KR101143061B1 - Bake hardenable cold rolled steel sheet having superior fprmability and method of manufacturing the same - Google Patents
Bake hardenable cold rolled steel sheet having superior fprmability and method of manufacturing the same Download PDFInfo
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
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- C21D8/0421—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing characterised by the working steps
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- C21D—MODIFYING THE PHYSICAL STRUCTURE OF FERROUS METALS; GENERAL DEVICES FOR HEAT TREATMENT OF FERROUS OR NON-FERROUS METALS OR ALLOYS; MAKING METAL MALLEABLE, e.g. BY DECARBURISATION OR TEMPERING
- C21D8/00—Modifying the physical properties by deformation combined with, or followed by, heat treatment
- C21D8/02—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips
- C21D8/04—Modifying the physical properties by deformation combined with, or followed by, heat treatment during manufacturing of plates or strips to produce plates or strips for deep-drawing
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Abstract
자동차, 가전제품 등의 소재로 사용되는 소부경화형 냉연강판과 그 제조방법이 제공된다. 이 냉연강판은, 중량%로 C:0.003-0.005%, S:0.003-0.025%, Al:0.01-0.08%, N:0.004%이하, P:0.015%이하, Cu:0.01-0.2%를 포함하고, 상기 Cu와 S가 조건 0.5*Cu/S:1-10를 만족하고, 나머지 Fe 및 기타 불가피한 불순물로 조성되며, CuS석출물의 평균크기가 0.1㎛이하로 이루어지는 것이다. 본 발명에 의하면 미세한 CuS석출물에 의해 결정립내 고용탄소량을 줄여 소부경화특성과 가공성이 개선된다.A cold-rolled steel sheet of a hardening type for use as a material for automobiles, home appliances, and the like, and a method of manufacturing the same. The cold-rolled steel sheet contains 0.003-0.005% of C, 0.003-0.025% of S, 0.01-0.08% of Al, 0.004% or less of N, 0.015% or less of P and 0.01-0.2% , Cu and S satisfy the condition 0.5 * Cu / S: 1-10, the balance Fe and other unavoidable impurities, and the average size of CuS precipitates is 0.1 탆 or less. According to the present invention, the amount of dissolved carbon in the crystal grains is reduced by fine CuS precipitates, and the hardening properties and the workability are improved.
냉연강판, 소부경화, 소성이방성 지수, CuS석출물 Cold rolled steel sheet, Sinter hardening, Plastic anisotropy index, CuS precipitate
Description
도 1은 CuS석출물의 크기에 따른 결정립내 고용탄소량의 변화를 나타내는 그래프이다. Fig. 1 is a graph showing changes in the amount of solid carbon in the crystal grains depending on the size of the CuS precipitate.
도 2는 냉각속도에 따른 CuS석출물의 크기를 나타내는 그래프로서,2 is a graph showing the size of CuS precipitates with respect to the cooling rate,
도 2(a)는 0.5*Cu/S:2.56의 경우이고,2 (a) shows the case of 0.5 * Cu / S: 2.56,
도 2(b)는 0.5*Cu/S:8.1의 경우이며,2 (b) shows the case of 0.5 * Cu / S: 8.1,
도 2(c)는 0.5*Cu/S:28의 경우이다.2 (c) shows the case of 0.5 * Cu / S: 28.
본 발명은 자동차, 가전제품 등의 소재로 사용되는 소부경화형 냉연강판과 그 제조방법에 관한 것이다. 보다 상세하게는, 미세한 CuS석출물에 의해 결정립내 고용탄소량을 조절하여 소부경화특성과 가공성이 개선된 냉연강판과 그 제조방법에 관한 것이다.
BACKGROUND OF THE INVENTION 1. Field of the Invention [0002] The present invention relates to a cold-rolled steel sheet of a bake-hardening type used as a material for automobiles, household appliances and the like, and a manufacturing method thereof. More particularly, the present invention relates to a cold-rolled steel sheet having improved hardening properties and processability by controlling the amount of solid carbon in the crystal grains by fine CuS precipitates, and a method for producing the same.
자동차 등의 외판 소재에는 내덴트성을 향상하기 위하여 소부경화형 냉연강판이 많이 사용되고 있다. 소부경화형 냉연강판은 강판중에 적정량의 고용탄소를 잔존시켜 프레스 성형시에 생성된 전위를 도장소부시의 열을 이용하여 고용탄소를 고착하여 항복점을 높인 강이다.
In order to improve the dent resistance, cold-rolled steel sheets of bake-hardening type are often used for the outer plate materials of automobiles and the like. The bake-hardening type cold-rolled steel sheet is a steel in which an adequate amount of solid carbon is left in the steel sheet and the yield point is raised by fixing the solid carbon produced by the heat of the bobbin to the electric potential generated at the press molding.
소부경화형 냉연강판에는 상소둔재인 Al-Killed강과 IF강(Interstitial Free Steel)이 있다. There are Al-Killed steel and IF steel (Interstitial Free Steel), which are sintered blast furnace.
상소둔재인 Al-Killed강의 경우에는 적은 양의 고용탄소가 잔존하고 있어 비시효특성을 확보하면서 소부처리후 10~20MPa 정도의 소부경화능을 가진다. 상소둔재의 경우 소부처리후 상승하는 항복강도가 낮고, 장시간 소둔하므로 생산성이 낮다는 단점이 있다. In the case of Al-Killed steel, the amount of solid carbon remaining is small, and it has a hardening ability of 10 ~ 20 MPa after baking while securing the non-aging property. In the case of unsintered blast furnace, there is a disadvantage in that the yield strength rising after baking treatment is low and the productivity is low due to long-time annealing.
IF강의 경우에는 Ti, Nb을 첨가하여 강중에 고용된 탄소 또는 질소를 완전히 석출하여 성형성을 향상시킨 강종으로, 이 IF강에 소부경화특성을 부여한 것이 소부경화형 IF강이다. 소부경화형 IF강은 Ti 또는 Nb의 첨가량과 탄소의 첨가량을 제어하여 적당한 양의 탄소를 강중에 잔존하게 하여 소부경화특성을 부여한 것이다. 소부경화형 IF강의 경우 적당한 양의 탄소를 고용하기 위해서는 첨가되는 탄소의 양 뿐만 아니라, 첨가되는 Ti 또는 Nb의 양은 물론, Ti, Nb과 반응하여 석출물을 생성하는 황, 질소의 양도 매우 좁은 범위에서 제어를 해야하므로 안정적인 품질확보가 어려우며, 생산비용도 많이 드는 단점이 있다. In the case of the IF steel, Ti and Nb are added to improve the formability of carbon or nitrogen completely dissolved in the steel to improve the formability. It is a hardening type IF steel in which the IF hardening property is given to the IF steel. The hardening type IF steel is obtained by controlling the amount of addition of Ti or Nb and the amount of carbon added so that a suitable amount of carbon remains in the steel to give a curing property. In the case of the hardening type IF steel, it is necessary to control not only the amount of carbon added but also the amount of Ti or Nb to be added as well as the amount of sulfur and nitrogen which react with Ti and Nb to form a precipitate in a very narrow range It is difficult to secure stable quality, and the production cost is high.
본 발명은 Ti, Nb을 첨가하지 않으면서 소부경화특성을 갖고 가공성이 우수한 냉연강판과 그 제조방법을 제공하는데, 그 목적이 있다. An object of the present invention is to provide a cold-rolled steel sheet having excellent curing properties and excellent workability without addition of Ti and Nb, and a manufacturing method thereof.
상기 목적을 달성하기 위한 본 발명의 냉연강판은, 중량%로 C:0.003-0.005%, S:0.003-0.025%, Al:0.01-0.08%, N:0.004%이하, P:0.015%이하, Cu:0.01-0.2%를 포함하고, 상기 Cu와 S가 조건 0.5*Cu/S:1-10(Cu, S의 함량은 중량%임)를 만족하고, 나머지 Fe 및 기타 불가피한 불순물로 조성되며, CuS석출물의 평균크기가 0.1㎛이하로 이루어진다.
In order to achieve the above object, the cold-rolled steel sheet of the present invention comprises 0.003-0.005% of C, 0.003-0.025% of S, 0.01-0.08% of Al, 0.004% or less of N, : 0.01-0.2%, wherein the Cu and S satisfy the condition 0.5 * Cu / S: 1-10 (the content of Cu, S is% by weight), the balance Fe and other unavoidable impurities, CuS The average size of the precipitates is 0.1 mu m or less.
또한, 본 발명의 냉연강판의 제조방법은, 중량%로 C:0.003-0.005%, S:0.003-0.025%, Al:0.01-0.08%, N:0.004%이하, P:0.015%이하, Cu:0.01-0.2%를 포함하고, 상기 Cu와 S가 조건 0.5*Cu/S:1-10를 만족하고, 나머지 Fe 및 기타 불가피한 불순물로 조성되는 강을 1100℃이상의 온도로 재가열한 후 마무리 압연온도를 Ar3변태점 이상으로 하여 열간압연하고 300℃/min이상의 속도로 냉각하고 700℃이하의 온도에서 권취한 다음, 냉간 압연하고, 연속소둔하는 것을 포함하여 이루어진다.
The method of manufacturing a cold-rolled steel sheet according to the present invention is a method of producing a cold-rolled steel sheet, comprising: 0.003 to 0.005% of C, 0.003 to 0.025% of S, 0.01 to 0.08% of Al, 0.004% or less of N, 0.01 to 0.2%, and the Cu and S satisfy the condition 0.5 * Cu / S: 1-10 and the remaining Fe and other unavoidable impurities are reheated to a temperature of 1100 ° C or higher, and then the finish rolling temperature Rolled at an Ar 3 transformation point or more, cooled at a rate of 300 ° C / min or more and wound at a temperature of 700 ° C or lower, cold rolled, and continuously annealed.
이하, 본 발명을 상세히 설명한다. Hereinafter, the present invention will be described in detail.
본 발명자들은 Ti, Nb을 첨가하지 않으면서 소부경화 특성을 개선하기 위한 연구과정에서 다음과 같은 새로운 사실을 밝혀내었다. 탄소함량을 적정량으로 제어 하면서 석출물을 미세하게 분포시키면 항복강도도 높으면서 소부후의 항복강도가 크게 증가한다는 것이다. 이는 미세한 CuS석출물이 결정립내 고용탄소량에 영향을 미친다는 연구에 기반한 것이다. The inventors of the present invention have found the following new facts in the process of improving the hardening properties of the resin without addition of Ti and Nb. When the precipitates are finely distributed while controlling the carbon content in an appropriate amount, the yield strength after the baking is greatly increased while the yield strength is high. This is based on the research that fine CuS precipitates affect the amount of solid carbon in the crystal grains.
도 1에 나타난 바와 같이, 석출물이 미세하게 분포할수록 결정립내의 고용탄소량이 줄어드는 것을 확인할 수 있다. 결정립내 잔존하는 고용탄소는 이동이 비교적 자유롭기 때문에 가동전위와 결합하여 상온시효특성에 영향을 미치게 된다. 이에 반해, 결정립계나 석출물의 주변과 같이 보다 안정된 위치에 편석하여 존재하는 탄소들은 도장소부처리와 같은 고온에서 활성화되어 소부경화특성에 영향을 주게 된다. 이와 같이, 결정립내의 고용탄소량이 줄어드는 것은 보다 안정된 위치 즉, 결정립계나 미세한 석출물들의 주변에서 탄소가 편석하여 소부경화특성에 영향을 미친다는 것이다. As shown in Fig. 1, it can be seen that as the precipitates are finely distributed, the amount of the solid carbon in the crystal grains decreases. The remaining solid carbon in the crystal grains is relatively free to move, and therefore, affects the aging characteristics at room temperature in combination with the movable potential. On the other hand, carbons segregated at a more stable position such as the grain boundaries or the vicinity of the precipitate are activated at a high temperature such as a paint baking treatment, thereby affecting the baking hardening characteristics. As described above, the decrease in the amount of solid carbon in the crystal grains is attributable to the more stable position, that is, the segregation of carbon in the vicinity of grain boundaries and fine precipitates, which affects the curing properties.
본 발명에 따르면, 소부경화특성은 탄소의 함량을 0.003~0.005%로 하면서 CuS석출물의 평균크기가 0.1㎛이하가 될 때 안정적으로 확보될 수 있다. 이와 같은 새로운 사실에 주목하여 CuS를 미세하게 분포시키는 방안에 대하여 연구하게 되었다. 그 결과,(1) Cu의 함량을 0.01~0.2%로 하고 S의 함량을 0.003~0.025%로 하면서 이들의 성분비(0.5*Cu/S)를 1-10으로 조절하는 것이 필요하며, (2) 이와 함께 압간압연이 끝난 후 냉각속도를 300℃/min이상으로 하면 0.2㎛이하의 미세한 CuS의 석출물을 얻을 수 있다는 것이다. According to the present invention, the bake hardening characteristics can be stably ensured when the average size of the CuS precipitates is 0.1 탆 or less while the content of carbon is 0.003 to 0.005%. With this new fact in mind, we have been studying the possibility of finely distributing CuS. As a result, (1) it is necessary to adjust the composition ratio (0.5 * Cu / S) to 1-10 by setting the content of Cu to 0.01 to 0.2% and the content of S to 0.003 to 0.025% In addition, when the cooling rate is 300 ° C / min or more after completion of the intermetallic rolling, fine CuS precipitates of 0.2 μm or less can be obtained.
즉, 도 2(a)는 0.004%C-0.01%P-0.008%S-0.05%Al-0.0014%N-0.041%Cu인 강(0.5*Cu/S:2.56)을 열간압연후 냉각속도에 따른 석출물의 크기를 조사한 그래프이 다. 도 2(a)의 그래프를 보면, Cu와 S의 성분비(0.5*Cu/S)가 1-10를 만족하는 경우에 대해 냉각속도를 조절하면 CuS의 석출물 크기가 0.1㎛이하를 만족할 수 있음을 확인할 수 있다.
That is, FIG. 2 (a) is a graph showing the relationship between the cooling rate after hot rolling of steel (0.5 * Cu / S: 2.56) containing 0.004% C-0.01% P-0.008% S-0.05% Al-0.0014% N-0.041% The size of the precipitate is a graph. The graph of FIG. 2 (a) shows that when the cooling rate is adjusted for the case where the composition ratio of Cu and S (0.5 * Cu / S) satisfies 1-10, the precipitate size of CuS can be 0.1 μm or less Can be confirmed.
이러한 관점에 기초하여 완성된 본 발명의 냉연강판과 그 제조방법을 이하에서 구체적으로 설명한다. The cold-rolled steel sheet of the present invention which has been completed on the basis of this viewpoint and the manufacturing method thereof will be described in detail below.
[본 발명의 냉연강판][Cold rolled steel sheet of the present invention]
탄소(C)의 함량은 0.003~0.005%가 바람직하다.The content of carbon (C) is preferably 0.003 to 0.005%.
본 발명에서는 미세한 석출물에 의해 결정립내 고용탄소량이 줄어들게 되면서 그 만큼 소부경화량이 커지게 된다. 즉, 강판중에 총탄소량이 많아질수록 결정립내 보다 소부경화특성에 유효한 결정립계나 석출물의 주변에서 편석하는 탄소의 함량이 많아지게 된다. 이를 고려할 때, 탄소(C)의 함량은 0.003%이상되어야 소부경화특성을 확보하게 된다. 소부경화량을 보다 크게 하기 위해서는 탄소의 함량을 0.0031%이상으로 하는 것이 바람직하다. 탄소의 함량이 0.005%초과의 경우에는 성형성이 급격히 저하된다.
In the present invention, as the amount of solid carbon in the crystal grains is reduced by fine precipitates, the amount of hardening of the sintered compact is increased correspondingly. That is, as the amount of the total carbon in the steel sheet increases, the amount of carbon segregated in the vicinity of grain boundaries or precipitates effective for the curing properties of the cement is higher than in the crystal grains. Considering this, the content of carbon (C) should be 0.003% or more to secure the curing property of the resin. In order to increase the baking hardening amount, it is preferable that the content of carbon is 0.0031% or more. When the content of carbon is more than 0.005%, the moldability drastically decreases.
황(S)의 함량은 0.003-0.025%가 바람직하다.The content of sulfur (S) is preferably 0.003-0.025%.
황(S)의 함량이 0.003%미만의 경우에는 CuS 석출량이 적을 뿐만 아니라 석출되는 CuS의 크기가 매우 조대해져 소부경화특성에 좋지 않다. 황의 함량이 0.025% 초과의 경우에는 고용된 황의 함량이 많아 연성 및 성형성이 크게 낮아지며, 적열 취성의 우려가 있기 때문이다. 황의 함량은 0.02초과~0.025%이하의 범위일 때 CuS의 석출물 크기를 원하는 범위로 조절하기가 용이해진다.
When the content of sulfur (S) is less than 0.003%, not only CuS precipitation amount is small but also the size of precipitated CuS becomes very large, which is not good for the curing property. When the content of sulfur is more than 0.025%, the content of sulfur dissolved is large, so that ductility and moldability are greatly lowered, and there is a fear of heat brittleness. When the sulfur content is in the range of more than 0.02 to 0.025%, it becomes easy to adjust the precipitate size of CuS to a desired range.
알루미늄(Al)의 함량은 0.01-0.08%가 바람직하다.The content of aluminum (Al) is preferably 0.01-0.08%.
알루미늄은 탈산제로 첨가하는 원소이지만 본 발명에서는 강중 질소를 석출하여 고용질소에 의한 성형성 저하방지를 위해 첨가한다. 알루미늄의 함량이 0.01%미만의 경우에는 고용질소의 양이 많아 성형성이 저하하고, 알루미늄의 함량이 0.08%초과의 경우에는 고용 상태로 존재하는 알루미늄의 양이 많아 연성을 저하한다.
Aluminum is an element to be added as a deoxidizing agent, but in the present invention, nitrogen is precipitated in the steel so as to prevent degradation of the formability due to solid nitrogen. When the content of aluminum is less than 0.01%, the amount of solid solution nitrogen is large and the formability is deteriorated. When the content of aluminum is more than 0.08%, the amount of aluminum existing in a solid state is large.
질소(N)의 함량은 0.004%이하가 바람직하다.The content of nitrogen (N) is preferably 0.004% or less.
질소는 제강중 불가피하게 첨가되는 원소로 0.004%초과의 경우에는 성형성이 저하하므로 0.004%이하가 바람직하다.
Nitrogen is an element that is inevitably added during steelmaking. When it exceeds 0.004%, the formability is lowered. Therefore, it is preferably 0.004% or less.
인(P)의 함량은 0.015%이하가 바람직하다. The content of phosphorus (P) is preferably 0.015% or less.
인의 함량이 0.015% 초과의 경우에는 연성 및 성형성이 저하하므로 0.015%이하로 하는 것이 바람직하다.
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%.
구리(Cu)의 함량은 0.01~0.2%가 바람직하다.The content of copper (Cu) is preferably 0.01 to 0.2%.
구리는Cu와 S의 함량비 그리고 열간압연공정에서 권취전의 냉각속도가 적절 해지는 경우 0.1㎛이하의 매우 미세한 CuS가 석출되고 이 CuS석출물의 주변에는 탄소가 석출되어 석출된 탄소는 도장소부처리과정에서 용해되어 소부경화능을 부여한다는 연구에 기초하여 0.01~0.2% 첨가한다. 구리의 함량이 0.01%이상되어야 미세하게 CuS석출할 수 있고 0.2%초과하면 조대한 CuS가 석출하여 소부경화특성이 열악해진다.
Copper has very fine CuS precipitates of 0.1 탆 or less when the content ratio of Cu and S and the cooling rate before winding in the hot rolling process become appropriate, and the carbon deposited on the periphery of the CuS precipitate and precipitated carbon is treated in the coating baking process 0.01 to 0.2% is added based on the study that it is melted to give a resin hardening ability. If the content of copper is 0.01% or more, CuS can be precipitated finely. If it is more than 0.2%, coarse CuS precipitates and the hardening property of the resin hardens.
상기 Cu와 S의 중량비는 0.5*Cu/S:1~10를 만족하는 것이 바람직하다.The weight ratio of Cu to S is preferably 0.5 * Cu / S: 1 to 10.
Cu와 S은 결합하여 CuS로 석출되는데, 이 CuS석출물은 Cu와 S의 첨가량에 따라 석출상태가 달라져 소부경화특성, 소성이방성지수, 면내이방성 지수에 영향을 미친다. 본 발명의 연구에 따르면 Cu과 S의 첨가비(0.5*Cu/S, 여기서, Cu, S의 함량은 중량%)가 1이상이 되어야 유효한 CuS석출물이 석출하게 되며, 10초과의 경우에는 CuS석출물이 조대하여 소부경화특성이 좋지 않으며, 소성이방성지수, 면내이방성 지수의 특성이 좋지 않다. 0.1㎛이하의 CuS를 안정적으로 확보하기 위해 보다 바람직한 0.5*Cu/S의 값은 1~3이다.
Cu and S bond with each other and precipitate as CuS. This CuS precipitate has a different precipitating state depending on the amount of Cu and S added, which affects the curing properties, the plastic anisotropy index, and the in-plane anisotropy index. According to the study of the present invention, effective CuS precipitates are precipitated when the addition ratio of Cu and S (0.5 * Cu / S, wherein the content of Cu and S is in weight%) is 1 or more, The sintering property is poor, and the properties of plastic anisotropy index and in-plane anisotropy index are not good. A more preferable value of 0.5 * Cu / S is 1 to 3 for stably securing CuS of 0.1 탆 or less.
CuS석출물의 평균크기는 0.1㎛이하가 바람직하다. The average size of the CuS precipitates is preferably 0.1 탆 or less.
본 발명의 연구결과에 따르면 CuS석출물의 크기가 소부경화특성, 소성이방성지수, 면내이방성지수에 직접적으로 영향을 미치는데, CuS의 평균크기가 0.1㎛ 초과의 경우에는 특히 소부경화특성이 떨어지며, 소성이방성지수, 면내이방성지수가 좋지 않다. 따라서, CuS 석출물의 평균크기는 0.1㎛ 이하가 바람직하다. According to the results of the present invention, the size of the CuS precipitates directly affects the sintering property, the plastic anisotropy index and the in-plane anisotropy index. When the average size of the CuS exceeds 0.1 μm, the sintering property is particularly poor, Anisotropy index and in-plane anisotropy index are not good. Therefore, the average size of the CuS precipitates is preferably 0.1 탆 or less.
본 발명의 냉연강판에는 추가로 몰리브덴(Mo)이 포함될 수 있다. 이러한 Mo의 함량은 0.01~0.2%가 바람직하다.몰리브덴은 가공성확보를 위해 첨가되는데, 그 함량이 0.01%이상되어야 가공성이 좋아지며, 0.2%를 초과하면 가공성은 더 이상 좋아지지 않고 열간취성을 일으킬 우려가 있다.
The cold-rolled steel sheet of the present invention may further contain molybdenum (Mo). Molybdenum is added to ensure workability. When the content of molybdenum is 0.01% or more, the workability is improved. If the content is more than 0.2%, the workability is not improved any more and causes hot brittleness. There is a concern.
[냉연강판의 제조방법] [Production method of cold-rolled steel sheet]
본 발명은 상기한 강조성을 만족하는 강을 열간압연과 냉간압연을 통해 냉간압연판에 CuS석출물의 평균크기가 0.1㎛ 이하를 만족하도록 하는데 특징이 있다. 냉간압연판의 CuS석출물의 크기는 Cu/S의 비와 제조공정에 영향을 받으나 특히 열간압연후의 냉각속도에 직접적인 영향을 받는다. The present invention is characterized in that an average size of CuS precipitates in the cold rolled steel sheet is 0.1 mu m or less through hot rolling and cold rolling. The size of the CuS precipitates in the cold rolled sheet is affected by the ratio of Cu / S and the manufacturing process, but is directly affected by the cooling rate especially after hot rolling.
[열간압연조건][Hot rolling condition]
본 발명에서는 상기한 강조성을 만족하는 강을 재가열하여 열간압연한다. 재가열온도는 1100℃이상이 바람직하다. 재가열온도가 1100℃미만의 경우에는 재가열온도가 낮아 연속주조중에 생성된 조대한 CuS가 완전히 용해되지 않은 상태로 남아있어 열간압연후에도 조대한 CuS가 많이 남아있기 때문이다.
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 less than 1100 ° C, the coarse CuS produced during the continuous casting remains in a completely undissolved state due to the low reheating temperature, and a large amount of coarse CuS remains after the hot rolling.
열간압연은 마무리압연온도를 Ar3변태온도 이상의 조건에서 행하는 것이 바람직하다. 마무리압연온도가 Ar3변태온도 미만의 경우에는 압연립의 생성으로 가공성이 저하할 뿐만아니라 연성이 크게 저하기 때문이다. 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.
열간압연후 권취전 냉각속도는 300℃/min이상으로 하는 것이 바람직하다. 본 발명에 따라 Cu과 S의 성분비(0.5*Cu/S)를 10이하로 하더라도 냉각속도가 300℃/min미만이면 CuS의 석출물 크기가 0.1㎛를 초과해 버린다. 즉, 냉각속도가 빨라질수록 많은 수의 핵이 생성하여 CuS석출물이 미세해지기 때문이다. Cu와 S의 성분비(0.5*Cu/S)를 10초과의 경우에는 재가열공정에서 미용해된 조대한 CuS석출물이 많아 냉각속도가 빨라지더라도 새로운 핵이 생성되는 수가 적어 석출물은 미세해지지 않는다(도 2c, 0.0039%C-0.01%P-0.005%S-0.03%Al-0.0015%N-0.28%Cu). 도 2의 그래프를 보면, 냉각속도가 빨라질수록 CuS석출물의 크기가 미세해지므로 냉각속도의 상한을 제한할 필요는 없으나, 냉각속도가 1000℃/min이상이라도 석출물 미세화 효과가 더 이상 커지지 않으므로 냉각속도는 300~1000℃/min가 보다 바람직하다. 도 2a와 도 2b(0.0043%C-0.01%P-0.005%S-0.03%Al-0.0024%N-0.081%Cu)는 0.5*Cu/S의 값이 3이하의 경우와 3초과의 경우에 대한 것으로, 0.5*Cu/S의 값이 3이하일 때 보다 안정적으로 0.1㎛이하의 CuS석출물이 얻어지는 것을 알 수 있다.
The cooling rate before hot rolling is preferably 300 DEG C / min or more. According to the present invention, even if the composition ratio of Cu and S (0.5 * Cu / S) is 10 or less, if the cooling rate is less than 300 캜 / min, the precipitate size of CuS exceeds 0.1 탆. That is, as the cooling rate is increased, a large number of nuclei are generated and the CuS precipitates become finer. When the composition ratio of Cu and S (0.5 * Cu / S) is more than 10, precipitated CuS precipitates are abundant in the reheating step, so that even when the cooling rate is increased, new nuclei are generated and the precipitates are not fine , 0.0039% C-0.01% P-0.005% S-0.03% Al-0.0015% N-0.28% Cu). 2, it is not necessary to limit the upper limit of the cooling rate since the size of the CuS precipitates becomes finer as the cooling rate increases. However, even if the cooling rate is 1000 캜 / min or more, the precipitate refinement effect does not further increase, Is more preferably 300 to 1000 占 폚 / min. In the case where the value of 0.5 * Cu / S is less than or equal to 3 and the case where the value is more than 3, . When the value of 0.5 * Cu / S is 3 or less, CuS precipitates of 0.1 탆 or less can be stably obtained.
[권취조건][Winding condition]
상기와 같이 열간압연한 다음에는 권취를 행하는데, 권취온도는 700℃이하가 바람직하다. 권취온도가 700℃초과의 경우에는 CuS석출물이 너무 조대하게 성장하여 소부경화특성이 좋지 않다.
After hot rolling as described above, winding is performed, and the winding temperature is preferably 700 DEG C or lower. If the coiling temperature exceeds 700 ° C, the CuS precipitate grows too coarsely and the bake hardenability is poor.
[냉간압연조건] [Cold rolling conditions]
냉간압연은 50~90%의 압하율로 행하는 것이 바람직하다. 냉간압하율이 50%미만의 경우에는 소둔재결정 핵생성양이 적기 때문에 소둔시 결정립이 너무 크게 성장하여 소둔 재결정립의 조대화로 강도 및 성형성이 저하한다. 냉간압하율이 90%초과의 경우에는 성형성은 향상되지만 핵생성 양이 너무 많아 소둔 재결정립은 오히려 너무 미세하여 연성이 저하한다.
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.
[연속소둔][Continuous Annealing]
연속소둔 온도는 제품의 재질을 결정하는 중요한 역할을 한다. 본 발명에서는 500~900℃의 온도범위에서 행하는 것이 바람직하다. 연속소둔 온도가 500℃미만의 경우에는 재결정립이 너무 미세하여 목표로 하는 연성값을 확보할수 없으며, 소둔온도가 900℃초과의 경우에는 재결정립의 조대화로 강도가 저하된다. 연속소둔시간은 재결정이 완료되도록 유지하는데, 약 10초이상이면 재결정이 완료된다.
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 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.
이하, 본 발명을 실시예를 통하여 보다 구체적으로 설명한다.Hereinafter, the present invention will be described in more detail with reference to examples.
실시예에서 기계적특성은 냉연강판은 ASTM규격(ASTM E-8 standard)에 의한 표준시편으로 가공하여 측정하였다. 기계적특성은 인장시험기(INSTRON사, Model 6025)를 이용하였다. 소부후의 항복강도는 시편에 2%의 스트레인을 가한 후 170℃에서 20분간 열처리후 항복강도를 측정한 것이다. 소성이방성 지수(rm값), 면내이방성 지수(△r값)은 다음의 식으로 구하였다. In the examples, the mechanical properties were measured by processing the cold-rolled steel sheet with a standard specimen according to ASTM E-8 standard. Mechanical properties were measured using a tensile testing machine (INSTRON, Model 6025). The yield strength after baking was measured by applying a 2% strain to the test specimen, and then measuring the yield strength after heat treatment at 170 ° C for 20 minutes. The plastic anisotropy index ( rm value) and in-plane anisotropy index (? R value) were obtained by the following formulas.
rm=(r0+2r45+r90)/4, △r=(r0-2r45+r 90)/2 r m = (r 0 + 2r 45 + r 90) / 4, △ r = (r 0 -2r 45 + r 90) / 2
석출물의 평균크기와 석출물의 분포수는 기지내 존재하는 모든 석출물의 크기와 분포수를 측정한 것이다. The average size of the precipitates and the number of precipitates are the sizes and distribution numbers of all the precipitates present in the matrix.
[실시예 1][Example 1]
표 1의 강슬라브를 1200℃에서 재가열하여 마무리열간압연한 후 400℃/min의 속도로 냉각하고 650℃에서 권취한 다음, 75%의 압하율로 냉간압연과 연속소둔처리하였다. 이때의 마무리압연온도는 Ar3변태점이상인 910℃이며, 연속소둔은 10℃/초의 속도로 750℃로 40초 동안 가열하여 행하였다. 표 1에서 A7은 강슬라브를 1050℃에서 재가열하여 마무리열간압연한 후 400℃/min의 속도로 냉각하고 750℃에서 권취한 것이다. The steel slabs shown in Table 1 were reheated at 1200 캜 and subjected to finish hot rolling, followed by cooling at a rate of 400 캜 / min and winding at 650 캜, followed by 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. In Table 1, A7 indicates that the steel slab was reheated at 1050 占 폚, finished hot-rolled, cooled at a rate of 400 占 폚 / min, and wound at 750 占 폚.
Cu
0.06
0.083
0.05
0.25
0.064
(㎛)Average size of precipitates
(탆)
(MPa)Yield strength
(MPa)
(MPa)The tensile strength
(MPa)
(%)Elongation
(%)
(△r)In-plane anisotropy index
(R)
(MPa)Yield strength after baking
(MPa)
(DBTT-℃)Secondary brittle
(DBTT-C)
표 1, 2에 나타난 바와 같이, A1~A4(발명강)는 화학성분 및 제조조건이 본 발명에서 제시하는 범위에 포함되어 연신율 45%이상으로 소성이방성 지수 1.75이상, 면내이방성 지수 0.41이하로 매우 우수한 성형성을 가지며, 소부경화특성도 우수하다. 이러한 재질특성을 나타내는 것은 CuS의 석출물의 크기를 0.1㎛이하로 제어함으로써 가능하다. As shown in Tables 1 and 2, A1 to A4 (inventive steel) have a plastic anisotropy index of 1.75 or more and an in-plane anisotropy index of 0.41 or less at an elongation of 45% or more, It has excellent moldability and excellent curing properties. This material property can be exhibited by controlling the size of the precipitate of CuS to 0.1 mu m or less.
한편, A5(비교강)의 경우 탄소함량이 낮아 소부하 항복강도가 낮고, A6(비교강)은 석출물의 크기가 커서 소부후 항복강도가 낮다. A7(비교강)은 탄소함량이 높아 연신율 및 소성이방성지수가 낮아 성형가공시 파단이 일어날 가능성이 크다.On the other hand, A5 (comparative steel) has a low carbon yield and a small shear yield strength, and A6 (comparative steel) has a large precipitate size and low yield strength after baking. A7 (comparative steel) has a high carbon content, and the elongation and the plastic anisotropy index are low, so that there is a high possibility of fracture during molding.
[실시예 2][Example 2]
표 3의 강슬라브를 1200℃에서 재가열하여 마무리열간압연한 후 450℃/min의 속도로 냉각하고 650℃에서 권취한 다음, 75%의 압하율로 냉간압연과 연속소둔처리하였다. 이때의 마무리압연온도는 Ar3변태점이상인 910℃이며, 연속소둔은 10℃/초의 속도로 750℃로 40초 동안 가열하여 행하였다.
The steel slabs shown in Table 3 were reheated at 1200 ° C and subjected to finish hot rolling, followed by cooling at a rate of 450 ° C / min and winding at 650 ° C, followed by 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.
Cu/S0.5 *
Cu / S
0.080.01-
0.08
0.20.01-
0.2
0.20.01-
0.2
(㎛)Average size of precipitates
(탆)
(MPa)Yield strength
(MPa)
(MPa)The tensile strength
(MPa)
(%)Elongation
(%)
(MPa)Yield strength after baking
(MPa)
(DBTT-℃)Secondary brittle
(DBTT-C)
표 3, 4에 나타난 바와 같이, B1~B3은 본 발명을 만족하는 발명강으로 소부경화특성을 갖으면서 가공성이 우수하다. B4의 경우에는 Mo의 첨가량이 본 발명의 조건 보다 많은 것으로 가공성이 좋지 않다. As shown in Tables 3 and 4, B1 to B3 are inventive steels satisfying the present invention and have excellent bending hardening properties and excellent workability. In the case of B4, the addition amount of Mo is larger than that of the present invention, and the workability is poor.
상술한 바와 같이, 본 발명에 따라 제공되는 냉연강판은 소부경화특성이 우수하고 소성이방성지수가 높으면서 면내이방성 지수가 낮아서 가공시 주름 발생이 적으며 가공후에는 귀발생이 적은 유용한 효과가 있다. As described above, the cold-rolled steel sheet provided according to the present invention has a small effect of hardening curing, a high anisotropic index of plasticity, a low in-plane anisotropic index, and a small effect of reducing wrinkles during processing.
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| Publication number | Priority date | Publication date | Assignee | Title |
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| JPH0617141A (en) * | 1992-07-03 | 1994-01-25 | Nippon Steel Corp | Production of cold rolled steel sheet excellent in workability and shape |
| KR20020035653A (en) * | 2000-08-04 | 2002-05-13 | 아사무라 타카싯 | Cold rolled steel sheet and hot rolled steel sheet excellent in bake hardenability and resistance to ordinary temperature aging and method for their production |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPH0617141A (en) * | 1992-07-03 | 1994-01-25 | Nippon Steel Corp | Production of cold rolled steel sheet excellent in workability and shape |
| KR20020035653A (en) * | 2000-08-04 | 2002-05-13 | 아사무라 타카싯 | Cold rolled steel sheet and hot rolled steel sheet excellent in bake hardenability and resistance to ordinary temperature aging and method for their production |
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