KR101143098B1 - High strength cold rolled steel sheet having aging resistance and superior workability, and process for producing the same - Google Patents

Abstract

A high strength cold rolled steel sheet used as a material for automobiles, home appliances and the like, and a manufacturing method thereof. The cold-rolled steel sheet contains 0.003% or less of C, 0.05 to 0.2% of Mn, 0.005 to 0.03% of S, 0.01 to 0.1% of Al, 0.005 to 0.02% of N, 0.01 to 0.2% of Mo, : 0.03 to 0.06%, wherein the Mn and S satisfy the condition 0.58 * Mn / S? 10, the balance Fe and other unavoidable impurities, and the average size of the MnS precipitates is 0.2 탆 or less. According to the present invention, by the addition of Mo, the firing anisotropy index is high. In addition, MnS precipitates provide high yield strength and improved anti-aging properties.

Cold rolled steel sheet, high nitrogen, high strength, plastic anisotropy index, MnS precipitate

Description

 BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high strength cold rolled steel sheet having a high workability,

Fig. 1 is a graph showing changes in the amount of solid carbon in the crystal grains depending on the size of MnS precipitates.

2 is a graph showing the magnitude of MnS precipitates with respect to the cooling rate,

2 (a) shows the case of 0.58 * Mn / S < 10,

2 (b) shows a case of 0.58 * Mn / S > 10.

The present invention relates to a high-strength cold-rolled steel sheet used as a material for automobiles, household appliances, and the like, and more particularly, to a high-strength cold-rolled steel sheet excellent in workability by the addition of Mo and a method for producing the same. Further, in the cold-rolled steel sheet of the present invention, the yield strength is high and the anti-aging property is improved by the fine MnS precipitates.

Cold rolled steel sheets used in automobiles and home appliances are required to have strength and moldability as well as resistance to aging. Aging is a kind of deformation aging phenomenon that causes defects such as stretcher strain due to hardening as the interstitial elements C and N stick to the electric potential over time.

The durability of the cold-rolled steel sheet can be ensured by the application of aluminum-killed steel, but it has a disadvantage that the productivity is low due to the long annealing time and the material variation is large in each part. Therefore, Interstitial Free Steel, which is continuously annealed by adding strong carbon and nitride forming elements such as Ti and Nb, is mainly used.

In order to produce an IF steel, Ti, Nb, etc., which are strong carbon and nitride forming elements, are added, and these elements must be annealed at a high temperature because they raise the recrystallization temperature. This lowers productivity and increases energy costs. In addition, when annealing is performed at a high temperature, various defects such as fine scratches and shape defects are likely to occur. Also, since Ti and Nb are highly oxidative, many nonmetallic inclusions are generated in steel making to cause surface defects of the steel sheet. In addition, the IF steel has a disadvantage in that so-called secondary processing brittleness occurs in which the grain boundary is weak and brittleness is generated after processing. To prevent this, an element such as B is added to prevent brittle secondary processing. Particularly, in the case of IF steel, there are disadvantages that many defects are generated in a product which performs surface treatment such as plating and painting.

In order to solve this problem, Ti and Nb-free alloys without Ti or Nb have been proposed. For example, in JP-A Nos. 6-093376, 6-093377 and 6-212354, Ti and Nb are not added and instead, 0.0001 to 0.003% of B is added to 0.0001 to 0.003% It is a technology to improve Hyosung. However, in this prior art, endurance is not sufficient and quenching after annealing is recommended in order to secure endurance. In this case, since most of the water is cooled, a pickling treatment is performed again to remove the oxide film The surface is not good and costs extra. In addition, these steel types have disadvantages of low strength, high in-plane anisotropy, wrinkles, and ear inconvenience.

On the other hand, the present inventors have proposed a method of manufacturing a cold-rolled steel sheet having improved ductility without adding Ti and Nb to Korean Patent Publication No. 2000-0039137. A method for producing a cold-rolled steel sheet comprising the steps of: C: 0.0005 to 0.002%, Mn: 0.05 to 0.3%, S: 0.015% or less, P: 0.015% %, A steel slab containing C + N + S + P of 0.025% or less and containing the remaining Fe and other inevitably contained elements. The cold-rolled steel sheet is excellent in endurance and ductility while maintaining the plastic anisotropy index at a certain level or higher. However, since the cold-rolled steel sheet strictly controls C + N + S + P to 0.025%, the desulfurization and denitrifying ability must be strengthened in the manufacturing process, which is very disadvantageous in terms of productivity and cost. In terms of materials, the yield strength is too low to use a thicker material, and the plastic anisotropy index is 1.8, so the workability is not good.

On the other hand, a cold rolled steel sheet having a high yield strength and having an increased strength of Mn and P added to the ultra low carbon steel and Ti added thereto is known. This cold-rolled steel sheet has a ductile-brittle transition temperature of 0 to 30 ° C, which is not good enough to cause secondary working brittle fracture at impact even at room temperature.

An object of the present invention is to provide a high strength cold rolled steel sheet having resistance to aging without secondary addition of Ti and Nb and excellent secondary workability and processability, and a manufacturing method thereof.

In order to achieve the above object, the cold-rolled steel sheet of the present invention is characterized by containing 0.003% or less of C, 0.05 to 0.2% of Mn, 0.005 to 0.03% of S, 0.01 to 0.1% of Al, 0.005 to 0.02% of N, 0.01 to 0.2% of Mo, 0.03 to 0.06% of P, the Mn and S satisfy the condition 0.58 * Mn / S? 10, the balance Fe and other unavoidable impurities, and the average size of the MnS precipitates is 0.2 탆 .

The method of manufacturing a cold-rolled steel sheet according to the present invention is a cold-rolled steel sheet producing method comprising the steps of: C: 0.003% or less; Mn: 0.05 to 0.2%; S: 0.005 to 0.03%; Al: 0.01 to 0.1%; N: 0.005 to 0.02% 0.01 to 0.2%, P: 0.03 to 0.06%, the steel slab satisfying the condition of 0.58 * Mn / S? 10 and containing the remaining Fe and other unavoidable impurities is reheated to a temperature of 1100 占 폚 or more Rolled at a finishing rolling temperature of not less than the Ar ₃ transformation point, hot rolled, cooled at a rate of 200 ° C / min or more, rolled at a temperature of 700 ° C or lower, cold rolled and continuously annealed.

In the present invention, it is more preferable that Al and N satisfy the following condition: 0.52 * Al / N: 1 to 5.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention have found the following new facts in a research process for securing high strength properties with secondary processing characteristics and having endurance properties without addition of Ti and Nb to high-nitrogen cold-rolled steel sheets. That is, in the high nitrogen component system having the N content of 0.005 to 0.02%, the precipitates of MnS affect the yield strength and the secondary machining characteristics by precipitation strengthening as well as the aging property, and when the Mo is added together, the plastic anisotropy index It is remarkably high.

As shown in Fig. 1, as the precipitates of MnS are finely distributed in the high-strength cold-rolled steel sheet to which Mo is added together as a high nitrogen, the amount of the solid carbon in the crystal grains is reduced and the aging resistance characteristics are improved. Since the solid carbon remaining in the crystal grains is relatively free to move, it binds with the movable potential and affects the aging characteristics. Therefore, if the amount of the solid carbon in the crystal grains is reduced to a certain level or less, the aging resistance is improved. The amount of the solid carbon in the crystal grains is at least 20 ppm or less, preferably 15 ppm or less in terms of securing the aging property.

Thus, in order to control the amount of solid carbon in the crystal grains to 20 ppm or less, it is important to finely disperse the precipitates of MnS while keeping the content of carbon added to the steel to 0.003% or less. In the present invention, by using a precipitate of fine MnS, the content of carbon in steel can be increased to 0.003%, which is a small load in the steelmaking process.

With this new fact in mind, we have been studying the possibility of finely distributing MnS in high strength Mo-added cold rolled steel sheets. As a result, it is necessary to (1) adjust the content of Mn (0.58 * Mn / S) to 10 or less while adjusting the content of Mn to 0.05 to 0.2% and the content of S to 0.005 to 0.03% When the cooling rate is 200 [deg.] C / min or more after the intermetallic rolling, a fine MnS precipitate of 0.2 [micro] m or less can be obtained.

That is, FIG. 2 (a) is a steel in which 0.002% C-0.10% Mn-0.04% P-0.010% S-0.05% Al- 0.008% N- Is 5.8, and the size of the precipitate according to the cooling rate after the hot rolling. The graph of FIG. 2 (a) shows that when the cooling rate is adjusted for the case where the composition ratio of Mn and S (0.58 * Mn / S) satisfies 10 or less, the precipitate size of MnS can satisfy 0.2 탆 or less .

The cold-rolled steel sheet of the present invention and its manufacturing method will be described in detail below.

[Cold rolled steel sheet of the present invention]

The content of carbon (C) is preferably 0.003% or less.

When the content of carbon is 0.003% or more, it is difficult to secure endurance due to a large amount of carbon in solid in the steel, and the crystal grains of the annealed plate become finer and the ductility is greatly lowered. Therefore, the content of carbon (C) is preferably 0.003% or less, and more preferably the content of carbon (C) is 0.0005 to 0.003%. When the content of carbon (C) is less than 0.0005%, the crystal grains of the hot-rolled steel sheet have a reduced strength and an in-plane anisotropy. In the present invention, since the amount of dissolved carbon in the crystal grains can be lowered by the MnS precipitate, the content of carbon can be increased up to 0.003%, so that the decarburization treatment for decreasing the content of carbon can be omitted. To 0.003% or less.

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

Manganese is known as an element to prevent hot shortness due to sulfur by precipitating solid sulfur in MnS. In the present invention, it is preferable to set the content of manganese to 0.05 to 0.2% on the basis of the result of research that improvement of yield strength and in-plane anisotropy is ensured by basically securing the anti-aging property by depositing very fine MnS when the content of manganese and sulfur is made appropriate Do. If the content of manganese is less than 0.05%, the amount of residual sulfur in the solid state is large, so that the embrittlement of brittleness may occur. When the content of manganese is more than 0.2%, the content of manganese is high and coarse MnS precipitates are formed, Hyosung is getting worse.

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

When the content of sulfur (S) is less than 0.005%, not only the MnS precipitation amount is small but also the magnitude of precipitated MnS becomes very large and the anti-aging property is poor. When the content of sulfur is more than 0.03%, the content of sulfur dissolved is large, so that the ductility and formability are greatly lowered, and there is a fear of heat brittleness. When the content of sulfur is in the range of 0.005 to 0.03%, it becomes easy to control the size of the precipitate of MnS to a desired range. The more preferable content of S is 0.016 to 0.03%.

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

Aluminum is added as a deoxidizing agent. It is added for the purpose of strengthening effect by precipitating nitrogen in the steel. When the addition amount is less than 0.01%, the amount of AlN precipitation is small and the strengthening effect is not obtained.

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 purpose of strengthening by AlN precipitation. When the amount of addition exceeds 0.02%, the formability is lowered. Therefore, it is preferably 0.005 to 0.02%.

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

Molybdenum is added as an element to increase the plastic anisotropy index. When the content is more than 0.01%, the plastic anisotropy index becomes large. When the content exceeds 0.2%, plastic anisotropy index does not increase any more and may cause hot brittleness.

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

When the content of phosphorus is less than 0.03%, it is difficult to obtain the intended strength. When the content of phosphorus is more than 0.06%, the ductility and formability are lowered, so that the content is preferably in the range of 0.03 to 0.06%.

It is preferable that the weight ratio of Mn and S satisfies 0.58 * Mn / S? 10.

Manganese and sulfur bind to MnS precipitates. The MnS precipitates vary in precipitation depending on the amount of manganese and sulfur added, affecting the age index, yield strength, and in-plane anisotropy index. According to the study of the present invention, when the addition ratio of manganese and sulfur (0.58 * Mn / S, wherein the content of Mn and S is% by weight) is more than 10, MnS precipitates have a larger aging aging index, The characteristics of the index are not good.

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

According to the results of the present invention, the size of the MnS precipitates directly affects the aging index, yield strength and in-plane anisotropy index. When the average size of MnS exceeds 0.2 μm, the aging index increases sharply and the in- . Therefore, the average size of the MnS precipitates is preferably 0.2 mu m or less.

[Production method of cold-rolled steel sheet]

The present invention is characterized in that an average size of MnS precipitates in the cold-rolled steel sheet is made to be 0.2 탆 or less through hot rolling and cold rolling, wherein the steel satisfying the above-described stress is obtained. The size of the MnS precipitates in the cold-rolled sheet is affected by the ratio of Mn / S and the manufacturing process, but is directly influenced by the cooling rate especially after hot rolling.

[Hot rolling condition]

In the present invention, the steel satisfying the above-mentioned stress is reheated and hot-rolled. The reheating temperature is preferably 1100 DEG C or higher. When the reheating temperature is lower than 1100 ° C, the reheating temperature is low, so that the coarse MnS generated during the continuous casting remains in a completely undissolved state, and a large amount of coarse MnS remains after the hot rolling.

The hot rolling is preferably carried out under the condition that the finishing rolling temperature is equal to or higher than the Ar ₃ transformation temperature. When the finishing rolling temperature is lower than the Ar ₃ transformation temperature, not only the workability is lowered due to the production of the pressure-relief but the ductility is greatly reduced.

The cooling rate before hot rolling is preferably 200 占 폚 / min or more. According to the present invention, even if the composition ratio of Mn and S (0.58 * Mn / S) is 10 or less, the precipitate size of MnS exceeds 0.2 탆 when the cooling rate is less than 200 캜 / min. That is, as the cooling rate is increased, a large number of nuclei are generated and the MnS precipitates become finer. When the composition ratio of Mn and S (0.58 * Mn / S) is more than 10, even if the coarse MnS precipitates unreacted in the reheating step are increased in number and the cooling rate is increased, the number of new nuclei is small and the precipitates are not fine 0.04% P-0.009% S-0.04% Al-0.008% N-0.05% Mo). 2, it is not necessary to limit the upper limit of the cooling rate since the size of the MnS precipitates becomes finer as the cooling rate is increased. However, even if the cooling rate is 1000 占 폚 / min or more, the effect of refining the precipitate does not further increase, so that the cooling rate is more preferably 200 to 1000 占 폚 / min.

[Winding condition]

After hot rolling as described above, winding is performed, and the winding temperature is preferably 700 DEG C or lower. When the coiling temperature is higher than 700 ° C, the MnS precipitates grow too coarse and the anti-aging properties deteriorate.

[Cold rolling conditions]

The cold rolling is preferably performed at a reduction ratio of 50 to 90%. When the cold rolling reduction rate is less than 50%, the amount of annealed recrystallized nuclei is small, so that the grain size grows too large during annealing and the strength and formability are lowered due to coarsening of the annealed recrystallized grains. If the cold rolling reduction ratio exceeds 90%, the formability is improved but the amount of nucleation is too large, so that the annealed recrystallized grains are rather too fine and the ductility is lowered.

[Continuous Annealing]                     

The continuous annealing temperature plays an important role in determining the material of the product. In the present invention, it is preferable to carry out the reaction in a temperature range of 500 to 900 ° C. When the continuous annealing temperature is less than 500 캜, the recrystallized grains are too fine to secure a desired ductility value, and when the annealing temperature exceeds 900 캜, the strength is lowered due to coarsening of the recrystallized grains. The continuous annealing time is maintained so that the recrystallization is completed. When the time 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 1]

이다. The slabs shown in Table 1 were reheated at 1200 ° C and finishing hot-rolled, cooled at a rate of 200 ° C / min, rolled at 650 ° C, and then subjected to cold rolling 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 annealed sheets were processed into standard specimens according to ASTM E-8 standard to investigate mechanical properties. The specimens were measured for yield strength, tensile strength, elongation, plastic anisotropy index ( _rm value), in-plane anisotropy index (Δr) and aging index (AI) using a tensile tester (INSTRON Model 6025) . Where _{r m = (r 0 + 2r} 45 + r 90) / 4, △ r = (r 0 -2r 45 + r 90) /

to be.

Specimen Number Chemical composition (% by weight) C Mn P S Al N Mo


0.58
*
Mn / S 0.52
*
Al / N ≤0.003 0.05
~ 0.2 0.03 to 0.06 0.005
~ 0.3 0.01 ~
0.1 0.005 ~
0.02 0.01
~ 0.2 ≤10 1-5 One 0.0015 0.11 0.042 0.01 0.055 0.012 0.028 6.38 2.25 2 0.0025 0.1 0.04 0.01 0.045 0.009 0.06 5.8 2.6 3 0.0022 0.08 0.038 0.008 0.034 0.0075 0.13 5.8 2.36 4 0.0018 0.09 0.04 0.01 0.045 0.01 0.17 5.22 2.34 5 0.0026 0.08 0.039 0.007 0.033 0.0009 0.26 6.63 1.9 6 0.0019 0.1 0.04 0.008 0.042 0.015 - 6.5 1.46 7 0.0028 0.09 0.042 0.007 0.04 0.0068 - 7.73 3.06 8 0.0023 0.11 0.04 0.010 0.05 0.0082 - 5.8 3.17

Sample number Mechanical property Precipitation
water
Average
size
(탆) Remarks surrender
burglar
(MPa) Seal
burglar
(MPa) year
God
rate
(%) Plasticity
Anisotropy
Indices
(r _m ) In-plane
Anisotropy
Indices
(R) prescription
Indices
(AI- (MPa) Secondary
Processing
Brittle
(DBTT-C) One 225 355 47 2.15 0.31 12 - 80 0.08 Invention river 2 235 362 45 2.05 0.29 27 - 80 0.1 Invention river 3 233 353 45 2.06 0.33 27 - 80 0.12 Invention river 4 224 344 47 2.19 0.35 19 - 70 0.09 Invention river 5 230 359 44 1.82 0.28 28 - 80 0.11 Comparative steel 6 231 352 46 1.78 0.31 22 - 70 0.07 Comparative steel 7 229 344 48 1.82 0.38 25 - 70 0.09 Comparative steel 8 235 348 48 1.83 0.31 22 - 70 0.09 Comparative steel

As shown in Tables 1 and 2, Specimens 1 to 4 are inventive steels satisfying the present invention, and have a high plastic anisotropy index and excellent second-order brittleness.

In the case of specimen 5, Mo is added excessively and the plastic anisotropy index is low. In the case of Specimen 6-8, the workability was not good because no Mo was added.

INDUSTRIAL APPLICABILITY As described above, according to the present invention, there is provided a high-strength cold-rolled steel sheet having not only high firing anisotropy index but also excellent secondary workability.

Claims (4)

0.001 to 0.03% of Al, 0.005 to 0.02% of N, 0.01 to 0.2% of Mo, 0.005 to 0.03% of Cr, 0.003 to 0.03% of Cr, 0.05 to 0.2% of Mn, : 0.03 to 0.06%, Mn and S satisfy the condition 0.58 * Mn / S? 10, the balance Fe and other unavoidable impurities, the average size of the MnS precipitates is 0.2 탆 or less and the plastic anisotropy index High-strength, weather-resistant, cold-rolled steel sheet with excellent workability of 2.05 or more. The cold rolled steel sheet according to claim 1, wherein Al and N satisfy the following condition: 0.52 * Al / N: 1 to 5. 0.001 to 0.03% of Al, 0.005 to 0.02% of N, 0.01 to 0.2% of Mo, 0.005 to 0.03% of Cr, 0.003 to 0.03% of Cr, 0.05 to 0.2% of Mn, : 0.03 ~ 0.06% Mn and S ,, the condition that 0.58 * Mn / satisfies S≤10, and the balance of Fe and other after re-heating the steel slab which is the composition as inevitable impurities to a temperature at least 1100 ℃ finish rolling temperature of Ar ₃ Rolled at a temperature equal to or higher than the transformation point, cooled at a rate of 200 DEG C / min or more and wound at a temperature of 700 DEG C or lower, and then subjected to cold rolling and continuous annealing, and having a high anisotropy index of 2.05 or higher, A method for producing an aged cold rolled steel sheet. 4. The method according to claim 3, wherein Al and N satisfy the following condition: 0.52 * Al / N: 1 to 5.

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