JP2002194506A - Stainless steel sheet and production method for the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a metastable austenitic stainless steel sheet having high strength and high fatigue characteristic, and superior workability as well, and also provide the method to produce the same without needing additional costs. SOLUTION: The stainless steel sheet is characterized in that it contains by mass % C: 0.01-0.13%, Si: 0.1-3.2%, Mn: 2.8% or less, Cr: 13-20%, Ni: 3-12%, N: 0.1-0.2%, Nb: 0.01-0.1% and at least any one from an among Mo: 4% or less, Cu: 4% or less, or Al: 3% or less, with the Md value in the range of 20-60, and that the metal structure of the steel consists of recrystallized structure ranging from 50 to 99% in terms of area % with the remainder being substantially non-recrystallized structures and precipitated micro structures.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a metastable austenitic stainless steel sheet which is required to have high strength and high fatigue properties and to be excellent in workability, and a method for producing the same. It is suitable for a wide variety of spring components to which a variable stress is applied, such as springs for electronic device components, retractor springs, and gaskets and metal packings for automobiles and motorcycles.

[0002]

2. Description of the Related Art Conventionally, SUS301 and SUS30 belonging to a metastable austenitic system have been used for stainless steel for springs.
4 has been widely used. These stainless steels are generally used after temper rolling and impart high strength by work hardening accompanied by work-induced martensitic transformation. It also shows relatively high corrosion resistance.

Another example is SU, which is a precipitation-strengthened stainless steel.
S631 and the like are also used, and are used after further aging treatment after temper rolling.

However, these stainless steels for springs are:
Although the required high strength can be obtained relatively easily, there is a problem that the ductility sharply decreases and it becomes difficult to process the product.
In addition, fatigue characteristics are also reduced due to the occurrence of defects such as cracks due to insufficient workability. For example, it has been found that the fatigue characteristics against repeated fluctuating stresses increase with increasing strength in the flat plate after rolling, but often decrease after forming into a product.

[0005] That is, although the stainless steel for springs manufactured by the conventional method satisfies the minimum required high strength, at present, workability and fatigue properties are insufficient.

JP-A-4-214484, JP-A-5-2-2
No. 79802 and Japanese Unexamined Patent Publication No. Hei 5-117213 disclose that the surface of a sheet at the time of product processing due to deterioration in ductility after temper rolling by making crystal grains after final annealing into fine and uniform crystal grains of 10 μm or less. The invention discloses a metastable austenitic stainless steel in which the generation of defects such as cracks in steel is suppressed and the fatigue characteristics are improved, and a method for producing the same.

However, these production methods require temper rolling, aging treatment, and the like, and the production cost is high. The obtained stainless steel sheet does not always have a sufficient balance of strength, fatigue characteristics and workability for springs. I can't say.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a metastable austenitic stainless steel sheet having high strength and high fatigue properties and excellent workability, and a manufacturing method which does not increase the manufacturing cost. It is in.

[0009]

Means for Solving the Problems The present inventors have developed a stainless steel sheet having higher strength, superior fatigue characteristics and workability than conventional stainless steel for springs, and a simpler production of the stainless steel sheet. The following findings were obtained as a result of various experiments and investigations to develop a manufacturing method using a process.

A) The metal structure after the final annealing is a mixed structure composed of a recrystallized structure having high ductility and a non-recrystallized structure having a high-strength martensite phase, and if the ratio thereof is appropriately adjusted, the metal structure can have high strength. Excellent workability and fatigue properties are obtained.

B) In order to stably obtain such a mixed structure, a stainless steel whose chemical composition has been adjusted so as to have an Md value of 20 to 60 has a reduction ratio of 60% or more in final cold rolling. It is sufficient to adjust the annealing temperature and the annealing time after saturation of the highly induced martensite.

C) The mixed structure comprises an extremely fine austenite recrystallized portion having a grain size of 1 to 2 μm or less and an unrecrystallized portion of austenite and martensite phases surrounded by these crystal grains. Has a much finer and relatively more uniform structure than the uniform fine particles obtained after the recrystallization of In addition, the fatigue characteristics are further improved thereby. Further, it is considered that the soft recrystallized austenite grains are first deformed during molding into a product, and the deformed grains are transformed into a high-hardness martensite phase, whereby the processed portion is hardened and excellent strength is obtained.

D) With the above-mentioned mixed structure, required strength can be obtained, and it is not necessary to perform temper rolling or aging treatment after final annealing, thereby simplifying the manufacturing process.

The present invention has been made based on such findings, and the gist is as follows.

(1) In mass%, C: 0.01 to 0.13
%, Si: 0.1 to 3.2%, Mn: 0.4 to 2.8
%, Cr: 13-20%, Ni: 3-12%, N: 0.
1 to 0.2%, Nb: 0.01 to 0.2%, Mo: 0 to 4%, Cu: 0 to 4%, Al: 0 to 3%,
Ca: 0 to 0.05%, rare earth element: 0 to 0.05%,
B: contains 0 to 0.05%, the balance is Fe and impurities, the Md value represented by the following formula is 20 to 60, the recrystallized structure of 50 to 99% in area%, and the balance is substantially Stainless steel sheet having a metallic structure consisting of a non-recrystallized structure and a fine precipitate.

Md = 500-458 (C + N) -9 (Si + Mn) -14Cr-22Ni-21C
u-20Mo (2) A method for hot-rolling a stainless steel having the chemical composition described in (1) above, followed by cold rolling and annealing to produce a steel sheet. And a final annealing in a temperature range of 750 to 950 ° C.

Here, “substantially” means that inclusions unavoidable in production may be present. The final cold rolling means cold rolling before annealing in the method of performing cold rolling and annealing once, respectively, and final annealing and the method in the method of performing cold rolling and annealing twice or more each. This refers to cold rolling between the previous annealing. The rolling reduction refers to the total rolling reduction in the final cold rolling.

[0018]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the chemical composition and production conditions of the stainless steel sheet of the present invention will be described in detail.

1. Chemical composition of stainless steel sheet (% indicates mass%) C: 0.01 to 0.13% C is an element for solid solution strengthening the austenite matrix and the work-induced martensite phase, and the strength required after annealing. To obtain at least 0.01% or more. Further, it is an austenite stabilizing element, and suppresses martensitic transformation with an increase in the amount of addition. Furthermore, if excessively contained, carbide precipitation occurs after final annealing performed at a relatively low temperature to obtain the mixed structure,
Deterioration of workability and corrosion resistance. Therefore, the upper limit of C is set to 0.13%. Preferably it is 0.02-0.12%.

Si: 0.1 to 3.2% Si is an important element in the steel sheet of the present invention, and has an effect of solid solution hardening the austenite matrix phase and the martensite phase. Also, when Si is contained, the high-temperature strength increases,
It is considered that the difference between the recrystallization start and end temperatures is widened, and it becomes easy to obtain a mixed structure. To obtain these effects, 0.1% or more is required. However, 3.2%
Exceeding the above results in poor workability. Therefore, the upper limit is set to 3.
2%. Preferably it is 0.3 to 3%.

Mn: 0.4 to 2.8% Mn is an austenite stabilizing element and also an element that improves hot workability. Although it is contained in a balance with other austenite stabilizing elements, it is necessary to contain 0.4% or more to obtain the above effect. However, 2.8%
If the content exceeds 0.1%, a work-induced martensite phase may not be obtained, and the ductility of the material may be reduced due to generation of inclusions and the like. Therefore, the upper limit is set to 2.8%.
Preferably it is 0.6 to 2.6%.

Cr: 13 to 20% Cr is a basic element of stainless steel, and is contained in an amount of 13% or more to ensure corrosion resistance. Further, it is a ferrite stabilizing element, and if contained in excess of 20%, the ferrite phase remains in the steel, so the upper limit was made 20%. Preferably it is below 15-19%.

Ni: 3 to 12% Ni is an austenite stabilizing element. It is an essential element to make an austenite phase at room temperature,
In the steel sheet of the present invention, metastable austenite is required at room temperature, and it is necessary to contain 3% or more in order to obtain good workability. However, if the content exceeds 12%, work-induced martensitic transformation does not occur, so the upper limit is 12%.
%. Preferably it is 4 to 11%.

N: 0.1 to 0.2% N is an important element in the steel sheet of the present invention, and is an element for solid solution strengthening the austenite matrix and the work-induced martensite phase. In addition, as described later, N
Since the growth of recrystallized grains is suppressed by the precipitation of the bN compound, fine grains can be easily obtained. To obtain these effects, 0.1% or more is required. It is also an austenite stabilizing element and has the effect of suppressing martensitic transformation with increasing content. However, 0.2
%, The hot workability is impaired, and ear cracks and the like are generated. Therefore, the upper limit is set to 0.2%. Preferably, it is 0.11 to 0.16%.

Nb: 0.01 to 0.2% Nb has an effect of suppressing grain growth by precipitating an Nb-N compound. Therefore, it becomes easy to obtain fine recrystallized grains by containing Nb. Therefore, it is an important element for the steel of the present invention. In order to obtain these effects, it is necessary to contain at least 0.01%. However, it is an extremely expensive element, and if added in a large amount, it becomes expensive steel and causes a reduction in ductility of the steel. Therefore, the upper limit is set to 0.
2%. Preferably, it is 0.03 to 0.16%.

In addition to the above-mentioned elements, the following elements may be contained as necessary, and the inclusion does not particularly affect the workability and the fatigue properties.

Mo, Cu, Al: These elements are elements contained as necessary to harden steel mainly by precipitation strengthening. In addition, it has an effect of improving corrosion resistance and also has an effect as a deoxidizing agent. When they are contained, it is preferable to contain 0.6% or more in order to obtain the effect of precipitation strengthening.

Mo is also a ferrite stabilizing element, and
%, The ferrite phase remains in the steel, so the upper limit was made 4%.

Further, when Cu and Al are contained in large amounts, the hot workability is impaired, and segregation at grain boundaries and the like causes cracking. Therefore, the upper limits are set to 4% and 3%, respectively.

Ca, Rare Earth Element (REM) Ca and REM can be used as a deoxidizing agent at the time of melting, if necessary. In order to obtain the effect, the content is preferably 0.006% or more. However, when the content exceeds 0.05%, the inclusions increase and the fatigue characteristics decrease, so that the content is set to 0.05% or less, respectively.

B: 0.05% or less B has an effect of improving hot workability, and is contained as necessary. In order to obtain the effect, the content is preferably 0.004% or more. If the content exceeds 0.05%, inclusions (borides) may increase and the fatigue properties may decrease. Is preferably contained at 0.05% or less.

2. Md value: Md = 500-458 (C + N) -9 (Si + Mn) -1
4Cr-22Ni-21Cu-20Mo = 20 to 60 Md (° C) is 5% of the total when 30% tensile deformation is given.
0% at which martensitic transformation occurs (generally M
d30) is an empirical formula that is corrected based on a series of tests as in the following formula.

Md = 500-458 (C + N) -9 (Si + Mn) -14Cr-22Ni-21C
In order to adjust the amount of martensite remaining in the unrecrystallized portion after u-20Mo final annealing, it is necessary to saturate and stabilize the amount of induced martensite after cold rolling at a working ratio of 60% or more. For that purpose, Md needs to be 20 to 60.

That is, if the Md value is less than 20, the austenite structure is stable, so that it is difficult to transform into a martensite phase by cold rolling. . Therefore, the Md value was defined as 20 to 60. 3. Metallographic structure The structure of the stainless steel sheet of the present invention is 50 to 99% in area%.
The reason why the percentage is limited to the recrystallized structure, the remainder being the unrecrystallized structure substantially leaving the influence of the pre-processing, and the fine precipitate dispersed throughout will be described with reference to some test results.

A hot-rolled steel sheet having the chemical composition of Sample No. (2) shown in Table 1 was rolled at a final rolling reduction of 62%.
The area ratio, hardness, and elongation of recrystallized grains of a steel sheet having a thickness of 0.4 mm which was finally annealed at various temperatures within a temperature range of 750 to 900 ° C. were measured.

[0036]

[Table 1] FIG. 1 shows the relationship between the area ratio of recrystallized grains and the hardness and elongation as a result of the measurement. As is apparent from the figure, the elongation increases while the hardness decreases relatively slowly with the increase in the area ratio of recrystallized grains. Thus, the steel of the present invention achieves both high hardness and high ductility at the same area ratio of 50 to 99%. In addition, when it exceeds 99%, the hardness sharply decreases.

The hardness is measured using a micro-Vickers hardness meter specified in JIS-Z-2244.
It was measured by a tensile test using an Instron type tester specified in IS-Z-2201 and 2241. Further, the recrystallized grain area ratio is an average value calculated by observing the structure using an optical microscope and an electron microscope. Naturally, small crystal grains should be observed with an electron microscope, and large crystal grains should be observed with an optical microscope.

From the steel plate having a thickness of 0.4 mm, a fatigue test piece was prepared and subjected to a fatigue test to determine the relationship between fatigue characteristics and crystal grain size.

FIG. 2 is a diagram for explaining a method of evaluating fatigue characteristics. FIG. 2A shows a method of manufacturing a fatigue test piece.
(B) is a diagram for explaining a test method.

As shown in FIG. 2 (a), the fatigue test piece 3 is obtained by processing a strip-shaped steel plate having a width of 10 mm and a length of 40 mm into a V shape at an angle of 90 degrees by upper and lower molds 1 and 2. is there. The test was performed using a pulsating flat bending tester in which one side of a test piece 3 was fixed by a fixture 4 and the other end was vibrated as shown in FIG.
After repeating bending and unbending ^six times, the amplitude at which the strip-shaped test piece was broken was obtained and evaluated.

FIG. 3 shows the relationship between the results of the fatigue test and the area ratio of the recrystallized structure. As is clear from FIG. 3, the amplitude leading to fracture sharply increases at the same area ratio of 50% or more, and the fatigue characteristics are greatly improved.

Based on the above test results, the recrystallized structure was defined as 50 to 99 area%.

FIG. 4 shows a state of observation of the metal structure of the steel of the present invention after the final annealing by an electron microscope.

The metallographic structure is composed of fine recrystallized grains having a size of 1 to 2 μm or less, which hardly contain defects such as dislocations, and an unrecrystallized portion surrounded by these grains. Further, it is considered that fine Nb-N compounds having a diameter of about 100 nm or less are dispersed as precipitates, and the precipitation suppresses the growth of recrystallized grains. The preferred area ratio of the recrystallized grains is 60 to 95%.

4. Manufacturing method Reduction rate in final cold rolling: Reduction rate in final cold rolling is as follows:
The rolling reduction was set to 60% or more in order to saturate and stabilize the amount of induced martensite after cold rolling. By saturating and stabilizing the amount of induced martensite, the amount of martensite remaining in the unrecrystallized portion after final annealing is also stabilized. Martensite remaining in the non-recrystallized portion has the effect of improving strength. In addition, when the content exceeds 60%, the amount of martensite is saturated, but when the rolling reduction is further increased, the hardness gradually increases, and the fineness is gradually reduced after recrystallization annealing.
%, Recrystallization proceeds more rapidly, making it difficult to control. The preferred rolling reduction is 60 to 70%.

Final annealing: The final annealing is performed, and the processed structure produced by the cold rolling is made into a mixed structure composed of a recrystallized structure having high ductility and an unrecrystallized structure having a martensitic phase in which the remainder is substantially high in strength. Is what you do. The cold-rolled steel sheet having the chemical composition specified in the present invention is subjected to an annealing temperature of 75.
By annealing within a temperature range of 0 to 900 ° C., a mixed structure of a recrystallized structure and an unrecrystallized structure can be obtained. The amount of the recrystallized structure can be controlled by adjusting the annealing temperature and the annealing time. If the temperature is lower than 750 ° C., it takes a long time to obtain a target mixed structure, which is not practical. If the temperature exceeds 900 ° C., recrystallization is completed by short-time annealing, and the desired mixed structure cannot be obtained. Therefore, the annealing temperature was set to 750 to 900 ° C.

When a steel sheet is annealed by applying a tension during annealing, more martensite can be left even with the same recrystallized structure area. It is preferable to perform annealing while annealing.

Using a hot-rolled steel sheet having the chemical composition shown in Sample No. (2) in Table 1, the final cold rolling was performed at a rolling reduction of 64% at which the amount of work-induced martensite was saturated and stabilized, and
It was investigated how the area ratio of the recrystallized grains and the amount of residual martensite change with the change of the annealing temperature between the case where the tension is applied during the annealing and the case where the annealing is performed without applying the tension.

FIG. 5 shows the relationship between the annealing temperature, the area ratio of recrystallized grains, and the amount of martensite in the unrecrystallized portion.

In the absence of tension, recrystallization was partially observed at 750 ° C.
At 0 ° C., almost only recrystallized grains are obtained, and the recrystallization is completed.
In addition, when there is tension, the amount of martensite in the non-recrystallized portion increases as compared to when there is no tension, and the recrystallization temperature is 25 to 50 ° C.
And the completion of recrystallization also exceeds 900 ° C. This is presumably because the transformation of the martensite phase into the austenite matrix with the decrease in volume was suppressed by the tension.

Naturally, the increase in the amount of martensite remaining after annealing increases the strength of the material after annealing as compared to the case with no tension. The tension was 49 N / mm ² per sectional area, and the holding time during annealing was 10 seconds after the test piece reached the set temperature.

The amount of martensite was determined by using an X-ray diffractometer.
It was calculated from the ratio (%) of the intensity of the martensite phase to the integrated intensity of the diffraction peak of the austenite phase and the martensite phase. X-rays used were Cu-Kα rays.

A preferable annealing temperature is 76 in a non-tension state.
0 to 890 ° C. and 760 to 9 when a tension is applied.
30 ° C.

The holding time during annealing to obtain a mixed structure is from 1 to 120 seconds, and when held at a high temperature for a long time, only recrystallized grains are formed. Preferably, from the economic point of view
90 seconds. These finish annealings can also be performed in a continuous annealing line on an industrial scale.

The stainless steel finally annealed by the production method of the present invention may be further subjected to aging treatment, temper rolling and aging treatment in the same manner as in the conventional method.

[0056]

EXAMPLE Stainless steel having the chemical composition shown in Table 1 was melted in a vacuum melting furnace to form a 15 kg ingot, and after hot rolling, annealing, descaling (pickling), cold rolling and annealing were repeated. The final cold rolling was performed at a draft of 62% or more, and after the final annealing, a thin steel sheet having a thickness of 0.4 mm was manufactured. The final annealing was also performed by applying tension at 49 N / mm ² . The holding time of the final annealing was all 10 seconds.

Using the obtained thin steel sheet, the recrystallized grain area ratio, the amount of martensite, the hardness, the elongation, and the fatigue properties were examined.

The fatigue characteristics were measured by the test method described with reference to FIG. 10 ⁶ bends at an amplitude of 2.0 mm,
The evaluation was performed by examining whether or not the processed test piece after breaking was bent. Here, the minimum hardness and elongation required for the spring material are set to Hv300 or more and 10% or more, respectively.

Table 2 shows the manufacturing conditions and test results.

[0060]

[Table 2] As is clear from the table, as shown in (1) -1 to (10) -1, the steel of the present invention has a high hardness exceeding Hv300 and an elongation exceeding 10% at a recrystallized grain area ratio of 50% or more. It is compatible. Also, regarding the fatigue properties, no break after the test was confirmed. On the other hand, (11)-
In Comparative Examples 1 to (14) -2, a recrystallized grain area ratio exceeding 50% may not be obtained in some cases, and even when obtained, the hardness or elongation is inferior. In particular, when the elongation was inferior, breakage in a fatigue test was also confirmed.

[0061]

According to the present invention, it is possible to stably supply a stainless steel sheet having high strength, high fatigue properties and excellent workability, which is suitable as a material for a spring.
Can contribute to thinning etc.

[Brief description of the drawings]

FIG. 1 is a diagram showing the relationship between the area ratio of recrystallized grains and hardness and elongation.

FIG. 2 is a diagram for explaining a fatigue test method.

FIG. 3 is a diagram showing a relationship between a recrystallized grain area ratio and a stress amplitude.

FIG. 4 is a diagram showing an example of an observation state of a metal structure after final annealing.

FIG. 5 is a diagram showing a relationship between an annealing temperature, a recrystallized grain area ratio, and an amount of martensite in an unrecrystallized portion.

 ──────────────────────────────────────────────────続 き Continued on the front page F term (reference) 4K037 EA01 EA02 EA05 EA06 EA09 EA12 EA13 EA15 EA16 EA17 EA18 EA19 EA20 EA21 EA27 EA28 EA36 EB14 FG03 FG10 FJ05 FJ06 JA06

Claims (2)

[Claims] C .: 0.01 to 0.13% by mass, S:
i: 0.1 to 3.2%, Mn: 0.4 to 2.8%, C
r: 13 to 20%, Ni: 3 to 12%, N: 0.1 to
0.2%, Nb: 0.01 to 0.2%, and M
o: 0 to 4%, Cu: 0 to 4%, Al: 0 to 3%, C
a: 0 to 0.05%, rare earth element: 0 to 0.05%,
B: contains 0 to 0.05%, the balance is Fe and impurities, the Md value represented by the following formula is 20 to 60, the recrystallized structure of 50 to 99% in area%, and the balance is substantially A stainless steel sheet characterized by having a non-recrystallized structure and a metal structure composed of fine precipitates. Md = 500-458 (C + N) -9 (Si + Mn) -14Cr-22Ni-21Cu-20Mo 2. A method for producing a steel sheet by subjecting a stainless steel having the chemical composition according to claim 1 to hot rolling, followed by cold rolling and annealing to produce a steel sheet. A rate of 60% or more, and then a final annealing within a temperature range of 750 to 950 ° C.