.

Al: 0.08% or less When Al is added as a deoxidizer at the stage of steelmaking, 0.02% or more is contained. On the other hand, if the Al content exceeds 0.08%, an adverse effect on fatigue resistance becomes obvious due to the influence of inclusions such as alumina. Therefore, the Al content is 0.08% or less. Preferably it is 0.07% or less.

N: 0.008% or less N combines with Nb to form coarse carbonitride. For this reason, the amount of Nb contributing to strengthening is reduced, which causes a reduction in steel sheet strength. Moreover, since coarse carbonitride reduces fatigue resistance, it is preferable to reduce N as much as possible, and N content was made into 0.008% or less. Preferably it is 0.006% or less.

Nb: 0.10% to 2.0% Nb is an element that forms fine carbides and increases the strength of the steel sheet. In order to obtain the yield strength of 1000 MPa or more required in the present invention, it is necessary to contain at least 0.10% of Nb. On the other hand, if it exceeds 2.0%, an adverse effect of grain growth of the carbide after casting becomes obvious. Therefore, the Nb content is set to 2.0% or less. Preferably they are 0.12% or more and 1.8% or less.

  The above is the basic composition in the present invention. Furthermore, you may contain the following composition.

V: 0.01% or more and 1.0% or less, Ti: 0.01% or more and 2.0% or less V and Ti, like Nb, are finely dispersed as carbides. It is an element that contributes to crystallization. On the other hand, if excessively contained, the coarsening of the carbide is promoted, which causes a decrease in fatigue resistance. From the above viewpoint, when V or Ti is contained, V: 0.01% to 1.0% and Ti: 0.01% to 2.0% are preferable. More preferably, V: 0.05% to 0.8% and Ti: 0.02% to 1.6%. In order to obtain a yield strength of 1000 MPa or more, it is preferable to contain a total amount of Nb, V and Ti of 0.6% or more.

One or more of B, Ca, Mg, Cr, Co, Ni, Cu, Sb, Zr, Y, REM, Hf, Ta, and W in total, 0.0001% or more and 0.1% or less If the total of one kind or two or more kinds is 0.0001% or more and 0.1% or less, the characteristics of the present invention are not inhibited. In particular, Cr, Ni, and Cu are inevitably mixed in production. On the other hand, Ca, Mg, REM, and Y have the effect of improving fatigue resistance by controlling the shape of inclusions. A preferred range is 0.0001% or more and 0.06% or less in total of one or more of the above components.

  Components other than the above are Fe and inevitable impurities.

  Next, the reason for limiting the metal structure of the cold-rolled steel sheet of the present invention will be described.

The segregation width of Mn generated in the central portion of the plate thickness is 1 μm or less. The segregation width of Mn is obtained by analyzing the spectral intensity of Mn in the plate thickness direction using an electron beam microanalyzer. Specifically, when the spectral intensity at the 1/4 position of the plate thickness from the center of the steel sheet surface width is used as the background, the spectral intensity at the center of the plate thickness (1/2 position of the plate thickness from the center of the steel sheet surface) is used as the background. When the spectral intensity at the center of the plate thickness (1/2 t position in the plate thickness direction from the center of the steel plate surface) is 1.5 times or more of the background as the segregation portion, The total peak width at a position 1.5 times the peak height of the peak obtained is defined as the Mn segregation width. If the Mn segregation width exceeds 1 μm, the punchability is significantly deteriorated. When the punched end face properties are poor, stress concentration occurs at the interface between the segregated portion and the normal portion, and as a result, the fatigue characteristics deteriorate. For this reason, in the present invention, the Mn segregation width is limited to 1 μm or less. Preferably, it is 0.5 μm or less.

The number density of carbides containing Nb is 2.3 × 10 ²² particles / m ³ or more As the number of particles per unit volume increases, the particle dispersion strengthening for strengthening the steel sheet with fine carbides increases. In order to obtain the yield strength of 1000 MPa or more obtained in the present invention, the number density is 2.3 × 10 ²² pieces / m ³ or more. Preferably, it is 2.5 × 10 ²² pieces / m ³ or more. Here, the target carbide containing Nb is a carbide observable by TEM. A transmission electron microscope may be used to measure the number density. Note that a magnification of 150,000 times may be selected so that fine carbides having a particle size of 5 nm or less can be observed, and at least five different crystal grains may be observed. The film thickness in the observation field is determined using an electron energy loss spectrometer. If a TEM sample is prepared by a focused ion beam apparatus and the influence of the subsequent thickness reduction by electropolishing is known, the value may be used as a film thickness.

The total of martensite content and residual austenite content is 1.5% or less. Martensite is observed as a lath-like structure with a scanning electron microscope. Moreover, a retained austenite is specified by the X-ray diffraction analysis as described in an Example. Since martensite has many movable dislocations, its fatigue resistance is not good. In addition, when martensite or retained austenite is generated, it indicates that the transformation from ferrite to austenite has started, and thus coarsening of the carbide is inevitable. Therefore, it is preferable to reduce these structures as much as possible. In the present invention, the total of the martensite area ratio (content ratio) and the retained austenite fraction (content ratio) is set to 1.5% or less. Preferably, it is less than 1.0%.

  The balance other than martensite and retained austenite is ferrite: 97.5% or more, and bainite and cementite are 1% or less.

In mass%, Fe: 5.0% or more and 20.0% or less, Al: 0.001% or more and 1.0% or less, and Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr , Co, Ca, Cu, Li, Ti, Be, Bi, REM containing a total of one or more selected from 0% to 30%, with the balance being Zn and inevitable impurities. In the invention, when having a plating layer on the steel sheet surface, it contains Fe: 5.0% or more and 20.0% or less, Al: 0.001% or more and 1.0% or less in mass%, and further Pb, Sb , Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, REM, or a total of 0% to 30%, and the balance It has a plating layer composed of Zn and inevitable impurities. And are preferred. As the plating layer, any one of a hot dip galvanized layer, an alloyed hot dip galvanized layer and an electrogalvanized layer is preferable.

  Next, the manufacturing method of the steel plate of this invention is demonstrated.

  The present invention casts a steel material (slab) having the above composition with a continuous casting machine, and starts rough rolling within 1 hour from the time of slab cutting while maintaining the temperature of the cast steel material at 1000 ° C. or higher. After finishing rolling at a finish rolling temperature of 800 ° C. or higher, a hot rolling step of winding at a winding temperature of 720 ° C. or lower, a cold rolling step of cold rolling a hot-rolled sheet after the hot rolling step, and annealing A process. For the annealing step, the cold-rolled sheet after the cold rolling step is heated at an annealing temperature of 750 ° C. or higher and 850 ° C. or lower, and then cooled to 680 ° C. at an average cooling rate of 5 ° C./s or The cold-rolled sheet after the hot rolling process is box-annealed at an annealing temperature of 600 ° C. or more and less than 680 ° C.

  In the present invention, the method for melting steel is not particularly limited, and a known melting method such as a converter or an electric furnace can be employed. Further, secondary refining may be performed in a vacuum degassing furnace. Then, it is preferable to use a slab (steel material) by a continuous casting method from the viewpoint of productivity and quality.

  First, the manufacturing method of hot rolling is demonstrated.

Casting with a continuous caster, and starting rough rolling within 1 hour from the time of slab cutting while maintaining the temperature of the cast steel material at 1000 ° C or higher C and Nb are cooled to room temperature after casting and heated again In the process, particles grow as coarse carbides. When C and Nb are contained in large amounts, they cannot be dissolved in the slab heating step, and the desired number density of carbides cannot be obtained. Therefore, in this invention, it is necessary to start rough rolling within 1 hour from the time of a slab cut, maintaining the steel raw material (slab) cast with the continuous casting machine at 1000 degreeC or more. Here, the reason why the temperature is set to 1000 ° C. or more is that when the temperature of the slab is lowered to less than 1000 ° C., the precipitation driving force of the carbide containing Nb increases, and the carbide precipitates and grows particles. The reason why rough rolling is started within one hour from the time of slab cutting is that if it exceeds one hour, carbide containing Nb precipitates and grows even if the slab temperature is 1000 ° C. or higher. Preferably, rough rolling is started within 40 minutes from the time of slab cutting while maintaining 1100 ° C. or higher.

Finishing rolling temperature: 800 ° C. or more When the finishing rolling temperature is lower than 800 ° C., ferrite transformation starts during finish rolling, and a structure in which ferrite grains are extended is formed, and portions having extremely different mechanical properties are generated. This deteriorates the sheet thickness accuracy during cold rolling, and causes breakage trouble during manufacturing. Accordingly, the finish rolling temperature is 800 ° C. or higher. Preferably it is 820 degreeC or more. Note that the finish rolling temperature is preferably 1000 ° C. or less because of restrictions on the production line.

Winding temperature: 720 ° C. or less When the winding temperature exceeds 720 ° C., carbides containing Nb are coarsened, and the number density of carbides required in the present invention cannot be obtained. Therefore, the winding temperature is set to 720 ° C. or lower. Preferably it is 680 degrees C or less. In view of the plate shape, the coiling temperature is preferably 350 ° C. or higher.

Cold rolling process for cold rolling the hot rolled sheet after the hot rolling process In order to obtain a desired sheet thickness, it is necessary to cold roll the hot rolled sheet after the hot rolling process. Although there is no restriction | limiting in a cold rolling rate, 20-80% of a cold rolling rate is preferable from the restriction | limiting of a production line.

The cold-rolled sheet after the cold rolling process is heated at an annealing temperature of 750 ° C. or higher and 850 ° C. or lower. It is necessary to recover the ductility lost by the cold rolling in the annealing process. In the case of continuous annealing, it hardly recovers by annealing below 750 ° C. If the dislocations are not recovered, the ductility is remarkably lowered and the number of movable dislocations is so great that the fatigue resistance is lowered. On the other hand, in annealing exceeding 850 ° C., the transformation from ferrite to austenite proceeds, and the coarsening of the carbide proceeds. From the above, the annealing temperature in the annealing step was set to 750 ° C. or higher and 850 ° C. or lower.

Cool at an average cooling rate of 5 ° C / s or lower to 680 ° C or lower When the steel sheet is held at a high temperature, the carbide containing Nb becomes coarser, so immediately after the ductility is restored, the temperature does not become coarser. It needs to be cooled. In order to prevent the carbides from becoming coarse, the average cooling rate may be reduced to 680 ° C. or lower at a rate of 5 ° C./s or higher. Preferably, the average cooling rate is 10 ° C./s or higher up to 580 ° C. or lower. The average cooling rate in the present invention is calculated by (cooling start temperature−cooling stop temperature) / cooling time.

In the case of box annealing using a box-type annealing furnace, an average cooling rate of 5 ° C./s or more cannot be obtained, so the annealing temperature must be set to less than 680 ° C.
In addition, since the particle growth of carbide is also influenced by time, it is preferable that the time maintained in the range from 600 ° C. to the annealing temperature satisfies the following formula (1). If the left side of the formula (1) is 28000 or less, an adverse effect due to the coarsening of the carbide does not become obvious. Therefore, in order to obtain a yield strength of 1000 MPa or more and a fatigue limit of 650 MPa or more, it is preferable that the left side of the formula (1) is 28000 or less. In order to recover the ductility more stably, it is desirable that the left side of the formula (1) is 24000 or more.

In the formula (1), T: annealing temperature (° C.), t: annealing time (s).

  Moreover, since the characteristic of the steel plate of this invention will not be changed if cooling stop temperature is a temperature range below 600 degreeC, the heat history after cooling is not prescribed | regulated in particular.

  The cold-rolled steel sheet of the present invention can have a plating layer on the surface. In order to provide the plating layer, a plating process may be performed after annealing at the annealing temperature. In the present invention, by mass%, Fe: 5.0% or more and 20.0% or less, Al: 0.001% or more and 1.0% or less, and further Pb, Sb, Si, Sn, Mg, Mn , Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, REM, containing a total of 0% to 30%, with the balance being Zn and inevitable impurities In order to obtain a desired plating layer, preferable conditions may be adopted as appropriate. Suitable examples of the plating layer include galvanized layers such as hot dip galvanized, alloyed galvanized layers such as alloyed hot dip galvanized, and electrogalvanized layers. When producing an alloyed hot-dip galvanized steel sheet, the characteristics of the steel sheet do not change unless the temperature of the alloying treatment exceeds 650 ° C.

  Examples of the present invention will be described below.

The steel material having a thickness of 250 mm having the composition shown in Table 1 was subjected to cold rolling with a hot rolling steel sheet under the hot rolling conditions shown in Table 2 and a cold rolling rate of 10% to 70%. Then, under the conditions shown in Table 2, cold rolled steel sheet (CR), hot dip galvanized steel sheet (GI) in a box annealing furnace (BAF), continuous annealing line (CAL), or continuous hot dip galvanizing line (CGL), Alternatively, an alloyed hot-dip steel sheet (GA) was manufactured. The temperature of the plating bath immersed in the continuous annealing hot dip galvanizing line (plating composition: Zn-0.13 mass% Al) is 460 ° C., and the amount of plating adhesion is 45 to 65 g / m ² per side for both GI and GA. The amount of Fe contained in the plating layer was in the range of 6 to 14% by mass.

  A test piece was collected from the cold-rolled steel sheet obtained as described above, and the structure was observed by the following method to evaluate the performance.

(I) Microstructure observation From a cold-rolled steel sheet, the section parallel to the rolling direction is cut out to become an observation surface, and the central part of the sheet thickness appears to corrode with 1% nital, and the sheet thickness is enlarged by 2000 times with a scanning electron microscope. The 1/4 t position (t: plate thickness) was photographed for 10 fields of view. The martensite phase is a structure in which no carbide is observed in the grains and is observed with a white contrast compared to the ferrite phase. The area ratio occupied by the martensite phase with respect to the observation visual field area was defined as the area ratio.

  The segregation width of Mn generated at the center of the plate thickness was determined by an electron beam microanalyzer with an acceleration voltage of 15 kV and a beam diameter of 0.5 μm. Specifically, when the spectral intensity at the 1/4 t position (t is the thickness of the steel sheet) in the thickness direction from the center of the steel sheet surface width is used as a background, the center of the thickness (from the center of the steel sheet to the thickness direction 1 / The peak width at a position 1.5 times the peak height of the background peak is defined as the segregation part where the spectral intensity at 2t position is 1.5 times or more of the background. The total of was measured.

  An X-ray diffraction analysis was performed on a sample in which the steel plate surface was chemically polished by 0.3 μm from the steel plate surface in the plate thickness direction, and the amount of retained austenite was determined. The fraction of retained austenite was calculated from the integrated intensity of the (200) α, (211) α, (200) γ, (220) γ, (311) γ diffraction peaks measured by the Mo-Ka line.

  The number density of carbides containing Nb was measured using a transmission electron microscope. The central part of the steel sheet in the thickness direction is the object of observation, and is magnified 300,000 times. The observed precipitates are identified with carbides containing Nb using an energy dispersive X-ray analyzer attached to the TEM. The number density was determined by selecting 5 fields of view.

(Ii) Tensile test A JIS No. 5 tensile test piece was produced from the obtained steel sheet in a direction perpendicular to the rolling direction, and a tensile test in accordance with the provisions of JIS Z 2241 (2011) was conducted five times to obtain an average yield strength. (YS), tensile strength (TS), and total elongation (El) were determined.

(Iii) Fatigue test No. 1 test piece (test piece width 20 mm, test piece shape symbol 1-20) defined by JIS Z 2275 is prepared so that the longitudinal direction of the test piece is perpendicular to the rolling direction, and the frequency is 30 Hz. , maximum repetition number 10 ⁷ times, stress ratio in both shake fatigue test of -1, was determined fatigue limit. The fatigue limit required in the present invention was 650 MPa or more.

  The results obtained as described above are shown in Table 3.

  It can be seen that all of the inventive examples have good fatigue resistance at a yield strength of 1000 MPa or more. On the other hand, in many of the comparative examples outside the scope of the present invention, the yield strength does not reach 1000 MPa or more, or a steel plate with good fatigue resistance has not been obtained.

Claims (9)

In mass%, C: 0.10% to 0.50%, Si: 2.0% or less, Mn: 0.9% or less, P: 0.05% or less, S: 0.01% or less, Al : 0.08% or less, N: 0.008% or less, Nb: 0.10% or more and 2.0% or less, with the balance being Fe and inevitable impurities, segregation of Mn occurring in the center of the plate thickness The width is 1 μm or less, the number density of carbides containing Nb is 2.3 × 10 ²² pieces / m ³ or more, the total content of martensite and residual austenite is 1.5% or less, and the yield strength is 1000 MPa. A cold-rolled steel sheet having a fatigue limit of 650 MPa or more.   Furthermore, it contains 1 type or 2 types of V: 0.01% or more and 1.0% or less, and Ti: 0.01% or more and 2.0% or less in the mass%, It is characterized by the above-mentioned. Cold rolled steel sheet.   Further, in terms of mass%, one or more of B, Ca, Mg, Cr, Co, Ni, Cu, Sb, Zr, Y, REM, Hf, Ta, and W are combined in a total of 0.0001% or more and 0.001% or more. The cold-rolled steel sheet according to claim 1 or 2, characterized by containing 1% or less.   The surface of the cold-rolled steel sheet according to any one of claims 1 to 3 contains, in mass%, Fe: 5.0% to 20.0%, Al: 0.001% to 1.0%. Furthermore, a total of one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM is 0% to 30% in total. A cold-rolled steel sheet comprising a galvanized layer having a composition comprising the following, the balance being composed of Zn and inevitable impurities.   The cold-rolled steel sheet, wherein the plated layer according to claim 4 is any one of a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, and an electrogalvanized layer.   Within 1 hour from the time of slab cutting, the steel material having the component composition according to any one of claims 1 to 3 is cast by a continuous casting machine, and the temperature of the cast steel material is maintained at 1000 ° C or higher. After rough rolling is started and finish rolling is finished at a finish rolling temperature of 800 ° C. or higher, a hot rolling step of winding at a winding temperature of 720 ° C. or lower, and a cold rolling of cold-rolling the hot-rolled sheet after the hot rolling step A cold rolling sheet after the cold rolling process and an annealing process in which the cold rolled sheet is heated at an annealing temperature of 750 ° C. or higher and 850 ° C. or lower and then cooled to 680 ° C. at an average cooling rate of 5 ° C./s or higher. A method for producing a cold-rolled steel sheet.   Within 1 hour from the time of slab cutting, the steel material having the component composition according to any one of claims 1 to 3 is cast by a continuous casting machine, and the temperature of the cast steel material is maintained at 1000 ° C or higher. After rough rolling is started and finish rolling is finished at a finish rolling temperature of 800 ° C. or higher, a hot rolling step of winding at a winding temperature of 720 ° C. or lower, and a cold rolling of cold-rolling the hot-rolled sheet after the hot rolling step A method for producing a cold-rolled steel sheet, comprising: a cold-rolling process, and an annealing process in which the cold-rolled sheet after the cold-rolling process is subjected to box annealing at an annealing temperature of 600 ° C or higher and lower than 680 ° C.   After the annealing step, it contains Fe: 5.0% or more and 20.0% or less, Al: 0.001% or more and 1.0% or less in mass%, and Pb, Sb, Si, Sn, Mg, A plating layer containing one or more selected from Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM in a total of 0% to 30%, with the balance being Zn and inevitable impurities A method for producing a cold-rolled steel sheet according to claim 6 or 7, further comprising a galvanizing treatment step for performing the treatment.   The method for producing a cold-rolled steel sheet according to claim 8, wherein the plated layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer.