KR100482199B1 - A cold rolled steel sheet with extra deep drawability and its manufacturing method - Google Patents

Abstract

Provided are a cold rolled steel sheet excellent in drawing property and a method of manufacturing the same.

The present invention is, by weight%, C: 0.001% to 0.004%, Si: 0.02% or less, Mn: 0.2% or less, P: 0.003 to 0.02%, S: 0.0005 to 0.005%, N: 0.002% or less, and acid value Al 0.1 to 0.3%, Ti: 0.015 to 0.1% and the rest of Fe, and satisfy [4C + 1.5S + 3.8N + 0.03] ≤ Ti ≤0.1% and [4C + S + 0.1Al] ≥0.02% Steel sheet excellent in drawing ability,

Providing a steel slab formed as described above; Cooling the steel of the composition after finishing hot rolling at an Ar ₃ to Ar ₃ + 50 ° C. temperature; Winding the cooled steel sheet at 600-750 ° C. and cooling it to room temperature, followed by cold rolling at a reduction ratio of 75 to 90%; And a step of annealing the cold rolled steel sheet at a temperature of 830 to 900 ° C. for 10 to 180 seconds.

Description

Cold rolled steel sheet with excellent drawing property and its manufacturing method {A cold rolled steel sheet with extra deep drawability and its manufacturing method}

     BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel sheet having excellent drawing property and a method for manufacturing the same, which are used for automobile panel parts subjected to severe processing. More particularly, the drawing is optimized by optimizing steel composition and suppressing grain growth of hot rolled sheet. The present invention relates to a steel sheet capable of securing sex and a method of manufacturing the same.

Recently, as a method of improving the drawability of cold rolled steel sheets for automobiles, crystal grains having a (111) texture structure which is advantageous for drawing property when annealing by reducing the trace elements in the steel such as carbon (C), sulfur (S), and nitrogen (N) as much as possible Increasing the way to promote growth has been used. However, if the content of such trace elements is too low, the grain growth rate becomes very fast immediately after hot rolling, and thus coarse grains similar to columnar crystals are formed in the thickness direction, resulting in a drawability of the cold rolled steel sheet.

In addition, since the finer grain size of the hot rolled sheet tends to increase the drawability in the rolling direction and the 45 degree direction, the grain size of the hot rolled sheet and the drawability of the cold rolled steel sheet are known to be closely related. Therefore, as a method for suppressing and minimizing grain growth immediately after hot rolling, a method of controlling the cooling rate while raising the cumulative reduction ratio on the rolling exit side or starting cooling within a short time immediately after rolling has been proposed. Invention of Unexamined-Japanese-Patent No. 61-276930, Unexamined-Japanese-Patent No. 1-77322, and Unexamined-Japanese-Patent No. 7-70650 is mentioned.

However, in the invention disclosed in the above-mentioned patent publication, since a measuring device such as a thermometer or a thickness meter is installed on the exit side of the rolling mill, there is a problem that grain growth cannot be avoided to some extent until the start of cooling immediately after rolling.

On the other hand, titanium (Ti), niobium (Nb), or titanium-niobium (Nb) is added to the aluminum-kilted ultra low carbon steel to precipitate invasive solid atoms (C, S, N) from the hot rolled sheet to give drawing properties. In manufacturing a cold rolled steel sheet [Interstitial Free (IF) cold rolled steel sheet], it is generally known that the added grains of Nb or B are finely grained. However, this is limited to the effect of inhibiting grain growth in the cooling process or the winding process after hot rolling, and there is a limitation in suppressing grain growth at a high temperature immediately after hot rolling.

 Accordingly, the present invention is to solve the above-mentioned problems of the prior art, and to optimize the steel composition to suppress grain growth of the hot rolled sheet, as well as to control the manufacturing conditions such as annealing temperature, etc. And to provide a method for producing the object.

The present invention for achieving the above object, in weight%, C: 0.001 ~ 0.004%, Si: 0.02% or less, Mn: 0.2% or less, P: 0.003 ~ 0.02%, S: 0.0005 ~ 0.005%, N: 0.002% or less, including acid value Al: 0.1 to 0.3%, Ti: 0.015 to 0.1%, and the remaining Fe, and [4C + 1.5S + 3.8N + 0.03] ≤ Ti ≤0.1% and [4C + S + 0.1Al ] It relates to a cold rolled steel sheet having excellent drawability formulated to satisfy ≥0.02%.

In addition, the present invention is, in weight%, C: 0.001% to 0.004%, Si: 0.02% or less, Mn: 0.2% or less, P: 0.003 to 0.02%, S: 0.0005 to 0.005%, N: 0.002% or less, and acid. Soluble Al: 0.1-0.3%, Ti: 0.015-0.1% and the remaining Fe, and [4C + 1.5S + 3.8N + 0.03] ≤ Ti ≤0.1% and [4C + S + 0.1Al] ≥0.02% Providing a steel slab configured to satisfy; Cooling the steel of the composition after finishing hot rolling at an Ar ₃ to Ar ₃ + 50 ° C. temperature; Winding the cooled steel sheet at 600-750 ° C. and cooling it to room temperature, followed by cold rolling at a reduction ratio of 75 to 90%; And annealing the cold rolled steel sheet at a temperature of 830 to 900 ° C. for 10 to 180 seconds. The present invention relates to a cold rolled steel sheet manufacturing method having excellent drawing property for deep processing.

Hereinafter, the present invention will be described.

In the present invention, in order to sufficiently remove carbon, sulfur and nitrogen present in the ultra low carbon steel in order to secure the drawing property, the addition amount of Ti is satisfied so as to satisfy the compositional formula [4C + 1.5S + 3.8N + 0.03] ≦ Ti ≦ 0.1%. It is characterized by relatively controlling for the content of the carbon and the like. However, when the content of carbon and sulfur in the steel is reduced as much as possible according to the control of the amount of Ti added, as described above, the grain growth rate is increased immediately after hot rolling, whereby coarse grains such as columnar crystals are formed in the thickness direction below the surface layer. Growing phenomenon may occur and deterioration of drawing property may occur.

Therefore, in order to solve this problem, the present invention is characterized by mutually controlling the addition amount of carbon, sulfur and aluminum so as to satisfy the compositional formula [4C + S + 0.1Al]? 0.02%.

First, the specific composition component of the steel plate which concerns on this invention is demonstrated.

Carbon (C) is required to minimize the amount of addition because it inhibits the development of the (111) aggregate structure of the cold rolled steel sheet to reduce the drawing. However, if the addition amount is too small, the grain growth rate becomes very fast immediately after hot rolling, and coarse grains are formed, thereby degrading the drawing property of the annealing plate. In addition, in the present invention, by adding carbide forming elements such as Ti, Nb, Mo to precipitate carbon in the steel as carbide before cold rolling, if the carbon content is excessive, carbide precipitation increases to increase the recrystallization temperature in the annealing process, accordingly (111 By suppressing the growth of crystal grains having an aggregate structure, the drawing property can be deteriorated.

Therefore, in the present invention, the amount of carbon (C) added is limited to 0.001% to 0.004% by weight.

Since silicon (Si) may act as an oxide in steel in the present invention and inhibit ductility, the addition amount is limited to 0.02% or less, and manganese (Mn) is limited to 0.2% because it increases strength but decreases ductility. This is preferred.

Phosphorus (P) is an essential element in molten steel, and if the concentration is extremely low during refining, the lower limit is 0.003%, and the lower limit is 0.003%. The upper limit is limited to 0.02% because the recrystallization temperature is increased in the process to lower the drawing property.

Sulfur (S) has a problem that there is almost no sulfide (Ti ₄ C ₂ S ₂ ) that suppresses grain growth of the hot rolled sheet or sulfur that is intergranular when the addition amount is less than 0.0005%. In addition, when the content exceeds 0.005%, the titanium sulfide formed before hot rolling is changed from relatively fine Ti ₄ C ₂ S ₂ to coarse TiS. Accordingly, solid solution carbon is present during hot rolling, thereby changing the hot rolled structure into a bad component in drawing property. Furthermore, since the solid carbon forms fine TiC carbide in the cooling process after rolling, there is a problem of suppressing the grain growth during the annealing process to lower the drawing property. Therefore, in the present invention, the amount of sulfur added is limited to 0.0005 to 0.005%.

Since nitrogen (N) combines with Ti in steel to reduce ductility and drawability, the content of the nitrogen (N) is limited to 0.002% or less.

Acid value Al suppresses grain growth using grain boundary segregation at high temperature immediately after hot rolling and is added for deoxidation. In the present invention, the addition amount is limited to 0.10 to 0.3%. This is because if it is less than 0.10%, there is no effect of inhibiting grain growth, and if it exceeds 0.3%, the elongation is drastically lowered and, moreover, it is uneconomical for the alloy cost.

Titanium (Ti) is an element which completely combines nitrogen, sulfur, and carbon in steel or exists as solid solution Ti in steel to increase drawing property, and in the present invention, the amount thereof is limited to 0.015 to 0.1%. If the addition amount is less than 0.015%, it is insufficient to debond the elements such as carbon, and if the content exceeds 0.1%, oxide (TiO ₂ ) is formed in the playing process and adheres to the nozzle, thereby degrading workability. Because there is.

On the other hand, in the present invention, in order to sufficiently precipitate carbon, sulfur and nitrogen in the steel to secure the drawing property, the amount of T i should be limited to satisfy the compositional formula [4C + 1.5S + 3.8N + 0.03] ≤ Ti ≤0.1%. If Ti is smaller than [4C + 1.5S + 3.8N + 0.03], the drawability is very degraded.

In addition, the present invention, as described above, the grain growth rate is increased immediately after hot rolling, and thus coarse grains such as columnar crystals grow in the thickness direction under the surface layer, thereby preventing the drawing property from deteriorating. The amount of carbon, sulfur and aluminum added should be limited to satisfy [4C + S + 0.1Al] ≥0.02%. If the value of [4C + S + 0.1Al] is less than 0.02%, the grain growth rate immediately after hot rolling is increased, and coarse grains are generated accordingly, resulting in poor drawing.

In the present invention, the addition of Nb and / or Mo to the composition component prepared as described above is more preferable for preventing secondary processing brittleness.

Nb is dissolved in steel and delays recrystallization after hot rolling. In addition, NbC carbides are formed during cooling to refine crystal grains in hot-rolled sheets to improve the drawability in the rolling direction and the 45 degree direction, and improve the secondary workability of the annealing plate. In the present invention, it is preferable to limit the amount of Nb added to 0.003 to 0.015%, which is not expected to be effective when the amount is less than 0.003%, and exceeds 0.015%. This is because the yield strength is increased by increasing the recrystallization temperature of, and thus the elongation may be lowered.

Mo not only refines the grain size of the hot rolled sheet by complex precipitation with TiC to NbC in the cooling process after hot rolling, but some solid solution Mo is an element that improves secondary workability due to grain boundary segregation of P. In the present invention, it is preferable to limit the amount of Mo added to 0.005 ~ 0.04%, because the effect of the addition can not be expected at less than 0.005%, if exceeding 0.04% may result in a decrease in workability and cost increase .

On the other hand, in the present invention, when adding Nb and / or Mo having the above component range, Nb and Mo to satisfy the composition formula 4C + 0.03 ≤ [(Ti-0.015) + 0.52 Nb + 0.5Mo)] ≤ 0.1% It is desirable to control the amount of participation of. If the value defined by [(Ti-0.015) + 0.52Nb + 0.5Mo)] is less than (4C + 0.03%), solid carbon will remain and the drawing performance will be deteriorated. This may cause an increase.

In addition, B is an element that segregates at the grain boundary, and suppresses grain boundary movement during hot rolling to contribute to grain refinement and prevents grain embrittlement due to high purity, and the addition thereof is optional in the present invention. If it is added, it is preferable to limit the amount to 0.0001 to 0.0020%, since the effect of the addition is less than 0.0001%, and if it exceeds 0.0020%, the recrystallization temperature of the annealing plate is increased to decrease the workability. .

Next, a method of manufacturing a cold rolled steel sheet excellent in drawing property using a steel slab having a composition component as described above will be described.

First, in the present invention, the steel slab of the composition is hot rolled, in which the finishing hot rolling temperature is limited to the range of Ar ₃ ~ Ar ₃ + 50 ℃. Because being a finishing temperature low, ferrite transformation after which is received the rolling deformation problem that the drawability deterioration, Ar ₃ + higher than 50 ℃ increase the crystal grain growth immediately after hot rolling to obtain a mouth determines a coarse than Ar _3, This is because workability may deteriorate.

And after winding the hot rolled steel sheet as described above, in the present invention, the winding temperature is limited to 600 ~ 750 ℃. This is because, if the coiling temperature is less than 600 ° C, the drawability is poor due to insufficient bonding of carbon. If the temperature is higher than 750 ° C, the drawing property in the rolling direction and the 45 degree direction decreases due to grain growth.

The rolled steel sheet is cooled to room temperature, and then subjected to a normal pickling process, followed by cold rolling. In the present invention, the cold reduction rate is preferably limited to 75 to 90% at this time. This is because, if the reduction ratio is less than 75%, the desired level of drawability cannot be obtained, while if the reduction ratio exceeds 90%, the (111) aggregate structure of the annealing plate may decrease again, leading to deterioration of the drawing property.

Thereafter, the cold rolled steel sheet is heated and annealed, wherein the annealing temperature is an important factor that directly affects the formation and development of the (111) aggregate structure, which is beneficial for drawing property. Therefore, in the present invention, the annealing temperature is limited to 830 ~ 900 ℃, because the (111) aggregate structure is not sufficiently developed at less than 830 ℃, because the austenite is formed over 900 ℃ to reduce the drawing.

In addition, in the present invention, it is preferable to limit the annealing time to 10 to 180 seconds as the time required for forming and developing the (111) texture. This is because if the annealing time is less than 10 seconds, sufficient annealing does not occur. If the annealing time exceeds 180 seconds, excessive grain growth occurs and orange peel-like surface defects occur during forming of the steel sheet.

As described above, in the present invention, by appropriately controlling the composition components and optimizing the cold reduction rate and annealing conditions, it is easy to grow crystal grains having a low recrystallization temperature and having a (111) texture in the annealing process, thereby ranking Rankford (r). It is possible to effectively produce cold rolled steel sheet having excellent drawing property of 2.6-3.0 value and 53% or more elongation.

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

(Example 1)

To prepare a steel slab having a composition as shown in Table 1 below. Each steel slab thus prepared was heated at 1200 ° C. for 1 hour, and then hot rolled at a temperature range of Ar ₃ to Ar ₃ + 50 ° C., followed by winding at 720 ° C.

Each of the wound steel sheets was cooled to room temperature and then pickled, and then cold rolled to a thickness of 0.8 mm at a reduction ratio of 80%. Each cold rolled steel sheet was heated and annealed at 860 ° C. for 50 seconds.

Yield strength, tensile strength, elongation, Rankford (r) value, and ductile-brittle transition temperature were measured for each of the annealed steel sheets manufactured under the above-described manufacturing conditions, and the results are shown in Table 2 below.

           Ingredients (% by weight) Formula 1 Formula 2 Formula 3 Remarks  C Mn  P  S  N  Sol. Al  Ti Nb Mo B One 0.0014 0.05 0.009 0.0013 0.0019 0.16 0.075 O O - foot persons River 2 0.0024 0.05 0.010 0.0008 0.0018 0.14 0.079 O O - 3 0.0031 0.05 0.010 0.0010 0.0018 0.13 0.076 O O - 4 0.0014 0.08 0.006 0.0033 0.0015 0.17 0.081 O O - 5 0.0010 0.08 0.008 0.0013 0.0016 0.20 0.051 0.005 O O O 6 0.0016 0.05 0.010 0.0009 0.0017 0.15 0.079 0.007 O O O 7 0.0013 0.07 0.009 0.0015 0.0017 0.17 0.049 0.020 O O O 8 0.0018 0.05 0.011 0.0008 0.0015 0.15 0.085 0.022 O O O 9 0.0010 0.08 0.008 0.0021 0.0018 0.16 0.044 0.01 0.021 O O O 10 0.0011 0.08 0.009 0.0030 0.0015 0.22 0.056 0.0008 O O O 11 0.0014 0.06 0.010 0.0027 0.0019 0.19 0.051 0.008 0.0006 O O O 12 0.0010 0.05 0.007 0.0014 0.0016 0.22 0.059 0.015 0.0008 O O O 13 0.0012 0.05 0.010 0.0018 0.0021 0.20 0.055 0.007 0.0010 O O O 14 0.0014 0.06 0.010 0.0012 0.0017 0.12 0.048 X O - ratio School River 15 0.0012 0.05 0.007 0.0008 0.0025 0.11 0.060 X O - 16 0.0017 0.06 0.010 0.0019 0.0017 0.15 0.030 O X - 17 0.0015 0.05 0.008 0.0045 0.0019 0.14 0.041 O X - 18 0.0014 0.06 0.010 0.0020 0.002 0.04 0.050 - - - 19 0.0045 0.06 0.010 0.0019 0.002 0.15 0.030 - - - 20 0.0015 0.05 0.008 0.0076 0.002 0.14 0.041 - - - 21 0.0015 0.10 0.010 0.0048 0.002 0.15 0.012 - - -

 * In Table 1, Formula 1 is [4C + S + 0.1Al] ≥0.02%, Formula 2 is [4C + 1.5S + 3.8N + 0.03] ≤ Ti ≤0.1%, and Formula 3 is 4C + 0.03 ≤ [( Ti-0.015) + 0.52Nb + 0.5Mo)] ≦ 0.1%, and the value obtained by such a composition formula is within the scope of the present invention (O), and the deviation is indicated by (X).

YS (kg / mm ² ) TS (kg / mm ² )    El (%)   r value Ductile-brittle transition temperature (℃) Remarks One 12.2 26.9 55.4 2.97 -50 foot persons River 2 13.4 27.9 54.1 2.83 -50 3 14.1 28.5 53.0 2.67 -55 4 12.6 27.0 54.8 2.90 -50 5 13.2 27.5 55.0 2.71 -55 6 13.8 27.8 53.8 2.89 -60 7 13.1 27.7 55.9 2.81 -60 8 13.3 28.1 56.1 2.88 -65 9 13.6 28.4 54.9 2.91 -65 10 13.0 27.4 55.0 2.88 -70 11 13.5 27.8 54.1 2.85 -70 12 13.6 28.0 55.0 2.80 -70 13 14.0 28.5 54.0 2.77 -70 14 15.1 28.7 52.5 2.35 -40 ratio School River 15 13.9 27.8 51.6 2.42 -50 16 16.1 28.1 53.1 2.21 -50 17 14.1 27.9 51.9 2.29 -40 18 14.1 28.3 51.0 2.25 -50 19 16.5 29.2 49.4 2.13 -60 20 13.6 27.2 52.8 2.33 -55 21 18.3 27.5 52.3 2.06 -60

As shown in Table 1 and Table 2, in the invention steels (1 to 13) that optimally controlled not only the composition component but also the amount of addition between the composition components, the r value showing the drawing property was 2.67 to 3.06, and the elongation was elongated. 53% or more showed very excellent workability. In addition, in the drop impact test performed after the cylindrical cup was prepared with the drawing ratio of 2.16, the ductile-brittle transition temperature was lower than -50 ° C.

In particular, in the case of the inventive steels (5 to 13) in which Nb and / or Mo is added and satisfying 4C + 0.03 ≤ [(Ti-0.015) + 0.52 Nb + 0.5Mo)] ≤ 0.1%, a lower ductile-brittle transition It can be seen that the temperature is advantageous in preventing secondary processing brittleness.

On the contrary, although the composition is within the scope of the present invention, the comparative steels 14 to 15, which do not satisfy the composition formula [4C + S + 0.1Al] ≥0.02%, have coarse grains formed immediately after hot rolling, so that the r value is higher than that of the inventive steel. It was low and the ductile-brittle transition temperature, which shows secondary processing brittleness, was also relatively high. In addition, the comparative steels (16 to 17), which did not satisfy the composition formula [4C + 1.5S + 3.8N + 0.03] ≤ Ti ≤0.1%, had a low r value due to the depletion of solid solution Ti and a high ductile-brittle transition temperature.

And Sol. Comparative steels (18 to 21) of the addition amount of Al, C, S and Ti out of the present invention all showed a very low r value, but ductile-brittle transition temperature was similar to that of the inventive steel due to some residual solid carbon. Levels were indicated.

(Example 2)

Steel slabs having the same components as in Steel 1 of Table 1 above were prepared. And the prepared steel slab was prepared by annealed steel sheet by varying its manufacturing conditions as shown in Table 3 below, and also yield strength, tensile strength, elongation, Rankford (r) value and ductile-brittle transition for each of the prepared annealed steel sheet The temperature was measured and the results are also shown in Table 3 below.

 Hot Rolling Condition (℃) Annealing Temperature (℃) YS (kg / mm ² ) TS (kg / mm ² ) El (%)  r value Ductile-brittle transition temperature (℃) Remarks  Finish rolling temperature Coiling temperature 1-1 925 720 830 13.3 27.6 54.5 2.81 -50 Invention steel 1-2 925 720 860 12.0 26.5 55.0 3.06 -50 1-3 870 720 880 12.5 27.2 52.5 2.43 -45 Comparative steel 1-4 919 560 880 13.0 27.5 52.6 2.51 -40 1-5 925 720 800 14.5 28.1 52.0 2.33 -50

As shown in Table 3, the invention steel (1-1,1-2), the hot rolling conditions and the annealing temperature are properly controlled, all showed excellent drawing, but in the case of comparative steel outside the scope of the present invention This deteriorated.

As described above, the present invention appropriately controls the amount of C, S and Al to suppress grain growth immediately after hot rolling, and to reduce the C, S, P in steel as much as possible, and to appropriately reduce Ti, Nb, Mo, and B. By removing the adverse effects of impurities in the steel by inputting, it is possible to provide a cold rolled steel sheet with excellent drawing and secondary processing brittleness. It can be usefully used.

Claims (6)

By weight%, C: 0.001 to 0.004%, Si: 0.02% or less, Mn: 0.2% or less, P: 0.003 to 0.02%, S: 0.0005 to 0.005%, N: 0.002% or less, acid value Al: 0.1 to 0.3 %, Ti: 0.015-0.1% and the remaining Fe, and drawability formulated to satisfy [4C + 1.5S + 3.8N + 0.03] ≤ Ti ≤ 0.1% and [4C + S + 0.1Al] ≥ 0.02% This excellent cold rolled steel sheet. The method of claim 1, further comprising at least one of Nb: 0.003-0.015% and Mo: 0.005-0.04%, wherein 4C + 0.03 ≦ [(Ti-0.015) + 0.52Nb + 0.5Mo)] ≦ 0.1% Cold rolled steel sheet with excellent drawability formulated to satisfy. The cold rolled steel sheet having excellent drawing property according to claim 1, further comprising B: 0.0001 to 0.0020%. By weight%, C: 0.001 to 0.004%, Si: 0.02% or less, Mn: 0.2% or less, P: 0.003 to 0.02%, S: 0.0005 to 0.005%, N: 0.002% or less, acid value Al: 0.1 to 0.3 Steel slab comprising%, Ti: 0.015-0.1% and the remaining Fe and satisfying [4C + 1.5S + 3.8N + 0.03] ≤ Ti ≤0.1% and [4C + S + 0.1Al] ≥0.02% Providing a; Cooling the steel of the composition after finishing hot rolling at an Ar ₃ to Ar ₃ + 50 ° C. temperature; Winding the cooled steel sheet at 600-750 ° C. and cooling it to room temperature, followed by cold rolling at a reduction ratio of 75 to 90%; And Annealing the cold-rolled steel sheet at a temperature of 830 ~ 900 ℃ for 10 ~ 180 seconds; excellent cold rolled steel sheet manufacturing method comprising a. The steel slab of claim 4, wherein the steel slab further comprises at least one of Nb: 0.003-0.015% and Mo: 0.005-0.04%, and 4C + 0.03 ≦ [(Ti-0.015) + 0.52Nb + 0.5Mo). ] Cold drawing steel sheet manufacturing method with excellent drawability, characterized in that the composition to satisfy ≤0.1%. The method of claim 4, wherein the steel slab is B: 0.0001 to 0.0020% of the cold rolled steel sheet manufacturing method with excellent drawability, characterized in that the composition further comprises.