JP2795799B2 - Manufacturing method of high strength bolts with excellent delayed fracture resistance - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a high-strength bolt having a tensile strength of 125 kgf / mm ² or more and excellent in delayed fracture resistance.

[0002]

2. Description of the Related Art High-strength bolts are widely used in machines, automobiles, bridges and buildings, and are also used in many parts such as automobile parts. However, the tensile strength was 12
It is well known that the risk of delayed fracture increases when the pressure exceeds 5 kgf / mm ² , and at present, the strength of bolts actually used is 110 kgf / mm ² class. However, in recent years, with the increase in size of structures, there is a high demand for higher strength bolts for the purpose of improving joint efficiency and reducing weight. There is a strong demand for higher strength.

[0003] The delayed fracture of high strength members is attributed to hydrogen in steel. In particular, it is considered that diffusible hydrogen, which can easily move in the vicinity of room temperature, accumulates at the crystal grain boundary in the tensile stress concentration portion, and promotes grain boundary cracking, which causes delayed fracture. Therefore, when high strength mechanical structural steel is used, it must be resistant to hydrogen, especially diffusible hydrogen.

[0004] The inventors of the present invention examined the effects of alloying elements and tempering temperature on delayed fracture resistance.
Compared to steel for machine structural use, it has been found that reduction of Si, Mn, P, increase of Mo, and tempering at 400 ° C. or more are effective. In addition, they have found that the addition of V, Ti, and Nb can further improve the delayed fracture resistance.
In JP-A-323146, the amount of diffusible hydrogen that has a tensile strength of 125 kgf / mm ² or more and does not lead to delayed fracture by adjusting the chemical composition of the steel and the tempering temperature (hereinafter referred to as the critical diffusible hydrogen) We proposed a method for forming steel for machine structural parts and machine parts that can increase the steel.

[0005]

SUMMARY OF THE INVENTION However, Japanese Patent Application No. Hei.
In the bolt manufacturing method described in No. 23146, a bolt is formed by cold forging after spheroidizing annealing, followed by quenching and tempering, and heat treatment is performed twice. Spheroidizing annealing is a step performed to soften the steel material and prevent early damage of the mold during cold forging, but requires heating and holding at 700 to 800 ° C for 10 hours or more,
Energy costs are enormous. In recent years, there has been a strong demand for reduction in processing cost, and the omission of this spheroidizing annealing has been strongly demanded. In the bolt manufacturing method described in Japanese Patent Application No. 3-323146, quenching and tempering are performed after the forging of the bolt, so that the metal flow effective for improving the delayed fracture resistance has disappeared.

Therefore, as a forming method in which the spheroidizing annealing is omitted and the metal flow is left in order to improve delayed fracture resistance, forging forming of a steel material immediately after heating during quenching and tempering may be considered. On the other hand, Sekiguchi et al. 24 No. 271 (1983) proposes tempering warm forging as a similar forming method. However, plasticity and processing Vol. 24 No. 271 (1983) aims at improving toughness after forging, and does not mention improvement in delayed fracture resistance due to residual metal flow. Further, simply performing warm forging immediately after heating during tempering may lead to shape defects.

[0007]

SUMMARY OF THE INVENTION The present invention has been made in view of the above findings and problems, and is equivalent to the method described in Japanese Patent Application No. 3-323146 even if spheroidizing annealing having high energy cost is omitted. 125kgf / mm ² which can be molded with forging die life and can improve delayed fracture resistance
This method is capable of producing a high-strength bolt having the above-described tensile strength and excellent in delayed fracture resistance.

That is, the gist of the present invention is as follows. (1) C: 0.15 to 0.50% by weight, Si: 0.
05 to 0.5%, Mn: 0.1 to 0.6%, P: 0.0
15% or less, S: 0.02% or less, Cr: 0.1 to 2.
0%, Mo: 0.2 to 2.0%, Al: 0.005 to
0.05%, N: 0.01% or less, the balance being Fe
When tempering after quenching a rolled steel bar or wire consisting of unavoidable impurities, in forging performed immediately after tempering heating, the raw material temperature immediately before forging is set to 450 ° C. or higher, and at an average processing speed of 200 mm / sec or more, the forging is performed immediately before forging. A method for producing a high-strength bolt excellent in delayed fracture resistance and having a tensile strength of 125 kgf / mm ² or more, which is forged into a predetermined bolt shape using a punch having a surface temperature of 100 ° C. or less.

(2) C: 0.15 to 0.50 by weight%
%, Si: 0.05 to 0.5%, Mn: 0.1 to 0.6
%, P: 0.015% or less, S: 0.02% or less, C
r: 0.1 to 2.0%, Mo: 0.2 to 2.0%, A
l: 0.005 to 0.05%, N: 0.01% or less, V: 0.001 to 0.20%, Ti: 0.
001 to 0.050%, Nb: 0.001 to 0.050
%, And the above-mentioned quenching (1) and bolt forming immediately after holding the tempering heating temperature are performed by using a rolled steel bar or a wire rod containing at least one of Fe and inevitable impurities. A method for manufacturing a high-strength bolt having a tensile strength of 125 kgf / mm ² or more and excellent in delayed fracture resistance.

(3) When a rolled steel bar or a wire having the composition described in the above (1) or (2) is quenched and tempered, the material temperature is 45 in the forging performed immediately after the tempering heating.
Tempering heating to 0 ° C or more, and in subsequent forging, lubricating liquid or the like is blown out so that the surface of the steel material immediately before forging is 200 ° C or less, and heat is removed. A method for producing a high-strength bolt having a tensile strength of 125 kgf / mm ² or more and excellent in delayed fracture resistance, characterized by being forged.

The alloy composition of the steel used in the present invention was determined for the following reasons. C is required to be 0.15% or more in order to obtain high strength by quenching and tempering, but if it is too much, it is an element that deteriorates toughness and delayed fracture resistance. did. Si is required at least 0.05% to increase the deoxidation and strength of steel.
Because it is an element that increases material strength and impairs forgeability,
0.5% or less.

Mn is required to be 0.1% or more in order to ensure the deoxidation and hardenability of steel. However, Mn is an element that segregates at the grain boundary during heating in the austenite region, embrittles the grain boundary and deteriorates delayed fracture resistance. Therefore, the content is set to 0.6% or less. P
Is effective as a quenchable element, but is 0.015% because it segregates microscopically during solidification, segregates at the grain boundaries during heating in the austenite region, embrittles the grain boundaries, and deteriorates delayed fracture resistance. It was as follows. Although S is an unavoidable impurity, it is segregated at the grain boundary during heating in the austenite region, embrittles the grain boundary, and deteriorates the delayed fracture resistance, so that the content of S is set to 0.02% or less.

[0013] In order to obtain the hardenability of steel, Cr is 0.1%.
% Is necessary, but if it is too large, it is an element that causes deterioration of toughness. Mo is an element that is 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 at a tempering temperature of 450 ° C. or more. The content is required to be 2% or more, but if the content is too large, the effect is saturated and the cost is increased. Al is an element effective for deoxidation of steel, and therefore needs to be 0.005% or more. However, if it is too much, toughness is deteriorated.

N is an element which segregates at the grain boundary during austenite heating, embrittles the grain boundary and also deteriorates the delayed fracture resistance, so that N is set to 0.01% or less. V, Ti, Nb
Is an element that contributes to the refinement of crystal grains and has a high affinity for hydrogen and is an element effective for improving delayed fracture resistance by suppressing diffusion and accumulation of hydrogen in steel.
001% or more is required. However, if the content is too large, the effect is saturated and the element is rather deteriorated in toughness. Therefore, V: 0.2% or less, Ti: 0.05% or less, Nb:
0.05% or less.

On the other hand, when tempering after quenching a rolled material having this component, the temperature of the raw material immediately before forging is 450 ° C. or higher in forging performed immediately after tempering heating. As described in Japanese Patent Application No. 3-323146, the present inventors examined the effects of alloying elements and tempering temperature on the delayed fracture resistance, and found that the effect of Si was higher than that of steel for machine structural use. , Mn, P decrease, Mo increase and 400
This is because tempering at a temperature of not less than ° C. is effective, and in forging immediately after heating by tempering, at a lower temperature, the mold life is reduced. Processing speed during forging is 200mm / average
The reason for setting the time to seconds or longer is that if the processing speed is slower than this, the temperature of the steel material decreases during forging, and the life of the mold decreases.

The reason why the punch temperature is controlled is that the lower part of the punch is so softened by the heat generated by the working of the steel that the shape becomes poor after forming.
It is necessary to control the heat transfer from the steel material to the punch as follows. Further, in forging immediately after tempering heating, the same effect as controlling the punch temperature can be obtained by cooling the surface layer of the material after inserting the material into the mold. In this case, a lubricating liquid or the like is sprayed and the heat is removed so that the surface of the steel material immediately before forging becomes 200 ° C. or less. However, the temperature of the steel material surface layer is set to 200 ° C. or less. Due to softening, shape defects are caused after molding. In addition to heat removal, in addition to liquid,
It is also possible to use a substantially non-oxidizing gas.

[0017]

EXAMPLES The chemical components of the test steel are shown in Table 1. A to H are according to the steel for bolts of the present invention, and I to M are comparative steels. These rolled steel bars of φ22 mm and length 120 mm
After heating and holding for 00 × 1 hour, the steel was quenched by oil cooling, heated to each tempering temperature, and immediately after holding for 1 hour, a head equivalent to an M22 trimming bolt was formed by forging and then water-cooled. The tempering temperature is such that the tensile strength after molding is 1
It was set to be 50 kgf / mm ² or more.

The forging was carried out at a predetermined working speed with a servo type hydraulic compression tester, the forming load was measured and the forging was carried out.
The mold life evaluation was estimated from the relationship between the molding load and the mold life shown in FIG. The solid line in FIG.
This is a straight line connecting the experimental values at Y5, Y9, and Y10. Symbols Y9 and Y10 in Table 2 are molding methods described in Japanese Patent Application No. 3-323146. In evaluating the mold life, the rolling coil is subjected to spheroidizing annealing, and the head is formed using a bolt former. went.

The raw material temperature immediately before forging was measured with a radiation thermometer. As shown in FIG. 2, a heater was embedded in the punch, and the temperature was controlled through a water-cooled pipe.
A graphite lubricant was sprayed on the punch surface to prevent seizure and control the punch surface temperature. The punch temperature was measured by a thermocouple embedded at a position 2 mm from the punch surface as shown in FIG. 2, and the temperature immediately before forging was used as the punch temperature. The mold shape is as shown in FIG.

Table 2 shows the results of the molding experiment. Symbol X1
To X10 are the cases according to the method of the present invention, and symbols Y1 to Y8
Is the case of the comparison method. Symbols Y9 and Y10 are results obtained by cold forging after spheroidizing annealing described in Japanese Patent Application No. 3-323146. Comparison method Y3, Y4, Y7
After molding, as shown in FIG. 3, the shape of the head side became stepped, resulting in a shape defect. Therefore, the measurement of the mold life was not performed. In the comparative methods Y1, Y2, Y5, Y6, and Y8, the mold life was 40,000 or less, which was about half that of the conventional methods Y9 and Y10. In contrast, in the method of the present invention, the number of molds was 80,000 or more in each case, and molding was possible with a mold life equal to or longer than that of the conventional methods Y9 and Y10.

Next, in order to evaluate the delayed fracture property, the bolt-shaped material formed by the method of the present invention was quenched and tempered at the temperature shown in Table 2, and 4 mmV was immediately below the neck of the M22 bolt shown in FIG. A test piece provided with a circumferential notch was manufactured. Further, the comparative steels I to M were also molded by the method of the present invention,
The test piece of FIG. 4 was manufactured. The conventional methods Y9 and Y10
As for the test piece, after quenching and tempering after forging, a test piece having the same shape as that of FIG. 4 and having a circumferential notch of 4 mmV directly below the neck of the bolt was manufactured. The method for obtaining the limit hydrogen amount will be described below.

In order to enrich the hydrogen in a set of two test pieces shown in FIG. 4, the amount of hydrogen in the test pieces is changed by immersion in 20 to 36% HCl for 20 to 120 minutes. One of them was immersed in HCl and allowed to stand in the air for 30 minutes, then the amount of hydrogen was measured by thermal analysis. The other was allowed to stand in the air for 30 minutes after immersion, and the test shown in FIG. Perform a delayed fracture test on the machine. In FIG. 5, 1 indicates a test piece, 2 indicates a balance weight, and 3 indicates a fulcrum. The test load in the delayed fracture test was 7% of the rupture load of each specimen before immersion in the HCl solution.
It was kept constant at 0%.

Table 3 shows the relationship between the obtained amount of diffusible hydrogen and the rupture time in the delayed fracture test when the concentration of HCl and the immersion time were variously changed in accordance with the above procedure. In the same table, when the upper limit diffusible hydrogen amount that does not cause delayed fracture after 4000 minutes is estimated as the limit diffusible hydrogen amount for each steel type, Table 4 is obtained. From this table, X1 to X10 formed by the method of the present invention using the developed steels A to H are shown.
It can be seen that the test piece of No. has a higher critical hydrogen content than Z1 to Z5 using comparative steels I to M, and is less likely to be fractured with delay. Also, it can be seen that the critical hydrogen content becomes higher than in the case of spheroidizing annealing and quenching / tempering after cold forging described in Japanese Patent Application No. 3-323146 using developed steels D and B.

Table 5 shows molding in a case where a mist-like graphite-based lubricating liquid is injected into a test piece and heat is removed after inserting a material subjected to tempering and heating into a mold. As the material temperature before forging, the temperature at the time of inserting into a mold after tempering and heating was measured by a radiation thermometer. The temperature at the time of heat removal of the heated material is determined by a thermocouple embedded at a position of 2.2 mm from a surface layer of a φ22 × 120 mm test piece used for load measurement and a thermocouple attached to the surface layer. The material temperature at the time of injection was measured. Then, the flow rate and the pressure of the ejected lubricating liquid were set so as to satisfy the predetermined temperature conditions shown in Table 5. For load measurement, a test piece with a thermocouple attached was used, and after confirming that it was under a predetermined temperature condition, forging was performed as it was. The mold life evaluation was estimated from the measured load using FIG.

From Table 5, it can be seen that the method of the present invention can be formed with a mold life four times or more longer than that of the comparative methods Y11 and Y12 without the occurrence of a shape defect, and is slightly lower than those of the conventional methods Y9 and Y10. Are equivalent. The method X11 of the present invention,
The critical diffusible hydrogen was measured for X12, and the result was 0.75 ppm for X11 and 0.77 pp for X12.
m and the same level as the method of the present invention in Table 4, and
9, which had higher delayed fracture resistance than Y10.

[0026]

[Table 1]

[0027]

[Table 2]

[0028]

[Table 3]

[0029]

[Table 4]

[0030]

[Table 5]

[0031]

According to the present invention, a high-strength bolt having a tensile strength of 125 kgf / mm ² or more and having excellent delayed fracture resistance can be formed without performing spheroidizing annealing. As a result, the joint efficiency of the bolt is improved, and it is possible to contribute to the weight reduction of automobiles and the like, and the industrial effect is great.

[Brief description of the drawings]

FIG. 1 is a chart showing a relationship between a molding load and a mold life during bolt molding.

FIG. 2 is an explanatory diagram of a die shape, punch temperature control, and temperature measurement during forging.

FIG. 3 is a cross-sectional view of a test piece showing a state of a shape defect during forging.

FIG. 4 is an explanatory view of a test piece shape.

FIG. 5 is an explanatory diagram of a delayed fracture test apparatus.

[Explanation of symbols]

 Reference Signs List 1 test piece 2 punch 3 die 4 water cooling pipe 5 heater 6 drill hole for thermocouple installation

Continued on the front page (51) Int.Cl. ⁶ Identification code FI C22C 38/00 301 C22C 38/00 301Z 38/22 38/22 (72) Inventor Fumio Ishikawa 20-1 Shintomi, Futtsu Nippon Steel Corporation (58) Investigated field (Int.Cl. ⁶ , DB name) B21K 1/44 B21J 1/06 B21J 5/00

Claims (3)

(57) [Claims] C: 0.15 to 0.50%, Si: 0.05 to 0.5%, Mn: 0.1 to 0.6%, P: 0.015% or less by weight, S : 0.02% or less, Cr: 0.1 to 2.0%, Mo: 0.2 to 2.0%, Al: 0.005 to 0.05%, N: 0.01% or less, the balance being Fe When tempering after quenching a rolled steel bar or wire consisting of unavoidable impurities, in forging performed immediately after tempering heating, the raw material temperature immediately before forging is set to 450 ° C. or higher, and at an average processing speed of 200 mm / sec or more, the forging is performed immediately before forging. A method for producing a high-strength bolt excellent in delayed fracture resistance, wherein the bolt is forged into a predetermined bolt shape using a punch having a surface temperature of 100 ° C. or less and has a tensile strength of 125 kgf / mm ² or more. 2. One or more of V: 0.001 to 0.20%, Ti: 0.001 to 0.050%, and Nb: 0.001 to 0.050% by weight%. The method for producing a high-strength bolt excellent in delayed fracture resistance according to claim 1, characterized in that: 3. When a rolled steel bar or a wire rod having the composition according to claim 1 or 2 is quenched and tempered, the material temperature is set to 45 in forging performed immediately after tempering heating.
Tempering heating to 0 ° C or more, and in subsequent forging, lubricating liquid or the like is blown out so that the surface of the steel material immediately before forging is 200 ° C or less, and heat is removed. Forged to 125kgf /
A method for producing a high-strength bolt excellent in delayed fracture resistance, having a tensile strength of not less than mm ² .