JP6390572B2 - Cold-rolled steel sheet, plated steel sheet, and production method thereof - Google Patents

Description

  TECHNICAL FIELD The present invention relates to a cold-rolled steel sheet, a plated steel sheet, and a method for producing them, which are useful for the use of a skeleton member for automobiles and have both high strength of yield strength (YS) of 1000 MPa or more and excellent fatigue resistance.

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. When using a high-strength steel plate, generally a steel plate thinner than a low-strength steel plate is used. For this reason, the steel sheet is required to have excellent fatigue resistance 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 is known. Although particle dispersion strengthening is a useful strengthening method, carbides are coarsened in the annealing process, which is an indispensable process for producing cold-rolled steel sheets or plated steel sheets. difficult. Therefore, 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, residual austenite and cementite is limited to 5% or less, the average grain size of ferrite is limited to 20 μm or less, and occupies the ferrite By restricting 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, the ferrite phase consists of 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, as shown in Table 1 of Examples, the Mn content is 1.3% by mass or more, and the problem of center segregation cannot be avoided. In addition, since Mo contained in the steel sheet of Patent Document 1 is an element that rapidly increases the recrystallization temperature, there is a problem that the allowable range of the annealing temperature that can recover ductility and suppress carbide particle growth is extremely narrow.

  In the technique proposed in Patent Document 2, the number of carbides of 20 nm or less is specified. Carbides of 10 nm or more have a small contribution to strength increase. Therefore, the high-strength cold-rolled steel sheet proposed in Patent Document 2 is mainly strengthened by tempered martensite. In order to obtain this tempered martensite, it was 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.

  In the technique proposed in Patent Document 3, it is considered that the particle size of the carbide is large and the strength is obtained by martensite and bainite. Bainite and martensite are highly sensitive to thermal history after annealing, and, as in Patent Document 2, a high-strength plated steel sheet having a thermal history different from that of a cold-rolled steel sheet cannot be obtained.

  In the technique proposed in Patent Document 4, a technique for suppressing the coarsening of carbide is insufficient, and unless the annealing temperature is set to an extremely low temperature of 600 ° C., a steel sheet having good strength and ductility cannot be obtained.

  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.

  The present invention has been made to solve the above-described problems, and an object of the present invention is to provide a cold-rolled steel sheet, a plated steel sheet having high strength and excellent fatigue resistance, and a production method thereof. .

  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, in producing cold-rolled steel sheets or plated steel sheets having a yield strength of 1000 MPa or more as required in the present invention, two major problems have emerged. First, in order to obtain many fine carbides, it is necessary to contain a large amount of carbide constituent elements C and Ti, but if these are contained in a large amount, a slab (steel material) heating step before the hot rolling step Since coarse carbides remain and cannot be completely dissolved, the number of carbides finer than expected cannot be obtained. Second, carbide particles grow by high-temperature heating in the annealing process, and the number of carbides decreases as fine carbides disappear. 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. In addition, regarding the suppression of carbide coarsening in the annealing process, when transformation from ferrite to austenite during annealing, the solubility product of carbide containing Ti changes, and it has been found that grain growth is further promoted 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 limit the content of Mn to be produced and not to contain Mo unlike Patent Document 1. On the other hand, regarding the problem of fine carbides disappearing and forming coarse 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 plate strength are not necessarily It was found that there was no strong correlation. In addition, the amount of strengthening of particle dispersion varies depending on the distance between particles. Therefore, as a result of paying attention to the number of particles per unit volume (number density of carbides containing Ti), 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% A component composition containing Al: 0.08% or less, N: 0.008% or less, Ti: 0.22% or more and 2.0% or less, with the balance being Fe and inevitable impurities, and the following method The segregation width of Mn generated in the center portion of the plate thickness measured in step 1 is 1 μm or less, the number density of carbides containing Ti is 2.3 × 10 ²² pieces / m ³ or more, the content of martensite and residual austenite A cold-rolled steel sheet having a steel structure with a total content of 1.5% or less, a yield strength of 1000 MPa or more, and a fatigue limit of 650 MPa or more.
(Measuring method)
Using an electron beam microanalyzer, the spectral intensity of Mn is analyzed from the center of the steel sheet surface width to the sheet thickness direction, and the spectral intensity at the 1/4 t position (t is the thickness of the steel sheet) from the center of the steel sheet surface width to the sheet thickness direction is determined as the background. When the spectral intensity at the center of the plate thickness (1 / 2t position in the plate thickness direction from the center of the steel plate surface width) is a portion where the spectral intensity is 1.5 times or more of the background, the segregated portion is the peak. Measure the total peak width at a position 1.5 times the peak height of the background peak.

  [2] The cold rolled steel sheet according to [1], wherein the component composition further includes, by mass%, V: 0.01% to 1.0%.

  [3] The component composition further includes one or two or more of B, Ca, Mg, Cr, Co, Ni, Cu, Sb, Zr, Y, REM, Hf, Ta, and W in mass%. The cold-rolled steel sheet according to [1] or [2], which is contained in an amount of 0.0001% to 0.1%.

  [4] On the cold-rolled steel sheet according to any one of [1] to [3] and on the cold-rolled steel sheet, Fe: 5.0% to 20.0%, Al: 0.001 % Or more and 1.0% or less, and one or two selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM A plated steel sheet comprising: a plating layer containing a total of 0 to 30% of seeds and the balance of Zn and inevitable impurities.

  [5] The plated steel sheet according to [4], wherein the plated layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer.

  [6] The steel material having the component 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 after slab cutting, The rough rolling is started within one hour, the finish rolling is finished at a finish rolling temperature of 800 ° C. or higher, and then the hot rolling step is taken up at a winding temperature of 720 ° C. or less, and the hot rolled sheet is cooled after the hot rolling step. A cold rolling step for cold rolling, a continuous annealing step for heating the cold-rolled sheet after the cold rolling step to 750 ° C. or more and 850 ° C. or less, and cooling to 680 ° C. at an average cooling rate of 5 ° C./s or more, The manufacturing method of the cold-rolled steel plate which has this.

  [7] A steel material having the component 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 more, and after slab cutting, The rough rolling is started within one hour, the finish rolling is finished at a finish rolling temperature of 800 ° C. or higher, and then the hot rolling step is taken up at a winding temperature of 720 ° C. or less, and the hot rolled sheet is cooled after the hot rolling step. A method for producing a cold-rolled steel sheet, comprising: a cold-rolling step for cold rolling, and a box annealing step for heating and cooling the cold-rolled sheet after the cold-rolling step to 600 ° C or higher and lower than 680 ° C.

  [8] On the cold-rolled steel sheet obtained by the method for producing a cold-rolled steel sheet described in [6] or [7], Fe: 5.0 to 20.0%, Al: 0.001 by mass%. One or two selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM The manufacturing method of the plated steel plate which has the plating process which contains the above 0 to 30% in total, and forms the plating layer which remainder consists of Zn and an unavoidable impurity.

  [9] The method for producing a plated 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 high-strength cold-rolled steel sheet and a high-strength plated steel sheet having good fatigue resistance suitable for the use of automobile structural members and the like are obtained, and the weight reduction and reliability of automobile parts are improved. The effect is remarkable.

  Moreover, in this invention, since Mo: 0.05-0.5 mass% is not included, the problem that the tolerance | permissible_range of an annealing temperature becomes very narrow does not arise.

  Hereinafter, embodiments of the present invention will be described. In addition, this invention is not limited to the following embodiment.

  The component composition of the cold-rolled steel sheet of the present invention is, by mass, C: 0.10% or more and 0.50% or less, Si: 2.0% or less, Mn: 0.9% or less, P: 0.05% Hereinafter, S: 0.01% or less, Al: 0.08% or less, N: 0.008% or less, Ti: 0.22% or more and 2.0% or less, with the balance being Fe and inevitable impurities Become.

  The component composition of the cold-rolled steel sheet of the present invention may further contain V: 0.01% or more and 1.0% or less in mass%.

  Moreover, the component composition of the cold-rolled steel sheet of the present invention is one or two of B, Ca, Mg, Cr, Co, Ni, Cu, Sb, Zr, Y, REM, Hf, Ta and W in mass%. You may contain more than seed | species 0.0001% or more and 0.1% or less in total.

  Hereinafter, the reason for limiting the content of the elements contained in the component composition will be described. “%” In the description of the content means “% by mass” unless otherwise specified.

C: 0.10% or more and 0.50% or less C is an element that combines with Ti 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 obtained in the present invention, the C content needs to be at least 0.10%. 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. Desirably, it is 0.16% or more and 0.45% or less.

Si: 2.0% or less Si is an element that contributes to high strength by solid solution strengthening, and raises the transformation temperature from ferrite to ferrite and suppresses the 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 bad influence of chemical conversion treatment property and plating property fall will become obvious. The upper limit of the Si content in the present invention is 2.0% or less. Desirably, it is 1.5% or less. Although the lower limit is not particularly defined, 0.01% Si may inevitably be mixed into the steel.

Mn: 0.9% or less Mn lowers the transformation temperature from ferrite to austenite. For this reason, the larger the Mn content, the more easily the coarsening of the carbide proceeds due to the austenite phase generation. Moreover, since Mn raises recrystallization temperature, it becomes a cause of ductility fall and Mn segregation. In order to make the segregation width of Mn 1 μm or less, in the present invention, it is necessary to reduce the Mn content to 0.9%, so the upper limit of the Mn content is set to 0.9%. Desirably, 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, in order to obtain this effect, the Mn content is preferably 0.1% or more.

P: 0.05% or less P segregates at the grain boundary to cause grain boundary cracking during repeated stress loading, thereby reducing fatigue resistance. For this reason, it is preferable to reduce P content as much as possible. In the present invention, the P content is acceptable up to 0.05%. The P content is preferably 0.03% or less. Although it is desirable to reduce the P content as much as possible, 0.001% may be inevitably mixed in production.

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. Although it is desirable to reduce the S content as much as possible, 0.0001% may be inevitably mixed in production.

Al: 0.08% or less When Al is added as a deoxidizer in the steelmaking stage, the Al content is 0.02% or more. 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 Ti to form coarse nitrides. When the nitride is formed, the Ti content that contributes to an increase in strength is reduced, so the formation of the nitride causes a reduction in the strength of the steel sheet. Further, since coarse nitrides reduce fatigue resistance, the N content is preferably reduced as much as possible, and the upper limit of the N content is set to 0.008%. The N content is preferably 0.006% or less.

Ti: 0.22% or more and 2.0% or less Ti is an element that forms fine carbides and increases the strength of the steel sheet. In order to obtain the yield strength required in the present invention: 1000 MPa or more, the Ti content needs to be at least 0.22%. On the other hand, if the Ti content exceeds 2.0%, the adverse effect of carbide particle growth after casting becomes obvious, so the upper limit of the Ti content was set to 2.0%. The Ti content is preferably 0.24% or more and 1.8% or less.

  The above is the basic composition in the present invention, but it may contain the following instead of a part of Fe of the basic composition described above.

V: 0.01% or more and 1.0% or less V, like Ti, is an element that contributes to increasing the strength of the steel sheet because it becomes a carbide and finely disperses. On the other hand, when V is contained excessively, coarsening of the carbide is promoted, which causes a decrease in fatigue resistance. From the above viewpoint, when V is contained in order to obtain the above effect, the V content is set to 0.01% or more and 1.0% or less. Preferably, it is 0.05% or more and 0.8% or less.

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 When the content of one kind or two or more kinds is 0.0001% or more and 0.1% or less in total, 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. Inevitable impurities include components included within a range that does not impair the effects of the present invention, in addition to components inevitably mixed during production. Examples of components that do not impair the effects of the present invention include V: less than 0.01% and a total of one or more of B and the like: less than 0.0001%.

Next, the steel structure of the cold rolled steel sheet according to the present invention will be described. In the steel structure of the cold-rolled steel sheet of the present invention, the segregation width of Mn generated in the center part of the plate thickness measured by the method described later is 1 μm or less, and the number density of carbides containing Ti is 2.3 × 10 ²² pieces / and the m ³ or more, and the total content and the content of residual austenite in the martensite is 1.5% or less.

The segregation width of Mn generated in the central part of the plate thickness is 1 μm or less. The segregation width of Mn is analyzed by analyzing the spectral intensity of Mn in the plate thickness direction using an electron beam microanalyzer, and 1/4 t from the center of the steel plate surface width to the plate thickness direction. When the spectral intensity at the position (t is the thickness of the steel sheet) is defined as the background, the spectral intensity at the center of the plate thickness (1 / 2t position in the thickness direction from the center of the steel sheet surface width) is 1.5 times or more of the background The part is measured as a segregation part. The total peak width at the position 1.5 times the peak height of the peak as the background is measured for the peak as the segregation part. The total of the peak widths is defined as the 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 fatigue characteristics are deteriorated. Therefore, 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 Ti is 2.3 × 10 ²² particles / m ³ or more. As the number of particles per unit volume increases, the effect of increasing the strength by particle dispersion strengthening that strengthens 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. The number density is specified by measurement with a transmission electron microscope. Note that a magnification of 30,000 times is selected so that fine carbides can be observed. In addition, at least five crystal fields are observed for different crystal grains. 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 the martensite content and the retained austenite content is 1.5% or less. Martensite is specified as a lath-like structure by a scanning electron microscope. Since martensite has many mobile dislocations, the fatigue resistance cannot be increased by martensite. Moreover, a retained austenite is specified by the X-ray diffraction analysis as described in an Example. 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 steel of the present invention, the martensite area ratio (content ratio) and the retained austenite fraction (content ratio) are set to 1.5% in total. Preferably, it is less than 1.0%.

  The phases other than the martensite phase and the retained austenite are bainite and cementite in total of 1.0% or less, and the balance is the ferrite phase.

  Then, the plated steel plate of this invention is demonstrated. The plated steel sheet of the present invention includes the cold rolled steel sheet and a plating layer. The plating layer contains, by mass%, Fe: 5.0 to 20.0%, Al: 0.001 to 1.0%, and Pb, Sb, Si, Sn, Mg, Mn, Ni, One or two or more selected from Cr, Co, Ca, Cu, Li, Ti, Be, Bi, and REM are contained in a total of 0 to 30%, and the balance is composed of Zn and inevitable impurities.

  In addition, the plating layer is a hot dip galvanized layer, an alloyed hot dip galvanized layer (a plated layer mainly comprising an Fe—Zn alloy formed by diffusing Fe in steel during galvanizing by an alloying reaction). It may be. Further, the plating layer may be an electrogalvanized layer.

  Then, the manufacturing method of the cold rolled sheet steel of this invention is demonstrated.

  The manufacturing method of the cold-rolled steel sheet of the present invention includes a hot rolling process, a cold rolling process, and an annealing process.

  The hot rolling process is a process in which a steel material having the above composition is cast by a continuous casting machine, and rough rolling is started within one hour after slab cutting while maintaining the temperature of the cast steel material at 1000 ° C or higher. And after the completion of finish rolling at a finish rolling temperature of 800 ° C. or higher, winding is performed at a winding temperature of 720 ° C. or lower.

  The melting method of 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.

  In the hot rolling process, rough rolling is started within one hour after slab cutting while maintaining the temperature of the cast steel material at 1000 ° C. or higher by casting with a continuous casting machine.

  C and Ti grow into particles as coarse carbides in the process of cooling to room temperature after casting and reheating. When C and Ti 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 after slab cutting, maintaining the steel raw material (slab) cast with the continuous casting machine at 1000 degreeC or more. Here, the reason why the temperature of the steel material is set to 1000 ° C. or higher is that when the temperature is lowered to less than 1000 ° C., the precipitation driving force of the carbide containing Ti is increased, and the carbide is precipitated and particles are grown. The reason why rough rolling is started within one hour is that when it exceeds one hour, carbide containing Ti precipitates and grows even if the slab temperature is 1000 ° C. or higher. It is preferable to start rough rolling within 40 minutes 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. Further, the finish rolling temperature is preferably 1000 ° C. or less because of manufacturing restrictions.

Winding temperature: 720 ° C. or less When the winding temperature exceeds 720 ° C., carbides containing Ti are coarsened, and the number density of carbides required in the present invention does not fall within a desired range. Therefore, the winding temperature is set to 720 ° C. or lower. Preferably it is 680 degrees C or less. Moreover, the coiling temperature is preferably 350 ° C. or higher from the viewpoint of the plate shape.

  After the hot rolling process, a cold rolling process is performed. A cold rolling process is a process of cold rolling a hot-rolled sheet after a 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, the cold rolling rate shall be 20% or more and 80% or less from the restriction | limiting of a production line.

  An annealing step is performed after the cold rolling. An annealing process is a process which heats the cold-rolled sheet after a cold rolling process to 650 degreeC or more and 850 degrees C or less, and cools to 680 degrees C or less with an average cooling rate of 5 degrees C / s or more.

Heating at 750 ° C or higher and 850 ° C or lower (in case of continuous annealing)
It is necessary to recover the ductility lost by cold rolling in the annealing process. The heating temperature of 750 ° C. or more and 850 ° C. or less is an annealing temperature. In the case of continuous annealing, the lost ductility is hardly recovered by annealing at an annealing temperature lower than 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 and a desired fatigue limit cannot be obtained. On the other hand, when the annealing temperature exceeds 850 ° C., the transformation from ferrite to austenite proceeds and the coarsening of the carbide proceeds. From the above, the heating temperature in the annealing step was set to 750 ° C. or higher and 850 ° C. or lower.

Cooling to 680 ° C at an average cooling rate of 5 ° C / s or more (in the case of continuous annealing)
When the steel sheet is annealed at a high temperature, carbides containing Ti are coarsened. Therefore, after recovering ductility by annealing, it is necessary to immediately cool to a temperature range where the carbides do not coarsen. In order to prevent the carbides from becoming coarse, the carbide may be cooled to at least 680 ° C. at an average cooling rate of 5 ° C./s or more. Preferably, the average cooling rate is 10 ° C./s or higher up to 580 ° C. or lower. The term “immediately” means within 10 seconds after the heating in the annealing process. The average cooling rate means a value calculated by ((cooling start temperature) − (cooling stop temperature)) / cooling time.

  The cooling stop temperature of the cooling is 680 ° C. or lower, preferably 580 ° C. or lower. In the temperature range of 650 ° C. or lower, the characteristics of the steel of the present invention are not changed, so that the heat history after cooling need not be specified.

Heating at 600 ° C or higher and lower than 680 ° C (in case of box annealing)
Since the box-type annealing furnace cannot obtain an average cooling rate of 5 ° C./s or more, it is necessary to set the annealing temperature to less than 680 ° C. In the box-type annealing, the annealing time is longer than that of the continuous annealing, and the ductility recovers when the annealing time is 600 ° C. or higher. Therefore, the lower limit of the annealing temperature is set to 600 ° C. Preferably, they are 620 degreeC or more and 670 degrees C or less.

  Further, since the particle growth of carbide is also affected by time, the residence time (t (sec)) staying in the range from 650 ° C. to the annealing temperature (T (° C.)) satisfies the formula (1). preferable. If the left side of the formula (1) is 28000 or less, it is a range in which an adverse effect due to coarsening of the carbide does not become apparent. Therefore, in order to obtain a yield strength of 1100 MPa or more and a fatigue limit of 700 MPa or more, it is preferable to set the left side of the formula (1) to 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.

  Then, the manufacturing method of the plated steel plate of this invention is demonstrated. The method for producing a plated steel sheet includes a plating step after the annealing step.

  The plating step is mass% and contains Fe: 5.0-20.0%, Al: 0.001% -1.0%, Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr A plating layer containing a total of 0 to 30% of one or more selected from Co, Ca, Cu, Li, Ti, Be, Bi and REM, with the balance being Zn and inevitable impurities. It is the process of forming in the surface of the cold-rolled steel plate obtained by. The method for the plating treatment is not particularly limited, and a general method such as hot dip galvanization or electrogalvanization may be employed, and the conditions may be set as appropriate. Moreover, you may perform the alloying process heated after hot dip galvanization.

  For example, when performing hot dip galvanizing frequently used for automotive steel sheets as plating treatment, the above annealing is performed in a continuous hot dip plating line, and the cold rolled steel sheet is immersed in a hot dip galvanizing bath following cooling after annealing. Thus, a galvanized layer may be formed on the surface. Moreover, you may perform the alloying process of a plating layer as needed after the said galvanization. The heating temperature in the alloying treatment needs to be 650 ° C. or less from the viewpoint of not changing the characteristics.

  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%. Next, under the conditions shown in Table 2, box annealing (BAF), continuous annealing (CAL), or continuous annealing fusion (CGL) plating is performed, cold rolled steel sheet (CR material), hot dip galvanized steel sheet (GI material), Alternatively, an alloyed hot-dip galvanized steel sheet (GA material) was produced. 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.

  Test pieces were collected from the cold-rolled steel sheet, hot-dip galvanized steel sheet, and alloyed hot-dip galvanized 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, a section parallel to the rolling direction is cut out to become an observation surface, and the central part of the plate thickness appears to corrode with 1% nital, and is magnified 2000 times with a scanning electron microscope. Images were taken for 10 visual fields at the 1/4 t position (t is the thickness of the steel plate) in the thickness direction from the center of the surface. 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 plate) in the plate thickness direction from the steel plate surface center is used as the background, the plate thickness center portion (1/2 t from the steel plate surface center to the plate thickness direction). A portion where the spectral intensity at position) is 1.5 times or more of the background was defined as a segregation portion, and the total half-value width of the peak as the segregation portion was measured.

  An X-ray diffraction analysis was performed on a sample in which the steel plate surface was reduced by 0.3 μm in the thickness direction from the center of the steel plate surface by chemical polishing, and the amount of retained austenite was determined. The residual γ fraction 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 Ti was measured using a transmission electron microscope. Using a focused ion beam apparatus as a target for observation in the center of the steel sheet in the thickness direction, a sample with a film thickness of 50 nm is produced, magnified to 300,000 times, and the observed dispersion is accompanied by energy dispersion attached to the TEM. A carbide containing Ti was identified using a type X-ray analyzer. Further, the number density was derived by dividing the number of carbides containing Ti with respect to the observation visual field area by the film thickness using the observation result and the film thickness.

(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 based on the provisions of JIS Z 2241 (2011) was performed five times. Table 3 shows TS, YS, and El obtained in this test. The values listed in Table 3 are average values of the results of five tests.

(Iii) Fatigue test No. 1 test piece (test piece width 20 mm) defined in JIS Z 2275 was prepared so that the longitudinal direction of the test piece was perpendicular to the rolling direction, and the frequency was 30 Hz and the maximum number of repetitions was 10 ⁷ times. The fatigue limit was determined in a double-sided fatigue test with a stress ratio of -1. The results are shown in Table 3. The fatigue limit required in the present invention was 650 MPa or more.

  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 having a steel structure outside the scope of the present invention, the yield strength does not reach 1000 MPa, or a steel plate with good fatigue resistance is not obtained.

Claims (10)

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, Ti: 0.08% or more and 2.0% or less, V: 0.01% or more and 1.0% or less, the balance being Fe and inevitable Component composition of impurities,
The segregation width of Mn generated in the central portion of the plate thickness measured by the following method is 1 μm or less, and the number density of carbides containing Ti is 2.3 × 10. ²² Pieces / m ³ A steel structure in which the total content of martensite and residual austenite is 1.5% or less, and
A cold-rolled steel sheet having a yield strength of 1000 MPa or more and a fatigue limit of 650 MPa or more.
(Measuring method)
Using an electron beam microanalyzer, the spectral intensity of Mn is analyzed from the center of the steel sheet surface width to the sheet thickness direction, and the spectral intensity at the 1/4 t position (t is the thickness of the steel sheet) from the center of the steel sheet surface width to the sheet thickness direction When the spectral intensity at the center of the plate thickness (1 / 2t position in the plate thickness direction from the center of the steel plate surface width) is a portion where the spectral intensity is 1.5 times or more of the background, the segregated portion is the peak. Measure the total peak width at a position 1.5 times the peak height of the background peak. 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, Ti: 0.22% or more and 2.0% or less, with the balance being composed of Fe and inevitable impurities,
The segregation width of Mn generated in the central portion of the plate thickness measured by the following method is 1 μm or less, the number density of carbides containing Ti is 2.3 × 10 ²² pieces / m ³ or more, and the content ratio of martensite And a steel structure having a total content of residual austenite of 1.5% or less,
A cold-rolled steel sheet having a yield strength of 1000 MPa or more and a fatigue limit of 650 MPa or more.
(Measuring method)
Using an electron beam microanalyzer, the spectral intensity of Mn is analyzed from the center of the steel sheet surface width to the sheet thickness direction, and the spectral intensity at the 1/4 t position (t is the thickness of the steel sheet) from the center of the steel sheet surface width to the sheet thickness direction is determined as the background. When the spectral intensity at the center of the plate thickness (1 / 2t position in the plate thickness direction from the center of the steel plate surface width) is a portion where the spectral intensity is 1.5 times or more of the background, the segregated portion is the peak. Measure the total peak width at a position 1.5 times the peak height of the background peak. The said component composition is a cold-rolled steel plate of Claim 2 which contains V: 0.01% or more and 1.0% or less further by the mass%. The component composition is further in terms of mass%, and one or more of B, Ca, Mg, Cr, Co, Ni, Cu, Sb, Zr, Y, REM, Hf, Ta, and W are added in a total amount of 0.0. The cold-rolled steel sheet according to any one of claims 1 to 3, which is contained in a range of 0001% to 0.1%. The cold-rolled steel sheet according to any one of claims 1 to 4 ,
On the cold-rolled steel sheet, 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, 1 to 2 or more types selected from Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM are contained in total, and the balance is composed of Zn and inevitable impurities. A plated steel sheet comprising: a plated layer having a composition. The plated steel sheet according to claim 5 , wherein the plated layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer. It is a manufacturing method of the cold-rolled steel plate in any one of Claims 1-4,
A steel material having the component composition according to any one of claims 1 to 4 is cast by a continuous casting machine, and the temperature of the cast steel material is maintained at 1000 ° C or higher and roughened within 1 hour after slab cutting. A hot rolling process in which rolling is started and finished at a finishing rolling temperature of 800 ° C. or higher and finished at a winding temperature of 720 ° C. or lower;
A cold rolling step of cold rolling the hot-rolled sheet after the hot rolling step;
A method of manufacturing a cold-rolled steel sheet, comprising: heating a cold-rolled sheet after the cold-rolling process to 750 ° C. or more and 850 ° C. or less, and cooling to 680 ° C. at an average cooling rate of 5 ° C./s or more. It is a manufacturing method of the cold-rolled steel plate in any one of Claims 1-4,
A steel material having the component composition according to any one of claims 1 to 4 is cast by a continuous casting machine, and the temperature of the cast steel material is maintained at 1000 ° C or higher and roughened within 1 hour after slab cutting. A hot rolling process in which rolling is started and finished at a finishing rolling temperature of 800 ° C. or higher and finished at a winding temperature of 720 ° C. or lower;
A cold rolling step of cold rolling the hot-rolled sheet after the hot rolling step;
A cold-rolled steel sheet having a cold-rolled steel sheet after the cold-rolling process is heated to 600 ° C. or higher and lower than 680 ° C. and cooled. On the cold-rolled steel sheet obtained by the method for producing a cold-rolled steel sheet according to claim 7 or 8 , Fe: 5.0-20.0%, Al: 0.001-1.0% in mass%. In addition, one or more selected from Pb, Sb, Si, Sn, Mg, Mn, Ni, Cr, Co, Ca, Cu, Li, Ti, Be, Bi and REM are 0 in total. A method for producing a plated steel sheet having a plating step of forming a plating layer containing 30 % and the balance being Zn and inevitable impurities. The method for producing a plated steel sheet according to claim 9 , wherein the plating layer is a hot-dip galvanized layer, an alloyed hot-dip galvanized layer, or an electrogalvanized layer.