TW201542835A - Medium-high carbon steel sheet and method for manufacturing thereof - Google Patents

Abstract

A medium-high carbon steel sheet according to an embodiment of the present invention is a steel sheet having a chemical composition comprising: C: 0.10 to 1.50%; Si: 0.01 to 1.00%; Mn: 0.01 to 3.00%; P: 0.0001 to 0.1000%; S: 0.0001 to 0.1000% and a remainder including Fe and impurities, wherein the steel sheet includes a structure in which a total volume ratio of a martensite, a bainite, a pearlite, and a retained austenite is 5% or less and a remainder is a ferrite and a carbide, a spheroidizing ratio of a particle of the carbide is 70% or more and 99% or less, and a ratio of a number of the particle of the carbide including a boundary of which orientation difference is 5DEG or more to a total number of the particle of the carbide is 20% or less.

Description

Medium and high carbon steel plate and manufacturing method thereof Field of invention

The present invention relates to a high carbon steel sheet and a method for producing the same, which are formed at a high strain rate and have excellent shrinkage.

The present application claims priority based on Japanese Patent Application No. 2014-045689, filed on Jan.

Background of the invention

Medium and high carbon steel sheets are used as materials for drive systems such as chains, gears, and clutches of automobiles, as well as saws and cutting tools. A steel sheet cut from a steel strip or a steel strip of medium-high carbon steel is formed into a material of a predetermined shape, and is formed into a part shape by plastic working such as deep drawing processing, hole expanding processing, thickening processing, and thickness reduction processing. . Cold forging, which performs various processes at the same time or several of them, is partially formed at a high strain rate of about 10/sec. The steel sheet used as the material is required to be deformed even at high strain speed. It is capable of having excellent formability, that is, having excellent shrinkage.

There have been many proposals for techniques for improving the shrinkage rate of medium and high carbon steel sheets (see, for example, Patent Documents 1 to 6).

For example, Patent Document 1 discloses an invention as a method for producing a high-carbon steel sheet having excellent deep extensibility, which is a hot-rolled steel sheet or an annealed steel sheet having a C: 0.20 to 0.90% by mass, at least in the last pass of the rolling. The working roll having a surface roughness Ra of 0.20 to 1.50 μm is subjected to finishing rolling under the condition that the total rolling ratio is set to 20 to 70%, followed by finishing annealing. However, the technique disclosed in Patent Document 1 is a technique for improving the shrinkage rate by improving the roughness of the surface of the steel sheet, and is not a technique for improving the shrinkage rate by controlling the material obtained by the microstructure of the steel material, and may not be able to The desired effect of the invention is obtained.

Further, Patent Document 2 discloses an invention of a high carbon steel sheet composed of a carbide and an equiaxed ferrite, and a high toughness high carbon steel sheet having excellent workability contains C: 0.6 to 1.3% by mass. Si: 0.5% by mass or less, Mn: 0.2% to 1.0% by mass, P: 0.02% by mass or less, and S: 0.01% by mass or less, and the remainder is substantially a composition of Fe; by adjusting hot rolling conditions, cold rolling conditions, and The annealing conditions are such that the maximum length of the carbide is 5.0 μm or less, the spheroidization ratio of the carbide is 90% or more, and the volume of the spherical carbide having a particle diameter of 1.0 μm or more is 20 of the total spheroidal carbide volume. %the above.

Patent Document 3 discloses an invention as having an excellent depth Extensible medium-high carbon steel, which is designed to have a C content of 0.10 to 0.90% by mass and a carbide grain boundary ratio (F value) of 30% or more, so that the carbide is dispersed in the ferrite iron In the organization of the formation.

Patent Document 4 discloses an invention as a high-carbon cold-rolled steel strip having a small anisotropy in a deep extension plane, which has a steel composition of C: 0.25 to 0.75%, and satisfies an average particle diameter of 0.5 μm in the steel. As described above, the spheroidization rate is 90% or more, and the aggregate structure is a mathematical formula "(222)/(200) is 6-8.0 x C (%)".

Patent Document 5 discloses an invention as having deep extensibility A high carbon steel strip which is excellent in imparting high hardness and excellent wear resistance is characterized in that the C content is 0.20 to 0.70% by mass, and 50% by area or more of the swarf carbon iron in the steel is graphitized.

Patent Document 6 discloses a technique as an excellent The method for producing a high-carbon cold-rolled steel sheet containing C: 0.1 to 0.65%, Si: 0.01 to 0.3%, Mn: 0.4 to 2%, sol. Al: 0.01 to 0.1%, and N: 0.002 to 0.008 %, B: 0.0005~0.005%, Cr: 0~0.5, Mo: 0~0.1 high carbon steel is hot rolled and coiled at 300~520 °C, and box annealing at 650~(Ac1-10) °C (box annealing), cold rolling at a rolling reduction ratio of 40 to 80% and box annealing at 650 to (Ac 1-10) °C.

However, any of these patent documents relates to suppression at high speeds. When forming under the degree of the steel, the cracks in the ferritic carbon iron inside the steel and the crack growth due to the occurrence of cracks. The knowledge and technology that link the low shrinkage rate are not revealed at all.

Prior technical literature Patent literature

Patent Document 1: Japanese Laid-Open Patent Publication No. 2003-293042

Patent Document 2: Japanese Patent Laid-Open Publication No. 2003-147485

Patent Document 3: Japanese Laid-Open Patent Publication No. 2002-155339

Patent Document 4: Japanese Laid-Open Patent Publication No. 2000-328172

Patent Document 5: Japanese Patent Laid-Open No. 6-108158

Patent Document 6: Japanese Patent Laid-Open No. Hei 11-61272

Summary of invention

In view of the above circumstances, an object of the present invention is to provide a high carbon steel sheet having excellent shrinkage ratio at a high strain rate and a method for producing the same.

The inventors of the present invention conducted intensive studies in order to solve the above problems. As a result, the inventors of the present invention have found that cracks (voids) generated in the carbides are grown and connected to each other during deformation, so that the shrinkage rate at the high strain rate is lowered. Further, the inventors of the present invention have found that the crack generated in the carbide is generated from the crystal interface existing in the carbide particles which were previously regarded as one particle. The present inventors have found that by reducing the amount of the crystal interface in the carbide particles, it is possible to exhibit excellent shrinkage even when deformed at a high strain rate, and simultaneously perform deep drawing processing, hole expanding processing, and thickening. In the case of plastic working such as processing and thickness reduction processing and cold forging in several of these processes, a high carbon steel sheet exhibiting excellent moldability can be obtained.

Further, the inventors of the present invention have obtained the following knowledge by accumulating various studies and have completed the present invention: the steel sheet having the above-described characteristics is difficult to manufacture in individual underground operations such as hot rolling conditions and annealing conditions, and there are only The so-called consistent process such as hot rolling and annealing steps can be optimized to be manufactured.

The gist of the present invention is as follows.

(1) A high carbon steel sheet in one aspect of the invention having the following composition: In terms of mass%, C: 0.10 to 1.50%, Si: 0.01 to 1.00%, Mn: 0.01 to 3.00%, P: 0.0001 to 0.1000%, S: 0.0001 to 0.1000%, and the remainder consists of Fe and impurities; And the steel sheet has the following structure: the volume ratio of the granulated iron, the tough iron, the ferritic iron, and the residual Worth iron is 5.0% or less, and the remaining part is ferrite iron and carbide; The spheroidization ratio of the particles is 70% or more and 99% or less. The number of the carbide particles having a crystal interface having a difference in orientation of 5 or more in the carbide particles is the total number of the carbide particles. 20% or less.

(2) The high carbon steel sheet according to (1) above, wherein the front side of the steel sheet Further, the component may further contain one or more of the following: Al: 0.001 to 0.500%, N: 0.0001 to 0.0500%, O: 0.0001 to 0.0500% Cr: 0.001 to 2.000%, Mo: 0.001~2.000%, Ni: 0.001~2.000%, Cu: 0.001~1.000%, Nb: 0.001~1.000%, V: 0.001~1.000%, Ti: 0.001~1.000%, B: 0.0001~0.0500%, W: 0.001 ~1.000%, Ta: 0.001~1.000%, Sn: 0.001~0.020%, Sb: 0.001~0.020%, As: 0.001~0.020%, Mg: 0.0001~0.0200%, Ca: 0.001~0.020%, Y: 0.001~ 0.020%, Zr: 0.001 to 0.020%, La: 0.001 to 0.020%, and Ce: 0.001 to 0.020%.

(3) a method for producing a high carbon steel sheet according to another aspect of the present invention, After directly or temporarily cooling the steel sheet having the above-mentioned component (1) or (2), finishing hot rolling is performed in a temperature range of 600 ° C or more and 1000 ° C or less when heating and hot rolling; Will be above 350 ° C and below 700 ° C The coiled hot-rolled steel sheet is subjected to box annealing; and cold rolling of 10% or more and 80% or less is performed; then, the annealing temperature is 650 ° C or more and 780 ° C or less, and the holding time is 30 seconds or more on the continuous annealing line. The cold rolled sheet annealing is performed for 1800 seconds or less.

According to the present invention, it is possible to provide a high carbon steel sheet having excellent shrinkage ratio at a high strain rate and a method for producing the same.

Fig. 1 is a view showing the shape of a test piece for measuring the shrinkage rate at a high strain rate.

Fig. 2 is a view showing a state in which cracks are generated from a crystal interface located in carbide particles at the time of deformation.

Fig. 3 is a graph showing the relationship between the number ratio of carbide particles containing a crystal interface and the shrinkage ratio at the time of a tensile test at a high strain rate.

Form for implementing the invention

Hereinafter, the present embodiment will be described in detail.

First, the reason for limiting the chemical composition of the steel sheet of the present embodiment will be described. Here, the "%" of the component means the mass %.

(C: 0.10~1.50%)

C is an element that increases the strength of steel by heat treatment by quenching. The medium-high-carbon steel sheet is used as a part by heat treatment by quenching and quenching and tempering before being used as a drive system component such as a chain, a gear, or a clutch of a car, and a material such as a saw or a cutting tool. Necessary strength Or resilience. Since the C content is less than 0.10%, the strength cannot be increased by quenching, so 0.10% is made the lower limit of the C content. On the other hand, since the C content is more than 1.50%, the ratio of the number of carbides having a crystal interface inside the particles increases after cold rolling annealing, so that the shrinkage rate at a high strain rate is lowered, so the upper limit of the C content is imposed. Set to 1.50%. Preferably, the C content is from 0.15 to 1.30%.

(Si: 0.01~1.00%)

The Si system functions as a deoxidizing agent, and further suppresses the coarsening and bonding of carbide particles in the hot-rolled sheet annealing and the cold-rolled sheet annealing. When the carbide particles are austenitic in the cold-rolled sheet annealing, the crystal interface is introduced into the carbide particles when two or more particles are in contact with each other in the vicinity. When the steel sheet is deformed, the crystal interface in the carbide particles becomes a starting point for crack generation. In order to suppress this phenomenon, it is necessary to lower the growth rate of the carbide which is annealed in the hot rolled sheet and annealed in the cold rolled sheet. The Si system is one of the elements which is representative of a decrease in the growth rate of carbides. When the content of Si is less than 0.01%, the above effects cannot be obtained, so the lower limit of the Si content is made 0.01%. On the other hand, since the Si content is more than 1.00%, the ferrite iron is liable to cause cracking damage, and the shrinkage rate at a high strain rate is lowered. Therefore, the upper limit of the Si content is set to 1.00%. The Si content is preferably 0.05% or more and 0.80% or less, more preferably 0.08% or more and 0.50% or less.

(Mn: 0.01~3.00%)

Similarly to Si, the Mn system suppresses the coarsening and bonding of carbide particles in the hot-rolled sheet annealing and the cold-rolled sheet annealing. When the Mn content is less than 0.01%, the above effects cannot be obtained, so the lower limit of the Mn content is made 0.01%. On the other hand, when the Mn content is more than 3.00%, the carbide is not easily spheroidized during hot-rolled sheet annealing and cold-rolled sheet annealing, but is deformed at a high strain rate, and cracks are generated by using needle-shaped carbide as a starting point. The shrinkage rate is low. Therefore, the upper limit of the Mn content is set to 3.00%. The Mn content is preferably 0.30% or more and 2.50% or less, more preferably 0.50% or more and 1.50% or less.

(P: 0.0001~0.1000%)

P is an impurity element that embrittles the ferrite grain boundary. The smaller the P content, the better, but when the P content is less than 0.0001% in the refining step and the steel is made highly purified, the time required for refining becomes long, and the manufacturing cost is greatly increased, so the P content is increased. The lower limit is set to 0.0001%. On the other hand, when the P content is more than 0.1000%, when deformed at a high strain rate, cracks are remarkably generated from the ferrite grain boundary, and the shrinkage rate is remarkably lowered. Therefore, the upper limit of the P content is made 0.1000%. The P content is preferably 0.0010% or more and 0.050% or less, more preferably 0.0020% or more and 0.0300% or less.

(S: 0.0001~0.1000%)

S forms an impurity element of a non-metallic inclusion such as MnS. Since the non-metallic inclusion becomes a starting point for cracking when deformed at a high strain rate, the S content is preferably as small as possible. However, since the S content is reduced to less than 0.0001%, the refining cost is greatly increased, so the lower limit of the S content is made 0.0001%. On the other hand, when S is more than 0.1000%, the shrinkage rate becomes remarkable, so the upper limit of the S content is made 0.1000% or less. The S content is preferably 0.0003% or more and 0.0300% or less.

In the present embodiment, the above components are used as the basic structure of the steel sheet. And, in order to improve the mechanical properties of the steel plate, it is also optional. One or two or more of the elements described below are optionally contained. However, since the elements described below are not necessarily contained, the lower limit of the elements described below is 0%.

(Al: preferably 0.001 to 0.500%)

Al is an element of the action of the deoxidizer of steel. When the Al content is less than 0.001%, the effect of the inclusion cannot be sufficiently obtained, so the lower limit of the Al content can be made 0.001%. On the other hand, when the Al content is more than 0.500%, the grain boundary of the ferrite iron is embrittled, and the shrinkage rate at the high strain rate is lowered. Therefore, the upper limit of the Al content can be set to 0.500%. The Al content is preferably 0.005% or more and 0.300% or less, more preferably 0.010% or more and 0.100% or less.

(N: preferably 0.0001 to 0.0500%)

The N system promotes the toughening iron metamorphism of steel, and contains a large amount of elements which cause iron embrittlement of the fertilizer. The smaller the N content, the better, but since the N content is reduced to less than 0.0001%, the refining cost is increased, so the lower limit of the N content can be made 0.0001%. On the other hand, when the N content is more than 0.0500%, cracking of the ferrite iron is caused when the strain is deformed at a high strain rate, so the upper limit of the N content can be made 0.0500%. The N content is preferably 0.0010% or more and 0.0250% or less, more preferably 0.0020% or more and 0.0100% or less.

(O: preferably 0.0001 to 0.0500%)

Since the O system is contained in a large amount to promote the formation of a coarse oxide element in the steel, the O content is preferably as small as possible. However, since the O content is reduced to less than 0.0001% by hour, the refining cost is increased, so the lower limit of the O content can be made 0.0001%. On the other hand, since the O content is more than 0.0500%, A coarse oxide is formed in the steel, so that a crack with a coarse oxide as a starting point is generated when deformed at a high strain rate, so the upper limit of the O content can be made 0.0500%. The O content is preferably 0.0005% or more and 0.0250% or less, more preferably 0.0010% or more and 0.0100% or less.

(Cr: preferably 0.001 to 2.000%)

Similarly to Si and Mn, the Cr system is an element which suppresses coarsening and bonding of carbide particles which are annealed in a hot-rolled sheet and annealed in a cold-rolled sheet. However, since the above effect cannot be obtained because the Cr content is less than 0.001%, the lower limit of the Cr content can be made 0.001%. On the other hand, since the Cr content is more than 2.000%, the carbide is not easily spheroidized in the hot-rolled sheet annealing and the cold-rolled sheet annealing, and the crack is caused by the needle-shaped carbide as the starting point when deformed at a high strain speed. Since the rate is low, the upper limit of the Cr content can be set to 2.000%. The Cr content is preferably 0.005% or more and 1.500% or less, more preferably 0.010% or more and 1.300% or less.

(Mo: preferably 0.001 to 2.000%)

Similarly to Si, Mn, and Cr, Mo is an element that suppresses coarsening and bonding of carbide particles which are annealed in hot-rolled sheet and annealed in cold-rolled sheet. Since the Mo effect is less than 0.001%, the above effects cannot be obtained, so the lower limit of the Mo content can be made 0.001%. On the other hand, since the Mo content is more than 2.00%, the carbide is not easily spheroidized in the hot-rolled sheet annealing and the cold-rolled sheet annealing, and the crack is caused by the needle-shaped carbide as the starting point when deformed at a high strain speed. Since the rate is low, the upper limit of the Mo content can be set to 2.00%. The Mo content is preferably 0.005% or more and 1.000% or less, more preferably 0.008% or more and 0.800% or less.

(Ni: preferably 0.001 to 2.000%)

Ni is an effective element for improving the toughness of parts and improving hardenability. In order to effectively exert the effect, Ni is preferably contained in an amount of 0.001% or more. On the other hand, since the Ni content is more than 2.000%, the carbide is not easily spheroidized in the hot-rolled sheet annealing and the cold-rolled sheet annealing, and the crack is caused by the needle-shaped carbide as the starting point when deformed at a high strain speed. Since the rate is low, the upper limit of the Ni content can be set to 2.000%. The Ni content is preferably 0.005% or more and 1.500% or less, more preferably 0.005% or more and 0.700% or less.

(Cu: preferably 0.001 to 1.000%)

Cu is an element which increases the strength of a steel material by forming fine precipitates. In order to effectively exert the strength increasing effect, it is preferable to contain 0.001% or more of Cu. On the other hand, since the Cu content is more than 1.00%, the carbide is not easily spheroidized in the hot-rolled sheet annealing and the cold-rolled sheet annealing, and the crack is caused by the needle-shaped carbide as the starting point when deformed at a high strain speed. The rate is low, so the upper limit of the Cu content can be set to 1.00%. The Cu content is preferably 0.003% or more and 0.500% or less, more preferably 0.005% or more and 0.200% or less.

(Nb: preferably 0.001 to 1.000%)

Nb forms a carbonitride and suppresses coarsening and bonding of carbide particles in hot-rolled sheet annealing and cold-rolled sheet annealing. Since the above effect cannot be obtained because the Nb content is less than 0.001%, the lower limit of the Nb content can be made 0.001%. On the other hand, since the Nb content is more than 1.000%, the carbide is not easily spheroidized in the hot-rolled sheet annealing and the cold-rolled sheet annealing, and the crack is caused by the needle-shaped carbide as the starting point when deformed at a high strain speed. The rate is low, and the upper limit of the Nb content can be set to 1.000%. The Nb content is preferably 0.005% or more and 0.600% or less, more preferably 0.008% or more and 0.200% or less.

(V: preferably 0.001 to 1.000%)

Similarly to Nb, V is an element which forms a carbonitride and coarsens and bonds the carbide particles which are annealed in the hot-rolled sheet and annealed in the cold-rolled sheet. When the V content is less than 0.001%, the above effects cannot be obtained, and the lower limit of the V content can be made 0.001%. On the other hand, since the V content is more than 1.000%, the carbide is not easily spheroidized in the hot-rolled sheet annealing and the cold-rolled sheet annealing, and the crack is caused by the needle-shaped carbide as the starting point when deformed at a high strain speed. Since the rate is low, the upper limit of the V content can be set to 1.000%. The V content is preferably 0.001% or more and 0.750% or less, more preferably 0.001% or more and 0.250% or less.

(Ti: preferably 0.001 to 1.000%)

Similarly to Nb and V, Ti is an element which forms a carbonitride and coarsens and bonds carbide particles which are annealed in a hot-rolled sheet and annealed in a cold-rolled sheet. When the Ti content is less than 0.001%, the above effects cannot be obtained, so the lower limit of the Ti content can be made 0.001% or more. On the other hand, since the Ti content is more than 1.000%, the carbide is not easily spheroidized in the hot-rolled sheet annealing and the cold-rolled sheet annealing, and the crack is caused by the needle-shaped carbide as the starting point when deformed at a high strain rate. Since the rate is low, the upper limit of the Ti content can be set to 1.000%. The Ti content is preferably 0.001% or more and 0.500% or less, more preferably 0.003% or more and 0.150% or less.

(B: preferably 0.0001 to 0.0500%)

B is an element that improves the hardenability of a part during heat treatment. Because of B content When the amount is less than 0.0001%, the above effect cannot be obtained, so the lower limit of the B content can be made 0.0001%. When the B content is more than 0.0500%, a coarse Fe-B-C compound is formed, and when it is deformed at a high strain rate, it becomes a starting point of cracking, and the shrinkage rate is lowered, so the upper limit of the B content can be made 0.0500%. The B content is preferably 0.0005% or more and 0.0300% or less, more preferably 0.0010% or more and 0.0100% or less.

(W: preferably 0.001 to 1.000%)

Similarly to Nb, V, and Ti, W is an element which forms a carbonitride and coarsens and bonds carbide particles which are annealed in a hot-rolled sheet and annealed in a cold-rolled sheet. Since the W effect is less than 0.001%, the above effects cannot be obtained, so the lower limit of the W content can be made 0.001%. On the other hand, since the W content is more than 1.000%, the carbide is not easily spheroidized in the hot-rolled sheet annealing and the cold-rolled sheet annealing, and the crack is caused by the needle-shaped carbide as the starting point when deformed at a high strain speed. Since the rate is low, the upper limit of the W content can be set to 1.000%. The W content is preferably 0.001% or more and 0.450% or less, more preferably 0.001% or more and 0.160% or less.

(Ta: preferably 0.001 to 1.000%)

Similarly to Nb, V, Ti, and W, Ta is an element which forms a carbonitride and coarsens and bonds carbide particles which are annealed in a hot-rolled sheet and annealed in a cold-rolled sheet. Since the above effect cannot be obtained when the Ta content is less than 0.001%, the lower limit of the Ta content can be made 0.001%. On the other hand, since the Ta content is more than 1.000%, the carbide is not easily spheroidized in the hot-rolled sheet annealing and the cold-rolled sheet annealing, and the crack is caused by the needle-shaped carbide as the starting point when deformed at a high strain speed. The rate is low, so the upper limit of the Ta content can be set to 1.000% or less. The Ta content is preferably 0.001% or more and 0.750% or less, more preferably 0.001% or more and 0.150% or less.

(Sn: preferably 0.001 to 0.020%)

Sn is an element contained in steel when scrap is used as a steel raw material, and the Sn content is preferably as small as possible. Since the reduction of the Sn content is less than 0.001%, the refining cost is increased, so the lower limit of the Sn content can be made 0.001%. Further, since the Sn content is more than 0.020%, the ferrite iron is embrittled and the shrinkage rate is lowered when deformed at a high strain rate, so the upper limit of the Sn content can be made 0.020%. The Sn content is preferably 0.001% or more and 0.015% or less, more preferably 0.001% or more and 0.010% or less.

(Sb: preferably 0.001 to 0.020%)

In the case where the Sb system uses scrap as a steel material in the same manner as Sn, the Sb content is preferably as small as possible. Since the reduction of the Sb content to less than 0.001% causes an increase in the refining cost, the lower limit of the Sb content can be made 0.001%. Further, since the Sb content is more than 0.020%, the ferrite iron is embrittled and the shrinkage rate is lowered when deformed at a high strain rate, so the upper limit of the Sb content can be made 0.020% or less. The Sb content is preferably 0.001% or more and 0.015% or less, more preferably 0.001% or more and 0.011% or less.

(As: preferably 0.001 to 0.020%)

As in the case of Sn and Sb, the As system is an element contained when scrap is used as a steel raw material, and the As content is preferably as small as possible. Since the content of As is reduced to less than 0.001%, the refining cost is increased, so the lower limit of the As content can be made 0.001%. Moreover, since the As content is more than 0.020%, the ferrite iron is embrittled and deforms at a high strain rate, resulting in a low shrinkage rate, so the As content can be The upper limit is set to 0.020% or less. The As content is preferably 0.001% or more and 0.015% or less, more preferably 0.001% or more and 0.007% or less.

(Mg: preferably 0.0001 to 0.0200%)

The Mg-based element can control the form of the sulfide even if the content is a trace amount, and can be contained as necessary. Since the Mg content is less than 0.0001%, the effect cannot be obtained, and the lower limit of the Mg content can be made 0.0001%. On the other hand, when Mg is excessively contained, the grain boundary of the ferrite iron is embrittled and the shrinkage rate is lowered when deformed at a high strain rate, so the upper limit of the Mg content can be made 0.0200%. The Mg content is preferably 0.0001% or more and 0.0150% or less, more preferably 0.0001% or more and 0.0075% or less.

(Ca: preferably 0.001 to 0.020%)

In the same manner as Mg, the Ca system can control the element of the form of the sulfide even in a small amount, and can be contained as necessary. Since the effect cannot be obtained when the Ca content is less than 0.001%, the lower limit of the Ca content can be made 0.001%. On the other hand, when Ca is excessively contained, the grain boundary of the ferrite iron is embrittled and the shrinkage rate is lowered when deformed at a high strain rate, so the upper limit of the Ca content can be made 0.020%. The Ca content is preferably 0.001% or more and 0.015% or less, more preferably 0.001% or more and 0.010% or less.

(Y: preferably 0.001 to 0.020%)

In the same manner as Mg and Ca, the Y system can control the form of the sulfide form even in a small amount, and can be contained as necessary. When the Y content is less than 0.001%, the effect cannot be obtained because the lower limit of the Y content can be 0.001%. On the other hand, when Y is excessively contained, the grain boundary of the ferrite iron is embrittled and the shrinkage rate is lowered when deformed at a high strain rate, so Y can be used. The upper limit of the content is set to 0.020%. The Y content is preferably 0.001% or more and 0.015% or less, more preferably 0.001% or more and 0.009% or less.

(Zr: preferably 0.001 to 0.020%)

In the same manner as Mg, Ca, and Y, the Zr system can control the element of the form of the sulfide even in a small amount, and can be contained as necessary. Since the effect cannot be obtained when the Zr content is less than 0.001%, the lower limit of the Zr content can be made 0.001%. On the other hand, when Zr is excessively contained, the grain boundary of the ferrite iron is embrittled and the shrinkage rate is lowered when deformed at a high strain rate, so the upper limit of the Zr content can be made 0.020%. The Zr content is preferably 0.015% or less, more preferably 0.010% or less.

(La: preferably 0.001 to 0.020%)

In the same manner as in the case of Mg, Ca, Y, and Zr, the La system is effective in controlling the form of the sulfide even in a small amount, and may be contained as necessary. Since the effect is not obtained when the La content is less than 0.001%, the lower limit can be made 0.001%. On the other hand, when La is excessively contained, the grain boundary of the ferrite iron is embrittled and the shrinkage rate is lowered when deformed at a high-speed La content, so the upper limit of the La content can be made 0.020%. The La content is preferably 0.001% or more and 0.015% or less, more preferably 0.001% or more and 0.010% or less.

(Ce: preferably 0.001 to 0.020%)

Similarly to Mg, Ca, Y, Zr, and La, the Ce-based element can control the form of the sulfide in a small amount, and can be contained as necessary. Since the effect cannot be obtained when the Ce content is less than 0.001%, the lower limit of the Ce content can be made 0.001%. On the other hand, when Ce is excessively contained, it is fat The grain boundary of the granular iron is embrittled and the shrinkage rate is lowered when deformed at a high strain rate, so the upper limit of the Ce content can be made 0.020%. The Ce content is preferably 0.001% or more and 0.015% or less, more preferably 0.001% or more and 0.010% or less.

Further, in the steel sheet according to the embodiment, the components described above are The remainder is Fe and impurities.

The steel sheet according to the embodiment has the above composition In addition, because it performs the best hot rolling and annealing, it has a structure mainly composed of ferrite iron and carbide, and the volume ratio of the granulated iron, the tough iron, the bund iron, and the residual Worth iron 5% or less, the spheroidization ratio of the carbide particles is 70% or more and 99% or less, and the carbide particles contain a carbide particle at a crystal interface having a difference in orientation of 5 or more with respect to the total number of carbide particles. The number ratio is 20% or less. According to this feature, when plastic working such as stretching, hole expansion, thickening, and thickness reduction is performed at a high strain rate, or when such cold forging is combined, a steel sheet having excellent formability can be obtained. This is the novel knowledge discovered by the inventors and the like.

The steel of the present embodiment has substantially ferrite iron and carbide Organization. In addition, the carbide is a compound of alloys of Mn and Cr, and a compound of Mn, Cr, etc., which is substituted with Fe atoms in ferritic carbon iron, and alloys, in addition to ferritic carbon iron (Fe3C) of a compound of iron and carbon. Carbide (M23C6, M6C, MC. Further, M-based Fe and other alloying elements). In the structure, it is preferable that the volcanic iron, the toughening iron, the ferritic iron, and the residual Worth iron are not contained, and the total volume ratio at the time of inclusion is set to 5.0% or less. The lower limit of the total amount of the granulated iron, the toughening iron, the ferritic iron, and the residual Worth iron is not regulated. When a tissue is observed at 3000 times using a scanning electron microscope, any of the tissues is not detected at all, because the total amount of the granulated iron, the toughened iron, the bund iron, and the residual Worth iron can be regarded as a total amount. Since the volume is 0.0% by volume, the lower limit of the total amount of the granulated iron, the toughened iron, the ferritic iron, and the residual Worth iron can be set to 0.0%.

Explain that the Ma Tian loose iron, toughened iron, Bora iron, and residual Wo The reason for the total measurement of the company. In the present embodiment, the granulated iron, the toughened iron, the bund iron, and the residual Worth iron are the steel sheets which are heated in the cold-rolled sheet and heated to the two-phase region of the ferrite iron and the Worth iron. The tissue generated from Worth Iron during the process of being cooled to room temperature. Therefore, the granulated iron, the toughening iron, and the Bora iron are located at the grain boundary of the ferrite iron, and the residual Worth iron is present at the lath interface or the boundary of the slab iron and the tough iron. First, when the Worth iron is metamorphosed to the granulated iron, the toughened iron, or the ferritic iron, the stress is left at the grain boundary of the ferrite iron because of the volume expansion. Since the residual stress locally in the grain boundary of the ferrite iron promotes the formation of voids near the grain boundary when the steel sheet is deformed by the stress load, the stress remaining at the grain boundary of the ferrite iron is at a high strain rate. When the deformation occurs, the shrinkage rate is lowered. In addition, the residual Worthite iron system causes a process-induced metamorphosis in the course of deformation of the steel sheet to become a granulated iron, and further increases the stress to the ferrite grain boundary, thereby contributing to the decrease in the shrinkage rate. For the above reasons, in order to increase the shrinkage rate at the time of deformation at a high strain rate, the structure of the steel sheet is made substantially the structure of the ferrite iron and carbide, and does not contain the granulated iron, the tough iron, and the ferrite. And the residual Worthfield iron is better, and the total amount of the granulated iron, the toughening iron, the bund iron, and the residual Worthite iron must be combined. The volume ratio is set to 5.0% or less. Moreover, when the Borne iron metamorphosis occurs, the ratio of acicular carbides also increases. The influence of acicular carbides will be described later. Further, since the carbide does not undergo a phase change state and no stress is generated between the carbide and the base material, it is possible to suppress the decrease in the shrinkage ratio.

Next, the spheroidization ratio of the carbide is set to 70% or more. 99% or less. When the spheroidization ratio of the carbide is less than 70%, the stress is concentrated on the acicular carbide, the carbide is cracked and voids are formed, and the fracture surface is formed by the connection of the voids, causing the shrinkage at the time of deformation at a high strain rate. The rate is low. Therefore, the lower limit of the spheroidization ratio of the carbide is set to 70%. Further, although the spheroidization rate is preferably as high as possible, in order to control the spheroidization rate to 100%, it is necessary to perform annealing for a very long time, which causes an increase in manufacturing cost, so the upper limit of the spheroidization rate is Less than 100% is preferable and set to 99% or less.

Moreover, the description should be based on the total number of carbide particles, carbon The number of carbide particles having a crystal interface having a crystal orientation difference of 5 or more in the compound particles is set to be 20% or less. The crack of the carbide at the time of deformation is mainly generated from a crystal interface existing in a carbide having a difference in crystal orientation of 5° or more in the carbide which is regarded as a particle in the prior art. When deformed at a high strain rate, cracks are generated at the crystal interface of the carbide, and the voids are joined and the fracture surface is formed, resulting in a low shrinkage rate. Although the ratio of the carbide having a crystal interface having a crystal orientation difference of 5 or more is preferably less, in order to control the ratio of the number of carbides having a crystal interface having a crystal orientation difference of 5 or more, relative to the carbide The total number of particles is less than 0.1%, in continuous casting, hot rolling, hot-rolled sheet annealing, cold rolling, And the consistent quality design management of the cold-rolled sheet annealing becomes necessary and causes a decrease in the yield. Therefore, with respect to the total number of carbide particles, carbides having a crystal interface having a crystal orientation difference of 5 or more are used. The lower limit of the number ratio is preferably set to 0.1%, more preferably 0.2%. In addition, when the number of carbides having a crystal interface having a crystal orientation difference of 5° or more (hereinafter, abbreviated as a number ratio) is more than 20% with respect to the total number of carbide particles, the strain is high. The shrinkage ratio at the time of deformation at a speed is remarkably lowered, so the upper limit of the number ratio is set to 20%, preferably 15%, more preferably 10%.

Next, the observation and measurement of the above-specified tissues will be described. law.

The observation of ferrite iron, carbide, 麻田散铁, toughened iron, and ferritic iron was carried out using a scanning electron microscope. Prior to observation, the sample for observation of the tissue was subjected to wet grinding using a sandpaper, and grinding was performed using diamond abrasive grains having an average particle size of 1 μm, whereby the observation surface was finished into a mirror surface. Next, the observation surface was etched in advance using a 3% nitric acid-alcohol solution. The observation magnification is among 1000 to 10000 times, and the magnification of each tissue capable of identifying ferrite iron, carbide, 麻田散铁, toughened iron, and ferritic iron is selected. In this embodiment, 3000 times is selected. Using the selected magnification, 16 sheets of a field of 30 μm × 40 μm with a thickness of 1/4 layer were randomly photographed. The volume fraction of each tissue is obtained by using point statistics. On the photographed tissue photograph, grid lines spaced at intervals of 2 μm were drawn in the horizontal and vertical directions, and the number of tissues at the intersection of the grid lines and the ratio of the number of tissues were measured, and each photographed photograph was measured. The ratio of each organization of Zhang. Subsequently, the values obtained by averaging the measurement results of the ratios of all the tissues of the 16 tissue photographs were obtained as the volume fraction of the tissue of each sample.

Also, the so-called Ma Tian loose iron and toughened iron are based on It is distinguished by no fine carbides. It is mainly located on the grain boundary of the ferrite iron. The structure without carbide is the granulated iron, and the structure containing carbide is the toughened iron. Moreover, when the granulated iron is tempered in the granulated iron, the tempered granulated iron contains carbides inside, so there is a possibility that it is mistaken for toughening iron. However, in the steel of the present embodiment, it is clear that a good shrinkage ratio can be obtained by setting the volume ratio of the total of the granulated iron, the toughening iron, the ferritic iron, and the residual Worth iron to 5%. Therefore, the mistaken effect of the tough iron on the shape of the steel of the last embodiment is very small. Further, it is preferable that the fat iron is set to have a volume fraction of 70% or more.

The volume fraction of the residual Worthite iron is measured by X-ray diffraction. set. The strain layer on the surface of the sample after the observation surface was finished into the mirror surface by the above procedure was prepared by measuring the residual Worthite iron by using electric field polishing. Electric field polishing was carried out using a 5% perchloric acid-acetic acid solution and applying a voltage of 10 V. The X-ray tube system uses Cu and is based on the strength of each of the (200), (220), (311) and the (200) and (211) surfaces of the ferrite iron. The volume ratio of the stone.

The observation of the carbide was carried out using a scanning electron microscope. The tissue observation sample was subjected to wet grinding using a sandpaper and grinding using a diamond abrasive grain having a particle size of 1 μm , and after finishing the mirror finish of the observation surface, using a picric acid (Picric acid) The saturated alcohol solution is prepared by etching. The observation magnification is 1000 to 10000 times. In the present embodiment, at a magnification of 3000 times, a field of view having 16 or more carbon crystals is selected on the tissue observation surface to obtain a tissue image. For the obtained tissue image, the image analysis software represented by Winro F., Ltd. (Win ROOF) was used to measure the area of each carbide contained in the region in detail. From the area of each carbide, the circle-equivalent diameter of each carbide ("circle equivalent diameter" = 2 × ("area" / 3.14) 1/2) was determined, and the average value thereof was defined as the carbide particle diameter. Further, in order to suppress the influence of noise on the measurement error, the carbide having an area of 0.01 μm ² or less is removed from the evaluation object.

A preferred range of the carbide particle diameter is 0.30 μm or more and 1.50 μm or less. When the carbide particle diameter is less than 0.30 μm, the ferrite iron particle diameter becomes fine, so the lower limit of the carbide particle diameter is set to 0.30 μm. When the carbide particle diameter is more than 1.50 μm, the carbide tends to generate voids in the vicinity of the deformation of the steel sheet and the deformation ability is lowered. Therefore, the upper limit of the carbide particle diameter is set to 1.50 μm. Further, a carbide having a ratio of a major axis length to a minor axis length of 3 or more is recognized as a needle-like carbide, and a carbide having a ratio of a major axis length to a minor axis length of less than 3 is recognized as a spherical carbide. The value obtained by dividing the number of spheroidal carbides by the number of total carbides is defined as the spheroidization ratio of carbides (such as Xueming carbon iron).

Whether there are crystal boundaries in the carbide particles with a crystal orientation difference of 5° or more The face was investigated using EBSD. The sample for evaluation is obtained by using a discharge wire processing machine to cut the cut plate or the blanked blank plate cut from the steel strip and the steel strip without applying strain, and the surface of the steel sheet is perpendicular to the surface of the steel sheet. The surface is set as the observation surface. Since the measurement accuracy of EBSD is affected by the flatness of the observation surface and the strain applied by the polishing, after the observation surface is finished into a mirror surface by wet grinding and diamond abrasive grinding, Correction grinding is performed on the observation surface. The correction polishing was carried out using a vibration polishing apparatus (VIBROMET 2 manufactured by Buehler) under the conditions of 40% output and a polishing time of 60 minutes. The type of device of the SEM and Kikuchi line detector is not particularly limited as long as SEM-EBSD is used. Four fields of view were measured at a measurement displacement interval of 0.2 μm in a region of 1/4 layer thickness, 100 μm in the plate thickness direction, and 100 μm in the plate width direction, and image information from the obtained crystal orientation was counted in each of the snowy carbons. The difference in orientation of the crystal interface existing in iron and the number of crystal interfaces having 5 or more. It is preferable to use the TIM company's OIM analysis software for the analysis of the measurement data. In order to remove the data impact of the noise on the measurement error, the cleanup is not performed, but the reliability index (COINCIDENCE INDEX: CI) The data whose value is 0.1 or less is removed and analyzed.

By setting the particle size of the ferrite iron of the structure after annealing the cold-rolled sheet to 5 μm or more and 60 μm or less, it is possible to suppress a decrease in shrinkage rate at the time of deformation at a high strain rate. Since the ferrite iron particle size is less than 5 μm , the deformability is low, so the lower limit of the ferrite iron particle size is set to 5 μm . In addition, since the particle size of the ferrite iron is more than 60 μm , in the initial stage of deformation, the surface of the pear is streaked, and the surface unevenness generated here is used as a starting point to promote the fracture, resulting in a low shrinkage rate. The upper limit of the diameter is set to 60 μm or less. The particle size of the ferrite is measured by grinding the observation surface into a mirror surface by the above-described procedure, and then etching is performed using a 3% nitric acid-alcohol solution, and the tissue is observed by an optical microscope or a scanning electron microscope, and applied. The linear analysis method is performed by measuring the captured image. The particle size of the ferrite is preferably 10 μm or more and 50 μm or less.

Next, the measurement of the shrinkage rate at the time of deformation at a high strain rate will be described. Method.

In order to determine the deformation of the steel plate at a strain rate of 10 mm/sec In the case of the shrinkage at the time of the fracture, a special test piece having a parallel portion of 1.5 mm as shown in Fig. 1 must be used. By performing a tensile test at a stroke speed of 900 mm/min by a special test piece having a parallel portion of 1.5 mm, it is possible to provide a strain rate very close to 10 mm/sec to the parallel portion of the test piece. Further, in order to accurately evaluate the failure of the steel sheet generated when the actual component is formed, it is necessary to strictly manage the ratio of the thickness to the width of the parallel portion of the tensile test piece. When the tensile deformation of the tensile test piece is performed, necking deformation occurs in two directions in the thickness direction and the width direction. Needless to say, of course, when the actual part is fractured, the necking deformation in the thickness direction dominates the fracture, and the influence of the necking deformation in the width direction is very small. Therefore, since the influence of the necking deformation in the width direction must be removed by the evaluation using the tensile test piece, the ratio of the width of the parallel portion to the thickness of the parallel portion must be 2 or more. The ratio of the width/thickness is preferably as large as possible, and is preferably 4 or more, more preferably 6 or more. Further, the shrinkage ratio can be calculated from the thickness change before and after the tensile fracture and using the formula (1).

"Shrinkage ratio (%)" = (("Sheet thickness before test" - "Sheet thickness after fracture") / "Sheet thickness before test") × 100. . . (1)

Further, the thickness before the test was measured by measuring the thickness of the center portion of the width of the parallel portion by using a micrometer, and separating the thickness from the center portion by two points perpendicular to the stretching direction and parallel to the width direction. And the measured values of 3 points are averaged to obtain. The measurement of the thickness of the sample after the fracture is, for example, The KEYENCE microscope (VHX-1000) was used, and the center of the width of each fracture surface of the sample separated into two pieces by the fracture and the position separated by 1 mm in the width direction were measured in the same manner as before the test. The thickness and the average of the measured values at 6 o'clock were set as the thickness after the test. A sample having a high shrinkage ratio of 10% or more in the above test was evaluated as a sample having "excellent shrinkage ratio".

Next, a method of manufacturing the steel sheet according to the embodiment will be described.

The technical idea of the method for producing a steel sheet according to the present embodiment is It is characterized in that the materials in the above-mentioned range of ingredients are used and the conditions of hot rolling and annealing are consistently managed.

The characteristics of the specific manufacturing method of the steel sheet according to the present embodiment are As below.

Hot rolling is characterized in that a steel preform having a predetermined composition is continuously After the casting, the hot rolling is finished at a temperature region of 600 ° C or more and less than 1000 ° C at the time of hot rolling after directly or temporarily cooling using a usual method. By cooling the rolled steel strip on the output stage (ROT) and cooling at a cooling rate of 10 ° C / sec or more and 100 ° C / sec or less, at a temperature range of 350 ° C or more and less than 700 ° C Hot rolled steel coils are obtained by coiling. The hot-rolled steel coil is subjected to hot-rolled box-type annealing, and secondly, cold-rolled at a cold rolling rate of 10% or more and 80% or less, and subjected to cold-rolled sheet annealing to obtain excellent shrinkage at high strain speed. The rate is high carbon steel plate.

Hereinafter, the manufacturer of the steel sheet according to the embodiment will be specifically described. law.

(hot rolling)

After continuously casting a steel preform (steel sheet) having a predetermined composition, after directly or temporarily cooling and then heating, when hot rolling is performed, finishing hot rolling is performed in a temperature region of 600 ° C or more and less than 1000 ° C. The obtained steel strip was taken up at a temperature range of 350 ° C or more and less than 700 ° C.

The heating temperature of the steel embryo is set to 950 ° C or more and 1250 ° C or less. The heating time is set to 0.5 hours or more and 3 hours or less. Since the heating temperature is greater than 1250 ° C or the heating time is more than 3 hours, the decarburization from the surface layer of the steel blank becomes remarkable, and even if the heat treatment by quenching is performed, the hardness of the surface layer is low and the wear resistance required as a part cannot be obtained. . Therefore, the upper limit of the heating temperature is 1250 ° C or lower, and the upper limit of the heating time is set to 3 hours or less. Further, when the heating temperature is less than 950 ° C or the heating time is less than 0.5 hours, microsegregation and large segregation formed during casting cannot be eliminated, and a region where the alloying elements such as Si and Mn are partially concentrated in the steel material remains. , causing the region to shrink at a high strain rate when the shrinkage rate is low. Therefore, the lower limit of the heating temperature is set to 950 ° C or higher, and the lower limit of the heating time is set to 0.5 hour or longer.

Finishing hot rolling is completed at 600 ° C or higher and 1000 ° C or lower It is better. Since the finishing hot rolling temperature is less than 600 ° C, the rolling load is remarkably increased due to an increase in the deformation resistance of the steel, and the amount of wear of the erecting roller is increased and the productivity is lowered. Therefore, the finishing hot rolling temperature is set to 600 ° C or higher. Moreover, when the finishing hot rolling temperature is greater than 1000 ° C, the steel sheet will form thick rust in the RunOutTable, and the grain boundary of the ferrite iron or the ferrite is oxidized after the coiling because the rust becomes an oxygen source. , causing on the surface Produces fine irregularities. Since the fine unevenness is used as a starting point, the steel sheet is broken at an early stage when it is deformed at a high strain rate, and the fine concavities and convexities cause a decrease in shrinkage. Moreover, since the finishing hot rolling temperature is higher than 1000 ° C, the alloying elements of Si, Mn, and the like are promoted to segregate at the Worthfield iron grain boundary after the hot rolling, so that the concentration of the alloying elements in the Worthfield iron particles is low. In the portion where the concentration of the alloying element is thin, the progress of the carbide agglomeration during the annealing of the hot rolled sheet and the annealing of the cold rolled sheet causes the ratio of the number of carbides having a crystal interface to increase. Therefore, the finishing hot rolling temperature is set to 1000 ° C or lower.

After finishing hot rolling, the cooling rate of the steel strip in the ROT is set to 10 ° C / sec or more and 100 ° C / sec or less. When the cooling rate is less than 10 ° C / sec, because the cooling rate is slow, the growth of ferrite iron is promoted, and the structure of the ferrite iron, the ferrite, and the toughened iron in the thickness direction of the steel strip is in the heat. Rolling plate formation. Since such a structure remains after cold rolling annealing and causes a shrinkage rate of the steel sheet to be lowered, the cooling rate is set to 10 ° C /sec or more. Further, when the steel strip is cooled at a cooling rate of more than 100 ° C / sec over the entire thickness of the sheet, the outermost layer portion is excessively cooled and low-temperature metamorphosed structures such as toughened iron and granulated iron are generated. After the coiling, when the steel coil cooled to 100 ° C to room temperature is taken up, minute cracks are generated in the aforementioned low temperature metamorphic structure. In the subsequent pickling and cold rolling steps, it is difficult to remove cracks which cause the shrinkage of the steel sheet after annealing of the cold rolled sheet to be low. Therefore, the cooling rate is set to 100 ° C / sec or less. Further, the cooling rate specified in the above is a point from the point when the steel strip after the hot rolling is finished without passing through the water injection section and then is cooled by the water in the water injection section, and is cooled to the target temperature of the coiling on the ROT. At the point in time, the cooling capacity received from the cooling equipment in each water injection section is It does not mean the average cooling rate from the start of the water injection to the winding up by the coiler.

The coiling temperature is set to 350 ° C or more and 700 ° C or less. Because of the volume When the temperature is less than 350 °C, the untransformed Worth iron in the finishing rolling will be transformed into the granulated iron, and even after the cold-rolled sheet is annealed, the fine ferrite iron and swarf carbon iron will shrink. The rate is low, so the coiling temperature is set to 350 ° C or higher. Moreover, since the coiled temperature is greater than 700 ° C, the untransformed Worth iron will be transformed into a rough layer of lamella, and the thick needle-like ferritic carbon remains after the cold-rolled sheet is annealed. And caused the shrinkage rate to be low. Therefore, the coiling temperature is set to 700 ° C or lower.

Direct or pickling of hot rolled steel coils produced under the foregoing conditions After the box annealing is performed. The annealing temperature is 670 ° C or more and 770 ° C or less, and the holding time is set to 1 hour or more and 100 hours or less.

The box annealing temperature is set to 670 ° C or more and 770 ° C or less. It is better. When the annealing temperature is less than 670 ° C, the coarsening of the ferrite particles and the carbide particles is insufficient, and the shrinkage rate is lowered when deformed at a high strain rate. Therefore, the annealing temperature is set to 670 ° C or higher. Moreover, since the annealing temperature is higher than 770 ° C, the microstructure ratio of the ferrite iron which is annealed in the 2-phase domain of the ferrite iron and the Vostian iron becomes too small, even in the case of box annealing, very slow cooling at 1 ° C / hr. When the temperature is cooled to room temperature, it is impossible to avoid generation of a large amount of intercalated iron, so that the spheroidization rate after annealing of the cold-rolled sheet is lowered, and the shrinkage rate at the time of deformation at a high strain rate is lowered. Therefore, the annealing temperature is set to 770 ° C or lower. The annealing temperature is preferably 685 ° C or more and 760 ° C or less.

The holding time of the box annealing is set to be 1 hour or more and 100 Below the hour is better. Because the holding time is less than 1 hour, in the hot rolled sheet The spheroidization of the annealed carbide is insufficient, and the spheroidization rate after annealing of the cold-rolled sheet is also low, resulting in a low shrinkage rate. Therefore, the holding time of the box annealing is set to 1 hour or longer. When the holding time is longer than 100 hours, the productivity is lowered and the interface due to the combination or contact of the carbides is formed. Therefore, the holding time of the box annealing is set to 100 hours or less. The lower limit of the holding time of the box annealing is preferably 2 hours, more preferably 5 hours, and the upper limit is preferably 70 hours, more preferably 38 hours.

Moreover, the environment of box annealing is not particularly limited, more than 95% Any of the nitrogen atmosphere, the environment of 95% or more of nitrogen, or the atmosphere.

Next, the description is carried out at a cold rolling ratio of 10% or more and 80% or less. The reason for cold rolling. In the step of annealing the hot-rolled hot-rolled sheet, the hot-rolled sheet is annealed at any of the hot-rolled sheets before and after annealing, and the cold-rolled rate is 10% or more and 80% or less. Cold rolling. When the cold rolling rate is less than 10%, the number of cores of re-crystallization of the ferrite iron in the cold-rolled sheet annealing is small, resulting in coarsening of the ferrite-grained iron particle size, and at the surface of the steel sheet when deformed at a high strain rate. The resulting pear skin markings break as a starting point, resulting in a low shrinkage rate. Therefore, the lower limit of the cold rolling ratio is set to 10%. Further, when the cold rolling ratio is more than 80%, since the number of cores for recrystallization of the ferrite iron is large, the particle size of the ferrite iron obtained after annealing the cold rolled sheet becomes excessively fine and the deformability is low, resulting in a low deformation property. When deformed at high strain speeds, the shrinkage rate is lowered. Therefore, the upper limit of the cold rolling ratio is set to 80%.

By carrying out cold rolling of the steel strip after the cold rolling rate described above Cold rolled sheet annealing, which is excellent when deformed at high strain speed High carbon steel plate in the expansion ratio.

Further, in the cold-rolled sheet annealing, lattice defects such as dislocations introduced by cold rolling are present, and the diffusion frequency of each element in the steel is increased. As a result, in the cold-rolled sheet annealing, the carbide particles are austenite grown and the coarsened carbide particles are brought into contact with each other to form one particle, and a change in the crystal interface is likely to occur in the inside of the carbide particles. Since the change of the above-mentioned carbide particles is further remarkable at the time of annealing for a long period of time, it is preferred that the cold-rolled sheet is annealed in a continuous annealing furnace.

Next, the conditions for annealing the cold-rolled sheet subjected to continuous annealing will be described. The continuous annealing is preferably carried out at an annealing temperature of 650 ° C or more and 780 ° C or less, and a holding time of 30 seconds or more and 1800 seconds or less. Since the annealing temperature is less than 650 ° C, the ferrite iron obtained after annealing the cold rolled sheet is fine in size and has low deformability, resulting in a low shrinkage rate when deformed at a high strain rate. Therefore, the lower limit of the annealing temperature is set to 650 °C. Further, when the annealing temperature is higher than 780 ° C, the ratio of the Worthite iron generated during the annealing is excessively increased, so that it is impossible to suppress the formation of the granulated iron, the tough iron, the bund iron, and the residual Worth iron after the cooling. And caused the shrinkage rate to be low. Therefore, the upper limit of the annealing temperature was set to 780 °C. Moreover, since the retention time is less than 30 seconds, the size of the ferrite iron obtained after annealing the cold-rolled sheet becomes fine, resulting in a low shrinkage rate. Therefore, the lower limit of the hold time is set to 30 seconds. Further, when the holding time is longer than 1800 seconds, during the growth of the carbide particles in the cold-rolled sheet annealing, the carbide particles come into contact with each other to have a crystal interface in the particles, resulting in a low shrinkage ratio. Therefore, the upper limit of the annealing time is 1800 seconds or less. Moreover, the heating rate and cooling rate of the cold rolled sheet annealing, although the OA area (overaged area) The temperature system is not particularly limited. However, in the test of the present embodiment, it is possible to confirm that the heating rate is 3.5 ° C / sec or more and 35 ° C / sec or less, and the cooling rate is 1 ° C / sec or more and 30 ° C / In the case where the temperature in the OA region is not more than sec and the temperature in the OA region is 250° C. or more and 450° C. or less, the form of the steel sheet of the present embodiment which is the target can be sufficiently obtained.

According to the method of manufacturing a steel sheet according to the above embodiment, The ferrite iron and the carbide are used as the main structure, and the volume ratio of the granulated particles of the granulated iron, the toughened iron, the ferritic iron, and the residual Worth iron is 5% or less. 70% or more and 99% or less, and the number of carbide particles having a crystal interface having a difference in orientation of 5° or more in the carbide particles is 20% or less with respect to the total number of the carbide particles. High-carbon steel sheets exhibiting excellent moldability can be obtained by performing plastic working such as stretching, hole expansion, thickening, and thickness reduction at a high strain rate, or in combination with such cold forging.

Example

Next, the effects of the present invention will be described by way of examples.

The level of the embodiment is an example of the conditions of implementation used to confirm the implementation possibilities and effects of the present invention, and the present invention is not limited by the example of the condition. The present invention can be applied to various conditions without departing from the gist of the present invention.

A continuous casting slab (steel block) having the composition shown in Table 1 was heated at 1,140 ° C for 1.6 hr, and then hot rolled, and the steel slab having a thickness of 250 mm was roughly rolled to a thickness of 40 mm. The thick strip of the finished hot-rolled material is heated to 36 ° C and the finishing hot rolling is started, and the finishing is performed at 880 ° C. After hot rolling, it was cooled to 520 ° C at a cooling rate of 45 ° C / sec on the ROT and coiled at 510 ° C, thereby preparing a hot rolled steel coil having a thickness of 4.6 mm. The hot-rolled steel coil is pickled and the steel coil is placed in a box-type annealing furnace, and after the environment is controlled to 95% hydrogen-5% nitrogen, the heating rate from room temperature to 500 ° C is set to 100 ° C / hour. Heating, maintaining at 500 ° C for 3 hours to homogenize the temperature distribution in the coil, heating to 705 ° C at a heating rate of 30 ° C / hour, and maintaining at 705 ° C for 24 hours, then cooling in the furnace (furnace Cooling) to room temperature. The steel coil subjected to the hot-rolled sheet annealing is cold-rolled at a rolling reduction ratio of 50%, and is subjected to cold-rolled sheet annealing at 720 ° C for 900 seconds and subjected to temper rolling at a rolling reduction ratio of 1.2%. Sample for evaluation of characteristics. The structure of the sample and the shrinkage rate at the time of deformation at a high strain rate were measured by the above method.

Table 2-1 and Table 2-2 show that the prepared sample is at high speed. The evaluation result of the shrinkage rate at the time of deformation. As shown in Table 2-1 and Table 2-2, No. B-1, C-1, D-1, E-1, F-1, G-1, H-1, I-1 of the inventive example, J-1, M-1, N-1, P-1, Q-1, R-1, S-1, U-1, X-1, Y-1, Z-1, AA-1, AB- 1. AC-1, any of which is a total volume ratio of 麻田散铁, toughened iron, ferritic iron, and residual Worth iron, which is 5% or less, and the spheroidization ratio of carbide particles is 70. % or more and 99% or less, the number of carbide particles having a crystal interface having a difference in orientation of 5 or more in the carbide particles is 20% or less with respect to the total number of the carbide particles, and the deformation system is at a high strain rate. Shows excellent shrinkage.

On the other hand, Comparative Example A-1 has carbonization at the crystal interface. The ratio of the material is small and the deformation system exhibits excellent shrinkage at high strain rates. However, since the content of C was small, the quenching step using the part could not be made high, so the evaluation was unacceptable. In Comparative Example K-1, since the Mn content was small, the austenite growth of the carbide at the time of annealing the cold rolled sheet was promoted, and the ratio of the carbide having a crystal interface was increased to cause a decrease in shrinkage. In Comparative Example L-1, since the content of P is large, the ferrite grain boundary is embrittled, and when it is deformed at a high strain rate, cracks and propagation occur from the ferrite grain boundary, and the shrinkage rate is lowered. In Comparative Example O-1, since Mn content is high, spheroidization of carbides in hot-rolled sheet annealing and cold-rolled sheet annealing is suppressed, resulting in cracking and propagation from acicular carbides when deformed at high strain speeds. , so the shrinkage rate is low. Comparative Example T-1 promotes the austenite growth of the carbide at the time of annealing the cold-rolled sheet because the Si content is small, so that the ratio of the carbide having a crystal interface is increased and the shrinkage rate is lowered. In Comparative Example V-1, since a large amount of S is present, many coarse inclusions such as MnS are present in the steel, and cracks and progress are caused by inclusions as a starting point, so that the shrinkage rate is lowered. In Comparative Example W-1, since the Si content is large, the Worth Iron produced in the cold-rolled sheet annealing is less likely to cause ferrite-iron deformation during cooling and promotes toughening iron and Borne iron metamorphism due to the fat particles. The ratio of the structure other than iron and carbide increases, causing stress to concentrate on the ferrite grain boundary and the shrinkage rate is low. In Comparative Example AD-1, since the content of C and the volume fraction of carbide were large, the number ratio of carbides having a crystal interface could not be controlled to 20% or less, resulting in a decrease in shrinkage ratio.

Second, in order to investigate the allowable content range of other elements, Continuous casting slab (steel block) consisting of the components shown in Table 3-1, Table 2-3, and Table 3-3 and Table 4-1, Table 2-4, and Table 4-3, at 1180 After heating at ° C for 0.7 hr, the steel sheet having a thickness of 250 mm obtained by hot rolling was hot-rolled to a thickness of 45 mm, and then the thick strip of the finished hot-rolled material was heated at 48 ° C to start finishing hot rolling. After finishing hot rolling at 870 ° C, it was cooled to 510 ° C on a ROT at a cooling rate of 45 ° C / sec and taken up at 500 ° C, thereby preparing a hot rolled steel coil having a thickness of 2.6 mm. The hot-rolled steel coil is pickled and the steel coil is placed in a box-type annealing furnace, and after the environment is controlled to 95% hydrogen-5% nitrogen, the heating rate from room temperature to 500 ° C is set to 100 ° C / hour. Heating, maintaining at 500 ° C for 3 hours to homogenize the temperature distribution in the coil, heating to 705 ° C at a heating rate of 30 ° C / hour, and holding at 705 ° C for 24 hours, cooling in the furnace to the chamber Warm up. The steel coil subjected to the hot-rolled sheet annealing is cold-rolled at a rolling reduction ratio of 50%, and is subjected to cold-rolled sheet annealing at 700 ° C for 900 seconds and subjected to temper rolling at a rolling reduction ratio of 1.0%. Sample for evaluation of characteristics.

Table 5-1~Table 5-6 shows the prepared specimen at high strain rate Evaluation result of shrinkage rate at the time of deformation. As shown in Table 5-1 to Table 5-6, No. AE-1, AF-1, AL-1, AM-1, AN-1, AR-1, AS-1, AV-1 of the invention examples, AW-1, AX-1, BC-1, BD-1, BF-1, BH-1, BI-1, BJ-1, BK-1, BM-1, BN-1, BT-1 In one case, the volume ratio of the granulated iron, the tough iron, the ferritic iron, and the residual Worth iron is 5% or less (including 0.0%), and the spheroidization ratio of the carbide particles is 70% or more. 99% or less, with respect to the total number of carbide particles, the inclusion of carbide particles The number ratio of the carbide particles at the crystal interface having a difference of 5° or more is 20% or less, and the strain system exhibits an excellent shrinkage ratio at a high strain rate.

In contrast, Comparative Examples AG-1, AH-1, AO-1, AT-1, AU-1, AZ-1, BA-1, BB-1, BO-1, and BS-1 are mainly contained in the contents of Ce, Ca, Y, Al, Mg, As, Zr, Sn, Sb, and La, The grain boundary of the ferrite iron is caused to be embrittled and the shrinkage rate is low when deformed at a high strain speed. Comparative Examples AI-1, AJ-1, AK-1, AQ-1, BE-1, BG-1, BL-1, BQ-1, and BR-1 are based on Nb, W, Ti, Ni, Cr, Mo The content of V, Cu, and Ta is large, so that the spheroidization of carbides during annealing of the hot rolled sheet and annealing of the cold rolled sheet is suppressed, and cracking occurs from the acicular carbide when deformed at a high strain rate and Spread, so the shrinkage rate is low. In the comparative example AP-1, since the content of N is large, the Worthite iron system formed in the cold-rolled sheet annealing is less likely to cause ferrite-grain metamorphism during cooling and promotes toughening iron and Borne iron metamorphism due to fertilizer. The ratio of the structure other than the granular iron and the carbide increases, and the stress concentration causes the shrinkage rate to decrease at the ferrite grain boundary. In Comparative Example AY-1, a large oxide was formed in steel because of a large content of O, and when it was deformed at a high strain rate, cracks and propagation were caused by using a coarse oxide as a starting point, and the shrinkage rate was lowered. In Comparative Example BP-1, coarse Fe-B-Carbide (iron-boron carbide) was formed in steel due to a large B content, and cracks and propagation occurred as a starting point of Fe-B-Carbide, and the shrinkage rate was lowered.

Then in order to investigate the influence of manufacturing conditions, the casting has Table 1. Tables 3-1 to 3-3 and Tables 4-1 to 4-3 show No. B, C, D, E, F, G, H, I, J, M, N, P, Q, R, S, U, X, Y, Z, AA, AB, AC, AE, AF, AL, AM, AN, AR, AS, AV, AW, AX, BC, BD, BF, BH, BI, BJ, Steel embryos of BK, BM, BN, and BT are temporarily cooled, and are shown in Table 6-1-1, Table 6-1-2, Table 6-2-1, Table 6-2-2, Table 7-1. -1, Table 7-1-2, Table 7-2-1, Table 7-2-2, Table 8-1-1~Table 8-1-3, Table 8-2-1~Table 8-2- 3. Table 9-1-1~Table 9-1-3, and Table 9-2-1~Table 9-2-3 (hereinafter referred to as Tables 6, 7, 8, and 9) Under hot rolling conditions, a hot-rolled steel strip having a thickness of 3.5 mm was formed and subjected to hot-rolled sheet annealing, pickling, cold rolling, and cold-rolled sheet annealing to prepare a sample for evaluation of characteristics.

Tables 6, 7, 8, and 9 also show that the prepared sample is at high speed. The evaluation result of the shrinkage rate at the time of deformation. Inventive examples No. B-2, C-2, D-2, E-2, J-2, N-2, Q-2, X-2, Y-2, Z-2, AB-2, AC -2, AL-2, AN-2, AS-2, AV-2, BC-2, BD-2, BH-2, BI-2, BJ-2, BN-2, F-3, G-3 , H-3, I-3, M-3, N-3, P-3, R-3, S-3, U-3, AA-3, AB-3, AE-3, AF-3, AM -3, AR-3, AW-3, AX-3, BF-3, BK-3, BM-3, BT-3, as shown in Table 8, any one of them is the Shitian iron, change The volume ratio of the toughness iron, the ferritic iron, and the residual Worthite is 5% or less, and the spheroidization ratio of the carbide particles is 70% or more and 99% or less, and the carbonization is based on the total number of the carbide particles. The number ratio of the carbide particles having a crystal interface having a difference in orientation of 5 or more in the particles is 20% or less, and the strain system exhibits an excellent shrinkage ratio at a high strain rate.

In contrast, Comparative Examples AA-2, BK-2, C-3, and BJ-3 are as follows. As shown in Tables 6 and 7, since the finishing hot rolling temperature is higher, the ratio of the number of carbides having a crystal interface increases, and the thick rust generated between the cooling to the coiling becomes oxygen. The supply source oxidizes the grain boundary after the coiling, and fine cracks are formed on the surface. Therefore, when deformed at a high strain rate, the crack propagates as a starting point of cracks in the surface layer, causing a decrease in shrinkage rate. In the comparative examples R-2, BM-2, X-3, and BC-3, since the finishing hot rolling temperature is low, the rust is caught during hot rolling, causing irregularities on the surface of the steel sheet during rolling, and is high. When the strain is deformed at a strain rate, cracks are generated from the surface unevenness and progress, and the shrinkage rate is lowered. Comparative Examples U-2, AR-2, Y-3, and AL-3 are formed by needle-forming and having a large thickness of carbide in the hot-rolled sheet because the coiling temperature is high, and annealing in the cold-rolled sheet. Spheroidization of post-acicular carbides No progress, but cracks and propagation occur as a starting point for acicular carbides, resulting in a low shrinkage rate. In Comparative Examples H-2, AM-2, Q-3, and BI-3, since the coiling temperature is low, the microstructure of the hot-rolled sheet is fine, and the structure after annealing of the cold-rolled sheet is also fine, so that the deformation ability is low and the height is high. The shrinkage rate at the time of deformation at the strain rate is low.

Comparative Examples G-2, AE-2, J-3, and BD-3 are shown in Tables 6 and 7. It is shown that since the cold rolling ratio is high, the microstructure after annealing of the cold rolled sheet becomes fine, so that the deformability is lowered, and the shrinkage rate is lowered. In Comparative Examples S-2, AW-2, AC-3, and BH-3, since the cold rolling rate is low, the grain size of the ferrite iron after annealing the cold rolled sheet becomes coarse, and when deformed at a high strain rate, The surface layer produces pear skin streaks, which cause cracks and progress based on the formed surface irregularities, resulting in a low shrinkage rate. In Comparative Examples M-2, BT-2, Z-3, and AS-3, since the temperature at which the cold-rolled sheet is annealed is high, the phase ratio of the Worthite iron generated during the annealing is increased because In the cooling process, it is impossible to suppress the metamorphism of the granulated iron, the toughened iron, and the ferritic iron, so the shrinkage rate is low when deformed at a high strain speed. Comparative Examples P-2, BF-2, E-3, and BN-3 are due to the low temperature of the cold-rolled sheet annealing, which causes the fine-grained iron to have a fine particle size, resulting in low deformation ability and deformation at high strain speed. The shrinkage rate is low. In Comparative Examples I-2, AX-2, D-3, and AN-3, since the cold rolled sheet has a long annealing time, the carbide particles contact each other during the roughening process and have a crystal interface inside the particles, causing shrinkage. low. In Comparative Examples F-2, AF-2, B-3, and AV-3, since the annealing time of the cold rolled sheet was short, the ferrite iron was fine, and the deformation ability was low, resulting in a low shrinkage rate at the time of deformation at a high strain rate.

Figure 1 shows the steel plate used to evaluate deformation at high strain speed. The shape of the test piece of shrinkage. The parallel portion of the test piece was 1.5 mm, and the test piece was stretched at a stroke speed of 900 mm/min to break the test piece, and the shrinkage ratio of the steel plate was determined from the change in the thickness of the center of the parallel portion before and after the test.

Figure 2 shows the elongation at the time of deformation at high strain speed After 13.4% cessation, the microstructure of Example U-1 was observed by etching the sample with a 3% nitric acid-alcohol solution to cause ferrite iron and carbide. It is clear that the cracks in the carbides originate from the crystalline interface present in the carbide particles.

3 is an invention example and a comparative example of Table 2-1 and Table 2-2, And the inventive examples and comparative examples of Tables 5-1 to 5-6, Table 6, Table 7, Table 8, and Table 9, showing the shrinkage rate at a high strain rate and the number of total carbides The relationship between the number ratio of carbides having a crystal interface in the carbide particles. It has been found that the shrinkage ratio can be remarkably improved by adjusting the composition to the invention and setting the number ratio of the carbide having a crystal interface to 20% or less.

Claims (3)

A medium-high carbon steel sheet characterized by having the following components: C: 0.10 to 1.50%, Si: 0.01 to 1.00%, Mn: 0.01 to 3.00%, P: 0.0001 to 0.1000%, S: 0.0001 by mass% ~0.1000%, and the remainder is composed of Fe and impurities; and the steel sheet has the following structure: the volume ratio of the granulated iron, the tough iron, the bund iron, and the residual Worth iron is 5.0% or less, and the remaining Part of the ferrite iron and carbide; and the spheroidization rate of the carbide particles is 70% or more and 99% or less; and the carbide particles have a difference in orientation of 5 or more with respect to the total number of the carbide particles. The number ratio of the aforementioned carbide particles at the crystal interface is 20% or less. In the high-carbon steel sheet according to the above-mentioned item 1, the component of the steel sheet further contains one or more of the following in terms of mass%: Al: 0.001 to 0.500%, N: 0.0001 to 0.0500%, and O: 0.0001 ~0.0500%, Cr: 0.001~2.000%, Mo: 0.001 to 2.000%, Ni: 0.001 to 2.000%, Cu: 0.001 to 1.000%, Nb: 0.001 to 1.000%, V: 0.001 to 1.000%, Ti: 0.001 to 1.000%, B: 0.0001 to 0.0500%, W : 0.001 to 1.000%, Ta: 0.001 to 1.000%, Sn: 0.001 to 0.020%, Sb: 0.001 to 0.020%, As: 0.001 to 0.020%, Mg: 0.0001 to 0.0200%, Ca: 0.001 to 0.020%, Y: 0.001 to 0.020%, Zr: 0.001 to 0.020%, La: 0.001 to 0.020%, and Ce: 0.001 to 0.020%. A method for producing a medium-high carbon steel sheet, characterized in that the steel sheet having the above-mentioned composition according to claim 1 or 2 is directly or temporarily cooled, and then heated and hot rolled at 600 ° C or higher and 1000 ° C or lower. Finishing hot rolling is performed in a temperature region; hot-rolled steel sheets which have been taken up above 350 ° C and below 700 ° C are subjected to box annealing; Cold rolling is performed at 10% or more and 80% or less; then, the cold rolling sheet annealing is performed on the continuous annealing line at an annealing temperature of 650 ° C or more and 780 ° C or less, and a holding time of 30 seconds or more and 1800 seconds or less.