KR100401979B1 - MANUFACTURING METHOD OF 35 kg/mm¬2 CLASS HIGH TENSILE STRENGTH COLD ROLLED STRIP FOR DEEP DRAWING HAVING SUPERIOR WIRE BENDING PREVENTING CHARACTERISTICS - Google Patents

Abstract

PURPOSE: Provided is a manufacturing method of 35 kg/mm¬2 class high tensile strength cold rolled strip for deep drawing having superior wire bending preventing characteristics by properly designing steel component system and controlling process variables. CONSTITUTION: The method comprises the steps of heating an ultra-low carbon Al-killed steel slab comprising 0.005 wt.% or less of C, 0.55 to 0.65 wt.% of Mn, 0.015 wt.% or less of S, 0.065 wt.% or less of soluble Al, 0.004 wt.% or less of N, 0.03 to 0.05 wt.% of P, 0.04 to 0.06 wt.% of Ti and a balance of Fe and other inevitable impurities to the temperature range of 1,200 deg.C or more; hot finish rolling the heated steel slab in the temperature range of 900 to 920 deg.C that is directly above the Ar3 temperature; coiling the hot finish rolled strip in the temperature range of 500 to 570 deg.C; cold rolling the coiled strip at a reduction ratio of 60 to 80%; continuous annealing the cold rolled strip in the temperature range of 800 to 850 deg.C; and performing controlled rolling on the continuous annealed strip at a reduction ratio of 0.5 to 1.5%.

Description

Manufacturing method of high strength cold rolled steel sheet for deep processing of 35kg / mm2 class with excellent linear bending prevention

The present invention relates to a method for manufacturing a high-tension cold-rolled steel sheet for 35 kg / mm2 deep processing used in the interior and exterior of a vehicle, and more particularly, a method for manufacturing a high-strength cold rolled steel sheet for 35 kg / mm2 deep processing excellent in line bending protection It is about.

The development history of high strength cold rolled steel sheets used in the interior and exterior of automobiles has been changed with close relations with various laws and regulations surrounding the automobile industry such as automobile safety regulation, fuel economy regulation, and exhaust gas regulation law. For example, due to the petroleum surge in the 70s, fuel economy regulation has become a concern for automakers. With this trend, R & D has been accelerated and many kinds of high strength steel sheets have been developed.

On the other hand, typical steel grades of high strength steel for automotive and exterior plating that require workability include high tensile cold rolled steel for deep processing. The high strength steel for deep processing is extremely low carbon (≤50ppm) in order to completely fix solid elements and improve moldability. ) Is a cold rolled steel sheet having excellent strength characteristics by adding Ti, which is a carbon and nitride forming element, and a large amount of P. However, this steel sheet has a number of excellent characteristics, but the demand is a problem that the irregularities are generated on the surface of the steel sheet during press work, that is, the linear bending defects occur during processing. If such a defect occurs, it is impossible to apply to a board or a plate that strictly manages the surface, and there is a problem that demands of constant complaints and claims are raised.

Accordingly, the present invention has been proposed based on the results of the metallurgical research and experiments to solve the above problems, the present invention is excellent in the prevention of linear bending by controlling the appropriate design and process parameters of the steel component system To provide a high strength cold rolled steel sheet for deep processing 35kg / ㎡ class, its purpose is to provide.

1 is a slab structure photograph of the invention and comparative steel

Figure 2 is a cold rolled coil structure photograph of the invention steel and comparative steel

3 is a linear bending segregation zone in a cold rolled coil

The present invention for achieving the above object in the manufacturing method of high-strength cold-rolled steel sheet for 35kg / mm2 deep processing, in weight%, C: 0.005% or less, Mn: 0.55-0.65%, S: 0.015 or less. Sol-Al: 0.06% or less, N: 0.004% or less, P: 0.03-0.05%, Ti: 0.04-0.06%, residual Fe and other unavoidable impurities. After heating to the temperature range, hot-rolled at a temperature range of 900-920 ° C., which is directly above the Ar ₃ temperature, and then wound at a temperature range of 500-570 ° C., followed by cold rolling at a reduction ratio of 60-80%, then 800 The present invention relates to a method of manufacturing a high-strength cold rolled steel sheet for deep processing of 35 kg / mm2 deep cores, which is continuously annealed at a temperature range of -850 ° C., and then adjusted to a reduction ratio of 0.5-1.5%.

Hereinafter, the present invention will be described in detail.

As a result of analyzing the cause of the linear bending on the surface of the conventional steel sheet, it can be seen that the melting point added to secure the strength is due to the addition of a large amount of P, which will be described in detail as follows.

Various mechanical behaviors act on the solidification interface in the casting solidified during continuous casting.For example, bugging between continuous casting rolls, uneven alignment between casting rolls, bending and freezing of castings by casting bending rolls. Tensile and compressive stresses due to bending, etc., act on the slab, which causes stress unevenness and encourages internal cracks in the slab. At this time, due to the difference in solubility of the solute element as the cast is solidified, the solute element is discharged from the solid phase into the liquid phase and concentrated between the dendrites, and the molten steel containing the solute element promotes cracks and fills the inner cracks. Segregated within. The segregation of the infiltrated solute remains segregated even in the state of cold rolled coil, which is the final product, and the demand causes deformation unevenness during processing, causing linear bending defects. The solute elements include P, Ti, S and the like, of which P has the greatest influence.

Therefore, the present invention is characterized by constructing a steel component system such as adding Mn for strength correction while reducing P, which forms a segregation zone of the cast steel, while being a strength increase element. To this end, the C is at least 0.005%, and the yield strength is increased to induce processing defects, and also due to the increase in solid solution carbon, the yield point is elongated after the final annealing, so that a stretcher strain is generated. It is preferable to limit it to 0.005% or less, and the lower the carbon content, the better.

The S is an element included in the aggregates of P, Ti, and S that cause linear bending in the final cold rolled state, and the lower the range of components, the better. However, the S is preferably limited to 0.015% in consideration of steelmaking capability.

Al is a component added for deoxidation of steel, and if the amount is 0.06% or more, it causes material hardening. Therefore, the Al content is preferably limited to 0.06% or less.

The Ti serves to suppress the occurrence of stretch strain by lowering the yield strength and eliminating the yield point stretching by depositing solid elements, ie, C, N, and S with TiC, TiN, and TiS. In general, the stretcher strain generated during yield-strengthening after annealing increases in proportion to the amount of solid solution present in the steel. However, when the Ti content is less than 0.04%, the solid solution cannot be effectively precipitated. If there is a risk that the increase in strength due to the generation of a large amount of precipitates, it is preferable to limit the amount of Ti to 0.04% -0.06%.

The N is to inhibit the invasive element {111} texture, impair the workability, impede the growth of particles, and lower the elongation, so the lower the management is better, the lower the limit to 0.004% in order to minimize the aging phenomenon as an aging element Good to do.

P is a substitution type alloy element having the largest solidification effect, and acts as a main cause of linear bending, limiting the P component to 0.03% -0.05% in order to secure strength and prevent linear bending at the same time. It is desirable to. At this time, the content of P was adjusted to the component range by evaluating the relationship between the slab internal crack index, segregation index, linear bending tendency.

When Mn is 0.65% or more, the material hardens or deteriorates formability due to solid solution hardening of Mn. When Mn is less than 0.55%, strength is secured (tensile strength ≥ 35 kg / mm) and slab embrittlement and segregation that cause linear bending are caused. It is preferable to limit the content of Mn to 0.55-0.65% because S, which is a factor, cannot be sufficiently caught.

The slab continuously cast after dissolving in the converter with the composition as described above is heated at 1200 ° C where the austenite structure can be sufficiently homogenized, and then finish hot rolling at 900-920 ° C, which is directly above the Ar ₃ temperature. At this time, when the temperature of the hot finishing is less than the Ar ₃ temperature, it is rolled in the ferrite + perlite abnormal structure of the hot rolled coil, which causes abnormal coarse and accordingly causes the defect in the processing of the product. Since it is not good, it is preferable to finish-roll in the temperature range of 900-920 degreeC.

After hot rolling as described above, when the winding temperature is a high temperature, a large amount of scale may occur, which may cause a problem of surface quality, and the fraction of the {111} direction of the tissue may be lowered, which may cause deterioration of workability, but at a low temperature. In the case of working, since the structure increases the {111} direction fraction in the structure, it is preferable to limit the range to 530-570 ° C.

After winding under the above conditions, it is pickled by a conventional method and then subjected to continuous annealing after cold rolling at a reduction ratio of 60-80%. The annealing temperature is 800-850 where recrystallization is completed and sufficient ferrite grain growth can occur. ℃ is suitable. When the annealing temperature is higher than 850 ° C, workability is deteriorated due to high temperature annealing, which makes it difficult to control tension during continuous annealing or burner life is reduced.

After continuous annealing as described above, temper rolling is carried out at a reduction ratio of 0.5-1.5% for roughness and shape control.

Hereinafter, the present invention is specifically determined through examples.

Example

The steels of Table 1 were prepared according to hot rolling, cold rolling, and continuous annealing of Table 2, and the internal crack index, segregation index, linear bending press test results, and mechanical properties of each slab were evaluated, and the results are shown in Table 3 below. .

TABLE 1

TABLE 2

TABLE 3

As shown in Tables 1, 2 and 3, the lower the P component was, the lower the internal crack index and segregation index of the slab were, and the internal crack index and segregation index dropped sharply when the P content was less than 0.05%. It was found that the occurrence of linear bending was prevented. The inventive material (1-3) has a tensile strength (TS) of 35-40 kg / mm2 and exhibits an elongation (EI) of 39-41%, thereby making it possible to produce a cold rolled steel sheet capable of preventing linear bending with excellent moldability. I could see that. On the other hand, in the case of the comparative material (4) it is possible to prevent the linear bending due to the low content of P, it was impossible to secure the tensile strength (≥35kg / ㎜) to 33kg / ㎜. In the case of the comparative material (1-4), linear bending occurred due to high internal crack index and segregation index, which were outside the upper limit of P.

On the other hand, Fig. 1, Fig. 2, and Fig. 3 shows the segregation state of the cracks and cold-rolled steel sheet of the slab of the invention steel and the comparative steel, it can be seen that the cracks and segregation in the slab or cold rolled steel of the comparative steel was intensified.

As described above, the present invention can provide a 35kg / ㎜ class high tensile cold rolled steel sheet excellent in the prevention of linear bending through the steel component design of low P and high Mn content, the provided steel sheet can be applied to the interior and exterior of automobiles That has a useful effect.

Claims (1)

In the manufacturing method of high strength cold rolled steel sheet for 35kg / ㎡ class deep processing, By weight%, C: 0.005% or less, Mn: 0.55-0.65%, S: 0.015% or less, Sol-Al: 0.065% or less, N: 0.004% or less, P: 0.03-0.05%, Ti: 0.04-0.06% , Ultra-low carbon Al-killed steel slab composed of residual Fe and other unavoidable impurities is heated to a temperature range of more than 1200 ℃, and then hot-rolled at a temperature range of 900-920 ℃, which is directly above Ar ₃ , and then 500- After winding at a temperature range of 570 ° C, cold rolling at a reduction ratio of 60-80%, followed by continuous annealing at a temperature range of 800-850 ° C, followed by a controlled rolling at a reduction ratio of 0.5-1.5%. Manufacturing method of high strength cold rolled steel sheet for 35kg / mm2 deep processing with excellent bending resistance.