JP3218442B2 - Manufacturing method of mechanical structural steel with excellent delayed fracture resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION The present invention relates to 125 kgf / mm ²
The present invention relates to a method for producing a steel for machine structural use having the above tensile strength and excellent in delayed fracture resistance.

[0002]

2. Description of the Related Art High-strength steel for machine structural use is widely used, for example, as high-strength bolts in machines, automobiles, bridges and buildings, and is also widely used as PC steel wire and automobile parts. However, for all varieties, the tensile strength is 1
It is known that when the pressure exceeds 25 kgf / mm ² , the risk of delayed fracture increases. Therefore, the strength of currently used bolts is limited to 110 kgf / mm ² class.

[0003] However, in recent years, with the increase in the size of the structure, there has been an increasing demand for higher strength bolts from the viewpoint of improving joint efficiency and reducing weight. On the other hand, in automobiles that need to improve fuel efficiency from the viewpoint of protection of the global environment, high strength of various components is required in order to achieve a reduction in vehicle weight that directly affects fuel efficiency. Therefore, in order to satisfy these requirements, the strength is 125 kgf / m
The problem of delayed fracture of steel for machine structural use exceeding m ² must be solved. It is believed that delayed fracture of high strength members is caused by hydrogen in steel. In particular, it is considered that diffusible hydrogen, which can easily move around normal temperature, accumulates at the crystal grain boundary in the tensile stress concentrated portion and causes delayed fracture to promote grain boundary cracking.

Accordingly, high-strength structural steel must be resistant to hydrogen, especially diffusible hydrogen. On the other hand, in the manufacturing process of high-strength steel for mechanical structural use, for example, steel bars and wires, when the material is cooled at a normal cooling rate, a low-temperature transformation structure having high delayed fracture susceptibility starts from a portion where P is concentrated by microsegregation in the surface layer. There is also a problem in that it is formed and delayed fracture occurs after winding of the material, thereby lowering the product yield.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to provide a method for producing a steel for machine structural use having a tensile strength of 125 kgf / mm ² or more, which is excellent in delayed fracture resistance.

[0006]

The gist of the present invention is as follows. (1) C: 0.15 to 0.50% by weight, Si ≦
2.0%, Mn ≦ 0.6%, P ≦ 0.015%, S ≦
0.02%, Cr: 0.1 to 3.0%, Mo: 0.2 to
1.2%, Al: 0.005 to 1.0%, N ≦ 0.03
V: more than 0.20%, 0.50% or less, Ti: more than 0.05%, 0.50% or less, Nb: 0.
After hot-rolling a steel containing one or more of not less than 05% and not more than 0.50%, the balance being Fe and unavoidable impurities, the steel is cooled under conditions satisfying Kl ≦ 130 defined by the following equation. After performing spheroidizing annealing, forming into a predetermined shape by cold working, and then performing quenching and tempering, tempering is performed in a temperature range of 400 ° C. or more, 125 kgf / mm ^2. A method for producing a steel for machine structural use having the above tensile strength and excellent in delayed fracture resistance.

Kl = Dl + 260 × 1 log ₁₀ (CR) Dl = (5 × [% C] +3) × (1 + 0.64 × [% S
i]) × (1 + 4.10 × [% Mn]) × (1 + 2.8
3 x [% P]) x (1-0.62 x [% S]) x (1+
2.33 × [% Cr]) × (1 + 0.52 × [% N
i]) × (1 + 3.14 × [% Mo]) (unit: m
m, [% X]: weight% of element X) CR: cooling rate of steel material (unit: ° C./sec) (2) By weight, C: 0.15 to 0.50%, Si ≦
2.0%, Mn ≦ 0.6%, P ≦ 0.015%, S ≦
0.02%, Cr: 0.1 to 3.0%, Mo: 0.2 to
1.2%, Al: 0.005 to 1.0%, N ≦ 0.03
V: more than 0.20%, 0.50% or less, Ti: more than 0.05%, 0.50% or less, Nb: 0.
After hot rolling to a rolling finish temperature of 650 to 900 ° C. on a steel containing one or more of not less than 05% and 0.50% or less and the balance being Fe and unavoidable impurities, After cooling under conditions satisfying Kl ≦ 130 defined by the following formula, then performing spheroidizing annealing, forming into a predetermined shape by cold working, and then performing quenching / tempering treatment, tempering at 400 ° C. A method for producing a steel for machine structural use having a tensile strength of 125 kgf / mm ² or more and excellent in delayed fracture resistance, which is performed in the above temperature range.

Kl = Dl + 260 × 1 log ₁₀ (CR) Dl = (5 × [% C] +3) × (1 + 0.64 × [% S
i]) × (1 + 4.10 × [% Mn]) × (1 + 2.8
3 x [% P]) x (1-0.62 x [% S]) x (1+
2.33 × [% Cr]) × (1 + 0.52 × [% N
i]) × (1 + 3.14 × [% Mo]) (unit: m
m, [% X]: weight% of element X) CR: cooling rate of steel material (unit: ° C./sec) (3) By weight, C: 0.15 to 0.50%, Si ≦
2.0%, Mn ≦ 0.6%, P ≦ 0.015%, S ≦
0.02%, Cr: 0.1 to 3.0%, Mo: 0.2 to
1.2%, Ni ≦ 2.0%, Al: 0.005 to 1.0
%, N ≦ 0.03%, and V: 0.20%
More than 0.50%, Ti: more than 0.05%, 0.50%
Hereinafter, after hot rolling a steel containing one or two or more kinds of Nb: more than 0.05% and 0.50% or less, the balance being Fe and unavoidable impurities, K defined by the following formula
After cooling under conditions satisfying l ≦ 130, and then performing spheroidizing annealing, forming into a predetermined shape by cold working, and then performing quenching / tempering treatment,
125 kgf /
A method for producing steel for machine structural use having a tensile strength of not less than ² mm and excellent in delayed fracture resistance.

Kl = Dl + 260 × 1 log ₁₀ (CR) Dl = (5 × [% C] +3) × (1 + 0.64 × [% S
i]) × (1 + 4.10 × [% Mn]) × (1 + 2.8
3 x [% P]) x (1-0.62 x [% S]) x (1+
2.33 × [% Cr]) × (1 + 0.52 × [% N
i]) × (1 + 3.14 × [% Mo]) (unit: m
m, [% X]: weight% of element X) CR: cooling rate of steel material (unit: ° C./sec) (4) By weight, C: 0.15 to 0.50%, Si ≦
2.0%, Mn ≦ 0.6%, P ≦ 0.015%, S ≦
0.02%, Cr: 0.1 to 3.0%, Mo: 0.2 to
1.2%, Ni ≦ 2.0%, Al: 0.005 to 1.0
%, N ≦ 0.03%, and V: 0.20%
More than 0.50%, Ti: more than 0.05%, 0.50%
Hereinafter, Nb: hot-rolled steel containing one or more of 0.05% or more and 0.50% or less, and the balance consisting of Fe and unavoidable impurities, with a rolling finish temperature of 650 to 900 ° C. After rolling, Kl defined by the following equation:
After cooling under conditions satisfying ≦ 130, and then performing spheroidizing annealing, forming into a predetermined shape by cold working, and then performing quenching / tempering treatment, tempering at 400 ° C.
125 kgf / m, which is performed in the above temperature range
A method for producing a steel for machine structural use having a tensile strength of at least m ² and excellent resistance to delayed fracture.

Kl = Dl + 260 × 1 log ₁₀ (CR) Dl = (5 × [% C] +3) × (1 + 0.64 × [% S
i]) × (1 + 4.10 × [% Mn]) × (1 + 2.8
3 x [% P]) x (1-0.62 x [% S]) x (1+
2.33 × [% Cr]) × (1 + 0.52 × [% N
i]) × (1 + 3.14 × [% Mo]) (unit: m
m, [% X]: weight% of element X) CR: cooling rate of steel material (unit: ° C / sec) Hereinafter, the present invention will be described in detail.

As a result of repeated studies to solve the above problems, the inventors of the present invention have found that adjustment of the chemical composition of steel, in particular, reduction of Mn and P contents, increase of Mo and Ni contents, and By adding one or more of V, Ti, and Nb, and adjusting the tempering temperature, it was found that the critical diffusion hydrogen amount that does not lead to delayed fracture can be increased. The inventors have found that the frequency of delayed fracture of a material can be greatly reduced by controlling the cooling rate of the material, thereby completing the present invention. According to the present invention, it is possible to obtain a steel for machine structural use having a tensile strength of 125 kgf / mm ² or more, which shows a higher critical diffusion hydrogen amount than conventional steel.

The present inventors have investigated the effects of alloying elements on delayed fracture resistance and the effect of tempering temperature in heat treatment. As a result, the inventors have found that the Mn and P contents are reduced as compared with conventional steels for machine structural use. , Mo, and Ni, increasing the amount of V, Ti, and Nb is effective, and tempering in a temperature range of 400 ° C. or more is effective.

It has also been found that by adjusting the cooling rate of the material after hot rolling in accordance with the hardenability of the material, the occurrence of cracks in the wire coil or bar coil can be suppressed. Furthermore, it has been found that by setting the finishing temperature in the hot rolling within the range of 650 to 900 ° C., the occurrence of cracks in the wire coil or the bar coil can be further suppressed.

Next, the reasons for limiting the alloy composition of the material in the present invention will be described. C must be contained in an amount of 0.15% or more in order to obtain high-strength steel by quenching and tempering. On the other hand, a large amount of C exceeding 0.50%
In addition, not only does the toughness of the steel deteriorate, but also the delayed fracture resistance deteriorates. Si is an element necessary for deoxidizing steel and improving the strength of steel.
Large amounts of addition exceeding 0% impair the cold workability of the steel.

Mn is an element necessary for deoxidizing the steel and ensuring the hardenability of the steel. However, when the steel is heated to the austenite region, it not only segregates at the grain boundaries but also embrittles the grain boundaries. In order to deteriorate the delayed fracture resistance of steel, the content is set to 0.6% or less. P is effective as a hardenability improving element, but is an element that segregates microscopically during solidification, segregates at the grain boundaries during heating in the austenite region, embrittles the grain boundaries, and deteriorates delayed fracture resistance. 0.015%
It was as follows.

S is an unavoidable impurity, but segregates at the grain boundaries during heating in the austenite region, embrittles the grain boundaries, and degrades the delayed fracture resistance.
02% or less. Cr is required to be 0.1% or more in order to obtain the hardenability of steel, but if it is too large, it is an element that causes deterioration of toughness and cold workability, so that Cr is set to 3.0% or less.

Mo is an element necessary for obtaining the hardenability of steel, has a tempering softening resistance, and is effective for obtaining a tensile load of 125 kgf / mm ² or more stably at a tempering temperature of 400 ° C. or more. However, if the amount is too large, the effect is saturated and the cost is increased. Al is an effective element for deoxidizing steel, so
Although it is necessary to be 5% or more, if it is too much, the toughness is deteriorated.

N is an element that segregates at the grain boundary during austenite heating to embrittle the grain boundary and also deteriorates the delayed fracture resistance, so that N is set to 0.03% or less. Ni is an element that is added as needed and improves the toughness and the delayed fracture resistance. However, when the content exceeds 2.0%, the effect is saturated, and the cost is rather increased.

V, Ti, and Nb contribute to the refinement of crystal grains and have a high affinity for hydrogen.
Since these elements are effective for improving delayed fracture resistance by suppressing accumulation, V: more than 0.20% and Ti:
It is necessary to add more than 0.05% and Nb: more than 0.05%. However, if the content is too large, the effect saturates and the element is rather deteriorated in toughness.
0.50% or less, Ti: 0.50% or less, Nb: 0.5
0% or less.

The heat treatment conditions are 125 kgf / mm
The lower limit of tempering temperature was set to prevent delayed fracture at high strength of ² or more. That is, if the temperature is lower than 400 ° C., the grain boundary embrittlement becomes remarkable and the delayed fracture resistance deteriorates. On the other hand, the cooling rate after rolling is determined to improve the delayed fracture resistance of steel bars and wire rod coils, suppress the occurrence of coil cracks, increase the coil yield, and finish the rolling to further suppress coil cracks. The temperature was also determined. In other words, with regard to the cooling rate after rolling, it is necessary to gradually cool to suppress the generation of low-temperature transformation structure with high delayed fracture susceptibility and reduce cracking caused by delayed fracture. Considering the cooling rate, the cooling rate (CR) was limited to a range satisfying Kl ≦ 130.

Where Kl = Dl + 260 × 1 log ₁₀ (CR) Dl = (5 × [% C] +3) × (1 + 0.64 × [% Si]) × (1 + 4.10 × [% Mn]) × (1 + 2 .83 × [% P]) × (1 + 0.62 × [% S]) × (1 + 2.33 × [% Cr]) × (1 + 0.52 × [% Ni]) × (1 + 3.14 × [% Mo] ]) (Unit: mm, [% X]: weight% of element X) CR: cooling rate (unit: ° C / sec) In addition, regarding the rolling finishing temperature, the fine grain size is further improved to improve the delayed fracture resistance of the coil. However, the effect is ineffective at a temperature exceeding 900 ° C., and the effect is saturated at a temperature lower than 650 ° C., but rather, the rolling finishing temperature is reduced to 650 to 900 ° C. to inhibit productivity. And

[0022]

EXAMPLES The chemical components of the test steel are shown in Table 1. (A) ~
(J) is a steel of the present invention, and (K) to (O) are comparative steels.
You. Using these 20 mmφ steel bars, the tensile strength is 1
50 ~ 160kgf / mm^TwoHeat treatment (quenching
Tempering). Heat treatment conditions and tensile strength
The degrees are shown in Table 2. How these steels resist delayed fracture
Degree of diffusible hydrogen is acceptable, i.e.
The elementary quantity was examined.

Hereinafter, a method for obtaining the limit hydrogen amount will be described. A test piece provided with a circumferential notch of 2 mmV in the shaft portion with M10 bolt shown in FIG. 1 is prepared, and immersed in 20 to 36% HCl for 20 to 120 minutes to enrich hydrogen in a set of two pieces. Thus, the amount of hydrogen in the test piece was changed.
One of them was immersed in HCl and left in the air for 30 minutes, then the amount of hydrogen was measured by thermal analysis, and the other was left in the air for 30 minutes after immersion, as shown in FIG. Rupture test was carried out using a test machine. In FIG. 2, 1 is a test piece, 2
Indicates a balance weight, and 3 indicates a fulcrum. The test load in the delayed fracture test was constant at 70% of the fracture load of each test piece before immersion in the HCl solution.

Table 3 shows the relationship between the amount of diffusible hydrogen obtained and the rupture time in the delayed fracture test when the concentration of HCl and the immersion time are variously changed in accordance with the above procedure. From the table, the upper limit diffusible hydrogen amount that does not cause delayed fracture of each steel,
That is, when the amount of critical diffusible hydrogen is estimated, Table 4 is obtained. From this table, (A) to (J) in the range of the composition and tempering temperature of the present invention have a higher critical hydrogen content than the comparative materials (K) to (O), and are less susceptible to delayed fracture. Is evident.

Next, when the steel of the present invention is heated to 1200 ° C., and the rolling finish temperature and the cooling rate after the rolling are changed, 100 kg of a coil having a cross section of 25 mmφ and a diameter of 1400 mm is produced within 100 hours after the production. Table 5 shows the state of occurrence of coil cracks. When cooling is performed at a cooling rate of Kl> 130, which is outside the range of the cooling conditions of the present invention, the crack occurrence rate of the coil is 0.17% or more regardless of the rolling finishing temperature, whereas the Kl of the present invention is When cooled at a cooling rate that satisfies ≦ 130, the generation rate is suppressed to 0.05% or less, and it is clear that it is effective to perform slow cooling that satisfies Kl ≦ 130 to reduce the crack generation rate. . Further, by lowering the rolling finish temperature from 1000 ° C. to 850 ° C., the crack occurrence rate of the coil is suppressed to 0.02% or less, and lowering the rolling finish temperature to 900 ° C. or less is effective for reducing the coil cracking rate. It is clear that.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

[0030]

[Table 5]

[0031]

According to the present invention, a bolt having a tensile strength of 125 kgf / mm ² or more and excellent in delayed fracture resistance can be expected. As a result, the joint efficiency of the bolt is improved, which contributes to weight reduction of automobiles and the like. In addition, it is possible to suppress the occurrence of cracks after the coil is manufactured, and it is also possible to solve the operational problem of lowering the yield. Therefore, the industrial effect is great.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram of a shape of a test piece.

FIG. 2 is an explanatory diagram of a delayed fracture test apparatus.

──────────────────────────────────────────────────続 き Continued on the front page (58) Field surveyed (Int. Cl. ⁷ , DB name) C21D 8/00-8/10 C22C 38/00-38/60

Claims (4)

(57) [Claims] 1. C: 0.15 to 0.50% by weight, S
i ≦ 2.0%, Mn ≦ 0.6%, P ≦ 0.015%, S
≦ 0.02%, Cr: 0.1-3.0%, Mo: 0.2
１．２1.2%, Al: 0.005 to 1.0%, N ≦ 0.0
3%, V: more than 0.20%, 0.50% or less, Ti: more than 0.05%, 0.50% or less, Nb: 0.
After hot-rolling a steel containing one or more of not less than 05% and not more than 0.50%, the balance being Fe and unavoidable impurities, the steel is cooled under conditions satisfying Kl ≦ 130 defined by the following equation. After performing spheroidizing annealing, forming into a predetermined shape by cold working, and then performing quenching and tempering, tempering is performed in a temperature range of 400 ° C. or more, 125 kgf / mm ^2. A method for producing a steel for machine structural use having the above tensile strength and excellent in delayed fracture resistance. Kl = Dl + 260 × 1 log ₁₀ (CR) Dl = (5 × [% C] +3) × (1 + 0.64 × [% S
i]) × (1 + 4.10 × [% Mn]) × (1 + 2.8
3 x [% P]) x (1-0.62 x [% S]) x (1+
2.33 × [% Cr]) × (1 + 0.52 × [% N
i]) × (1 + 3.14 × [% Mo]) (unit: m
m, [% X]: weight% of element X) CR: cooling rate of steel material (unit: ° C / sec) 2. C: 0.15 to 0.50% by weight, S
i ≦ 2.0%, Mn ≦ 0.6%, P ≦ 0.015%, S
≦ 0.02%, Cr: 0.1-3.0%, Mo: 0.2
１．２1.2%, Al: 0.005 to 1.0%, N ≦ 0.0
3%, V: more than 0.20%, 0.50% or less, Ti: more than 0.05%, 0.50% or less, Nb: 0.
After hot rolling to a rolling finish temperature of 650 to 900 ° C. on a steel containing one or more of not less than 05% and 0.50% or less and the balance being Fe and unavoidable impurities, After cooling under conditions satisfying Kl ≦ 130 defined by the following formula, then performing spheroidizing annealing, forming into a predetermined shape by cold working, and then performing quenching and tempering treatment, tempering at 400 ° C. A method for producing steel for machine structural use having a tensile strength of 125 kgf / mm ² or more and excellent in delayed fracture resistance, characterized by being carried out in the above temperature range. Kl = Dl + 260 × 1 log ₁₀ (CR) Dl = (5 × [% C] +3) × (1 + 0.64 × [% S
i]) × (1 + 4.10 × [% Mn]) × (1 + 2.8
3 x [% P]) x (1-0.62 x [% S]) x (1+
2.33 × [% Cr]) × (1 + 0.52 × [% N
i]) × (1 + 3.14 × [% Mo]) (unit: m
m, [% X]: weight% of element X) CR: cooling rate of steel material (unit: ° C / sec) 3. C: 0.15 to 0.50% by weight, S
i ≦ 2.0%, Mn ≦ 0.6%, P ≦ 0.015%, S
≦ 0.02%, Cr: 0.1-3.0%, Mo: 0.2
-1.2%, Ni≤2.0%, Al: 0.005-1.
0%, N ≦ 0.03%, V: 0.20%
More than 0.50%, Ti: more than 0.05%, 0.50%
Hereinafter, after hot rolling a steel containing one or two or more kinds of Nb: more than 0.05% and 0.50% or less, the balance being Fe and unavoidable impurities, K defined by the following formula
After cooling under conditions satisfying l ≦ 130, and then performing spheroidizing annealing, forming into a predetermined shape by cold working, and then performing quenching / tempering treatment,
125 kgf /
A method for producing steel for machine structural use having a tensile strength of not less than ² mm and excellent in delayed fracture resistance. Kl = Dl + 260 × 1 log ₁₀ (CR) Dl = (5 × [% C] +3) × (1 + 0.64 × [% S
i]) × (1 + 4.10 × [% Mn]) × (1 + 2.8
3 x [% P]) x (1-0.62 x [% S]) x (1+
2.33 × [% Cr]) × (1 + 0.52 × [% N
i]) × (1 + 3.14 × [% Mo]) (unit: m
m, [% X]: weight% of element X) CR: cooling rate of steel material (unit: ° C / sec) 4. C: 0.15 to 0.50% by weight, S
i ≦ 2.0%, Mn ≦ 0.6%, P ≦ 0.015%, S
≦ 0.02%, Cr: 0.1-3.0%, Mo: 0.2
-1.2%, Ni≤2.0%, Al: 0.005-1.
0%, N ≦ 0.03%, V: 0.20%
More than 0.50%, Ti: more than 0.05%, 0.50%
Hereinafter, Nb: hot-rolled steel containing one or more of 0.05% or more and 0.50% or less, and the balance consisting of Fe and unavoidable impurities, with a rolling finish temperature of 650 to 900 ° C. After rolling, Kl defined by the following equation:
After cooling under conditions satisfying ≦ 130, and then performing spheroidizing annealing, forming into a predetermined shape by cold working, and then performing quenching / tempering treatment, tempering at 400 ° C.
125 kgf / m, which is performed in the above temperature range
A method for producing a steel for machine structural use having a tensile strength of at least m ² and excellent resistance to delayed fracture. Kl = Dl + 260 × 1 log ₁₀ (CR) Dl = (5 × [% C] +3) × (1 + 0.64 × [% S
i]) × (1 + 4.10 × [% Mn]) × (1 + 2.8
3 x [% P]) x (1-0.62 x [% S]) x (1+
2.33 × [% Cr]) × (1 + 0.52 × [% N
i]) × (1 + 3.14 × [% Mo]) (unit: m
m, [% X]: weight% of element X) CR: cooling rate of steel material (unit: ° C / sec)