KR950003807B1 - Making method of cold rolling sheet - Google Patents

Abstract

The method reduces the manufacturing cost and improves productivity, surface quality of cold rolled steel plate, and formability by lowering Mn content. The method comprises; (A) hot rolling Al-killed steel and coiling at the temperature range of 600-700 deg.C; (B) cold rolling; and (C) continuous annealing at 850 deg.C for above 30 seconds.

Description

Manufacturing method of cold rolled steel sheet for high ductility

Fig. 1: Transmission electron micrograph showing the form of precipitates in Ti-added ultra low carbon steels.

2 is a graph showing the mechanical properties of the Mn content and the hot rolled coil temperature change.

Figure 3: Workability Evaluation Index Graph showing values and machinability evaluation index (r ^* ) values.

The present invention relates to a method for manufacturing a super-cold cold rolled steel sheet used in a material such as car body core processing member and integral molding of an automobile, and more particularly, to produce a cold rolled steel sheet for super-ductile super-core processing by hot rolling low temperature winding It is about a method.

The method of manufacturing cold rolled steel sheet for deep processing is roughly divided into annealing and continuous annealing, and the continuous annealing method has the advantages of high productivity, uniform material, and performance, thus making up the main role of the recent annealing equipment. Since continuous annealing is characterized by rapid heating and short time cracking, the recrystallized grain becomes fine when manufacturing cold-rolled steel sheets for deep processing by the continuous annealing method, and the yield strength is not effectively precipitated because invasive solid elements such as carbon and nitrogen cannot be effectively deposited. (2) High tensile strength and difficulty in securing sufficient processability, and there is a problem in that mechanical properties deteriorate with time due to aging. As a means to solve this problem, a method of precipitating solid solution carbon through rapid cooling and aging treatment and a method of removing solid solution carbon and nitrogen from the annealing process by adding Ti and Nb, which are powerful carbonitride forming elements, are alloy elements. It is commercially adopted. However, since these alloying elements are very expensive, a method of reducing the amount of carbon fixed as precipitates, i.e., reducing the amount of alloying elements added by lowering the carbon content to 0.01% by weight (hereinafter referred to as%) in the steelmaking process Is commercialized. This is called Ti-added ultra-low carbon steel, and is usually manufactured by continuous annealing and can be given better workability than the normal annealing material. Therefore, it is widely used in deep processing members such as automobile bodies. However, the steelmaking cost is much higher than that of the annealing material, and in order to secure sufficient processability, the hot rolled winding temperature of about 720 ° C is applied, so that the oxide layer grows thick at high temperature, and the oxide layer does not dissolve well in acid during cold rolling. There is a problem that causes surface defects. The present invention solves the above problems and gives better workability by reducing the steelmaking cost and hot rolling temperature for deep-rolled cold rolled steel sheet made of Ti-added ultra low carbon steel, and in detail the role of the Ti component in the steel in detail. Based on this, by appropriately changing the components that can be easily adjusted in the steelmaking process among the components of solid steel in the steel, to provide a cold rolled steel sheet for manufacturing cost lower than the existing similar steel grades, and high-ductility super-machining with extremely excellent workability There is a purpose.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention is, by weight%, C: 0.01% or less, Mn: 0.1% or less, acid value Al: 0.01-0.04%, N: 0.005% or less, S: 0.015% or less, atomic ratio of effective Ti / C: 1.0 -4.0 (Effective Ti is 3-48 · N / 14-48 · S / 32), remainder Fe and other unavoidably contained aluminum-kilted steels hot rolled and then wound in a temperature range of 600-700 ° C. Then, after cold rolling by a conventional method, and the continuous annealing for more than 30 seconds at a recrystallization temperature or more 850 ℃ or less relates to a method for manufacturing a cold rolled steel sheet for super-ductile super-core processing.

Hereinafter, the above components and the manufacturing conditions thereof will be described in detail.

Since the carbon acts as an invasive solid solution and develops a recrystallized texture which is detrimental to workability during annealing, it is not preferable to exceed 0.01%, and since the Ti component that solidifies and removes solid carbon as a precipitate increases in proportion to the solid solution carbon, Since the burden of steelmaking costs increases, it is desirable to limit the amount to 0.01% or less. The upper limit of the Mn is preferably limited to 0.1%. Such reduction of the Mn content is sufficiently manufacturable in accordance with the recent development of steelmaking technology. In spite of being 0.1% or less, Mn alloy iron is added to molten steel to adjust its components in the range of 0.15-0.3%. The reason for this is that the existing metallurgical position on Mn is that the solid solution S, which cannot be precipitated as MnS without sufficient Mn in the steel, acts as a liquid film to the austenite grain boundary and acts as a hot brittle factor during hot rolling. Because it is considered.

Therefore, Mn is an element that prevents hot brittleness during hot rolling, and it has been known until now that a proper amount or more must be contained in steel. However, in the steel added Ti as in the present invention, most of the steel grade S is higher than the MnS precipitation temperature. Since Ti is preferentially precipitated by Ti as Ti emulsion or Ti carbon emulsion at temperature (see FIG. 1), Mn mostly exists as a substituted solid solution element. When Mn is present as a substitutional solid solution, grain growth is suppressed, and thus the grain size is reduced, thereby decreasing workability and elongation. In the present invention, the Mn component in the Ti-added ultra low carbon steel acts as a substitution-type solid solution element, unlike the conventional metallurgical position, so that the smaller the content, the better the workability and ductility.

Therefore, the content of Mn is preferably limited to 0.1% or less.

Aluminum in the present invention, unlike ordinary steel, is a component added for the purpose of deoxidation.

In other words, in general steel, aluminum forms AlN to reduce solid solution nitrogen, and thus workability is improved. However, in the present invention, since Ti forms a more stable nitride at a much higher temperature than Al, most steels are formed. Al exists in the solid solution state.Soluble aluminum is not preferable because the atomic radius with iron is greatly different, which causes grain growth inhibition during strengthening or annealing, thereby degrading the workability and ductility of the cold rolled steel sheet. In addition, since aluminum contains other disadvantages of manufacturing cost increase, it is preferable to limit the upper limit to 0.04% in the present invention steel.

However, if the Al content is too low, the Ti component added for the purpose of removing and removing solid solutions such as carbon and nitrogen will form a Ti oxide preferentially, resulting in a loss. Therefore, the lower limit of the Al content is sufficient to deoxidize, that is, Al _2. It is preferable to make the acid-soluble Al excluding 0.01% of the acid-insoluble Al content such as O ₃ . Solid solution nitrogen develops a recrystallized texture which is detrimental to workability during annealing, and in the present invention steel, solid solution nitrogen is completely removed as a precipitate by Ti, but when the content of nitrogen is increased, the amount of Ti added increases, which is not preferable in terms of manufacturing cost. Therefore, the upper limit is preferably limited to 0.005%. When the S is present as a solid element, it causes hot brittleness during the hot rolling process. However, in the present invention steel, since S mostly exists as a Ti emulsion in the hot rolling process, there is no problem of generating hot brittleness. However, when the content of S increases, the amount of Ti added increases, which is disadvantageous in terms of manufacturing cost, so the upper limit is preferably limited to 0.015%. In the present invention, Ti is a precipitate forming element, and the precipitates are mainly nitrides, emulsions, carbides, and composite precipitates thereof. Therefore, the Ti content in the steel can completely remove solid solution carbon as a precipitate when the atomic ratio of effective Ti / C is 1 or more. Where the effective Ti is the amount remaining after removing nitrogen and S as a precipitate, that is, Ti% -48N)%) / 14-48S (%) / 32. Therefore, if the atomic ratio of effective Ti / C is less than 1, the removal of invasive solid-solution elements is insufficient. As a result, the formation of recrystallized aggregates that are detrimental to workability is promoted, and the effective Ti / C atomic ratio exceeds 4. In addition, the addition of Ti does not only increase the manufacturing cost, but also increases the annealing temperature during continuous annealing due to an increase in the recrystallization temperature due to the increase in the density of fine precipitates and solid solution Ti. It is preferable to limit to 1.0-4.0 in atomic ratio.

The steel produced as described above is hot rolled as usual, but the winding after hot rolling is preferably limited to 600-700 ° C. Thus, the hot rolling temperature is set to 700 ° C or lower, which is lower than the 720 ° C, which is the winding temperature for ordinary equivalent materials. When the hot rolling temperature exceeds 700 ° C, the hard acid pickling hot oxide layer is thickened and is not easily removed. This is because the surface defects of cold-rolled products are increased. However, workability is reduced by lowering the hot-rolled coiling temperature to 700 ° C or lower, and this workability can be compensated to the degree of workability improvement obtained by lowering the Mn content than conventional steel grades. to be. However, very low hot rolling temperature is difficult to secure the target processability, and in order to lower the coiling temperature, the amount of cooling water to be poured on the run out table prior to hot rolling needs to be increased, thereby lowering the lower limit of hot rolling temperature. It is preferable to limit to 600 ° C. Thereafter, the cold rolled coil is manufactured by pickling and cold rolling as in the usual method, and then continuously annealed. The temperature at the time of continuous annealing is sufficient to be cracked for 30 seconds or more at the temperature of steel recrystallization temperature or more and 850 degrees C or less.

In addition, in the present invention, in order to obtain a more accurate workability, the following formula for determining the workability evaluation index (r ^* ) value is determined by regression analysis of the workability evaluation index and manufacturing process variables for the present invention steel,

r ^* = 94.8 CT ^0.01 -0.493Mn-0.951SRT / 1000 + 2.11 (AT / 1000) ⁹ -98.3

According to the above formula can determine the continuous annealing temperature of the cold rolled steel sheet to secure the target workability evaluation index.

In the above formula, CT is hot rolling temperature (℃), Mn is Mn content (%), SRT is temperature just before reheating or hot rolling (℃), and AT is continuous annealing temperature (℃, crack for 30 seconds or more). In the equation, the adjustment crystal coefficient is 0.86 or more for the inventive steel. The r ^* value, which is the workability evaluation index required for the cold rolled steel sheet for initial processing, is 1.9 or more. In the present invention, unlike the conventional manufacturing method, the workability evaluation index prediction equation is given to give more flexibility to the manufacturing process. For example, in the present invention steel containing 0.07% Mn, when the slab reheating temperature is high as 1270 ℃ in cold rolling process and the hot rolling temperature is inevitably set to 600 ℃ due to the concern about the surface quality, cold rolled steel for the initial processing of the workability evaluation index 1.9 It can be seen that the annealing temperature for producing a steel sheet is 827 ℃ or more. As such, the present invention has excellent flexibility in the manufacturing process and almost no defects in the manufacturing process because the target processability can be sufficiently obtained by appropriately controlling the main factors of the post-process even in the case of mistakes in the previous process or unavoidable working conditions in the manufacturing flow. As a result, product yield improvement is remarkable.

As described above, in the present invention, since solid solutions such as carbon and nitrogen are sufficiently fixed and removed by Ti as a precipitate, there is no need for quenching and overaging in continuous annealing, and the yield point stretching phenomenon according to solid carbon and nitrogen after continuous annealing is eliminated. Therefore, the level of temper rolling quality and surface roughness is sufficient.

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

Example 1

To manufacture aluminum-kilted steel by vacuum induction melting and commercial converter-continuous casting in the composition as shown in Table 1, followed by hot rolling and winding under the conditions as shown in Table 2, followed by cold rolling and continuous annealing The properties were measured, and the measurement results are shown in Table 2 below.

At this time, the hot finishing and rolling temperature was 920 ° C., the cold rolling rate was 75%, and the continuous annealing time was 30 seconds.

Meanwhile, specimens of the inventive steel (1) and the comparative steel (2) shown in Table 1 were taken from the center of the hot-rolled hot rolled sheet at a slab reheating temperature of 1200 ° C., finishing finishing temperature of 920 ° C., and hot rolling temperature of 650 ° C. The result was observed with a transmission electron microscope (TEM) by the extraction replica method, and the observation result (micrograph) was shown in FIG.

TABLE 1

1) Al ^* : Content of acid value Al

2) Effective Ti / C (atomic ratio): 0.25 Effective Ti% / C%

3) Effective Ti%: Ti% -48N% / 14-48S% / 32

TABLE 2

r ^* : Machinability index predicted value

(94.8 CT ^0.01 -0.493Mn-0.951SRT / 1000 + 2.11 (AT / 1000) ⁹ -98.3

As shown in Table 2, the invention steel (1-3) of the present invention material (aj) in accordance with the present invention is wound at a low hot rolling temperature of 700 ℃ or less as 1.9 or more On the contrary, the comparative materials (a, c, d) outside the scope of the present invention are wound at a low hot rolling temperature of less than 700 ℃ ℃ comparative steel (1-2), it is difficult to secure an F value of 1.9 or more, 1.9 or more also in the case of the comparative material (b) which hot-rolled the comparative steel (1) at 720 degreeC It can be difficult to secure.

However, the comparative material (e) is obtained by hot-rolling the comparative steel (2) at 700 ° C and continuously annealing at 850 ° C. Indicates.

Therefore, by using a steel grade outside the scope of the present invention to be wound at a winding temperature of 700 ℃ or less to 1.9 or more It can be seen that high temperature annealing of 850 ° C. or higher is necessary in continuous annealing in order to ensure.

In addition, all of the present invention materials (aj) exhibited an elongation of 50% or more, whereas in the case of the comparative materials (ae), all of them exhibited an elongation of 50% or less. In contrast, it can be seen that the material of the high smoker can be more suitably applied to the process molding material for the super-core processing.

On the other hand, Figure 1 (a) and Figure 1 (b) is a micro-precipitated photograph of the hot rolled sheet hot rolled invention steel (1) having a Mn content of 0.02%, respectively, and comparative steel having a content of Mn of 0.25% (2 ) Shows a picture of the fine precipitate of the hot rolled sheet is hot-rolled, the lower picture in Figures 1 (a) and 1 (b) is an enlarged view of the Ti carbonitride of the upper part (10,000 times magnification). .

As shown in FIG. 1, both the inventive steel (1) and the comparative steel (2) did not precipitate in the form of MnS as seen in ordinary carbon steel, but precipitated in Ti emulsion, and in different steel grades. It can be seen that much less than MnS of.

Precipitates in the hot rolled steel of these steels are mainly composed of Ti emulsion of 1500-2000Å size or the composite precipitate of Ti carbonitride and fine Ti carbonitride within 1000Å and the size and distribution of these precipitates regardless of Mn content. The density appeared similar, and therefore, it can be seen that most of the Mn component in the steel exists as a substituted solid solution element, because the forming power of the Ti emulsion is stronger than that of Mn. In addition, although the hot rolled coiling temperature was 650 ° C., the AlN precipitate was not observed as in the general steel because the Ti nitride had a stronger forming force, and thus, it was found that it existed in solid solution in the steel like Mn.

Example 2

Among the steel grades shown in Table 1 of Example 1, the inventive steels (a) and (c) and comparative steels (b) were subjected to hot finishing rolling temperatures of 920 ° C., slab reheating temperature of 1200 ° C., and hot rolled winding temperatures of 600-700 ° C. After hot rolling under the conditions, cold rolling was performed at a cold reduction rate of 75% to prepare a cold rolled plate having a thickness of 0.8 mm.

The cold rolled sheet manufactured as described above was continuously annealed at a cracking temperature (annealing temperature) of 830 ° C. for 30 seconds, and then measured mechanical properties according to Mn content and hot rolled coil temperature change, and the results are shown in FIG. 2. It was.

As shown in Figure 2, the machinability index As the Mn content decreases and the hot rolling temperature increases, the value increases. Therefore, to reduce the surface defects of cold rolled products, lower the hot rolling temperature and The easiest and most economical way to ensure that the Mn content will be as low as possible.

In addition, the tensile strength, yield strength, and elongation are more dependent on the Mn content than the hot rolling temperature, and in particular, the elongation is at least 50% when the Mn content is 0.1% or less.

Therefore, if the Mn content of Ti-added ultra low carbon steel is lowered, super cold-rolled cold rolled steel sheets having high ductility and resistance properties are manufactured, and even if a hot rolled winding temperature lower than that of conventional steel is applied, equivalent or more materials can be secured. It can be seen that.

Example 3

Inventive steels (a), (b) and (c) of the steel grades shown in Table 1 of Example 1 were subjected to slab reheating temperature of 1100-1250 ° C, hot finishing rolling temperature of 920 ° C, and hot-rolling winding temperature of 600-720 ° C. After hot rolling, cold rolling at 75% and continuous annealing at 790-850 ° C for 30 seconds or more. The value was measured, and the measurement result showed the workability evaluation index predicted value (r ^* ) calculated | required by the following formula about each condition in FIG.

r ^* = 94.8 CT ^0.01 -0.493Mn-0.951 SRT / 1000 + 2.11 (AT / 1000) -98.3

(Here, CT: coiling temperature, SRT: slab reheating temperature, AT: continuous annealing temperature)

In FIG. 3, the standard deviation of the equation is 0.44 and the coefficient of crystal is 0.86. You can see that and r ^* are almost identical.

Therefore, it is necessary to commercially available cold rolled steel sheet Is 1.7 or more, but in the present invention, It is preferable to set the target value of the predicted value to 1.9 or more so that the target value can be used. Since it can obtain, the flexibility of a manufacturing process can be provided.

As described above, the present invention not only lowers the manufacturing cost of Ti-added ultra low carbon steel, which is similar to conventional steels by lowering the Mn content, Since 49% and 1.9, respectively, the cold rolled steel sheet for super-ductile super-machining, which far surpasses the conventional workability, it can be suitably applied to the material fields of automobile body core processing member and integral molding, and especially in the manufacturing method. Since it can lower the hot rolled cold winding than conventional materials, it is effective to improve the surface quality, product yield and productivity of cold rolled steel sheet.

Claims (1)

In the method of manufacturing a high ductility cold rolled steel sheet by hot-rolling and winding the aluminum-kilted steel, followed by continuous annealing of the cold-rolled and cold-rolled steel sheet, the aluminum-kilted steel, C: 0.01% or less, Mn: 0.1% or less, acid value Al: 0.01-0.04%, N: 0.005% or less, S: 0.015% or less, atomic ratio of effective Ti / C: 1.0-4.0 (effective Ti is Ti-48N / 14- 48 · S / 32), the balance Fe and other inevitably contained impurities; The coiling temperature of the hot rolled steel sheet is 600-700 ° C; The continuous annealing temperature is not less than the recrystallization temperature of 850 ° C. and the continuous annealing time is 30 seconds or more; And the coiling temperature CT, Mn (%), slab reheating temperature (SRT), and continuous annealing temperature (AT) are such that the workability evaluation index predicted value (r ^* ) expressed as follows is 1.9 or more, r ^* = 94.8 CT ^0.01 -0.493Mn (%)-0.951SRT / 1000 + 2.11 (AT / 1000) ⁹ -98.3 Method for producing a high ductility cold rolled steel sheet, characterized in that selected.