JP2002235156A - High strength steel having excellent stress corrosion cracking resistance and production method therefor - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce an inexpensive high strength stainless steel which has a strength level of the Vickers hardness >500 Hv, excellent toughness and stress corrosion cracking resistance, and to provide a production method therefor. SOLUTION: A steel having a chemical composition containing, by mass, 12 to 16% Cr, 0.1 to 0.3% Ni, 0.07 to 0.13% C, 0.04 to 0.085% N and <=0.1% Cu is subjected to primary quenching treatment. The steel is next cold-rolled, and is thereafter subjected to secondary quenching treatment to form a metallic structure containing a martensitic phase of, by volume, 50 to 75% and a ferritic phase of 50 to 25% and having an average grain size of <=10 μm.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a steel having excellent stress corrosion cracking resistance, high strength and high toughness, such as molded springs for automobiles and looms, and a method for producing the same.

[0002]

2. Description of the Related Art As a steel strip having corrosion resistance and excellent stress corrosion cracking resistance, a quenching / tempering material of martensitic stainless steel such as SUS420 is used.
Although a heat-treated steel strip of J2 is known, there is a problem that productivity of this heat treatment line is low and production cost is high.

To solve such a problem, Japanese Patent Laid-Open No.
Japanese Patent Application Publication No. 265907 discloses a tough steel excellent in stress corrosion cracking resistance and characterized by high productivity, which can utilize ordinary stainless steel production equipment as it is. However, the maximum strength of the steel is about Hv420, and SUS4
Considering that the quenching / tempering material of 20J2 can cover the strength region up to Hv500, further improvement in strength is desired.

[0004]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and has as its object the purpose of the present invention.
It is an object of the present invention to provide an inexpensive high-strength stainless steel having a toughness and stress corrosion cracking resistance excellent at a strength level exceeding 500 and a method capable of manufacturing this steel using an existing manufacturing line.

[0005]

The above objects of the present invention have been attained by a tough steel having the following constitution and excellent stress corrosion cracking resistance, and a method for producing the same. (1) In mass%, Cr: 12 to 16%, Ni: 0.1
0.3%, C: 0.07-0.13%, N: 0.04
0.085%, Cu: 0.1% or less of chemical composition, 50 to 75% by volume of martensite phase and 50 to 75%
25% ferrite phase having an average crystal grain size of 10
A tough steel excellent in stress corrosion cracking resistance, having a metal structure of μm or less.

(2) Cr: 12 to 16% by mass%
Ni: 0.1 to 0.3%, C: 0.07 to 0.13%,
First quenching treatment is performed on steel having a chemical composition of N: 0.04 to 0.085% and Cu: 0.1% or less, then cold rolling is performed, and then second quenching treatment is performed. Go to
50% to 75% martensite phase and 50% to 50% by volume
Having a ferrite phase of 25% and an average crystal grain size of 10 μm
A method for producing a tough steel excellent in stress corrosion cracking resistance, characterized by having a metal structure of not more than m.

(3) A method for producing a tough steel excellent in stress corrosion cracking resistance, characterized by subjecting the quenched steel obtained in (2) to low-temperature annealing at 200 ° C. to 550 ° C.

(4) A method for producing a tough steel excellent in stress corrosion cracking resistance, characterized by subjecting the quenched steel obtained in (2) to cold working.

(5) The quenched steel obtained in (2) is subjected to low-temperature annealing at 200 ° C. to 550 ° C. after cold working, and is excellent in stress corrosion cracking resistance. Method of manufacturing tough steel.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have studied the heat treatment of ferritic or martensitic stainless steel having the ability to form an austenite phase at high temperatures. To ensure stress corrosion cracking resistance, the addition of Ni must be suppressed. Japanese Patent Application Laid-Open No. H10-265907 points out that the heat treatment in the two-phase region of the austenite phase and the ferrite phase causes the crystal grain coarsening and the decrease in toughness. It has been proposed that two quenching steps sandwiching the rolling step are effective for grain refinement and securing toughness.

In the present invention, a steel having a specific chemical composition is cold-rolled to a predetermined thickness, and then a ferrite ferrite adjusted to a grain size of 10 μm or less is subjected to two quenching treatments including a cold-rolling step. Phase (α phase) and martensite phase (α 'phase) (hereinafter referred to as (α + α') two-stage two-phase treatment), and then a low-temperature annealing treatment for toughness and high elasticity , Or through a step of performing a low-temperature annealing treatment after performing a predetermined cold rolling after the two-stage two-phase treatment, a tough steel excellent in stress corrosion cracking resistance and toughness of Vickers hardness Hv 380-550. can get.

The steel of the present invention contains, by mass%, Cr: 12 to 16
%, Ni: 0.1 to 0.3%, C: 0.07 to 0.13
%, N: 0.04 to 0.085%, Cu: 0.1% or less, and the metal structure after the two-stage two-phase treatment has an average crystal grain size of 50 to 75% by volume. Has a martensite phase of 10 μm or less and a ferrite phase having an average crystal grain size of 50 to 25% by volume and 10 μm or less.

Hereinafter, the steel of the present invention and a method for producing the same will be described in detail.

(Chemical composition of steel) In mass%, Cr: 12-
16%, Ni: 0.1-0.3%, C: 0.07-0.
13%, N: 0.04 to 0.085%, Cu: 0.1%
And a chemical composition in which the volume fraction α ′ (%) of the martensite phase represented by the following formula (I) is 50 to 75%.

Α ′ (%) = 407.4% C−46.53% N + 88.69% Mn− 15.75% Cr + 22.31% Ni-11.15% Si + 8.73% Cu-1 1.64% Mo + 227.05 (I)

Ni: 0.1 to 0.3% In the present invention, in order to secure stress corrosion cracking resistance, Ni
The amount added is limited to 0.1-0.3%. Within this composition range, the crack formation time in a 42% magnesium chloride corrosion test under a load stress condition of an elastic limit is 100%.
More time.

Cr: 12 to 16% As described above, in the present invention, the ability to form a γ phase in a high Cr region is limited because the amount of Ni added is limited as much as possible. Therefore, the upper limit of the amount of added Cr is set to 16%. Also,
The lower limit was set to 12% in consideration of corrosion resistance.

C: 0.07 to 0.13%, N: 0.04
0.085% Both elements are α ′% after (α + α ′) two-stage two-phase treatment
% C-0.114% N = f (Hv (Q), da, df,
…) In the present invention, the upper limit of the amount of added nitrogen is set to 0.08% from the viewpoint of commercial production. Further, the reason why the lower limit value of the nitrogen addition amount is set to 0.04% is that the strength Hv (Q) after the two-stage two-phase treatment is set to Hv (Q)> 380.
The concentration range of carbon is linked to the amount of nitrogen.

Cu: 0.1% or less Cu is an element that deteriorates stress corrosion cracking resistance similarly to Ni, and the effect is half that of Ni. In the present invention, the upper limit is 0.1 to mean that there is no intentional addition of Cu.
%.

(Metal Structure) The steel of the present invention has a martensite phase matrix (75 to 50% by volume) having an average crystal grain size of 10 μm or less and an average crystal grain size of 25 to 50% by volume ratio being 10 μm. It has the following ferrite phase. Controlling the average crystal grain size to 10 μm or less and restricting the amount of martensite transformation after (α + α ′) two-stage two-phase treatment to 75% or less improves the toughness of the matrix and the buffer effect of the remaining ferrite phase. For this reason, the production can be efficiently performed using ordinary stainless steel production equipment, such as separating the quenching treatment and the low-temperature annealing treatment, or performing a cold rolling treatment in the meantime. On the other hand, if the amount of martensite transformation is less than 50%, sufficient strength cannot be obtained. In the present invention, the term “volume ratio” refers to an area ratio at an arbitrary cut surface.

(Method of Manufacturing Steel) Next, a method of manufacturing steel having the above-described chemical composition and metal structure will be described. First,
A steel having a chemical composition within the above set range in which the metal structure is a ferrite phase is cold-rolled to a predetermined thickness, and (α +
α ′) Provide for two-stage two-phase treatment. Here, it is not necessary to pay special attention to the cold rolling reduction before the two-stage two-phase treatment because the subsequent rolling reduction does not particularly affect the properties.
It is about 0 to 80%. The biphasic treatment temperature is the same for both the first quenching treatment and the second quenching treatment, and as shown in FIG.
Is preferred.

The control of the crystal grain size is interposed between the first quenching process and the second quenching process in the two-stage two-phase treatment (hereinafter, also referred to as “between the two-stage two-phase treatment”). It is performed by setting the rolling ratio in the cold rolling process. Here, two-stage two phase average crystal grain size of the martensite phase after treatment (d _m; equal to the average grain size d _a two-phase treatment temperature range austenite phase) and ferrite phase average grain diameter ( d _f ) are substantially equal, and have a relationship represented by the following formula (II) with the true rolling strain R interposed during the two-stage two-phase treatment. _{d f (μm) = d m} (μm) = 2.24 (1 / R) +3.70 (II) where a R = Ln {1 / (1 -r)}, r is two-stage two-phase It shows the rolling ratio in the cold rolling process between the oxidizing treatments.

From the viewpoint of toughness, the average crystal grain size is 10 μm
From the formula (II), it is necessary that the rolling ratio in the rolling step between the two-stage two-phase processing be 30% or more. The upper limit of the rolling reduction is determined by the thickness of the product and economic reasons, but is generally 80% or less.

Next, (a) low-temperature annealing at 200 to 550 ° C. (hereinafter referred to as Tempered finishing), or (b) cold-working, followed by low-temperature annealing at 200 to 550 ° C. ( Hereinafter, referred to as R & T finishing).

The low-temperature annealing is performed for the purpose of toughening and high elasticity. In the present invention, the preferred annealing temperature is 2
00-550 ° C. The effect of the annealing treatment can be evaluated by observing the recovery behavior of the Kb value (elastic limit), and FIG. 2 shows an example of the case where the aging time is as short as 2 minutes. In the case of the short-time aging in FIG. 2, the temperature range of 450 to 550 ° C. is a suitable low-temperature annealing condition, and the Kb value is 100
It has recovered to a high level of 0 N / mm ² . The preferred low-temperature annealing treatment temperature for the long-term aging of 30 minutes or more shifts to the low-temperature side, but does not fall below 200 ° C. 200 ° C
The Kb value below 400 is not more than 400 N / mm ² . Cold rolling is performed in order to increase the strength, and the rolling reduction is preferably about 20 to 70%. (A) Tempered finish and (b) R &
The T finish is appropriately selected depending on the desired strength value. The former is intended for high-strength steel of Hv 380-420 level,
In the case of a tough steel having higher strength, the latter step is performed.

The flexural toughness of the Tempered finish and the R & T finish was evaluated by a 90 ° V-bending test, and the results are shown in Table 1. From this, Tempered finish and R & T finish are
It was found that both exhibited high bending toughness despite high strength. Note that (R ₀ / t) in the table is a value obtained by dividing the minimum bending radius R ₀ by the plate thickness t, and is a dimensionless quantity indicating bending toughness.

[0027]

[Table 1]

[Determination of Vickers hardness after (α + α ') two-stage two-phase treatment] Here, in order to obtain a tough steel having a desired strength level, (α + α') two-stage two-phase two-phase treatment A method for calculating the Vickers hardness of steel will be described.

The relationship between the rolling reduction and the Vickers hardness increment in the cold rolling after the (α + α ′) two-stage two-phase treatment is expressed by equation (1), and the Vickers hardness increment in the subsequent low-temperature annealing treatment is expressed by equation (2). Depends on.

ΔHv (R) = 58.5 LogR + 93.5 (1) ΔHv (A) = 20 (2) where ΔHv (R): Vickers by rolling after two-stage two-phase processing Hardness increment R: True rolling strain = Ln {1 / (1-r)} r: Rolling rate ΔHv (A): Vickers hardness increment in low-temperature annealing treatment Therefore, for example, Vickers hardness H after R & T finish
The hardness Hv (Q) of the steel with v = 500 after (α + α ′) two-stage two-phase treatment is ΔHv (R) in equation (1), where _{r = 0.5, r = 0.5} And Equation (2) are obtained as Equation (3).

Hv (Q) = 500−ΔHv (R) _{r = 0.5−} ΔHv (A) = 500−84.2−20 = 395.8 (3)

[(Α + α ′) Vickers Hardness and Chemical Composition after Two-Stage Dual-Phase Treatment] Next, a method for determining a chemical composition for obtaining a two-stage (α + α ′) duplex stainless steel having a desired Vickers hardness will be described. I do. The Vickers hardness Hv (Q) of the two-step (α + α ′) duplex stainless steel can be calculated from the average particle size of the α phase and α ′ phase, their volume fraction and chemical composition, and (4)
Follow the formula.

[0033] _{Hv (Q) = 0.3 {V} f σ y F + (1-V f) σ y M} ... (4) where, _{V f:} volume fraction of the ferrite phase _1-V f = V _M : Volume ratio of martensite phase = 4.074% C-0.4653% N + 0.8869% Mn-0. 1575% Cr + 0.2231% Ni- 0.1115% Si + 0.0873% C u-0.1164% Mo + 2.2705 ... (5) σ y F: Strength of ferrite phase ^{(N / mm 2) = 174} + 250% P + 26.8 % Si + 61% Ni + 64.8 % Mo + 21 (d f) -1/2 ... (6) d f: average ferrite grain size (mm) σ _y ^M: strength of martensite phase ^{(N / mm 2) = σ} y M ( 0) + σ _y ^A (7) σ _y ^M (0): Martensite proof stress as Fe—CN system = 3.33 × 10 ³ (% C +% N) ^0.425 (8) σ _y ^A : Strength increase due to solid solution strengthening and grain size strengthening of austenite phase = 390% C + 710% N + 21.6% Si + 6.3% Cr-2. 6% Ni-6.4% Cu- 1.1% Mn + 66 + 8.6 (d a) -1/2 ... (9) d a: average austenite grain size (mm) (4) determining the chemical composition required from equation Table 2 shows the Vickers hardness H after the (α + α ′) two-stage two-phase treatment.
1 shows an example of a chemical composition necessary for obtaining v400. It can be determined by using the designated conditions and preconditions shown in Table 2 in the equation (4). Table 2
As can be seen from the results shown in the above, a significant feature is that the composition becomes high nitrogen based.

[0034]

[Table 2]

In the present invention, the Vickers hardness after the (α + α ′) two-stage two-phase treatment is Hv 380-420, the α ′ ratio is 50-75%, and the average grain size is 7 μm (two-stage two-phase treatment). (Rolling ratio between treatments: 50%) and (I) based on the above formulas (1) to (4) and the deoxidation conditions during steelmaking.
Solving the equation gives the overall chemical composition. A particularly preferred chemical composition range of the present invention within the above set range is 13% Cr ferritic stainless steel in Table 3.

[0036]

[Table 3]

The relationship between the cold rolling ratio and the hardness of the two-stage dual phase-treated steel after the cold rolling, and the cold rolling ratio of the steel aged at 500 ° C. for 2 minutes after the cold rolling. FIG. 3 shows the relationship between the hardness. The relationship between the cold rolling reduction and hardness after the two-stage two-phase treatment is as follows:
There is a relationship similar to the above-mentioned formula (1), and the hardness increase due to age hardening is ΔHv = 20 to 3 in the region where the rolling reduction is 20% or more.
Between 0.

[0038]

EXAMPLES The resistance to stress corrosion cracking of the steel of the present invention will be described below with reference to examples. However, the present invention is not limited to this embodiment.

Materials of each composition shown in Table 4 were melted in vacuum.
Ingot casting → hot rolling (finished plate thickness: 5mm) →
Pickling → batch annealing (750 ° C × 2 hours) → cold rolling (finished plate thickness: 2.6 mm, rolling ratio: 50%) → 1st
Times (α + α ') two-phase treatment (1050 ° C x 2 minutes) →
Cold rolling (finished thickness: 1.3 mm, rolling ratio: 50%)
→ 2nd (α + α ') two-phase treatment (1050 ° C × 2 minutes) → Cold rolling (finished sheet thickness: 0.57mm / rolling ratio 5)
6%) → R & T finishing step of low-temperature annealing (500 ° C. × 2 minutes), and subjected to a boiling 42% magnesium chloride corrosion test. The stress corrosion cracking resistance was evaluated at a critical load stress that can withstand 100 hours, and the results are shown in Table 5 together with the R & T finish hardness.

[0040]

[Table 4]

[0041]

[Table 5]

The steel of the present invention has been subjected to (α + α ') two-stage two-phase treatment.
Cold-rolled at a rolling rate of 56%,
Hv> 5 with R & T finishing that performs low-temperature annealing treatment for 2 minutes at
00 and still cause stress corrosion cracking
Field load stress is 1360N / mm ²And the elastic limit of the steel type
Minute level to cover. That is, the steel of the present invention has Hv
Avoid stress corrosion cracking practically in the strength range over 500
It was shown to be able to.

[0043]

As described above, according to the present invention, H
It is possible to provide an inexpensive high-strength stainless steel which aims at a strength level exceeding v500 and is excellent in toughness and stress corrosion cracking resistance, and a method for producing the same.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between a two-phase treatment temperature and hardness in a first quenching treatment.

FIG. 2 is a graph showing a relationship between a heating temperature and Kb in a low-temperature annealing process.

FIG. 3 is a graph showing a relationship between true rolling strain after two-stage two-phase treatment and hardness after low-temperature annealing treatment.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Koji Kobayashi, Inventor Nippon Metal Co., Ltd. 4-1-1, Funatari, Itabashi-ku, Tokyo (72) Inventor Masaru Tamura 3-6-1, Kamiya, Kita-ku, Tokyo Japan Metal Co., Ltd. F term (reference) 4K032 AA04 AA13 AA14 AA16 AA19 AA21 AA23 AA31 CG01 CG02 CH06 CL01 CL02 CM01

Claims (5)

[Claims] Claims: 1. A mass% of Cr: 12-16%, Ni:
0.1-0.3%, C: 0.07-0.13%, N:
Average crystal grain size having a chemical composition of 0.04 to 0.085%, Cu: 0.1% or less, and having a volume fraction of 50 to 75% martensite phase and 50 to 25% ferrite phase. A tough steel excellent in stress corrosion cracking resistance, characterized by having a metal structure of 10 μm or less. 2. In mass%, Cr: 12 to 16%, Ni:
0.1-0.3%, C: 0.07-0.13%, N:
First quenching treatment is performed on steel having a chemical composition of 0.04 to 0.085% and Cu: 0.1% or less, then cold rolling is performed, and then second quenching treatment is performed. And having a metal structure having a martensite phase of 50 to 75% by volume and a ferrite phase of 50 to 25% by volume and having an average crystal grain size of 10 μm or less. Method of manufacturing tough steel. 3. The quenched steel obtained in claim 2,
A method for producing a tough steel having excellent resistance to stress corrosion cracking, comprising performing a low-temperature annealing treatment at 200 ° C to 550 ° C. 4. The quenched steel obtained in claim 2,
A method for producing a tough steel excellent in stress corrosion cracking resistance, characterized by performing cold working. 5. The quenched steel obtained in claim 2,
A method for producing a tough steel excellent in stress corrosion cracking resistance, comprising performing low-temperature annealing at 200 ° C. to 550 ° C. after cold working.