JPH083640A - Production of high-tensile non-heat treated bolt - Google Patents

Abstract

PURPOSE:To produce a high-tensile bolt from whose production process spheroidizing and hardening and tempering stages are eliminated, and excellent in tensile strength for automobiles and industrial machines. CONSTITUTION:A steel having a compsn. contg., by weight, 0.15 to 0.30% C, 0.03 to 0.55% Si, 0.80 to 2.0% Mn and 0.010 to 0.060% Al or in which the content of P is furthermore limited to <=0.010%, S to <=0.010% and N to <=0.0060%, and the balance Fe with inevitable impurities is subjected to hot rolling to form into a steel, which is air-cooled for 3 to 10 sec. After that, the wire rod cooled in a hot water bath is subjected to drawing at 55 to 85% reduction rate of area, is thereafter subjected to shearing, head forming and thread rolling and is subjected to blueing or baking. Thus, the cost of the heat treatment can remarkably be reduced.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high tensile non-heat treated bolt having a tensile strength of 900 MPa or more for automobiles and industrial machines.

[0002]

2. Description of the Related Art High-tensile bolts having a tensile strength of 900 MPa or more, which are used for automobile engine bolts, etc., are made by subjecting low alloy steel to spheroidizing annealing and wire drawing, and then bolt forming by cold forging. After that, it is usual to give a predetermined strength and toughness by carrying out quenching and tempering. However, when performing softening or spheroidizing annealing before the cold forging, and further performing quenching / tempering after cold forming,
The heat treatment has the drawbacks that the processing steps are long and the manufacturing cost is high.

However, recently, for the purpose of energy saving and reduction of manufacturing cost, low carbon-boron steel among these heat treatments in which spheroidizing annealing is omitted, and low carbon steel in which heat treatment before and after bolt forming is omitted by constant temperature transformation treatment ( JP-A-6
0-155622) and low-carbon steel in which heat treatment before and after bolt forming is similarly omitted by adjusting cooling and wire drawing (Japanese Patent Publication No. 5-30884, JP-A-2-274810).
Has been reported. However, in the first low carbon-boron steel, it is difficult to omit the quenching and tempering process, and in the second low carbon steel subjected to the isothermal transformation treatment, the material strength becomes high, so the tool life during cold working Is significantly reduced, and there is a problem that bluing at 300 to 400 ° C. is indispensable.

The low carbon steel which is strengthened by the third adjusted cooling and wire drawing is capable of omitting the spheroidizing annealing and quenching and tempering, but it is difficult to increase the strength to 900 MPa or more, and further, the breaking elongation after the bolt forming. In addition, there is a problem that bluing at 300 to 400 ° C. is indispensable for improving the permanent elongation. Generally, most high-strength bolts for automobiles and industrial machines are plated, and thereafter, baking (dehydrogenation treatment) at 200 ° C. or lower is further performed.
Therefore, if a new bluing process is required, a new capital investment will be required for the furnace.

[0005]

DISCLOSURE OF INVENTION Problems to be Solved by the Invention According to the present invention, high-strength bolts produced by a conventional spheroidizing annealing followed by cold working and then quenching and tempering treatment are subjected to spheroidizing annealing and quenching and tempering before and after bolt forming. Omitting tensile strength 9
It is intended to provide a method for manufacturing a high-strength non-heat treated bolt having 00 MPa or more.

[0006]

The present invention, in weight percent, comprises:
C: 0.15% or more and 0.30% or less, Si: 0.03%
Or more and 0.55% or less, Mn: 0.80% or more and 2.0% or less, Al: 0.010% or more and 0.060% or less, or P: 0.0010% or less, S: 0.0.
010% or less, N: 0.0060% or less, the rest of which is Fe and unavoidable impurities, is hot-rolled into a wire rod, wound on a coil, allowed to cool for 3 to 10 seconds, and then hot water bath. It is characterized in that the wire rod that is cooled in the ferrite-pearlite structure is subjected to drawing with a surface reduction rate of 55% or more and 85% or less, and then shearing, head forming, thread rolling, bluing or baking. Tensile strength 900
It is a method for manufacturing a high-strength non-heat treated bolt having MPa or more.

[0007]

[Operation] In the process of manufacturing bolts after cold drawing from non-heat treated wire rods, the bolts are formed by deformation in the opposite compression direction after drawing, so a kind of Bauschinger effect acts during this and deformation resistance Is known to go down. The present inventors conducted an experimental study not only on the control of the conventional wire drawing method, but also on various metallurgical factors such as the composition, the structure, and the controlled cooling after hot rolling. As a result, in order to maximize the Bauschinger effect, it is optimal to increase the amount of C and to make a fine ferrite-pearlite structure with a small ferrite fraction by rapid cooling such as hot water bath cooling rather than low carbon bainite structure. It was also confirmed that, even in the wire rods of these component systems, the higher the reduction ratio of drawing, the more the Bausinger result.

Therefore, by effectively combining these factors, even if the tensile strength of the wire-drawn material increases with the increase of the wire-drawing rate, the deformation resistance during the bolt-forming process is spheroidized. It was found that the material can be maintained at the same level as that of the material, and it is sufficiently
It was also found that a strength of 00 MPa or more can be achieved. In addition, it was also confirmed that the bolts thus produced had a quality equal to or higher than that of the conventional quench-hardened and tempered wire bolts and non-heat treated bolts. In addition, in order to achieve the high ductility and high strength (improvement of permanent elongation) required for bluing omission,
The material has a high ductility and a fine ferrite / pearlite structure, and by strengthening the wire drawing beyond the tensile strength of conventional non-heat treated wire, and increasing the tensile strength and yield strength, sufficient breaking elongation at a baking treatment of 200 ° C or less. It was found that it is possible to suppress permanent elongation. Furthermore, in order to prevent a decrease in ductility due to stronger wire drawing than the conventional one, P is 0.010% or less in the low carbon steel, S is 0.010% or less, and N is N in the steel.
Of 0.0060% or less to soften the
It has been found that it is possible to suppress age hardening and reduce MnS generation, resulting in high ductility, and after hot rolling a steel adjusted to this component range into a wire rod, rapid cooling using hot water bath cooling enables fine ferrite pearlite. It was also found that the organization of the material and the reduction of ductility during the hard wire drawing can be suppressed, and the present invention has been completed.

The reasons for limiting the chemical components and manufacturing conditions in the present invention will be described below. C has a great influence on the Bauschinger effect in a ferritic / pearlitic steel. The reason for this is that the Bauschinger effect is influenced by the movable dislocation density in ferrite, and by increasing the C content, the ferrite fraction is reduced and the introduced dislocation density is increased. Therefore, in order to enhance the Bauschinger effect and reduce the deformation resistance, this object is not achieved if the C content is less than 0.15%, and if the Bausinger effect exceeds 0.30%, the ductility deteriorates, Since the deformation resistance becomes excessive and the tool life becomes short, 0.15
% To 0.30%.

Si is not only used for deoxidation but also forms a solid solution with iron to increase the yield point and tensile strength, but if it is less than 0.03%, the deoxidizing effect is insufficient and 0.55%. If it exceeds 1.0, the tensile strength is increased, but the increasing tendency of the Bauschinger effect is smaller than that of C, the ductility is deteriorated, and the cold forgeability is deteriorated, so the content was made 0.03% or more and 0.55% or less. .

Mn remarkably contributes to the increase in strength and improves the microstructure of the wire to facilitate cold forging. If Mn is less than 0.80%, the improvement in strength is insufficient. However, Mn is not an element that increases the Bauschinger effect, so excessive addition of more than 2.0% is not preferable because it significantly increases the deformation resistance. Therefore, it is set to 0.8% or more and 2.0% or less.

Al is added in order to prevent the coarsening of crystal grains and to achieve high ductility, and it is necessary to add 0.010% or more to obtain the effect. On the other hand, if it exceeds 0.060%, the effect is not obtained. Saturate. Therefore, it is set to 0.010% or more and 0.060% or less.

P, S and N are extremely important elements for achieving high ductility and high strength (improvement of permanent elongation) necessary for omitting bluing. That is, in order to omit bluing, by strengthening the wire drawing beyond the tensile strength of the conventional non-heat treated wire material and increasing the tensile strength and the yield strength, it is possible to sufficiently suppress the breaking elongation and permanent elongation by the baking treatment at 200 ° C or less. Since it is possible, it is necessary to prevent the decrease in ductility due to the stronger wire drawing than ever.

On the other hand, P is an element that enhances the strength and hinders the ductility of the hot-rolled wire and the drawn wire. Therefore, in order to suppress the deterioration of ductility during the material and hard wire drawing, 0.01
It is necessary to keep it at 0% or less. However, 0.003%
If less than 0.005%, the effect of improving ductility is almost saturated, and the cost for reduction is significantly increased.
It is preferably 10% or less.

S is an element that forms MnS and reduces ductility. For this reason, it is necessary to keep the content at 0.010% or less in order to suppress the deterioration of the ductility during the material and the hard wire drawing. However, if it is less than 0.003%, the ductility improvement effect is almost saturated, and the cost for reduction is significantly increased.
It is preferable to set it to 0.003% or more and 0.010% or less.

N is an element that accelerates age hardening and hinders the ductility of the wire drawing material. Therefore, in order to suppress the decrease in ductility, it is necessary to keep it at 0.0060% or less. However,
If it is less than 0.0010%, the cost for reduction remarkably increases, so it is preferable to set it to 0.0010% or more and 0.0060% or less. Other impurities are acceptable within the range normally present in this type of steel.

Next, cooling after hot rolling will be described. After rolling, it is wound on a coil and allowed to cool for 3 to 10 seconds in order to make the wire temperature uniform during this time, align the crystal grains that are not uniform due to rolling, and at the same time adjust the scale with good peelability. Is. If this time is too short, the effect cannot be obtained, and if it is too long, the crystal grains become coarse and the scale becomes thick and strong, which is not preferable. The first purpose of cooling the wire in the hot water bath is to increase the ferrite fraction by the hot water bath cooling, which has a faster cooling rate than the forced air cooling in order to increase the Bauschinger effect that reduces the deformation resistance during bolt forming. It is to adjust to a ferrite-pearlite structure with few defects. The second purpose is to make the cooling rate uniform by cooling in a hot water bath and to make the strength variation extremely small. The temperature of the hot water according to the present invention is preferably 90 ° C or higher. The reason for this is to stabilize the boiling film in warm water and improve the uniformity of the cooling rate. The hot water cooling start temperature may be 900 ° C, which is higher than the temperature at which the ferrite and pearlite transformation starts, but it is rapidly cooled to about 700 ° C just above the start of pearlite transformation with a cooling trough,
When gradually cooled or allowed to cool for 30 to 10 seconds and then cooled with hot water, the amount of ferrite decreases, the lamellar spacing becomes finer, the strength increases, and the scale becomes thin and easy to peel off.

Next, the drawing process before bolt forming will be described. Usually, this drawing process is carried out in order to improve the dimensional accuracy, and the tempering material is also subjected to a surface reduction rate of about 10%. However, in the case of a non-heat treated wire, there is also an effect that the strength of the hot-rolled wire applied by controlled cooling is further work-hardened in this step to adjust it to a predetermined strength. The Bausinger effect is increased by increasing the drawing rate. Therefore, in order to enhance the tensile strength of 900 MPa or more and the Bausinger effect and reduce the deformation resistance,
If the area reduction rate is less than 55%, this objective cannot be achieved.
%, The ductility deteriorates, so the content was made 55% to 85%.

After that, shearing, bolt head forming and thread rolling are carried out by a known method, and then the final strength, elongation and drawing are adjusted by a bleuing treatment. In this case, the bluing temperature is preferably 200 ° C. or higher and 400 ° C. or lower, and the baking temperature is preferably 150 ° C. or higher and 250 ° C. or lower, whereby elongation, improvement of drawing and dehydrogenation effects can be obtained.

[0020]

[Examples] Examples will be further described below. Table 1
Indicates the chemical composition of the test material. In the table, A1 to A10 are steels of the present invention, and B1 to B9 are comparative steels. Of these, B1-B
6 is a non-heat treated steel which is out of the scope of the present invention, B7 is a heat treated type in which annealing of low carbon-boron steel is omitted, and B8 is one in which quenching and tempering after bolt forming is omitted by a prior isothermal transformation treatment. is there. Further, B9 is an existing heat-treated steel, which is bolt-formed after spheroidizing annealing and then quenched and tempered. These steels are melted and then forged or rolled to 16
No 2 mm square steel piece, 8.5, 9, 12, 1 shown in Table 2
Rolled to 5.5 and 18mmφ wire rod, 97 ℃ ~ 99 ℃
Cooling in a hot water bath or normal cooling was performed.

[0021]

[Table 1]

[0022]

[Table 2]

The results of the tensile test are shown in Table 3. As shown in the table, the materials A1 to A10 according to the present invention have high tensile strength, remarkably good elongation at break and drawing, and very small variation in strength within the coil. On the other hand, among B1 to B6 which are non-heat treated steels outside the scope of the present invention, B1 has a low tensile strength,
B2-B6 have low elongation at break and low drawing. In addition, among B7 to B9 in which quenching and tempering are performed after the bolt forming, B7 of the low carbon-boron steel is judged to have low tensile strength and good cold forgeability,
Although B8 and B9 have large variations in material strength, they cannot be said to have a problem with the material because they are subjected to constant temperature treatment or spheroidizing annealing before drawing.

[0024]

[Table 3]

[0025]

[Table 4]

Table 4 shows that in A1 to A10 and B1 to B6, after drawing with a surface reduction rate of 69.1 to 85.0% without heat treatment, with B7, a surface reduction rate of 11.4% without heat treatment. , B8 is 11.4% drawn after isothermal transformation, B9
Shows the tensile properties after the spheroidizing annealing treatment and drawing with a surface reduction rate of 11.4%. The constant temperature transformation process is 950 ° C.
After heating, it was kept in a lead bath at 540 ° C. for 5 minutes and air-cooled. The deformation resistance shown here is measured using each of the test steel wires before bolt forming, and it has been known that the lower this value is, the longer the tool life during cold forging is. Each test steel wire is cut off by a lathe, and a cylindrical test piece for upsetting [however, adjusted to an upsetting ratio (height / diameter) of 1.5] is created, and the strain rate of the universal test is set. Upset processing was performed at 1 / second (however, a constrained cemented carbide plate with concentric circular grooves was used as the upsetting pressure plate), and the deformation resistance during the processing was measured. The deformation resistance was obtained by logarithmic strain [lnH ₀ / H (where H ₀ and H represent the initial test piece length and the test piece length after upsetting, respectively)] 1.5, and the upsetting load Is divided by the cross-sectional area of the test piece after deformation.

According to this, the present invention steels A1 to A10 (N
o. 1 to No. In all cases 14), the deformation resistance is almost the same as that of B9 drawn after spheroidizing annealing, and it can be said that the value is sufficient to withstand actual heading. However, B1 to B6 outside the scope of the present invention have high deformation resistance, and particularly B8 which is subjected to constant temperature treatment and has high strength before bolt forming has extremely high deformation resistance, and significantly shortens tool life and is not suitable for practical use. I understand.

[0028]

[Table 5]

Table 5 shows that after molding this into a hexagon bolt,
A6, B1 to B4 and B8 are 350 ° C bluing treatment, and A7 to A10 and B5 and B6 are 200 ° C.
Shows the tensile strength, elongation at break and permanent elongation after baking treatment of. However, B7 and B8 are 8 as described above.
After heating at 80 ℃, quench in an oil bath at 60 ℃, 500 ℃
Quenched in. The tensile test of the bolt was carried out by using wedge tension with an angle of 10 °, and the strength and the presence or absence of head jump were examined. As a result, the bolt of the present invention has a tensile strength of 900 MPa or more, no head jump, a large elongation at break, and a permanent elongation as low as that of B8 which has been quenched and tempered.
It can be seen that (1991) "9.8" and "10.9" steel bolts and machine screws have good characteristics. Comparative Examples B1 and B4 have low tensile strength, and B2 and B3
Has a low elongation at break, and B5 and B6 have a low elongation at break and a large permanent elongation, and therefore do not satisfy the JIS standard. On the other hand, B7 and B9 that have been quenched and tempered in the same manner as conventional processes
And B8 subjected to constant temperature treatment have a tensile strength of 900 MPa
That is all.

[0030]

As described above, the high tension non-heat treated bolt manufactured according to the present invention can achieve high speed, high toughness and extension of tool life at 900 MPa or more while omitting the annealing and tempering steps. This is a bolt, which brings about a great reduction in heat treatment cost. Furthermore, while conventional non-heat treated bolts were blued after bolt molding,
Since the bolt manufactured according to the present invention can be replaced by baking after plating, it is possible to omit spheroidizing annealing and quenching / tempering steps, which are inevitable in non-tempering, and to omit unavoidable bluing. It brings savings and the industrial effect is extremely remarkable.

Claims (2)

[Claims] 1. C: 0.15% or more and 0.30 by weight%
% Or less, Si: 0.03% or more and 0.55% or less, Mn:
Steel containing 0.80% or more and 2.0% or less, Al: 0.010% or more and 0.060% or less, with the balance being Fe and inevitable impurities, after hot rolling into a wire rod and after winding into a coil After being left to cool for 3 to 10 seconds, it is cooled in a hot water bath and drawn into a ferrite-pearlite structure, which is then subjected to drawing with a surface reduction rate of 55% or more and 85% or less, followed by shearing, head forming, screw rolling. A method for producing a high-strength non-heat treated bolt having a tensile strength of 900 MPa or more, which is characterized by producing and bluing. 2. C: 0.15% or more and 0.30 in% by weight
% Or less, Si: 0.03% or more and 0.55% or less, Mn:
Contains 0.80% or more and 2.0% or less, Al: 0.010% or more and 0.060% or less, and P: 0.0010% or less, S: 0.010% or less, N: 0.0060% A steel having a balance of Fe and inevitable impurities, which is limited to the following, is hot-rolled into a wire rod, wound on a coil, allowed to cool for 3 to 10 seconds, and then cooled in a hot water bath to form a ferrite-pearlite structure. Of a high-strength non-heat treated bolt having a tensile strength of 900 MPa or more, which is characterized by subjecting the formed wire rod to a drawing process with a surface reduction rate of 55% or more and 85% or less, and then performing shearing, head forming, thread rolling, or baking. Production method.