KR20060022540A - Bake hardening cold rolled steel sheet having superior workability and process for producing the same - Google Patents

Abstract

The present invention relates to a hardened hardened cold rolled steel sheet used as a material for automobiles, home appliances, and the like. The cold rolled steel sheet is, by weight, C: 0.003-0.005%, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less, P: 0.015% or less, Mo: 0.01-0.2%, wherein the weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S ≦ 10, It is composed of the remaining Fe and other unavoidable impurities, the average size of the MnS precipitate is less than 0.2㎛. In addition, a method for producing this cold rolled steel sheet is also provided.

In the cold rolled steel sheet of the present invention, by controlling the content of carbon in the steel in which fine MnS is precipitated, the baking hardening property is secured, and the addition of Mo results in a high plastic anisotropy index and a low in-plane anisotropy index.

Cold Rolled Steel, Hardening Hardening, Mo, Plastic Anisotropy Index, MnS

Description

 BAKE HARDENING COLD ROLLED STEEL SHEET HAVING SUPERIOR WORKABILITY AND PROCESS FOR PRODUCING THE SAME}

1 is a graph showing the change in the amount of solid solution carbon in the grain according to the size of the MnS precipitate,

2 is a graph showing the size of the MnS precipitates according to the cooling rate.

The present invention relates to a hardening hardened cold rolled steel sheet used as a material for automobiles, home appliances, etc. More specifically, by controlling the content of fine MnS precipitates and carbon to secure the hardening hardening properties, cold rolling is excellent by the addition of Mo It relates to a steel sheet and a method of manufacturing the same.

In order to improve the dent resistance, exterior hardening type cold rolled steel sheets are frequently used for exterior materials such as automobiles. The hardened hardened cold rolled steel is a steel in which a yield of solid solution carbon is retained in the steel sheet and the dislocation generated during press molding is fixed by using solid heat of the coating furnace to increase the yield point.

There are Al-Killed steel and IF steel (Interstitial Free Steel).

In the case of Al-Killed steel, which is an ordinary annealing material, a small amount of solid carbon remains, and it has a hardening hardening capacity of about 10 to 20 Mpa after the quenching treatment while securing aging characteristics. In the case of the annealing material, the yield strength rising after the baking treatment is low, and there is a disadvantage in that the productivity is low because it is annealed for a long time.

In the case of IF steel, Ti and Nb are added to completely precipitate the carbon or nitrogen dissolved in the steel to improve moldability. The hardening hardening characteristic is given to the IF steel by hardening. The baking hardening type IF steel controls the adding amount of Ti or Nb and the adding amount of carbon so that an appropriate amount of carbon remains in the steel to give the baking hardening characteristic. In the case of small hardening IF steel, in order to employ an appropriate amount of carbon, not only the amount of carbon added, but also the amount of Ti or Nb added, as well as the amount of sulfur and nitrogen that react with Ti and Nb to form precipitates are controlled within a very narrow range. Because of this, it is difficult to secure stable quality and costs a lot of production.

The present invention provides a cold rolled steel sheet having a bake hardening property and excellent workability without adding Ti and Nb, and an object thereof.

Cold rolled steel sheet of the present invention for achieving the above object, by weight% C: 0.003-0.005%, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less, P: 0.015% or less, Mo: 0.01% to 0.2%, the weight ratio of Mn and S satisfies the following conditions 0.58 * Mn / S ≤ 10, and is composed of the remaining Fe and other unavoidable impurities, MnS precipitate The average size is made less than 0.2㎛.

In addition, the manufacturing method of the cold rolled steel sheet of this invention is C: 0.003-0.005%, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less by weight%, : 0.015% or less, Mo: 0.01% to 0.2%, the weight ratio of Mn and S satisfy the following conditions 0.58 * Mn / S ≤ 10, the steel is composed of the remaining Fe and other unavoidable impurities 1100 ℃ or more After reheating to a temperature, hot rolling is performed at a finish rolling temperature of at least Ar ₃ transformation point, cooled at a rate of 200 ° C./min or more, wound at a temperature of 700 ° C. or lower, and then cold rolled and continuously annealed.

Hereinafter, the present invention will be described in detail.

The present inventors have discovered the following new facts in the course of research to improve the baking hardening properties without adding Ti and Nb. The fine distribution of MnS precipitates while controlling the carbon content to an appropriate amount basically increases the yield strength and increases the yield strength after baking, which means that the fine MnS precipitates affect the solid solution carbon. In addition, when Mo is added to the steel sheet in which the fine MnS precipitates are distributed, the plastic anisotropy index is increased and the in-plane anisotropy index is lowered, thereby significantly improving workability.

As shown in FIG. 1, the finer the precipitate of MnS, the smaller the amount of solid solution carbon in the grains. If the carbon content is 0.003 ~ 0.005% in the steel with finely distributed MnS precipitates, it will have a hardening characteristic. This suggests that when the MnS precipitates are minutely distributed, carbon precipitates around the MnS precipitates, which does not cause aging at room temperature, but dissolves the carbon in the coating sub-heat treatment to significantly increase the yield strength.

For this purpose, the content of carbon should be adjusted to 0.003 ~ 0.005%, the size of the fine precipitate of MnS was determined to be less than 0.2㎛.

Paying attention to these new facts, we have studied how to finely distribute MnS. As a result, (1) it is necessary to adjust the content ratio (0.58 * Mn / S) of 10 or less while (2) Mn content of 0.05 to 0.2% and S content of 0.005 to 0.03%. In addition, when the cooling rate is 200 ℃ / min or more after the end of the rolling rolling, it is possible to obtain a fine MnS precipitate of 0.2 ㎛ or less.

That is, Figure 2 (a) is a steel of 0.004% C-0.15% Mn-0.008% P-0.015% S-0.03% Al-0.0012% N-0.05% Mo and the component ratio of Mn and S (0.58 * Mn / S) It is a graph that investigates the size of precipitates according to the cooling rate after hot rolling steel with composition of 5.8. Looking at the graph of Figure 2 (a), the Mn and S component ratio (0.58 * Mn / S) when the cooling rate is adjusted for the case of less than 10 confirms that the precipitate size of MnS can satisfy 0.2㎛ or less Can be.

Based on this aspect, the cold rolled steel sheet of the present invention and a method of manufacturing the same will be described in detail below.

[Cold rolled steel sheet of the present invention]

The content of carbon (C) is preferably 0.003% to 0.005%.

If the content of carbon is less than 0.003%, the amount of hardening of baking in steel is small, and if the content of carbon is more than 0.005%, the moldability decreases rapidly.

The content of manganese (Mn) is preferably 0.05-0.2%.

Manganese is known as an element that precipitates solid sulfur in steel as MnS to prevent hot shortness caused by solid sulfur. In the present invention, when the content of manganese and sulfur is appropriate, very fine MnS is precipitated, and carbon is precipitated around the MnS precipitate, and the precipitated carbon is dissolved in the coating baking treatment to give baking hardening capacity. It is preferable to make content of manganese into 0.05 to 0.2%. If the content of manganese is less than 0.05%, red brittleness may occur due to the large amount of sulfur remaining in solid solution. If the content of manganese is more than 0.2%, coarse MnS precipitates are formed due to high content of manganese. Curing properties are poor.

The content of sulfur (S) is preferably 0.005-0.03%.

When the content of sulfur (S) is less than 0.005%, not only the amount of MnS precipitates but also the size of precipitated MnS is very coarse, which is not good for baking hardening characteristics. If the content of sulfur is more than 0.03%, the content of solute is high so that the ductility and moldability is greatly lowered, there is a fear of red brittleness. When the content of sulfur is in the range of 0.005 ~ 0.03%, it becomes easy to adjust the precipitate size of MnS to the desired range.

The content of aluminum (Al) is preferably 0.01-0.1%.

Aluminum is an element added as a deoxidizer, but in the present invention, nitrogen is added in order to prevent formability deterioration due to solid nitrogen by precipitating nitrogen in steel. If the aluminum content is less than 0.01%, the amount of solid solution is high because the amount of solid solution decreases, and when the aluminum content is more than 0.1%, the amount of aluminum in the solid solution state is high and the ductility is lowered.

The content of nitrogen (N) is preferably 0.004% or less.

Nitrogen is an element inevitably added during steelmaking, and in the case of more than 0.004%, the moldability is lowered, so 0.004% or less is preferable.

The content of phosphorus (P) is preferably 0.015% or less.

If the content of phosphorus is more than 0.015%, the ductility and moldability are lowered, it is preferable to be 0.015% or less.

The content of molybdenum (Mo) is preferably 0.01 to 0.2%.

Molybdenum is added to ensure the processability, the content is more than 0.01% to improve the processability, when the content exceeds 0.2% there is a fear that the workability is no longer improved and cause hot brittleness.

The weight ratio of Mn and S preferably satisfies 0.58 * Mn / S ≦ 10.

Manganese and sulfur combine to precipitate as MnS, which affects the hardening hardening ability, yield strength, and in-plane anisotropy index, depending on the amount of manganese and sulfur added. According to the present invention, when the addition ratio of manganese and sulfur (0.58 * Mn / S, where the content of Mn, S in weight%) is more than 10, MnS precipitates are coarse, and the hardening hardening property is lowered, yield strength, in-plane Anisotropy index is not good.

The average size of the MnS precipitates is preferably 0.2 μm or less.

According to the results of the present invention, the size of MnS precipitate directly affects the hardening hardening, yield strength and in-plane anisotropy index. Especially, when the average size of MnS is more than 0.2 ㎛, the hardening hardening characteristic is rapidly dropped and the in-plane anisotropic index is lower. Also increases. Therefore, the average size of the MnS precipitates is preferably 0.2 μm or less.

[Manufacturing method of cold rolled steel sheet]

The present invention is characterized in that an average size of MnS precipitates in a cold rolled sheet is hot and cold rolled to satisfy the above-described stress. The size of MnS precipitates in the cold rolled plate is affected by the ratio of Mn / S and the manufacturing process, but in particular by the cooling rate after hot rolling.

[Hot Rolling Condition]

In the present invention, the steel that satisfies the above-mentioned emphasis is reheated and hot rolled. The reheating temperature is preferably 1100 ° C or more. If the reheating temperature is lower than 1100 ℃, the reheating temperature is low, the coarse MnS produced during the continuous casting is not completely dissolved, the coarse MnS remains even after hot rolling.

Hot rolling is preferably performed at a finish rolling temperature above Ar ₃ transformation temperature. This is because when the finish rolling temperature is lower than the Ar ₃ transformation temperature, not only the workability is degraded due to the formation of the rolled grain but also the ductility is greatly reduced.

After hot rolling, the cooling rate before winding is preferably 200 ° C / min or more. According to the present invention, even if the component ratio (0.58 * Mn / S) of Mn and S is 10 or less, the precipitate size of MnS exceeds 0.2 µm when the cooling rate is less than 200 ° C / min. In other words, as the cooling rate increases, a large number of nuclei are generated and MnS precipitates become finer. In the case where the composition ratio of Mn and S (0.58 * Mn / S) is more than 10, there are many coarse MnS precipitates unresolved in the reheating process, and even if the cooling rate is fast, new nuclei are generated less and the precipitates do not become fine (Fig. 2b). , 0.0038% C-0.43% Mn-0.011% P-0.009% S-0.035% Al-0.0043% N-0.05% Mo). Referring to the graph of Figure 2, the faster the cooling rate is the size of the MnS precipitate becomes finer, so there is no need to limit the upper limit of the cooling rate, even if the cooling rate is more than 1000 ℃ / min cooling rate is no longer increased 200-1000 degreeC / min is more preferable.

[Coiling condition]

Winding is performed after hot rolling as above, but the winding temperature is preferably 700 ° C or lower. If the coiling temperature exceeds 700 ℃ MnS precipitates grow too coarse, so the hardening hardening characteristics are not good.

[Cold rolling condition]

Cold rolling is preferably performed at a reduction ratio of 50 to 90%. If the cold reduction rate is less than 50%, the amount of nucleation of the annealing recrystallization is small, so that grains grow too large during annealing, resulting in a decrease in strength and formability due to coarsening of the annealing recrystallization grains. If the cold reduction ratio is more than 90%, the moldability is improved, but the nucleation amount is too high, so the annealing recrystallized grain is too fine to decrease the ductility.                     

[Continuous Annealing]

Continuous annealing temperature plays an important role in determining the material of the product. In this invention, it is preferable to carry out in the temperature range of 500-900 degreeC. If the continuous annealing temperature is less than 500 ° C., the recrystallized grain is too fine to obtain a target ductility value. If the annealing temperature is higher than 900 ° C., the strength decreases due to coarsening of the recrystallized grain. The continuous annealing time keeps the recrystallization complete. If it is about 10 seconds or more, the recrystallization is completed.

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

EXAMPLE

The steel slabs of Table 1 were reheated, rolled up after finishing hot rolling, and then cold-rolled and continuous annealing at a reduction ratio of 75%. The finish rolling temperature of not less than Ar ₃ transformation point is 910 ℃, continuous annealing was performed by heating for 40 seconds to 750 ℃ to 10 ℃ / second. The obtained annealing plate was processed into a standard specimen according to ASTM E-8 standard to investigate the mechanical properties. The specimen was measured for yield strength, tensile strength, elongation, plastic anisotropy index (r _m value) and in-plane anisotropy index (Δr) using a tensile tester (INSTRON, Model 6025). Where r _m = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4 and Δr = (r ₀ −2r ₄₅ + r ₉₀ ) / 2. The quench hardening properties are shown in Table 2 as the yield strength measured after quenching after 20% heat treatment at 200 ° C. after adding 2% of the strain to the specimen.

sample Chemical composition (% by weight) 0.58 * Mn / S Reheating Temperature (℃) Cooling rate (℃ / min) Coiling temperature (℃) C Mn P S Al N Mo Ti Scope of the invention 0.003-0.005 0.05-0.2 ≤0.015 0.005-0.03 0.01-0.1 ≤0.004 0.01-0.2 - One 0.0042 0.11 0.012 0.01 0.032 0.0018 0.02 - 6.38 1200 200 650 2 0.0038 0.1 0.01 0.008 0.035 0.0025 0.16 - 7.25 1200 200 650 3 0.0045 0.08 0.011 0.011 0.04 0.0011 0.064 - 4.22 1200 200 650 4 0.0044 0.08 0.01 0.01 0.025 0.003 0.092 - 4.64 1200 200 650 5 0.0046 0.09 0.012 0.012 0.04 0.0015 0.27 - 4.35 1200 200 650 6 0.0035 0.1 0.01 0.009 0.04 0.0021 - - 6.44 1200 200 650 7 0.0041 0.10 0.009 0.012 0.05 0.0032 - - 4.83 1200 200 650 8 0.0038 0.08 0.011 0.012 0.035 0.0041 - - 3.87 1200 200 650 9 0.0044 0.1 0.01 0.006 0.02 0.0039 - - 9.67 1200 200 650 10 0.0024 0.4 0.07 0.01 0.04 0.0016 - 0.02 11.6 1200 200 650

Sample Number Mechanical properties Average size of precipitates (㎛) Remarks Yield strength (MPa) Tensile strength (MPa) Elongation (%) Plastic Anisotropy Index (r _m ) In-plane anisotropy index (△ r) Post-burst yield strength (MPa) Secondary process brittleness (DBTT- ℃) One 225 312 49 2.13 0.35 273 -80 0.12 Invention steel 2 229 310 48 2.21 0.28 270 -80 0.13 Invention steel 3 235 318 47 2.09 0.31 281 -80 0.11 Invention steel 4 236 320 46 2.01 0.33 304 -80 0.1 Invention steel 5 232 328 46 1.73 0.35 305 -80 0.12 Comparative steel 6 221 310 49 1.83 0.41 275 -70 0.13 Comparative steel 7 241 315 47 1.75 0.36 288 -70 0.14 Comparative steel 8 233 312 47 1.73 0.38 279 -70 0.12 Comparative steel 9 245 328 45 1.69 0.31 301 -70 0.1 Comparative steel 10 182 292 48 1.83 0.58 215 -10 Conventional Steel

As shown in Table 2, Sample Nos. 1 to 4 are invention steels satisfying the present invention and have excellent workability while having a hardening hardening characteristic.

In the case of sample No. 5, the addition amount of Mo was higher than the conditions of the present invention, and the workability was not good. In the case of Sample No. 6-9, the steel was not added with Mo and the plastic anisotropy index was 1.9 or less, resulting in poor workability. In the case of Sample No. 10, the workability is poor as a conventional IF hardened steel.

As described above, the cold rolled steel sheet provided in accordance with the present invention has a small hardening property, a high plastic anisotropy index, a low in-plane anisotropy index, and thus less wrinkles during processing, and has less useful effects after processing.

Claims (2)

C: 0.003-0.005%, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less, P: 0.015% or less, Mo: 0.01-0.2% by weight% Including, the weight ratio of the Mn and S satisfies the following conditions 0.58 * Mn / S ≤ 10, and is composed of the remaining Fe and other unavoidable impurities, the small size hardening type with excellent workability, the average size of the MnS precipitate is less than 0.2㎛ Cold rolled steel sheet. C: 0.003-0.005%, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.004% or less, P: 0.015% or less, Mo: 0.01-0.2% by weight% Including, the weight ratio of Mn and S satisfy the following conditions 0.58 * Mn / S ≤ 10, and after reheating the steel composed of the remaining Fe and other unavoidable impurities to a temperature of 1100 ℃ or more after finishing rolling temperature Ar ₃ transformation point A method for producing a hardened hardened cold rolled steel sheet having excellent workability, including hot rolling, cooling at a rate of 200 ° C./min or more, winding at a temperature of 700 ° C. or less, and then cold rolling and continuous annealing.

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