KR850000103B1 - Method of high tension cold steel sheet - Google Patents

Abstract

This invention relates to high-tension cold-rolling steel sheet, which has a low yield point, low yield ratio, high ductility and 40- 80 kg/mm2 tensile strength. The steel sheet, which contains 0.01- 0.12% carbon, under 1.2% silicon, 1.0-1.8% Mn, 0.010.10% Sol. Al, and the remainder Fe and impurities, is first hot-rolled. The hot-rolled steel sheet is then cold-rolled. The hot-rolled steel sheet is then heated at 30-800≰C, for 20 sec.2min. Continuous annealing treatment is then performed on the sheet at 30-300≰C/sec. cooling velocity to a temperature under 250≰C.

Description

Manufacturing method of composite structure high tensile cold rolled steel sheet with excellent workability

1 (a), 1 (b), and 1 (c) are graphs showing the cooling rate and the tendency of complex organization for the combination of the amount of C and Mn.

The present invention relates to a method for producing a high tensile cold rolled steel sheet having a resistance yield point, a resistance ratio and high ductility, and a tensile strength of 40-80kg / mm ² .

In recent years, the automobile industry has been promoting full-scale use of high-strength steel sheets that can be used by thinning the sheet thickness instead of the conventional soft steel sheet as a part of safety measures while reducing the body weight. In order to respond to the demands of the automotive industry as the steel industry, there is a need to provide a high-strength cold-rolled steel sheet having a material property of the phase and at the same time low price.

Conventionally, a composite structure high tensile cold rolled steel sheet and a manufacturing method thereof are described, for example, in Japanese Patent Application Laid-Open No. 50-98419. The invention of the publication mainly consists of C and Mn, and Si as needed is 0.1. Ferat and the steel obtained by heating the steel containing% -0.7% in the α, γ2 phase coexistence zone in the state diagram, and relatively quenching so that the average cooling rate to 500 ° C is 0.5-30 ° C / sec. It is to obtain a composite steel sheet containing a structure of quench transformation, which also contains some residual austenite.

The present invention is to provide a novel manufacturing method for a composite high-strength cold rolled steel sheet which is economically workable by a combination of cooling conditions following a crack in continuous annealing and composition of a starting material. .

Steel sheet obtained by the present invention has a tensile strength of 40-80kg / mm ² while showing a resistance point, and has a high ductility has the following effects. In other words, the yield point is related to the spring back of the material at the time of press molding. The yield point (resistance ratio) does not increase the pressure on the air in order to have affinity for the press die and improve the shape of the molded article. By manufacturing the high-strength composite steel sheet at low cost by lowering the alloying amount by the manufacturing method of the present invention, it contributes further to the high tension of the steel sheet for automobiles. Moreover, the steel plate obtained by this invention has high ductility. Since the ductility is natural and high in the sense of withstanding high processing, the steel sheet according to the present invention is remarkably advantageous in this respect.

On the other hand, the thickness of the steel sheet used on the outer plate for automobiles was generally about 0.8mm, but in recent years, the movement to reduce it further to reduce the weight is active. In such a case, the dent resistance indicating resistance to deformation of the member becomes a problem. Dent resistance (resistance to locally retracting) depends on sheet thickness and strength. The reason why the high strength thin steel sheet is used as an exterior plate for automobiles is to improve this dent resistance. In this sense, according to the present invention, a steel sheet capable of securing workability with a resistance point and high elongation and having improved dent resistance with high tensile strength is obtained. There is great significance in that it is used in place of the conventional soft steel sheet sufficiently satisfying the demands of the automotive industry.

Hereinafter, the content of the present invention will be described in detail.

One of the features of the present invention is as described below, and employs a very fast cooling rate of 30 ° C.-300 ° C./sec up to a temperature of 250 ° C. or less upon cooling in continuous annealing. In order to achieve the object of the present invention in such rapid cooling, first, the composition of the material requires the following conditions.

The following conditions are required.

態) 오스테나이트상(相)으로 된다.If the amount of C is not more than 0.01%, the amount of quenching tissue generated by quenching is insufficient. On the other hand, if the amount of C exceeds 0.12%, the quench transformation product is excessively increased, making it difficult to obtain a composite structure having a high ductility intended by the present invention. In this case, the quench transformation product is martensite or untransformation.

(Iii) It becomes an austenite phase.

Next, the amount of Mn increases the quenchability of the γ phase, obtains the quench transformation product during the cooling process, and has the effect of enhancing the ductility by strengthening the ferrite substrate. Hardness is insufficient at less than 1.0%. An Mn amount of more than 1.8% is not preferable in terms of economical efficiency while deteriorating the negative contact.

Al is needed as a deoxidation material, it is insufficient in less than 0.01%, and if it is 0.01% -0.10%, the objective of deoxidation can fully be achieved.

Si does not basically need to be contained and is good as Si≤0.1%, but in order to increase the ductility of the composite structure, it is added in the range of 1.2% or less. If more than the amount, since it adversely affects the paintability and corrosion resistance, it was limited to 1.2% or less.

In addition, in order to further increase the strength, Ca, REM, and Zr may be contained in the range of 0.1% or less, respectively, in order to improve Cr, Cu, and Ni by 1% or less, and to improve the bending property.

Thus, the raw material whose composition is adjusted is made into a slab by the continuous casting method or the ingot-destructive method according to a conventional method, and made into a hot rolled and cold rolled plate, and performs the characteristic continuous annealing of this invention next. Uniformity of continuous annealing requires 730 ° C-800 ° C x 20 seconds-2 minutes for the following reasons. If the amount of γ necessary for obtaining the desired retained austenite and concentration of C and Mn in γ is insufficient when the residual amount of austenite is less than 730 ° C and less than 20 seconds, the desired composite structure cannot be obtained even if quenching is performed. Can't get a good grater. On the other hand, in the crack exceeding 800 ° C., the amount of γ is excessively large, and C and Mn in γ become lean. As a result, even if quenching is reused, the desired residual austenite is not obtained, hard martensite is increased, and steel sheets having a good strength-ductility balance are not obtained. The upper limit of crack time may be longer, but the longer the length of the furnace is, the more limited it is to 2 minutes in consideration of the economics of the installation.

The cooling performed after the cracking is performed up to 250 ° C at a rate of 30 ° C-300 ° C / sec, preferably 100 ° C-2300 ° C / sec. This is because the quenching end point is 250 ° C. first because when quenching is stopped at a temperature higher than that, the residual austenide is transformed into martensite, and thus, the compound aid is not obtained.

Next, about the cooling rate, when the speed | rate is less than 30 degreeC / sec, it is difficult to obtain a composite structure with the low alloy amount prescribed | regulated by this invention, and when a cooling rate is 300 degreeC / sec or more, ductility will fall. In other words, the ductility hardly changes at a cooling rate in the range of 30 ° -300 ° C / sec. Such controlled cooling rate is difficult to obtain in gas cooling and submerged cooling, and is easily obtained by cooling by gas and water by injection of a gas-liquid mixer.

Moreover, according to the cooling method by this gas and water, uniform cooling of the width direction of a steel strip is possible, and it also has the characteristic that a uniform material is obtained.

In this way, the shape of the material after continuous annealing is usually corrected as a reduction ratio of about 1.0%.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated based on an Example.

Steel having various components as shown in Table 1 was melted to prepare a hot rolled steel strip having a thickness of 2.5 mm by hot rolling. This was cold rolled again to a cold rolled steel sheet of 0.7 mm, and after continuous cracking at 770 ° C. for 40 seconds, the cooling rate was changed from 10 ° C. to water cooling (W.Q about 1000 ° C./sec) and cooled to room temperature. The material of the obtained steel plate is shown in a 1st table | surface.

[Table 1]

[Table 1] (continued)

In the conventional continuous annealing method, the cooling rate after cracking is about 10 ° C / sec. In the case of such a slow cooling rate, if the amount of the alloying component (Mn) is not large, as shown in Table D, the yield point is low, so that a composite steel sheet having a low yield ratio is not obtained. In addition, with the increase of the Mn amount, there is a difficulty in that the weldability is deteriorated and the price is increased.

On the contrary, when the cooling rate is 30 ° -300 ° C./sec as in the present invention, a composite structure is obtained even if the amount of alloy addition is reduced, such as A-C and E-I. Moreover, the ductility thereof is hardly changed in the scope of the present invention. On the other hand, when the cooling rate is increased to 350 ° C./sec (water spray) and 1000 ° C. (water immersion), the ductility deteriorates remarkably and does not meet the object of the present invention.

1 (a), 1 (b), and 1 (c) show the relationship between the cooling rate and the composite texture tendency for various combinations of carbon amount and Mn amount.

The numbers in the figure represent the yield ratio (%), and the 50% yield ratio is the boundary of complex organization.

As can be seen from the figure, as the cooling rate increases, it can be seen that even if the amount of alloying elements is small, it is composited. Moreover, as shown in the first table, in the range of the cooling rate 30 ° -300 ° C / sec of the present invention, no bad state in which ductility deteriorates occurs.

Claims (1)

C 0.01-0.12%, Si 1.2% or less, Mn 1.0-1.8%, Sol.Al 0.01-0.10%, hot rolled steel with remainder Fe and impurities, cold rolled hot rolled steel sheet, and then this cold rolled steel sheet The processability is characterized by performing a continuous annealing treatment in which the film is cracked at 730 DEG -800 DEG C for 20 seconds-2 minutes and then cooled to a temperature of 250 DEG C or lower at a cooling rate of 30 DEG -300 DEG C / sec. Excellent composite structure high tensile cold rolled steel sheet manufacturing method.

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