JPH05255738A - Production of steel for machine structural use excellent in delayed fracture resistance - Google Patents

Abstract

PURPOSE:To produce a high strength steel for machine structural use excellent in delayed fracture resistance by subjecting a steel having a specific composition consisting of C, Si, Mn, P, S, Cr, Mo, Al, N, and Fe to hot rolling and then to specific cooling and tempering. CONSTITUTION:A steel which has a composition consisting of, by weight, 0.15-0.50% C, <=0.5% Si, <=0.6% Mn, <=0.015% P, <=0.02% S, 0.1-2.0% Cr, 0.2-1.2% Mo, 0.005-1.0% Al, <=0.03% N, and the balance Fe with inevitable impurities and further containing, if necessary, one or more kinds among 0.001-0.20% V, 0.001-0.050% Ti, and 0.001-0.050% Nb and further containing <=2.0% Ni is hot-rolled, finish-rolled, preferebly, at 650-900 deg.C, and then cooled under the condition that the value of K represented by equation is <=130. The resulting hot rolled stock is subjected to spheroidizing annealing, to forming, and then to quench-and-temper treatment at <=400 deg.C. By this method, the high strength steel for machine structural use having >=125kgf/mm<2> tensile strength and excellent in delayed fracture resistance can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is applied to a method for producing a mechanical structural component having a tensile strength of 125 kgf / mm ² or more and an excellent delayed fracture resistance.

[0002]

2. Description of the Related Art High-strength mechanical structural steel is widely used as high-strength bolts in machines, automobiles, bridges, and buildings.
It is also widely used in PC steel rods and automobile parts. However, the tensile strength of all varieties is 125 kgf /
It is well known that the risk of delayed fracture increases when it exceeds mm ² , and for example, the strength of currently used bolts is currently 110 kgf / mm ² class as the upper limit.

[0003]

However, with the recent increase in size of structures, there is a strong demand for higher strength bolts for the purpose of improving joint efficiency and reducing weight. In addition, in automobiles that need to improve fuel efficiency due to global environmental issues, there is a strong demand for higher strength of various parts in order to achieve weight reduction that leads to fuel efficiency. Therefore, it is necessary to solve the problem of delayed fracture of mechanical structural steel having a strength exceeding 125 kgf / mm ² .

Hydrogen in steel is considered to be the cause of delayed fracture of high strength members. In particular, it is considered that diffusible hydrogen, which can easily move near room temperature, accumulates at the grain boundaries of the tensile stress concentration portion and promotes intergranular cracking, resulting in delayed fracture. Therefore, if high strength mechanical structural steel is used, it must be resistant to hydrogen, especially diffusible hydrogen.

On the other hand, in the manufacturing process of the material (bar steel / wire), when it is cooled at a normal cooling rate, a low temperature transformation structure having a delayed fracture susceptibility is formed on the surface layer due to microsegregation and having a higher delayed fracture susceptibility than the P concentrated portion. Therefore, there is a problem that delayed fracture occurs in the coil after winding and the product yield is reduced, and it is also necessary to improve the manufacturing method.

An object of the present invention is to solve the above-mentioned problems and to provide a method for producing high strength mechanical structural steel for imparting excellent delayed fracture resistance.

[0007]

DISCLOSURE OF THE INVENTION The inventors of the present invention have adjusted the chemical composition of steel, in particular, lowering Si, Mn, P, increasing Mo and Ni, and adjusting heat treatment so that the diffusivity is the limit that does not lead to delayed fracture. It was found that the amount of hydrogen (hereinafter referred to as critical diffusible hydrogen) can be increased, and that the frequency of delayed fracture occurrence in the coil can be suppressed by controlling the cooling rate after rolling the steel bar. .. The present invention has been made on the basis of the above findings, and by heat treatment, at a high strength of 125 kgf / mm ² or more, shows a critical diffusible hydrogen that is higher than that of conventional steel, and delayed fracture resistance. It manufactures high-strength mechanical structural steel with excellent properties.

That is, in the present invention, it is effective to investigate the influence of alloying elements on the delayed fracture resistance and to reduce Si, Mn, P and increase Mo, Ni as compared with the conventional steel for machine structural use. And that the occurrence of coil cracks can be suppressed by adjusting the cooling rate after rolling according to the hardenability, and C: 0.15 to 0.50% by weight%. , Si: 0.5% or less, Mn: 0.6% or less, P: 0.015% or less, S: 0.02% or less, Cr: 0.1 to 2.0
%, Mo: 0.2 to 1.2%, Al: 0.005
1.0%, N: 0.03% or less is contained, Ni: 2.0% or less is added if necessary, and V: 0.001 to 0.20%, Ti: 0.001 to 0.001 if necessary.
0.050%, Nb: 0.001 to 0.050% of one or more kinds of steel, with the balance being Fe and inevitable impurities, after hot rolling, or at a rolling finishing temperature of 650 to 900 ° C. After rolling, K ≦ 130
Steel bar or wire rod obtained by cooling under the condition satisfying the condition is subjected to spheroidizing annealing, and after being formed into a predetermined shape, tempering is performed at 400 ° C. or more when quenching / tempering is performed. 125 kgf / mm ² or more It is a method for producing high-strength mechanical structural steel having tensile strength and excellent delayed fracture resistance. However, K = D ₁ + 260 × log ₁₀ (CR) D ₁ = (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) The present invention is described in detail below. First, the alloy composition range of the steel for machine structural use in the present invention was determined for the following reason. C
Is 0.1 in order to obtain high strength by quenching and tempering.
It is necessary to be 5% or more, but if it is too much, it is an element that deteriorates the toughness and also the delayed fracture resistance, so the content was made 0.50% or less.

Si is an element necessary for deoxidizing the steel and increasing the strength, but since it is an element that impairs cold workability, it is segregated at the grain boundaries during austenite heating and embrittles the grain boundaries. Also, since it is an element that deteriorates the delayed fracture resistance, it is set to 0.5% or less. Mn is an element necessary for ensuring deoxidation and hardenability of steel, but it is an element that segregates at grain boundaries during austenite heating, embrittles the grain boundaries, and deteriorates delayed fracture resistance. % Or less.

Although P is effective as a hardenability element, it is an element that causes microsegregation during solidification and segregates to grain boundaries during austenite heating to embrittle the grain boundaries and deteriorate delayed fracture resistance. It was set to 0.015% or less. Although S is an unavoidable impurity, it is an element that segregates at the grain boundaries during austenite heating to embrittle the grain boundaries and deteriorates the delayed fracture resistance.

In order to obtain the hardenability of steel, Cr is 0.
It is required to be 1% or more, but if it is too much, it is an element that causes deterioration of toughness and cold workability, so it was made 2.0% or less. ◎ Mo is an element that is necessary for obtaining the hardenability of steel and has tempering softening resistance and is effective for stably obtaining a tensile load of 125 kgf / mm ² or more at a tempering temperature of 400 ° C or more. However, if it is too large, the effect is saturated and the cost is increased, so the content is made 1.2% or less.

[0012] Al is an element effective for deoxidizing steel, so 0.005% or more is necessary. However, if too much, it causes deterioration of toughness, so 1.0% or less. N is an element that segregates at the grain boundaries during austenite heating, embrittles the grain boundaries, and deteriorates the delayed fracture resistance as well, so N was made 0.03% or less.

Ni is an element that is added as needed to improve toughness and delayed fracture resistance. However, if it exceeds 2.0%, the effect is saturated and the cost is rather increased, so the content is made 2.0% or less. V, T
Since i and Nb are added as necessary and contribute to the refinement of crystal grains and improve delayed fracture resistance, 0.001% or more of each is necessary. However, when the amount is too large, the effect is reduced and rather the element deteriorates the toughness, so V: 0.20% or less, Ti: 0.050% or less, N
b: 0.050% or less.

On the other hand, the delayed fracture resistance of the product is improved,
The rolling and heat treatment conditions were determined in order to increase the yield.
That is, regarding the cooling rate after rolling, slow cooling is essential in order to suppress the formation of a low temperature transformation structure with high delayed fracture sensitivity and reduce cracks due to delayed fracture.・ Considering the material diameter, the cooling rate was limited to the range satisfying K ≦ 130. However, K = D ₁ + 260 × log ₁₀ (CR) D ₁ = (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]: wt% of element X) CR: Cooling rate (Unit: ° C / sec) Also, regarding rolling finishing temperature, the delayed fracture resistance of the product can be determined by grain refinement. Although it is an effective means for further improving, the effect becomes ineffective at a temperature higher than 900 ° C., and the effect is saturated at a temperature lower than 650 ° C., rather, the rolling finishing temperature is 650 to 650 in order to hinder the productivity. 900 ° C
And

[0015]

[Examples] Table 1 shows the chemical composition of the test steel. In the table (A)
~ (J) is according to the steel for bolts of the present invention,
(K) to (O) are comparative steels. Using these 20 mmφ steel bars, the tensile strength is 150 kgf / mm ^{2 to} 160 kgf / mm
Heat treatment (quenching-tempering) was performed with the goal of ² . Table 2 shows the heat treatment conditions and the tensile strength at this time.

The extent of diffusible hydrogen that these steels can tolerate against delayed fracture, that is, the limiting hydrogen content of each steel was investigated. The method of obtaining the limit hydrogen amount will be described below. Using M10 bolt shown in Fig. 4, make a test piece with a circumferential notch of 2 mmV on the shaft and make a set of two pieces, and dip them in 20 to 36% HCl for 20 to 60 minutes to enrich hydrogen. By doing so, the amount of hydrogen in the test piece is changed.
One of them was immersed in HCl and left in the atmosphere for 30 minutes, and then the amount of hydrogen was measured by a thermal analysis method. The other one was left in the atmosphere for 30 minutes after the immersion and then shown in FIG. The delayed destructive test is performed with the testing machine. In FIG. 2, 1 is a test piece, 2 is a balance weight, and 3 is a fulcrum. Further, the test load in the delayed fracture test was constant at 70% of the breaking load of each test piece before being immersed in the HCl solution.

Table 3 shows the relationship between the amount of diffusible hydrogen obtained and the breaking time in the delayed fracture test when the concentration of HCl and the immersion time were variously changed according to the above procedure. From the table, the upper limit of diffusible hydrogen content that does not cause delayed fracture of each steel,
That is, Table 4 shows the estimation of the critical diffusible hydrogen content. From this table, (A) to (J) in the composition and tempering temperature range of the present invention are comparative materials (K) to (O).
It was revealed that the limit hydrogen content was higher than that of No. 1, and delayed fracture was less likely to occur. Steel (O) is a comparative steel having a low tempering temperature as shown in Table 2 and a low limit hydrogen content. Therefore, it is necessary to set the tempering temperature to 400 ° C. or higher in order to obtain steel that is hard to undergo delayed fracture.

[0018]

[Table 1]

[Table 2]

[Table 3]

[Table 4]

[Table 5] Next, using the steels (A), (C), (F), and (H) of the present invention, after heating to 1200 ° C., the rolling finish temperature and the cooling rate after rolling were changed, and a coil having a cross section of 25 mmφ and a diameter of 1400 mm was 100 kg. Table 5 shows the situation of occurrence of coil cracks within 100 hours after the manufacturing. In the case of cooling at a cooling rate satisfying K> 130, which is outside the range of the cooling conditions of the present invention, the crack occurrence rate of the coil is 0.19% or more regardless of the rolling finish temperature, whereas K ≦ 13
When cooled at a cooling rate that satisfies 0, the occurrence rate is suppressed to 0.06% or less, and K ≦ 130 to reduce the crack occurrence rate.
It was revealed that it is effective to carry out slow cooling that satisfies the above conditions. Furthermore, the rolling finishing temperature is 1000 ° C to 850 ° C.
It was clarified that the occurrence rate of coil cracking can be suppressed to 0.04% or less by lowering the value to, and lowering the rolling finishing temperature to 900 ° C. or less is effective in reducing the rate of coil cracking.

[0019]

According to the present invention, a bolt having a tensile strength of 125 kgf / mm ² or more and excellent delayed fracture resistance can be expected. This improves the efficiency of the bolt joints and contributes to the weight reduction of automobiles and the like. Further, it is possible to suppress the occurrence of cracks after the coil is manufactured, and it is possible to solve the problem in operation that the yield is reduced. Therefore, the industrial effect is great.

[Brief description of drawings]

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

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

Claims (8)

[Claims] 1. C: 0.15 to 0.50% by weight, Si: 0.5% or less, Mn: 0.6% or less, P: 0.015% or less, S: 0.02% or less , Cr: 0.1 to 2.0%, Mo: 0.2 to 1.2%, Al: 0.005 to 1.0%, N: 0.03% or less, with the balance being Fe and unavoidable. Steel made of mechanical impurities is hot-rolled and then cooled under conditions satisfying K ≦ 130, spheroidizing annealing is performed on the obtained steel bar or wire rod, and after tempering / tempering is performed, tempering is performed to 400 times. ℃
A method for producing a high-strength mechanical structural steel having a tensile strength of 125 kgf / mm ² or more and excellent in delayed fracture resistance, which is characterized by the above. However, K = D ₁ + 260 × log ₁₀ (CR) D ₁ = (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) 2. C: 0.15 to 0.50% by weight, Si: 0.5% or less, Mn: 0.6% or less, P: 0.015% or less, S: 0.02% or less , Cr: 0.1 to 2.0%, Mo: 0.2 to 1.2%, Al: 0.005 to 1.0%, N: 0.03% or less, and V: 0.0. 001 to 0.20%, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050%, containing one or more kinds, and the balance being Fe and inevitable impurities. Characterized in that steel bar or wire rod obtained by cooling under the condition of satisfying K ≦ 130 after slab rolling is subjected to spheroidizing annealing, and after being formed into a predetermined shape, tempering is performed at 400 ° C. or higher when quenching / tempering. To 125
A method for producing a high-strength mechanical structural steel having a tensile strength of kgf / mm ² or more and excellent in delayed fracture resistance. However, K = D ₁ + 260 × log ₁₀ (CR) D ₁ = (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) 3. C: 0.15 to 0.50% by weight, Si: 0.5% or less, Mn: 0.6% or less, P: 0.015% or less, S: 0.02% or less , Cr: 0.1 to 2.0%, Mo: 0.2 to 1.2%, Al: 0.005 to 1.0%, N: 0.03% or less, with the balance being Fe and unavoidable. Rolling temperature of 650 to 9 when hot-rolling steel consisting of mechanical impurities
Steel bar or wire rod obtained by cooling at a temperature of 00 ° C. and satisfying K ≦ 130 is spheroidized and annealed to form a predetermined shape, and then tempered at 400 ° C. for quenching and tempering.
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 characterized by the above. However, K = D ₁ + 260 × log ₁₀ (CR) D ₁ = (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) 4. C: 0.15 to 0.50% by weight%, Si: 0.5% or less, Mn: 0.6% or less, P: 0.015% or less, S: 0.02% or less , Cr: 0.1 to 2.0%, Mo: 0.2 to 1.2%, Al: 0.005 to 1.0%, N: 0.03% or less, and V: 0.0. 001 to 0.20%, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050%, containing one or more kinds, and the balance being Fe and inevitable impurities. When rolled into a wire rod at a rolling finish temperature of 650 to 900 ° C. and cooled under conditions satisfying K ≦ 130, the steel bar or wire rod is spheroidized and annealed, and then quenched and tempered. When performing, tempering is performed at 400 ° C. or higher 125
A method for producing a mechanical structural steel having a tensile strength of kgf / mm ² or more and excellent in delayed fracture resistance. However, K = D ₁ + 260 × log ₁₀ (CR) D ₁ = (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) 5. By weight%, C: 0.15 to 0.50%, Si: 0.5% or less, Mn: 0.6% or less, P: 0.015% or less, S: 0.02% or less. Cr: 0.1 to 2.0%, Mo: 0.2 to 1.2%, Ni: 2.0% or less, Al: 0.005 to 1.0%, N: 0.03% or less A steel bar or wire rod, which is contained and whose balance is Fe and unavoidable impurities, is hot-rolled, cooled under conditions satisfying K ≦ 130, spheroidized and annealed, and then quenched into a predetermined shape.・ When tempering, temper at 400 ℃
A method for producing a high-strength mechanical structural steel having a tensile strength of 125 kgf / mm ² or more and excellent in delayed fracture resistance, which is characterized by the above. However, K = D ₁ + 260 × log ₁₀ (CR) D ₁ = (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) 6. C: 0.15 to 0.50% by weight, Si: 0.5% or less, Mn: 0.6% or less, P: 0.015% or less, S: 0.02% or less Cr: 0.1 to 2.0%, Mo: 0.2 to 1.2%, Ni: 2.0% or less, Al: 0.005 to 1.0%, N: 0.03% or less In addition, one or more of V: 0.001 to 0.20%, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050% are contained, and the balance is Fe and unavoidable. Steel made of mechanical impurities is hot-rolled and then cooled under conditions satisfying K ≦ 130, spheroidizing annealing is performed on the obtained steel bar or wire rod, and after tempering / tempering is performed, tempering is performed to 400 times. 125 characterized by being performed above ℃
A method for producing a high-strength mechanical structural steel having a tensile strength of kgf / mm ² or more and excellent in delayed fracture resistance. However, K = D ₁ + 260 × log ₁₀ (CR) D ₁ = (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) 7. By weight%, C: 0.15 to 0.50%, Si: 0.5% or less, Mn: 0.6% or less, P: 0.015% or less, S: 0.02% or less. Cr: 0.1 to 2.0%, Mo: 0.2 to 1.2%, Ni: 2.0% or less, Al: 0.005 to 1.0%, N: 0.03% or less When the steel containing and the balance of Fe and unavoidable impurities is hot rolled into a wire rod, a rolling finishing temperature of 650 to 9
Steel bar or wire rod obtained by cooling at a temperature of 00 ° C. and satisfying K ≦ 130 is spheroidized and annealed to form a predetermined shape, and then tempered at 400 ° C. for quenching and tempering.
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 characterized by the above. However, K = D ₁ + 260 × log ₁₀ (CR) D ₁ = (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) 8. C: 0.15 to 0.50% by weight, Si: 0.5% or less, Mn: 0.6% or less, P: 0.015% or less, S: 0.02% or less Cr: 0.1 to 2.0%, Mo: 0.2 to 1.2%, Ni: 2.0% or less, Al: 0.005 to 1.0%, N: 0.03% or less In addition, one or more of V: 0.001 to 0.20%, Ti: 0.001 to 0.050%, Nb: 0.001 to 0.050% are contained, and the balance is Fe and unavoidable. Of steel made of static impurities into a wire rod during hot rolling is performed at a rolling finish temperature of 650 to 900 ° C. and cooled under conditions satisfying K ≦ 130, and then obtained by spheroidizing annealing to obtain a predetermined shape. 125, which is characterized by carrying out tempering at 400 ° C. or higher when quenching / tempering after forming into
A method for producing a mechanical structural steel having a tensile strength of kgf / mm ² or more and excellent in delayed fracture resistance. However, K = D ₁ + 260 × log ₁₀ (CR) D ₁ = (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)