JPH0775846A - Production of high strength bolt excellent in delayed fracture resistance - Google Patents

Abstract

PURPOSE:To provide the production method of a high strength bolt excellent in delayed fracture resistance by using the steel of specified component and adjusting harm forging after annealing and tempering temp. CONSTITUTION:After the steel, in which C, Si, Mn, P, S, Cr, Mo, Al, V, Ti, Nb, N are specified, is subjected to supheroidizing, it is subjected to bolt forming with heating to 200-400 deg.C and at a working speed of >=200mm/sec, at executing subsequent quenching-tempering, the tempering temp. is >=400C. Further, in the case of forging with heating to >=400 deg.C, the punch having a surface temp. of <=100 deg.C is used, further after heat is withdrawn so that a steel surface temp. is <=100 deg.C and a temp. at 1/10 of stock diameter from the surface layer is >=250 deg.C, it is subjected to bolt forming at a working speed of average >=200mm/ sec, the tempering temp. at subsequent quenching.tempering is >=400 deg.C.

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 an excellent delayed fracture resistance.

[0002]

2. Description of the Related Art High-strength bolts are widely used in machines, automobiles, bridges and buildings, as well as in PC steel rods and automobile parts. However, it is well known that the risk of delayed fracture increases when the tensile strength exceeds 125 kgf / mm ² for all types, and the maximum strength of bolts actually used is 110 kgf / mm ² class. Is the current situation. However, with the increase in size of structures in recent years,
There is a strong demand for higher strength of bolts for the purpose of improving joint efficiency and weight reduction, and also for automobiles that are required to improve fuel consumption, higher strength of bolts is strongly demanded for achieving weight reduction.

Hydrogen in steel is 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 crystal grain boundaries in the tensile stress concentration portion and promotes grain boundary cracking, resulting in delayed fracture. Therefore, if high strength mechanical structural steel is used, it must be resistant to hydrogen, especially diffusible hydrogen.

Then, the present inventors investigated the influence of alloying elements and tempering temperature on the delayed fracture resistance, and found that
Compared with machine structural steel, Mn, P decrease, Mo increase,
It was found that addition of V, Ti, and Nb and tempering at 400 ° C or higher are effective, and in Japanese Patent Application No. 4-127801, tensile strength of 125 kgf / mm ² or more was adjusted by adjusting the chemical composition of steel and tempering temperature. We have proposed a method for forming mechanical structural steel and mechanical parts that has a high diffusible hydrogen content (hereinafter referred to as "critical diffusible hydrogen") and that does not cause delayed fracture.

[0005]

[Problems to be Solved by the Invention] Japanese Patent Application No. 4-12780
As described in No. 1, when forming a high-strength bolt by cold forging, the material strength before forging is high, so that the die life during forging is short and it is not practically used. The present invention has been made in view of the above knowledge and problems, and has an excellent delayed fracture resistance in combination with the adjustment of the chemical composition of steel, the forging method, and the adjustment of the tempering temperature.
It is a manufacturing method of a high strength bolt having a tensile strength of 25 kgf / mm ² or more and capable of being manufactured with a long mold life.

[0006]

The gist of the present invention is as follows. (1) C: 0.15 to 0.50% by weight, Si:
0.05-2.0%, Mn: 0.1-0.6%, P:
0.015% or less, S: 0.02% or less, Cr: 0.1
~ 3.0%, Mo: 0.2-2.0%, Al: 0.00
5 to 0.05%, N: 0.03% or less, and V: more than 0.10 to 0.50%, Ti: 0.
Steel bar or wire rod containing 01 or more than 0.10% and Nb: 0.01 to 0.10% of one or two kinds, and the balance of Fe and unavoidable impurities is spheroidized and annealed, and the steel is forged. Immediately after uniform heating to a temperature of 200 ° C or higher and lower than 400 ° C, forging is performed into a predetermined bolt shape at a processing speed of 200 mm / sec or more on average, and then tempering is performed at 400 ° C or higher for quenching and tempering. A method for producing a high-strength bolt having a tensile strength of 125 kgf / mm ² or more and excellent delayed fracture resistance.

(2) A bar steel or wire rod having the composition described in (1) above is annealed by spheroidizing, and this steel material is uniformly heated to a temperature of 400 ° C. or more immediately before forging, and then an average of 20
The surface temperature immediately before forging is 100 at a processing speed of 0 mm / sec or more.
Forged into a predetermined bolt shape using a punch below ℃, and then temper 40
A method for producing a high-strength bolt having a tensile strength of 125 kgf / mm ² or more and excellent delayed fracture resistance, which is characterized in that the temperature is 0 ° C. or more.

(3) A steel bar or wire rod having the composition described in (1) above is spheroidized and annealed, and this steel material is uniformly heated to a temperature of 400 ° C. or more immediately before forging, and the surface of the steel material immediately before forging becomes 100. Below ℃, 1/10 of the material diameter from the surface layer
At a temperature of 250 ° C or higher, a lubricant is blown to remove heat, and after forging into a predetermined bolt shape at an average processing speed of 200 mm / sec or higher, tempering is performed at 400 ° C or higher when performing quenching and tempering. Characterized by 125
It is a method for producing a high-strength bolt having a tensile strength of kgf / mm ² or more and excellent in delayed fracture resistance.

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, so it is 0.50% or less. did. Si is required to be 0.05% or more, which is necessary for deoxidizing and increasing the strength of steel, but is 2.0% or less because it is an element that increases the material strength and impairs forgeability.

Mn is required to be 0.1% or more in order to secure deoxidation and hardenability of steel, but it is an element that segregates at the grain boundaries during heating in the austenite region to embrittle the grain boundaries and deteriorate delayed fracture resistance. Therefore, it is set to 0.6% or less. P
Is effective as a hardenability element, but is 0.015% because it is an element that microsegregates during solidification and segregates to grain boundaries during heating in the austenite region to embrittle the grain boundaries and deteriorate delayed fracture resistance. Below.

Although S is an unavoidable impurity, it is 0.02 because it is an element that segregates to the grain boundaries during heating in the austenite region, embrittles the grain boundaries, and deteriorates delayed fracture resistance.
% Or less. In order to obtain the hardenability of steel, Cr is 0.1%.
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 the content was made 3.0% or less. Mo is an element that is necessary for obtaining hardenability of steel and has temper softening resistance, and is effective in obtaining a tensile load of 125 kgf / mm ² or more stably at a tempering temperature of 400 ° C. or more, If it is too large, the effect is saturated and the cost is increased, so the content is made 2.0% 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 0.05% or less. Since N is an element that segregates at the grain boundaries during austenite heating to embrittle the grain boundaries and deteriorates the delayed fracture resistance, the content of N is set to 0.03% or less. V, Ti and Nb contribute to the refinement of crystal grains,
In addition, since it is an element which is rich in affinity with hydrogen and effective in improving delayed fracture resistance by suppressing diffusion and accumulation of hydrogen in steel, V: more than 0.10% and Ti: 0.01, respectively.
%, Nb: more than 0.01% is required. However, when the amount is too large, the effect is saturated and rather deteriorates the toughness, so V: 0.5% or less, Ti: 0.1% or less, and Nb: 0.1% or less, respectively.

On the other hand, the strength of the steel material is reduced and the life of the die during forging can be improved by spheroidizing and annealing the steel material so that the temperature just before forging is 200 ° C. or higher, but the heating temperature lower than this Then, the effect of improving the mold life is small. The average processing speed of 200 mm / sec or more is
This is because if the processing speed becomes slower than this, the temperature of the steel material decreases during forging.

When the temperature just before forging is heated to 400 ° C. or higher, the steel material is severely softened by the heat generated by working at the lower part of the punch, which causes a defective shape after forming. Need to control movement. When heating the material, it is possible to perform partial heating in which only the material region corresponding to the bolt head molding portion that determines the life of the mold is heated.

Further, when the temperature immediately before forging is heated to 400 ° C. or higher, the same effect as controlling the punch temperature can be obtained by cooling the surface layer of the material after inserting the heated material into the mold. it can. In this case, the temperature of the steel material just before forging is 100 ° C or less, and the surface diameter is 1/1
The temperature of the steel material surface layer is set to 100 ° C or less because the temperature of the surface layer of the steel material is 100 ° C or less. This is because it causes a defective rear shape. The temperature from the surface layer to 1/10 of the material diameter is set to 250 ° C. or higher, because if the temperature is lower than this, the molding load becomes large and the die life is shortened. In addition to liquid, it is possible to use substantially non-oxidizing gas for heat removal.

[0016]

[Examples] Table 1 shows the chemical composition of the test steel. A to E are according to the steel for bolts of the present invention, and F to J are comparative steels. After spheroidizing and annealing these φ22 mm steel bars, M
A head portion equivalent to 22 trimming bolts was formed. Spheroidizing annealing was held at 770 ° C for 3 hours and then 710 at 0.5 ° C / min.
The mixture was gradually cooled to ℃, held at 710 ℃ for 4 hours and then air-cooled. After annealing, these steel materials were lubricated with zinc phosphate.

[0017]

[Table 1]

The test material for measuring the load during forging has a length of 1
After cutting it to 00 mm, it was uniformly heated by a high frequency at a speed of 10 to 20 ° C./sec, and molded at a predetermined processing speed by a servo type hydraulic compression tester. Further, when performing die life evaluation, an annealing coil was used to uniformly heat the steel material between the bolt forming part former and the coil by high frequency, and after cutting, the bolt head was formed. The material temperature was measured by a radiation thermometer in both cases.

A heater was embedded in the punch as shown in FIG. 1, and the temperature was controlled through a water cooling pipe. When heating the pre-forging temperature to 400 ° C. or higher, a graphite-based lubricant was sprayed on the punch surface to prevent seizure and control the temperature of the punch surface. The punch temperature was measured by a thermocouple embedded at a position 2 mm from the punch surface as shown in FIG. 1, and the temperature immediately before forging was defined as the punch temperature. The mold shape is as shown in FIG.

[0020]

[Table 2]

Table 2 shows the results of the molding experiment. Symbol X1
~ X7 is the case of the method of the present invention, and the symbols Y1 to Y9 are the case of the comparison method. Symbols Y1 and Y5 are Japanese Patent Application No. 4-12
It is the result of the cold forging method described in No. 7801,
The maximum load is 180 or more, and the die life is 5000 or less. Since the punching temperature was high in the symbols Y4, Y7, and Y9, in any case, the head side surface became stepped as shown in FIG. Therefore, the symbols Y4, Y7, Y9
The mold life was not evaluated for the above. On the other hand, in the method of the present invention, a shape defect does not occur in any case,
Further, the mold life was 170,000 or more, which was twice as long as that of the comparative method.

Next, in order to evaluate the delayed fracture characteristics, the bolt-shaped material formed by the method of the present invention was quenched and tempered at the temperatures shown in Table 2, and the M10 bolt shown in FIG. A test piece having a circumferential notch was manufactured. Further, the comparative steels F to J were also formed by the method of the present invention, and after quenching and tempering, the test pieces of FIG. 3 were manufactured. In each case, the quenching temperature was 900 ° C., and the tempering temperature was set so that the tensile strength after tempering was 150 to 160 kgf / mm ² .

A method for obtaining the limit hydrogen amount will be described below. In order to enrich the hydrogen by making the test piece shown in FIG. 3 into two sets, it is immersed in 20 to 36% HCl for 20 to 120 minutes to change the amount of hydrogen in the test piece. One of these is HC
After being immersed in 1 and left in the air for 30 minutes, the amount of hydrogen was measured by a thermal analysis method, the other one was left in the air for 30 minutes after the immersion, and then delayed fracture was performed with the tester shown in FIG. Perform the test. In FIG. 4, 1 is a test piece, 2 is a balance weight,
3 shows a fulcrum. The test load in the delayed fracture test is 70% of the breaking load of each test piece before being immersed in the HCl solution.
And made it constant.

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 changed according to the above procedure. From the table, the upper limit of the amount of diffusible hydrogen that does not cause delayed fracture after 4000 minutes is estimated as the limit diffusible hydrogen amount for each steel type. From this table, developed steel A
It can be seen that the test pieces of X1 to X7 molded by the method of the present invention using ~ E have a higher limit hydrogen amount and are less prone to delayed fracture than Z1 to Z5 using comparative steels FJ.

[Table 3]

[0025]

[Table 4]

[0026]

[Table 5]

Table 5 shows the case where heat is applied uniformly by high-frequency heating so that the temperature before forging is 400 ° C. or higher, the material is inserted into a mold, and then a mist-like graphite type lubricant is sprayed to remove heat. Shows the molding of. The material temperature during high frequency heating was measured by a radiation thermometer. In addition, the temperature during heat removal of the heated material is sprayed with a lubricating fluid by a thermocouple embedded at a position of 2.2 mm from the surface layer in a φ22 * 100 mm test piece used for load measurement and a thermocouple attached to the surface layer. The material temperature at that time was measured.

Then, the flow rate and the liquid pressure of the jetted lubricating liquid were set so that the predetermined temperature conditions shown in Table 5 were obtained. For load measurement, a thermocouple was used and a test piece was used. After confirming that the test piece was in a predetermined temperature condition, forging was performed as it was. In the die life evaluation, forging was performed after confirming the surface temperature with a radiation thermometer using the set flow rate and liquid pressure of the lubricating liquid.

From Table 5, it can be seen that the method of the present invention enables molding with a mold life that is at least twice as long as that of the comparative method. With respect to the methods X8 and X9 of the present invention, quenching and tempering were performed at a quenching temperature of 900 ° C and a tempering temperature of 525 ° C, and the limiting diffusible hydrogen was measured. As a result, the critical diffusible hydrogen estimated amount is 0.74 pp.
m was the same level as the method of the present invention in Table 4, and the delayed fracture resistance was higher than that of the comparative method.

[0030]

According to the present invention, a high-strength bolt having a tensile strength of 125 kgf / mm ² or more and excellent delayed fracture resistance can be molded with a long mold life. As a result, the joint efficiency of the bolt can be improved, and it can contribute to the weight reduction of automobiles and the like, which has a great industrial effect.

[Brief description of drawings]

FIG. 1 is an explanatory view of a die shape, punch temperature control, and temperature measurement during forging.

FIG. 2 is a sectional view of a test piece showing a state of defective shape during forging.

FIG. 3 is an explanatory view of the shape of a test piece.

FIG. 4 is an explanatory diagram of a delayed fracture test device.

[Explanation of symbols]

 1 Test piece 2 Punch 3 Die 4 Water cooling pipe 5 Heater 6 Thermocouple mounting drill hole

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁶ Identification number Office reference number FI technical display location // B21K 1/46 Z 8824-4E

Claims (3)

[Claims] 1. C: 0.15 to 0.50% by weight%, Si: 0.05 to 2.0%, Mn: 0.1 to 0.6%, P: 0.015% or less, S : 0.02% or less, Cr: 0.1 to 3.0%, Mo: 0.2 to 2.0%, Al: 0.005 to 0.05%, N: 0.03% or less, and steel As a component, one or two kinds of V: more than 0.10 to 0.50%, Ti: more than 0.01 to 0.10%, and Nb: more than 0.01 to 0.10% are contained, and the balance is Fe and A steel rod or wire rod made of unavoidable impurities is spheroidized and annealed, and this steel material is uniformly heated to a temperature of 200 ° C or more and less than 400 ° C immediately before forging, and then a predetermined bolt shape at an average processing speed of 200 mm / sec or more. Forged and then quenched /
A method for producing a high-strength bolt having excellent delayed fracture resistance, characterized in that tempering is performed at 400 ° C. or higher when tempering is performed. 2. A steel bar or a wire rod having the composition according to claim 1 is spheroidized and annealed, and the temperature immediately before forging of the steel bar is 400.
Immediately after being uniformly heated to ℃ or more, average 200 mm /
Delay resistance characterized by forging into a predetermined bolt shape using a punch with a surface temperature immediately before forging of 100 ° C or less at a processing speed of at least 2 seconds, and then tempering at 400 ° C or more when performing quenching / tempering. A method for manufacturing a high-strength bolt having excellent fracture characteristics. 3. A steel bar or wire rod having the composition according to claim 1 is spheroidized and annealed, and the temperature immediately before forging of the steel bar is 400.
After uniformly heating to ℃ or more, the surface of the steel material just before forging is 100 ℃ or less, and 1/10 of the material diameter is sprayed from the surface layer to heat it to 250 ℃ or more. A method for producing a high-strength bolt having excellent delayed fracture resistance, which comprises forging into a predetermined bolt shape at a processing speed of not less than 1 / second and then tempering at 400 ° C. or more when performing quenching and tempering.