JP3289506B2 - Method for producing low alloy steel with excellent wire drawing workability - 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 efficiently producing a low-alloy steel material having excellent drawability at low cost.

[0002]

2. Description of the Related Art Low carbon content Si-Mn steel and low carbon content Si
-Low alloy steel such as Mn-Ti steel is used, for example, as a carbon dioxide arc welding wire, and its composition is generally C: 0.02 to 0.15% and Si: 0.20%.
-1.65%, Mn: 1.0-2.0%, and Ti: 0.01-0.30% if necessary, with the balance being iron and unavoidable impurities.

[0003] This low-alloy steel material is subjected to hot rolling and then drawn to a steel wire having a diameter of about 1 mm by wire drawing. However, if the hot-rolled steel is drawn as it is, the wire is broken during the wire drawing. For this reason, usually, annealing is performed before wire drawing, that is, after hot rolling or at the stage of an intermediate finish wire. However, this annealing treatment raises production costs such as heat treatment costs.

The above-mentioned disconnection is caused by the difference in the deformability between the ferrite phase present in the hot-rolled steel and the hard island-like martensite phase present in the ferrite phase. It is known that cracks are formed at the interface between both phases or inside martensite during wire drawing, and the cracks propagate and reach a broken state. Therefore, if the formation of island-like martensite in the ferrite phase can be suppressed, the hot-rolled steel material can be subjected to wire drawing as it is, without performing the above-described high-cost production annealing treatment. It is considered that the final product can be obtained by this. Therefore, various methods have been studied in which the formation of island-like martensite can be suppressed without performing an annealing treatment.

[0005] For example, R & D Kobe Steel Engineering Reports (April 1985
The method described in Moon, Vol. 35, No. 2, pp. 52-54) uses a low-carbon containing Si—Mn—Ti steel wire, and uses a wire heating temperature and a rolling temperature in an intermediate stage and a finishing stage. The rolling is performed while keeping the temperature low, and the subsequent cooling process is gradually controlled to make the material uniform. However, this method has a problem in that the content condition of island-like martensite (area content rate: 1.0% or less) necessary for obtaining good wire drawing workability cannot be completely achieved.

[0006] Japanese Patent Application Laid-Open No. Sho 62-164821 discloses a method of performing a cryogenic rolling treatment using a very low carbon steel material. This is intended to reduce the hot deformation resistance at 750 to 900 ° C. by suppressing C to 0.02% or less, and to enable the hot rolling treatment at such an extremely low temperature, resulting in island martensite. Is to be suppressed. However, since this method can be applied only to ultra-low carbon steel materials, when such ultra-low carbon steel is used as a welding wire, the C concentration in the weld metal is diluted and the strength of the weld metal decreases. There is a problem of doing.

Further, Japanese Patent Publication No. Sho 62-54364 discloses that
After hot rolling the mild steel wire rod, it is cooled to 750-650 ° C. at a cooling rate of 10 ° C./sec or less, then cooled to room temperature at a cooling rate of 15 ° C./sec or more, and further 300-500 ° C.
For 0.5 to 10 minutes, and then cooled to room temperature again to improve drawability. According to this method, the intermediate annealing step during wire drawing can be omitted, but after cooling to room temperature, a step of raising the temperature to 300 to 500 ° C. and holding the temperature is provided, so that productivity is reduced. There's a problem.

[0008] In addition, Japanese Patent Publication No. 1-181515 relates to a rod composed of a case hardening steel composition and a tough steel composition, which is heated to a temperature higher than the Ac3 transformation point, hot-rolled, and further transformed into an Ar3 transformation. After performing finish rolling at a total area reduction rate of 40% or more in a temperature range of the temperature from
A method of gradually cooling at a cooling rate of 0.05 to 0.29 ° C./sec is disclosed. However, according to this method, the area content of the island-like martensite cannot be suppressed to 1.0% or less, and the load at the time of hot rolling is increased, so that productivity is greatly impaired or the cooling rate is reduced. Must be strictly adjusted and controlled, and in reality, the annealing process has not been omitted.

[0009]

SUMMARY OF THE INVENTION The present invention has been made in view of the above circumstances, and an object of the present invention is to provide a wire drawing process capable of efficiently suppressing the formation of island martensite at low cost. An object of the present invention is to provide a method for producing an excellent low alloy steel material, which can secure the strength of a weld metal portion when welding is performed using the same.

[0010]

Means for Solving the Problems The method for producing a low alloy steel material excellent in wire drawing workability of the present invention which can solve the above-mentioned problems is as follows.
C: 0.02 to 0.15%, Si: 0.20 to 1.65
%, Mn: 1.0 to 2.0%, Ti: 0.01 to 0.3
In hot rolling a steel material containing 0% and the balance consisting of iron and inevitable impurities, after hot rolling at a temperature higher than the Ac3 transformation point, the temperature range up to the Ar3 transformation point is 7 ° C.
Cooling at a rate of less than 7 ° C./sec, cooling the temperature range between the Ar3 transformation point and the Ar1 transformation point at a rate of 7 ° C./sec or more, and then cooling the temperature range below the Ar1 transformation point at a rate of less than 7 ° C./sec. It has a gist where it cools with.

[0011]

The present inventors have intensively searched for how to suppress the formation of island-like martensite, which causes breakage during wire drawing, and found that the temperature range up to the Ar3 transformation point was 7 ° C. After cooling at a rate of less than / sec, the temperature range from austenite to ferrite transformation, that is, the temperature range between the Ar3 transformation point and the Ar1 transformation point, is cooled at a high cooling rate of 7 ° C / sec or more. 7 ℃ / s in the following temperature range
The inventors have found that cooling at a rate less than ec is effective and completed the present invention.

Hereinafter, the constitution and operation of the present invention will be described in detail, and a steel material itself having the following composition is known. First, the reasons for limiting the components of the steel material used in the present invention will be described. C: 0.02 to 0.15% C is an element that controls the strength and ductility of the weld metal when the steel material of the present invention is used as a welding wire.
%, The required strength cannot be obtained. On the other hand, if it exceeds 0.15%, the toughness decreases.
0.15%.

Si: 0.20 to 1.65% Si is both a strong deoxidizing element and a solid solution strengthening element. In order to increase the strength of the weld metal portion by reducing the amount of impurity inclusions, the lower limit must be set to 0.20%. However, if the amount is too large, the toughness is reduced. Therefore, the upper limit is set to 1.6.
Reduced to 5%.

Mn: 1.0 to 2.0% Mn has a deoxidizing effect, and also has an effect of improving the strength of a weld metal and suppressing susceptibility to hot cracking. In order to effectively exert such an effect, it is necessary to set the lower limit to 1.0%. However, if the amount is too large, the elongation of the weld metal tends to deteriorate. And

Ti: 0.01 to 0.30% Ti is effective for softening a welding arc in a high current region and preventing a decrease in strength and toughness of a weld metal portion due to an increase in heat input during welding. Can be selectively added according to In order to exert the above-mentioned effects effectively, 0.01
% Or more, but the Ti content is 0.30%
If the ratio exceeds 1, the strength becomes too large and the wire drawing processability deteriorates. Therefore, the upper limit is set to 0.30%.

The steel material used in the present invention is a steel material containing the above elements as essential components, with the balance being iron and unavoidable impurities.

As described above, in the production method of the present invention, when hot rolling is performed using a steel material having the above composition,
After hot rolling at a temperature above the Ac3 transformation point, the temperature range up to the Ar3 transformation point is cooled at a rate of less than 7 ° C./sec, and the temperature range between the Ar3 transformation point and the Ar1 transformation point is 7 ° C./sec or more. High-speed cooling at a speed of 7 ° C / s in the temperature range below the Ar1 transformation point
It is characterized in that it is cooled at a speed less than ec.

The island-like martensite, which causes breakage during wire drawing, is generated near the center of the wire, and its generation mechanism is considered as follows. In other words, there is an Mn solidification deflection zone near the center of the wire,
n is concentrated, and austenite is stabilized to a low temperature range during cooling after hot rolling. Therefore, when the surroundings are transformed from austenite to ferrite, C diffuses into this portion and is concentrated. As a result, during the subsequent cooling process, C is transformed into martensite and a hard martensite phase is formed.

The inventors of the present invention have conducted repeated basic experiments for the purpose of suppressing the formation of martensite. As a result, the temperature range between the Ar3 transformation point and the Ar1 transformation point was 7 ° C./s.
It has been found that cooling at a high cooling rate of ec or more is effective. That is, when the cooling rate is set to 7 ° C./sec or more, the degree of supercooling of the transformation increases, and as a result, the nucleation rate of ferrite increases, and ferrite nucleates even in the Mn segregated portion. In addition, since there is not enough time for C to diffuse, martensite is less likely to be generated in the segregation part because of the reason that the concentration of C in the segregation part is suppressed. Alternatively, even if it is generated, it becomes fine, so that it does not cause a disconnection during wire drawing.
The temperature range from the above-mentioned Ar3 to the Ar1 transformation point varies depending on the composition of the steel material used and the cooling rate.
To 600 ° C, and this temperature range is 7 ° C /
What is necessary is just to cool at high speed more than sec. The cooling method is not particularly limited, and includes a method of blowing air on a conveyor after hot rolling, a method of cooling after hot rolling and immersing in a water tank, and a method of spraying cooling water.

Further, according to the present invention, after completion of hot rolling, Ar3
The temperature range up to the transformation point and the temperature range below the Ar1 transformation point
Cool at a rate of less than 7 ° C / sec. The reason is that Ti
This is due to the fact that TiC is precipitated in the cooling process in the contained steel material. That is, the precipitation of TiC occurs in both the austenite phase and the ferrite phase, but the TiC precipitated in the ferrite phase causes an increase in strength, and thus the wire drawing workability of the wire may be deteriorated. To prevent this, it is necessary to precipitate as much TiC as possible in the austenitic phase, and it is necessary to gradually cool the temperature range from the end of hot rolling to the Ar3 transformation point at a cooling rate of less than 7 ° C./sec.
On the other hand, if the TiC in the ferrite phase is coarsened and the distance between the particles is increased, the strength rise is also reduced. Therefore, in the temperature range below the Ar1 transformation point, it is possible to gradually cool at a cooling rate of less than 7 ° C./sec. is necessary.

FIG. 1 is a graph showing the relationship between the precipitation state of TiC and various cooling rate curves in the temperature range from the end of hot rolling to the Ar3 transformation point. In the figure, Ps is a curve representing the precipitation start temperature of TiC, and Pf is a curve representing the precipitation end temperature of TiC. Between the curves Ps and Pf,
The longer the cooling rate curves overlap, the more Ti
This means that a large amount of C precipitates. As can be seen from FIG. 1, the precipitation rate of TiC is most increased when the cooling rate is 1.5
° C / sec, then 3 ° C / sec.
Even when the cooling rate is less than 7 ° C./sec, since the precipitation of TiC is large and the formation of island martensite is hardly observed, the upper limit is set to 7 ° C./sec in the present invention. On the other hand, when the cooling rate is 10 ° C./sec, the precipitation of TiC decreases and the generation of island martensite increases, which is not preferable.

Hereinafter, the present invention will be described in more detail with reference to Examples. However, the following Examples do not limit the present invention, and all changes and implementations without departing from the spirit of the present invention will be described. Within the technical scope of

[0023]

EXAMPLES Example 1 C: 0.08%, Si: 0.78%, Mn: 1.50
%, Ti: 0.14% and about 10%
After being heated to 00 ° C. and hot-rolled, the steel was cooled under the cooling conditions shown in Table 1 to obtain a steel wire (φ: 5.5 mm). The following items were measured for each steel wire thus obtained. (I) Tensile strength (kgf / mm ² ): JIS Z 2241 method (ii) Tensile strength of welded metal part at heat input (30 kJ / cm) (kgf / mm ² ): JIS Z 2241 method (iii) Island shape Martensite content (%): The wire is cut along a plane passing through the diameter, and the area ratio of island martensite on the plane is observed with an optical microscope and measured by the point counting method. (Iv) Limiting wire drawing size (mmφ): Diameter of die where wire is broken for the first time during wire drawing The obtained results are also shown in Table 1.

[0024]

[Table 1]

As is apparent from Table 1, high-speed cooling at a temperature range of 840 to about 600 ° C. at a rate of 10 to 30 ° C./sec, which is within the scope of the present invention, as in Nos. 1, 2 and 4 to 5 And
If other temperature regions were cooled at a rate of 0.2 to 5 ° C./sec, the formation of island martensite could be suppressed and wire drawing could be performed to φ: 1.0 mm or less. However, like No.3, other temperature ranges are 7 ° C /
When cooled at a speed of at least sec, the strength of the steel wire slightly increases, and the wire drawing limit size is φ: 1.2 mm.
The drawability was slightly reduced.

On the other hand, 840 like Nos. 6 and 7
The temperature range of ~ 700 ° C is 3 ° C / sec which is out of the range of the present invention.
When cooled at a cooling rate of c or 0.5 ° C./sec,
The production of island-like martensite increased, and the wire drawing workability also remarkably decreased.

[0027]

The production method of the present invention is configured as described above, and can suppress the generation of island-like martensite, so that an annealing treatment is not required. It turns out that it is. The low alloy steel material obtained by the method of the present invention is excellent in wire drawing workability,
The strength of the weld metal part when welding using this is also good.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the precipitation state of TiC and various cooling rate curves in a temperature range from the end of hot rolling to the Ar3 transformation point.

────────────────────────────────────────────────── ─── Continued on the front page (72) Inventor Hiroaki Mori 1 Kanazawa-cho, Kakogawa City, Hyogo Prefecture Kobe Steel, Ltd. Kakogawa Works (56) References JP-A-3-6325 (JP, A) JP-A Heihei 1-139719 (JP, A) JP-A-53-51120 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C21D 8/06-8/08 C21D 9/52 C22C 38 / 00-38/60

Claims (1)

(57) [Claims] 1. C: 0.02 to 0.15% (% by weight, the same applies hereinafter), Si: 0.20 to 1.65%, Mn: 1.0
-2.0%, Ti: 0.01-0.30% ,
When hot rolling a steel material whose part consists of iron and inevitable impurities , hot rolling at a temperature above the Ac3 transformation point, then cooling the temperature range up to the Ar3 transformation point at a rate of less than 7 ° C / sec.
And retirement, the temperature range between the Ar1 transformation point from Ar3 transformation point 7 ° C. / sec
After cooling at the above rate, the temperature range below the Ar1 transformation point is cooled at a rate of less than 7 ° C / sec.
A method for producing a low-alloy steel material having excellent drawability, which is characterized in that: