KR101143240B1 - Non aging cold rolled steel sheet having superior workability and process for producing the same - Google Patents

Abstract

A 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 comprises 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, 0.004% or less of N, To 0.2%, and V: 0.01 to 0.2%, wherein the Mn and S satisfy the condition 0.58 * Mn / S? 10, the remaining Fe and other unavoidable impurities, and the average size of the MnS precipitates is 0.2 탆 or less . According to the present invention, the firing anisotropy index is increased by adding Mo while generating fine MnS precipitates, and the addition of V improves the non-aging properties.

Cold rolled steel sheet, Workability, Non-aging, MnS precipitate

Description

 [0001] The present invention relates to a non-aged cold-rolled steel sheet having excellent workability and a method of manufacturing the same.

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 a case of 0.58 * Mn / S < 10,

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

More particularly, the present invention relates to a cold-rolled steel sheet having fine MnS precipitates and excellent in workability and non-aging properties by addition of Mo and V, and a method of manufacturing the same .

Cold rolled steel sheets used in automobiles and home appliances are required to have strength and formability as well as non-aging properties. The aging is a kind of deformed aging phenomenon which causes defects called "stretcher strain" as hardening occurs as the solid element (C, N) sticks to the potential.

The non-vitrification of the cold-rolled steel sheet can be ensured by the application of aluminum-killed steel. It has the disadvantage that the productivity is low due to the long annealing time and the material variation is very 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 the prior art, non-vitrification is not sufficient and quenching after annealing is recommended to ensure non-vitrification. In this case, since most of the water is cooled, the 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. This cold-rolled steel sheet has excellent ductility while maintaining the plastic anisotropy index at a certain level or more, and has an anti-aging property. However, the cold-rolled steel sheet has a problem that the yield strength is too low in terms of the material, so that it is required to use a thicker material, and the plastic anisotropy index is 1.8, so that the workability is not good and the osmosis index OMPa is not completely non-viable.

An object of the present invention is to provide a cold-rolled steel sheet having excellent non-aging properties and high plastic anisotropy index without addition of Ti and Nb and having excellent processability and a method for producing the same.

In order to achieve the above object, the cold-rolled steel sheet according to the present invention comprises 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, 0.004% or less of N, : 0.015% or less, Mo: 0.01 to 0.2%, and V: 0.01 to 0.2%, wherein the Mn and S satisfy the condition 0.58 * Mn / S? 10, the balance Fe and other unavoidable impurities, MnS The average size of the precipitates is 0.2 mu m or less.

The method of manufacturing a cold-rolled steel sheet according to the present invention is characterized in that it 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, 0.004% or less of N, % Of Mo, 0.01 to 0.2% of Mo, and 0.01 to 0.2% of V, wherein the Mn and S satisfy the condition 0.58 * Mn / S? 10 and the remaining Fe and other unavoidable impurities, Or more and then hot rolled at a finishing rolling temperature of not lower than the Ar ₃ transformation point, cooling at a temperature of 200 ° C / min or more and winding at a temperature of 700 ° C or lower, followed by cold rolling and continuous annealing.

Hereinafter, the present invention will be described in detail.

The inventors of the present invention have found the following facts in the research process for securing excellent workability and non-aging properties without addition of Ti and Nb in a high-strength cold-rolled steel sheet. As shown in FIG. 1, as the precipitates of MnS are finely dispersed, the amount of solid carbon in the crystal grains is reduced, thereby improving the aging resistance characteristics. The solid carbon remaining in the crystal grains is relatively free to move, and therefore, affects the aging characteristics in combination with the movable potential. 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 a desired level, it is important to finely distribute 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.

This new fact has been paid attention to, and we have been studying how to distribute MnS finely. 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 0.0018% C-0.15% Mn-0.008% P-0.015% S- 0.03% Al-0.0012% N-0.05% Mo- Mn / S) of 5.8 was subjected to hot rolling to examine the size of the precipitate according to the cooling rate. 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 .

When V is added to the steel in which fine MnS precipitates are distributed as described above, the remaining solid carbon can be precipitated in the carbonitride to ensure the non-aging properties. Also, when Mo is added, the plastic anisotropy index is increased and the workability is improved. The distribution of fine MnS precipitates in the present invention positively influences the yield strength and in-plane anisotropy index.

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 ensure non-vitrification 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 that the content of manganese is adjusted to 0.05 to 0.2% based on the result of research on improvement of yield strength and in-plane anisotropy while basically ensuring non-viability due to precipitation of very fine MnS when manganese and sulfur are appropriately contained Do. In order to obtain such an effect, the content of manganese should be not less than 0.05%. When the content of manganese is more than 0.2%, the content of manganese is high and coarse MnS precipitates are formed.

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 an element to be added as a deoxidizing agent, but in the present invention, nitrogen is added to prevent precipitation of nitrogen in the steel to prevent aging by solid nitrogen. When the content of aluminum is less than 0.01%, the amount of solute nitrogen is large and the aging phenomenon can not be prevented. When the content of aluminum is more than 0.1%, the amount of aluminum existing in a solid state is large.

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

Nitrogen is an element which is inevitably added during steelmaking. When it exceeds 0.004%, the aging index becomes high.

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

When the content of phosphorus is more than 0.015%, the ductility and formability are lowered, and therefore, it is preferable that the content is less than 0.015%.

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 vanadium (V) is preferably 0.01 to 0.2%.

Vanadium is added to precipitate solute C to ensure non-aging properties. When the content is more than 0.01%, non-aging properties can be obtained. When the content exceeds 0.2%, the plastic anisotropy index is lowered.

It is more preferable that the weight ratio (V / C) of V and C satisfies 1 to 20. If the weight ratio of V and C is less than 1, the precipitation effect of the solid solution C is not large. If the weight ratio of V and C is more than 20, the plastic anisotropy index is low.

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 cooling rate is increased due to a large amount of coarse MnS precipitates unreacted in the reheating step, new nuclei are generated and the precipitates are not fine , 0.024% C-0.43% Mn-0.011% P-0.009% S-0.035% Al-0.0043% N-0.05% Mo-0.05% V). 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 exceeds 700 ° C, the MnS precipitates grow too coarse and the non-vitrification deteriorates.

[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]

이다. The steel slabs shown in Table 1 were reheated at 1,200 DEG C, finishing hot-rolled, cooled at a rate of 200 DEG C / min, rolled at 650 DEG C, and 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.

city
side
time
number Chemical composition (% by weight) 0.58
*
Mn / S 0.25
* V / C C Mn P S Al N Mo V ≤0.003 0.05
~ 0.2 ≤0.015 0.005
~ 0.03 0.01
~ 0.1 ≤0.004 0.01 ~
0.2 0.01
~ 0.2 ≤10 1 to 20 One 0.0023 0.11 0.011 0.011 0.023 0.0017 0.017 0.025 5.8 2.72 2 0.0027 0.09 0.01 0.009 0.037 0.0027 0.074 0.082 5.8 7.59 3 0.0025 0.08 0.009 0.012 0.032 0.0031 0.15 0.16 3.87 16 4 0.0019 0.12 0.012 0.013 0.022 0.0013 0.22 0.24 5.35 31.6 5 0.0028 0.09 0.012 0.006 0.032 0.0032 0 0.05 8.7 4.46 6 0.0025 0.11 0.009 0.011 0.025 0.0017 0.052 0 5.8 0 7 0.0021 0.38 0.01 0.008 0.031 0.0031 0.08 0.12 27.6 9.52 8 0.0023 0.08 0.01 0.005 0.04 0.0015 0 0 9.28 0 9 0.0018 0.10 0.011 0.012 0.05 0.0026 0 0 4.83 0 10 0.0018 0.15 0.008 0.015 0.03 0.0012 0 0 5.8 0 11 0.0027 0.09 0.012 0.025 0.035 0.0018 0 0 2.09 0

city
side
time
number Mechanical property Precipitate
Average size
(탆) Remarks surrender
burglar
(MPa) Seal
burglar
(MPa) year
God
rate
(%) Plasticity
Anisotropy
Indices
(r _m ) In-plane
Anisotropy
Indices
(R) Aging index
(AI- (MPa) One 158 290 50 2.19 0.35 0 0.07 Invention river 2 162 288 49 2.22 0.39 0 0.08 Invention river 3 172 292 49 2.08 0.29 0 0.11 Invention river 4 178 310 45 1.82 0.29 0 0.09 Comparative steel 5 172 292 45 1.82 0.35 0 0.11 Comparative steel 6 172 292 49 1.98 0.29 23 0.11 Comparative steel 7 158 281 50 2.08 0.35 25 0.11 Comparative steel 8 211 309 49 1.83 0.21 23 0.05 Comparative steel 9 209 311 52 1.93 0.24 22 0.12 Comparative steel 10 201 295 54 1.94 0.26 21 0.15 Comparative steel 11 223 319 48 1.88 0.26 27 0.14 Comparative steel

As shown in Tables 1 and 2, Samples 1 to 3 are inventive steels satisfying the present invention, and have high anti-abdominal strength, high plasticity anisotropy index, and non-aging properties.

As described above, the cold-rolled steel sheet provided according to the present invention has an excellent non-aging property, excellent workability, and a high yield strength, so that the sheet thickness can be reduced, thereby reducing the weight.

Claims (4)

0.003% or less (excluding 0 is excluded), Mn: 0.05 to 0.2%, S: 0.005 to 0.03%, Al: 0.01 to 0.1% (Excluding 0), Mo: 0.01 to 0.2%, V: 0.01 to 0.2%, wherein the Mn and S satisfy the condition 0.58 * Mn / S? 10 and the balance Fe and other unavoidable impurities And an average size of the MnS precipitates is 0.2 占 퐉 or less. The non-agglomerated cold-rolled steel sheet according to claim 1, wherein the weight ratio of V and C (0.25 * V / C) satisfies 1 to 20. 0.003% or less (excluding 0 is excluded), Mn: 0.05 to 0.2%, S: 0.005 to 0.03%, Al: 0.01 to 0.1% (Excluding 0), Mo: 0.01 to 0.2%, V: 0.01 to 0.2%, and the Mn and S satisfy the condition 0.58 * Mn / S? 10 and the balance Fe and other unavoidable impurities after the re-heating the steel to a temperature of at least 1100 ℃ by a finish rolling temperature above the Ar ₃ transformation point, hot-rolled and cooled by more than 200 ℃ / min speed and take-up at a temperature below 700 ℃ then to cold and rolling, continuous annealing By weight based on the total weight of the non-agglomerated cold-rolled steel sheet. 4. The method for producing a non-agglomerated cold-rolled steel sheet according to claim 3, wherein the weight ratio of V and C (0.25 * V / C) satisfies 1 to 20.

Images