JP2014189802A - LOW Ni AUSTENITIC STAINLESS STEEL SHEET EXCELLENT IN AGE HARDENING PROPERTY AND METHOD OF PRODUCING THE SAME - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a low Ni austenitic stainless steel sheet which has high age hardening property usable for applications to which 300 series stainless steel for high strength is applied, while contents of Ni and Mn are suppressed to minimum requirements.SOLUTION: There is provided the low Ni austenitic stainless steel sheet excellent in age hardening property containing, by mass%, C:0.03 to 0.30%, Si:1.50% or less, Mn:2.0 to 5.0%, P:0.06% or less, S:0.005% or less, Ni:1.0 to 4.0%, Cr:15.0 to 19.0%, Cu:1.0 to 3.5%, N:0.03 to 0.30%, Sn:0.02%, B:0.001 to 0.010% and the balance Fe with inevitable impurities. The low Ni austenitic stainless steel sheet excellent in age hardening property has a surface hardness at any parts of 400 HV or more, a deformation-induced martensite content of 10% or more, and a full-width at half maximum in a crystal orientation (211) of the deformation-induced martensite of 1.5° or more at X-ray analysis at 40 kV and 200 mA in a target of Co.

Description

  The present invention relates to a low Ni austenitic stainless steel sheet having high age-hardening ability that can be used in applications where high-strength 300 series stainless steel is applied while suppressing Ni and Mn to the necessary minimum contents.

  Conventionally, there are work hardening type and precipitation hardening type high strength stainless steel in applications requiring high strength, and SUS301 temper rolled material or the like is used as work hardening type high strength stainless steel. Thereafter, it is molded and subjected to an aging treatment to increase the strength. Recently, 200 series stainless steel containing 5% or more of Mn as an austenite forming element instead of Ni as described in the following Patent Documents 1 to 3 is being provided as an alternative to 300 series stainless steel. . In addition, a technique of austenitic stainless steel in which Ni that does not contain a large amount of Mn as in Patent Document 4 is saved is also proposed. The work-hardening type metastable austenitic stainless steel represented by SUS301 and SUS304 is frequently used as a material for high-strength stainless springs because high strength is obtained by cold working.

  In the technology containing a large amount of Mn as in Patent Documents 1 to 3, the surface quality of the steel sheet is deteriorated, productivity such as annealing pickling property and bright annealing is impaired, and these are produced despite the reduction of Ni. There is a problem that the effect is offset in terms of the total cost due to the decrease in performance. Further, Patent Document 4 has a problem that some steels do not contain a large amount of Mn but have insufficient age-hardening ability.

Until now, the technique shown to patent documents 5-7 is shown as an example of a use of the high intensity | strength austenitic stainless steel which saved Ni. In Patent Document 5, stainless steel having excellent corrosion resistance and workability in terms of recyclability, productivity due to surface quality, and material characteristics, Patent Document 6 expresses excellent bending workability and is essential as a high-strength stainless steel spring. Stainless steel for springs that has both sag resistance and corrosion resistance, Patent Document 7 expresses excellent deep drawability and overhanging properties, and includes automobile bodies such as trucks, structural members and reinforcing materials. It is disclosed.
However, both of these techniques have the problem that some steels lack age hardenability.

Japanese Patent No. 4606113 Japanese Patent No. 4333131 Japanese Patent No. 4116134 Japanese Patent No. 4327030 Japanese Patent No. 5014915 Japanese Patent No. 5091732 Japanese Patent No. 5091733

  This invention makes it a subject to improve the age hardening ability of the low Ni austenitic stainless steel which was not able to be solved with the steel materials of the said patent document description.

  As a result of various studies on requirements for aging treatment after temper rolling using steels of various component ranges as raw materials, the present inventors adjusted the rolling rate of the material in addition to the component specification range, and set the aging treatment conditions. It has been found that a low Ni austenitic stainless steel sheet with high age-hardening ability can be provided by controlling.

  The above-mentioned problems are in mass%, C: 0.03 to 0.30% or less, Si: 1.50% or less, Mn: 2.0 to 5.0%, P: 0.06% or less, S: 0 0.005% or less, Ni: 1.0 to 4.0%, Cr: 15.0 to 19.0%, Cu: 1.0 to 3.5%, N: 0.03 to 0.30%, Sn : 0.02% or less, B: 0.001 to 0.010% included, the balance being substantially composed of Fe and inevitable impurities, the surface hardness of any part is 400HV or more, processing induced martensite amount Is 10% or more, and when the X-ray analysis is performed at 40 kV and 200 mA with the target Co, the half width in the crystal orientation (211) of the processing induced martensite phase is 1.5 ° or more. Or, the difference in hardness before and after the aging treatment when the aging treatment is performed in the range of 400 ° C. or more and 60 minutes is 50 HV A low Ni austenitic stainless steel sheet or an annealed material characterized by the above being rolled at a cold rolling rate of 40% or more and then subjected to an aging treatment in the range of 400 ° C. or more and 60 minutes. This is achieved by a method for producing an austenitic stainless steel sheet.

  According to the present invention, there is provided a low Ni austenitic stainless steel sheet that is excellent in age hardening characteristics while avoiding the addition of a large amount of Mn content while reducing the Ni content. Products manufactured and processed using this steel plate can be used for high-strength applications in which the material is 300 series stainless steel.

The relationship between the half width and ΔHV before and after aging treatment is shown. The relationship between the Mn content in the rolled material with a cold rolling rate of 40% and the half width of the crystal orientation (211) of the work-induced martensite phase is shown. The relationship between a cold rolling rate and a half width is shown.

  The present inventors have found that ΔHV before and after the aging treatment and the half width of the material are closely related, and that the half width can be increased and ΔHV can be increased by adjusting the component range and the rolling ratio of the material. It was.

  FIG. 1 shows the relationship between the half width and ΔHV before and after aging treatment. ΔHV indicates the hardness difference before and after the aging treatment. The half width was measured by X-ray analysis at 40 kV and 200 mA with the target Co, measured with the crystal orientation (211) of the work-induced martensite phase, and the width value at half the intensity was measured. It can be seen that ΔHV increases with an increase in the half width, and that an ΔHV of 50 or more can be obtained at a half width of 1.5 ° or more.

  In order to obtain a half width of 1.5 ° or more, it is necessary to control the component range and the cold rolling rate. FIG. 2 shows the relationship between the Mn content in the rolled material with a cold rolling rate of 40% and the half width of the crystal orientation (211) of the work-induced martensite phase. It can be seen that the full width at half maximum increases as the Mn content increases, and that the full width at half maximum is 1.5 ° C. or more when the Mn content is 2.0 mass% or more. The reason why Mn increases the half width is unknown, but it is considered that the increase in the amount of N dissolved by Mn or the decrease in stacking fault energy contributes.

  FIG. 3 shows the relationship between the cold rolling rate and the half width. In Example A1 of the steel of the present invention, a half width in the crystal orientation (211) of the martensite phase was 1.5 ° or more at a cold rolling rate of 40% or more. On the other hand, in Example B5 of the comparative steel, the half width in the crystal orientation (211) of the martensite phase cannot be obtained 1.5 ° or more even when the cold rolling rate is 40% or more.

Hereinafter, the alloy components contained in the steel of the present invention and the reasons for limiting the content range will be described.
C and N are austenite-forming elements, and are useful elements for strengthening the work-induced martensite phase by solid solution strengthening and for expressing age-hardening ability. If the content of these elements is too small, the amount of δ ferrite phase produced increases and hot workability decreases. In order to obtain strength, it is important to secure a content exceeding 0.03% by mass for both C and N. On the other hand, if the contents of C and N are too large, the contents become excessively hard and the workability is hindered. Therefore, the upper limit of the contents of C and N is set to 0.30% by mass.

  Si is an element useful for deoxidation in steelmaking and an element contributing to solid solution strengthening. If the content exceeds 1.5% by mass, the steel becomes hard and the workability is impaired. Further, since Si is a ferrite-forming element, excessive addition causes a large amount of δ-ferrite phase to be generated at a high temperature range, thereby impairing hot workability. Therefore, the Si content is specified to be 1.5% by mass or less.

  Mn is a useful austenite-forming element that is less expensive than Ni and can substitute for the function of Ni. It is also an effective element for increasing the solid solution amount of N and expressing age-hardening ability. As described above, it is necessary to secure a Mn content of 2.0% or more in order to increase the half width of the work-induced martensite phase of the rolled material and improve the age hardening ability. On the other hand, when the Mn content is excessive, it causes a decrease in productivity due to surface properties and a decrease in workability and corrosion resistance due to the formation of inclusions such as MnS. For this reason, the upper limit of the Mn content is regulated to 5.0% by mass.

  P and S are mixed as unavoidable impurities, but the lower the content, the more desirable. P is 0.06% by mass or less for P as a range that does not have a great adverse effect on processability and other material properties and manufacturability. Was defined as 0.005 mass% or less.

  Ni is an essential element for austenitic stainless steel. In order to obtain good hot workability, for example, it is necessary to contain Ni so that it becomes a γ single phase at a heating temperature of 1200 ° C., and the lower limit is 1.0 mass%. In the present invention, component design is performed to keep the Ni content as low as possible from the viewpoint of cost reduction, and the upper limit is defined as 4.0% by mass.

  Cr is an essential element for forming a passive film that ensures the corrosion resistance of stainless steel. In the present invention, in order to sufficiently secure the corrosion resistance, the lower limit of the Cr content is set to 15.0% by mass. However, since Cr is a ferrite-forming element, the heating temperature before hot rolling becomes a (γ + δ) two-phase region due to excessive Cr content, and after heating, a large amount of δ-ferrite is generated, which is a factor that impairs hot workability. It is not preferable. Therefore, the upper limit of Cr content is defined as 19.0% by mass.

  Since Cu is an austenite-generating element, the degree of freedom in setting the Ni content increases with an increase in Cu content, and component design that suppresses Ni becomes easy. In order to improve age hardening ability, it is necessary to secure a Cu content of 1.0% or more. However, a large amount of Cu exceeding 3.5 mass% tends to hinder hot workability. For this reason, Cu content was prescribed | regulated to 1.0-3.5 mass 9%.

  Sn may be mixed as an unavoidable impurity, but in steel containing Cu, a Cu—Sn phase of a low melting point compound is generated to significantly reduce hot workability. Therefore, the upper limit of the Sn content is defined as 0.02% by mass.

  B is an element added to improve hot workability and softening, and a stable effect can be obtained by adding 0.001% by mass or more. However, since the compound of B will precipitate when it adds excessively and hot workability will deteriorate, the upper limit was prescribed | regulated to 0.010 mass%.

  The steel of the present invention can be manufactured by a general austenitic stainless steel sheet manufacturing process. Specifically, the component-adjusted molten steel is cast continuously or batchwise, and the resulting cast slab is heated and extracted, and then hot-rolled with a continuous hot rolling mill or a reverse hot rolling mill. Can be used. It is desirable to perform intermediate annealing or finish annealing after hot rolling in the range of 1050 to 1100 ° C. Moreover, after finish annealing, the temper rolling according to target hardness is given, for example, it can be set as the temper rolled steel plate of plate thickness 0.1-3.0mm. Thereafter, shape correction and continuous aging treatment in the above-described aging temperature range may be appropriately performed.

  Various stainless steels having the compositions shown in Table 1 were melted. In Table 1, A1 to A5 are steels of the present invention having chemical components defined by the present invention, and B1 to B5 are comparative steels. The chemical component content of the underlined portion of the comparative steel is out of the range defined in the present invention.

  For inventive steels A1 to A5 and comparative steels B1 to B5, cold-rolled steel sheets were prepared. After obtaining 100 kg of steel ingot for each steel, hot rolled plates having a thickness of 3.0 mm were manufactured by hot rolling at an extraction temperature of 1230 ° C. Each steel hot-rolled sheet having a thickness of 3.0 mm is annealed at 1080 ° C. for 1 minute soaking, and then rolled at a cold rolling rate of 40% or more, so that the hardness is 460 ± 10 HV, A temper rolled steel sheet having a thickness of 1.0 mm was obtained. In addition, the temper rolling rate at which the hardness after temper rolling becomes 460 HV is examined in advance for each steel, the thickness before cold rolling is set based on the temper rolling rate, and the thickness is set to that thickness. After annealing at 1080 ° C. for 1 minute soaking, temper rolling to a thickness of 1.0 mm was performed. The temper rolling was performed after heating so that the plate temperature became 70 ° C.

  Using the temper rolled material having a plate thickness of 1.0 mm, the amount of work-induced martensite was measured. After collecting a disk having a diameter of 5 mm, a sample obtained by electropolishing the edge in phosphoric acid sulfuric acid was used, and four sheets were stacked and measured with a vibrating sample magnetometer.

  Further, an aging treatment was performed using a temper rolled material having a plate thickness of 1.0 mm. The aging treatment was performed at 450 ° C. for 60 minutes, and the hardness of the sample surface before and after the aging treatment was measured according to JISZ2244 with a load of 10 kg using a Vickers hardness tester. Further, the half width of the crystal orientation (211) of the martensite phase was measured by X-ray analysis at 40 kV and 200 mA for the temper rolled material. Table 2 shows the rolling rate, the amount of work-induced martensite, the half width, and the hardness before and after the aging treatment of the temper rolled material.

  As shown in Table 2, the steels A1 to A5 of the present invention are manufactured at a cold rolling rate of 40% or more, the amount of work-induced martensite is 10% or more, and the half width is 1.5 ° or more. Since it is within the range, ΔHV after aging treatment is high. On the other hand, the comparative steels B1 to B5 have a B2 of less than 40%, B3 of less than 10% of work-induced martensite, and B1 to B5 of less than 1.5 ° half width within the scope of the present invention. Therefore, ΔHV after aging treatment is low.

  From this result, the steel sheet was rolled at a cold rolling rate of 40% or more, the surface hardness of any part was 400 HV or more, the amount of work-induced martensite was 10% or more, and X-ray analysis was performed at 40 kV and 200 mA at the target Co. The half-value width in the crystal orientation (211) of the processing-induced martensite phase is 1.5 ° or more, and the hardness before and after the aging treatment when aging treatment is performed at 450 ° C. for 60 minutes. It was confirmed that the difference was 50 HV or more. As described above, it has been found that by adjusting the component range and the rolling ratio of the material, the half width is increased and a low Ni austenitic stainless steel sheet having excellent age hardening characteristics can be obtained.

Claims (3)

% By mass
C: 0.03 to 0.30% or less Si: 1.50% or less Mn: 2.0 to 5.0% or less P: 0.06% or less S: 0.005% or less Ni: 1.0 to 4 0.0% or less Cr: 15.0 to 19.0% or less Cu: 1.0 to 3.5% or less N: 0.03 to 0.30% or less Sn: 0.02% or less B: 0.001 0.010% or less, the balance being substantially composed of Fe and inevitable impurities, the surface hardness of any part is 400 HV or more, the amount of work-induced martensite is 10% or more, and the target Co is 40 kV , Low Ni austenitic stainless steel with excellent age-hardening characteristics characterized in that the half-value width in the crystal orientation (211) of the work-induced martensite phase is 1.5 ° or more when X-ray analysis is performed at 200 mA steel sheet. The low Ni austenitic stainless steel with excellent age-hardening characteristics according to claim 1, wherein the difference in hardness before and after aging treatment is 50HV or more when aging treatment is performed in a range of 400 ° C or more and 60 minutes. steel sheet. An annealed steel material comprising the component range of claim 1 is rolled at a cold rolling rate of 40% or more, and then subjected to an aging treatment in a range of 400 ° C. or more and 60 minutes. Method.

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