JP2018003139A - Stainless steel - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a stainless steel capable of effectively being manufactured and excellent in strength and ductility.SOLUTION: A stainless steel contain, 0.05 mass% to 0.30 mass% of C, 1.50 mass% o less of Si, 0.1 mass% to 2.0 mass% of Mn, 0.06 mass% or less of P, 0.005 mass% or less of S, 5.0 mass% to 7.0 mass% of Ni, 15.0 mass% to 19.0 mass% of Cr, 0 mass% to 2.0 mass% of Mo, 0 mass% to 2.0 mass% of Cu, 0.05 mass% to 0.30 mass% of N, C+N≥0.20 mass% and the balance Fe (iron) with inevitable impurities. It has a value of Mdof 5 to 30, a value of SFE of 15 or more and less than 25, a value of δcal of 1.0 or less and a multiphase structure of an austenite phase and a process induction martensite phase.SELECTED DRAWING: None

Description

  The present invention relates to a stainless steel excellent in strength and ductility.

  Due to the progress of weight reduction and miniaturization of products and parts corresponding to environmental problems and effective utilization of resources, a thin plate material having high strength is also demanded for steel sheets used as materials for these products and parts.

  In addition, the shape of products and parts is becoming more complicated, and as a material for these products and parts, stainless steel having excellent ductility and good workability is required.

  However, generally, when the strength is improved, the ductility is lowered, and it is difficult to achieve both of these properties.

  Stainless steel has excellent corrosion resistance and is widely used as a component material for springs such as metal gaskets.

  As stainless steel for springs, SUS420J2-CSP, which is a quenched / tempered martensitic stainless steel, SUS631-CSP and SUS632J1-CSP, which are precipitation hardening type stainless steels, and work hardening type austenitic stainless steels as defined in JIS G 4313. Steels such as SUS301-CSP and SUS304-CSP are known, and the mechanical properties such as hardness, bendability, strength, and elongation are standardized.

  Quenched and tempered martensitic stainless steel and precipitation hardened stainless steel are heat treated after product processing to improve strength.

  However, the stainless steel for springs accompanied by these heat treatments is difficult to adjust to a desired strength, and the ductility and workability are greatly lowered due to the increase in strength.

  In addition, processing manufacturers are concerned about an increase in cost and a decrease in productivity due to heat treatment load.

  Further, in the quenched / tempered martensitic stainless steel, the corrosion resistance due to Cr carbide precipitation during the heat treatment is likely to occur.

  Work hardening type austenitic stainless steel improves the strength by generating work induced martensite by processing such as cold rolling after solution treatment.

  However, the high-strength stainless steel obtained by processing may be difficult to process into a predetermined shape due to a decrease in ductility as the strength increases.

  As described above, stainless steel used as a spring material is required to have high strength, excellent ductility, and good workability due to the progress of weight reduction, downsizing, and complexity of products and parts. However, at present, the strength is improved by heat treatment and cold rolling while maintaining a certain degree of workability, and it is difficult to achieve both high strength and good workability.

  Specifically, as a material for this type of spring, for example, as described in Patent Document 1, a multiphase structure of a martensite phase and a retained austenite phase is obtained by quenching, and then C is austenite phase by heat treatment. Stainless steel manufactured by a method of adjusting the balance between strength and ductility by diffusing in is known.

  In addition, as described in Patent Documents 2 to 4, in metastable austenitic stainless steel, a method of ensuring bending workability while adjusting hardness and strength by heat-treating the material after final rolling is known. ing.

JP 2011-184780 A JP 7-216450 A JP 2010-144231 A JP 2006-249564 A

  The stainless steel of Patent Document 1 secures an excellent balance between strength and ductility by setting the product of tensile strength and elongation to 17300 or more, but the strength is evaluated by tensile strength.

  Usually, in a spring application, since the 0.2% yield strength value is more important than the tensile strength, there is a possibility that sufficient strength (0.2% yield strength) cannot be obtained by the method of Patent Document 1.

  The stainless steel of Patent Document 2 ensures and adjusts the desired hardness and bendability by softening a material strengthened by cold rolling by heat treatment at 650 to 900 ° C.

  However, since Patent Document 2 is softened by heat treatment at a relatively high temperature, the strength is likely to change sensitively, and it is considered difficult to stably obtain the desired strength and ductility. In the heat treatment, it is preferable that predetermined characteristics can be obtained without annealing in consideration of the problem of the sensitization phenomenon.

  The stainless steel of Patent Document 3 improves the shape stability and the spring limit value by subjecting a material reinforced by cold rolling to aging heat treatment at a low temperature after removing residual stress with a roller leveler.

  However, there is a possibility that good ductility cannot be ensured by obtaining a high spring limit value. In addition, since the roller leveler processing and the aging heat treatment are performed, the manufacturing load due to the processing after the final rolling is large, and there is a concern that the productivity is lowered.

  The stainless steel of Patent Document 4 controls the crystal grain size in the final recrystallization annealing and the rolling rate in the final rolling within a predetermined range, and sets the material arrival temperature in the strain relief annealing to 200 to 500 ° C. By controlling the speed to 20 ° C./second or more, high strength and good bending workability are ensured.

  However, the heat treatment load due to the control of the grain size and the heating rate and the stress relief annealing after cold rolling causes a decrease in productivity and an increase in cost.

  As described above, in the methods of Patent Documents 1 to 4, for example, stainless steel having excellent strength and ductility may not be efficiently manufactured as a spring material.

  Therefore, there has been a demand for stainless steel that can be produced efficiently and has excellent strength and ductility.

  This invention is made | formed in view of such a point, and it aims at providing the stainless steel which can be manufactured efficiently and was excellent in intensity | strength and ductility.

The stainless steel described in claim 1 is C: 0.05% by mass or more and 0.30% by mass or less, Si: 1.50% by mass or less, Mn: 0.1% by mass or more and 2.0% by mass or less, P: 0.06 mass% or less, S: 0.005 mass% or less, Ni: 5.0 mass% or more and 7.0 mass% or less, Cr: 15.0 mass% or more and 19.0 mass% or less, Mo: 0 mass% or more and 2.0 mass% or less, Cu: 0 mass% or more and 2.0 mass% or less and N: 0.05 mass% or more and 0.30 mass% or less, and C content and N content Is 0.20% by mass or more, and the balance is Fe and inevitable impurities, and Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18. the value of _{Md 30} is austenite stability index indicated by 5Mo is 5 to 30 The value of SFE, which is a stacking fault energy generation index represented by SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32, is 15 or more and less than 25, and δcal = -15-44.91C-0.88Mn-2.31Ni + 2 The value of δcal, which is a δ ferrite formation index after heating at 1230 ° C. for 2 hours, as shown by .20Cr-1.08Cu-28.8N, is 1.0 or less, and a multiphase structure of an austenite phase and a work-induced martensite phase. I have it.

  The stainless steel according to claim 2 is the stainless steel according to claim 1, which is at least one of a spring and a metal gasket.

According to the present invention, in the alloy composition defined within a predetermined range, C + N ≧ 0.20 mass%, the value of Md ₃₀ is 5 or more and 30 or less, the value of SFE is 15 or more and less than 25, and the value of δcal Is adjusted to be 1.0 or less, it can be produced efficiently, and the strength and ductility are good.

It is a graph which shows the relationship between 0.2% yield strength and elongation in an Example.

  Hereinafter, the configuration of an embodiment of the present invention will be described in detail.

  In general, effective means to obtain high strength based on austenitic stainless steel is to provide work such as cold rolling to work harden the austenite phase and harden part of the austenite phase. The so-called work-induced transformation plasticity (TRIP) phenomenon that transforms into a new work-induced martensite phase. Further, also in the molding process, the portion to which processing strain is applied is cured by the TRIP phenomenon.

  The ease with which such a TRIP phenomenon occurs is affected by the austenite stability.

  However, although curing by the TRIP phenomenon is effective for increasing the strength, it causes a decrease in workability.

  Therefore, in order to increase the strength while maintaining excellent workability, it is important to adjust the austenite stability and the amount of work-induced martensite.

  In addition, the workability has a certain correlation with the elongation evaluated by a tensile test or the like. This elongation is closely related to C and N, which are solid solution strengthening elements that affect the strength of the TRIP phenomenon and the processing-induced martensite phase.

  That is, in order to improve the bendability in order to ensure the workability, it is necessary to adjust the austenite stability, the C amount, and the N amount within appropriate ranges.

  And when the value of SFE, which is a generation index of stacking fault energy, is large, work hardening of the austenite phase is difficult to occur, so the hardness difference between the work-induced martensite phase and the austenite phase is large, and the value of SFE is small. Since work hardening of the austenite phase tends to occur, the ductility of the austenite phase decreases.

  Both the increase in the hardness difference between the work-induced martensite phase and the austenite phase and the decrease in the ductility of the austenite phase are factors that reduce workability. To ensure good workability, It is important to adjust the value.

  Hereinafter, the stainless steel according to an embodiment of the present invention is 0.05 mass% or more and 0.30 mass% or less of C (carbon), 1.50 mass% or less of Si (silicon), 0.1 mass%. More than 2.0 mass% Mn (manganese), 0.06 mass% or less P (phosphorus), 0.005 mass% or less S (sulfur), 5.0 mass% or more and 7.0 mass% or less. Ni (nickel), 15.0 mass% or more and 19.0 mass% or less Cr (chromium), 0 mass% or more and 2.0 mass% or less Mo (molybdenum), 0 mass% or more and 2.0 mass% or less Cu (copper) and 0.05 mass% or more and 0.30 mass% or less of N (nitrogen) are contained, and the total of C content and N content is 0.20 mass% or more (C + N ≧ 0. 20% by mass), and the balance is composed of Fe (iron) and inevitable impurities.

Further, in the range of the content of each element, austenite represented by the formula (1) of Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo Components are adjusted so that the value of Md ₃₀ as a stability index is 5 or more and 30 or less.

  Furthermore, in the range of the content of each element, the value of SFE, which is a stacking fault energy generation index represented by the formula (2) of SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32, is 15 or more and less than 25. The ingredients are adjusted so that

  Further, within the range of the content of each element described above, δcal = −15−44.91C−0.88Mn−2.31Ni + 2.20Cr−1.08Cu−28.8N, represented by the expression (3) at 1230 ° C. for 2 hours The components are adjusted so that the value of δcal, which is a δ ferrite formation index after heating, is 1.0 or less.

  Further, the stainless steel has a multiphase structure of an austenite phase and a work-induced martensite phase.

  In addition, the content of each element contained in the stainless steel is substituted for the element symbol of each formula, and among the elements included in each formula, 0 is substituted for the additive-free element.

  C and N are austenite-generating elements. If the content of these elements is too small, the amount of δ ferrite phase increases and hot workability decreases. C and N are useful elements for strengthening the solution-induced martensite phase by solid solution strengthening. And, it is important for both the C content and the N content to be 0.05% by mass or more in order to stably obtain a remarkable ductility improving effect. On the other hand, when C and N are contained excessively exceeding 0.30 mass%, the steel becomes excessively hard and may be a factor that hinders workability. Accordingly, the C content and the N content are both 0.05% by mass and 0.30% by mass.

  In addition, when generating the work-induced martensite phase, C + N (total content of C and N) needs to be 0.20% by mass or more in order to develop sufficient ductility due to the TRIP phenomenon. Therefore, C and N are set to C + N ≧ 0.20 mass% in the respective content ranges.

  In addition, when C + N exceeds 0.40 mass%, there exists a possibility of inhibiting the workability by hardening. Therefore, C + N is preferably 0.40% by mass or less, and the C content and N content are both 0.05% by mass and 0.15% by mass, and C + N is 0.20% by mass or more. More preferably, it is 0.30 mass% or less.

  Si is an element useful for deoxidation in steelmaking and an element contributing to solid solution strengthening. However, if the content exceeds 1.50% 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 set to 1.50 mass% or less.

  Mn is a useful austenite forming element that is less expensive than Ni and can replace the action of Ni. It is also a useful element for solid solution strengthening of steel. In order to utilize the action, it is necessary to contain 0.1% by mass or more. On the other hand, when Mn is contained excessively exceeding 2.5 mass%, it becomes a factor which inhibits hot workability. Therefore, the Mn content is 0.1% by mass or more and 2.0% by mass or less, and more preferably 1.0% by mass or more and 1.5% by mass or less.

  P and S are mixed as inevitable impurities, but the lower the content, the better. In consideration of adverse effects on workability and other material characteristics and manufacturability, the P content is 0.06% by mass or less (including no addition), and the S content is 0.005% by mass. % Or less (including no additive).

  Ni is an essential element for austenitic stainless steel and is effective in improving ductility and toughness. In order to fully exhibit the effect | action, it is necessary to contain 5.0 mass% or more. On the other hand, if Ni is contained excessively in excess of 7.0 mass%, it becomes a factor of deteriorating strength characteristics, and economical efficiency is also reduced due to an increase in cost. Therefore, the Ni content is 5.0% by mass or more and 7.0% by mass or less.

  Cr is an element essential for the formation of a passive film that ensures the corrosion resistance of stainless steel. By containing 15.0% by mass or more, sufficient corrosion resistance can be secured. On the other hand, since Cr is a ferrite-forming element, if it is excessively contained, the heating temperature before hot rolling becomes a two-phase region (γ + δ), and after heating, a large amount of δ-ferrite is generated, and the hot workability is impaired. It becomes. In this embodiment, the content of the austenite-generating element can be adjusted to 19.0% by mass. Therefore, the Cr content is 15.0 mass% or more and 19.0 mass% or less.

  Mo is an element that is useful for improving corrosion resistance and contributes to solid solution strengthening. However, if it is excessively contained in an amount exceeding 2.0% by mass, hot workability is impaired. Therefore, the Mo content is set to 0% by mass or more and 2.0% by mass or less (including no addition).

  Cu is an effective element that can reduce the load of the manufacturing process because it suppresses work hardening due to the formation of a work-induced martensite phase. On the other hand, when Cu is excessively contained, it leads to a decrease in hot workability. Therefore, the Cu content is set to 0% by mass or more and 2.0% by mass or less (including no addition).

Here, the larger the value of Md ₃₀ that is the austenite stability index represented by the formula (1), the easier the transformation from the austenite phase to the work-induced martensite phase occurs. High strength can be obtained and excellent ductility can be secured. In addition, even when molding is performed, a portion to which processing strain is applied, such as a bent portion, can easily obtain higher strength due to the TRIP phenomenon. Such an effect appears remarkably when the value of Md ₃₀ is 5 or more. On the other hand, if the value of Md ₃₀ exceeds 30, the amount of work-induced martensite generation in the part subjected to the bending process becomes too large, so that cracking may be induced and the bendability may deteriorate. Therefore, in order to stably ensure high strength and good ductility, the value of Md ₃₀ which is an austenite stability index is 5 or more and 30 or less, and more preferably 8 or more and 28 or less.

  Moreover, when the value of SFE, which is the stacking fault energy generation index represented by the formula (2), is large, specifically, when the value of SFE is 25 or more, the work hardening of the austenite phase becomes small. Cracks are likely to occur due to the hardness difference between the work-induced martensite phase and the austenite phase that occurs during processing. On the other hand, when the value of SFE is less than 15, work hardening of the austenite phase becomes excessive, and ductility may be reduced. Therefore, the value of SFE, which is a stacking fault energy index, is 15 or more and less than 25 in order to prevent the occurrence of cracks due to the hardness difference and the reduction in ductility.

  Further, δcal represented by the expression (3) indicates the amount of δ ferrite in the slab after the continuous casting and heat treatment at 1230 ° C. for 2 hours. When the value of δcal exceeds 1.0, the ear cracks are likely to occur during hot rolling. In addition, in the final product material, it also affects the deterioration of mechanical properties and fatigue properties. Therefore, for example, in order to ensure good hot workability, mechanical characteristics and fatigue characteristics as a material for spring stainless steel, the value of δcal is set to 1.0 or less.

  The stainless steel according to the above embodiment can be manufactured by a general austenitic stainless steel plate manufacturing process.

  Specifically, the steel whose components are adjusted as described above is melted by a steel making facility to form a slab, and then hot rolled to obtain a hot rolled steel sheet. In addition, the slab heating temperature before hot rolling should just be the range of 1100 degreeC or more and 1350 degrees C or less.

  The hot-rolled steel sheet is subjected to intermediate annealing, and then subjected to finish annealing by reducing the sheet thickness by cold rolling. In addition, pickling is performed after intermediate annealing and finish annealing. Moreover, you may give an intermediate annealing in the middle of cold rolling as needed. The intermediate annealing and the finish annealing after the hot rolling are preferably performed in the range of 900 ° C. or higher and 1100 ° C. or lower.

  In addition, after finish annealing, temper rolling according to the target hardness is performed, and for example, a temper rolled steel sheet having a thickness of 0.1 mm to 3.0 mm can be obtained. Further, after temper rolling, shape correction is performed as necessary.

According to the one embodiment, by adjusting the components so that the value of Md ₃₀ is 5 or more and 30 or less, the TRIP phenomenon can easily occur from the austenite phase to the work-induced martensite phase. In addition, it is possible to prevent a decrease in workability by suppressing a decrease in bendability.

  In addition, by adjusting the components so that the SFE value is 15 or more and less than 25, it is possible to suppress a decrease in ductility due to excessive work hardening of the austenite phase, and cracks due to a hardness difference between the austenite phase and the work-induced martensite phase. Generation can be suppressed and deterioration of workability can be prevented.

  Furthermore, by adjusting the components so that the value of δcal is 1.0 or less, it is possible to suppress the occurrence of cracking during hot rolling and prevent the hot workability from being lowered, and the mechanical properties due to the δ ferrite phase. The deterioration of characteristics and fatigue characteristics can be prevented.

  Moreover, by making C + N 0.20 mass% or more, ductility can be expressed by the TRIP phenomenon, and workability can be improved.

Therefore, within the above alloy composition range, the value of Md ₃₀ is 5 or more and 30 or less, the value of SFE is 15 or more and less than 25, the value of δcal is 1.0 or less, and C + N is 0.20% by mass or more. Since the components are adjusted as described above, the strength and ductility are good.

  That is, for example, the balance between strength and ductility is good as compared with a conventional SUS301-CSP / H material of 300 series stainless steel.

  Further, for example, heat treatment or the like is not required after final rolling for adjusting the balance between strength and ductility, and the production can be efficiently performed.

  And since the said stainless steel is excellent in intensity | strength and ductility, it is suitable as materials for springs, such as a metal gasket, for example.

  Hereinafter, this example and a comparative example will be described.

  First, stainless steel having the composition shown in Table 1 was melted. In Table 1, a1 to a8 are invention steels (this example) having chemical components defined in the present invention, b1 to b5 are comparative steels (comparative examples), and c1 is SUS301- of conventional steel (comparative examples). CSP / H material.

Incidentally, b1, the value of _{Md 30} is outside the range specified in the present invention. b2 is out of the range defined by the present invention in the value of SFE. b3 is out of the range defined by the present invention in terms of Md ₃₀ , C + N, and δcal. b4 is outside the range defined by Md ₃₀ and δcal in the present invention. b5 is outside the range defined by the present invention in terms of C + N and δcal.

  Each steel obtained a steel ingot of 100 kg, and then hot rolled at an extraction temperature of 1230 ° C. to produce a hot rolled steel strip having a thickness of 3 mm.

  This hot rolled steel strip was subjected to intermediate annealing at 1080 ° C. for 5 minutes soaking, and then cold rolling and annealing at 1080 ° C. for 1 minute were repeated to obtain an intermediate steel strip. Further, the rolling rate at which the thickness after temper rolling becomes 0.2 mm is examined in advance for each steel, and the thickness at the time of finish annealing is set based on the temper rolling rate, and the thickness After performing cold rolling to 980 ° C., finish annealing was performed at 1080 ° C. for 1 minute.

  Furthermore, temper rolling was performed to a sheet thickness of 0.2 mm after finish annealing. This temper rolling was performed for 7 to 10 passes after heating the steel sheet to 70 ° C.

  Using the temper rolled material having a thickness of 0.2 mm for each steel manufactured as described above, the mechanical properties were investigated.

  The mechanical properties were evaluated by using the tensile strength and 0.2% proof stress as measured by the tensile test of JIS Z 2241 for the strength and the elongation for the ductility.

  Table 2 and FIG. 1 show the results of the mechanical characteristics relating to this example and the comparative example.

  This example had a better balance of strength and ductility than the conventional steel SUS301-CSP / H material (c1 as a comparative example).

  Specifically, in this example, in the case of a temper rolling ratio of about 40% (this example group on the right side in FIG. 1), the strength is equivalent to that of SUS301-CSP / H and 3 It was more than twice as ductile. Further, in the case of a temper rolling ratio of about 50% (this example group on the left side in FIG. 1), it had high strength and good ductility that was at least twice as high as that of the SUS301-CSP / H material. .

On the other hand, compared with SUS301-CSP / H material, comparative steels (b1 to b5 as comparative examples)
Most of the steel types are inferior in strength and ductility, the same in strength but inferior in ductility, the inferior in strength but inferior in strength, and the same in both strength and ductility. The mechanical properties (strength and ductility) were lower. That is, all of the comparative examples were inferior in balance between high strength and high ductility as compared with the present example.

  Therefore, after adjusting to the composition prescribed | regulated by this invention, in the steel strengthened by temper rolling, it was confirmed that high intensity | strength and high ductility can be made compatible.

Claims (2)

C: 0.05 mass% or more and 0.30 mass% or less, Si: 1.50 mass% or less, Mn: 0.1 mass% or more and 2.0 mass% or less, P: 0.06 mass% or less, S: 0.005 mass% or less, Ni: 5.0 mass% or more and 7.0 mass% or less, Cr: 15.0 mass% or more and 19.0 mass% or less, Mo: 0 mass% or more and 2.0 mass% or less, Cu: 0% by mass or more and 2.0% by mass or less and N: 0.05% by mass or more and 0.30% by mass or less, and the total of C content and N content is 0.20% by mass or more, The balance consists of Fe and inevitable impurities,
Md ₃₀ = 551-462 (C + N) -9.2Si-8.1Mn-29 (Ni + Cu) -13.7Cr-18.5Mo is an austenite stability index represented by Md ₃₀ of 5 or more and 30 or less.
SFE = 2.2Ni + 6Cu-1.1Cr-13Si-1.2Mn + 32 is the value of SFE which is a stacking fault energy generation index indicated by 15 or more and less than 25,
δcal = -15-44.91C-0.88Mn-2.31Ni + 2.20Cr-1.08Cu-28.8N The value of δcal, which is a δ ferrite formation index after heating at 1230 ° C. for 2 hours, is 1.0. Below,
A stainless steel characterized by having a multiphase structure of an austenite phase and a work-induced martensite phase. The stainless steel according to claim 1, wherein the stainless steel is at least one of a spring and a metal gasket.

Images