KR20050059817A - Cold rolled steel sheet having homogeneous plastic deformation property and manufacturing method thereof - Google Patents

Abstract

The present invention r _m value is close to 1 Δr value is excellent in low almost shape fixability as well as gajyeoseo a value close to 0 to a plastic deformation in the stretching mode, which typically occurs during the processing of the vehicle outer panel deformed uniformly cold-rolled steel sheet, and It relates to a manufacturing method.

According to the present invention, by weight% C: 0.1% or less, Mn: 0.1 to 1.0%, P: 0.1% or less, S: 0.02% or less, Si: 0.05% or less, acid value Al: 0.08% or less, N: 0.01 A cold rolled steel sheet comprising% or less, Ti: 0.0025 to 0.10%, Zr: 0.10% or less, and the balance consisting of Fe and inevitable impurities is provided. In addition, reheating the steel slab having the composition to 1050 ℃ or more, finishing temperature and winding temperature of Ar ₃ or more 500 Cold rolled steel sheet consisting of hot rolling at 100 ℃, cold rolling at a 50 ~ 80% reduction rate, annealing at a crack zone temperature of Ar ₁ or less, and temper rolling at a 0.5 ~ 2% reduction rate Provided are methods for preparing

The steel of the present invention is particularly suitable for use in automobile shells and the like to which a stretching mode deformation is applied.

Description

Cold Rolled Steel Sheet Having Homogeneous Plastic Deformation Property And Manufacturing Method Thereof}

The present invention relates to a cold rolled steel sheet having excellent workability and a method of manufacturing the same, and in particular, the average plasticization ratio (r _m ) is close to 1 and the plastic strain ratio is low, so that it is easy to press-process the outer shell of automobiles, which is advantageous for molding automobile parts having a complex shape. The present invention relates to a cold rolled steel sheet having a low anisotropy coefficient Δr and a method of manufacturing the same.

As a material of an automobile outer plate, a cold rolled steel sheet having excellent workability is required in order to perform press molding without processing defects and to smoothly manufacture a part having a desired shape after molding. That is, it is advantageous to manufacture a part having a complicated shape in a desired shape by forming a vehicle outer plate using a cold rolled steel sheet having a high elongation, low yield strength and uniform plastic deformation in the plane direction. The elongation is a value representing the property of the steel sheet to be elongated without cracking when tensile, so when the elongation is large, the allowable deformation of the steel sheet is large. Elongation is a mechanical property of steel that does not change significantly once the steel grade is determined. The plastic strain ratio r value is a value defined as the ratio of the strain in the width direction to the strain in the thickness direction. If the steel sheet with a large plastic deformation ratio is assumed to have a constant deformation in the width direction, when the plate is tensioned in an arbitrary direction by a certain amount of deformation, the strain in the thickness direction is small, so that the material can be processed without causing necking until a large deformation. it means. In the case of automobile inner plates, cold drawing steels having a high elongation and plastic strain ratio are advantageous because of deep drawing deformation, but in the case of automobile outer panels, stretching deformation mainly occurs, so the plastic deformation ratio is advantageous in terms of processing. The reason is that the lower the plastic deformation ratio, the lower the biaxial yield strength, and thus plastic deformation can be performed with less deformation. On the other hand, the plastic strain ratio has a different value depending on the tensile direction because of the anisotropic property of the plate. The average plastic strain ratio r _m and the plastic strain ratio anisotropy r are shown. In general, the plastic deformation ratio is measured by measuring r ₀ , r ₄₅ , and r ₉₀ of each sheet and then substituting the following equations to determine r _m and Δr.

r _m = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4, Δr = (r ₀ -2r ₄₅ + r ₉₀ ) / 2

Where r ₀ , r ₄₅ and r ₉₀ are tensile directions of 0 ⁰ , 45 ⁰ , It means the plastic strain ratio value in the 90 ⁰ direction. In order for the value of Δr to be small, the difference in plastic strain ratio should be small when it is tensioned in each direction. The small value of Δr means that the distribution of strain in the press molding is uniform, which is advantageous for molding from deformation in stretching mode to large deformation. Steels with low Δr and _rm values close to 1 improve shape freezing during machining of automotive shells, where stretching strains occur predominantly.

Japanese Laid-Open Patent Publication No. 9-296226 describes the formation of automotive cold rolled steel sheets by adding Ti or Nb alone or in combination to ultra low carbon cold rolled steel sheets to precipitate solid solution C and N in the form of carbides and nitrides, thereby increasing elongation and plastic strain ratio. Techniques for improving the properties are known. German Patent DE3847732,3803064 and U.S. Pat.No. 5,395,803, US Pat. This is known. However, the steel with Ti and Nb has a relatively high Δr and r _m value, which shows excellent workability in the deformation of the deep drawing mode, but high anisotropy and high isoaxial yield strength in the deformation of the stretching mode. There is a problem in terms of workability. On the other hand, in Japanese Laid-Open Patent Publication No. 6-346149, in order to solve this problem, heat treatment is performed after degassing by adding only Ti or Ti and Nb to ultra low carbon steel. However, there is a problem that the r value in the rolling direction and the width direction is different when using this technique.

An object of the present invention is to maintain a high elongation even in the carbon composition range corresponding to the low and medium carbon steel, the r _m value is close to 1 and the Δr value is low to nearly 0, the stretching mode that occurs mainly during the processing of automotive shells It is to provide a cold rolled steel sheet having a uniform plastic deformation in the deformation of and excellent in shape freezing and a method of manufacturing the same.

In order to achieve the above object, in the present invention, Zr, which is not used in the known art, is added to the medium and low carbon steel, but Ti and Zr are added in combination.

According to the present invention, by weight% C: 0.1% or less, Mn: 0.1 ~ 1.0%, P: 0.1% or less, S: 0.02% or less, Si: 0.05% or less, acid value Al: 0.08% or less, N: 0.01 % Or less, Ti: 0.1% to 0.0025%, Zr: 0.1% or less, and the remainder is composed of Fe and inevitable impurities, average plastic strain ratio (r _m ) is 1.1 or less, and plastic strain ratio anisotropy coefficient (Δr) is 0.15 Provided is a cold rolled steel sheet for an automotive exterior plate having the following values.

In addition, according to the present invention, the step of heating the steel slab having the composition to 1050 ℃ or more, the finishing temperature and winding temperature 500 of Ar ₃ or more Hot rolling at 100 ° C., cold rolling at 50-80% reduction rate, batch annealing at a crack zone temperature of Ar ₁ or less, and temper rolling at 0.5-2% reduction rate. Provided is a method of manufacturing a cold rolled steel sheet for an automotive exterior plate.

The present invention is described in detail below.

In the present invention, while the steel C is present in the form of invasive solid solution element and cementite, it has a great influence on the formation of texture of the steel sheet during cold rolling and annealing. In the case of a large amount of C in the steel, C is combined with Fe to form cementite, and thus C may be stably present in the steel. In processes with slow heating and cooling rates, such as annealing, cementite is likely to be thermodynamically stable. During annealing, C in the steel combines with Ti to precipitate TiC, thereby restoring the grains having an orientation (<111> // ND) in which the ND (normal direction; vertical direction in the rolling direction) is parallel to the <111> direction. And the recrystallization rate is lowered, so that the fraction of grains having the <111> // ND orientation is lowered. This leads to an increase in the r _m value and a decrease in the Δr value. Some C may combine with Zr to form ZrC, but the precipitates are relatively coarse than TiC, which is advantageous for elongation.

Mn in steel is an effective element for the solid-solution strengthening effect, and in particular, S precipitates steel in MnS to suppress the occurrence of polarized fracture and high temperature embrittlement by S during hot rolling. However, according to the experiment of the present invention, when the Mn content is less than 0.3% by weight, the effect of increasing strength cannot be obtained, and there is a problem in securing formability because the steel S cannot be precipitated completely by MnS.

The higher the content of P in the steel, the better the strength. However, the addition of excess P increases the possibility of brittle fracture, which increases the probability of slab breakage during hot rolling, and facilitates diffusion and segregation to the grain boundary after annealing. As a result, the problem of the occurrence of secondary processing brittleness during molding is increased, and therefore, it is necessary to limit the content thereof. In the present invention, the content of P is limited to less than 0.1 wt% by precipitation strengthening by TiC and ZrC.

It is important to keep S and N as low as possible because they are inevitably included as impurities in the steel. But. The less it manages, the higher the refining cost of the steel. Therefore, it is important to manage S to 0.01% by weight or less, which is a range of possible operating conditions. Since the N content forms TiN at high temperature and changes the effective Ti to combine with C, the total Ti content may vary depending on the N content. have.

Since Si acts as an oxide in steel and inhibits ductility, the smaller the amount is, the more preferable. However, Si is limited to 0.05 wt% in consideration of the economical efficiency in steelmaking refining step.

The amount of acid value Al effectively acts as a decarbonization element of molten steel. However, when Al is excessively added, it adversely affects workability, so it is preferable to control the content to 0.08% by weight or less.

Ti has an effect of inhibiting AlN formation by bonding Ti to steel to form TiN. AlN produced during hot rolling has a problem of increasing the shape anisotropy of the sheet by stretching the hot rolled structure. The amount of Ti added to precipitate TiC during annealing (Ti-N x 48/14) It is preferred to add 0.0025% by weight. However, Zr As the Ti addition of Ti addition amount increases without adding it appears that the r _m values increase, Zr and compound, when added, but is almost no change of the r _m of the Ti addition amount to a low r _m values for Ti content of 0.1 wt. It has been found to be limited to less than%.

Zr combines with C in steel to form ZrC. As the amount of Zr is increased, it is effective in terms of anisotropy of the steel, but at 0.10% by weight or more, the effect increase is insignificant, and a problem arises in the recovery rate of Zr.

In order to obtain a cold rolled steel sheet of medium and low carbon steel having uniform plastic deformation and excellent shape freezing in the deformation of the stretching mode, it is very important to add both of them in combination with adjusting the Ti and Zr content as described above.

On the other hand, the cold-rolled steel sheet having the steel component is required to undergo an appropriate heat treatment process in order to exhibit a uniform plastic deformation and excellent shape freezing properties under the deformation conditions applied during the manufacture of automobile exterior plates. That is, the steel slab having the composition is reheated to 1050 ° C. or higher and the finishing temperature and winding temperature of Ar ₃ or higher 500. After hot rolling at 100 ℃, cold rolling at 50 ~ 80% reduction rate, batch annealing at a cracking zone temperature of less than Ar ₁ , and temper rolling at 0.5 ~ 2% reduction ratio should be produced cold rolled steel sheet . At this time, it is important that the coiling temperature of the coil after hot rolling is kept at a low temperature as mentioned above to suppress precipitation of precipitates as much as possible in the wound state.

The following is an embodiment of the present invention.

Each steel of Table 1 component was melted and hot-rolled. The reheating temperature at the time of hot rolling was 1200 degreeC, the finishing temperature was 890 degreeC, and the coiling temperature was 500 degreeC. After removing the surface oxide layer of the hot rolled plate by pickling, cold rolling was performed at a 70% reduction rate. The cold rolled steel sheet was annealed in a continuous annealing furnace. The temperature of the heat treatment crack was 690 ° C. After the heat treatment, temper rolling was carried out at a reduction ratio of 1.5%.

 Table 1

   C Si Mn P S Sol-Al Ti Zr N Inventive Example 1 Invention Example 2 Invention Example 3  0.034 0.01 0.19 0.01 0.01 0.05 0.01 0.0014 0.003 0.034 0.01 0.19 0.01 0.01 0.07 0.016 0.0064 0.003 0.035 0.01 0.19 0.01 0.01 0.07 0.016 0.013 0.003 Comparative Example 1 Comparative Example 2 Comparative Example 3  0.034 0.01 0.19 0.01 0.01 0.043 0.01 0.003 0.0032 0.005 0.36 0.03 0.011 0.015 0.05 0.0019 0.0032 0.004 0.35 0.03 0.011 0.022 0.058 0.0012

Table 2 shows the results of measuring the mechanical properties of the cold rolled steel sheet manufactured by the components of Table 1. As a comparative example, Ti-added low carbon steel and Ti-added ultra low carbon steel were used. Comparing Inventive Example 1 with Comparative Example 1 in Table 2, it can be seen that the case of Inventive Example 1 in which a small amount of Zr was added has a low Δr value. Inventive Example 2 and Inventive Example 3 is a case where the Zr content is increased compared to Inventive Example 1, it can be seen that as the Zr content increases, the r _m value does not change significantly but the Δr value decreases. However, when the Zr content was 0.015 wt% or more, the Δr value reduction rate was not large. The improvement effect of this anisotropic property is believed to be due to the formation of coarse ZrC than TiC due to the addition of Zr. From Comparative Examples 2 and 3, it can be seen that, in the case of the ultra low carbon steel, the r _m value has a larger value than that of the low carbon steel, and the large Δr value has a large defect in in-plane anisotropy. It has been shown to be desirable to improve the anisotropic properties of the steel. It can be seen that the steel of the present invention has an r _m value of 1.1 or less and a Δr value of 0.15 or less.

 Table 2

   Tensile Strength Total Elongation r_m    (Kgf / mm²) (%)  Invention Example 1 Invention Example 2 Invention Example 3      32.9 39.75 1.09 0.13 33.2 41.07 1.09 0.14 33.5 39.44 1.08 0.02  Comparative Example 1 Comparative Example 2 Comparative Example 3      35.74 39.2 1.04 0.24 31.19 45.1 1.73 0.56 32.26 44.9 1.74 0.34

FIG. 1 shows a crystallographic orientation map measured using Field Emission (FE) -SEM and Electron Backscattered Direction (EBSD) of Inventive Example 3. FIG. Compared with the color of the reverse pole viscosity at the top, in the present invention, the aggregates of α-fiber (grains in which the rolling direction is parallel to the <110> direction) and γ-fibers (grains in which the ND is parallel to the <111> direction) You can see that it is developing together. Figure 2 shows the results of theoretical calculation of the effect of texture on the plastic strain ratio anisotropy. It can be seen from FIG. 2 that the α-fiber and γ-fiber textures have different effects on the plastic strain ratio. Comparing FIG. 1 and FIG. 2, it can be determined that the steel of Inventive Example 3 has a value of plastic strain ratio close to 1 and steel having a small plastic strain ratio anisotropy coefficient. Figure 3 compares the ear development after deep drawing of Inventive Example 3 and Comparative Example 1. Steel of Inventive Example 3 has no earing, but Comparative Example 1 shows that the earing develops severely, and thus the steel of the present invention shows excellent deformation characteristics in stretching mode deformation.

As described above, according to the present invention, steel having excellent workability having an r _m value of 1.1 or less and a Δr value of 0.15 or less can be obtained. These steels exhibit excellent processing characteristics in deformation of the stretching mode, and thus are suitable for use in automobile shells and the like.

1 is a micrograph and a crystallographic orientation map of the microassembly measured after the annealing of Inventive Example 3.

2 is a theoretical calculation of the effect of texture on r value anisotropy.

3 is a comparative photograph of ear development after deep drawing for Comparative Example 2 and Inventive Example 3. FIG.

Claims (3)

By weight% C: 0.1% or less, Mn: 0.1 ~ 1.0%, P: 0.1% or less, S: 0.02% or less, Si: 0.05% or less, acid value Al: 0.08% or less, N: 0.01% or less, Ti: Cold rolled steel sheet with uniform plastic deformation characteristics characterized by 0.0025 to 0.10%, Zr: 0.10% or less, and the remainder composed of Fe and unavoidable impurities The method of claim 1 wherein the Ti content is (Ti-N x 48/14) Cold rolled steel sheet having a uniform plastic deformation characteristics, characterized in that added in 0.0025% by weight relationship. Heating the steel slab having the composition according to claim 1 to at least 1050 ° C .; Finishing temperature and winding temperature over Ar ₃ 500 Hot rolling at 100 ° C .; Cold rolling at a rate of 50-80% reduction; Annealing at a crack zone temperature below Ar ₁ ; And Temper rolling at a rate of 0.5-2% reduction; Method for producing a cold rolled steel sheet having a uniform plastic deformation characteristics, characterized in that consisting of.