JP2006249458A - High strength steel with excellent delayed fracture resistance and method for producing the same - Google Patents

Abstract

The present invention provides a high-strength steel excellent in delayed fracture resistance with a suitable tensile strength of 1500 MPa or more for automobiles and various industrial machine parts, and a method for producing the same.
SOLUTION: In mass%, C: more than 0.4 to 0.6%, Si: less than 0.2%, Mn: 0.1 to 1.0%, Ni: more than 5 to 12%, sol.Al: 0.01 to 0.1%, Ti: 0.001 to 0.1%, N: 0.02% or less, P: 0.01% or less, S: 0.01% or less, Cu: 0.1-1%, Cr: 0.1-1%, Mo: 0.02-1%, B: 0.0001-0.005% , Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.005%, or one or more of them, the balance being Fe and inevitable impurities, preferably the prior austenite grain size is 15 μm or less.
[Selection figure] None.

Description

  The present invention relates to high-strength steel and a method for producing the same, and more particularly to a steel having a tensile strength of 1500 MPa or more and excellent delayed fracture resistance, and suitable for automobiles and various industrial machine parts.

  There is a growing need for high-strength steel to reduce the weight and performance of automobiles and various industrial machines. On the other hand, increasing the strength of steel materials is a concern for delayed fracture characteristics. Bolts used for automobiles, various industrial machines, and bridges are restricted in use because the risk of delayed fracture increases when the strength becomes 1200 MPa or higher.

  In addition, various springs used in various transportation equipment such as automobiles are required to increase the strength of parts for weight reduction in order to achieve improved fuel efficiency of transportation equipment, resulting in deterioration of delayed fracture characteristics. Has become a concern.

  In general, delayed fracture is considered to occur because hydrogen that has entered the steel from the external environment accumulates at the grain boundaries of the tensile stress concentration part and promotes grain boundary cracking. Corrosion → Hydrogen penetration → Hydrogen accumulation at grain boundaries → Crack formation at grain boundaries → Propagation of cracks → Destruction.

  Various proposals for improving delayed fracture characteristics have been made based on this mechanism. For example, Patent Document 1 discloses refinement of prior austenite grains for the purpose of grain boundary strengthening, Patent Document 2 and Patent Document 3 indicate P, It has been proposed to reduce grain boundary embrittlement elements such as S.

Further, Patent Document 4 proposes a technique in which it is effective to spheroidize the carbide by tempering at a high temperature of 550 ° C. or higher in order to alleviate the stress concentration on the carbide precipitated at the prior austenite grain boundaries. .
Japanese Patent Publication No. 64-4566 Japanese Patent Publication No. 05-59967 Japanese Unexamined Patent Publication No. 03-243744 JP 2001-288538 A

  However, both proposals are specialized for grain boundary strengthening. For ultra-high-strength materials of 1500MPa or more, for example, 1800-2000MPa, measures are required in each process from hydrogen penetration to crack propagation. It is thought that it does not reach sufficient delayed fracture resistance.

  The present invention has been made paying attention to the above situation, and an object of the present invention is to provide a high-strength steel having good delayed fracture characteristics and particularly having a strength of 1500 MPa or more, and a method for producing the same.

The present inventors obtained the following knowledge as a result of intensive studies to achieve the above-mentioned problems.
That is, as steel composition
a) Addition of corrosion-resistant elements to reduce the amount of intrusion hydrogen (suppression of hydrogen intrusion)
b) Reduction of P and S of grain boundary embrittlement elements, and reduction of Mn and Si to promote concentration of these elements at grain boundaries
c) Addition of elements to improve toughness (inhibition of crack propagation)
As a manufacturing method
a) Reduction of carbide precipitation at grain boundaries due to low-temperature tempering, and reduction of concentration of P and S, which are grain boundary embrittlement elements (inhibition of crack generation),
d) Reducing the microstructure and reducing the concentration of P and S at the crystal grain boundaries by lowering the quenching temperature and / or high-frequency rapid heating (crack generation, crack propagation suppression)
As a result, it was found that excellent delayed fracture characteristics can be obtained.
The present invention was made by further study based on the obtained knowledge, that is, the present invention is
1% by mass, C: more than 0.4 to 0.6%, Si: less than 0.2%, Mn: 0.1 to 1.0%, Ni: more than 5 to 12%, sol. Al: 0.01 to 0.1%, Ti: 0.001 to 0.1 %, N: 0.02% or less, P: 0.01% or less, S: 0.01% or less, high-strength steel excellent in delayed fracture resistance consisting of the balance Fe and inevitable impurities.

  2% by mass, C: more than 0.4 to 0.6%, Si: less than 0.2%, Mn: 0.1 to 1.0%, Ni: more than 5 to 12%, sol. Al: 0.01 to 0.1%, Ti: 0.001 -0.1%, N: 0.02% or less, P: 0.01% or less, S: 0.01% or less, Cu: 0.1-1%, Cr: 0.1-1%, Mo: 0.02-1%, B: 0.00011- A high-strength steel excellent in delayed fracture resistance, containing one or more of 0.005%, Ca: 0.0005-0.005%, Mg: 0.0005-0.005%, the balance being Fe and inevitable impurities.

  3. A high strength steel excellent in delayed fracture resistance according to 1 or 2, wherein the prior austenite grain size is 15 μm or less.

4. A method for producing a high strength steel part having excellent delayed fracture resistance, characterized by quenching and tempering a steel having the composition described in 1 or 2 at a quenching temperature ≦ Ac ₃ + 60 ° C. and a tempering temperature: 400 ° C. or less. .
5. The method for producing a high-strength steel part having excellent delayed fracture resistance according to 4, wherein the quenching and tempering treatment is performed by high-frequency heating.
A part using high strength steel of any one of 61 to 3.

  According to the present invention, by taking appropriate measures at each stage from hydrogen intrusion to crack propagation, it is possible to provide a high strength steel excellent in delayed fracture resistance even at a high strength of 1500 MPa or more and a method for producing the same. In addition, high strength parts excellent in delayed fracture resistance suitable for automobiles and various industrial machine parts can be obtained, which is extremely useful industrially.

Hereinafter, the component composition, microstructure and quenching / tempering treatment of the present invention will be described.
1 Component composition (% is% by mass)
C: 0.4 to 0.6%
C is an important element for securing the strength of steel, and if it is less than 0.4%, a desired strength cannot be obtained.
On the other hand, if it exceeds 0.6%, not only the workability is deteriorated but also the toughness is lowered. Therefore, it is limited within the range of more than 0.4 to 0.6%.

Si: Less than 0.2% Si is an important element as a deoxidizing material for steel. However, in order to promote grain boundary segregation of impurity elements such as P and S, it is made less than 0.2%.

Mn: 0.1 to 1.0%
Mn is not only necessary for deoxidation but also an effective element for improving hardenability, but if it is less than 0.1%, the effect cannot be obtained. To help. Therefore, it is limited within the range of 0.1 to 1.0%.

P: 0.01% or less P segregates at austenite grain boundaries, weakens grain boundary strength, and promotes delayed fracture. Therefore, it is 0.01% or less.

S: 0.01% or less S, like P, segregates at austenite grain boundaries, weakens the grain boundary strength, and promotes delayed fracture. Therefore, it is 0.01% or less.

Ni: More than 5 to 12%
Ni is an element effective for imparting hardenability to steel, increasing static strength, and further improving toughness and corrosion resistance, and more than 5% is necessary to obtain the effect. However, even if added in a large amount, the effect is saturated and the alloy cost increases. Therefore, it exceeds 5 to 12%.

sol.Al: 0.01 to 0.1%
Al is an element necessary as a deoxidizing material, and also has the effect of fixing N as well as the effect of preventing coarsening of austenite grains by forming AlN during heat treatment. The effect is not exhibited, and even if it exceeds 0.1%, the effect is saturated, so it is limited to the range of 0.01 to 0.1%.

Ti: 0.001 to 0.1%
Ti has an effect of refining crystal grains, and further, N in steel is fixed as a nitride and mitigates grain boundary embrittlement. In order to obtain the effect, 0.001% or more is necessary. However, if added in a large amount, on the contrary, carbonitrides are excessively increased to reduce toughness and workability, and alloy costs also increase. Therefore, it is 0.001 to 0.1%.

N: 0.02% or less N also segregates at the austenite grain boundary to weaken the grain boundary strength and contribute to grain boundary embrittlement. Therefore, it is made 0.02% or less.

The above is the basic component composition of the present invention, but when improving properties such as strength and toughness, one or more of Cu, Cr, Mo, B, Ca and Mg are added.
Cu: 0.1 to 1%
Cu is an element effective for improving the hardenability of steel, increasing the strength of the steel, and further improving the corrosion resistance. If the amount is less than 0.1%, this effect cannot be sufficiently exerted. On the other hand, if added in a large amount, surface flaws are likely to occur during hot rolling, and cold workability is lowered. Therefore, when adding, it is 0.1 to 1%.

Cr: 0.1 to 1%
Cr is an element effective in improving hardenability and increasing the strength of steel, and further suppressing S segregating at the grain boundary as one of the sulfide-forming elements. However, the effect is saturated even if added over 1%. Therefore, when adding, it is 0.1 to 1%.

Mo: 0.02-1%
Mo is an element effective for preventing the segregation of P to the grain boundary and increasing the grain boundary strength. However, if it is less than 0.02%, the effect cannot be sufficiently exerted, while adding over 1%. Even so, the effect is saturated, resulting in an increase in alloy costs. Therefore, when adding, it is 0.02 to 1%.

B: 0.0001 to 0.005%
B improves hardenability with a small amount of addition, increases the strength of the steel and mitigates grain boundary embrittlement, but the effect cannot be fully exhibited at 0.0001% or less, while it is added in a large amount. The effect is only saturated. Therefore, when adding, it is 0.0001 to 0.005%.

Ca: 0.0005 to 0.005%, Mg: 0.0005 to 0.005%
Ca and Mg are sulfide-forming elements that suppress S segregating at the austenite grain boundaries and increase the grain boundary strength. Although 0.0005% or more is necessary to fully exhibit the effect, on the other hand, even if added in a large amount, the effect is not only saturated but also castability is impaired. Therefore, when both elements are added, the content is made 0.0005 to 0.005%.

2 Prior austenite grain size: 15 μm or less Making the austenite grain size fine is an effective means for improving delayed fracture characteristics because it has the effect of improving the grain boundary strength and reducing the crack propagation rate. In order to sufficiently exhibit this effect, the prior austenite grain size after quenching and tempering is set to 15 μm or less, more preferably 10 μm or less.

3 Quenching and tempering treatment Quenching temperature ≦ Ac ₃ + 60 ° C, tempering temperature: 400 ° C or less If the quenching temperature is too high, the austenite grain size becomes coarse, the grain boundary strength decreases, and the delayed fracture property deteriorates. Is limited to Ac ₃ temperature + 60 ° C.

  The tempering temperature is set to 400 ° C. or lower in order to suppress cementite precipitation on the austenite grain boundaries, further reduce the concentration of impurity elements on the austenite grain boundaries, and improve delayed fracture characteristics.

  In the present invention, the quenching and tempering treatment method is not particularly limited, but when high-frequency induction heating is used, rapid and short-time heating is possible, and austenite grain refinement and concentration of impurity elements on the austenite grain boundary can be reduced. This is preferable because the delayed fracture characteristics are further improved.

Hereinafter, a description will be given based on examples.
Steel having the component composition shown in Table 1 was vacuum melted (50 kg), and a Φ25 material was prepared by hot forging. In Table 1, steel no. Nos. 1 to 13 are steels according to the present invention, steel Nos. 14-20 are comparative steels whose component composition is outside the scope of the present invention.

The obtained forged material was adjusted to a strength of 1500 to 1850 MPa by quenching and tempering, and the delayed fracture characteristics were evaluated. The quenching and tempering treatment was performed by induction heating at a quenching temperature: (Ac ₃ + 50 ° C.) * 30 minutes → WQ, and a tempering temperature: T ° C. * 60 minutes → WC.

  Delayed fracture characteristics are evaluated by measuring the material's critical diffusible hydrogen content (Hc) and environmental intrusion hydrogen content (He / Hc). He / Hc> 1 is poor (×), and He / Hc ≦ 1 is good (○).

  For the critical diffusible hydrogen content (Hc), the test piece shown in Fig. 1 was used, hydrogen was infiltrated by the cathodic charging method, a static tensile stress 0.9 times the tensile strength was applied, the time until fracture was examined, From the relationship between time and the amount of diffusible hydrogen, the amount of diffusible hydrogen that does not break was determined.

  The test time was 100 hours maximum, and the test piece was plated after cathodic charging in order to avoid hydrogen release from the test piece during the test. The amount of hydrogen intrusion into the test piece was controlled by changing the cathode charge conditions in various ways, and the constant load test was started from 24 hours after plating in order to make the hydrogen distribution in the test piece uniform.

  In the analysis of hydrogen, the test piece is heated in a carrier gas (Ar) at a constant heating rate, and the gas is analyzed by gas chromatography for 1 minute every 5 minutes, and the amount of hydrogen released up to 300 ° C is integrated. The value was defined as diffusible hydrogen, and the relationship with the fracture time was arranged.

  For conditions that broke in less than 100 hours, infiltrate hydrogen again under the same conditions and perform plating.After leaving for 24 hours, further, peel off the plating and perform hydrogen analysis. The plating was peeled off and hydrogen analysis was performed.

  The amount of hydrogen penetrating into the environment (He) was measured by a 5% NaCl salt spray-> drying acceleration test. Corrosion conditions for the accelerated test were 7 cycles of a process in which salt spray (5% NaCl, 25 ° C.) was performed for 16 hours and then left in the atmosphere (humidity 60%, 25 ° C.) for 8 hours. After 7 cycles, the surface rust was removed by shot blasting, hydrogen analysis was performed in the same manner as described above, and the amount of diffusible hydrogen was measured.

  Table 2 shows the results obtained. Steel No. The steels of the present invention of 1 to 13 were all good with He / Hc ≦ 1. On the other hand, Steel No. No. 14 has a high C content exceeding the upper limit of the range of the present invention, so that a large amount of cementite precipitates at the grain boundaries, lowering toughness, Hc is low, and delayed fracture characteristics are poor.

  Steel No. No. 15 has high amounts of Si and Mn exceeding the upper limit of the range of the present invention, so that toughness is lowered, Hc is low, and delayed fracture characteristics are poor. Steel No. Nos. 16 and 17 have high amounts of P and S exceeding the upper limit of the range of the present invention, respectively, so that toughness is reduced, Hc is low, and delayed fracture characteristics are poor.

Steel No. In No. 18, since the Ni content is lower than the lower limit of the range of the present invention, the toughness is lowered and the delayed fracture characteristics are poor.
Steel No. In No. 19, since Al is lower than the lower limit of the range of the present invention, cleanliness is poor due to insufficient deoxidation. As a result, toughness is deteriorated and delayed fracture characteristics are poor.

  Steel No. In No. 20, the Ti content is lower than the lower limit of the range of the present invention, and the N amount is higher than the upper limit of the range of the present invention.

  ★

★
[Effect of prior austenite grain size on delayed fracture characteristics]
Steel No. shown in Table 1 3 was used to investigate the effect of prior austenite grain size on delayed fracture characteristics. The delayed fracture characteristics were evaluated by measuring Hc and He in the same manner as described above and using the value of He / Hc.

  Table 3 shows the results. As the prior austenite grain size becomes smaller, the He / Hc becomes lower and the delayed fracture property becomes better. The characteristic example-1 shows the best results because the grain boundary strength is improved by refinement of the old γ grains and the He / Hc is lowered. In the characteristic example-2, the oldest γ grains were the largest, and as a result, Hc mainly decreased and the delayed fracture characteristics slightly deteriorated. In addition, the He / Hc of the characteristic examples-1, 2, and 3 were all 1 or less.

★
[Heat treatment conditions]
Steel No. shown in Table 1 3 was used to investigate the effects of quenching temperature, tempering temperature, and induction heat treatment on delayed fracture characteristics. Table 4 shows the results. Delayed fracture characteristics were evaluated by measuring Hc and He, and evaluating the magnitude relationship of He / Hc.

  Production Examples 1, 3, 5, and 6 have a quenching temperature of AC3 + 60 ° C. or less and a tempering temperature of 400 ° C. or less within the scope of the present invention, and the quenching temperature is outside the scope of the present invention. Compared with Production Example 4 which is outside the scope of the present invention, He / Hc is low, and delayed fracture characteristics are good.

  In particular, Production Example 5 was obtained by quenching and tempering by high-frequency heating, and the old γ grain boundaries were finer than Production Examples 1, 3, and 6, and delayed fracture characteristics were improved. In all cases, He / Hc was 1 or less.

  ★

The figure which shows the test piece (Example) for a delayed fracture test.

Claims (6)

In mass%, C: more than 0.4 to 0.6%, Si: less than 0.2%, Mn: 0.1 to 1.0%, Ni: more than 5 to 12%, sol. Al: 0.01 to 0.1%, Ti: 0.001 to 0.1%, N : High strength steel excellent in delayed fracture resistance consisting of 0.02% or less, P: 0.01% or less, S: 0.01% or less, balance Fe and inevitable impurities. By mass%, C: more than 0.4 to 0.6%, Si: less than 0.2%, Mn: 0.1 to 1.0%, Ni: more than 5 to 12%, sol. Al: 0.01 to 0.1%, Ti: 0.001 to 0.1% N: 0.02% or less, P: 0.01% or less, S: 0.01% or less, Cu: 0.1-1%, Cr: 0.1-1%, Mo: 0.02-1%, B: 0.0001-0.005%, Ca: A high-strength steel excellent in delayed fracture resistance, containing one or more of 0.0005 to 0.005%, Mg: 0.0005 to 0.005%, with the balance being Fe and inevitable impurities. The high strength steel having excellent delayed fracture resistance according to claim 1 or 2, wherein the prior austenite grain size is 15 µm or less. A method for producing a high-strength steel part having excellent delayed fracture resistance, comprising quenching and tempering the steel having the component composition according to claim 1 at a quenching temperature ≦ Ac ₃ + 60 ° C. and a tempering temperature of 400 ° C. or less. . The method for producing a high-strength steel part having excellent delayed fracture resistance according to claim 4, wherein the quenching and tempering treatment is performed by high-frequency heating. A part using the high-strength steel according to any one of claims 1 to 3.

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