Patent Description:
As weight reduction and safety are required in the automotive industry, the market has an increasing demand for higher-strength steel plates. Dual-phase steel has excellent properties such as low yield strength, high tensile strength and high initial work hardening rate, and is widely used in the production of automotive parts. In view of the rebound of some automotive parts such as car seats in practical use, there is a high demand for <NUM> grade dual-phase steel having a high yield ratio (a yield ratio of greater than <NUM>) in the market.

In the prior art, <CIT>, "High-plasticity 700MPa grade cold-rolled weather-resistant dual-phase steel and preparation method thereof' discloses a weather-resistant dual-phase steel having a chemical composition in mass percentages of <NUM>-<NUM>% C, <NUM>-<NUM>% Si, <NUM>-<NUM>% Mn, <<NUM>% P, <<NUM>% S, <NUM>-<NUM>% Cr, <NUM>-<NUM>% Cu, <NUM>-<NUM>% Ni, <NUM>-<NUM>% Nb, <NUM>-<NUM>% Ti, and a balance of Fe and other unavoidable impurities. The method for manufacturing the steel plate comprises heat preservation at <NUM>, finish rolling at <NUM>-<NUM>, annealing at <NUM>-<NUM>, rapid cooling from <NUM>-<NUM> at a rapid cooling rate of <NUM>/s, and termination of rapid cooling at a temperature of <NUM>, wherein a <NUM>-<NUM> MPa steel plate having a yield strength of <NUM>-<NUM> MPa and an elongation of <NUM>-<NUM>% is obtained. In the design of the composition of the steel plate, relatively large amounts of alloying elements such as Cr, Cu, Ni are used, and the content of Si is relatively high.

Further, <CIT>, "<NUM> MPa grade low yield ratio hot-rolled dual-phase steel plate and manufacturing method thereof' discloses a <NUM> MPa grade low yield ratio hot-rolled dual-phase steel plate having a chemical composition in mass percentages of <NUM>%-<NUM>% C, <NUM>%-<NUM>% Si, <NUM>%-<NUM>% Mn, <NUM>%-<NUM>% Al, <NUM>%-<NUM>% Cr, and a balance of Fe. The cast slab used for manufacturing the steel plate is heated in a heating furnace and rolled through a hot continuous rolling unit. After rolling, a laminar cooling process is used for staged cooling, and an ultra-high strength hot-rolled dual-phase steel having a tensile strength of <NUM> MPa is obtained at the end.

Moreover, <CIT> and <CIT> disclose steel plates and manufacturing methods of the same according to prior art.

In summary, the dual-phase steel products in the prior art are mainly classified into two types: (<NUM>) cold-rolled, annealed dual-phase steel plates containing relatively large amounts of such elements as Cu, Ni, Cr, etc.; and (<NUM>) low-yield ratio hot-rolled steel plates. These two types of products contain relatively large amounts of alloying elements, while the yield ratio is rather low.

In view of this situation, it is desirable to provide a dual-phase steel that contains less alloying elements and has a higher yield ratio to meet the market demand for dual-phase steel having a high yield ratio.

One of the objects of the present invention is to provide a cold-rolled dual-phase steel having a high yield ratio, wherein the dual-phase steel has a low cost, contains less alloying elements, and has a higher strength and a higher yield ratio, so that it can satisfy the market demand for dual-phase steel having a high yield ratio.

In order to attain the above object, the present invention provides a cold-rolled dual-phase steel plate having a high yield ratio, comprising the following chemical elements in mass percentages:.

In the technical solution of the present invention, the various chemical elements are designed according to the following principles:.

Further, in the cold-rolled dual-phase steel having a high yield ratio according to the present invention, the microstructure is a complex phase structure of martensite+ferrite+[NbxTiy(C,N)z] carbonitride.

Still further, in the cold-rolled dual-phase steel having a high yield ratio according to the present invention, the phase proportion of the martensite is <NUM>-<NUM>%, and the martensite is in the shape of long strips-islands (it is island-shaped when observed under a low-magnification metallographic microscope; it is lath or long strip-shaped when observing the fine structure of the martensite).

In the cold-rolled dual-phase steel having a high yield ratio according to the present invention, the phase proportion of the martensite is <NUM>-<NUM>%, and the martensite is in the shape of long strips-islands. The martensite has a function of phase transformation strengthening. If the phase proportion of the martensite is too high or too low, the strength of the steel will be unduly high or low. Therefore, the present invention limits the phase proportion of the martensite in the cold-rolled dual-phase steel having a high yield ratio to <NUM>-<NUM>%.

Further, in the cold-rolled dual-phase steel having a high yield ratio according to the present invention, the [NbxTiy(C,N)z] carbonitride has an irregular spherical shape and is uniformly distributed in the ferrite grains. The phase proportion of the [NbxTiy(C,N)z] carbonitride is <NUM>-<NUM>%, wherein x+y=z.

In the cold-rolled dual-phase steel having a high yield ratio according to the present invention, the [NbxTiy(C,N)z] carbonitride has an irregular spherical shape and is uniformly distributed in the ferrite grains to achieve dispersion precipitation strengthening and increase the yield ratio.

If the phase proportion of the [NbxTiy(C,N)z] carbonitride is less than <NUM>%, it cannot achieve the effect of increasing the yield ratio. After the phase proportion of the [NbxTiy(C,N)z] carbonitride is increased to be higher than <NUM>%, the yield ratio of the steel will not change much. Therefore, the present invention limits the phase proportion of the [NbxTiy(C,N)z] carbonitride in the cold-rolled dual-phase steel having a high yield ratio to <NUM>-<NUM>%.

Further, in the cold-rolled dual-phase steel having a high yield ratio according to the present invention, the [NbxTiy(C,N)z] carbonitride has a size of less than <NUM>.

In the cold-rolled dual-phase steel having a high yield ratio according to the present invention, among the unavoidable impurities, the mass percentages of the P, S and N elements are: P≤<NUM>%; S≤<NUM>%; N≤<NUM>%, according to the following principles:
P: P is an impurity element in steel. The lower the mass percentage of P, the better. With the requirements of both the production cost and process conditions taken into account, the present invention limits the mass percentage of P in the cold-rolled dual-phase steel having a high yield ratio to P≤ <NUM>%.

S: S is an impurity element in steel. The lower the mass percentage of S, the better. With the requirements of both the production cost and process conditions taken into account, the present invention limits the mass percentage of S in the cold-rolled dual-phase steel having a high yield ratio to S≤ <NUM>%.

N: N is an impurity element in steel. If its amount is too high, the surface of a slab tends to crack. Therefore, the lower the mass percentage of N, the better. With the requirements of both the production cost and process conditions taken into account, the present invention limits the mass percentage of N in the cold-rolled dual-phase steel having a high yield ratio to N≤ <NUM>%.

Further, the cold-rolled dual-phase steel having a high yield ratio according to the present invention has a yield ratio of greater than <NUM>.

Further, the cold-rolled dual-phase steel having a high yield ratio according to the present invention has a yield strength of <NUM>-<NUM> MPa, a tensile strength of ≥<NUM> MPa, and an elongation at break of ≥ <NUM>%.

Accordingly, another object of the present invention is to provide a method for manufacturing the above-mentioned cold-rolled dual-phase steel having a high yield ratio. The cold-rolled dual-phase steel having a high-yield ratio obtained by this method has a higher strength and a higher yield ratio.

To attain the above object, the present invention proposes a method for manufacturing a cold-rolled dual-phase steel plate having a high yield ratio, comprising the following steps:.

In the manufacturing method of the present invention, in Step (<NUM>), in order to ensure the stability of the rolling load, the temperature for heating the cast blank is controlled to be <NUM> or higher. On the other hand, with the solid solubilities of Ti(C, N) and Nb(C, N) ) in austenite taken into consideration, in order to ensure that the carbonitrides Ti(C,N) and Nb(C,N) can be precipitated at a high temperature, the upper limit of the temperature for heating the cast blank is controlled to be <NUM>. That is, the cast blank is controlled to be soaked at a temperature of <NUM>-<NUM>, preferably for a soaking time of <NUM>-<NUM> hours, followed by rolling. In addition, in view of the formability after the annealing and the possibility that coarse grains will result in a nonuniform structure, the finish rolling temperature is controlled to be <NUM>-<NUM>. After the rolling, cooling is performed at a rate of <NUM>-<NUM>/s, preferably to <NUM>-<NUM>, and then coiling is performed. The coiling temperature may be viewed as the precipitation temperature of the carbonitrides in ferrite, and the precipitation temperature is one of the main factors that control the size of the precipitates. The lower the precipitation temperature, the smaller the critical nucleus size for precipitation nucleation, and the finer the precipitates. In addition, the diffusion of Ti and Nb is slow. As a result, the growth rate of Ti and Nb is also small. From the perspective of kinetics, due to the high diffusion activation energies of Ti and Nb, the precipitation process of Ti(C,N) and Nb(C,N) is a result of long-range diffusion, and full precipitation needs sufficient time. If the cooling rate is too fast, the precipitation process of the second phase particles will be inhibited, and at the same time, the solid solution content will be increased. This is unfavorable for the precipitation process of Ti(C,N) and Nb(C,N), and the precipitation amount will be reduced. The coiling temperature is preferably <NUM>-<NUM>.

In addition, in Step (<NUM>), the annealing soaking temperature and annealing time determine the degree of austenitization, and ultimately determine the phase proportions of martensite and ferrite in the steel structure. If the annealing soaking temperature is too high, the phase proportion of martensite will be so high that the strength of the final steel plate will be unduly high. If the annealing soaking temperature is too low, the phase proportion of martensite will be so low that the strength of the final steel plate will be unduly low. In addition, if the annealing soaking time is too short, the degree of austenitization will be insufficient; and if the annealing soaking time is too long, the austenite grains will become coarse. Therefore, in the manufacturing method according to the present invention, in Step (<NUM>), the annealing soaking temperature is controlled to be <NUM>-<NUM>; the annealing time is <NUM>-<NUM>; and then cooling is performed at a rate of <NUM>-<NUM>/s. The starting temperature of the cooling is <NUM>-<NUM>; the aging temperature is <NUM>-<NUM>; and the over-aging time is <NUM>-<NUM>.

Further, in the manufacturing method according to the present invention, in Step (<NUM>), the cold rolling reduction rate is controlled to be <NUM>-<NUM>%; and/or in Step (<NUM>), the temper rolling reduction rate is controlled to be <NUM>-<NUM>%.

In the manufacturing method according to the present invention, in Step (<NUM>), in some embodiments, the mill scale on the steel surface may be removed by pickling, and then cold rolling is performed. In order to produce more polygonal ferrite in the steel structure, the cold rolling reduction rate is controlled to <NUM>-<NUM>%. In addition, in Step (<NUM>), in order to ensure the flatness of the steel plate, the steel plate needs to be temper rolled to a certain degree. If it's temper rolled excessively, the yield strength will increase unduly. Therefore, in the manufacturing method according to the present invention, in Step (<NUM>), the temper rolling reduction rate is controlled to be <NUM>-<NUM>%.

Compared with the prior art, the cold-rolled dual-phase steel having a high yield ratio and the manufacturing method thereof according to the present invention have the following beneficial effects:.

<FIG> is a microstructure diagram of a cold-rolled dual-phase steel having a high yield ratio in Example <NUM>.

The cold-rolled dual-phase steel having a high yield ratio according to the present invention and the method for manufacturing the same will be further explained and illustrated with reference to the accompanying drawing of the specification and the specific examples. Nonetheless, the explanation and illustration are not intended to unduly limit the technical solution of the present invention.

Table <NUM>-<NUM> and Table <NUM>-<NUM> list the mass percentages (wt%) of the chemical elements in the high-yield-ratio cold-rolled dual-phase steels of Examples <NUM>-<NUM> and Comparative Examples <NUM>-<NUM>.

The method for manufacturing the high-yield-ratio cold-rolled dual-phase steels of Examples <NUM>-<NUM> and Comparative Examples <NUM>-<NUM> is as follows (the specific process parameters are listed in Table <NUM>-<NUM> and Table <NUM>-<NUM>):.

The high-yield-ratio cold-rolled dual-phase steels of Examples <NUM>-<NUM> and Comparative Examples <NUM>-<NUM> were tested for their properties. The test results are listed in Table <NUM>.

As can be seen from Table <NUM>, the high-yield-ratio cold-rolled dual-phase steels of Examples <NUM>-<NUM> have a tensile strength of ≥ <NUM> MPa, an elongation at break of ≥ <NUM>%, and a yield ratio of greater than <NUM>. Thus, it can be seen that the cold-rolled dual-phase steel having a high yield ratio according to the present invention has the advantages of high strength, low carbon equivalent and high yield ratio.

As can be seen from <FIG>, the microstructure of the high-yield-ratio cold-rolled dual-phase steel of Example <NUM> is a complex phase structure of martensite + ferrite + [NbxTiy(C,N)z] carbonitride, wherein the martensite has a phase proportion of <NUM>-<NUM>%, and has a function of phase transformation strengthening. The martensite structure is in the shape of long strips-islands (it is island-shaped when observed under a low-magnification metallographic microscope; it is lath or long strip-shaped when observing the fine structure of the martensite). Meanwhile, the [NbxTiy(C,N)z] carbonitride has an irregular spherical shape and is uniformly distributed in the ferrite grains. The carbonitride has a size of less than <NUM>, and has a function of dispersion precipitation strengthening in the structure.

Claim 1:
A cold-rolled dual-phase steel plate having a high yield ratio, comprising the following chemical elements in mass percentages:
C: <NUM>-<NUM>%, Mn: <NUM>-<NUM>%, Si: <NUM>-<NUM>%, Nb: <NUM>-<NUM>%, Ti: <NUM>-<NUM>%,
Al: <NUM>-<NUM>%, P ≤ <NUM>%, S ≤ <NUM>%, N ≤ <NUM>%, and a balance of Fe and other unavoidable impurities; wherein
the cold-rolled dual-phase steel plate has a microstructure which is a complex phase structure of martensite+ferrite+[NbxTiy(C,N)z] carbonitride, wherein x + y = z;
the martensite has a phase proportion of <NUM>-<NUM>%, and the martensite is in the shape of long strips-islands;
the [NbxTiy(C,N)z] carbonitride has an irregular spherical shape and is uniformly distributed in ferrite grains, and the [NbxTiy(C,N)z] carbonitride has a phase proportion of <NUM>-<NUM>%; and
the cold-rolled dual-phase steel plate has a yield strength of <NUM>-<NUM> MPa, a tensile strength of ≥ <NUM> MPa, an elongation at break of ≥ <NUM>%, and a yield ratio of greater than <NUM>.