JP5977609B2 - Saving Ni-type austenitic stainless steel - Google Patents

Description

  The present invention relates to a Ni-saving austenitic stainless steel that is soft and has an appropriate work hardening property and excellent extrudability.

  As typical examples of austenitic stainless steel, 300 series (SUS301, SUS304, SUS316, etc.) standardized by JIS G 4305, 200 series (SUS201, SUS202, etc.) standardized by JIS G 4303, etc. ) Etc.

Of these, the 300 series austenitic stainless steel has good workability because it contains Mn of 2.0% by mass or less and Ni of about 6 to 15% by mass, and is also resistant to galling. Excellent. In particular, SUS304 has a metastable austenite phase, and after being solution-treated, it undergoes cold working such as cold rolling to produce work-induced martensite (α ^′ ). That is, since the martensite transformation occurs during the molding process and the work hardening is increased, the “extrusion property” is improved. However, as described above, the 300 series austenitic stainless steel contains a large amount of expensive Ni, and thus has a disadvantage of high raw material costs.

  On the other hand, the 200 series austenitic stainless steel is a Ni-saving (high Mn type) stainless steel in which the 300 series Ni is replaced with Mn. The 200 series austenitic stainless steel contains a large amount of C and N in addition to the Mn, and therefore has high strength and is nonmagnetic. In addition, the 200-series austenitic stainless steel has a lower raw material cost than the Ni-based austenitic stainless steel. However, high Mn type stainless steel represented by SUS201, SUS202, and the like contains a large amount of interstitial solid solution elements such as C and N as described above, and therefore is hard in the solution treatment state and has high strength. There is a problem of inferior moldability.

Therefore, as a technique for improving the workability of Ni-saving austenitic stainless steel represented by the 200 series, for example, Patent Documents 1 to 3 show that% C +% N is reduced to a specific region, A technique for providing a Ni-saving austenitic stainless steel with improved ductility and workability by adding a certain amount of Cu is disclosed. In these techniques, the “Md ₃₀ value”, which is an index indicating austenite stability, is adjusted so as to be within a predetermined range to improve the stretchability.

  Here, “% element symbol” such as “% C” and “% N” means “a ratio of a certain element in steel expressed in mass%” (hereinafter the same). .)

“Md ₃₀ ” is defined as “processing temperature at which 50% is transformed into martensite when 30% elongation strain is applied to austenitic stainless steel”. Based on the composition of the stainless steel, Md ₃₀ = 551-462 [% C +% N] -9.2 [% Si] -8.1 [% Mn] -29 [% Ni +% Cu] -13.7 [ % Cr] -18.5 [% Mo] -68 [% Nb] -1.42 [υ-8] (where υ is the grain size number). The

JP 52-119413 A Japanese Patent Publication No. 55-49150 Japanese Patent No. 4327030

  However, in the Ni-saving austenitic stainless steel with improved overhang, the following problems are concerned. That is, in austenitic stainless steel, there is a case in which delayed fracture called “time crack” occurs when processing exceeding the processing limit of the steel type occurs. It is known that the susceptibility of this time crack depends on% C +% N and austenite stability, but in the technique described in the above patent document, no specific examination has been made on the susceptibility to time crack, If the susceptibility to time cracking is inferior even though the stretchability is good, it can be said that it is practically unsuitable for press forming at a high processing rate.

  Therefore, the main problem of the present invention is that it is excellent in formability such as stretchability and excellent in cracking at the time, even though it is a Ni-saving component system with a Ni content of 4% by mass or less. It is to provide a reduced Ni type austenitic stainless steel.

The inventors have intensively studied to solve the above problems, and as a result, have completed the invention shown below. That is, the present invention
(1) By mass%, C ≦ 0.08%, N ≦ 0.08%, Si ≦ 1.0%, 8.05 ≦ Mn ≦ 10.0%, 14.0% ≦ Cr ≦ 16.0% 2.0% ≦ Ni ≦ 4.0%, 1.0% ≦ Cu ≦ 2.5% , Mo ≦ 0.5% , the balance being Fe and inevitable impurities,
(2) Md ₃₀ Mn = 551-462 ([% C] + [% N]) − 9.2 [% Si] −19.1 [% Mn] −13.7 [% Cr] −29 ([% Ni] + [% Cu])-18.5 [% Mo] Md ₃₀ Mn value satisfies (3) −45 ≦ Md ₃₀ Mn ≦ −5 (3) Ni-saving austenitic stainless steel ".

  Furthermore, in the above invention, in order to ensure press formability that allows the stretch processing at a high processing rate without causing a time crack, the elongation in the solution heat treatment state is 53% or more, and the Erichsen value is 12 It is preferably 5 mm or more.

  According to the present invention, since the blending ratio of expensive Ni is kept within the range of 2.0 mass% to 4.0 mass%, the predetermined austenitic stainless steel can be produced economically.

Further, since the Md ₃₀ Mn value correlated with the elongation and Erichsen value of the steel is in the range of −45 ≦ Md ₃₀ Mn ≦ −5, the obtained steel has formability such as elongation and stretchability. As well as excellent cracking susceptibility.

  In other words, according to the present invention, despite the Ni-saving component system with a Ni content of 4% by mass or less, it has excellent formability such as stretchability and excellent time-cracking susceptibility. Type austenitic stainless steel can be provided.

It is a graph which shows the relationship between austenite stability and elongation of steel. It is a graph which shows the relationship between austenite stability and the Erichsen value of steel. It is a graph which shows the relationship between austenite stability and the timetable generation | occurence | production number of steel.

  First, the reasons for limiting each component constituting the “Ni-saving austenitic stainless steel” (hereinafter also simply referred to as “steel”) according to the present invention will be described.

  1) C ≦ 0.08% by mass: C (carbon) is an extremely effective element for strengthening the martensite phase, and as austenite forming element, reduces δ ferrite formed during solidification and in a high temperature range, Reduces hot workability. However, excessive addition of C increases the susceptibility to intergranular corrosion and precipitates intergranular stress corrosion cracking in the heat affected zone of the weld and the hot rolled coil after coiling, thereby increasing the susceptibility to intergranular corrosion. Make it easier to do. Furthermore, the 0.2% proof stress of the steel is increased by solid solution strengthening, thereby reducing the cold workability. Therefore, the C content of steel needs to be 0.08% by mass or less.

  The lower limit of the C content is not particularly limited, but the C content is preferably 0.04% by mass or more in order to exhibit the C-containing effect remarkably.

  2) N ≦ 0.08 mass%: N (nitrogen) is an austenite-forming element like C. N is an element effective for stabilizing the austenite structure, strengthening the metal structure, and improving the corrosion resistance of the steel. However, since N has a high solid solution strengthening ability, the addition of N exceeding 0.08% by mass brings about significant hardening of the steel. Therefore, the upper limit of the N content is set to 0.08% by mass.

  The lower limit of the N content is not particularly limited, but the N content is preferably 0.04% by mass or more in order to exert the above N-containing effect remarkably.

  3) Si ≦ 1.0% by mass: Si (silicon) is an element that exerts an effect as a deoxidizer during steelmaking. However, if this Si is contained in a large amount, a lot of δ ferrite is generated at the time of solidification and in a high temperature range. That is, since it is disadvantageous for obtaining an austenite structure, the upper limit of the Si content of the steel is set to 1.0 mass% (more preferably 0.6 mass% or less).

The lower limit of the Si content is not particularly limited, but the Si content is preferably 0.4% by mass or more in order to exert the Si-containing effect remarkably. ) 7.0 ≦ Mn ≦ 10.0 mass%: Mn (manganese) is an element that can replace Ni as an austenite-forming element. By increasing the Mn content as much as possible, expensive Ni This is effective in reducing the product cost of steel. Therefore, in order to suppress the Ni content as much as possible and increase the cost reduction effect, the lower limit of the Mn content is set to 7.0% by mass (more preferably 7.5% by mass or more). On the other hand, excessive addition of Mn reduces the corrosion resistance of the steel, so the upper limit of its content was 10.0% by mass (more preferably 9.0% by mass or less).

  5) 14.0 ≦ Cr ≦ 16.0% by mass: Cr (chromium) is one of the most effective elements for enhancing the corrosion resistance of steel. In order to obtain sufficient corrosion resistance, 14.0% by mass or more Cr content is required. However, if the Cr content exceeds 16.0% by mass, a lot of δ ferrite is generated at the time of solidification and in a high temperature range, so that the hot workability of the steel is lowered. Therefore, the upper limit of the Cr content is set to 16.0% by mass.

  6) 2.0 ≦ Ni ≦ 4.0 mass%: Ni (nickel) is an austenite forming element. Ni is an essential element in the steel according to the present invention in order to obtain the stability of the austenite structure, the good hot workability of the steel, and the good cold workability of the steel. However, as described above, Ni is an expensive element and also causes metal allergy, so the upper limit of Ni content is 4.0% by mass and the lower limit is 2.0% by mass. did.

  7) Mo ≦ 0.5% by mass: Mo (molybdenum) is an element that can improve the corrosion resistance of steel. However, inclusion of Mo generates a lot of δ-ferrite during solidification and in a high-temperature region. There is a risk of reducing the hot workability. Therefore, the upper limit of the Mo content is set to 0.5% by mass so that the corrosion resistance of the steel can be improved while suppressing the decrease in hot workability of the steel.

  8) Cu ≦ 2.5% by mass: Cu (copper) is an austenite forming element and is effective as a substitute element for Ni. In addition, this Cu is an element that increases the corrosion resistance of the steel and contributes to softening. However, the addition of an excessive amount of Cu may reduce the hot workability and the stretchability. The upper limit was 2.5% by mass.

  The lower limit of the Cu content is not particularly limited, but the Cu content is preferably 1.0% by mass or more in order to exert the above Cu-containing effect remarkably.

9) −45 ≦ Md ₃₀ Mn ≦ −5: The Md ₃₀ Mn value is a value serving as an index of austenite stability in a Ni-saving austenitic stainless steel of a type in which Ni is replaced with Mn. That is, it is a value representing the ease of work-induced martensite (α ^′ ) transformation that occurs when cold-working the austenitic stainless steel of this component system, Md ₃₀ Mn = 551-462 ([% C] + [ % N])-9.2 [% Si] -19. 1 [% Mn] -13.7 [% Cr] -29 ([% Ni] + [% Cu])-18.5 [% Mo] The larger the value, the lower the austenite stability, and the more easily the processing-induced martensite (α ^′ ) transformation occurs during processing.

By the way, it is known that the ductility and stretch formability of austenitic stainless steel become the best by adjusting the austenite stability to a predetermined range. That is, when the Md ₃₀ Mn value, which is an index of austenite stability, reaches an upper limit within a certain range, an appropriate amount of processing-induced martensite phase (α ^′ ) occurs during processing, and work hardening of the deformed portion increases. Therefore, local deformation such as constriction is suppressed, large uniform elongation is obtained, and ductility and stretchability are improved. However, when the Md ₃₀ Mn value exceeds the upper limit of a certain range and the austenite stability becomes lower than a certain level, that is, when the austenite phase becomes unstable beyond the above certain range, Work hardening by martensitic transformation is completed, and after that, the above-mentioned necking prevention effect cannot be obtained, resulting in a decrease in ductility and stretchability.

In addition, austenitic stainless steel, when subjected to press working such as deep drawing, often causes a problem of delayed fracture called time cracking when the degree of work exceeds a limit. Factors that affect the susceptibility to time cracking include C and N content, hydrogen in steel, residual stress of the molded article, and austenite stability, etc., especially Md ₃₀ Mn value is large (that is, austenite stability It is known that steels that are more prone to work-induced martensite phase (α ^′ ) tend to be less susceptible to time cracking.

In summary, if the Md ₃₀ Mn value is large, the work-induced martensite phase (α ^′ ) is excessively larger than the appropriate amount when forming the product plate that has undergone solution treatment, such as pressing. As a result, ductility and overhanging properties are lowered, and the sensitivity to time cracking is also lowered. In the present invention, it has been found that Md ₃₀ Mn ≦ −5 is necessary to obtain a steel excellent in time cracking susceptibility while maintaining appropriate ductility and stretchability.

On the other hand, if the Md ₃₀ Mn value is small, there is no fear of time cracking after press forming, but if Md ₃₀ Mn is less than −45, the amount of work-induced martensite (α ^′ ) phase generated becomes extremely small. In addition, the effect of increasing the ductility using the processing-induced transformation cannot be obtained, and there is a concern that the formability of the product plate after the solution treatment is lowered. Therefore, the lower limit is preferably −45 (Md ₃₀ Mn ≧ −45).

That is, the predetermined range that makes the ductility and stretch formability of the austenitic stainless steel described above most favorable is a range of −45 ≦ Md ₃₀ Mn ≦ −5.

  In the above formula, “%” means “% by mass”, and “% element symbol” means “a ratio of elements in steel expressed in mass%”.

  Furthermore, in addition to the above elements and parameters, it is more preferable to adjust the content of the following elements.

  10) P ≦ 0.05% by mass: P (phosphorus) is an element that causes deterioration of the corrosion resistance and hot workability of steel, so the upper limit of its content was set to 0.05% by mass. Note that this P may be regarded as an inevitable impurity.

  11) S ≦ 0.01% by mass: S (sulfur) is an element that increases inclusions and decreases the resistance to galling of steel. Moreover, since the increase in the S content significantly decreases the hot workability, the upper limit of the S content is set to 0.01% by mass. The S may be regarded as an inevitable impurity.

  The steel according to the present invention composed of the elements as described above is manufactured by a general stainless steel manufacturing process. That is, solution heat treatment is performed after melting, casting, hot rolling and cold rolling.

  Below, the manufacturing method of a steel sample, the method of a test, and a result are demonstrated as an Example which concerns on this invention. In addition, this invention is not limited to the said Example.

1) Preparation of steel samples In order to obtain invention steels and comparative steels having the chemical compositions shown in Table 1, an ingot of 38 mm x 90 mm x 150 mm was manufactured in a high-frequency melting furnace, and this ingot was heated at 1200 ° C in an electric furnace. It was heated for 60 minutes and hot-rolled to a thickness of 3.0 mm with a four-high rolling mill to obtain a hot-rolled sheet.

Subsequently, this hot-rolled sheet was annealed at 1100 ° C. for 6 minutes, immersed in nitrohydrofluoric acid to remove the scale, and then cold-rolled to 1.0 mm with a four-high rolling mill, and further at 1100 ° C. for 2 minutes. Annealing and dipping in nitric hydrofluoric acid removed the scales to obtain cold-rolled annealed pickling plates as invention steel and comparative steel. The obtained invention steel and comparative steel were subjected to the following characteristic evaluation test.

2) Test method (1) Hardness A test piece was cut out from the cold-rolled annealed pickled steel plate used as the inventive steel and comparative steel prepared in the above-described steps, and a Vickers hardness test was performed according to JIS Z 2244. Hardness was evaluated. The obtained results are shown in Table 2.

(2) Tensile strength, 0.2% proof stress, elongation From the rolling direction of the cold-rolled annealed pickled steel plate used as the inventive steel and the comparative steel produced in the above-mentioned process, a JIS 13B test piece was cut out and conformed to JIS Z 2241 Then, a tensile test was performed, and tensile strength, 0.2% proof stress, and elongation were measured. The obtained results are shown in Table 2.

(3) Erichsen value From the cold-rolled annealed pickled steel plate used as the inventive steel and the comparative steel produced in the above-described process, a square test piece having a side of 90 mm was cut out and subjected to an Erichsen test in accordance with JIS Z 2247. The obtained results are shown in Table 2.

(4) Time cracking sensitivity A round test piece (φ100 mm) is taken from the cold-rolled annealed pickled steel plate as the invention steel and comparative steel manufactured in the above-mentioned process, and the test piece is cupd at a drawing ratio of 2.0. (Drawing test conditions: punch diameter: φ50 mm, die diameter: φ53 mm, drawing speed: 25 mm / min, wrinkle holding pressure 23.5 kN, test temperature: 25 ° C., number of tests: n = 3).

  Next, lubricating oil was applied to the test piece after drawing, and held in an environment of 40 ° C. for 500 hours, and the presence / absence and number of time cracks were confirmed. The obtained results are shown in Table 2. In addition, the blank (-) part in Table 2 shows what could not be drawn into a predetermined cup shape.

3) Test results As is clear from Table 2 above, the inventive steels 1 to 7 have an elongation of 53% or more and an Erichsen value of 12.5 mm or more, and press formability capable of being stretched at a high processing rate. have. In the inventive steels 1 to 7, no time cracking occurred as a result of the time crack susceptibility test. That is, the steel of the present invention is excellent not only in formability such as elongation and stretchability, but also in time cracking susceptibility.

The comparative steel 5 and the comparative steel 6 also had an Md ₃₀ Mn value within the range of the present invention, and an elongation of 53% or more and an Erichsen value of 12.5 mm or more were obtained. This steel has a Ni content exceeding 4.0% by mass (that is, outside the range of the component composition of the present invention), which increases the risk of Ni allergy and is also not economical. There is.

Here, FIG. 1 to FIG. 3 plot the relationship between the austenite stability and the elongation, Erichsen value, and number of time cracks of austenitic stainless steel using the various data. In addition, (a) in each figure is the “Md ₃₀ Mn value” according to the present invention as an index indicating austenite stability, that is, Md ₃₀ Mn = 551-462 ([% C] + [% N]) − 9. .2 [% Si] -19.1 [% Mn] -13.7 [% Cr] -29 ([% Ni] + [% Cu]) - 18.5 Md 30 Mn value determined in accordance with [% Mo] (B) uses the “Md ₃₀ value based on the improved field formula”, which is a general index indicating austenite stability. The region indicated by Usuzumi in (a) of each figure shows the Md ₃₀ Mn range of the steel according to the present invention.

As shown in these figures, in the Ni-saving and high-Mn raw material system including the steel of the present invention, the “Md ₃₀ value based on Nohara's modified formula”, which is a general indicator of austenite stability, and the elongation of steel Therefore, no correlation is obtained between the Erichsen value and the number of cracks. On the other hand, a high correlation is obtained between the “Md ₃₀ Mn value” according to the present invention and the steel elongation, Erichsen value and the number of time cracks, that is, the stretchability and the time crack sensitivity. That is, it can be seen that it is extremely effective to use the “Md ₃₀ Mn value” according to the present invention when designing the component of the Ni-saving high-Mn steel.

Therefore, when this “Md ₃₀ Mn value” is examined, as described above, when the “Md ₃₀ Mn value” is in the range of −45 ≦ Md ₃₀ Mn ≦ −5, In addition to formability such as stretchability, it is excellent in sensitivity to time cracking.

  As described above, according to the “Ni-saving austenitic stainless steel” of the present invention, although the Ni content is 4% by mass or less and is an economical Ni-saving component system, the extruding property, etc. It is possible to provide a Ni-saving austenitic stainless steel having excellent formability and excellent cracking susceptibility.

Claims (2)

% By mass, C ≦ 0.08%, N ≦ 0.08%, Si ≦ 1.0%, 8.05 % ≦ Mn ≦ 10.0%, 14.0% ≦ Cr ≦ 16.0%, 2 0.0% ≦ Ni ≦ 4.0%, 1.0% ≦ Cu ≦ 2.5% , Mo ≦ 0.5% , the balance being Fe and inevitable impurities,
Md ₃₀ Mn = 551-462 ([% C] + [% N]) − 9.2 [% Si] −19.1 [% Mn] −13.7 [% Cr] −29 ([% Ni] + Ni-saving austenitic stainless steel characterized in that the Md ₃₀ Mn value according to [% Cu])-18.5 [% Mo] satisfies −45 ≦ Md ₃₀ Mn ≦ −5. 2. The Ni-saving austenitic stainless steel according to claim 1 , wherein the elongation in the solution heat treatment state is 53% or more and the Erichsen value is 12.5 mm or more .