KR100896608B1 - Cold-Rolled Steel Sheet, Galvanized Steel Sheet Having High Yield Ratio and Yield Strength and Manufacturing Method Thereof - Google Patents

Abstract

In the present invention, C: 0.06 to 0.08%, Mn: 0.45 to 0.65%, Si: 0.07 to 0.13%, P: 0.03% or less, S: 0.01% or less, N: 0.005% or less, acid value Al: 0.02 It relates to a high yield ratio high strength cold rolled steel sheet, a hot dip galvanized steel sheet, an alloyed galvanized steel sheet and a method of manufacturing the same, characterized by consisting of ~ 0.05%, the remaining Fe and other unavoidable impurities.

When the high yield ratio high strength steel sheet is manufactured according to the present invention, expensive alloying elements are not added, thereby reducing the manufacturing cost of the steel sheet, and also reducing energy and logistics costs by eliminating the scarfing process. In addition, such a reduction in cost and yield strength variation can be reduced, and an appropriate yield strength and high yield ratio can be secured, thereby providing a steel sheet having a high value in the technical field.

High yield ratio, cold rolled steel sheet, cold rolled steel sheet, hot dip galvanized steel sheet, scarfing process

Description

High yield ratio high strength cold rolled steel sheet, hot dip galvanized steel sheet, alloyed galvanized steel sheet and manufacturing method thereof {Cold-Rolled Steel Sheet, Galvanized Steel Sheet Having High Yield Ratio and Yield Strength and Manufacturing Method Thereof}

The present invention relates to a steel sheet used as a structural member, such as automotive members, and a manufacturing method thereof, and more particularly, to a high yield ratio high strength cold rolled steel sheet, a hot dip galvanized steel sheet, an alloyed galvanized steel sheet and a yield strength of 260 MPa or more. It relates to a manufacturing method.

Recently, as the regulations on impact safety of automobiles spread, yield strength guarantee steel sheet is widely used for structural members such as members and light-worked parts to improve impact resistance of the vehicle body.

Yield strength guarantee steel is a steel plate designed to absorb collision energy in the event of a car collision. It is common that the yield strength (YS), that is, the yield ratio (YS / TS), is higher than the tensile strength (TS). This is because a material having a high strength is required along with an increase in the thickness of the steel sheet for the improvement of the impact resistance that effectively absorbs the impact energy generated during the automobile crash.

Cold rolled steel sheet known in the prior art in the field of such high strength high yield ratio steel, C: 0.06 to 0.08%, Mn: 0.45 to 0.55%, Si: 0.03% or less, P: 0.015 to 0.025% or less, S: 0.015% Or less, N: 0.005% or less, acid value Al: 0.02 to 0.05%, Mo: 0.04 to 0.05%, B: 5 to 15 ppm, and a steel slab composed of other inevitable impurities, and producing the steel slab at 1140 to 1200 ° C. Heated to 860-900 ° C. and hot-rolled, then wound in a temperature range of 580-620 ° C., cold rolled to 60% or more to produce a cold rolled steel sheet, and a temperature of 760-810 ° C. It may be produced by recrystallization annealing in the range and temper rolling at a reduction ratio of 1.0 to 1.6%.

According to the above-described prior art, it is possible to obtain a certain yield strength, but a failure rate of less than 260 MPa for the general yield strength reaches a minimum of 80%, and the slab corner cracks may occur due to the added Mo and B. An additional process that requires separate round scarfing of the corners of the slab is required, which is a problem for productivity. Furthermore, Mo and B are expensive alloy elements, so there is a disadvantage that high cost is required.

Therefore, there is a need for a stable and economically manufactured automobile steel sheet that satisfies the yield strength of 260 MPa while satisfying the component range defined by automobile companies around the world.

The present inventors conducted research and experiments to improve the problems of the prior art, and based on the results, the present invention is proposed, the present invention can omit a separate slab scarfing process, yield strength deviation An object of the present invention is to provide a high strength cold rolled steel sheet, a hot dip galvanized steel sheet, an alloyed galvanized steel sheet having a low yield strength of 260 MPa or more, and a method of manufacturing the same.

In the present invention, C: 0.06 to 0.08%, Mn: 0.45 to 0.65%, Si: 0.07 to 0.13%, P: 0.03% or less, S: 0.01% or less, N: 0.005% or less, acid value Al: It relates to a high yield ratio high strength cold rolled steel sheet, characterized by consisting of 0.02 ~ 0.05%, the remaining Fe and other unavoidable impurities.

Furthermore, the present invention, the heating step of heating the steel slab composed of the above components to 1150 ~ 1190 ℃, hot finishing rolling step of hot finishing rolling at 860 ~ 910 ℃, cold winding step of winding in the temperature range of 540 ~ 580 ℃ High yield ratio comprising a cold rolling step of rolling at a reduction rate of 60% or more, a recrystallization annealing step of recrystallization annealing at a temperature range of 770-830 ° C. and a temper rolling step of rolling at a reduction rate of 1.2-1.6% It relates to a method for producing a high strength cold rolled steel sheet.

Furthermore, in the present invention, C: 0.06 to 0.08%, Mn: 0.45 to 0.65%, Si: 0.07 to 0.13%, P: 0.03% or less, S: 0.01% or less, N: 0.005% or less, acid value Al : High yield ratio high strength hot dip galvanized steel sheet, characterized in that the hot-dip galvanized layer is formed on a cold rolled steel sheet containing 0.02 ~ 0.05%, the remaining Fe and other unavoidable impurities.

Furthermore, the present invention, the heating step of heating the steel slab composed of the above components to 1150 ~ 1190 ℃, hot finishing rolling step of hot finishing rolling at 860 ~ 910 ℃, cold winding step of winding in the temperature range of 540 ~ 580 ℃ , Cold rolling step of rolling at a reduction rate of 60% or more, recrystallization annealing step of recrystallization annealing at a temperature range of 770-830 ° C., temper rolling step rolling at a reduction rate of 1.2-1.6%, and hot dip galvanizing It relates to a method of producing a high yield ratio high strength hot dip galvanized steel sheet comprising a step.

Furthermore, the present invention is C: 0.06 to 0.08%, Mn: 0.45 to 0.55%, Si: 0.07 to 0.13%, P: 0.015 to 0.025%, S: 0.01% or less, N: 0.005% or less, acid value Al It relates to an alloyed hot-dip galvanized steel sheet, characterized in that hot-dip galvanized and alloyed on a cold rolled steel sheet containing 0.02% to 0.05% and the remaining Fe and other unavoidable impurities.

Furthermore, the present invention, the heating step of heating the steel slab composed of the above components to 1150 ~ 1190 ℃, hot finishing rolling step of hot finishing rolling at 860 ~ 910 ℃, cold winding step of winding in the temperature range of 540 ~ 580 ℃ , Cold rolling step of rolling at a reduction rate of 60% or more, recrystallization annealing step of recrystallization annealing at a temperature range of 770-830 ° C., temper rolling step rolling at a reduction rate of 1.2-1.6%, hot dip galvanizing It relates to a method of producing a high yield ratio high strength hot dip galvanized steel sheet comprising a step and an alloying heat treatment step of heating the surface temperature of the hot-dip galvanized steel sheet to 450 ~ 550 ℃.

However, the cold rolled steel sheet disclosed herein refers to a steel sheet produced only through a heating step, a hot finishing rolling step, a low temperature winding step, a cold rolling step, and a cold rolled steel sheet refers to recrystallization annealing of the cold rolled steel sheet and The steel sheet produced through the step of temper rolling is classified as meaning.

Hereinafter, the present invention will be described in detail.

The present invention improves the yield strength by utilizing the solidification effect that is added when adding 0.07 ~ 0.13% of Si without adding Mo and B, and high strength cold rolled steel sheet which can reduce the manufacturing cost when manufacturing ferroalloy slabs And a hot dip galvanized steel sheet, an alloyed galvanized steel sheet, and a manufacturing method thereof.

When the B component is added, the embrittlement temperature range is enlarged, and cracks may be generated at the corners of the slab when the slab surface temperature is included in the uncomfortable temperature range in the bending to unbending section when the slab is cast. Therefore, when the slab is extracted from a heating furnace and hot rolled, a scarfing process is required at the edge of the slab in order to prevent edge defects from occurring at the edge of the hot rolled material.

In the present invention, however, the B component is not added so that the scarfing process can be omitted, and at the same time, 0.07 to 0.13% of Si was added to compensate for the decrease in strength due to the absence of the B component.

In addition, the steel slab is heated to 1150 ~ 1190 ℃, hot-rolled at 860 ~ 910 ℃, wound in a temperature range of 540 ~ 580 ℃, then cold rolled to a rolling reduction of 60% or more, 770 ~ 830 ℃ By recrystallization annealing in the temperature range of can provide a high yield ratio high strength cold rolled steel sheet with a yield strength of 260MPa or more, and a manufacturing method thereof.

In addition, a high yield ratio high strength hot dip galvanized steel sheet having a yield strength of 260 MPa or more that satisfies a high yield ratio and has high strength through a process of temper rolling and hot dip galvanizing at a rolling reduction ratio of 1.2 to 1.6%, and a method of manufacturing the same. It may be, to provide an alloyed hot dip galvanized steel sheet prepared by alloying it.

Hereinafter, the reason for limiting the component system of the present invention will be described in detail.

In the present invention, C plays an important role as a precipitate forming element. If the content of C is less than 0.06%, a sufficient precipitation effect cannot be obtained, whereas if the content of C is more than 0.08%, it is difficult to manufacture the slab because it is included in the crack sensitive region where cracking of the cast is more likely to occur in the casting process. Therefore, the content of C is limited to 0.06 to 0.08%.

Mn precipitates S in MnS in steel and plays an important role in suppressing plate breakage and high temperature embrittlement caused by S during hot rolling. Furthermore, in the present invention, Mn serves to improve strength by solid solution strengthening. If the Mn content is less than 0.45%, it is difficult to secure the desired strength. If the Mn content exceeds 0.55%, the desired strength is secured, but the elongation decreases rapidly. Therefore, it is not good for surface cleanliness and oxidation resistance. Therefore, Mn is added by 0.45 to 0.55%.

Furthermore, when the weight ratio of Mn and Si, that is, Mn / Si is 4 or more, the plating workability of the alloyed hot-dip galvanized steel sheet becomes good and unplating does not occur. Therefore, the ratio of Mn and Si is properly adjusted.

Si is a component that improves strength through deoxidation and solid solution strengthening. If the content is less than 0.07%, the effect of improving strength cannot be obtained, whereas if it exceeds 0.13%, the plating property may deteriorate, so Si is 0.07. Add 0.13%.

P corresponds to an element that secures strength without significantly deteriorating formability. Increasing the P content in the steel is very advantageous for the strength increase, but if the P content exceeds 0.025%, the possibility of brittle fracture at room temperature is increased, which increases the possibility of plate break during hot rolling of the slab. Since the diffusion and segregation into the grain boundary becomes easy, the problem of the occurrence of secondary processing brittleness during the molding becomes large. On the other hand, when the P content is less than 0.015%, it is difficult to secure the strength. Therefore, the amount of P is limited to 0.015 to 0.025%.

S and N are inevitably added as impurities in the steel, so it is desirable to manage them as low as possible. However, the lower the content is managed, the higher the refining cost, so considering the economic feasibility, the S content is adjusted to 0.01% or less and N content is 0.005% or less.

Al is an element added for deoxidation. If the content is less than 0.02%, the effect of addition is not sufficient, and it is disadvantageous to secure the welding strength during welding after forming the steel sheet, and if it exceeds 0.05%, Due to the excessive formation of the steel sheet surface defects, alloying and hot-dip galvanized steel sheet surface quality and increase in manufacturing cost, the content is limited to 0.02 ~ 0.05%.

Hereinafter, the reason for limitation of the manufacturing conditions of the present invention will be described in detail.

The steel sheet of the present invention is produced by hot rolling, cold rolling and recrystallization annealing of steel slabs composed of the above-described component system. The recrystallization annealing may be provided by a continuous annealing treatment of the cold rolled steel sheet, it may be provided as a galvanized steel sheet or an alloyed galvanized steel plate after the plating.

First, in the present invention, after heating the steel slab composed of the above-described components to 1150 ~ 1190 ℃ hot-rolled and rolled at 870 ~ 910 ℃, and wound in a temperature range of 540 ~ 560 ℃ to produce a hot rolled steel sheet. The hot rolled steel sheet thus prepared is subjected to pickling as necessary, followed by cold rolling at a reduction ratio of 60% or more, and then to a cold rolled steel sheet through recrystallization annealing heat treatment and temper rolling.

Further, the cold rolled steel sheet is subjected to hot dip galvanizing at a temperature of 440 to 490 ° C. in a hot dip galvanizing bath containing Al: 0.12 to 0.23%, Fe: 0.04% or less, and Pb: 0.01% or less. Plated steel sheet can be produced. Furthermore, the steel sheet surface temperature immediately after the hot-dip galvanized steel sheet is manufactured to be heated to 480 to 550 ° C., thereby adding an alloying heat treatment to produce a high yield ratio galvanized steel sheet.

In the manufacturing process of the present invention, if the extraction temperature of the furnace is less than 1150 ° C., the rolling load may occur during hot-rolled finish rolling, and if it exceeds 1190 ° C., the scale defect may occur on the surface of the hot-combusted material, so the heating temperature is 1150 to 1190 ° C. Adjust with

In addition, when the hot finishing rolling temperature is less than 870 ℃, rolling load may occur, and if it exceeds 910 ℃, the surface defects such as scale may occur in the hot-combusted material, and also the particles are uneven and coarsened to produce the final product The strength of the steel may be lower than the appropriate level, so the hot finishing rolling temperature should be appropriately controlled.

In addition, due to the limitation of the coiling temperature, the solid solution C may not be completely precipitated at the coiling temperature of less than 540 ° C., thus reducing the precipitation strengthening effect, resulting in a problem of poor plate shape during the coiling, and the coiling temperature hit ratio. . On the other hand, if the coiling temperature exceeds 580 ° C, the precipitate tends to coarsen, and the precipitation strengthening effect is insufficient and the yield ratio may be lowered.

In addition, when hot-rolled low temperature winding is performed at 540 ~ 580 ℃, it is advantageous to increase yield strength and decrease yield strength deviation between tensile test coil length direction and width direction by miniaturization of ferrite particles, which satisfies the requirements of automobile companies around the world. Do.

The coiled steel sheet is cold rolled with a reduction ratio of 60% or more. When the cold reduction ratio is less than 60%, sufficient precipitates are not formed due to the reduction of the critical nucleation site for nucleation of precipitates, and the yield is increased due to the recrystallization temperature. The variation in intensity may increase.

Recrystallization annealing conditions are performed in the range of 770 ~ 830 ℃ by product, thickness group as shown in Table 1. Recrystallization annealing in this range can reduce the failure and deviation of yield strength. If the annealing temperature is lower than the unrecrystallized zone temperature of 770 ° C, the material variation in the coil may increase, which may exceed the appropriate yield strength. On the other hand, if the temperature exceeds 830 ° C, the precipitation coarsening tends to be sufficient due to the high temperature annealing. As the precipitation effect does not appear, a defect of less than the yield strength occurs.

product Thickness group (mm) Annealing Temperature (℃) Yield strength 260Mpa guaranteed cold rolled steel sheet, hot dip galvanized steel sheet T <0.8 800-830 0.8≤T <1.2 790-820 1.2≤T <1.8 780-810 1.8≤T <2.31 770-800 Yield strength 260Mpa guaranteed alloyed hot dip galvanized steel sheet T <0.8 800-830 0.8≤T <1.2 790-820 1.2≤T <1.8 780-810 1.8≤T <2.31 780-810

The annealing temperature range of Table 1 is similar to that of cold rolled steel and hot dip galvanized steel, but alloyed hot dip galvanized steel, but in the case of a thick steel sheet having a thickness of more than 1.81, in the case of alloyed hot dip galvanized steel considering the alloying process Only slightly increase the temperature is more efficient.

The rolling reduction ratio of temper rolling is 1.2 ~ 1.6%. If it is less than 1.2%, yield point extension shape occurs, causing the problem of exceeding the maximum criterion of yield strength. The problem is limited to 1.2 ~ 1.6% due to difficulties such as difficulty and elongation.

Further, the cold rolled steel sheet is molten zinc plated at a temperature of 440 to 490 ° C. in a hot dip galvanizing bath containing Al: 0.12 to 0.23%, Fe: 0.04% or less, and Pb: 0.01% or less, and then melted. Galvanized steel sheet can be produced. Further, by heating the surface temperature of the steel sheet immediately after the hot-dip galvanized steel sheet to 480 ~ 550 ℃ alloying heat treatment to produce a galvanized steel sheet. In this case, if the temperature of the hot dip galvanizing and alloying heat treatment is too low, the treatment rate is lowered, while if too high, the economical efficiency is lowered.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Each steel slab including the components of Table 2 was subjected to hot rolling, cold rolling, hot dip galvanizing and temper rolling under the same conditions as in Table 3 below, and mechanical properties were measured. Table 4 shows the conditions and measured results of each test.

division Chemical composition (% by weight) C Mn Si P S S-Al N (ppm) Mo B (ppm) Comparative steel component 0.07 0.50 Tr 0.20 0.010 0.035 40 0.04 10 Invention steel components 0.07 0.50 0.10 0.20 0.007 0.035 40 Tr Tr

division Cold rolled thickness (mm) Furnace Extraction Temperature (℃) Finish rolling temperature (℃) Hot rolled coiling temperature (℃) Annealing Temperature (℃) Temper reduction rate (%) CR 1 0.7 1170 890 560 815 1.4 CR 2 1.35 1170 890 560 795 1.4 CG 3 1.1 1170 890 560 800 1.4 CG 4 1.4 1170 890 560 800 1.4 CG 5 1.8 1170 890 560 790 1.4

Cold rolled annealing steel: CR, high yield ratio cold rolled steel sheet: CG

The values in Table 4 below are values representing mechanical properties for the specimens in which the tensile test specimen is 90 degrees to the rolling direction.

division  product Thickness (mm) Yield strength (Mpa) Tensile Strength (Mpa) Elongation (%)  High yield ratio Yield Strength Performance (260 ~ 310) Deviation Tensile Strength Performance (360 ~ 410) Deviation Elongation Performance (30 or more) Deviation Comparative steel CR 1 0.70 255 (bad) 19.0 378 8.2 40 2.3 dissatisfaction CR 2 1.35 238 (bad) 371 38 CG 1 1.0 259 (bad) 17.3 376 7.8 37 2.0 dissatisfaction CG 2 1.5 254 (bad) 385 37  Invention steel CR 1 0.7 289 (good) 14.3 404 4.4 38 0.6 satisfied CR 2 1.35 275 (good) 392 37 CG 3 1.1 297 (good)  13.4 395  5.0 37  1.2  satisfied CG 4 1.4 303 (good) 405 39 CG 5 1.8 296 (good) 403 38

As shown in Table 4, the inventive steel that meets the conditions of the present invention can be seen that not only high yield strength but also low yield strength deviation, showing a high yield ratio.

When manufacturing a steel sheet without the addition of Mo and B by the present invention, the production cost of the steel sheet is reduced by not adding expensive alloying elements, and energy and logistics costs are also reduced by eliminating the slab scarfing process. Furthermore, with such a reduction in costs, the yield strength variation is reduced, and an appropriate yield strength and high yield ratio can be secured, thereby providing a steel sheet having high utility in the art.

Claims (12)

delete delete By weight% C: 0.06 ~ 0.08%, Mn: 0.45 ~ 0.55%, Si: 0.07 ~ 0.13%, P: 0.015 ~ 0.025%, S: 0.01% or less, N: 0.005% or less, acid value Al: 0.02 ~ 0.05 Steel slab containing%, remaining Fe and other unavoidable impurities, A heating step of heating to 1150 to 1190 ° C; Hot finishing rolling step of hot finishing rolling at 860 ~ 910 ℃; Cold winding step of winding in the temperature range of 540 ~ 580 ℃; Cold rolling step of rolling at a rolling reduction of at least 60%; Recrystallization annealing step of recrystallization annealing at a temperature range of 770 ~ 830 ℃; And A temper rolling step of rolling at a rolling reduction of 1.2 to 1.6%; Method for producing a high yield ratio high strength cold rolled steel sheet comprising a. The method of claim 3, wherein the temperature of the recrystallization annealing step is relative to the thickness T of the steel sheet, 800 to 830 ° C. for T <0.8 mm; 790 ° C.-820 ° C. when 0.8 mm ≦ T <1.2 mm; 780 to 810 ° C. for 1.2 mm ≦ T <1.8 mm; And 770 to 800 ° C. for 1.8 mm ≦ T <2.31 mm; Method for producing a high yield ratio high strength cold rolled steel sheet, characterized by satisfying the conditions of. delete delete By weight% C: 0.06 ~ 0.08%, Mn: 0.45 ~ 0.55%, Si: 0.07 ~ 0.13%, P: 0.015 ~ 0.025%, S: 0.01% or less, N: 0.005% or less, acid value Al: 0.02 ~ 0.05 Steel slab containing%, remaining Fe and other unavoidable impurities, A heating step of heating to 1150 to 1190 ° C; Hot finishing rolling step of hot finishing rolling at 860 ~ 910 ℃; Cold winding step of winding in the temperature range of 540 ~ 580 ℃; Cold rolling step of rolling at a rolling reduction of at least 60%; Recrystallization annealing step of recrystallization annealing at a temperature range of 770 ~ 830 ℃; A temper rolling step of rolling at a rolling reduction of 1.2 to 1.6%; And Hot dip galvanizing step of hot dip galvanizing; Method for producing a high yield ratio high strength hot dip galvanized steel sheet comprising a. The method of claim 7, wherein the temperature of the recrystallization annealing step is relative to the thickness T of the steel sheet, 800 to 830 ° C. for T <0.8 mm; 790 ° C.-820 ° C. when 0.8 mm ≦ T <1.2 mm; 780 to 810 ° C. for 1.2 mm ≦ T <1.8 mm; And 770 to 800 ° C. for 1.8 mm ≦ T <2.31 mm; Method for producing a high yield ratio high strength hot-dip galvanized steel sheet, characterized by satisfying the conditions of. delete delete By weight% C: 0.06 ~ 0.08%, Mn: 0.45 ~ 0.55%, Si: 0.07 ~ 0.13%, P: 0.015 ~ 0.025%, S: 0.01% or less, N: 0.005% or less, acid value Al: 0.02 ~ 0.05 Steel slab containing%, remaining Fe and other unavoidable impurities, A heating step of heating to 1150 to 1190 ° C; Hot finishing rolling step of hot finishing rolling at 860 ~ 910 ℃; Cold winding step of winding in the temperature range of 540 ~ 580 ℃; Cold rolling step of rolling at a rolling reduction of at least 60%; Recrystallization annealing step of recrystallization annealing at a temperature range of 770 ~ 830 ℃; A temper rolling step of rolling at a rolling reduction of 1.2 to 1.6%; Hot dip galvanizing step of hot dip galvanizing; And An alloying heat treatment step of heating the surface temperature of the hot-dip galvanized steel sheet to 450˜550 ° C .; Method for producing an alloyed galvanized steel sheet comprising a. The method of claim 11, wherein the temperature of the recrystallization annealing step is relative to the thickness T of the steel sheet, 800 to 830 ° C. for T <0.8 mm; 790 ° C.-820 ° C. when 0.8 mm ≦ T <1.2 mm; 780 to 810 ° C. for 1.2 mm ≦ T <1.8 mm; And 780 to 810 ° C. for 1.8 mm ≦ T <2.31 mm; Method for producing an alloyed galvanized steel sheet, characterized in that to satisfy the conditions of.