KR101105007B1 - Cold rolled steel sheet having excellent baking hardenability and process for producing the same - Google Patents

Abstract

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

The cold rolled steel sheet of the present invention not only has a large increase in the yield strength after firing at 200Mpa or more, but also can reduce the product thickness to a yield strength-ductility balance of 10950 or more, thereby reducing the weight and in-plane anisotropy index of 0.41 or less. Less wrinkles are generated during processing and less ears occur after processing.

Cold rolled steel, hardened hardening, in-plane anisotropy index, MnS precipitate

Description

 COLD ROLLED STEEL SHEET HAVING EXCELLENT BAKING HARDENABILITY 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 MnS precipitates according to the cooling rate.

The present invention relates to a cold-rolled steel sheet used as a material for automobiles, and more particularly, to control solid-solution carbon by fine MnS precipitates, and thus, hard-curing type cold rolled steel having excellent bake hardenability, yield strength and strength-ductility balance and small in-plane anisotropy. 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, an annealing material, small amount of dissolved carbon remains, and it has a hardening hardening capacity of about 10 ~ 20Mpa after the baking treatment while securing the non-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 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 hardening hardened cold rolled steel sheet having a hardening hardening property without adding Ti and Nb and further excellent in yield strength and small in-plane anisotropy, and a method of manufacturing the same.

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, the weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S ≤ 10, is composed of the remaining Fe and other unavoidable impurities, the average size of the MnS precipitate is 0.2㎛ or less.

In addition, the cold rolled steel sheet manufacturing method of the present invention, 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, the weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S ≦ 10, and re-heats the steel composed of the remaining Fe and other unavoidable impurities to a temperature of 1100 ° C. or higher, followed by Ar ₃ Hot rolling over the transformation point, cooling at a rate of 200 ° C./min or more, winding at a temperature of 700 ° C. or lower, cold rolling at a reduction ratio of 50-90%, and continuous annealing at a temperature range of 500-900 ° C.

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 of the post-load. This means that the fine MnS precipitates affect the solid solution carbon.

As shown in FIG. 1, the finer the precipitate of MnS, the smaller the amount of solid solution carbon in the grains. When the carbon content of the cold rolled steel sheet is 0.003% to 0.005%, it has 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, FIG. 2 (a) is a steel having 0.004% C-0.15% Mn-0.008% P-0.015% S-0.03% Al-0.0012% N, and the composition ratio of Mn and S (0.58 * Mn / S) is 5.8 The graph shows the size of precipitates according to the cooling rate after hot rolling of steel. 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.

According to the invention 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 and the weight ratio of Mn and S If the average size of MnS precipitates is less than 0.2㎛ in the component system satisfying the following condition 0.58 * Mn / S≤10, the in-plane anisotropy index is satisfied while the yield strength meets 220MPa or more while basically securing the hardening characteristic (Δr) satisfies 0.41 or less. If the yield strength is high, the thickness of the steel sheet can be reduced, thereby reducing the weight, and also having low in-plane anisotropy, less wrinkles during processing, and less ears after processing. The baking hardening type cold rolled steel sheet of this invention and its manufacturing method are demonstrated 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 0.0005%, 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 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 precipitates 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 drops sharply and the in-plane anisotropic index is reduced. 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, thereby minimizing MnS precipitates. 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). Referring to the graph of FIG. 2, the faster the cooling rate, the smaller the size of the MnS precipitate is, so it is not necessary to limit the upper limit of the cooling rate. As for a speed | rate, 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 completed. If it is about 10 seconds or longer, the recrystallization is completed.

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

[Example]

The ingots 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 finishing rolling temperature at this time was 910 degreeC above Ar3 transformation point, and continuous annealing was performed by heating at 750 degreeC for 40 second at the speed of 10 degreeC / sec. The obtained annealing plate was processed into a standard specimen according to ASTM E-8 standard to investigate the mechanical properties. The specimens were tested for yield strength, tensile strength, elongation, plastic anisotropy index (r _m value), in-plane anisotropy index (Δr value), and secondary processing brittleness 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 170 ° C. after 2% strain of the specimen.

sample Chemical composition (% by weight) 0.58 *
Mn / S Reheat
Temperature
(℃) Cooling rate
(℃ / min.) Winding
Temperature
(℃) C Mn P S Al N Etc One 0.0035 0.1 0.01 0.009 0.04 0.0021 - 6.44 1200 200 650 2 0.0041 0.10 0.009 0.012 0.05 0.0030 - 4.83 1200 200 650 3 0.0038 0.08 0.011 0.012 0.035 0.0023 - 3.87 1200 200 650 4 0.0044 0.1 0.01 0.006 0.02 0.0032 - 9.67 1200 200 650 5 0.0022 0.1 0.009 0.011 0.04 0.0038 - 5.27 1200 200 650 6 0.0039 0.43 0.01 0.008 0.05 0.0038 - 31.2 1200 200 650 7 0.0067 0.1 0.008 0.01 0.04 0.0028 - 5.8 1200 200 650 8 0.0024 0.4 0.07 0.01 0.04 0.0016 Ti 0.02 11.6 1200 200 650

Sample Number Mechanical properties Average size of precipitate
(Μm) Remarks Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Plastic anisotropy
Indices
(r _m ) In-plane anisotropy
Indices
(△ r) Post-Seizure Yield Strength (MPa) 2nd processing brittleness
(DBTT- ℃) One 221 310 49 1.83 0.41 288 -70 0.13 Invention steel 2 241 315 47 1.75 0.36 292 -70 0.14 Invention steel 3 233 312 47 1.73 0.38 282 -70 0.12 Invention steel 4 245 328 45 1.69 0.31 301 -70 0.1 Invention steel 5 209 299 51 1.88 0.42 232 -70 0.11 Comparative steel 6 211 290 52 1.82 0.38 235  -70 0.59 Comparative steel 7 251 329 42 1.53 0.29 298  -70 0.12 Comparative steel 8 182 292 48 1.83 0.58 215 -10 0.21 Conventional Steel

As shown in Tables 1 and 2, Samples 1 to 4 (inventive steel) contained the chemical composition and manufacturing conditions in the range suggested by the present invention, yield strength was 220 MPa or more, and elongation was 45% or more. More than 10950 In addition, the plastic anisotropy index is 1.6 or more, the in-plane anisotropy index is 0.41 or less, it has very excellent moldability, and the yield strength after baking is also high. Such material characteristics can be exhibited by controlling the size of the precipitate to be 0.2 µm or less.

On the other hand, Sample 5 (comparative steel) has a low carbon content and low yield strength after baking. Sample 6 (comparative steel) had a large size of precipitates, resulting in low yield strength after baking. Sample 7 (comparative steel) has a high carbon content and a low elongation and plastic anisotropy index, so that fracture may occur during molding.

Sample 8 (conventional steel) is a conventional IF steel, which has a low yield strength after firing and a high secondary processing brittleness temperature, so that there is a high possibility of fracture at impact.

As described above, the cold rolled steel sheet provided in accordance with the present invention is excellent in hardening hardening properties and excellent in workability, yield strength and secondary workability.

Claims (6)

C: 0.003-0.005% by weight, 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, and the weight ratio of Mn and S A hardened hardened cold rolled steel sheet having the following conditions of 0.58 * Mn / S ≦ 10, consisting of remaining Fe and other unavoidable impurities, and having an average size of MnS precipitates of 0.2 μm or less. According to claim 1, wherein S is hardening type cold-rolled steel sheet, characterized in that 0.016 ~ 0.03%. According to claim 1, wherein the cold-rolled steel sheet has a yield strength of 220Mpa or more, strength-ductility balance is 10950 or more, in-plane hardened cold-rolled steel sheet characterized in that the in-plane anisotropy index is 0.41 or less. C: 0.003-0.005% by weight, 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, and the weight ratio of Mn and S After satisfying the following condition 0.58 * Mn / S≤10 and reheating the steel composed of the remaining Fe and other unavoidable impurities to a temperature of 1100 ℃ or more, hot rolling with a finish rolling temperature of Ar ₃ transformation point or more and 200 ~ 1000 ℃ Cooling at a rate of / min, wound at a temperature of 700 ℃ or less, followed by cold rolling at a rolling reduction rate of 50 ~ 90%, and continuous annealing in the temperature range of 500-900 ℃ a method for producing a hardened hardened cold rolled steel sheet. The method of claim 4, wherein S is 0.016 to 0.03%. The method of claim 4, wherein the cold rolled steel sheet is 220 Mpa or more, the strength-ductility balance is 10950 or more, and the in-plane anisotropy index is 0.41 or less.

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