KR101301351B1 - Method of manufacturing austenitic stainlesss steel having high strength and ductility - Google Patents

Abstract

In the present invention, cold rolling is carried out twice in the production of Cr-Ni-based and Cr-Mn-Ni-N-based austenitic stainless steels to control the reduction ratio during the primary and secondary rolling and in the subsequent annealing process. By controlling the temperature to obtain a fine and uniform austenite recrystallized structure provides a method for producing austenitic stainless steel excellent in strength, ductility and workability compared to the conventional temper rolling method.

Austenitic, Stainless, Reversible, Cold Rolled, Rolling Down, Recrystallized

Description

Method of manufacturing austenitic stainless steel having high strength and ductility

1 is a view showing a rolling ratio for each rolling pass during cold rolling according to the prior art.

2 is a view showing the temperature of the entrance material for each rolling pass during cold rolling according to the prior art.

3 is a view showing the cumulative martensite content for each rolling pass during cold rolling according to the prior art.

4 is a view showing a rolling reduction rate for each rolling pass during cold rolling according to an embodiment of the present invention.

5 is a view showing the temperature of the entry material for each rolling pass during cold rolling according to an embodiment of the present invention.

6 is a view showing the cumulative martensite content for each rolling pass during cold rolling according to an embodiment of the present invention.

FIG. 7 is a diagram illustrating an austenite inverse transformation due to heat treatment of a rolled material having martensite. FIG.

Figure 8 is a photograph showing the structure after the heat treatment of the rolled material STS 301L steel having 21% martensite.

9 is a photograph showing the structure after the heat treatment of the rolled material of STS 301L steel having 74% martensite, (A) heat treatment temperature 550 ℃, (B) heat treatment temperature 800 ℃.

10 is a photograph showing the structure after the heat treatment of the ASTM 204 steel having a 62% martensite, (A) heat treatment temperature 550 ℃, (B) heat treatment temperature 800 ℃.

11 is a view showing the change of the stress-strain curve according to the heat treatment temperature after cold rolling, (A) STS 301L steel, (B) ASTM 204 steel.

12 is a view showing the change of the stress-strain curve according to the grain size, (A) STS 301L steel, (B) ASTM 204 steel.

The present invention relates to a method for producing austenitic stainless steel, and more particularly to manufacturing a high-strength austenitic stainless steel having excellent strength and ductility in the annealing state and temper rolling state compared to conventional high strength austenitic stainless steel cold rolled products It is about how to.

Conventionally, temper rolling has generally been used as a method of increasing the strength of austenitic stainless steels. This method takes advantage of the phenomenon that high work hardening occurs as the austenite phase transforms into processed organic martensite during cold deformation. For example, metastable austenitic stainless steels that meet KS STS 301 and STS 301L standards can obtain various strengths in the range of 700 to 1000 MPa through temper rolling according to the specifications of LT, DLT, ST, MT, HT, etc. It is mainly used for corrosion resistant structures such as electric vehicle structural parts. Further, in general, STS304 steel and Cr-Mn-Ni-N-based metastable austenitic stainless steels have been subjected to temper rolling to obtain higher strength than the annealing state due to the demand of demand.

However, the product subjected to temper rolling has a drawback in that elongation and work hardening ability are drastically lowered and thus cannot be used for processing. For example, in the case of STS 301L steel, yield strength of about 800 Mpa can be obtained by 20% temper rolling, but the elongation is lowered to 20-25%, and subsequent processing is difficult.

As a method for solving the drawbacks of temper rolling, a method of refining grains has been proposed. In general, this method takes advantage of the fact that the reduction in ductility is relatively small when using a reinforcing mechanism by grain refining compared to work hardening. For example, Japanese Patent P2000-273538A proposes a technique for obtaining a yield strength of 400 Mpa or more and an elongation of 35% or more by cold rolling SUS304 stainless steel and then heat-treating it at a relatively low temperature of 700 to 950 ° C. to obtain a fine grain structure. . However, according to the present inventors' study on such a method, the austenite reverse transformation of martensite formed by cold rolling plays an important role in the reinforcement by the method, and in the rolling mill used for production, a sufficient amount of martensite It was confirmed that it is very difficult to obtain the method, not only difficult to apply the method described above, but also the strength increase effect is not great even if applied. Japanese Patent P2000-273538A mentioned above does not provide conditions for controlling martensite in order to achieve grain refinement and also a rolling method for realizing the same.

In order to solve the above problems, the present invention is cold-rolled in a rolling mill used for actual production of Cr-Ni-based and Cr-Mn-Ni-N-based austenitic stainless steels, and then manufactured as a cold-rolled steel strip through a subsequent annealing pickling process. In order to effectively reinforce the material and at the same time significantly reduce the ductility reduction and increase in yield ratio than the existing temper rolling method to provide a method for producing austenitic stainless steel that can produce a material with improved workability.

In order to achieve the above object, in the present invention, Cr-Mn-Ni-N-based standard steels such as KS standards STS304, STS304L, STS301, STS301L, Cr-Ni-Austenitic stainless steel and ASTM 201, 202, 204, 205, and Jindal J1 Of semi-stable austenitic stainless steels, including non-standard steels, including Jindal J3, Jindal J4, Nitronic 30, H400, NTK D-10, NTK K-4 and NTK K-1. In manufacturing cold rolled coil through hot rolling annealing, cold rolling and cold rolling annealing, the cold rolling is divided into primary and secondary rolling, and after rolling the first pass reduction ratio of 25% or more in primary rolling Secondary rolling is carried out at 40% or more under the total pressure without intermediate annealing, and after processing 50% or more of the processed organic martensite in the rolling structure, the subsequent cold rolling annealing process is performed by annealing temperature in the range of 800 to 1000 ° C. Homogeneous recrystallization with grain size less than 15μm Obtaining an austenitic structure provides a method for producing austenitic stainless steel having high strength, elongation and workability.

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

It is well known that fine grains can be refined by first cold working austenitic stainless steel and then heat treatment at relatively low temperatures. However, the present inventors have found that the final grain structure varies depending on the cold working structure during such processing and heat treatment, and the present invention has been proposed based on these results. In other words, when cold processing results in sufficient processing-organic martensite content, the modified martensite structure reverses transformation to recrystallized austenite at very low diffusion rates even at low temperatures, resulting in effective and uniform grain refinement. As a result, when the processed organic martensite content was not sufficient, the site transformed into martensite caused grain refinement due to the reverse transformation, and the site where the modified austenite remained did not undergo recrystallization of austenite as long as there was no concentration of deformation. In general, a non-uniform structure is formed in which fine austenite recrystallized tissue and coarse austenite processed tissue are mixed. In the latter case, when the heat treatment temperature is increased, recrystallization of the austenite processing structure occurs. At that time, coarsening of the austenite recrystallization structure already formed by reverse transformation occurs, and the overall grain size becomes coarse. Therefore, in order to prevent a decrease in ductility due to uneven structure and a decrease in strength due to grain coarsening, it is necessary to obtain a sufficient amount of processed organic martensite content as a result of cold working.

According to the results of the present inventors, in case of having a martensite content of 50% or more after cold working, the martensite phase is changed into a fine recrystallized austenite structure by reverse transformation, and at the same time, it is not transformed into martensite. Also, it was found that due to the presence of martensite in the periphery, sufficient strain concentration occurs and the driving force of the recrystallization is increased, so that rapid recrystallization occurs at a low temperature during subsequent heat treatment, resulting in a uniform and fine austenite recrystallized structure.

It is easy to obtain martensite content of 50% or more mentioned above in a laboratory mill, but not in a reversible mill or tandem continuous mill such as Sendzimir Mill used in actual production. This is because in the first pass of the rolling mill used in actual production, the processing organic martensite transformation occurs easily at room temperature, but in the subsequent pass, as the rolling amount increases, the temperature of the material increases and the reduction ratio increases. This is because the transformation of processed organic martensite becomes more difficult. In general, cold rolling of stainless steel is rolled using neat rolling oil to obtain surface gloss, which is lower in cooling than emulsion rolling oil used in carbon steel. It will rise to the 100-250 ℃ range. Because of this, the martensite content that can be obtained in primary rolling is limited to a maximum of 30-40%, which is mostly obtained in the first pass and almost no additional martensite transformation occurs due to plate temperature increase in subsequent passes. In particular, it is more difficult to obtain a sufficient amount of martensite in steels containing ASTM 204 steel and nitrogen of 0.2% or more and having relatively high austenite stability. On the other hand, the present inventors performed rolling in two or more times and carried out one pass of the primary rolling at a reduction ratio of 25% or more, and then, in the normal process management, the coil was sufficiently cooled for 24 hours and then put in the secondary rolling to reduce the total pressure. It has been found that the rolling of 40% or more facilitates martensite formation during the secondary rolling, so that the martensite content of 50% or more can be easily obtained as a whole.

The annealing temperature is 800-1000 ° C. lower than the normal annealing temperature of 1100 ° C. in the case of coils rolled to contain 50% or more of martensite by the above-described method, for example, by cold-rolling annealing by continuous cold annealing or bright annealing. In this case, a homogeneous recrystallized austenite structure having an average grain size of 15 μm or less can be obtained, and thus a steel sheet having a very high strength and elongation in the annealing state and higher strength can be obtained by subsequent temper rolling. The annealing temperature is limited to 1000 ° C. or lower to suppress the growth of grains refined by the austenite reverse transformation of martensite, and the limit to 800 ° C. or higher is to allow the reversed austenite to sufficiently recrystallize.

For the production of the high strength austenitic stainless steel provided by the present invention, no special technique is required for melting, casting and hot rolling. The molten steel is prepared by melting and argon-oxygen decarburization, which is a general manufacturing process of austenitic stainless steel, and slab is manufactured by continuous casting process under atmospheric pressure, and reheated to a temperature of 1100 ~ 1300 ℃, followed by hot rolling and annealing. After manufacturing hot rolled steel strip. It may also be prepared by rolling or forging after melting the ingot, and may also be produced by strip casting under atmospheric pressure as a more economical method.

(Example)

The following examples illustrate the present invention.

In order to illustrate the properties of the alloy of the present invention, a continuous casting slab of STS 301L steel (17% by weight of 17Cr-7Ni-0.03C-0.12N) and ASTM 204 steel (16Cr-9Mn-3Ni-0.04C-0.25N) was prepared. After reheating at a temperature of 1250 ° C., the result was hot rolled and hot rolled annealing to prepare a hot rolled coil.

1 shows the rolling reduction rate of each rolling pass when the hot rolled coil was first rolled by a conventional method in a Zenzimir reversible rolling mill and a 4 stance tandem continuous rolling mill as a comparative example. The star material temperature is shown. As shown in FIG. 1, the rolling reduction ratio per pass is typically 15 to 25% in rolling hot rolled coils having a thickness of 2.0 to 3.5 mm with heavy and thick cold rolled coils having a thickness of 0.6 mm or more and thin cold rolled coils having a thickness of less than 0.6 mm. Allocate from In the case of a thin material, the final pass may be performed at 15% or less to secure surface gloss and shape quality after rolling. In this rolling schedule, the temperature of the material to be rolled down by each pass is increased and partially cooled by the cooling function of the knitted rolling oil used in the rolling mill. The entrance material temperature for each pass formed through this process is shown in FIG. As described above, in one pass, the temperature rises rapidly from room temperature or two passes and rises to a temperature of about 70 ° C or more. Figure 2 shows the results of the material temperature for each pass of the rolling mill at the flow rate of 2250 l / min of the rolling mill in the rolling mill. .

FIG. 3 is a result of measuring the accumulated organic martensite accumulated content after rolling each pass in the conventional rolling reduction schedule. As a result, some martensite is formed in one pass, but martensite in two passes due to an increase in material temperature. Formation is very small and from 3 passes we see little additional martensite formation.

FIG. 4 is a hot rolled coil divided into primary and secondary as an example of the invention, and the pass-by-pass is performed when the first pass reduction ratio is 25% or more in the primary rolling and the total reduction ratio is 40% or more in the secondary rolling. Figure 5 shows the reduction ratio, Figure 5 shows the entrance temperature of each pass in the pass schedule, Figure 6 shows the results of measuring the martensite cumulative content after rolling per pass in the pass schedule. As shown in FIG. 5, the temperature of the coil during the secondary rolling is much lower than that of the conventional method, and thus promoted as shown in FIG. 6. As described above, when rolling is divided into primary and secondary rolling, the temperature of the coil is lowered during the secondary rolling, and the martensite transformation degree is increased according to the deformation amount during the secondary rolling. In the first rolling, there is almost no martensite transformation until about 15% of deformation, but when the amount of deformation increases, the so-called incubation deformation, which starts to increase the martensite content according to the deformation amount, is accumulated in the tissue during the second rolling. This is because the amount of martensite produced at the same amount of deformation increases because it is not necessary due to the deformation energy.

Figure 7 for the STS 301L steel and ASTM 204 steel rolled in the primary and secondary rolling method presented in the present invention above having a martensite of 50% or more and less than 50% martensite by rolling in a conventional manner The result of measuring the change of martensite content due to reverse transformation when the retained steel strip was heat-treated for 3 minutes at 400 ~ 800 ℃ temperature range. In all cases, it can be seen that martensite begins to undergo reverse transformation at about 500 ° C. and completes reverse transformation within the temperature range of 700 ° C. to 800 ° C. This reverse transformation causes a large change in tissue after reverse transformation, depending on the amount of martensite content in the initial tissue. FIG. 8 is a microstructure when the STS 301L steel having 21% martensite initially was heat treated at 550 ° C. during reverse transformation and at 800 ° C. after reverse transformation was completed. In the early stage, the site of martensite was changed to ultra-fine microstructure of several microns after reverse transformation, but the area of austenite around it was not micronized, showing that it is a hybrid of two tissues having different grain sizes. 9 is an example of the invention as a microstructure when the STS 301L steel having 74% martensite at an initial heat treatment at 550 ° C. undergoing reverse transformation and 800 ° C. after reverse transformation is completed. It shows that all sites are uniformly transformed into microns of several microns in size. In addition, FIG. 10 is also an example of the invention, when the ASTM 204 steel having 62% martensite is heat treated at 550 ° C. during the reverse transformation and 800 ° C. after the reverse transformation is completed. Shows that all sites are uniformly transformed into microns of several microns in size.

As described above, when a homogeneous and fine austenite recrystallized structure is obtained, the strength may be improved, and mechanical properties having a high elongation and a low yield ratio may be obtained as compared with the rough rolling method. 11 is rolled by the above-described method to have a martensite of 50% or more in STS 301L steel and ASTM 204 steel and heat-treated for 3 minutes at the comparative example 700 ℃, the invention example 800 ℃, and the comparative example and annealing temperature 1100 ℃ The stress-strain curves obtained by the post-tension test show that both the steel grades produced by the present invention exhibit significantly higher yield strength and tensile strength than those subjected to heat treatment at normal annealing temperatures. In addition, such an increase in strength does not involve a large decrease in ductility as in temper rolling, resulting in a very good elongation of 45 to 50%. However, when the heat treatment temperature is lower than the range of the present invention at 700 ℃, the increase in yield strength is very high, but the work hardening is greatly reduced and the yield ratio is large, which is not suitable for processing.

12 is a hot rolled steel sheet having a mean grain size of 180 microns or more by a conventional method of producing a stress-strain curve during tensile test of STS 301L steel and ASTM 204 steel prepared to have a fine and uniform grain structure having an average grain size of 4 microns according to the present invention. Compared with the annealing material and the cold rolled annealing material having an average grain size of 40-50 microns, the grain refinement according to the present invention shows a very good combination of strength and ductility compared to the conventional method. In addition, it can be seen that in the case of ASTM 204 steel having a higher nitrogen content than STS 301L, the increase in strength due to such grain refinement occurs more significantly, and thus the effect of the present invention is very high.

As described above, the present invention provides a method for producing austenitic stainless steel that can maintain high elongation and workability of 40% or more while increasing the strength of Cr-Ni-based and Cr-Mn-Ni-N-based austenitic stainless steels. do. The steel produced by the present invention has superior workability compared to steel produced by the conventional temper rolling method, thereby significantly reducing the use limitation of the tempered rolled material in various structural applications such as automobile structural parts and building structures. can do. In particular, the steel produced by the present invention has a high yield strength and also has a very high plastic deformation energy until breakdown, so it is also used for parts that require high impact energy absorbing ability along with high strength, such as an automobile bumper rail or a side member, and thus the structure of an automobile. It can increase the safety level and is suitable for lightening the structure due to high strength.

Claims (4)

Manufacture of austenitic stainless steels in which austenitic stainless steels are hot rolled coils produced by steelmaking, continuous casting, hot rolling, and hot-rolled annealing in cold rolled reversible, followed by cold-rolled annealing. In the method, the cold rolling is carried out by dividing the cold rolling into two or more times, but the rolling structure is formed by rolling the first pass reduction rate at the time of primary rolling to 25% or more and then performing the secondary rolling at a reduction rate of 40% or more under the total pressure without intermediate annealing. After obtaining 50% or more of the processed organic martensite in the subsequent cold rolling annealing process, the annealing temperature is in the range of 800 to 1000 ° C. to obtain a homogeneous recrystallized austenite structure having an average grain size of 15 μm or less. Method of manufacturing knight-based stainless steel. The method of claim 1, The cold rolling is carried out in two or more times in a tandem continuous rolling mill, and after rolling the first pass reduction rate at 25% or more in the primary rolling, the secondary rolling is performed at a reduction rate of 40% or more under the total rolling without intermediate annealing. A method for producing austenitic stainless steel. The method according to claim 1 or 2, A method for producing austenitic stainless steel, characterized in that the cold-rolled annealing step is performed by bright annealing. The method of claim 1, A method of manufacturing austenitic stainless steel, characterized in that for producing the hot rolled coil by a steel casting method.

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