JP2016160502A - Metastable austenitic stainless steel sheet and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stably produce a metastable austenitic stainless steel sheet having high reliability by efficiently controlling the structure of a metastable austenitic stainless steel sheet, mainly refining crystal grain sizes and further improving the excellent characteristics original in the metastable austenitic stainless steel sheet.SOLUTION: A metastable austenitic stainless steel sheet is cooled to a cooling temperature being a temperature of starting martensitic transformation with liquid nitrogen and is thereafter heated to a heating temperature being a temperature of starting the transformation of an austenitic host phase to control structural regulation, thus a metastable austenitic stainless steel sheet in which crystal grains are refined is produced.SELECTED DRAWING: Figure 1

Description

  The present invention relates to a metastable austenitic stainless steel sheet whose structure is adjusted and the crystal grain size is refined without plastic deformation of the material and a method for producing the same, and has excellent strength and balance of elongation and improved fatigue strength. The present invention relates to a metastable austenitic stainless steel plate and a manufacturing method thereof.

  In recent years, automobiles and parts thereof, as well as home appliances and parts thereof (hereinafter abbreviated as “parts”) have been promoted to be smaller and lighter from the viewpoints of environmental and energy problems and cost. Increasingly, those used after processing into a predetermined shape with strict accuracy are increasing.

  Many parts and the like are required to have high strength in order to compensate for a decrease in rigidity and the like due to miniaturization and weight reduction, making it difficult to achieve both workability and the like. Furthermore, durability is often required due to reliability problems of components and the like.

  For such applications, as disclosed in, for example, Patent Documents 1 to 3, many metastable austenitic (A) stainless steels centering on SUS301 and SUS304 have been used. Metastable austenitic stainless steel is relatively easy to obtain high strength due to work-induced transformation to hard martensite (M) phase, and deformation is propagated to the soft part due to large work hardening, making the entire material uniform. It is an excellent material that is deformed into both and has both excellent ductility.

  On the other hand, as exemplified by the fuel tank for automobiles disclosed in Patent Document 4, the shock absorbing member for automobiles disclosed in Patent Document 5, and the railway vehicle disclosed in Patent Document 6, There are many cases where welding is performed in a sbot or line. This is extremely effective in terms of reducing the number of parts, and is basically the same in other applications.

  In these cases, for example, although improvement of the welding method itself has been studied as disclosed in Patent Document 7, segregation of chemical components, precipitation of coarse compounds, and coarsening of crystal grains are caused by welding. Etc. occurred, and there was a problem that the characteristics of the product deteriorated.

  For this reason, for example, as disclosed in Patent Documents 8 to 10, a solution heat treatment that is heated to a relatively high temperature is often performed for the purpose of homogenizing the welded portion.

Japanese Patent No. 4360136 Japanese Patent No. 4475352 JP 2010-215953 A JP 2006-124807 A JP 2010-236560 A Japanese Patent No. 2863063 Japanese Patent No. 3627194 JP 62-267417 A Japanese Patent Publication No. 58-014847 Japanese Examined Patent Publication No. 62-003210

  In general, the solution heat treatment can be made more efficient by implementation after plastic deformation. However, it is often difficult to plastically deform products and parts. On the other hand, although it is considered that the crystal grain boundary itself promotes diffusion, as described above, the crystal grain becomes coarse in the welded portion, and the grain boundary density decreases. As a result, high-temperature and long-time solution heat treatment was performed, and the crystal grains of products and parts were coarsened.

  The present invention is metastable by making the structure of metastable austenitic stainless steel sheet whose size and complexity have been reduced and the shape of the steel plate become more complicated, and by adjusting the structure of the steel plate. An object of the present invention is to further improve the excellent properties inherent to an austenitic stainless steel sheet and to stably produce a highly reliable metastable austenitic stainless steel sheet.

  As a result of intensive studies in order to solve the above problems, the present inventors have obtained knowledge A and B listed below and completed the present invention.

  (A) Regarding the microstructure adjustment of metastable austenite (A) stainless steel, the martensite (M) transformation by cooling to a low temperature below room temperature and the reverse transformation to austenite phase by low temperature heating compared to solution heat treatment By utilizing it, the grain can be refined by efficiently adjusting the structure.

  (B) Furthermore, regarding the martensitic transformation by cooling to a low temperature, the martensitic transformation can be efficiently promoted by applying stress in the elastic deformation region that does not cause plastic deformation.

The present invention is listed below.
(1) Among the austenitic stainless steels defined in JIS-G-4313 (2011), after having a chemical composition that satisfies the metastable austenite state and after cooling to a cooling temperature equal to or lower than the temperature at which martensitic transformation starts. The difference between before and after the treatment by heating to a heating temperature equal to or higher than the temperature at which transformation to the austenite matrix starts is made. A metastable austenitic stainless steel sheet having a metal structure composed of one or more fine crystal grains (ΔG.S.No.).

  (2) The metastable austenitic stainless steel sheet described in item 1 which is SUS301, SUS301L, SUS304 or SUS304L as defined in JIS-G-4305 (2012).

  (3) The chemical composition is% by mass, C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.045% or less, S: 0.03% The metastable austenitic stainless steel sheet described in 1 or 2 below, which has Ni: 6 to 13%, Cr: 16 to 20%, N: 0.20% or less, and is composed of the remaining Fe and impurities.

  (4) The chemical composition has, in mass%, one or more selected from the group consisting of Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less. The metastable austenitic stainless steel sheet described in any one of items 1 to 3.

  (5) Tensile strength: 175 MPa or more, total elongation: 3% or more, and fatigue limit: a metastable austenitic stainless steel sheet described in any one of items 1 to 4 having mechanical properties of 90 MPa or more.

  (6) A metastable austenitic stainless steel sheet described in any one of items 1 to 5 used in a steel belt for a printer.

  (7) The microstructure is adjusted by heating the metastable austenitic stainless steel sheet to a heating temperature equal to or higher than the temperature at which transformation to the austenite matrix starts after cooling to a cooling temperature below the temperature at which martensitic transformation starts with liquid nitrogen. The method for producing a metastable austenitic stainless steel sheet according to any one of items 1 to 6, wherein:

  (8) The above-mentioned structure adjustment is performed before and after the adjustment with a crystal grain size No. defined by JIS-G-0551 (2013). The method for producing a metastable austenitic stainless steel sheet described in item 7, which is one or more grain refinements in terms of

  (9) The metastable austenitic stainless steel sheet according to 7 or 8, wherein stress is applied to the metastable austenitic stainless steel sheet at least during cooling to the martensite transformation start temperature or lower. Production method.

  (10) The metastable austenitic stainless steel sheet has an endless ring shape, and while applying the stress between two or more pulleys, bending and bending back with the pulley, a stress applying portion or bending, 10. The method for producing a metastable austenitic stainless steel sheet according to item 9, wherein one or more bent portions are cooled.

  (11) The method for producing a metastable austenitic stainless steel sheet according to any one of items 7 to 10, wherein the cooling temperature is not higher than the liquid nitrogen temperature and the heating temperature is 600 ° C. or higher.

  (12) The method for producing a metastable austenitic stainless steel sheet according to any one of items 9 to 11, wherein the stress is a stress less than 0.2% proof stress.

  According to the present invention, miniaturization and weight reduction have progressed, and the structure of the product made of metastable austenitic stainless steel sheet, which is complicated in shape with it, can be adjusted efficiently, and the balance between strength and formability is excellent 、 Mainly refined crystal grain size to improve fatigue strength, further improve the excellent characteristics inherent in metastable austenitic stainless steel, and provide stable and inexpensive metastable austenitic stainless steel sheet stably it can.

  According to the present invention, a metal made of a metastable austenitic stainless steel plate as a raw material and subjected to high-temperature heat treatment for welding or solid solution (homogeneous) can be used without causing unnecessary plastic deformation. By adjusting the structure, metastable austenitic stainless steel can be manufactured by a series of treatments using continuous equipment, and can be widely applied to various applications including automobiles and home appliances.

FIG. 1 is a metal structure photograph taken by an optical microscope of a material and each sample after cooling while applying tension.

The metastable austenitic stainless steel sheet and the manufacturing method thereof according to the present invention will be described.
1. Metastable austenitic stainless steel sheet according to the present invention (1-1) Chemical composition The metastable austenitic stainless steel sheet according to the present invention is defined in JIS-G-4313 (2011) and JIS-G-4305 (2012). It has a chemical composition corresponding to SUS301, SUS301L, SUS304, or SUS304L.

  Specifically, the chemical composition is C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.045% or less, S: 0.03% or less, Ni : 6-13%, Cr: 16-20%, N: 0.20% or less, if necessary, Ti: 0.5% or less, Nb: 0.5% or less and V: 0.5 % Or more selected from the group consisting of% or less, the balance Fe and impurities.

  That is, it is a metastable austenitic stainless steel whose basic composition is C: 0.15% or less, Cr: 16-20%, Ni: 6-13%, N: 0.20% or less. Elements other than the above are as follows.

[Si: 1.0% or less]
Since Si is an element that is used as a deoxidizer and is effective in improving oxidation resistance, it is usually contained in an amount of 0.2 to 1.0%. For this reason, it is preferable that the upper limit of Si content be 1.0%.

[Mn: 2.0% or less]
Mn is effective as a deoxidizer and is also an austenite forming element, so it is usually contained in an amount of 0.5 to 2.0%. However, when Mn is contained excessively, it also has the effect of reducing the corrosion resistance. For this reason, the upper limit of the Mn content is preferably set to 2.0%.

[P: 0.045% or less]
P has the effect of improving strength. In order to acquire this effect, it is preferable to make it contain 0.030% or more. However, if P is contained excessively, weldability is impaired, so the P content is preferably 0.045% or less.

[S: 0.03% or less]
S is an impurity and has the effect of reducing hot workability and toughness. For this reason, it is preferable that S content shall be 0.03% or less.

[One or more selected from the group consisting of Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less]
All of Ti, Nb, and V are 0.5% each of one or more of trace amounts of Ti, Nb, and V because of precipitation of compounds that are considered to be particularly effective in suppressing the growth of crystal grains. It may contain below, and it is still more preferable to contain 0.02-0.46%, respectively.
The balance other than the above is Fe and impurities.

(1-2) Metallographic structure The metastable austenitic stainless steel sheet according to the present invention has an austenite phase as a parent phase and has a grain size of No. 1 before and after grain size adjustment. (ΔG.S.No.) having a crystal grain size of 1 or more.

(1-3) Mechanical Properties The metastable austenitic stainless steel sheet according to the present invention has mechanical properties of tensile strength: 175 MPa or more, total elongation: 3% or more, and fatigue limit: 90 MPa or more, and stretches at a desired strength. In addition to excellent balance, it has excellent fatigue strength.

(1-4) Applications The metastable austenitic stainless steel sheet according to the present invention can be widely applied to various applications including automobiles and home appliances. Specifically, blades and gears used in printers and the like. It can be suitably used as a steel belt. In particular, steel belts used in printers are manufactured through a process of ring welding a metastable austenitic stainless steel plate, which is a raw material, into an endless ring shape. It tends to decrease. If the metastable austenitic stainless steel sheet according to the present invention is used as a material for a steel belt, as described above, the crystal grain size no. (ΔG.S.No.) has a metal structure composed of one or more fine crystal grains, and therefore, it is possible to suppress a decrease in strength due to the coarsening of the crystal grains even through the ring welding process. While maintaining the required strength, the balance of elongation is excellent, and excellent fatigue strength can be maintained.

2. Production method according to the present invention In the production method according to the present invention, the metastable austenitic stainless steel sheet having the above-described chemical components is cooled to a cooling temperature equal to or lower than the temperature at which martensitic transformation starts with liquid nitrogen, and then into the austenite matrix The metastable austenitic stainless steel sheet according to the present invention is manufactured through a process of adjusting the structure by heating to a heating temperature equal to or higher than the temperature at which the transformation starts. The reason for this will be explained.

  In consideration of versatility, the present inventors use SUS301, SUS301L, SUS304, and SUS304L, which are metastable austenitic stainless steels stipulated in JIS standards, and can be cooled with liquid nitrogen relatively easily. Investigating the change in properties by utilizing martensitic transformation due to heat treatment and heating above the reverse transformation temperature to the austenite matrix following this cooling (transformation to austenite phase at low temperature heating compared to solution heat treatment) did.

  As a result, the crystal grains of metastable austenitic stainless steel that has undergone such cooling and heating are refined, and further, at the time of cooling, generally in a tensile test specified in JIS-Z-2241 (2011). It was found that the martensitic transformation can be promoted by applying a stress of 0.2% proof stress or less. The 0.2% proof stress is generally measured in a tensile test of stainless steel, and is considered to be a value relatively close to the elastic limit among them.

As an example of the main process, the results of SUS301L shown in Table 2 of Examples described later will be described. First, Table 1 shows the amount of martensitic transformation before and after cooling with SUS301L in liquid nitrogen. Some samples were given a tension of 98 N / mm ² during cooling with liquid nitrogen.

  As shown in Table 1, the martensite transformation is caused by cooling with liquid nitrogen, the amount of martensite phase is 3% by area, and the amount of martensite transformation is increased by applying tension during cooling, resulting in 14% by area. Show. The amount of martensite was measured on the plate surface by X-ray diffraction, and was calculated by the ratio of the integrated intensity of the martensitic phase diffraction peak to the total diffraction peak including the austenite matrix.

  Furthermore, after heat-treating these samples at 800 ° C., a parallel section in the rolling direction was embedded in the resin, and then polished and corroded, and a metal structure was observed using an optical microscope. Fig. 1 shows a metallographic photograph of the material and the tensioned sample.

  The crystal grains of the sample cooled with liquid nitrogen while applying tension are made finer than the crystal grains of the material. When compared with the crystal grain size specified in JIS-G-0551, the tensioned material has a crystal grain size of 8 and the material has a crystal grain size of 7, whereas the crystal grain size has a decrease of 1. As shown, the average area of the crystal grains is about ½.

  As understood from the above description, the structure adjustment in the production method according to the present invention is performed using the crystal grain size No. defined by JIS-G-0551 before and after the adjustment. It is 1 or more crystal grain refinement | converting in conversion.

  In the present invention, the reason why the metastable austenitic stainless steel sheet is cooled to below the martensite transformation start temperature by liquid nitrogen and then heated to the austenite transformation start temperature or higher is that the metal structure is efficiently adjusted by those transformations, the crystal This is to make the grains finer. These effects are also promoted by increasing the number of repetitions of both transformations. Although the specific temperature depends on the composition of the raw material, it is advantageous to promote the martensitic transformation that the cooling temperature is lower.

  However, industrially, the temperature is preferably below the temperature of liquid nitrogen, which is easy to use. It should be noted that the martensitic transformation amount is expected to be accelerated by repeating the cooling to room temperature. Thus, in the manufacturing method according to the present invention, it is desirable that the cooling temperature is equal to or lower than the liquid nitrogen temperature.

  On the other hand, the heating temperature is set to 600 ° C. or more at which transformation into an austenite matrix occurs in a general metastable austenitic stainless steel. The heating temperature is preferably 650 ° C. or higher so that the crystal grains are sized by the minimum grain growth (the fluctuation of the grain size is reduced and becomes uniform). The upper limit of the heating temperature is equal to or lower than the solution treatment temperature, but the heating temperature is desirably low, preferably 900 ° C. or lower, in order to suppress crystal grain growth. Thus, in the manufacturing method according to the present invention, the heating temperature is desirably 600 ° C. or higher.

  The stress during cooling by liquid nitrogen is performed in order to promote martensitic transformation and efficiently utilize it. However, it is often impossible to plastically deform a material in a product, a part, or a manufacturing stage close to them. In the present invention, the martensitic transformation can be promoted even by applying stress within the elastic region. For this reason, it is set within the elastic deformation range and less than 0.2% proof stress measured by the tensile test of JIS standard considered to be closest thereto. As described above, in the production method according to the present invention, it is preferable to apply stress to the metastable austenitic stainless steel sheet at least during cooling to the martensite transformation start temperature or lower in order to promote grain refinement. Specifically, it is preferable to apply a stress of less than 0.2% proof stress.

  Further, as described above, when the metastable austenitic stainless steel plate according to the present invention is used as a material for a steel belt used in a printer or the like, endless ring shape is obtained by performing ring welding on the metastable austenitic stainless steel plate. In addition, by applying this endless ring-shaped metastable austenitic stainless steel sheet between at least two pulleys, stress is applied, and bending and unbending at the time of winding around the pulley are utilized. The metastable austenite system according to the present invention can be applied alternately and efficiently with tensile stress and compressive stress, and at least one stress applying portion between at least two pulleys is cooled with liquid nitrogen. It is preferable to use a stainless steel plate.

Examples of the present invention will be described.
Samples were prepared by cutting each small ingot with components adjusted to conform to SUS301, SUS304, SUS301L, SUS304L, SUS310S, and SUS430 of JIS-G-4305 into a predetermined shape, and then installing facilities at the laboratory level. Used to produce a thin plate in a general process.

  Specifically, after hot rolling, solution treatment and descaling, cold rolling and heat treatment (including descaled scale) are repeated one or more times, and heat treatment is performed at 1100 ° C. × 3 minutes equivalent to solution treatment in the final step. A thin plate having a thickness of 0.3 mm was obtained.

  Most of the test pieces were strip-shaped test pieces, some of which were in the form of elongated ribbons, and both ends were ring-shaped test pieces that were subjected to solution treatment at 1100 ° C. × 3 minutes after electron beam welding. Table 2 shows the results of chemical components of each sample.

  Next, the test piece was immersed in liquid nitrogen and cooled, and then heated to several levels of heating temperature. For cooling, the process of holding for 5 minutes in liquid nitrogen and holding for 5 minutes at room temperature were repeated three times.

The stress (tension) continued to give a tensile stress of 98 N / mm ² during repeated heating and cooling. The repeated bending was performed only on the ring-shaped test piece, and was applied by applying a tensile stress of 98 N / mm ² only during cooling and winding the pulley around once in 6 seconds. The diameter of the pulley is 40 mm.

  On the other hand, heating was carried out by holding at a predetermined temperature for 1 minute in an atmosphere of 100% by volume Ar gas. Then, the structure | tissue was observed with the test piece before implementation, the particle size number prescribed | regulated by JIS specification was measured, and the difference was computed. That is, the increase / decrease in the particle size number corresponding to crystal grain refinement was investigated.

The metal structure was embedded in a cross-section parallel to the rolling direction of the test piece, and after polishing and corrosion, it was observed using an optical microscope, a photograph at an average site was taken, and the crystal grain size was measured.
Table 3 shows the relationship between the processing conditions for each test piece and the particle size difference.

  In Table 3, those satisfying the conditions defined in the present invention were indicated as invention examples, and the others were indicated as comparative examples.

  As shown in Invention Examples 1 to 11, crystal grain refinement according to the present invention is confirmed. The crystal grain size becomes finer as the grain size number increases, and when the grain size number increases by 2, the crystal grain size becomes 1/2.

  On the other hand, in the case of SUS310S which is a stable austenitic stainless steel and SUS430 which is a ferritic stainless steel in which the material does not cause martensitic transformation as in Comparative Examples 13 to 16, the grain refinement effect according to the present invention is not recognized. .

Claims (12)

  Among austenitic stainless steels defined in JIS-G-4313 (2011), it has a chemical composition that satisfies the metastable austenite state, and after cooling to a cooling temperature below the temperature at which martensitic transformation starts, the austenite matrix The difference between before and after the treatment by heating to a heating temperature equal to or higher than the temperature at which the transformation into a crystal is started is the crystal grain size No. A metastable austenitic stainless steel sheet having a metal structure composed of one or more fine crystal grains (ΔG.S.No.).   The metastable austenitic stainless steel sheet according to claim 1, wherein the metastable austenitic stainless steel is SUS301, SUS301L, SUS304 or SUS304L defined in JIS-G-4305 (2012).   The chemical composition is, by mass, C: 0.15% or less, Si: 1.0% or less, Mn: 2.0% or less, P: 0.045% or less, S: 0.03% or less, Ni The metastable austenitic stainless steel sheet according to claim 1 or 2, comprising: 6 to 13%, Cr: 16 to 20%, N: 0.20% or less, and remaining Fe and impurities.   The said chemical composition has 1 type (s) or 2 or more types chosen from the group which consists of Ti: 0.5% or less, Nb: 0.5% or less, and V: 0.5% or less by the mass%. A metastable austenitic stainless steel sheet according to any one of claims 1 to 3.   The metastable austenitic stainless steel sheet according to any one of claims 1 to 4, having mechanical properties of tensile strength: 175 MPa or more, total elongation: 3% or more, and fatigue limit: 90 MPa or more.   The metastable austenitic stainless steel sheet according to any one of claims 1 to 5, which is used for a steel belt for a printer.   The microstructure is adjusted by heating the metastable austenitic stainless steel sheet to a heating temperature equal to or higher than the temperature at which transformation into the austenite matrix starts after cooling to a cooling temperature below the temperature at which martensitic transformation starts with liquid nitrogen. The method for producing a metastable austenitic stainless steel sheet according to any one of claims 1 to 6, wherein:   The structure adjustment is performed before and after the adjustment according to JIS-G-0551 (2013). The method for producing a metastable austenitic stainless steel sheet according to claim 7, wherein the refinement is one or more crystal grain refinements.   The production of the metastable austenitic stainless steel sheet according to claim 7 or 8, wherein stress is applied to the metastable austenitic stainless steel sheet at least during cooling to the martensitic transformation start temperature or lower. Method.   The metastable austenitic stainless steel sheet has an endless ring shape, and the stress is applied between two or more pulleys, and the pulley is bent and unbent to give a stress applying part or a bending or bending back part. The method for producing a metastable austenitic stainless steel sheet according to claim 9, wherein at least one of the above is cooled.   The method for producing a metastable austenitic stainless steel sheet according to any one of claims 7 to 10, wherein the cooling temperature is not higher than the liquid nitrogen temperature and the heating temperature is not lower than 600 ° C.   The method for producing a metastable austenitic stainless steel sheet according to any one of claims 9 to 11, wherein the stress is less than 0.2% proof stress.