JPS61130425A - Production of heat treatment omission type high tension steel wire or bar having excellent workability - Google Patents

Abstract

PURPOSE:To produce a heat treatment-omission type high tension steel wire or rod having excellent workability by hot working a steel contg. a specific comsn. ratio each of C, Si and Mn then subjecting the steel to adequate cooling to form specific composite structure. CONSTITUTION:A steel contg. 0.05-0.20wt% C, <=1.0% Si, 1.0-2.5% Mn and contg. further >=1 kinds of <=1.0% Cr, <=0.3% Mo or >=1 kinds among <=0.15 Al, <=0.1% Ti and <=0.3% Zr or >=1 kinds of <=0.05% PEM and <=0.003% Ca or <=1.0% Ni is heated to about 900-1,100 deg.C and is hot worked. The steel is thereafter cooled to a temp. range of 300-600 deg.C from 800-950 deg.C at 1-15 deg.C/sec cooling rate to form the composite structure of the low temp. transformation forming phase consisting of 40-75% volume content of ferrite and the balance martensite, bainite or the mixed structure thereof, by which the heat treatment omission type high tension steel wire or rod having excellent workability is obtd.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high-tensile steel wire bar without heat treatment, which has a tensile strength of 60 kgf/1 ms'' or more and a reduction of area of 50% or more, and has excellent workability.

Conventionally, medium-high carbon steel or low alloy steel with a C content of 0.30% by weight or more has been used for fastener parts for bolts and small screws with a tensile strength of 60 kgf/m@'' or more, and these parts usually have Spherical annealing treatment, cold wire drawing, cold bolt forming,
It is manufactured through a process of quenching and tempering. Also,
Parts such as chain link plates are also manufactured by cold punching steel strips, followed by quenching and tempering.

However, in recent years, against the backdrop of increasing demands for energy conservation, there has been a demand for high-tensile wire rod steel bars that can omit spherical annealing treatment and quenching and tempering treatment and that have excellent workability.

As a result of intensive research in response to such demands, the present inventors have already hot-worked low-carbon, low-alloy steel under predetermined conditions and then rapidly cooled it to transform the steel structure into a ferrite-martensitic composite structure. In this way, it has been discovered that it is possible to obtain a heat-treated high-tensile wire rod with excellent workability (Japanese Patent Publication No. 10442/1983).

However, even in the high-tensile wire rod steel bar having a composite structure obtained as described above, it is difficult to use the
For example, it has been found that there is still a tendency for cracking to occur during the manufacture of high tensile strength bolts, link plates, and the like. Therefore,
As a result of further research into the causes of such cracks, the present inventors discovered that this cracking is associated with a decrease in toughness in wire rod steel bars. After that, the steel structure is cooled to a predetermined temperature range at a predetermined cooling rate to form a composite structure of ferrite and a low-temperature transformation phase, improving its toughness and forming a spherical shape. We have discovered that it is possible to obtain a wire rod that can omit chemical annealing treatment and quenching and tempering treatment, and can prevent cracking during the production of products that are difficult to work with, such as high-tensile bolts. This led to the invention.

Therefore, an object of the present invention is to provide a method for manufacturing a high-tensile steel wire bar without heat treatment, which has excellent workability and can be particularly suitably used in applications requiring severe workability, such as bolts.

The method for manufacturing a high-strength wire rod steel bar without heat treatment with excellent workability according to the present invention includes CO, OS ~ 0.20%, Si 1.0% or less, and Mn 1.0-2.5% in weight percent. After hot working the steel containing steel, it is cooled from a temperature range of 800 to 950 °C to a temperature range of 300 to 600 °C at an average cooling rate of 1 to b, and the volume content of ferrite is 40 to 75% and the balance is malten. site,
It is characterized by having a composite structure with a low-temperature transformation-generated phase consisting of bainite or a mixed structure thereof.

First, the reasons for limiting the constituent elements in the steel according to the present invention will be explained.

C is necessary to give the steel the required strength through solid solution strengthening, and in the steel of the present invention, it is at least 0.05%.
Requires the addition of However, when adding too much,
C increases the deformation resistance of the steel, and in particular, when it exceeds 0.20%, the workability of the steel increases significantly, so the upper limit of the C content is set at 0.20%.

Like C, Si has a great effect of increasing the strength of steel through solid solution strengthening, but when added in excess, it causes a significant deterioration of toughness and also significantly increases decarburization.
The upper limit of the amount added is 1.0%.

Mn is added to strengthen the steel and obtain a desired structure during cooling after hot working, but when the amount added is less than 1.0%, the above effects cannot be sufficiently obtained. ,
On the other hand, when it exceeds 2.5%, workability decreases due to increased segregation of Mn during manufacturing. Therefore, the amount of Mn added is in the range of 1.0 to 2.5%.

In the method for manufacturing a high-tensile steel wire rod according to the present invention, steel having the above chemical composition is heated and first subjected to normal hot working into a wire rod. In the present invention, the thermal temperature of the steel ingot is not particularly limited, but preferably 900 to 11
It is in the range of 00°C. When the heating temperature exceeds 1100°C, coarsening of grains and decarburization may occur, while when the heating temperature is lower than 900°C, the deformation resistance of the steel increases during hot rolling, and the resulting wire rod steel bar This is because surface flaws may occur on the surface.

In the present invention, the thus heated steel is hot rolled, and the rolling finishing temperature is in the range of 850 to 1200°C,
Preferably it is in the range of 900 to 1150°C. When the rolling finishing temperature is a high temperature exceeding 1200°C, the crystal grains become coarse, so for example, the workability when forming the obtained wire rod into a bolt deteriorates, and the obtained bolt also has poor properties. That will happen. On the other hand, in terms of the behavior of grains, the rolling finishing temperature is lower, the grains become finer and the toughness of the steel improves, but on the other hand, the productivity decreases significantly, so in the present invention, The rolling finishing temperature is 850'
A grade of C or higher.

In the present invention, the cooling start temperature after the above-mentioned hot rolling is set in a temperature range of 800 to 950°C, and cooling is performed from this temperature range to a temperature range with an average cooling rate of 1 to b 00 to 600°C. When the cooling start temperature is a high temperature exceeding 950° C., it is difficult to adjust the amount of ferrite precipitation, and the crystal grains also become large, making it impossible to obtain the desired toughness.

On the other hand, when the cooling start temperature is lower than 800° C., it is also difficult to adjust the amount of ferrite precipitation, making it difficult to maintain the quality of the obtained product constant.

Next, by cooling from the above cooling start temperature at an average cooling rate of 1 to b, the obtained steel structure has a volume content of 4.
It has a composite structure of 0 to 75% ferrite and the balance is a low-temperature transformation product phase consisting of martensite, bainite, or a mixed structure thereof. When the cooling rate is slower than 1°C/sec, pearlite phase may precipitate following the precipitation of ferrite phase, which is undesirable. On the other hand, when it exceeds 15°C/sec, the required ferrite volume content is It becomes difficult to obtain a composite structure with a high degree of strength, and as a result, there is an increased tendency for variations in strength and toughness to increase.

Therefore, in the method of the present invention, the average cooling rate is
b In the present invention, the rolled wire rod steel bar is cooled to a temperature range of 300 to 600 ° C. at a predetermined average cooling rate as described above, and in order to achieve high toughness by the self-tempering effect due to the heat retained in the rolled wire rod steel bar. , it is important to maintain this temperature range. This cooling stop temperature is 300
When the temperature is lower than 6°C, the above self-tempering effect is poor, and
When the temperature is higher than 00°C, the above-mentioned effects are too strong, leading to a decrease in strength.

Therefore, in the method of the present invention, for example, in the case of a wire rod, it may be cooled to the above-mentioned predetermined temperature range, then wound and held in this temperature range, or it may be cooled to the above-mentioned temperature range, After being maintained within this temperature range, it may be rolled up, or alternatively, after being cooled to the above temperature range and maintained within this temperature range, it may be rapidly cooled and allowed to cool, and then wound up.

According to the present invention, the finally obtained steel structure is a composite structure of a low-temperature transformation-generated phase consisting of ferrite with a volume content of 40 to 75% and the remainder consisting of martensite, bainite, or a mixed structure thereof. Ferrite volume content is 40-7
When it is outside the range of 5%, variations in strength and toughness become large.

The wire rod obtained in this way has significantly improved toughness, and therefore can prevent cracking even in applications that require severe cold workability, such as high-tensile bolts.

Note that cooling to the temperature range of 300 to 600°C and cooling after holding are not particularly limited, but 9.
For example, considering the workability of bundling wire rods, etc., the temperature may be cooled to 100° C. or less within 3 hours.

Further, in the present invention, in order to strengthen the wire rod steel bar and increase its softening resistance through self-tempering, and to reduce variations in strength, At least one element can be added.

In addition, in order to improve toughness through grain refinement and reduction of solid solution N, from the group consisting of Ag 0.1% or less, Nb 0.1% or less, Ti 0.1% or less, and Zr 0.3% or less. At least one selected element can be added.

Furthermore, in order to control the shape of inclusions and increase workability, at least one element selected from the group consisting of REM 0.05% or less and Ca 0.003% or less may be added, Furthermore, in order to further improve the toughness, N
It is also possible to add i in a range of 1.0% or less.

As described above, the wire rod steel bar obtained by the method of the present invention has improved toughness and has excellent cold workability. After that, a bolt having the desired characteristics can be obtained by cold bolt forming including thread rolling, so that the spheroidizing annealing process can be omitted. Also, in the case of a link plate for a chain, the required properties can be obtained by performing cold flat pressure processing and cold punching, and therefore, the quenching and tempering process can be omitted.

The steel used in the present invention is melted, for example, in an LD converter, but the melting method is not limited at all.

EXAMPLES The present invention will be described in more detail below with reference to Examples and Comparative Examples.

Example 1 Steel having the chemical composition shown in Table 1 was melted in an LD converter,
After heating to 1050°C, it was hot rolled to a diameter of 10.3 mm or 8 fin wire at a finishing temperature of 1100°C, and then,
The wire rods were cooled under the conditions shown in Table 1 to obtain wire rods according to the present invention method and the comparative method. The mechanical properties and metallographic structure of these wires are shown. Although the wire rod according to the present invention is made of low carbon steel, it has a strength of 60 kgf/mm2 or more and an aperture value of 50% or more, and has a lower strength than the wire rod manufactured by the comparative method.
It is clear that the ductility balance is excellent.

In addition, after rolling to a diameter of 10.3 m as described above, the diameter was 9°0.
Table 2 shows the mechanical properties, cold compression test results, and cold open flat pressure test results of the steel wire drawn into 5 dragons.

It is clear that the steel wire produced by the method of the present invention has excellent cold compression workability and cold flat pressure workability.

Example 2 Steels A, C, and D shown in Table 1 were rolled and cold wire-drawn to a predetermined nominal diameter, and then formed into hexagonal bolts. Table 3 shows the rolling results and bolt performance. In addition, the oscillation fatigue test has an average stress of 40 kgf/n+
m'' up to 101 repetitions.

Conventionally, bolts with strength classes 8.8 and 9.8 specified in JIS B 1051 and ISO 898/1 have been mainly quenched and tempered, but according to the steel of the present invention, the bolts shown in Table 3 As such, it satisfies the above standards without requiring quenching and tempering treatment. It also has performance equivalent to or better than conventional bolts.

Claims (5)

[Claims] (1) After hot working steel containing 0.05 to 0.20% C, 1.0% or less Si, and 1.0 to 2.5% Mn by weight, a temperature of 800 to 950°C range from 300 to 6 at an average cooling rate of 1 to 15°C/sec.
Cooling to a temperature range of 00℃, volume content 40-75
% of ferrite and the balance is martensite, bainite, or a mixed structure of these, forming a composite structure of a low-temperature transformation-generated phase. (2) In addition to (a) C 0.05 to 0.20%, Si 1.0% or less, and Mn 1.0 to 2.5% by weight, (b) Cr 1.0% or less, After hot working steel containing at least one element selected from the group consisting of 0.3% or less of Mo, the steel is heated to 300 to 300°C at an average cooling rate of 1 to 15°C/sec from a temperature range of 800 to 950°C. 6
Cooling to a temperature range of 00℃, volume content 40-75
% of ferrite and the balance is martensite, bainite, or a mixed structure of these, forming a composite structure of a low-temperature transformation-generated phase. (3) In addition to (a) C 0.05-0.20%, Si 1.0% or less, and Mn 1.0-2.5% in weight%, (b) Al 0.1% or less, After hot working steel containing at least one element selected from the group consisting of Nb 0.1% or less, Ti 0.1% or less, and Zr 0.3% or less, a temperature range of 800 to 950 ° C. from 300 to 6 at an average cooling rate of 1 to 15°C/sec.
Cooling to a temperature range of 00℃, volume content 40-75
% of ferrite and the balance is martensite, bainite, or a mixed structure of these, forming a composite structure of a low-temperature transformation-generated phase. (4) In addition to (a) C 0.05-0.20%, Si 1.0% or less, Mn 1.0-2.5%, (b) REM 0.05% or less, and After hot working steel containing at least one element selected from the group consisting of Ca 0.003% or less, it is heated to 300 to 60% at an average cooling rate of 1 to 15°C/sec from a temperature range of 800 to 950°C.
Cooling to a temperature range of 00℃, volume content 40-75
% of ferrite and the balance is martensite, bainite, or a mixed structure of these, forming a composite structure of a low-temperature transformation-generated phase. (5) A steel containing (a) C 0.05 to 0.20%, Si 1.0% or less, Mn 1.0 to 2.5%, and Ni 1.0% or less is heated by weight. After processing, from a temperature range of 800 to 950 °C, an average cooling rate of 1 to 15 °C/sec is applied to 300 to 6
Cooling to a temperature range of 00℃, volume content 40-75
% of ferrite and the balance is martensite, bainite, or a mixed structure of these, forming a composite structure of a low-temperature transformation-generated phase.