JPH11229075A - High strength steel excellent in delayed fracture resistance and method of manufacturing the same - Google Patents

Abstract

[PROBLEMS] To provide an inexpensive high-strength steel excellent in delayed fracture resistance and toughness, and a method of manufacturing the same. (1) In a weight ratio, C: 0.12 to 0.4
5%, Si: 0.3% or less, Mn: 0.4 to 3.5%,
B: 0.0005 to 0.04%, Cu: 0.6% or less,
Ni: 6% or less, Cr: 0.8% or less, Mo: 2% or less, Nb: 0.1% or less, V: 0.08% or less, Ti:
0.03% or less, sol Al: 0.08% or less, 7
0% by volume or more is the martensite phase, of which 50%
A high-strength steel in which at least% volume is a martensite phase formed from an unrecrystallized austenite phase, and a method for producing the same.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high-strength steel having excellent delayed fracture resistance and used in, for example, industrial machines and a method for producing the same.

[0002]

2. Description of the Related Art In recent years, steel materials used for industrial machines and the like for the purpose of improving wear resistance and reducing weight have been increasingly required to have higher strength. The most common method of increasing the strength of a steel material by heat treatment is to use a martensite structure.
Martensitic steels containing 0.45% carbon (C) have been utilized. The high strength of the martensite phase is due to lattice distortion caused by supersaturated C atoms in the martensite phase. Since the lattice strain increases as the C content increases, the strength of the martensitic steel increases significantly by increasing the C content. In contrast,
Substitution elements such as Ni and Mn promote the formation of a martensite phase by increasing hardenability, but do not have the effect of increasing the hardness of the martensite phase itself.

[0003] Therefore, in the conventional wear-resistant martensitic steel, the C content is adjusted to a desired hardness, and the above-described hardenability required in accordance with a mass effect mainly determined by the wall thickness is ensured. A required amount of substitutional alloy elements such as Ni and Mn has been included. However, martensitic steel whose hardness is increased only by increasing the C content is extremely susceptible to delayed fracture due to hydrogen and has low toughness, so that it cannot be used as an important member.

For this reason, a large amount of expensive Ni is contained,
A low-carbon ultra-high-strength steel utilizing a high dislocation density in a shear-type reverse transformed austenite phase has been proposed (Japanese Patent Laid-Open No. 6-11 / 1994).
No. 6637). However, it is too expensive for use in industrial machines and the like that require wear resistance, and the wear resistance itself is not so high.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to supply a high-strength steel which is more excellent in delayed fracture resistance than a conventional high-carbon martensitic steel and has a certain degree of toughness at a low cost.

[0006]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies on a method for producing steel having excellent delayed fracture resistance, and as a result, increasing the dislocation density in the martensite phase has increased the number of hydrogen trap sites. It was confirmed that the delayed fracture resistance was improved even with the same strength. In order to obtain this high dislocation density martensite phase, an austenite phase before martensitic transformation (hereinafter referred to as “γ”)
Must be unrecrystallized γ also containing high-density dislocations. Here, “unrecrystallized γ” refers to γ including high dislocation density.
All of the above cases correspond to not only unrecrystallized γ in which γ grains are elongated by rolling, but also γ including high-density dislocations generated by shear-type reverse transformation caused by rapid heating near the Ac ₁ point. The γ generated by this shear-type reverse transformation contains high-density dislocations at the beginning of the reverse transformation, but the dislocations gradually disappear due to coalescence or the like. Therefore, in order to obtain a martensitic phase having a high dislocation density, it is necessary to heat the temperature range of _one or more points of Ac within a short time and to quench before γ is recrystallized.
By this rapid heating, the crystal grains of the shear-type reverse transformed austenite phase are also refined, and as a result, the toughness is also improved.

The martensite phase transformed from the above two types of unrecrystallized γ is fine, and as a result, has high strength, good toughness, and excellent delayed fracture resistance. Also,
In the normal martensite phase, the substitutional alloy element does not contribute to the increase in strength, but in the martensite phase from unrecrystallized γ, Mn, Ni and other alloy elements lower the Ac ₁ point and the Ac ₃ point. This is effective in maintaining a high dislocation density. Therefore, in the present invention, the substitutional alloy element has the effect of increasing the hardness of the martensite phase.

The present invention has been completed on the basis of the above-mentioned matters, and the gist of the present invention resides in the following high-tensile steel and a method for producing the same.

(1) By weight ratio, C: 0.12 to 0.4
5%, Si: 0.3% or less, Mn: 0.4 to 3.5%,
Ni: 6% or less, B: 0.0005 to 0.04%, C
u: 0.6% or less, Cr: 0.8% or less, Mo: 2% or less, Nb: 0.1% or less, V: 0.08% or less, Ti:
0.03% or less, sol Al: 0.08% or less and N:
0.006% or less, the balance being Fe and unavoidable impurities, and 70% by volume or more of the metal structure is a martensite phase or a tempered martensite phase, and 5% of the martensite phase or the tempered martensite phase.
A high-strength steel excellent in delayed fracture resistance, which is a martensite phase or a tempered martensite phase in which 0% by volume or more is formed from an unrecrystallized austenite phase (referred to as [Invention 1]).

(2) C: 0.12-0.4 by weight ratio
5%, Si: 0.3% or less, Mn: 0.4 to 3.5%,
Ni: 6% or less, B: 0.0005 to 0.04%, C
u: 0.6% or less, Cr: 0.8% or less, Mo: 2% or less, Nb: 0.1% or less, V: 0.08% or less, Ti:
0.03% or less, sol Al: 0.08% or less and N:
It includes 0.006% or less, after heating at the balance Fe and steel consisting of unavoidable impurities to a heating temperature above the Ac ₁ point from a temperature of less than Ac ₁ point heating rate 4 ° C. / s or higher cooling rate 1
A method for producing a high-strength steel excellent in delayed fracture resistance, which is cooled to a temperature range of 200 ° C. or less at 0 ° C./s or more (referred to as [Invention 2]).

[0011] (3) was subjected to rolling in the temperature range below Ac ₁ point, heating to a temperature in excess of Ac ₁ point from a temperature of less than Ac ₁ point, or during the heating above Ac ₁ point or after heating The manufacturing method (2) in which rolling is performed (referred to as [Invention 3]).

(4) The method of (2) or (3), which is further subjected to a tempering treatment (referred to as [Invention 4]).

The “unrecrystallized γ” in the above (1) corresponds to any γ including a high dislocation density. Not only the unrecrystallized γ in which γ grains are elongated by rolling but also _one point of Ac Unrecrystallized γ generated by shear-type reverse transformation that occurs when the vicinity is rapidly heated also corresponds to this.

In the above (2), the holding time at the heating temperature may be zero or shorter.

The "heating temperature exceeding the Ac ₁ point" in the above (2) is only required to exceed the Ac ₁ point, and may be ₃ Ac or more points. Ac ₁ and Ac ₃ can be easily obtained by calculation from the chemical composition of steel, and there is no great difference from the actually measured values. Ac ₁ and Ac ₃ are determined from the chemical compositions according to the following equations.

Ac ₁ (° C.) = 751-26.6C + 17.6Si-11.6Mn-22.9Cu
-23Ni + 24.1Cr + 22.5Mo-39.7V-5.7Ti + 233Nb-169A Ac ₃ (℃) = 937-476.5C + 56Si-19.7Mn-16.3Cu-26.6Ni-4.9Cr
+ 38.1Mo + 124.8V + 136.6Ti-19Nb + 198

[0017]

Next, the reason for limiting the present invention as described above will be described.

1. Chemical composition C: 0.12 to 0.45% C is extremely effective in increasing the strength of the martensite phase,
Also, it is effective to shift the Ac ₃ point to the low temperature side and maintain the unrecrystallized γ. For these reasons, C is 0.12%
Above. However, if the content exceeds 0.45%, the toughness of the steel is deteriorated, and the residual γ after quenching is reduced.
Remains, and the strength is rather reduced, so 0.4
The upper limit is 5%.

Si: 0.3% or less Si is an element effective for deoxidation, but if it exceeds 0.3%, the toughness of the heat affected zone is reduced, so the upper limit is made 0.3%. The content may be substantially 0. However, when the content of Si is set to 0, the loss of Al increases during Al deoxidation. Therefore, the content is usually such that Si deoxidation is performed and the content remains, for example, 0.01%. The degree is desirable as the lower limit.

Mn: 0.4 to 3.5% Mn is an element effective for lowering the transformation point, and is effective for increasing the strength of the martensite phase generated from unrecrystallized γ.
For that purpose, 0.4% or more is required. But,
If it exceeds 3.5%, residual γ is generated and the strength is reduced, so the upper limit is made 3.5%.

Ni: 6% or less Ni is effective in improving toughness and lowering the _three points of Ac. Therefore, it is desirable to include Ni particularly in the case of high strength and high toughness. 1% to clearly lower Ac ₃ points
It is desirable to make the above. However, when the content exceeds 6%, the residual γ increases and the strength is reduced.

B: 0.0005 to 0.04% B is effective not only to improve hardenability but also to suppress recrystallization of unrecrystallized γ during reverse transformation due to precipitation of Nb boride. 0.0005% or more is required. However, 0.0
If it exceeds 4%, the toughness deteriorates, so the upper limit is 0.04%.
And A desirable range is 0.002 to 0.02%.

Cu: 0.6% or less Cu may not be contained. Since Cu is effective in increasing the strength, it may be added particularly in order to further increase the strength. However, if it exceeds 0.6%, the toughness is degraded. When securing the strength without deteriorating the toughness, the content is preferably set to 0.2 to 0.4%.

Cr: 0.8% or less Cr may not be added. In the case where the strength is included for the purpose of lowering the transformation point, the content is preferably set to 0.3 to 0.7% in consideration of the balance with toughness. 0.8%
If it exceeds, the toughness deteriorates, so the upper limit is made 0.8%.

Mo: 2% or less Mo may not be contained. However, Mo is effective in suppressing the recrystallization of unrecrystallized γ, so it is desirable to add Mo.
However, if it exceeds 2%, the toughness deteriorates, so the upper limit is made 2%.

Nb: 0.1% or less Nb may not be contained. Nb is an element extremely effective in preventing recrystallization of unrecrystallized γ, and is desirably added. If the content is less than 0.01%, the effect is unclear. However, if it exceeds 0.1%, the toughness deteriorates.
%.

V: 0.08% or less V may not be added. However, to improve hardenability, 0.01%
It is desirable to contain about 0.05%. 0.08%
If it exceeds, the toughness deteriorates, so the upper limit is made 0.08%.

Ti: 0.03% or less Ti is used in a trace amount to fix N as TiN to refine crystal grains, and at the same time, to secure hardenability and the amount of solute B required for generation of shear-type reverse transformation γ. For this reason, it is desirable to contain 0.005% or more. In the case of Nb-containing steel, a small amount of Ti is effective in suppressing cracks on the surface of the continuously cast slab promoted by Nb. This effect is exhibited at 0.005% or more, but when it exceeds 0.03%, the toughness deteriorates, so the upper limit is made 0.03%.

Sol Al: 0.08% or less Sol Al refers to Al other than Al that forms an oxide by a deoxidation reaction, and is composed of solid solution Al and Al as a nitride. In the present invention, sol Al may be intentionally left, or only an oxide may be formed by a deoxidation reaction.
Al may not be included. That is, sol Al may be substantially zero. However, since the sheet thickness is large, the total rolling reduction of rolling [{(slab thickness-product sheet thickness) / slab thickness} × 100
(%)] Cannot be taken to 50% or more, and if pinhole crimping during solidification cannot be expected only under pressure, sol A in the solidified steel
It is desirable that 1 is left as 0.001% or more.
If the sol Al in the steel after solidification is less than 0.001%, the amount of Al necessary to prevent generation of pinholes by bonding with oxygen during solidification is insufficient.

N: 0.006% or less N combines with Al to form a nitride and is effective in refining crystal grains. However, excessive N impairs the toughness of the welded portion.
The upper limit is made 0.006%. Further, N has the effect of improving weldability. If this effect is expected, use 0.
You may make it contain 0015% or more.

Inevitable impurities: Of the inevitable impurities, P is desirably 0.01% or less. If it exceeds 0.01%, not only P but also C, Mn, S and the like are concentrated in the segregated portion, and the hardness is increased to deteriorate toughness and weldability.

S is desirably 0.007% or less. If it exceeds 0.007%, coarse MnS appears in the segregated portion.
Is generated, and serves as a starting point for welding low-temperature cracking and a starting point for hydrogen defects.

It is desirable that other impurities be reduced to levels obtained by conventional refining.

2. Metal Structure The volume ratio of the martensite phase in the metal structure is set to 70% or more in order to secure strength. The upper limit is not particularly defined, but may be a 100% martensite phase. The martensite phase transformed from unrecrystallized γ in the martensite phase must be at least 50% by volume. The upper limit is 100
% Is desirable.

The martensite phase can be easily distinguished from other phases, for example, a bainite phase, under an optical microscope or an electron microscope. Martensite transformed from unrecrystallized γ can be determined by measuring the dislocation density with an electron microscope. It is also possible to measure the half width of the X-ray diffraction line.

3. Heat treatment ([Invention 2]) Next, the heat treatment conditions will be described.

By rapidly heating the steel having the above composition between the Ac ₁ point where the reverse transformation of the austenite phase occurs and the heating temperature, austenite grains having a high dislocation density and finer than usual can be obtained. In order to improve the delayed fracture resistance, it is desirable to heat the entire thickness at this heating rate. However, even if only the surface layer is rapidly heated, the effect can be obtained.
The surface layer portion here refers to the front and back portions of about 10% of the plate thickness.

The heating rate is 5% of the above-mentioned martensite phase.
In order to transform 0% by volume or more from unrecrystallized γ, the temperature is set to 4 ° C./s or more so as not to allow time for recrystallization of γ to proceed. The higher the rate of temperature rise, the better, preferably 10 ° C./s or more, more preferably 20 ° C./s or more. Therefore, the upper limit of the heating rate is not particularly set.

The heating rate may be at the center of the thickness,
The above-mentioned surface layer part may be sufficient.

The heating temperature is desirably equal to or higher than the intermediate temperature between the Ac ₁ point and the Ac ₃ point in order to secure the volume ratio of the reverse transformed austenite phase. In order to have a uniform γ structure,
More desirably, it is desirable that Ac be ₃ points or more. However, the highest average hardness is obtained by martensitic transformation from unrecrystallized γ when heating is performed just below the Ac ₃ point. In order to prevent recrystallization and coarsening of the γ crystal grains, the upper limit of the heating temperature is desirably equal to or lower than (Ac ₃ points + 80 ° C.). The heating only needs to reach the above-mentioned heating temperature in the center of the thickness of the steel. At the heating temperature, it is desirable not to take a holding time in order to prevent the progress of recrystallization of γ generated by the shear type reverse transformation.

The rapid heating described above can be realized by direct current heating, high-frequency heating using a coil having a rectangular cross section, or the like. In the case of high frequency heating, it is suitable for rapid heating of the surface layer.

Since it is necessary to prevent the formation of a ferrite phase even at the center of the thickness, the cooling rate after heating is to be cooled to 200 ° C. or less at a cooling rate of 10 ° C./s or more. Although the upper limit of the cooling rate at the center of the thickness is not particularly set, it is preferably set to, for example, about 70 ° C./s. Although the lower limit of the cooling stop temperature is not particularly set, the temperature is preferably set to about 100 ° C. in order to prevent delayed destruction when the hydrogen content is high.

4. Rolling Conditions ([Invention 3]) In the above [Invention 2], in order to enhance the effects of miniaturization and increase in dislocation density, a material obtained by rolling in a low temperature range is used as a heat treatment material. The rolling process in this low temperature range is Ac ₁
It is effective to implement it below the point. Although the lower limit of the rolling temperature is not particularly set, it is preferably set to about 550 ° C. in consideration of the rolling mill's performance, flatness and the like. The working ratio at this time is 10% in terms of rolling reduction in order to sufficiently exhibit the above effects.
% Is desirable. The upper limit is not particularly set, but is preferably 35% or less in order to secure flatness.

5. Tempering ([Invention 4]) If necessary, a tempering treatment may be performed at an appropriate temperature of ₁ point or less of Ac to adjust the balance of strength and toughness.
In order to prevent the dislocation density introduced into the martensite from unrecrystallized γ from unnecessarily lowering, the tempering temperature is set at 65 ° C.
It is desirable that the temperature be 0 ° C. or lower.

[0045]

EXAMPLES Next, the effects of the invention 1 will be described with reference to examples (invention 1 obtained by applying the invention 2). As the test steel,
Thirteen types of steel were used by melting, casting and hot rolling by a conventional method. The plate thickness was 15 to 50 mm.

Table 1 shows the chemical compositions of these 13 types of steel, and the Ac ₁ point, Ac ₃ point and Ceq.

[0047]

[Table 1]

Table 2 shows the heat treatment conditions applied to the above steel sheet and the resulting metal structure.

[0049]

[Table 2]

Test pieces were taken from the center of the thickness of these steel sheets, and the following tests were performed.

Tensile test: Test piece JIS Z 2201 (longitudinal direction = rolling direction), according to method JIS Z 2241 Impact test: Test piece JIS Z 2202 (longitudinal direction = width direction), evaluation: Impact value at -20 ° C Delayed fracture test: The specimen was immersed in 3% NaCl at a potential difference of 1.5 V for 200 hours, and the breaking stress was measured by an SSRT test (Slow Strain Rate Tensile Test).

FIG. 1 shows a test piece of this SSRT test.
A pass can be determined if the rupture stress in this test is at least １ ／ of the room temperature tensile strength.

In Test No. 6, which is a comparative example, the composition of the steel is within the range defined in the present invention, but the volume rate of the unrecrystallized portion of the martensite phase is as small as 8% because of the low heating rate. The breaking stress in the delayed fracture test was 1
/ 2, and the toughness was low. In Comparative Example 7, the required volume ratio of the martensite phase was not obtained because the cooling rate was low, and all of the strength, delayed fracture resistance, and toughness were poor. In Comparative Example 8, the toughness was relatively good because C was low and the heating rate was low, but the strength and the delayed fracture resistance deteriorated. In Comparative Example 9, since C was too high, the strength was good and the delayed fracture resistance was relatively good, but the toughness was extremely poor. Comparative Example 10
The strength was good due to the high Si and the low heating rate, but the delayed fracture resistance and toughness were low. In Comparative Example 11, since Mn was low and the rate of temperature rise was low, excellent strength was obtained, but delayed fracture resistance was slightly lowered and toughness was deteriorated. In Comparative Example 12, since the amount of Cu was excessive and the rate of temperature rise was low, the strength was high, but the delayed fracture resistance was slightly reduced, and the toughness was significantly deteriorated. In Comparative Example 13, since the Ni content was excessive and the rate of temperature rise was high, the delayed fracture resistance was slightly deteriorated, and the toughness was poor. Comparative Example 1
Sample No. 4 resulted in inferior delayed fracture characteristics and toughness due to excessive Mo and low heating rate.

On the other hand, the breaking stress of steel within the definition range of the present invention was １ ／ of the room temperature strength in all test numbers.
It was over. In the tensile test, 1400M
A tensile strength of Pa or more is obtained, and in an impact test-
An impact value of 40 J or more was obtained at 20 ° C.

[0055]

According to the high-strength steel and the method for producing the same according to the present invention, high-strength steel excellent in delayed fracture resistance and toughness can be provided at low cost, and can contribute as a material for industrial machines and the like. Be expected.

[Brief description of the drawings]

Fig. 1 SSRT test (Slow Strain Rate Tensile Tes)
The test piece of t) is shown.

Claims (4)

[Claims] (1) a weight ratio of C: 0.12 to 0.45%,
Si: 0.3% or less, Mn: 0.4 to 3.5%, Ni:
6% or less, B: 0.0005 to 0.04%, Cu: 0.
6% or less, Cr: 0.8% or less, Mo: 2% or less, N
b: 0.1% or less, V: 0.08% or less, Ti: 0.0
3% or less, sol Al: 0.08% or less and N: 0.0
And at least 70% by volume of the metal structure is a martensite phase or a tempered martensite phase, and at least 50% by volume of the martensite phase or the tempered martensite phase. Is a martensitic phase formed from an unrecrystallized austenite phase or a tempered martensitic phase, characterized by having excellent delayed fracture resistance. 2. C: 0.12 to 0.45% by weight,
Si: 0.3% or less, Mn: 0.4 to 3.5%, Ni:
6% or less, B: 0.0005 to 0.04%, Cu: 0.
6% or less, Cr: 0.8% or less, Mo: 2% or less, N
b: 0.1% or less, V: 0.08% or less, Ti: 0.0
3% or less, sol Al: 0.08% or less and N: 0.0
Wherein 06% or less, after heating at the balance Fe and steel consisting of unavoidable impurities to a heating temperature above the Ac ₁ point from a temperature of less than Ac ₁ point heating rate 4 ° C. / s or higher cooling rate 10 ° C. / s
A method for producing a high-strength steel having excellent delayed fracture resistance, characterized by cooling to a temperature range of 200 ° C. or lower as described above. 3. After rolling in a temperature range of not more than _one point of Ac,
The process of claim 2 where the c ₁ point less than the temperature by heating to a temperature above _a point Ac, or characterized by applying heat during or after heating rolling over Ac ₁ point. 4. The method according to claim 2, wherein a tempering treatment is further performed.