JP2005240086A - High strength steel having excellent cold workability and delayed fracture resistance, and high strength steel component having excellent delayed fracture resistance - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide high strength steel whose delayed fracture resistance in an acid environment and an environment to which a thermal load is applied can be improved, and which has excellent cold workability as well, and to provide a high strength component obtained from the high strength steel. <P>SOLUTION: The high strength steel has a composition comprising, by mass, 0.30 to 0.6% C, ≤0.2% Si, 0.1 to 0.8% Mn, ≤0.02% P, ≤0.02% S, >0.5 to 0.95% Cr, 1.3 to 2.2% Mo, 0.05 to 0.15% Ti, 0.1 to 0.50% V, ≤0.5% Al and ≤0.02% N, and the balance Fe with inevitable impurities, and in which the Cr content, Mo content, Ti content and V content satisfy prescribed relations. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

The present invention relates to high-strength steel useful for producing drawn parts such as high-strength bolts, high-strength springs, high-strength PC steel wires, and high-strength rebars, and high-strength steel parts, and particularly has a tensile strength. The present invention relates to a high-strength steel and high-strength steel parts that are excellent in cold workability and delayed fracture resistance even if they are 1300 N / mm ² or more.

The 10～12T (about 980~1372N / mm ² approximately) High-strength bolt steel class, conventionally, SCM435 steel (C: 0.32~0.39%, Si: 0.15~0.35%, Mn: 0.55~0.90%, P: 0.030% or less, S: 0.030% or less, Cr: 0.85-1.25%, Mo: 0.15-0.35%), SCM440 steel (C: 0.42-0.49%, Si: 0.15-0.35%, Mn: 0.55-0.90) %, P: 0.030% or less, S: 0.030% or less, Cr: 0.85 to 1.25%, Mo: 0.15 to 0.35%) and the like (hereinafter, the SCM435 steel and the SCM440 steel are collectively referred to as “SCM435 steel, etc. "). In general, however, high strength bolts tend to cause delayed fracture when their tensile strength exceeds about 1200 N / mm ² , and their use range is limited.

Therefore, in order to improve delayed fracture characteristics in high-strength steel having a tensile strength of 1300 N / mm ² or more, for example, the technique of Patent Document 1 has been proposed. In this technique, the fatigue limit diffusible hydrogen amount and the delayed fracture limit diffusible hydrogen amount are increased by decreasing the Mn amount and increasing the Mo amount as compared with conventional steel types.

  In addition, in the Non-Patent Document 1, the present inventors have disclosed that although V can trap a large amount of diffusible hydrogen, the hydrogen trapping action is weak and hydrogen is released in a heat load environment. Announced to re-trap hydrogen released from V carbonitride traps and suppress delayed fracture. Combined addition of V-Ti improves delayed fracture resistance and is resistant in environments where thermal loads are applied. It has been clarified that the delayed fracture characteristics are further improved.

However, although Patent Document 1 and Non-Patent Document 1 describe improving the delayed fracture resistance of high-strength steel, no consideration was given to cold workability.
JP 2002-173739 A (see [Claims] and [0012]) Materials and Processes, Vol. 14, 2001, 1308 [CAMPS-ISIJ Vol. 14 (2001) -1308]

  The present invention has been made in view of such a situation, and its purpose is to improve delayed fracture resistance in an acidic environment or an environment where a thermal load is applied, and further to cold workability. It is to provide an excellent high strength steel.

  It is also an object of the present invention to provide a high-strength steel part that is excellent in delayed fracture resistance in an acidic environment or an environment where a thermal load is applied.

  Another object of the present invention is to provide a high-strength steel and a high-strength component obtained from the high-strength steel that can achieve both a delayed fracture resistance and a cold workability in an acidic environment or an environment in which a thermal load is applied. Is to provide.

The high-strength steel excellent in cold workability and delayed fracture resistance according to the present invention that has solved the above-mentioned problems is in mass% (hereinafter the same), C: 0.30 to 0.6%, Si: 0.2% or less, Mn: 0.1-0.8%, P: 0.02% or less, S: 0.02% or less, Cr: more than 0.5%, 0.95% or less, Mo : 1.3-2.2%, Ti: 0.05-0.15%, V: 0.1-0.50%, Al: 0.5% or less and N: 0.02% or less, The balance is steel composed of Fe and inevitable impurities, and the gist is that each component satisfies the following formulas (1) and (2).
0.3 ≦ 10 × {exp (0.4 × [Cr]) / [exp (2 × [Mo]) + exp (92 × [Ti] × [V])]} ≦ 0.6 (1)
[Ti] / [V] ≧ 0.1 (2)
In the above formula, [Cr], [Mo], [Ti], and [V] are the amount of Cr (mass%), the amount of Mo (mass%), the amount of Ti (mass%) and the amount of V contained in the steel, respectively. (Mass%) is shown.

In the steel, further, as other elements,
(1) Ni: 1.0% or less (not including 0%) and / or Cu: 1% or less (not including 0%),
(2) A total of 0.5% or less (not including 0%) of elements selected from the group consisting of Zr, W and Nb,
(3) B: 0.003% or less (excluding 0%),
And the like are preferred.

  Within the scope of the present invention, after hot rolling the above steel, spheroidizing annealing, then cold working into a predetermined shape, the cold worked steel is quenched at a heating temperature of 890-960 ° C, Subsequently, high strength steel parts having excellent delayed fracture resistance obtained by tempering at a heating temperature of 550 ° C. or higher are also included.

In addition, high-strength steel parts having the above-described component composition and excellent in delayed fracture resistance having the characteristics shown in the following (a) to (d) are also included in the scope of the present invention.
(A) Tensile strength: 1300 N / mm ² or more (b) Austenite grain size number: 9 or more (c) A crosshead speed of 2 × 10 with a test piece immersed in a 5 mass% NaCl aqueous solution having a pH of 3.0 the low strain rate test measures the elongation E ₁ by performing a ^{-3 mm} / min, also a non-performed in the atmosphere in the same way as in is the dipping low strain rate testing without immersion in NaCl solution the elongation E ₀ was measured by performing, measured E ₁ and E (3) below from a value of ₀ delayed fracture characteristic value calculated by the equation alpha is 0.50 or less delayed fracture property value alpha = (1-E ₁ / E ₀ ) × 100 (3)
(D) Elongation E ₂ is measured under the same conditions except that 80 ° C. distilled water is used in place of the NaCl aqueous solution in (c) above, and the atmosphere is not immersed in distilled water as in (c) above. The elongation E ₀ is measured in the sample, and the delayed fracture characteristic value β calculated by the following equation (4) from the measured values of E ₂ and E ₀ is 0.50 or less. Delayed fracture characteristic value β = (1−E ₂ / E ₀ ) × 100 (4)
The term “elongation” means “total elongation (elongation at break)”.

  ADVANTAGE OF THE INVENTION According to this invention, the high-strength steel which can improve the delayed fracture resistance in the acidic environment and the environment where a heat load is added, and also excellent in cold workability can be provided. Moreover, according to this invention, the high strength steel components excellent in the delayed fracture resistance in the acidic environment and the environment where a heat load is added can be provided. Furthermore, according to the present invention, a high-strength steel capable of achieving both high-level delayed fracture resistance and cold workability in an acidic environment or an environment where a thermal load is applied, and a high-strength component obtained from the high-strength steel are provided. Can be provided.

The inventors have made various studies in order to improve the cold workability and delayed fracture resistance of high-strength steel. as a result,
(I) The relationship between delayed fracture resistance and Cr, Mo, Ti, and V in an acidic environment, and the relationship between cold workability and Cr, Mo, Ti, and V can be unified in one equation. ,
(II) And according to this organized formula, even when Cr exceeds 0.5%, it is possible to achieve both a high level of delayed fracture resistance and cold workability in an acidic environment,
(III) Carbide is efficiently precipitated by adding Ti and V in combination, and since this carbide has a strong hydrogen trapping capability, it can improve delayed fracture resistance in an environment where a thermal load is applied,
The present invention has been completed. Hereinafter, the present invention will be described in detail. Note that “delayed fracture resistance in an acidic environment” and “delayed fracture resistance in an environment where a thermal load is applied” may be collectively referred to as “delayed fracture resistance”.

10~12T The (approximately 980~1372N / mm ² approximately) High-strength bolt steel class, but conventionally SCM435 steel or the like is used, combined addition Mo and Ti, and V with respect to the SCM435 steel etc. Even so, the improvement of delayed fracture resistance in an acidic environment is insufficient. This is because about 1% of Cr is added to the SCM435 steel or the like for the purpose of enhancing hardenability and ensuring high strength. That is, when about 1% of Cr is added, the steel material is corroded in a pit shape, and the pit-shaped portion becomes a stress concentration portion, which deteriorates the delayed fracture resistance in an acidic environment.

  On the other hand, when various steels are formed into parts, they are generally cold-worked. Conventionally, however, cold workability has not been considered so much, and forming is performed with a large deformation resistance. It was.

  Therefore, the present inventors have made extensive studies to improve delayed fracture resistance in an acidic environment and to improve cold workability. In the process, parts are manufactured by cold working using steels with different composition, and the deformation resistance during cold working is measured, and the delayed fracture characteristic value α in the acidic environment of the obtained parts is measured. Thus, cold workability and delayed fracture resistance in an acidic environment were evaluated. That is, C: 0.39%, Si: 0.06%, Mn: 0.50%, P: 0.006%, S: 0.005%, Cr: more than 0.5%, 0.95% or less, Mo: 1.3-2.2%, Ti: 0.05-0.15% V: 0.1 to 0.50%, Al: 0.037%, and N: 0.0037%, and the remaining steel composed of Fe and inevitable impurities was hot-rolled and then subjected to spheroidizing annealing. The spheroidizing annealing is performed by heating the wire obtained by hot rolling to 740 ° C. and holding it for 5 hours, and then gradually cooling to 650 ° C. (cooling rate: about 10 ° C./hr), The reaction was allowed to cool.

  Next, a compression test piece and a dumbbell-shaped test piece shown in the following examples are produced by cold working, and the obtained test piece is quenched at a heating temperature of 920 to 950 ° C., and then tempered at a heating temperature of 570 to 600 ° C. did. For each test piece obtained by variously changing the amount of Cr, Mo, Ti and V contained at this time, the deformation resistance during cold working and the delayed fracture characteristic value α in an acidic environment were measured under the following conditions, The relationship between delayed fracture resistance in cold workability or acidic environment and the amount of Cr, Mo, Ti and V was investigated.

For the evaluation of the deformation resistance during cold working, the stress (measured value) necessary for compressing the compression test piece while restraining the both end faces and setting the rolling reduction to 70% was used. Note that the smaller the deformation resistance, the better the cold workability. In particular, when the deformation resistance is 900 N / mm ² or less, it is possible to secure a mold life capable of mass production.

The delayed fracture characteristic value α was calculated by the following procedure. That is, the crosshead speed was set to 2 × 10 ⁻³ mm / min with the dumbbell-shaped test piece immersed in a 5 mass% NaCl aqueous solution obtained by adding NaCl to an aqueous solution adjusted to pH 3.0 using HCl. It was measured elongation E ₁ by performing; as minute low strain rate test (Slow strain rate Technique SSRT test). Further, the elongation E ₀ was measured by performing an SSRT test at a crosshead speed of 2 × 10 ⁻³ mm / min in the air without being immersed in the NaCl aqueous solution. The delayed fracture characteristic value α is calculated from the measured values of E ₁ and E _{0 by} the following equation (3). If the delayed fracture characteristic value α is 0.50 or less, the delayed fracture characteristic in an acidic environment is excellent.
Delayed fracture characteristic value α = (1−E ₁ / E ₀ ) × 100 (3)

FIG. 1 shows the relationship between the composite addition amount X expressed by the following formula (1a) and the measured deformation resistance or delayed fracture characteristic value α of the component. In the figure, ◆ indicates the deformation resistance during cold working, and ■ indicates the result of delayed fracture characteristic value α. In the following formula, [Cr], [Mo], [Ti] and [V] are the Cr amount (mass%), Mo amount (mass%), Ti amount (mass%) and V amount (mass%) is shown.
Compound addition amount X = 10 × {exp (0.4 × [Cr]) / [exp (2 × [Mo]) + exp (92 × [Ti] × [V])]} (1a)

As apparent from FIG. 1, when the value of the composite addition amount X is less than 0.3, the delayed fracture property value α is small, and the delayed fracture resistance value in an acidic environment is extremely excellent. Deformation resistance becomes too large and cold workability deteriorates. On the other hand, if the value of the composite addition amount X exceeds 0.6, the deformation resistance during cold working becomes small and the cold workability becomes extremely excellent, but the delayed fracture characteristic value α becomes too large, and the acidic environment Underlying delayed fracture resistance is poor. That is, as is clear from FIG. 1, the value calculated from the middle side of the following equation (1) (the value of the composite addition amount X) is a characteristic of both cold workability and delayed fracture resistance in an acidic environment. It becomes an index. In the high-strength steel of the present invention, it is necessary to adjust the amount of each element so that the value of the composite addition amount X is in the range of 0.3 to 0.6 as represented by the following formula (1). The preferred lower limit is 0.30, the more preferred lower limit is 0.31, the still more preferred lower limit is 0.32, the particularly preferred lower limit is 0.35, and the most preferred lower limit is 0.40. On the other hand, the preferable upper limit value is 0.60, the more preferable upper limit value is 0.59, the still more preferable upper limit value is 0.58, the particularly preferable upper limit value is 0.55, and the most preferable upper limit value is 0.50.
0.3 ≦ 10 × {exp (0.4 × [Cr]) / [exp (2 × [Mo]) + exp (92 × [Ti] × [V])]} ≦ 0.6 (1)

  As a result of the above, if the relationship among the Cr, Mo, Ti and V amounts contained in the steel among the component composition of the steel satisfies the above formula (1), the cold workability when processing into the test piece is improved. It was found that the obtained specimens had good delayed fracture resistance in an acidic environment and a high strength steel with a good balance between cold workability and delayed fracture resistance in an acidic environment. It was. That is, in order to adjust the balance between cold workability and delayed fracture resistance in an acidic environment, the balance between the Cr content (numerator in the formula) and the amounts of Mo, Ti, and V (denominator in the formula) It becomes important.

  By the way, V is useful in that fine VC can be efficiently generated by high-temperature quenching and high-temperature tempering, and strength can be improved by precipitation hardening. Furthermore, it has the effect of trapping hydrogen present in the steel. However, the hydrogen trapping capability of VC is relatively weak. When stress or heat is applied, the trapped hydrogen is released, and conversely, the amount of diffusible hydrogen is increased, resulting in delayed fracture resistance in an environment where heat load is applied. It becomes a factor to reduce. On the other hand, Ti is also a precipitation hardening type element, but its carbide has a strong trapping ability and has a property of not releasing hydrogen even when high stress or heat is applied.

  Therefore, in the present invention, a predetermined amount of Ti and V are added in combination. In other words, the hydrogen trapping capability is insufficient when V is added alone, and the addition of Ti is indispensable to make up for it. By adding Ti and V in combination, Ti-V composite carbide, Ti carbide, and V carbide can be formed, and carbide can be precipitated efficiently, and a carbide group having a strong trapping capability can be formed.

  From these viewpoints, the present inventors manufactured parts by cold working using steels having different component compositions, and measured the delayed fracture characteristic value β in an environment where a thermal load of the obtained parts is applied, Delayed fracture resistance was evaluated in an environment where heat load was applied. That is, the dumbbell-shaped test piece produced above was quenched and tempered under the same conditions as described above. For the test pieces obtained by variously changing the amounts of Cr, Mo, Ti and V contained at this time, the delayed fracture characteristic value β was measured under an environment where a thermal load was applied under the following conditions, and under the environment where the thermal load was applied The relationship between the delayed fracture resistance and the ratio of Ti to V (Ti / V) was examined.

The delayed fracture characteristic value β was calculated by the following procedure. That is, the elongation E ₂ is measured by performing an SSRT test with a crosshead speed of 2 × 10 ⁻³ mm / min with the test piece immersed in distilled water at 80 ° C. Further, the elongation E ₀ is measured by performing an SSRT test with the crosshead speed of 2 × 10 ⁻³ mm / min in the air without immersing the sample in distilled water. The delayed fracture characteristic value β is calculated from the measured values of E ₂ and E _{0 by} the following equation (4). If the delayed fracture characteristic value β is 0.50 or less, the delayed fracture resistance is excellent in an environment where a thermal load is applied.
Delayed fracture characteristic value β = (1−E ₂ / E ₀ ) × 100 (4)

FIG. 2 shows the relationship between the ratio of Ti to V ([Ti] / [V]) expressed by the following equation (2a) and the measured delayed fracture characteristic value β of the component. In the following formulae, [Ti] and [V] represent the Ti amount (mass%) and V amount (mass%) contained in the steel, respectively.
Ratio of Ti and V = [Ti] / [V] (2a)

As apparent from FIG. 2, when the ratio of Ti and V is less than 0.1, the delayed fracture characteristic value β becomes too large, and the delayed fracture resistance in an environment where a thermal load is applied becomes poor. In the high-strength steel of the present invention, it is necessary to adjust the amount of each element so that the ratio of Ti and V is 0.1 or more as shown in the following formula (2). Preferably it is 0.10 or more, More preferably, it is 0.100 or more, More preferably, it is 0.2 or more, Most preferably, it is 0.3 or more.
[Ti] / [V] ≧ 0.1 (2)

  Next, the reason for limiting the component composition of the high-strength steel according to the present invention will be described.

  The high-strength steel of the present invention includes C: 0.30 to 0.6%, Si: 0.2% or less, Mn: 0.1 to 0.8%, P: 0.02% or less, S: 0.02% or less, Cr: more than 0.5%, 0.95% or less, Mo: 1.3-2.2%, Ti: 0.05-0.15%, V: 0.1-0.50%, Al: 0.5% or less and N: 0.02% or less are satisfied.

C: 0.30-0.6%
C is an element necessary for ensuring the hardenability and strength of steel. In particular, the steel of the present invention contains precipitation hardening elements such as Mo, Ti, and V, as described later, and these elements are dissolved by high temperature quenching, and these elements are precipitated by high temperature tempering. Therefore, in order to ensure a predetermined tensile strength even after high temperature tempering, the C content is set to 0.30% or more. Preferably it is 0.35% or more, more preferably 0.38% or more. However, if C is excessive, the toughness of the steel deteriorates, so the delayed fracture resistance deteriorates, and further the cold workability deteriorates. Therefore, the upper limit of C content is 0.6%. Preferably it is 0.55% or less, More preferably, it is 0.48% or less, More preferably, it is 0.40% or less.

Si: 0.2% or less Since Si is added as a deoxidizer, it remains in the steel, but as the remaining amount of Si increases, the cold workability tends to decrease, and furthermore, heat treatment such as quenching. Grain boundary oxidation at the time is promoted, and the delayed fracture resistance is also easily deteriorated. Therefore, the Si content is 0.2% or less. It is preferably 0.1% or less, more preferably 0.07% or less, further preferably 0.05% or less, and particularly preferably 0.03% or less. Most preferably, it is 0%, but in reality, the Si amount never becomes 0%.

Mn: 0.1 to 0.8%
Mn is a hardenability improving element and is useful for achieving high strength. The amount of Mn is 0.1% or more, preferably 0.3% or more, more preferably 0.35% or more, and further preferably 0.40% or more. However, when Mn is excessive, cold workability is liable to deteriorate, and further segregates at the grain boundary to lower the grain boundary strength, which adversely affects the delayed fracture resistance. Therefore, the Mn content is 0.8% or less, preferably 0.6% or less, more preferably 0.55% or less, and still more preferably 0.50% or less.

P: 0.02% or less P is an element remaining in the steel as an impurity, and tends to cause segregation at the grain boundary to deteriorate the delayed fracture resistance. Therefore, the P content is 0.02% or less. Preferably it is 0.015% or less, More preferably, it is 0.010% or less, More preferably, it is 0.005% or less. Most preferably, it is 0%, but P is not practically 0%.

S: 0.02% or less S is also an element remaining in the steel as an impurity. MnS is formed and becomes a stress-concentrated portion, which tends to deteriorate the delayed fracture resistance. Therefore, the S content is 0.02% or less, preferably 0.015% or less, more preferably 0.010% or less, and still more preferably 0.005% or less. Most preferably, it is 0%, but in reality, the S amount never becomes 0%.

Cr: More than 0.5%, 0.95% or less In the high-strength steel of the present invention, a predetermined amount of Mo, V and Ti are added in combination, so that the grain boundaries are strengthened and the trapping ability of diffusible hydrogen is excellent. Further, when the Cr, Mo, Ti and V contents satisfy the above formula (1), it is possible to realize both cold workability and delayed fracture resistance in an acidic environment. However, if the Cr content is reduced to 0.5% or less, the cold workability is worsened. Therefore, the Cr content is over 0.5%. Preferably it is 0.52% or more, more preferably 0.55% or more. However, if the amount of Cr is excessive, the effect of improving delayed fracture resistance in an acidic environment is hindered even if the relationship of the above formula (1) is satisfied, so the upper limit of Cr amount is 0.95%. Preferably it is 0.80% or less, More preferably, it is 0.75% or less.

Mo: 1.3-2.2%
Mo is a hardenability improving element and is a precipitation hardening type element, so it is useful for securing strength. Mo has a grain boundary strengthening action, and improves the delayed fracture resistance in an acidic environment by adding it in a well-balanced manner together with Cr, Ti, and V as described above. Therefore, in the present invention, Mo is contained by 1.3% or more. Preferably it is 1.35% or more, more preferably 1.45% or more. However, when Mo becomes excessive, cold workability will fall. Therefore, Mo is 2.2% or less. Preferably it is 2.20% or less, More preferably, it is 2.0% or less, More preferably, it is 1.80% or less.

Ti: 0.05 to 0.15% and V: 0.1 to 0.50%
The amount of Ti added is 0.05% or more from the viewpoint of precipitation hardening action, hydrogen trap amount, and the like. Preferably it is contained in excess of 0.05%, more preferably 0.060% or more, more preferably 0.070% or more. Further, the amount of V added is also set to 0.1% or more from the viewpoint of precipitation hardening action and hydrogen trapping ability. Preferably it is 0.12% or more.

  However, when Ti or V is excessively added, the cold workability is remarkably lowered. Furthermore, the Ti and V giant carbides produced during the melting of the steel do not dissolve sufficiently during the heating of the quenching and deteriorate the toughness of the steel. For this reason, the delayed fracture resistance in an environment where a thermal load is applied may be lowered. Therefore, the Ti content is 0.15% or less. Preferably it is 0.10% or less, More preferably, it is 0.08% or less. V amount is 0.50% or less. Preferably it is 0.4% or less, More preferably, it is 0.3% or less.

Al: 0.5% or less When Al is added as a deoxidizer, it remains in the steel. Furthermore, since it is an element that can also be expected to improve the corrosion resistance by densification of rust, it remains actively. There is also. Although the lower limit of the amount of Al is not particularly limited, it is practically more than 0% when considering use as a deoxidizer, and from the viewpoint of exerting an effect of improving corrosion resistance, it is preferably 0.01% or more, more Preferably it is 0.02% or more, more preferably 0.03% or more. However, as the remaining amount of Al increases, the amount of oxide inclusions increases, and the delayed fracture resistance tends to deteriorate. Therefore, the Al content is 0.5% or less. Preferably it is 0.3% or less, More preferably, it is 0.1% or less, More preferably, it is 0.05% or less, Most preferably, it is 0.050% or less.

N: 0.02% or less N is harmful to delayed fracture characteristics, and it is desirable to reduce it as much as possible. Therefore, the N content is 0.02% or less. Preferably it is 0.015% or less, More preferably, it is 0.010% or less, More preferably, it is 0.007% or less, Most preferably, it is 0.005% or less. Most preferably, it is 0%, but in reality, the N amount never becomes 0%.

The balance of the high-strength steel according to the present invention is composed of Fe and inevitable impurities [for example, O (oxygen), etc.].
(A) As a corrosion resistance improving element, Ni: 1.0% or less (not including 0%) and / or Cu: 1% or less (not including 0%),
(B) As a fine carbonitride-forming element, a total of one or more selected from the group consisting of Zr, W and Nb is 0.5% or less (excluding 0%),
(C) B: 0.003% or less (excluding 0%),
Etc. may be included. The reason for specifying such a range is shown below.

Ni: 1.0% or less (excluding 0%)
Ni is an element that is desirably added positively in order to increase the toughness and hardenability of the steel and improve corrosion resistance and suppress hydrogen intrusion. When Ni is positively added (that is, when it exceeds 0%), the content is preferably 0.1% or more, more preferably 0.2% or more, and further preferably 0.3% or more. However, if Ni is added excessively, the effect is saturated and not only the cost is increased, but also cold workability is decreased. Therefore, even when Ni is positively added, the upper limit is made 1.0%. More preferably, it is 0.6% or less, more preferably 0.55% or less, and particularly preferably 0.50% or less.

Cu: 1% or less (excluding 0%)
Cu is an element that is desirably added positively because it is useful for enhancing the corrosion resistance of steel and suppressing the intrusion of hydrogen, which adversely affects delayed fracture resistance. When Cu is positively added (that is, when it is more than 0%), the content is preferably 0.1% or more, more preferably 0.3% or more. However, when Cu is added excessively, not only the effect is saturated, but also the toughness of the steel is reduced. Therefore, even when Cu is positively added, the upper limit is made 1%. More preferably, it is 0.7% or less, More preferably, it is 0.6% or less.

0.5% or less in total of elements selected from the group consisting of Zr, W and Nb (excluding 0%)
Zr, W and Nb, like Ti, form fine carbonitrides and contribute to the improvement of delayed fracture resistance, so it is desirable to add them positively. In addition, nitrides and carbides of these elements are effective for refining crystal grains. When Zr, W, and Nb are positively added (that is, when the content is more than 0%), the addition amount of these elements is preferably 0.03% or more, more preferably 0.05% or more. . However, when Zr, W and Nb are added excessively, delayed fracture resistance and toughness are hindered. Therefore, when Zr, W and Nb are positively added, the total addition amount is preferably 0.5% or less, more preferably 0.3% or less, and further preferably 0.1% or less.

B: 0.003% or less (excluding 0%)
B is an element desirably added positively because it is useful for improving hardenability, toughness after quenching and tempering, fatigue characteristics, and the like. When B is positively added (that is, when it exceeds 0%), the content is preferably 0.0005% or more, more preferably 0.0010% or more. However, if B is added excessively, the toughness is adversely affected, so even if B is positively added, the upper limit is made 0.003%. More preferably, it is 0.0025% or less, More preferably, it is about 0.0020% or less.

  Since the steel having the above chemical components is excellent in cold workability after spheroidizing annealing, it can be easily processed into various forms. For example, after hot working (for example, hot rolling), spheroidizing annealing treatment, and then cold working (for example, cold drawing, cold forging, etc.), various shapes of parts can be easily processed. it can. Since the high-strength steel according to the present invention is excellent in cold workability, it can be processed with a high yield without deteriorating the tool life during cold working. Note that the cold wire drawing and the cold forging exemplified as the cold working may be performed by adopting either one or a combination of both. It is also a preferable aspect that a film treatment (for example, pickling or lubricating film treatment) is performed between the spheroidizing annealing and the cold working.

  High-strength steel parts can be obtained by quenching and tempering the processed product obtained by cold working. Although steels having the above chemical components are potentially excellent in high strength and delayed fracture resistance, various high strength steel parts that effectively exhibit these high strength and delayed fracture resistance are manufactured. Therefore, it is necessary to appropriately set quenching conditions and tempering conditions.

  That is, since the steel contains a predetermined amount of precipitation hardening type elements such as Mo, V, and Ti, it is necessary to make the precipitation hardening type element form a solid solution during quenching heating and to precipitate as fine carbides during tempering. Further, fine carbides of Mo, V, and Ti are useful not only for improving strength but also for improving delayed fracture resistance. Furthermore, in order to achieve a predetermined delayed fracture resistance, the crystal grains must be prevented from coarsening during quenching.

  From such a viewpoint, specifically, the heating temperature during quenching is set to 890 to 960 ° C. If the quenching temperature is too low, the precipitation hardening type element is not sufficiently dissolved in the steel, and even if tempering is performed, a sufficient amount of precipitated carbide cannot be secured, and the tensile strength is lowered. Furthermore, since the structure before quenching is a spheroidized structure, the spheroidized carbide remains undissolved when heating is insufficient, and the tensile strength also decreases from this point. On the other hand, if the quenching temperature is too high, the crystal grains become coarse and the delayed fracture resistance deteriorates. Preferred heating temperatures are 900 ° C. or higher (more preferably 910 ° C. or higher, more preferably 920 ° C. or higher) and 950 ° C. or lower (more preferably 945 ° C. or lower, more preferably 940 ° C. or lower).

  As tempering conditions after quenching, the heating temperature during tempering is 550 ° C. or higher. If the tempering temperature is too low, the precipitation hardening type element does not sufficiently precipitate as fine carbides, and the tensile strength does not increase. In particular, if the fine carbides of Mo, Ti, and V are not sufficiently precipitated, the delayed fracture resistance is also deteriorated. A preferable tempering temperature is 570 ° C. or higher (preferably 580 ° C. or higher, more preferably 600 ° C. or higher). The upper limit of the tempering temperature is not particularly limited, but generally it is preferably about 650 ° C. or less, more preferably about 625 ° C. or less.

  The conditions other than the heating temperature at the time of quenching and the heating temperature at the time of tempering may be appropriately set in consideration of the characteristics of the precipitation hardening element, and can be selected from the following ranges, for example.

[Hardening conditions]
Holding time after heating: 10 minutes or more (for example, 20 minutes or more), 1 hour or less (for example, 40 minutes or less)
Cooling conditions: oil cooling, water cooling or air cooling [tempering conditions]
Holding time after heating: 30 minutes or more (for example, 70 minutes or more), 3 hours or less (for example, 2 hours or less)
Cooling conditions: oil cooling, water cooling or air cooling

  The high-strength steel parts obtained as described above are not only excellent in tensile strength, but also austenite crystal grains are not coarsened, so that they are also resistant to delayed fracture under high load stress and high temperature. Excellent and has the following physical properties.

[Tensile strength]
The high-strength steel part according to the present invention is a part having a tensile strength of 1300 N / mm ² or more. Among them 1400 N / mm ² or more of can be used, more preferably 1500 N / mm ² or more, further preferably 1600 N / mm ² or more.

[Austenite grain size number]
The high-strength steel part according to the present invention has an austenite grain size number of 9 or more. Of these, those of 10 or more are preferably used, more preferably 11 or more.

[Delayed fracture resistance in acidic environment]
The SSRT test was conducted at a crosshead speed of 2 × 10 ⁻³ mm / min with a test piece immersed in a 5 mass% NaCl aqueous solution obtained by adding NaCl to an aqueous solution adjusted to pH 3.0 using HCl. measuring the elongation E ₁ by performing. Further, the elongation E ₀ is measured by performing an SSRT test in the air at a crosshead speed of 2 × 10 ⁻³ mm / min without being immersed in an aqueous NaCl solution. The delayed fracture characteristic value α is calculated from the measured values of E ₁ and E _{0 by} the following equation (3), and the one having this value α of 0.50 or less is used as the high strength steel part according to the present invention.
Delayed fracture characteristic value α = (1−E ₁ / E ₀ ) × 100 (3)

[Delayed fracture resistance under heat load]
By performing SSRT test crosshead speed of 2 × 10 ^{-3 mm} / min in a state where the test piece was immersed in distilled water 80 ° C. to measure the elongation E _2. Further, the elongation E ₀ is measured by performing an SSRT test with the crosshead speed of 2 × 10 ⁻³ mm / min in the air without immersing the sample in distilled water. A delayed fracture characteristic value β is calculated from the measured values of E ₂ and E _{0 by} the following equation (4), and a component having this value β of 0.50 or less is defined as a high strength steel part according to the present invention.
Delayed fracture characteristic value β = (1−E ₂ / E ₀ ) × 100 (4)

  According to the high-strength steel part of the present invention, it has excellent delayed fracture resistance even in an acidic environment despite its high strength. The high-strength component of the present invention is excellent in delayed fracture resistance even in an environment where a thermal load is applied. Therefore, for example, a bolt can be used in a wide range without being restricted in use.

  Hereinafter, the present invention will be described in more detail with reference to examples. However, the following examples are not intended to limit the present invention, and may be implemented with appropriate modifications within a range that can meet the purpose described above and below. These are all possible and are within the scope of the present invention.

  Wires (diameter: 12 mm) were manufactured by hot working test steels containing chemical components shown in Tables 1 and 2 below. In Tables 1 and 2 below, the value of the middle side of the formula (1) (that is, the value of the composite addition amount X) and the value of the left side of the formula (2) are shown together.

  The obtained wire was heated to 740 ° C. and held for 5 hours, and then gradually cooled to 650 ° C. (cooling rate: about 10 ° C./hr), and then allowed to cool to the atmosphere for spheroidizing annealing.

  By cutting and cutting the wire obtained by spheroidizing annealing, a compression test piece having a diameter of 10 mm × height: 15 mm was produced, and cold workability was evaluated under the following conditions.

[Cold workability]
The compression test piece was compressed while restraining both end faces thereof, and the stress (deformation resistance) necessary to obtain a rolling reduction of 70% was measured. The smaller the deformation resistance, the better the cold workability. The measured deformation resistance is shown in Tables 3 to 4 below. The deformation resistance was evaluated as acceptable (determination: ◯) when 900 N / mm ² or less, and as unacceptable (determination: x) when exceeding 900 N / mm ² .

  Moreover, the wire obtained by the spheroidizing annealing treatment was quenched (retention time after heating: 30 minutes, cooling condition was water cooling) and tempering (retention time after heating: 90 minutes) under the conditions shown in Tables 3 to 4 below. The cooling conditions were water cooling).

  The austenite grain size number of the obtained quenched and tempered steel was measured according to JIS G 0551 “Austenite grain size test method for steel”. The measured austenite grain size numbers are shown in Tables 3 to 4 below.

Next, the quenched and tempered steel was cut and cut to produce a JIS Z2201 No. 14A test piece. The tensile strength was measured using this test piece, and the measurement results are shown in Tables 3 to 4 below. The case where the tensile strength was 1300 N / mm ² or more was evaluated as acceptable (determination: ◯), and the case where the tensile strength was less than 1300 N / mm ² was evaluated as unacceptable (determination: x).

  Next, the quenched and tempered steel was cut and cut to produce a delayed fracture test piece shown in FIG. Using this delayed fracture specimen, the “delayed fracture resistance in an acidic environment” and the “delayed fracture resistance in an environment where a thermal load is applied” were examined by the above procedure. The calculated delayed fracture characteristic values α and β are shown in Tables 3 to 4 below. For the delayed fracture characteristic value α or β, a value exceeding 0.50 was regarded as unacceptable (judgment: x), and a value of 0.50 or less was regarded as acceptable (determination: ◯).

  As apparent from Tables 3 and 4, Nos. 1 to 29 are examples that satisfy the requirements defined in the present invention, have low deformation resistance, and are excellent in cold workability. Further, the delayed fracture characteristic value α is small, and the delayed fracture characteristic is excellent in an acidic environment. Further, the delayed fracture resistance value β is small, and the delayed fracture resistance value is excellent even in an environment where a thermal load is applied.

  On the other hand, Nos. 30 to 52 are examples that do not satisfy any of the requirements defined in the present invention, and a desired effect is not obtained. That is, No. 30 is an example in which the amount of C is small, and the tensile strength cannot be secured. No. 31 is an example in which the amount of C is large, the deformation resistance increases, and the cold workability deteriorates. In addition, since the toughness of steel deteriorates, the delayed fracture resistance in an acidic environment also decreases. No. 32 is an example in which the amount of Mn is small, and the strength cannot be secured. No. 33 is an example in which the amount of Mn is large, the cold workability is poor, and the delayed fracture resistance in an acidic environment is also inferior. No. 34 is an example having a large amount of P and S, and is inferior in delayed fracture resistance in an acidic environment. No. 36 and No. 37 are examples in which the amount of Cr is small, deformation resistance increases, and cold workability is poor. No. 38 and No. 39 are examples with a large amount of Cr and have poor delayed fracture resistance. Nos. 40 to 46 are examples in which the balance of Cr, Mo, Ti, and V amounts is poor, and these contents do not satisfy the above formula (1) defined in the present invention. Since the value is smaller than the range defined in the present invention, the deformation resistance at the time of cold working becomes too large and the cold workability is poor. Nos. 47 and 53 to 55 are examples in which the balance of Cr, Mo, Ti, and V amount is poor and the above formula (1) defined in the present invention is not satisfied. Since it is larger than the range specified in the invention, the delayed fracture resistance is inferior. Nos. 48 to 51 are examples in which the balance between Ti and V amount is poor and the above formula (2) is not satisfied, and the value on the left side of formula (2) is smaller than the range defined in the present invention, so that the heat load The delayed fracture characteristics are poor. No. 52 has poor delayed fracture resistance due to the poor balance between Ti and V content.

  In addition, No. 35 is a reference example, and cold workability is deteriorated due to a large amount of Ni.

It is a graph which shows the relationship between the composite addition amount X and the measured deformation resistance or delayed fracture characteristic value α of a part. It is a graph which shows the relationship between ratio ([Ti] / [V]) of Ti and V, and delayed fracture characteristic value (beta). It is the schematic of the test piece used for the delayed fracture test.

Claims (6)

% By mass (hereinafter the same),
C: 0.30 to 0.6%,
Si: 0.2% or less,
Mn: 0.1 to 0.8%
P: 0.02% or less,
S: 0.02% or less,
Cr: more than 0.5%, 0.95% or less,
Mo: 1.3-2.2%,
Ti: 0.05 to 0.15%,
V: 0.1-0.50%,
Al: 0.5% or less and N: 0.02% or less,
And the balance is a steel composed of Fe and inevitable impurities,
A high-strength steel excellent in cold workability and delayed fracture resistance, characterized in that each component satisfies the following formulas (1) and (2).
0.3 ≦ 10 × {exp (0.4 × [Cr]) / [exp (2 × [Mo]) + exp (92 × [Ti] × [V])]} ≦ 0.6 (1)
[Ti] / [V] ≧ 0.1 (2)
In the formula, [Cr], [Mo], [Ti], and [V] are the amount of Cr (mass%), Mo (mass%), Ti (mass%), and V ( Mass%). Furthermore, as other elements,
The high-strength steel according to claim 1, which contains Ni: 1.0% or less (not including 0%) and / or Cu: 1% or less (not including 0%).   The high-strength steel according to claim 1 or 2, further comprising a total of 0.5% or less (not including 0%) of elements selected from the group consisting of Zr, W and Nb as other elements.   The high-strength steel according to any one of claims 1 to 3, further comprising B: 0.003% or less (not including 0%) as another element.   After hot rolling the steel according to any one of claims 1 to 4, spheroidizing annealing, then cold working into a predetermined shape, and quenching the cold worked steel at a heating temperature of 890-960 ° C Then, a high strength steel part having excellent delayed fracture resistance obtained by tempering at a heating temperature of 550 ° C. or higher. A high-strength steel part having the component composition shown in any one of claims 1 to 4 and excellent in delayed fracture resistance having the characteristics shown in the following (a) to (d).
(A) Tensile strength: 1300 N / mm ² or more (b) Austenite grain size number: 9 or more (c) A crosshead speed of 2 × 10 with a test piece immersed in a 5 mass% NaCl aqueous solution having a pH of 3.0 the low strain rate test measures the elongation E ₁ by performing a ^{-3 mm} / min, also a non-performed in the atmosphere in the same way as in is the dipping low strain rate testing without immersion in NaCl solution the elongation E ₀ was measured by performing, measured E ₁ and E (3) below from a value of ₀ delayed fracture characteristic value calculated by the equation alpha is 0.50 or less delayed fracture property value alpha = (1-E ₁ / E ₀ ) × 100 (3)
(D) Elongation E ₂ is measured under the same conditions except that 80 ° C. distilled water is used in place of the NaCl aqueous solution in (c) above, and the atmosphere is not immersed in distilled water as in (c) above. The elongation E ₀ is measured in the sample, and the delayed fracture characteristic value β calculated by the following equation (4) from the measured values of E ₂ and E ₀ is 0.50 or less. Delayed fracture characteristic value β = (1−E ₂ / E ₀ ) × 100 (4)

Images