KR101105098B1 - Bake-harding cold rolled steel sheet having excellent workability and high strength, process for producing the same - Google Patents

Abstract

The present invention relates to a hardened hardened high strength 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: 0.005 ~ 0.02%, P: 0.03% ~ 0.06%, the weight ratio of Mn and S satisfies the following condition 0.58 * Mn / S ≦ 10 and is composed of the remaining Fe and other unavoidable impurities , The average size of MnS precipitates is less than 0.2㎛. In addition, a method for producing this cold rolled steel sheet is also provided.

Cold rolled steel sheet of the present invention is not only a large increase in the yield strength after baking at 220Mpa or more, but also an excellent plastic anisotropy index of 1.68 or more, and the ductile-brittle transition temperature (DBTT) is -70 ℃ or less in extreme regions Can be used.

Cold rolled steel, hardened hardening, high strength, secondary processing brittleness, plastic anisotropy, MnS precipitate

Description

 BAKE-HARDING COLD ROLLED STEEL SHEET HAVING EXCELLENT WORKABILITY AND HIGH STRENGTH, 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 high-strength cold-rolled steel sheet used as a material for automobiles, and more particularly to a high-strength non-aging cold rolled steel sheet having excellent hardening hardenability and workability secondary workability by fine MnS precipitates in a high nitrogen AlN precipitation component system. It is about a method.

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. The upper annealing material has a disadvantage of low yield strength and low productivity after baking.

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 and a method of manufacturing the same, which have substantial baking hardening properties and excellent workability and secondary workability without adding Ti and Nb.

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: 0.005-0.02% , P: 0.03 ~ 0.06%, the weight ratio of Mn and S satisfies the following conditions 0.58 * Mn / S ≤ 10, is composed of the remaining Fe and other unavoidable impurities, the average size of the MnS precipitate is less than 0.2㎛ .

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: 0.005-0.02%, P : 0.03 ~ 0.06%, 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 and then finishes the rolling temperature. Hot rolling over the Ar ₃ transformation point, cooling at a rate of 200 ° C / min or more, winding at a temperature of 600 to 750 ° C, cold rolling at a rolling reduction of 50 to 90%, and continuous at a temperature range of 500 to 900 ° C. To anneal.

Hereinafter, the present invention will be described in detail.

The present inventors have discovered the following new facts in the course of research for securing the baking hardening property, workability and secondary processing characteristics without adding Ti and Nb as high nitrogen high strength cold rolled steel sheets. In the high nitrogen content system with N content of 0.005 ~ 0.02%, finely distributing the precipitate of MnS while controlling the carbon content to an appropriate amount basically yields a high yield strength and increases the yield strength of the post-bubble, which is a fine MnS precipitate. Will affect.

As shown in FIG. 1, the finer the distribution of MnS precipitates in the high-strength cold-rolled steel sheet of high nitrogen, the smaller the dissolved carbon content in the grains, and the carbon content of the cold-rolled steel sheet is 0.003 to 0.005%. 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 carbon content should be adjusted to 0.003 ~ 0.005%, and the size of MnS fine precipitate should be less than 0.2㎛.

By paying attention to these new facts, we have studied how to finely distribute MnS in high nitrogen steels. 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-0.10% Mn-0.04% P-0.010% S-0.05% Al-0.008% 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 present invention, the content of N in the basic component system of C: 0.003 ~ 0.005%, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, P: 0.03-0.06% is 0.005 ~ 0.02% If the weight ratio of Mn and S meets the following conditions 0.58 * Mn / S≤10 and the average size of MnS precipitates is less than 0.2㎛, it is basically secured at high strength and baked. The low anisotropy index is excellent in workability and excellent in secondary processing brittleness. If the precipitate becomes fine, the appropriate amount of carbon remains in the grain boundary to strengthen the grain boundary, thereby preventing brittle fracture that may occur due to the weak grain boundary after processing.

 The steel of the present invention satisfies the yield strength of 220 MPa or more, the yield strength-ductility balance of 9500 or more, while satisfying the in-plane anisotropy index Δr of 0.4 or less and plastic anisotropy index of 1.68 or more. 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 carbon content 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 drastically.

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. Curability becomes poor.

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

When the content of sulfur (S) is less than 0.005%, the amount of precipitated MnS is not only small but the size of precipitated MnS is very coarse, so that the hardening hardening characteristic is not good. 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. More preferable content of S is 0.016 to 0.03%.

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

Aluminum is an element added as a deoxidizer to precipitate nitrogen in the steel and is added for reinforcing effect. If the addition amount is less than 0.01%, AlN precipitation amount is small and there is no reinforcing effect.

The content of nitrogen (N) is preferably 0.005 to 0.02%.

Nitrogen is added in an amount of 0.005% or more for the strengthening effect of AlN precipitation, and if the amount exceeds 0.02%, the moldability is lowered, so 0.005 to 0.02% is preferable.

The content of phosphorus (P) is preferably 0.03 to 0.06% or less.

If the phosphorus content is less than 0.03%, it is difficult to obtain the target strength, and if it exceeds 0.06%, the ductility and formability are lowered, so it is preferable to set it in the range of 0.03 to 0.06%.

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 capacity, yield strength, secondary processing brittleness, 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 to lower the hardening hardenability, in-plane anisotropy index, Secondary brittleness is not good.

More preferably in the steel component system of the present invention, the addition ratio of Al and N (0.52 * Al / N) is set to 1 to 5. When the addition ratio of Al and N (0.52 * Al / N) is less than 1, aging may be caused by solid solution N. In the case of more than 5, there is little effect of strengthening strength due to coarsening of AlN precipitates.

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 and yield strength, the secondary processing brittleness, and the in-plane anisotropy index. Especially, when the average size of MnS is more than 0.2㎛, It sharply falls, the in-plane anisotropy index increases sharply, and the secondary processing brittleness is not good. 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-0.04% P-0.009% S-0.04% Al-0.008% N). 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]

After the hot rolling as described above, the winding is carried out, the winding temperature is preferably in the range of 600 ~ 700 ℃. When the coiling temperature is less than 600 ℃, precipitation of AlN precipitates does not occur completely, and there is almost no precipitation strengthening effect due to the precipitates. When the coiling temperature is higher than 700 ℃, MnS precipitates grow too coarsely, so that the hardening hardening characteristic is 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 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 tested for yield strength, tensile strength, elongation, plastic anisotropy index (r _m value), in-plane anisotropy index (Δr) and secondary work 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 characteristics are shown in Table 2 as the yield strength measured after quenching after 2% strain of the specimen after heat treatment at 200 ° C. for 2 minutes.

sample Chemical composition (% by weight) 0.58 *
Mn / S 0.52 * Al / N Reheating Temperature (℃) Cooling rate
(℃ / min) Winding
Temperature (℃)


C Mn P S Al N Etc One 0.0045 0.1 0.035 0.01 0.04 0.0135 - 5.8 1.54 1200 200 650 2 0.0038 0.11 0.044 0.007 0.055 0.0079 - 9.11 3.63 1200 200 650 3 0.0042 0.08 0.053 0.009 0.055 0.0065 - 3.87 4.4 1200 200 650 4 0.0018 0.10 0.042 0.01 0.04 0.0014 - 5.8 14.9 1200 200 650 5 0.0075 0.09 0.04 0.011 0.008 0.0067 - 6.53 0.46 1200 200 650 6 0.0035 0.4 0.04 0.009 0.04 0.0083 - 25.8 2.51 1200 200 650 7 0.0028 0.4 0.07 0.01 0.04 0.0016 Ti
0.022 11.6 13 1200 200 650

Sample Number Mechanical properties Average size of precipitate
(Μm) Remarks Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Plastic Anisotropy Index (r _m ) In-plane anisotropy index
(△ r) Post-Seizure Yield Strength (MPa) 2nd processing brittleness
(DBTT- ℃) One 242 358 44 1.71 0.31 283 -70 0.07 Invention steel 2 224 355 43 1.75 0.38 280 -70 0.09 Invention steel 3 239 360 40 1.68 0.29 302 -70 0.11 Invention steel 4 210 330 46 1.78 0.32 269 -70 0.11 Comparative steel 5 228 352 37 1.52 0.25 295 -70 0.12 Comparative steel 6 228 360 40 1.65 0.54 280 -70 0.41 Comparative steel 7 222 358 44 1.68 0.45 258 + 0 0.29 Conventional Steel

As shown in Tables 1 and 2, Samples 1 to 3 (inventive steel) contained the chemical composition and the manufacturing conditions in the range suggested by the present invention, and the yield strength was 220 MPa or more, and the yield strength and ductility balance was 40% or more. Is over 9500. In addition, the plastic anisotropy index is 1.68 or more, the in-plane anisotropy index is 0.4 or less, it has very excellent moldability, and the yield strength after baking is also high. In addition, it has excellent secondary processing brittleness with a ductile-brittle transition temperature of -70 ° C. or less. 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 4 (comparative steel) has a low carbon content, low yield strength after baking, and Sample 5 (comparative steel) has a high carbon content, which has a high elongation and plastic anisotropy index, which is likely to cause breakage during molding. Sample 6 (comparative steel) had a large size of precipitates, resulting in low yield strength after baking.

Sample 7 (conventional steel) is a conventional IF steel, not only has low yield strength after firing but also has a high secondary processing brittleness temperature, which is highly likely to cause breakage upon impact.

As described above, the cold rolled steel sheet provided according to the present invention has excellent baking hardening characteristics, and has excellent workability, yield strength, and secondary work brittleness characteristics.

Claims (8)

By weight% C: 0.003-0.005%, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.0065-0.02%, P: 0.03-0.06%, of Mn and S A small hardening type high strength cold rolled steel sheet having excellent workability, wherein the weight ratio satisfies the following condition 0.58 * Mn / S ≦ 10, is composed of remaining Fe and other unavoidable impurities, and the average size of MnS precipitate is 0.2 μm or less. According to claim 1, The S hardening type high strength cold rolled steel sheet having excellent workability, characterized in that 0.016 ~ 0.03%. According to claim 1, wherein Al and N satisfy the following conditions 0.52 * Al / N: 1 to 5, hardening type high strength cold rolled steel sheet excellent in workability, characterized in that. The cold rolled steel sheet has a yield strength of 220 Mpa or more, a yield strength-ductility balance of 9500 or more, a ductility-brittle transition temperature of -70 ° C. or less, an in-plane anisotropy index of 0.4 or less, and plastic anisotropy index. Hardened hardened high strength cold rolled steel sheet with excellent processability, characterized by more than 1.68. By weight% C: 0.003-0.005%, Mn: 0.05-0.2%, S: 0.005-0.03%, Al: 0.01-0.1%, N: 0.0065-0.02%, P: 0.03-0.06%, of Mn and S After the weight ratio satisfies the following condition 0.58 * Mn / S≤10, the steel composed of the remaining Fe and other unavoidable impurities is reheated to a temperature of 1100 ° C or higher, and hot-rolled at a finish rolling temperature of more than Ar ₃ transformation point. Excellent workability including cooling at a rate of 1000 ° C / min, winding at a temperature in the range of 600 to 700 ° C, cold rolling at a rolling reduction of 50 to 90%, and continuous annealing at a temperature in the range of 500 to 900 ° C. Method of producing a hardening hardened cold rolled steel sheet. The method of claim 5, wherein the S is 0.016 to 0.03%. The method for manufacturing a hardened hardened type high strength cold rolled steel sheet having excellent workability according to claim 5, wherein Al and N satisfy the following conditions of 0.52 * Al / N: 1 to 5. The cold rolled steel sheet has a yield strength of 220 Mpa or more, a yield strength-ductility balance of 9500 or more, a ductile-brittle transition temperature of -70 ° C. or less, an in-plane anisotropy index of 0.4 or less, and plastic anisotropy index Manufacturing method of high hardness hardened cold rolled steel sheet with excellent workability, characterized in that it is 1.68 or more

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