KR100530058B1 - Method for Manufacturing Cold Rolled Steel Sheet with Superior Elongation and Drawability - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a cold rolled steel sheet used in automobile large panel parts subjected to severe stretching and drawing processing, such as side panels and rear floors. It is an object of the present invention to provide a method for manufacturing a cold rolled steel sheet which can improve the ductility and drawing property while suppressing grain growth of hot rolled sheet in titanium-added high purity ultra low carbon steel by appropriately controlling the rolling reduction rate and rolling speed.

In the present invention, by weight% C: 0.001-0.003% or less, Mn: 0.5% or less, P: 0.003-0.02% or less, S: 0.003% or less, N: 0.002% or less, acid value Al: 0.01-0.05%, Ti : [4C + 1.5S + 3.8N + 0.005] ≤ Ti ≤ [4C + 1.5S + 3.8N + 0.020], 10 ℃ -100 ℃ per minute from 1300 ℃ to 900 ℃ during slab cooling of steel composed of remaining Fe After cooling at speed, it was heated to 1000-1250 ℃ and hot-rolled under the condition of finishing hot rolling at Ar ₃ or higher at 5% -15% of final rolling and 600m-1200m per minute, rolling speed at 650-750 ℃ After winding to room temperature, cold-rolled to 80% -90% after pickling, heated to 820 ℃ ~ 900 ℃ and annealed for 10-180 seconds to produce cold rolled steel sheet having excellent ductility and drawing property. Make a point.

Description

Method for Manufacturing Cold Rolled Steel Sheet with Superior Elongation and Drawability}

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing cold rolled steel sheets used in automobile large panel parts subjected to severe stretching and drawing processing, such as side panels and rear floors. It relates to an excellent cold rolled steel sheet manufacturing method.

In order to improve the ductility and drawing property of cold rolled steel sheet for automobiles, titanium, niobium or titanium-niobium is added to ultra low carbon steel to precipitate carbon and nitrogen in the steel during hot rolling to facilitate grain growth during annealing. Therefore, a method of securing ductility and promoting the growth of crystal grains having a (111) aggregate structure to simultaneously secure drawing property is mainly used.

However, in the continuous casting process to make titanium-added ultra-low carbon steel slab, the surface of the slab is cooled by water. At this time, the cooling rate of the surface layer is very fast in the high temperature range, which makes it difficult for titanium to combine with sulfur or nitrogen in the steel and the amount of unprecipitated titanium increases. Done. In general, the slab is heated to more than 1000 ℃ and then hot rolled. Since the solid titanium unprecipitated in the heating process combines with oxygen to form coarse titanium oxide, the formation of fine titanium precipitates after hot rolling decreases, resulting in hot rolling. In the high temperature region, grain growth cannot be suppressed, resulting in coarse grains in the surface layer.

Since these coarse grains deteriorate ductility and drawability, there is a problem in manufacturing high purity ultra low carbon steel containing titanium.

As a method for suppressing and minimizing grain growth immediately after hot rolling, as in Japanese Patent Application Laid-Open No. 61-276930, Japanese Patent Application Laid-Open No. 1-77322, and Japanese Patent Application Laid-open No. Hei 7-70650, a combination of niobium and titanium is added. There is a method of increasing the cumulative reduction ratio on the production side or controlling the cooling start time and cooling rate immediately after rolling.

In the case of adding niobium, which is known to refine the grains of a hot rolled sheet, carbides are precipitated after cooling after hot rolling or after winding, thereby suppressing grain growth. Therefore, there is no effect of suppressing grains at high temperatures as immediately after hot rolling.

In addition, the method of increasing the cumulative reduction ratio in hot rolling is effective in miniaturizing the central grains of the rolled plate, but has an adverse effect in miniaturizing the surface grains. That is, since the strain energy applied to the surface layer by rolling immediately after hot rolling causes grain growth (called the strain annealing effect), the application of a high cumulative reduction ratio causes a problem of making the grain coarse.

The present inventors conducted research and experiments to solve the above-mentioned problems of the prior art, and based on the results, the present invention proposes the present invention, the present invention is a chemical composition of steel, slab cooling rate, hot rolling reduction rate and rolling It is an object of the present invention to provide a method for manufacturing a cold rolled steel sheet capable of improving ductility and drawing property while suppressing grain growth of a hot rolled sheet in titanium-added high purity ultra low carbon steel by appropriately controlling the speed.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

In the present invention, by weight% C: 0.001-0.003% or less, Mn: 0.5% or less, P: 0.003-0.02% or less, S: 0.003% or less, N: 0.002% or less, acid value Al: 0.01-0.05%, Ti : [4C + 1.5S + 3.8N + 0.005] ≤ Ti ≤ [4C + 1.5S + 3.8N + 0.020], 10 ℃ -100 ℃ per minute from 1300 ℃ to 900 ℃ during slab cooling of steel composed of remaining Fe After cooling at speed, it was heated to 1000-1250 ℃ and hot-rolled under the condition of finishing hot rolling at Ar ₃ or higher at 5% -15% of final rolling and 600m-1200m per minute, rolling speed at 650-750 ℃ After winding to cold at room temperature, cold rolled to 80% -90% range after pickling, heated to 820 ℃ ~ 900 ℃ annealing for 10-180 seconds to produce a cold rolled steel sheet excellent in ductility and drawing will be.

In addition, the present invention relates to a method for producing a cold rolled steel sheet having excellent ductility and drawing property by adding one or two of Nb: 0.003-0.01% and B: 0.0001-0.0010% to the above-described steel.

Hereinafter, the chemical components and the production conditions of the present invention will be described in detail.

The present invention forms a titanium precipitate on the surface layer by controlling the cooling rate in the slab cooling process in the high purity ultra-low carbon aluminum-kilted (Al-killed) steel containing titanium (Ti), and the reduction rate and rolling speed in the hot rolling final rolling By controlling the growth rate of the surface crystal grains, excessive growth of the grains of the surface layer after hot rolling is suppressed.

In addition, the present invention by controlling the amount of carbon and sulfur in the steel and the coiling temperature after hot rolling to make a part of the precipitate of the hot rolled sheet into a fine titanium carbide that is easy to re-dissolve in the annealing process (111) ) Securing drawing structure and securing drawing property, eluting trace amount of carbon to strengthen grain boundary, greatly improving ductility.

The present invention also improves hot dip galvanizing properties.

If the content of carbon in steel is less than 0.001%, the grain growth rate is too fast immediately after hot rolling, and coarse grains are formed, and drawing property is deteriorated. If the amount is too large, a large amount of fine titanium carbide forms to damage ductility. Its content is preferably limited to 0.001-0.003%.

The manganese (Mn) is excessively added to inhibit the formation of titanium carbide to reduce the drawability, and decrease the ductility by strengthening the solid solution, the content is preferably limited to 0.5% or less.

Phosphorus (P) is essentially present in molten steel, but if it is lowered to an extreme concentration in the refining process, it is excessively costly, and also forms titanium phosphide during the high temperature winding to increase the recrystallization temperature in the annealing process to increase the ductility and drawability It is preferable to limit the content to 0.003 to 0.02% because it lowers.

When the content of sulfur (S) is 0.003% or more, since titanium sulfide is precipitated as TiS in Ti ₄ C ₂ S ₂ , the content thereof is preferably limited to 0.003% or less.

Since the nitrogen (N) is combined with titanium in the steel to reduce the ductility and drawability, the content is preferably limited to 0.002% or less.

In addition, if the content of acid-soluble Al is less than 0.01%, the deoxidation of molten steel is insufficient, resulting in bursting during the slag solidification process, and if it is more than 0.05%, segregation at grain boundaries lowers the ductility, so the content is 0.01-0.05%. It is preferable to limit to.

The titanium (Ti) does not add more than the theoretical amount (Ti = 4C + 1.5S + 3.8N) necessary to combine stoichiometrically with carbon, sulfur and nitrogen in the steel, so that the drawability is greatly lowered. It is preferable to lower the theoretical amount + 0.005% to the lower limit in consideration of the incomplete reaction of precipitation, and to set the upper limit to the theoretical amount + 0.020% because the amount of titanium is too high, which impairs the ductility. .

On the other hand, Nb delays recrystallization during hot rolling and forms titanium-niobium composite carbide during cooling after winding to refine grains of the hot-rolled sheet to increase drawing ability. The effect is very small at 0.003% or less and 0.01% or more. In the titanium-niobium composite carbide is excessively formed to decrease the ductility by raising the recrystallization temperature of the annealing plate, the content is preferably limited to 0.003-0.01%.

B is added to prevent grain boundary embrittlement by suppressing grain boundary movement during hot rolling as an element segregating at grain boundary, and to prevent grain embrittlement due to high purity, and when the amount is less than 0.0001%, there is no addition effect. If the content is 0.0010% or more, the recrystallization temperature of the annealing plate is increased to decrease the ductility and drawing property, so the content is preferably limited to 0.0001-0.0010%.

When cooling the steel slab formed as described above, it is preferable to limit the cooling rate at a rate of 10 ° C.-100 ° C. per minute from 1300 ° C. at which titanium sulfide and nitride starts to precipitate at 900 ° C. when precipitation is completed.

The reason why the cooling rate of the slabs is limited as described above is that when the slab cooling rate is too slow, a large amount of scale is generated on the surface of the slab, which is uneconomical and it is difficult to precipitate titanium sulfide and nitride when cooled at a higher rate.

The slab cooled as described above is heated to 1000-1250 ° C. in order to secure the hot rolling temperature. The reason is that when the heating temperature is less than 1000 ° C., the slab is difficult to secure the rolling temperature. It is because it becomes so large that it will reduce drawing property.

It is preferable to select a reduction ratio of 5-15% in the final rolling of the slab heated as described above during hot rolling.

The reduction ratio acts as a growth energy of the surface grains. When the reduction ratio is 5% or less, the size reduction due to hot rolling is small and shape control is difficult, whereas when the reduction ratio is 15% or more, the grain growth energy is so large that coarse grains are formed on the surface layer. Since it forms, it is preferable to limit a reduction ratio as mentioned above.

In addition, the hot rolling speed is related to the surface friction of the steel sheet, but if it is too slow, the friction is increased to increase the growth rate of the surface grains, and if it is too fast, it is preferable to set it to 600-1200m per minute because the load of the rolling mill is not economical.

In addition, when the finishing temperature of hot rolling is less than or equal to Ar ₃ , grains become coarse and drawing performance is reduced, and coarse Ti ₄ C ₂ S ₂ and TiC are precipitated by rolling and delayed re-dissolution during annealing. Since this falls, it is preferable to limit the finishing temperature of hot rolling to Ar ₃ or more.

If the hot rolling temperature is below 650 ° C, the precipitation of titanium carbide is slowed down and the drawing property is lowered. If the temperature is over 750 ° C, the drawing property decreases in the rolling direction and the 45 ° direction due to grain growth, so the winding temperature is 650-750 ° C. It is desirable to limit.

If the cold reduction rate is lower than 80%, since the introduction of strain energy during cold rolling is small, the developmental effect of the (111) aggregate structure is small during the annealing process, and the drawing property is low, and if it exceeds 90%, the grain size becomes too fine and the ductility decreases. The cold reduction rate is preferably limited to 80-90%.

The annealing temperature during annealing of the cold rolled cold rolled steel sheet as described above is limited to re-dissolve a part of titanium carbide in the steel to secure ductility and to form and develop a (111) aggregate structure which is advantageous for drawing property, and the annealing temperature is 820 ° C. or less. In this case, the precipitate hardly re-dissolves, so the ductility is low, and when it exceeds 900 ° C., transformation occurs into austenite, reducing ductility and drawing property, and therefore, the annealing temperature is preferably limited to 820-900 ° C.

In addition, in order to form and develop the grain growth and (111) aggregate structure during annealing of the cold rolled steel sheet, an appropriate annealing time is required. When the annealing time is 10 seconds or less, sufficient annealing does not occur. Orange peel-like surface defects (orange peel) occurs during the forming process of the steel sheet, the annealing time is preferably limited to 10-180 seconds.

As described above, the main feature of the present invention is to form a titanium precipitate on the surface layer by controlling the cooling rate in the slab cooling process in a high purity ultra-low carbon aluminum-kilted (Al-killed) steel with titanium (Ti), hot rolling final By controlling the reduction rate and rolling speed in rolling, limiting the growth rate of grains in the surface layer, it is possible to suppress excessive growth of grains in the surface layer after hot rolling, and to limit the amount of carbon and sulfur in the steel and the coiling temperature after hot rolling. Part of the precipitates are made of fine titanium carbide which can be easily re-dissolved during annealing, thus securing (111) aggregation structure during recrystallization during annealing, securing drawingability and eluting trace amounts of carbon to strengthen grain boundaries. Will be greatly improved.

Therefore, according to this invention, the cold rolled steel sheet which has the outstanding drawing property of Rankford value 2.5 or more and elongation of 56% or more can be manufactured.

Hereinafter, the present invention will be described in detail through examples.

(Example)

The slabs of the inventive steels (1-10) and the comparative steels (11-17) having the composition shown in Table 1 were cooled to 900 ° C at a cooling rate of 30 ° C per minute, and then heated at 1200 ° C for 1 hour to obtain a final reduction rate of 10 %, Rolling speed 800m per minute, hot-rolled at a temperature of Ar ₃ or higher, wound at 680 ° C, annealing at 860 ° C for 50 seconds after 85% cold rolling, and measuring the mechanical properties, the results are shown in Table 2 below. It was.

Table 2 below is one of the specimens (invention material 11-15) and the slab cooling rate, final rolling rate, rolling rate, winding temperature, and annealing temperature conditions that changed the manufacturing conditions within the scope of the invention for invention steel 1 The mechanical properties of the specimens (Comparative Materials 23-29) outside the scope of the present invention are shown.

Steel grade Chemical composition (% by weight) C Mn P S Sol.Al N Ti Nb B Invention steel One 0.0011 0.05 0.007 0.0015 0.025 0.0011 0.025 - - 2 0.0018 0.05 0.008 0.0011 0.024 0.0013 0.029 - - 3 0.0021 0.05 0.008 0.0010 0.023 0.0013 0.035 - - 4 0.0014 0.15 0.008 0.0009 0.019 0.0015 0.031 - - 5 0.0016 0.05 0.003 0.0009 0.026 0.0011 0.030 - - 6 0.0014 0.04 0.009 0.0025 0.031 0.0014 0.032 - - 7 0.0014 0.05 0.008 0.0010 0.025 0.0012 0.030 0.005 - 8 0.0012 0.05 0.009 0.0010 0.021 0.0015 0.035 - 0.0006 9 0.0016 0.10 0.010 0.0027 0.019 0.0019 0.031 - 0.0006 10 0.0014 0.08 0.008 0.0012 0.025 0.0012 0.030 0.005 0.0006 Comparative steel 11 0.0038 0.06 0.010 0.0012 0.032 0.0017 0.033 - - 12 0.0012 0.05 0.025 0.0008 0.036 0.0016 0.032 - - 13 0.0015 0.07 0.008 0.0045 0.034 0.0015 0.030 - - 14 0.0017 0.05 0.010 0.0020 0.08 0.0012 0.029 - - 15 0.0015 0.08 0.011 0.0015 0.025 0.0015 0.016 - - 16 0.0017 0.04 0.008 0.0011 0.029 0.0017 0.062 - - 17 0.0015 0.05 0.011 0.0018 0.045 0.0016 0.032 - 0.0015

Psalm No. Slab cooling rate (℃ / min) Final rolling reduction rate (%) Rolling speed (mpm) Winding temperature (℃) Cold rolling reduction (%) Annealing Temperature (℃) Elongation (%) r value Steel grade Invention One 30 10 800 680 82 860 58.6 2.82 Inventive Steel 1 2 30 10 800 580 82 860 57.3 2.76 Inventive Steel 2 3 30 10 800 680 82 860 57.0 2.72 Inventive Steel 3 4 30 10 800 680 82 860 56.5 2.65 Inventive Steel 4 5 30 10 800 680 82 860 58.2 2.76 Inventive Steel 5 6 30 10 800 680 82 860 57.8 2.83 Inventive Steel 6 7 30 10 800 680 82 860 56.9 2.75 Inventive Steel 7 8 30 10 800 680 82 860 56.5 2.72 Inventive Steel 8 9 30 10 800 680 82 860 56.1 2.62 Inventive Steel 9 10 30 10 800 680 82 860 56.0 2.65 Inventive Steel 10 11 80 10 800 680 82 860 57.2 2.77 Inventive Steel 1 12 30 10 1000 680 82 860 58.9 2.88 Inventive Steel 1 13 30 10 800 720 82 860 59.0 2.91 Inventive Steel 1 14 30 10 800 680 85 860 56.3 2.67 Inventive Steel 1 15 30 10 800 680 82 880 57.0 2.90 Inventive Steel 1 Comparative material 16 30 10 800 680 82 860 50.9 2.24 Comparative Steel 11 17 30 10 800 680 82 860 49.6 2.48 Comparative Steel 12 18 30 10 800 680 82 860 52.5 2.35 Comparative Steel 13 19 30 10 800 680 82 860 51.0 2.64 Comparative Steel 14 20 30 10 800 680 82 860 48.3 1.98 Comparative Steel 15 21 30 10 800 680 82 860 53.8 2.72 Comparative Steel 16 22 30 10 800 680 82 860 54.7 2.53 Comparative Steel 17 23 300 10 800 680 82 860 53.6 2.59 Inventive Steel 1 24 30 30 800 680 82 860 52.0 2.36 Inventive Steel 1 25 30 10 200 680 82 860 51.5 2.21 Inventive Steel 1 26 30 10 800 620 82 860 53.9 2.65 Inventive Steel 1 27 30 10 800 680 75 860 57.4 2.45 Inventive Steel 1 28 30 10 800 680 82 800 54.5 2.34 Inventive Steel 1 29 30 10 800 680 82 910 54.8 2.95 Inventive Steel 1

As shown in Table 2, in the case of the invention material (1-15) meeting the chemical composition and hot rolling conditions of the steel according to the present invention, the r value representing the drawing property is 2.62-2.91 level and the elongation is 56% or more. It can be seen that the ductility and drawing is very excellent.

In addition, in the case of the comparative material 11-17 made of the comparative steel, it can be seen that the elongation or r value is less than or equal to 56% and 2.5, respectively.

In addition, it can be seen that the elongation and r-value of the comparative steel materials 23-29 manufactured with the inventive steel 1 under the manufacturing conditions outside the scope of the present invention are partially lower than those of the inventive materials.

That is, in the case of the inventive material (1-15) according to the present invention, it can be seen that by controlling the crystal grains and precipitates of the titanium high purity ultra low carbon steel properly, better ductility and drawing property can be obtained than the comparative materials.

As described above, since the present invention can easily produce cold rolled steel sheet having excellent ductility and drawing property as compared to the conventional method, it is more effective in the field of manufacturing large panels of automobiles subjected to severe processing and the original plate of alloyed hot-dip plating for deep core processing. There is an effect that can be applied.

Claims (2)

By weight%, C: 0.001-0.003% or less, Mn: 0.5% or less, P: 0.003-0.02% or less, S: 0.003% or less, N: 0.002% or less, acid value Al: 0.01-0.05%, Ti: [ 4C + 1.5S + 3.8N + 0.005] ≤ Ti ≤ [4C + 1.5S + 3.8N + 0.020] at the rate of 10 ℃ -100 ℃ per minute from 1300 ℃ to 900 ℃ during slab cooling of steel composed of remaining Fe After cooling, it is heated to 1000-1250 ° C, hot rolling at the final rolling reduction rate of 5% -15% and the rolling speed of 600m-1200m per minute, under the condition that hot-rolled at Ar ₃ or higher and wound at 650-750 ° C. After cooling to room temperature, cold rolling after pickling in the range of 80% -90% and heating to 820 ℃ ~ 900 ℃ annealing for 10-180 seconds an excellent cold rolled steel sheet manufacturing method The method for manufacturing a cold rolled steel sheet having excellent ductility and drawing property according to claim 1, wherein Nb: 0.003-0.01% and B: 0.0001-0.0010% are further added to the steel.