KR940008064B1 - Making method of hot rolling steel plate - Google Patents

Abstract

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Description

Manufacturing method of hot rolled steel sheet for high processing

1 is a graph showing a hot rolling finish temperature and a winding temperature range of the present invention in which recrystallization occurs in a hot rolled coil.

2 is a graph showing the difference in the effective strain distribution in the thickness direction according to the reduction of the shear strain by lubrication rolling.

The present invention relates to a method for manufacturing a hot rolled steel sheet for high processing, and more particularly, to a method for producing a hot rolled steel sheet for continuous processing by continuous lubricating hot rolling.

When forming a thin steel sheet, it is common to have a combination of various processing forms such as stretching and deepdrawing. By the way, in the case of stretching processing, it is required to have excellent elongation among the mechanical properties of the steel sheet, and in the case of deep drawing processing, it is required to have excellent γavg value. To date, many studies have been conducted to improve the stretchability or the deep drawing property in order to improve the workability in hot rolled steel sheets, and few studies have been reported to satisfy both conditions simultaneously.

In general, the γavg value is known to be deeply related to the development of the (111) aggregate structure. As the degree of development of the (111) plane parallel to the rolled plate surface of the aggregate structure in the steel sheet increases the γavg value, the deep drawing property is improved. If the (100) plane is developed parallel to the rolled plate surface, the deep drawing property deteriorates. By the way, in the case of the normal austenite finish rolling material, after the hot rolling is completed, the transformation into ferrite occurs, and the aggregate structure developed in the austenite is greatly weakened during the transformation process, resulting in poor deep drawing property. Therefore, in the case of a general hot-rolled steel sheet, the γavg value is about 0.7-0.9.

Recently, in order to improve the deep drawing property of such hot rolled steel sheets, methods for improving γavg value by finishing hot rolling in a ferrite region have been proposed. For example, when the hot rolling is finished in a ferrite region, the strain rate is 300 /. The method of manufacturing a hot rolled steel sheet excellent in deep drawing property by more than a second (Korean Patent Publication No. 91-1606) can be mentioned.

However, in the above method, the strain rate of 300 / sec or more requires the reinforcement of the equipment in view of the fact that the strain rate is about 100 / sec at most in the current hot rolling mill.

In addition, Japanese Unexamined Patent Publication No. 59-226149 discloses a method of lubricating and adjusting the chemical composition of steel to manufacture hot rolled steel having excellent deep drawing property. not.

In addition, Hashimoto et al. Proposed a method of performing finish rolling in a ferritic station to improve the stretchability of hot rolled steel, but did not examine the deep drawing property (materials and processes, 89-1824).

Since the above methods terminate the hot rolling in the ferrite region, a separate recrystallization heat treatment is adopted, which has a disadvantage in that the manufacturing cost increases due to the addition of the process.

The present invention has been proposed to solve the above-mentioned disadvantages of the conventional method, by performing lubrication rolling at a low temperature below the Ar ₃ temperature, a method for directly manufacturing a hot rolled steel sheet having excellent stretch and deep drawing properties without a separate recrystallization heat treatment. I want to provide it, and its purpose is.

Hereinafter, the present invention will be described.

The present invention, in weight%, C: 0.01% or less, N: 0.01% or less, Mn: 1.0% or less, Si: 0.5% or less, Al: 0.1% or less, S: 0.015% or less, Ti and Nb alone or Composite: 0.04% to 0.06%, remainder Fe and other steels made of impurities inevitably contained after heating to 1150 ° C or less, hot rolling finish temperature: 700 to (winding temperature + 100) ℃, winding temperature: 650 to Rolling reduction at 700 ° C and 910 ° C or lower; The present invention relates to a method for producing a hot rolled steel sheet for continuous processing by hot rolling with continuous lubrication under a true strain of 2.2 or more.

Hereinafter, the reason for numerical limitation of the steel components will be described.

The C (carbon) and N (nitrogen) are similar to each other in the nature, such as the presence of infiltration-type impurity elements in the steel, or to form carbonitride of the added element, and in the case of cold-rolled steel sheet to lower the elongation and γavg value Known as Therefore, by adding Ti or Nb to fix these elements with carbonitrides to improve the workability, the same phenomenon is reported in the case of hot-rolled steel sheet, it is advantageous to workability to lower the content of carbon and nitrogen.

In the present invention, when the carbon content and the nitrogen content in the steel are each 0.01% or more, the Ti and Nb contents required for fixing them as carbonitrides are not compulsory due to an increase in the γavg value but the elongation is reduced, thereby reducing the carbon and nitrogen. The content of is preferably limited to 0.01% or less.

Si is generally an element used for deoxidation, but there is little effect of improving workability, but if the content is high, the surface quality of the hot rolled coil, the chemical conversion treatment and the spot weldability of the cold rolled product are deteriorated, so the content of Si is limited to 0.5% or less. It is preferable.

The Mn is an element exhibiting an effect of improving strength, but if the content thereof is high, the ferrite temperature range is lowered, so that the deformation resistance is increased when hot rolling is performed in the ferrite region. It is desirable to limit.

Al is an element mainly used as a deoxidation element of cold rolled steel, and serves to fix nitrogen as AIN. However, in the present invention steel, since nitrogen is fixed to Ti instead of Al, it is economical if the content of Al is high. In addition to the disadvantages, since the inclusions such as Al ₂ O ₃ is increased to deteriorate the surface quality, it is preferable to limit the content of Al to 0.1% or less.

S is an element fixed to Ti, and if the content is high, the content of Ti is increased correspondingly, so the elongation is lowered and it is economically disadvantageous. Therefore, the content is preferably limited to 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 alone or Nb alone or a combination thereof is added, and the Ti and Nb alone Alternatively, when the compounding amount is 0.04% or less, the carbonitride may not be formed sufficiently. If the amount of addition is too large, a considerable amount of the carbonitride may be present in solid solution, except for the formation of carbonitride. Although there is no problem in the property, since the elongation is lowered, the stretching property is deteriorated. Therefore, in the present invention, it is preferable to limit the amount of Ti and Nb content alone or in combination to 0.04 to 0.06%.

Hereinafter, the reason for limitation of hot rolling conditions is demonstrated.

The re-crystallization of the hot-rolled ferrite is a process in which recrystallized grains are formed and grown by driving the strain energy applied during rolling, so that the larger the amount of deformation, the higher the holding temperature after deformation, and the lower the rolling temperature, the easier the recrystallization.

If the rolling finish temperature is too high, the amount of strain energy applied at the time of rolling is extinguished by dynamic recovery at the same time as rolling, so that sufficient strain energy does not remain as necessary for recrystallization, so the upper limit is (winding temperature +100) ° C. It is preferable to limit to.

On the other hand, as the rolling finish temperature is lower, the amount of strain energy accumulated increases, which is advantageous for recrystallization. However, the lower limit thereof is at least 5 ° C in the cooling process from the finish rolling to the winding process in the hot rolling mill. Since the lower limit of a coiling temperature is 650 degreeC, the minimum of this finishing rolling temperature is limited to 700 degreeC.

On the other hand, if the coiling temperature is too low, even if the strain energy is sufficient, the diffusion rate is slow and recrystallization does not occur, so the lower limit is preferably limited to 650 ℃, if the coiling temperature is too high is extinguished by static recovery before recrystallization occurs Since the amount of strain energy is large and crystal grains develop into coarse grains, the upper limit of the winding temperature is preferably limited to 700 ° C.

The strain energy accumulated during hot rolling in the above temperature condition range is determined by the strain applied, and as the strain increases, recrystallization is likely to occur, so the rolling reduction at 910 ° C. or lower, which is the temperature at which ferrite transformation starts, is true. It is desirable to limit the strain to 2.2 or more.

To summarize the above hot rolling conditions, in order to recrystallize the microstructure of the hot rolled coil, when rolling with a true strain rate of 2.2 or more in the ferrite region, as shown in FIG. 1, the winding temperature ranges from 650 ° C to 700. When the coiling temperature is lower than or equal to 650 ° C and the winding temperature is 650 ° C, the hot rolling finish temperature must be controlled in the range of 700 ° C or higher and 750 ° C or lower. When the winding temperature is 700 ° C, the hot rolling temperature range is 700 ° C or higher and 800 ° C. It should be controlled within the following range, and in case the winding temperature is between 650 ℃ and 700 ℃, hot rolling finish temperature should be controlled to 700 ℃ or more (coiling temperature + 100) ℃.

At this time, the recrystallized microstructure is similar to that obtained in the hot rolling process of the annealing microstructure after cold rolling, and the processability or development tendency of the aggregate structure can be considered to be similar to that of the cold rolling material.

The lubrication rolling method applied to the present invention may be any lubrication rolling method used in the art, and this lubrication rolling method is a method of performing hot rolling while spraying lubricating oil together with water onto a rolling roll.

The hot rolled coil manufactured by the present invention is excellent in workability without the need for a separate recrystallization heat treatment because the microstructure is recrystallized in the hot rolling state, and the additional equipment reinforcement because the deformation rate during rolling is within the normal deformation rate range. It can be applied without and also has the advantage that it can be used in connection with thin slab casting or hot direct rolling in addition to the conventional slab manufacturing process due to the low heating temperature. High energy savings, low scale loss, high error rate.

The reason why the workability of the hot rolled hot rolled steel sheet according to the present invention is improved is as follows.

Compared to a coil in which hot rolling is finished in the normal austenite region, hot rolling is finished in the ferrite region, so that the texture of the texture can be easily controlled and the deep drawing property can be improved. In this case, it is thought that the improvement of deep drawing is made with the same mechanism as that of cold rolled steel sheet, and it is important to reduce the content of solid solution carbon and solid solution nitrogen, and because the solid solubility in ferrite is low compared to that of carbon and nitrogen in austenite, The hot rolling of not only effectively lowers the content of solid solution carbon and solid solution nitrogen, but also decreases the content of solid solution carbon, so that the effect of increasing the elongation is simultaneously produced, which has the effect of improving workability.

Moreover, in the present invention, by adding Ti and Nb alone or in combination, they form carbonitrides to further improve workability by lowering the content of solid solution carbon and solid solution nitrogen.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

After heating the steel material to be prepared as shown in Table 1 to 950 ℃, finish rolling temperature: 750 ℃, winding temperature: 700 ℃ and rolling pressure at 910 ℃ or less: lubrication and non-lubrication under the condition that the true strain is 2.2 After hot rolling, yield strength, tensile strength, elongation, and γavg were measured to investigate the effects of stretching and deep drawing on the lubrication during hot rolling. The results are shown in Table 2 below.

TABLE 1

TABLE 2

As shown in Table 2, the inventive material (a) and the comparative material (1) showed almost the same value in the elongation exhibiting the stretchability, from which it can be seen that the stretchability is independent of lubrication during hot rolling. have.

On the other hand, the inventive material (a) shows a larger value in the? Avg value indicating deep drawing property than the comparative material (1), whereby lubrication rolling is more effective on deep drawing than when lubrication rolling is not performed. It can be seen that it is excellent.

The improvement of the deep drawing property by lubrication rolling is caused by the friction between the roll and the material during hot rolling, and this is a product which shows the difference in the thickness direction effective strain distribution according to the reduction of shear deformation by lubrication rolling. If specifically described through 2 degrees are as follows.

As shown in FIG. 2, in the lubricated rolled material (invention material (a)), the strain distribution between the surface layer portion and the center portion is uniform, while in the rolled material (comparative material (1)) without lubrication, shear deformation in the surface layer portion due to friction. It can be seen that there is a concentrated effect, such a concentration of shear deformation encourages the development of (110) aggregates in the worms, and inhibits the development of (111) aggregates, worsening deep drawing.

In addition, it can be seen that the effect of the lubrication rolling has nothing to do with the chemical composition of the steel sheet.

Example 2

After heating the steel material as shown in Table 3 to 950 ℃ and finish rolling temperature: 750 ℃, winding temperature: 700 ℃ and 910 ℃ or less rolling rate: lubricated rolled to a condition that is 2.2 as true strain rate , Elongation and γavg value were measured, and the measurement results are shown in Table 4 below.

TABLE 3

TABLE 4

TABLE 5

As shown in Table 5 above, the invention materials (d to f) hot-rolled at a heating temperature of the steel material to 1150 ° C. or lower were applied to the comparative material (5) hot-rolled at a heating temperature of the steel material to 1250 ° C. Elongation and γavg values are larger than those of the present invention, indicating that stretching and deep drawing properties are excellent.

As described above, the present invention improves both the deep drawing and the stretchability at the same time, thereby improving the material that is easy to process in the complex processing pattern shown in the conventional processing form, and also eliminating the hot-rolled sheet recrystallization heat treatment process. It is possible to bring about cost reduction effect and the heating temperature of slab can be lowered because the rolling temperature is lower than that of hot rolling in austenite area, which is a common method. Since the amount of production is small, there is an effect that can improve the error rate of the hot rolled coil.

Claims (1)

By weight%, C: 0.01% or less, N: 0.01% or less, Mn: 1.0% or less, Si: 0.5% or less, Al: 0.1% or less, S: 0.015% or less, Ti or Nb alone or in combination: 0.04 to After heating the steel material composed of 0.06%, balance Fe and other unavoidable impurities below 1150 ° C, hot rolling finish temperature: 700 to (winding temperature + 100) ° C, winding temperature: 650 to 700 ° C and 910 Rolling amount below ℃: A method for producing a hot rolled steel sheet for high processing, characterized in that the continuous lubrication hot rolling under conditions of true strain of 2.2 or more.