KR100516459B1 - A method for manufacturing soft cold rolled steel sheet by hot direct rolling method - Google Patents

Abstract

The present invention relates to a method for manufacturing a soft cold rolled steel sheet for deep processing mainly used for automotive and exterior panels, in the manufacturing method of a soft cold rolled steel sheet made of ultra low carbon steel by hot direct rolling method, to improve workability It is an object of the present invention to provide a method for manufacturing a flexible cold rolled steel sheet which can provide excellent deep workability by controlling the rolling reduction ratio of rough rolling and finish rolling using a hot rolling schedule as a means.

In the present invention for achieving the above object, in the method of manufacturing a soft cold rolled steel sheet by hot direct rolling method after continuous casting of ultra low carbon steel, followed by rough rolling and finish rolling, followed by continuous annealing after pickling and cold rolling,

The finish rolling is carried out in a ferritic zone of 890 ° C. or less, and the rolling ratio distribution (rolling rate of rough rolling / rolling rate of finish rolling) in the rough rolling and finish rolling is 1.0 (50%: 50%) to 3.5 (78). %: 22%) to produce a soft cold rolled steel sheet by hot direct rolling method characterized in that the hot rolling schedule is adjusted to the technical gist.

Description

Manufacturing method of soft cold rolled steel sheet by hot direct rolling method {A METHOD FOR MANUFACTURING SOFT COLD ROLLED STEEL SHEET BY HOT DIRECT ROLLING METHOD}

The present invention relates to a method for manufacturing a soft cold rolled steel sheet for deep processing mainly used for automotive and exterior panels, and more particularly, hot rolling schedule for manufacturing soft cold rolled steel sheet using hot direct rolling method. By adjusting appropriately, the present invention relates to a method for economically manufacturing a soft cold rolled steel sheet having excellent deep workability.

The cold rolled steel sheet for deep processing, which is usually made of ultra-low carbon steel, is manufactured by heating a slab cast in a continuous casting process at a high temperature in a reheating furnace and finishing finishing rolling in an austenitic single-phase station above an Ar ₃ transformation point. In the manufacturing process, the rolling schedule during hot rolling, like other general steels, has been used only as a means for making a steel plate of the desired size at last, and there is no concept of using it for material control.

On the other hand, after the mid-1980s, a study on the ferritic reverse rolling, using the advantage that the rolling load is smaller when rolling in the ferritic station during the production of ultra-low carbon steel (related documents: Iron and steel 85-S1362, 85-S1366 , 86-S1343, CAMP-ISIJ vol.2 p.817, iron and steel vol.75 p.782, etc.). However, according to the above, in order to ensure good press formability, since lubrication rolling is essential at the time of ferritic reverse rolling, practical use has hardly advanced.

On the other hand, in the case of ferritic reverse rolling of ultra low carbon steel, the hot direct rolling (HDR) process, which is directly put into the hot rolling process after high temperature slab casting without undergoing a conventional reheating process, is a non-lubricated ferritic reverse rolling. As a technology capable of manufacturing continuous annealing soft cold rolled steel sheets for drawing processing, it has already been filed with Korean Patent Application Nos. 93-31645 and 94-11394. However, in the case of manufacturing a flexible cold rolled steel sheet using the above techniques in an actual hot rolling mill, compared to a product manufactured by a conventional method, that is, a finish rolling in an austenitic single-phase station after reheating, Since the r value (the plastic deformation value) (Lankford value), which is a measure, decreases somewhat, there is a problem that it is difficult to apply to the manufacture of a product for a hard-forming component which requires deep workability.

Accordingly, the present inventors have repeatedly conducted research and experiments to solve the above problems, and propose the present invention based on the results. The present invention is made of ultra-low carbon steel and soft cold rolling by hot direct rolling method. In the manufacturing method of the steel sheet, by using a hot rolling schedule to control the reduction ratio of rough rolling and finish rolling as a means for improving the workability, to provide a method for manufacturing a soft cold rolled steel sheet that can provide excellent deep workability, The purpose is.

In the present invention for achieving the above object, in the method of manufacturing a soft cold rolled steel sheet by hot direct rolling method after continuous casting of ultra low carbon steel, followed by rough rolling and finish rolling, followed by continuous annealing after pickling and cold rolling,

The finish rolling is carried out in a ferritic zone of 890 ° C. or less, and the rolling ratio distribution (rolling rate of rough rolling / rolling rate of finish rolling) in the rough rolling and finish rolling is 1.0 (50%: 50%) to 3.5 (78). %: 22%) to a method for manufacturing a soft cold rolled steel sheet by the hot direct rolling method characterized in that the hot rolling schedule is adjusted to be.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated in detail.

The present invention relates to a method for producing a soft cold rolled steel sheet by hot direct rolling method using ultra low carbon steel as a material, the material applied in the present invention can be produced as a soft cold rolled steel sheet by a hot direct rolling method Any conventional ultra low carbon steel can be used, and more preferably, the content of carbon in the steel is 100 ppm or less and Ti and Nb are contained 0.005 to 0.08% by weight alone or in combination. The C acts as an invasive solid solution and inhibits formation of {111} texture, which is advantageous for processability, during formation of the texture of the steel sheet during cold rolling and annealing. Since {100} and {110} aggregates, which are known to be poor in workability, are strongly formed in the hot rolled sheet and the influence remains even after annealing, the amount thereof is preferably limited to 100 ppm or less. In addition, the contents of Ti and Nb, which play a decisive role in the workability of cold rolled steel sheets, include sulfur, which is an impurity element to which these elements bind in steel (120 ppm or less in general ultra low carbon steel), and nitrogen (in general ultra low carbon steel). The nitrogen content of is set to 60ppm or less) and the carbon content, which is combined with all of these components to form a precipitate such as TiN, TiS, Ti ₄ C ₂ S ₂ , TiC, NbN, NbC, TiNbC and remain as excess. In order to be able to, it is preferable to add 0.005 to 0.08% by weight alone or in combination. If the content of Ti and Nb alone or in combination is less than 0.005%, the solid solution such as sulfur, nitrogen and carbon remain in the steel, resulting in poor workability and high annealing material crystal temperature. This may cause material degradation due to aging. Further, if the content is 0.08% or more, it is not preferable because it raises the recrystallization temperature and is very uneconomical.

When continuously casting ultra-low carbon steel thus formed into slabs and directly hot rolling without high temperature reheating, the rolling ratio of rough rolling and finish rolling is 1.0 (50%: 50%) to 3.5 (78%: 22 %) Is preferable because of the following reasons.

Ti, Nb, and the like added to the steel of the present invention combines with solid impurities such as nitrogen, sulfur, and carbon to form precipitates. The size and distribution of these precipitates greatly affect the processability of the final cold rolled product. That is, in the state where all impurity elements such as C, N, S, etc. in the hot rolled sheet are fixed as precipitates, very fine precipitates having a precipitate size of several tens of micrometers or less are hardly present and are distributed uniformly in the size of several hundred micrometers or more. As a result, the r value of the cold rolled steel sheet as a final product is greatly improved. On the other hand, since the temperature zone where these precipitates are actively precipitated coincides mainly with the temperature range during hot rolling, the size and distribution of precipitates in the ultra low carbon steel are largely dependent on the hot rolling temperature and the amount of rolling. Therefore, it is important to increase the amount of reduction in the temperature range that can promote the precipitation of these precipitates during rolling. In particular, when rolling ultra low carbon steel, which has a greater delay in precipitation than conventional rolling methods, by hot direct rolling, the higher the reduction amount of finish rolling is, the easier it is to form precipitates, and the size is more than several hundred microseconds. This predominance will be achieved.

That is, as in the present invention, the reduction ratio of the rough rolling and the finish rolling was set to 1.0 (50%: 50%) to 3.5 (78%: 22%) by increasing the amount of reduction in finish rolling. This is to increase the r value of the final product by adjusting the distribution of precipitates of hundreds of microns or more without the presence of employment elements. In addition, when the reduction ratio of the finish rolling is too large and the rolling reduction ratio is less than 1.0, the rolling load is greatly increased, and since the synergistic effect of the r value hardly appears at 3.5 or more, it is not preferable.

On the other hand, the finish rolling during hot rolling is preferably finished in the ferrite single-phase zone of 890 ℃ or less. The reason is that when the finish rolling temperature is 890 ° C or higher, rolling is performed in the two-phase region of ferrite and austenite, causing a mixed structure that greatly impairs the workability of the final product.

The hot rolled steel sheet rolled in the above manner removes the surface scale by pickling in a conventional manner, and then rolls it at a reduction ratio of 50 to 85% to form a cold rolled sheet, which is continuously annealed at a relatively low temperature of 720 ° C or higher.

The soft cold rolled steel sheet produced in this way can ensure excellent deep workability even by the non-lubricated ferrite reverse rolling during hot rolling.

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

(Example)

A hot rolled sheet was manufactured under the hot rolling conditions of Table 2, using the slabs having a composition as shown in Table 1 below. Then, the hot rolled sheet obtained above was pickled and cold rolled at a reduction ratio of 75%, followed by continuous annealing, and then the mechanical properties of the cold rolled sheet were measured, and the results are shown in Table 3 below.

division Chemical component (% by weight) C Mn S P Sol.Al N Ti Nb River 1 0.003 0.12 0.008 0.010 0.042 0.0025 0.047 - River 2 0.004 0.10 0.009 0.011 0.038 0.0023 0.032 0.010 River 3 0.003 0.09 0.007 0.009 0.032 0.0018 0.048 - River 4 0.005 0.11 0.006 0.012 0.045 0.0029 0.051 - River 5 0.003 0.10 0.009 0.009 0.044 0.0021 0.029 0.013 River 6 0.004 0.13 0.008 0.008 0.039 0.0020 0.045 -

division Steel grade used Slab reheating (℃) Finish rolling temperature (℃) Rolling rate distribution (%) Abhabi Continuous Annealing Condition Rough rolling Finish rolling Inventive Example 1 River 1 － 812 76 24 3.17 750 degrees Celsius * 40 second Inventive Example 2 River 2 843 71 29 2.45 Inventive Example 3 River 3 821 68 32 2.13 Comparative Example 1 River 4 798 86 14 6.14 Comparative Example 2 River 5 832 81 19 4.26 Conventional example River 6 1210 913 82 18 4.56 * Rolling ratio: Rolling rate of rough rolling / Finishing rolling rate

division Mechanical properties Yield strength (kg / mm ² ) Elongation (%) Rank value Inventive Example 1 15.3 47.0 1.86 Inventive Example 2 16.1 48.1 1.91 Inventive Example 3 15.7 47.8 1.92 Comparative Example 1 16.7 47.2 1.72 Comparative Example 2 16.0 46.9 1.74 Conventional example 29.3 32.0 0.8 As shown in Table 3 above, the inventive examples (1) to (3) of the present invention control the reduction ratio of rough rolling and finish rolling so as to perform lubrication-free ferrite reverse rolling. Compared with (2), it is excellent in workability, and it can be seen that the hot rolled material provides superior material properties compared to the conventional example prepared by completing finish rolling in austenite after heating.

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As described above, according to the present invention, by implementing a rolling method that actively uses the metallurgical behavior of the precipitates in the steel, while improving the productivity, shortening the manufacturing period, improve the strip sheet in the annealing furnace, energy saving, etc. Not only is it possible to manufacture the excellent soft cold rolled steel sheet, but it is also possible to further improve productivity by increasing the reduction ratio of the final finish rolling.

Claims (2)

In the method of manufacturing a soft cold rolled steel sheet by hot direct rolling method after continuous casting of ultra low carbon steel, followed by rough rolling and finish rolling, followed by continuous annealing after pickling and cold rolling, The finish rolling is carried out in a ferritic region of 890 ° C. or lower, and the rolling ratio distribution (rolling rate of rough rolling / rolling rate of finish rolling) in the rough rolling and finish rolling is 1.0 (50%: 50%) to 3.5 (78). %: 22%) to produce a flexible cold rolled steel sheet by the hot direct rolling method characterized in that to adjust the hot rolling schedule. The method of claim 1, wherein the ultra low carbon steel contains 0.005 to 0.08% by weight of C, Ti and Nb of 100 ppm or less alone or in combination.