JPH0432514A - Production of soft wire rod free from surface abnormal phase in as-rolled state - Google Patents

Abstract

PURPOSE:To produce a soft wire rod free from surface abnormal phase in an as-rolled state, requiring no softening, and easy of drawing, machining, and cold working, such as cold forging, by subjecting a billet of a steel with a specific composition to heat treatment and rolling under respectively specified temp. conditions. CONSTITUTION:A billet of a steel having a composition containing, by weight, 0.10-0.50% C, 0.01-0.40% Si, 0.25-1.70% Mn, 0.01-0.15% S, 0.015-0.05% Al 0.003-0.020% N, and <0.035% P or further containing one or >=2 kinds among <1.5% Cr, <3.5% Ni, <1.0% Mo, and <0.005% B is heated to 900-1250 deg.C, hot-rolled at >=880 deg.C at >50% total reduction of area, and subjected, at <880 deg.C, to rolling having more than one stage where the steel wire rod is successively subjected to rolling at >10% reduction of area, immediately to cooling so that the surface temp. of this steel wire rod temporarily reaches a temp. between the Ms point and 700 deg.C, and then to rolling at >10% reduction of area, and further, the surface temp. of the steel wire rod on the outlet side at the time of the final rolling is regulated to 750-880 deg.C. After finish rolling, the wire rod is rapidly cooled so that its surface temp. is regulated to 700-800 deg.C, followed by coiling. Then, this wire rod is cooled, in a coiled state, through a temp. range from 700 to 500 deg.C at 0.05-0.7 deg.C/se average cooling rate.

Description

[Detailed Description of the Invention] [Field of Application in Industry A] The present invention relates to a method for manufacturing a soft wire rod as rolled and free of surface abnormal phases, and more specifically, for manufacturing automobile parts, construction machinery parts, etc. This invention relates to a method for producing a soft wire rod that does not have any surface abnormalities as rolled and can be easily subjected to cold working such as drawing, cutting, and stoichiometric forging by omitting softening annealing.

[Conventional technology] Conventionally, parts for automobiles, parts for construction machinery, etc. were extracted,
Before cold working such as cutting and cold forging, softening annealing is performed for the purpose of improving cold workability. These annealing processes usually take several hours, and the cost of the annealing process has come to account for a large portion of the manufacturing cost of these mechanical parts as energy prices have skyrocketed in recent years.

For this reason, and from the viewpoint of improving productivity, there is a strong tendency to omit the softening annealing of cold working.

On the other hand, Japanese Patent Publication No. 59-31573 discloses that after hot rolling, the temperature is rapidly cooled to 700 to 850°C, and then to 700 to 700°C.
Hold at 75℃ or cool at 5℃/second or less, then 6
A direct heat treatment method for hot rolled wire rods is shown in which the temperature is 0.025 to 0.25°C per second.

In addition, Japanese Patent Publication No. 1-12815 discloses that steel having a specific composition is heated to the Ac3 transformation point or higher and hot rolled,
After finish rolling with a total area reduction of 40% or more at 3 transformation point to AC3 transformation point + 200° C., a cooling rate of 005 to b is shown.An excellent method for manufacturing low alloy steel is shown.

[Problem to be solved by the invention] However, the method disclosed in Japanese Patent Publication No. 59-31573 has poor productivity because it takes time to cool down after rolling, and if a cooling rate close to the upper limit is selected, the method does not reach the softening annealing level. There are drawbacks such as not softening sufficiently.

Furthermore, the method disclosed in Japanese Patent Publication No. 1-12815 aims to make the austenite crystal grain size 6 or more immediately after rolling finish, and to obtain a fine ferrite-pearlite structure as hot-rolled. However, the as-hot-rolled wire rod obtained by this method has insufficient cold workability compared to conventional softening annealing, and furthermore, the wire rod has an abnormal phase with a ferrite fraction of 90% or more on the surface. (hereinafter referred to as surface abnormal phase) appears.

As mentioned above, the current situation is that as-rolled soft wire rods have not yet been put into practical use.

An object of the present invention is to provide a method for producing a soft wire rod that guarantees excellent cold workability of the as-hot-rolled wire rod at the level of conventional softening annealing, and that has no abnormal surface phase.

[Means and effects for solving the problem] The present inventors have achieved a soft wire rod that guarantees excellent cold workability at the level of conventional softening annealing even after hot rolling, and has no surface abnormal phase. In order to achieve this goal, we conducted extensive research and obtained the following knowledge.

(1) In order to obtain the same level of cold workability as the conventional softened annealed material in the as-hot rolled material, the austenite grain size immediately after rolling finish should be set to 8 or more, and the bainite fraction in the as-hot rolled material should be 5. % or less and a fine ferrite-pearlite structure must be obtained.

(2) In order to make the austenite grain size No. 8 or higher immediately after rolling finishing, the following three points are essential.

(2) Use a steel material containing a specific amount of carbonitride-forming elements such as N, and limit the rolling heating temperature to 900 to 1250°C to prevent coarsening of austenite grains during rolling heating.

(2) Rolling is performed at a temperature range of 880° C. or higher with a total area reduction of 50% or more, and the austenite grains are refined to about No. 6 to 7 by recrystallization.

■ In a temperature range of less than 880℃, after rolling with an area reduction of 10% or more, immediately cool the steel surface temperature to the Ms point to 700℃, and then continue rolling with an area reduction of 10% or more. By performing rolling in a process that has one or more steps, the strain in each bath is accumulated, recrystallization becomes finer due to the cumulative roughness, and the surface temperature of the steel material on the exit side of the final rolling is lowered to 7.
By controlling the temperature to 50 to 880°C, growth and coarsening of austenite grains can be suppressed.

(3) Furthermore, in order to suppress the bainite fraction to 5% or less and have good cold workability, it is necessary to
The average cooling rate from 700 to 550°C is 0.05 to b.

(4) Also, the main cause of the abnormal phase on the surface is decarburization between the top rolling and winding, and after the top rolling, the cotton material should be rapidly cooled to a surface temperature of 700 to 800°C before winding. As a result, the occurrence of surface abnormal phases can be prevented.

The present invention has been made based on the above-mentioned new findings, and its gist is as follows: As a weight ratio, C: 0.10-050%, Si: 0.01-0.40%, Mn. Contains: 0.25-1.70%, S: 0.01-0.15%, Humanities: 0.015-0.05%, N: 0.003-0.020%, P: 0 .035% or less, and one or more of the following: Cr: 1.5% or less, Ni: 3.5% or less, Mo: 1.0% or less, B.0.005% or less Contains, and further contains one or more of Ti: 0.005~0.04%, Nb: 0.005~01%, V003~0.3%, and the balance is iron and inevitable impurities. When hot rolling steel made of
) A step of rolling with a total area reduction of 50% or more in a temperature range of 880°C or higher; (C) Thereafter, rolling with a total area reduction of 1
Immediately after rolling 0% or more, the steel surface temperature once reaches the Ms point.
Cool to 700℃, continue to bow, and reduce area by 10%
Rolling is performed in a process that has one or more steps of ``rolling as described above'', and the surface temperature of the steel material on the exit side of the final rolling is maintained at 750 to 880 t.
(D) After finish rolling, the surface temperature of the wire is 700 to 80.
It is characterized by comprising a step of rapidly cooling it to 0°C and then rolling it up, and (E) a step of thereafter coiling it in a temperature range of 700 to 500°C at an average cooling rate of 0.05 to b. The present invention provides a method for producing a soft wire rod without surface abnormal phases while still being rolled.

The present invention will be explained in detail below.

Is C an effective element for increasing the strength of the final product as a mechanical part? If it is less than 0.10%, the strength of the final product will be insufficient, and if it exceeds 0.50%, it will actually reduce the toughness of the final product. Since this may cause deterioration, the content should be reduced to 0.10-0.
It was set at 50%.

Next, Sl is added as a deoxidizing element and for the purpose of increasing the strength of the final product by solid solution hardening, but if it is less than 0.01%, these effects are insufficient; on the other hand, if it is less than 0.4% If the content exceeds 0.0, these effects will become saturated and the toughness of the final product will deteriorate.
The content was set at 1% to 0.4%.

Mn is an effective element for increasing the strength of the final product by improving hardenability, but if it is less than 0.25%, this effect is insufficient, and if it exceeds 17%, this effect is saturated. Rather, it causes deterioration of the toughness of the final product, so its content was set at 0.25% to 1.7%.

Further, S exists in the form of MnS in steel, and contributes to improving rigidity and refining the structure, but if it is less than 0.01%, the effect is insufficient. On the other hand, if it exceeds 0.15%, the effect will be saturated, rather leading to deterioration in toughness and increase in anisotropy. For the above reasons, the S content was set to 0.01 to 0.15.
%.

Next, An should be added as a deoxidizing element and a grain refining element; if it is less than 0.015%, the effect is insufficient; on the other hand, if it exceeds 0.05%, the effect is saturated;
Rather, it deteriorates the toughness, so the content should be reduced to 0.015
~005%.

Furthermore, N contributes to the refinement of the austenite grain/ferrite/pearlite structure through the precipitation behavior of AρN, or the effect is insufficient if it is less than 0.003%.
On the other hand, if it exceeds 0.020%, the effect will be saturated and the toughness will deteriorate, so the content should be reduced to N: 0.003%.
~0.020%.

On the other hand, P causes grain boundary segregation and center segregation in steel, causing toughness deterioration. In particular, when P exceeds 0.035%, the toughness deteriorates significantly, so 0.035% is set as the upper limit.

Next, in the steel used in the present invention, Cr, NiMo
One or more types of B can be contained. These elements are added to increase the strength of the final product by increasing its hardenability.

However, addition of large amounts of these elements causes bainite and martensitic structures during hot rolling, leading to increased hardness.
Moreover, since it is not preferable from an economic point of view, the contents are set to Cr: 1.5% or less, Ni: 3.5% or less, Mo: 1.0% or less, and B +0.005% or less.

Furthermore, in the present invention, for the purpose of particle size adjustment, Ti,
Nb, V (7) 1 fm or two or more types can be contained as essential elements. However, the effect is insufficient when the T content is less than 0.005%, the Nb content is less than 0.005%, and the V content is less than 0.03%; on the other hand, Ti: more than 0.040%. , Nb:
When the content exceeds 0.10% and V: 0.30%, the sono effect is saturated and the toughness is rather deteriorated.
i, 0.005-0.040%, Nb: 0
．． 005-0.1% ■003-0.3%.

Next, the reason for limiting the hot rolling conditions in the present invention will be described.

First, the heating temperature was set at 900 to 1250°C.
This is because at a heating temperature of less than 0°C, the rough rolling-intermediate rolling temperature becomes low and the austenite grains are insufficiently refined by recrystallization zone rolling, and at a heating temperature of 1250°C or higher, the austenite crystal grains become noticeably smaller. This is because it becomes coarser.

Next, rolling with a total area reduction of 50% or more in a temperature range of 880°C or higher is necessary to reduce the austenite grains by recrystallization.
The reason why the total area reduction rate is set to be 50% or more is because the effect of recrystallization to refine grains is small below this.

In addition, in a temperature range of less than 880℃, "After rolling with an area reduction rate of 10% or more, the surface temperature of the steel material is immediately raised to the Ms point to 700℃.
Rolling is performed in a process having one or more steps of "cooling the material so that
The reason why the surface temperature of the steel material on the exit side of the final rolling is set to 700 to 880°C is as follows. In rolling in a temperature range below 880° C., it is difficult to refine the austenite grains because recrystallization is usually difficult and release of working strain occurs mainly through recovery. On the other hand, if the steel is cooled immediately after rolling to suppress the release of processing strain due to recovery and accumulate strain, and substantially large strain remains, recrystallization becomes possible. This phenomenon can be solved by carrying out the process of ``immediately after rolling with an area reduction of 10% or more, the surface temperature of the steel material is once cooled to the Ms point ~ 700℃, and then rolling is performed with an area reduction of 10% or more.'' This can be achieved by rolling in the steps described above. Here, the reason why the area reduction ratio before and after cooling is set to be 10% or more is because if the area reduction ratio is less than 10%, the amount of accumulated strain is small and the effect on recrystallization grain refinement is insufficient. In addition, the reason why the surface temperature after cooling was set to Ms point ~ 700°C was because
This is because if the surface temperature after cooling exceeds 700° C., the release of processing strain is insufficiently suppressed, and if it is below the Ms point, a martensitic structure will occur in the surface layer. Note that the effect of suppressing the release of processing strain becomes particularly noticeable by cooling to less than 600°C, so if possible, it is desirable to set the surface temperature after cooling to less than 600°C above the Ms point. Furthermore, in the present invention, in a temperature range of less than 880°C,
``After rolling with an area reduction of 10% or more, immediately cool the steel material surface temperature to the Ms point ~ 700℃, and then continue rolling with an area reduction of 10% or more.'' Any rolling before or after the process. If this process is repeated two or more times, it may be performed continuously or with optional rolling in between. Next, the surface temperature of the steel material on the exit side of the final rolling was set to 75
The range of 0 to 880°C is because if the finishing temperature is less than 750°C, there is a risk that ferrite will be generated in the surface layer of the steel material before or during finish rolling, and if the finishing temperature exceeds 880°C. This is because there is a risk that the austenite grains will grow and coarsen immediately after rolling.

Next, the surface temperature of the stuffed wire rod after finish rolling is 700 to 800.
The reason why the film is rapidly cooled to ℃ and then rolled up is to prevent the formation of abnormal phases on the surface. If the temperature before winding exceeds 800°C, the effect of preventing surface abnormal phase formation will be insufficient, and if the temperature before winding is less than 700°C, there is a risk that fine pearlite will be generated and hardness will increase. Therefore, the winding temperature range was set to 700 to 800°C.

In addition, after winding, the coil is cooled at an average cooling rate of 0.05 to 0.7°C/sec in the temperature range of 700 to 550°C. This is because if the temperature exceeds 0.05°C/sec, the structure after rolling and cooling will be a structure containing 5% or more of bainite, while if it is below 0.05°C/sec, the slow cooling effect will be saturated and time will be wasted unnecessarily. . Note that an arbitrary cooling rate can be selected for cooling below 550°C. Further, as a method of controlled cooling, an appropriate method such as covering the coil conveyance line with a slow cooling cover may be used.

Below, the effects of the present invention will be illustrated in more detail with reference to Examples.

[Example] Table 1 shows the chemical composition of the sample materials.

All of these were cast by continuous casting after melting in a converter furnace. 1
After blooming into a 62 mm square steel billet, it was rolled into a 19 mm diameter wire rod under the rolling conditions shown in Table 2. Regarding rolling No. I of the present invention method and rolling No. Il of the comparative example, adjusted cooling was performed by placing a slow cooling cover on the coil conveyance line after rolling. In addition, only the materials rolled by the rolling method N o and Ill of the comparative example were subjected to softening annealing under the conditions of heating and cooling at 680° C. for 2 hours.

Hardness was evaluated as an index of cold workability. Table 3 shows the material properties of each steel material, comparing the present invention and a comparative example. As is clear from this, abnormal surface phases occur in Comparative Example Rolled No. and Il, but in the method of the present invention, generation of surface abnormal phases can be prevented, and furthermore, in the as-rolled material of the present invention, it is clear that the bainite fraction is 5. % or less, and the hardness level is approximately equal to that of the as-rolled material of the comparative example rolled No.
It was softened by 10 or more in IV, and as a result, it achieved a softening level that was almost the same as that of the "Comparative Example Rolling No. III (Conventional Method) Rolled Material - Softening Annealing" material.

[Effects of the invention] As described above, by using the present invention, it is possible to produce a soft wire rod that has no surface abnormal phases as hot-rolled and can guarantee excellent cold workability at the level of conventional softening annealing. and
When manufacturing automobile parts, construction machinery parts, etc., it is possible to omit softening annealing before cold working, making it possible to significantly reduce manufacturing costs and improve productivity, which has extremely significant industrial effects. There is.

4 others

Claims (1)

[Claims] 1 Weight ratio: C: 0.10-0.50%, Si: 0.01-0.40%, Mn: 0.25-1.70%, S: 0.01-0.01% Contains 0.15%, Al: 0.015-0.05%, N: 0.003-0.020%, P: limited to 0.035% or less, and the remainder is iron and unavoidable impurities. When hot rolling the steel, (A) heating to 900 to 1250°C; (B) 8
A step of rolling with a total area reduction of 50% or more in a temperature range of 80°C or more; (C) Then, rolling with a total area reduction of 1
After rolling by 0% or more, the steel surface temperature is immediately cooled to the Ms point to 700°C, and then rolled with an area reduction of 10% or more. The surface temperature of the steel material on the final rolling exit side is 750 to 880℃.
(D) After finish rolling, the surface temperature of the wire is 700 to 80.
(E) A step of cooling the coil in the temperature range of 700 to 500 degrees Celsius at an average cooling rate of 0.05 to 0.7 degrees Celsius per second. A method for producing a soft wire rod without surface abnormal phases as rolled, characterized by: 2. Claim in which the steel further contains one or more of the following: Cr: 1.5% or less, Ni: 3.5% or less, Mo: 1.0% or less, B: 0.005% or less 1. The method for producing a soft wire rod having no surface abnormal phase as rolled according to 1. 3. A claim in which the steel further contains one or more of Ti: 0.005 to 0.04%, Nb: 0.005 to 0.1%, and V: 0.03 to 0.3%. 3. A method for producing a soft wire rod as described in item 1 or 2, which has no surface abnormal phase as rolled.