KR100293218B1 - Method for manufacturing hot rolled steel sheet having superior deep drawability - Google Patents

Abstract

PURPOSE: A method for manufacturing a hot rolled steel sheet having superior deep drawability without recrystallization heat treatment is provided. CONSTITUTION: A method for manufacturing hot rolled steel sheet having superior deep drawability is characterized in that after finish lubrication rolling a steel slab comprising C 0.01wt.% or less, Mn 0.5wt.% or less, Al 0.05wt.% or less, S 0.015wt.% or less, N 0.015wt.% or less, 0.03-0.1wt.% of Ti or Nb, a balance of Fe and other inevitable impurities at a temperature range of 500 to 730deg.C, coiling the hot-rolled steel sheet at a temperature range of 680 to 730deg.C.

Description

Manufacturing method of hot rolled steel sheet with excellent deep drawing property

The present invention relates to a method for manufacturing a hot rolled steel sheet for deep drawing, and more particularly, to a method for manufacturing a hot rolled steel sheet excellent in deep drawing by continuous lubricating hot rolling.

In general, the body inner and outer plates used in automobiles are manufactured through plate forming processing such as deep drawing, the hot rolled steel sheet used should be excellent in workability.

The deep drawing property is improved as the degree of development of the (111) plane, which is an assembly structure parallel to the rolled plate surface, is increased, while the (100) plane becomes worse as it is developed parallel to the rolled plate surface.

In general, the method of manufacturing a hot rolled steel sheet, since the hot rolling finish temperature is terminated above the Ar ₃ temperature, the transformation of austenite to ferrite occurs after hot rolling, and the texture developed in the austenite is greatly weakened during the transformation process. This has a problem of deterioration.

In order to improve the deep drawing property of such a hot rolled steel sheet, a method of rolling by lowering the finishing temperature of the hot rolled steel sheet below the Ar ₃ temperature (Japanese Patent Laid-Open Publication No. Hei 5-140655, Korean Patent Publication 91-7, 91-1605) Although these methods are proposed, these methods mainly perform lubrication during hot rolling to obtain a uniform structure in the thickness direction, and finish hot rolling at an Ar ₃ temperature or lower to secure deep drawing. However, since the method has a low hot rolling temperature, deformed crystal grains are formed after hot rolling, and thus, recrystallization heat treatment must be performed after hot rolling. That is, since the steel sheet cooled to normal temperature must be heated to high temperature in order to perform recrystallization heat treatment, there exists a problem that manufacturing cost increases and a manufacturing process becomes complicated. Therefore, there have been attempts to produce a product without recrystallization heat treatment. To this end, it is necessary to manufacture steel with a very low carbon content (about 0.0005%) or to increase the strain rate during hot rolling to about 1000s ^-1 (Korean patent) Gazette 91-7; S. Matsoka et al., ISIJ Inter., 34 (1994), p. 77). In practical terms, however, lowering the carbon content of the steel is not only a factor of increasing manufacturing costs, but is also difficult to produce by the vacuum degassing method currently commonly used. In addition, increasing the strain rate during hot rolling is also difficult with existing rolling mills and is not practical because it requires a better steel sheet thickness and shape control system.

Accordingly, the present invention has been proposed to solve the above problems, and to provide a method for producing a hot rolled steel sheet having excellent deep drawing property without performing a separate recrystallization heat treatment by winding and winding lubricating hot rolling at low temperature, the object There is this.

1 is a graph showing the correlation between the finish rolling temperature and the deep drawing properties

Figure 2 is a photograph showing the change in microstructure with the heat treatment temperature of the invention steel and comparative steel

In order to achieve the above object, the present invention, in weight%, C: 0.01% or less, Mn: 0.5% or less, Al: 0.05% or less, S: 0.015% or less, N: 0.015% or less, Ti and Nb are single or Composite: A hot rolled steel sheet having excellent deep draw toughness, which is formed by finishing lubricating a slab of 0.03 to 0.1%, balance Fe and other inevitable impurities at a temperature of 500 to 730 ° C, and then winding at 680 to 730 ° C. It relates to a method of manufacturing.

Hereinafter, the reason for numerical limitation according to the composition of the present invention will be described.

First, in steel C, when the addition amount is too much, since deep drawing property falls, it is preferable to set it as 0.01% or less.

The elements of Mn, Al, S, and P have a high content, which increases the strength of the hot-rolled steel sheet and acts as a cause of poor workability or embrittlement, so Mn: 0.5% or less, Al: 0.05% or less, S: 0.015% Hereinafter, it is preferable to set it as P: 0.015% or less.

The Ti or Nb forms a carbonitride to lower the content of solid solution carbon or solid solution nitrogen to improve workability. In the present invention, Ti or Nb is added alone or in combination thereof. If the compound addition amount is 0.03% or less, the carbonitride is not sufficiently formed. If the addition amount is too large, the strength of the product increases, which makes the molding difficult and the manufacturing cost is high. Therefore, the amount of Ti and Nb added alone or in combination It is preferable to set it as 0.03 to 0.1%.

Hereinafter, the reason for limitation of the manufacturing conditions of this invention is demonstrated.

The slabs of the composition as described above can be hot rolled under the conditions of the present invention, both thin slabs produced by continuous casting or thin slab casting, and the lubricating rolling method commonly used in the art. Rolling by use, the lubrication rolling method is a method of hot rolling while spraying lubricating oil along with water on the rolling roll.

This lubrication during hot rolling serves to reduce the friction between the material and the roll, thereby suppressing the development of the (110) texture of the plate surface layer formed due to the friction between the roll and the material and consequently (111) aggregation. Tissues promote development to improve processability.

When rolling under the above conditions, when the finish rolling temperature is finished in the ferrite region of Ar ₃ or less, the control of the texture is easy and the deep drawing property can be improved.

This is because the disappearance of the processing energy due to the dynamic recovery during rolling, and thus the (111) aggregate structure is developed by accumulating sufficient deformation energy necessary for recrystallization.

Therefore, the finish rolling temperature is 500 ~ 730 ℃, when the finish rolling temperature is 500 ℃ or less, the rolling load increases and shape control is difficult, and when the finish rolling temperature is 730 ℃ or more, the average plastic deformation, which is an index of deep drawing property This is because the ratio (r _m ) is 1.4 or less, which causes deep drawing property to deteriorate.

As described above, when the hot rolled hot rolled sheet is wound, the cooling rate is very slow after winding, and the self-annealing effect is observed. However, in the ultra low carbon steel which is widely used as the steel for deep drawing, added Ti or Nb suppresses recrystallization Recrystallization heat treatment is required.

However, if the winding in the range of 680 ~ 730 ℃ 100% recrystallization occurs, the crystal grains are also fine can be produced hot rolled steel sheet having excellent deep drawing properties without additional recrystallization heat treatment, if the winding temperature is 680 ℃ or less recrystallization If it does not occur completely, if it is 730 ℃ or more, the grain size is coarse, which may cause surface roughness during press working. At this time, in order to wind up in the above temperature range, it is necessary to reheat the hot rolled sheet before winding up if the hot rolling temperature is low. Induction heating method may be suitable as a heating method, but another method capable of rapid heating Even if used, there is no difference in the effects of the present invention. In this way, additional heating process is required, but it is only necessary to install without changing the production line currently used, and the temperature of the steel sheet slightly dropped after hot rolling may be raised to a temperature suitable for winding.

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

Example 1

In order to find out the proper range of the finish rolling temperature, the slab formed as shown in Table 1 was subjected to finish rolling at various temperatures, and then the specimen cooled to room temperature was subjected to recrystallization heat treatment at 700 ° C. for 2 hours as in the conventional method. The average plastic strain ratio (r _m ), which is an index of sex, was measured and the results are shown in FIG. 1.

The said average plastic strain ratio r _m is also called the average value of Rankford value, and means the average value of the plastic strain ratio obtained from each direction of a tensile plastic deformation test piece, and is defined as follows. That is, a test piece of length L ₀ , width W ₀ , and thickness T ₀ is taken from the plate material, and the tensile plastic deformation is performed in the longitudinal direction, and the length is L, the width is W, and the thickness is T. The plastic strain ratio is defined as the logarithm of the logarithm of W / W ₀ divided by the logarithm of T / T _0. Actually, L ₀ , W ₀ , and T ₀ are measured in terms of volumetric schedules, and T is calculated by calculation. However, since the plastic deformation ratio varies depending on which direction on the surface of the material plate corresponds to the tensile direction of the test piece, in general, the test pieces are taken so as to be stretched in three different directions, and the average of the plastic deformation ratios obtained from the respective directions. Obtain This average value is referred to as the average plastic deformation ratio, and the larger this value is, the more excellent the deep drawing property is.

As shown in FIG. 1, it was found that the deep drawing property of the hot rolled steel sheet was improved as the finishing rolling temperature was lowered, and in order to satisfy the condition of r _m ≥ 1.4, which can be used as a steel sheet for processing as in the present invention. It was found that finish rolling should be carried out at a temperature of 730 ≧ or less. In addition, the lower limit of the finish rolling temperature was set to 500 ° C., which enables the shape and dimension control of the hot rolled steel sheet rather than the workability as described above.

Example 2

In order to determine the coiling temperature conditions, the slab having the composition shown in Table 2 was subjected to finish rolling at 700 ° C., and then subjected to heat treatment at 650 ° C., 700 ° C., and 750 ° C. using a specimen cooled to room temperature. It observed and the result is shown in FIG. Figure 2 (a) is 650 ℃, Figure 2 (b) is 700 ℃, Figure 2 (c) is a microstructure of the specimen heat-treated at 750 ℃.

As shown in FIG. 2, when the heat treatment temperature was low at 650 ° C., recrystallization did not occur completely (FIG. 2 (a)). When the heat treatment temperature was high at 750 ° C., recrystallization occurred completely, but the grain size of the crystal was increased. It can be seen that the coarseness is so high that surface roughness is likely to occur during press working (Fig. 2 (c)). On the other hand, when the heat treatment temperature is 700 ℃ 100% recrystallization occurred, the grain was also fine (Fig. 2 (b)). Therefore, the winding temperature range in this invention could be determined to 680 degreeC-730 degreeC.

Example 3

Determining the finish rolling temperature and the coiling temperature, respectively, as in the method described in Examples 1 and 2 may be somewhat error when the optimum manufacturing conditions by the combination of the two temperature conditions must be determined. However, the ultra-low carbon steel targeted by the present invention is expected to have no significant difference because the transformation temperature is high and the speed is high, so the transformation from austenite to ferrite is completed before finishing rolling within the scope of the present invention. In order to confirm again, it was tested by combining the finish rolling temperature range and the winding temperature range. At this time, the winding temperature may be higher than the finishing rolling temperature. In this case, the hot rolled sheet was heated in the induction furnace by the difference before winding. The slab having the composition as shown in Table 3 was prepared by lubricating hot rolling and winding under various conditions, and then evaluated the deep drawing property without heat treatment, and the results are shown in Table 4 below.

As shown in Table 4, the invention (1-3) of the finish rolling temperature and the coiling temperature produced according to the conditions of the present invention was excellent in the deep drawing property with the usual plastic deformation ratio (r _m ) of 1.4 or more. On the other hand, the finish rolling temperature corresponds to the present invention condition, but the comparative material (2) having the coiling temperature of 680 ° C. or less was found to have a low r _m of 1.21, and the deep drawing property was degraded. The comparative material 91) having a temperature of 730 ° C or higher showed that the deep drawing property was considerably bad as r _m was 0.82.

Example 4

In order to determine the effect of the composition and lubrication rolling of the present invention, the slab having the composition as shown in Table 5 below has the finish rolling temperature and the winding temperature as the conditions of Table 6, and the value of r _m according to the lubrication rolling and non-execution Was measured, and the results are shown in Table 6 below.

As shown in Table 6, in the case of the inventive material (4-5), it was found that the r _m was 1.47 or more and the excellent deep drawability was excellent as a result of lubrication rolling. On the other hand, the comparative material (3) rolled and wound without lubrication rolling with the inventive steel (3) had a r _m of 0.97, and the comparative material (4) manufactured without lubrication rolling with the comparative steel (1) was r _m was 0.85 and the comparative material (3.4) was significantly poor in deep drawing.

As described above, the present invention is capable of hot rolling in existing rolling equipment using steel materials in the carbon content range that can be usually manufactured, and in particular, can provide a method for producing hot rolled steel sheet having excellent deep drawing property without a separate recrystallization heat treatment. In addition, the steel sheet manufactured as described above has an effect that can be applied to the production of automotive plates by plate forming processing such as deep drawing.

Claims (1)

By weight%, C: 0.01% or less, Mn: 0.5% or less, Al: 0.05% or less, S: 0.015% or less, N: 0.015% or less, Ti and Nb alone or in combination: 0.03 to 0.1%, balance with Fe A method of manufacturing a hot rolled steel sheet having excellent deep drawing property, wherein the slab made of other inevitable impurities is lubricated by finishing lubrication at a temperature of 500 to 730 ° C, and then wound at 680 to 730 ° C.