JP2566068B2 - Method for manufacturing steel bar with excellent cold 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 producing a steel bar having excellent cold workability. More specifically, in the production of automobile parts, construction machine parts, etc., softening annealing is not required and rolling is performed as is. The present invention relates to a steel bar manufacturing method capable of easily performing cold working such as drawing, cutting, and cold forging.

[0002]

2. Description of the Related Art Conventionally, parts for automobiles, parts for construction machinery, etc. have been softened and annealed for the purpose of improving cold workability before cold working such as drawing, cutting and cold forging. .
These anneals usually take several hours, and the cost of the anneal treatment has come to occupy a large weight in the manufacturing costs of these mechanical parts with the recent increase in energy. Therefore, from the viewpoint of further improving productivity, there is a strong tendency to omit softening annealing before cold working. On the other hand, in Japanese Examined Patent Publication No. 1-181515, steel having a specific composition is heated to an A _c3 transformation point or higher and hot-rolled, and the total surface reduction is performed at the A _c3 transformation point to the A _c3 transformation point + 200 ° C. A method for producing a low alloy steel having excellent cold workability is shown, in which after finishing rolling with a rate of 40% or more, slow cooling is performed at a cooling rate of 0.05 to 0.29 ° C./sec.

[0003]

[Problems to be Solved by the Invention] Japanese Patent Publication No. 1-1
In the method disclosed in Japanese Patent No. 2815, the austenite grain size immediately after rolling finishing is set to 6 or more, and the aim is to obtain a fine ferrite-pearlite structure as hot-rolled. However, the cold workability of the as-hot-rolled steel obtained by this method is insufficient as compared with the conventional softening annealing, and it is the current situation that it has not been put to practical use.

An object of the present invention is to provide a method of manufacturing a steel bar which can guarantee the conventional cold workability at the softening annealing level as it is while hot rolling.

[0005]

[Means and Actions for Solving the Problems] The inventors of the present invention have conducted extensive studies in order to realize a steel bar capable of guaranteeing excellent cold workability at the level of conventional softening annealing as hot rolling. I got the knowledge of.

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

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

A steel material containing a specific amount of carbonitride-forming element such as Al or N is used, and the rolling heating temperature is 900 to 12
Limiting to 50 ° C to prevent coarsening of austenite grains during rolling heating.

A total area reduction rate of 5 at a temperature range of 880 ° C. or higher
Rolling by 0% or more and refining the austenite grains to a grain size of about 6 to 7.

In a temperature range of less than 880 ° C., “the steel material surface temperature is immediately cooled to Ms point to 700 ° C. immediately after rolling with a surface reduction rate of 10% or more, and then the surface reduction rate is 10%.
By performing rolling in a process having one or more steps of "performing rolling of at least%", strains in each pass are accumulated, recrystallization grain refinement due to accumulated large strain is promoted, and further steel material on the final rolling exit side Suppressing the growth coarsening of austenite grains by setting the surface temperature to 700 to 880 ° C.

(3) Further, in order to suppress the bainite fraction to 5% or less and to have good cold workability, after finishing rolling 700 to 500 ° C. at an average cooling rate of 0.05 to
It is necessary to cool at 0.7 ° C./sec.

The present invention has been made on the basis of the above new findings, and the gist thereof is as follows: C: 0.10 to 0.80%, Si: 0.01 to 0 as a weight ratio. 40%, Mn: 0.25 to 1.70%, S: 0.01 to 0.15%, Al: 0.015 to 0.05%, N: 0.003 to 0.020% P: 0.035% or less, or further, Cr: 1.5% or less, Ni: 3.5% or less, Mo: 1.0% or less, B: 0.005% or less, one kind Or containing two or more,
Furthermore, it contains one or more of Ti: 0.005 to 0.04%, Nb: 0.005 to 0.1%, V: 0.03 to 0.3%, and the balance is iron and When hot-rolling steel consisting of inevitable impurities, (A) a step of heating to 900 to 1250 ° C., and (B) a step of rolling at a total surface reduction rate of 50% or more in a temperature range of 880 ° C. or more, (C) After that, in a temperature range of less than 880 ° C., “after the rolling with a surface reduction rate of 10% or more, immediately, the steel surface temperature is once cooled to the Ms point to 700 ° C., and then the surface reduction rate is 10%.
% Rolling is performed at least once, and the steel surface temperature on the final rolling exit side is 700 to 880.
C., and (D) after finish rolling, cooling in a temperature range of 700 to 500 ° C. at an average cooling rate of 0.05 to 0.7 ° C./sec. It is a method of manufacturing a steel bar having excellent cold workability.

The present invention will be described in detail below.

First, C is an element effective for increasing the strength of the final product as a mechanical part, but if the content is less than 0.10%, the strength of the final product will be insufficient, and if it exceeds 0.80%. Since the toughness of the final product is deteriorated, the content is set to 0.10 to 0.80%.

Next, Si 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. If it exceeds 4%, these effects are saturated and rather the toughness of the final product is deteriorated, so the content was made 0.01 to 0.4%.

Mn is an element effective in increasing the strength of the final product by improving the hardenability, but 0.25
If it is less than 0.1%, this effect is insufficient, while if it exceeds 1.7%, this effect is saturated and rather the toughness of the final product is deteriorated. Therefore, the content is made 0.25 to 1.7%. .

Further, S exists as MnS in steel and contributes to improvement of machinability and refinement of the structure, but 0.01
%, The effect is insufficient. On the other hand, 0.15%
If it exceeds, the effect is saturated and rather causes deterioration of toughness and increase of anisotropy. For the above reasons, the S content is set to 0.
It was set to 01 to 0.15%.

Next, Al is added as a deoxidizing element and a crystal grain refining element, but if it is less than 0.015%, its effect is insufficient, while if it exceeds 0.05%, its effect is saturated. However, it rather deteriorates the toughness, so the content was made 0.015 to 0.05%.

In addition, N is the precipitation behavior of AlN,
Although it contributes to the refinement of the austenite grain / ferrite / pearlite structure, if it is less than 0.003%, the effect is insufficient, while if it exceeds 0.020%, the effect is saturated and rather the toughness deteriorates. , Its content is N: 0.
It was set to 003 to 0.020%.

On the other hand, P causes grain boundary segregation and center segregation in the steel and causes deterioration of toughness. In particular, when P exceeds 0.035%, deterioration of toughness becomes remarkable, so 0.035% was made the upper limit.

Next, in the steel used in the present invention, Cr,
One, two or more of Ni, Mo and B may be contained. These elements are added to increase the strength of the final product by increasing the hardenability. However, if a large amount of these elements is added, hot rolling causes bainite and martensite structure to increase hardness, and it is not preferable from the economical point of view, so that the content of Cr is 1.5% or less, Ni: 3.5% or less, Mo: 1.0% or less, B: 0.005% or less.

Further, in the present invention, one or more of Ti, Nb and V may be contained as an essential element for the purpose of adjusting the grain size. However, the Ti content is less than 0.005%, the Nb content is less than 0.005%,
If the V content is less than 0.03%, its effect is insufficient, while Ti: more than 0.040%, Nb: 0.10%.
If V exceeds 0.30%, the effect is saturated and rather the toughness is deteriorated.
05-0.040%, Nb: 0.005-0.1%,
V: 0.03 to 0.3%.

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

First, the heating temperature is set to 900 to 1250 ° C., because the heating temperature lower than 900 ° C. lowers the rough rolling-intermediate rolling temperature, and the austenite grains are not sufficiently refined by the recrystallization zone rolling. This is because the austenite crystal grains are remarkably coarsened at a heating temperature of 1250 ° C. or higher.

Next, rolling at a total area reduction rate of 50% or more in the temperature range of 880 ° C. or higher is to recrystallize the austenite grains into fine grains of about 6 to 7. The reason for setting the rate to 50% or more is that if it is less than 50%, the effect of recrystallizing and refining is small.

Further, in the temperature range of less than 880 ° C., “the surface temperature of the steel material is once Ms immediately after the rolling with a surface reduction rate of 10% or more.
The steel surface temperature on the delivery side of the final rolling is 700 to 8 after cooling in a process having one or more steps of "cooling to a temperature of ~ 700 ° C and then rolling with a surface reduction rate of 10% or more".
The reason why the temperature is 80 ° C. is as follows. In rolling in a temperature range of less than 880 ° C., recrystallization is usually difficult, and release of work strain mainly occurs by recovery, so that it is difficult to make austenite grains fine. On the other hand, if the strain is accumulated by suppressing the release of the processing strain due to the recovery by cooling immediately after rolling and leaving a substantially large strain, the recrystallized grains can be refined. This phenomenon is caused by one process of "immediately after rolling with a surface reduction rate of 10% or more, the steel surface temperature is once cooled to the Ms point to 700 ° C, and then rolling with a surface reduction rate of 10% or more" is performed once. It can be realized by rolling in the steps described above. Here, the surface reduction rate before and after cooling is 10
The reason why the content is set to be not less than 10% is that if the surface reduction rate is less than 10%, the amount of accumulated strain is small and the effect on recrystallization grain refinement is insufficient. Moreover, the surface temperature after cooling is Ms point to 700 ° C.
The reason is that when the surface temperature after cooling exceeds 700 ° C., the release of work strain is insufficiently suppressed, while when it is less than the Ms point, a martensite structure is generated in the surface layer. It should be noted that the effect of suppressing the release of processing strain becomes particularly remarkable by cooling to less than 600 ° C. Therefore, it is desirable to set the surface temperature after cooling to Ms point or more and less than 600 ° C. if possible. Further, in the present invention, in a temperature range of less than 880 ° C., “the steel material surface temperature is immediately cooled to Ms point to 700 ° C. immediately after rolling with a surface reduction rate of 10% or more,
It is possible to carry out any rolling before and after the step of "following the step of rolling with a surface reduction rate of 10% or more". Also, when this step is repeated twice or more, even if it is carried out continuously, any rolling can be performed. You can go there too. Next, the temperature of the steel material surface on the delivery side of the final rolling is set to 700 to 880 ° C.
If the finishing temperature is lower than, there is a risk that ferrite will be generated in the steel surface layer before or during finish rolling, and if the finishing temperature exceeds 880 ° C, the risk of growth and coarsening of austenite grains immediately after rolling will occur. Because there is. Here, the steel surface temperature at the final rolling exit side is
It depends on the reason. Steel bar size, rolling speed, processing before rolling
Heat temperature, steel surface temperature immediately after cooling during rolling (cooling
Stop temperature). Therefore, the surface temperature of the steel material on the delivery side of the final rolling
To 700-800 ° C, depending on the steel bar size,
The rolling speed, heating temperature and cooling stop temperature mentioned above are issued by the present application.
By operating within the range specified in the description. Cold in the middle
The same applies when the rejection is performed multiple times. Steel bar in the present invention
In rolling, remarkable processing heat is generated during rolling.
Therefore, the surface temperature of the steel material increases after rolling. Steel bar size,
The influence of the rolling speed is related to this processing heat value.
It is. Also, in the middle of rolling, it is forced by a cooling device.
By cooling, the surface temperature of the steel material is once set at Ms point to 70
0 ℃, but at this time, the internal temperature is higher than the steel surface temperature
Since it is also high, the surface temperature of the steel material is
It rises under the influence of temperature, that is, it recovers heat. for that reason,
To keep the surface temperature of the steel material on the delivery side of the final rolling within the specified range
The operation of this cooling stop temperature is particularly important. Figure 1
In a temperature range of less than 880 ° C, "a surface reduction rate of 10% or more
Immediately after rolling, the surface temperature of the steel material once becomes Ms point to 700 ° C.
To reduce the area reduction rate to 10% or more.
Rolling is performed in a process that has one process of "rolling"
Steel of the present invention having a surface temperature on the extension side of 700 to 880 ° C
An example of the temperature history of the material surface is shown. Processing heat generated during rolling
Therefore, the surface temperature of steel material increases after rolling A, B, F.
It In addition, after rolling A, the cooling device forcibly cools it.
By doing so, the surface temperature of the steel material is once Ms point to 700 ° C.
(T ₁ ), but after cooling, the steel surface
The surface temperature rises. Rolling F is the final rolling, and T _F is the maximum.
It is the surface temperature on the delivery side of the final rolling, which is 700 to 880 ° C.
And The rise of the surface temperature of steel from T ₁ to T _F
It is due to the recuperation after rejection and the heat generated by processing due to rolling. Therefore,
No heating is required during rolling. Next, FIG.
At a temperature range of less than 880 ° C, "a surface reduction rate of 10% or more
Immediately after rolling, the surface temperature of the steel material once becomes Ms point to 700 ° C.
To reduce the area reduction rate to 10% or more.
Rolling is performed in a process that has two steps of "rolling" and final pressure is applied.
Steel of the present invention having a surface temperature on the extension side of 700 to 880 ° C
It is an example of a temperature history of the material surface. Rolling 1A
~ Rolling 1B is one cycle of strain accumulation processing.
After repeating this twice, the final rolling of rolling F is performed.
It After performing the first strain accumulation process, that is, rolling 1
The surface temperature of the steel after B is due to recuperation and rolling after cooling.
Compared to the surface temperature of steel immediately after cooling due to heat generated by processing ,
It is rising. Therefore, the second strain accumulation continues.
Processing (rolling 2A to rolling 2B) can be performed continuously
It is. Therefore, again, heating is required during rolling.
It does not mean that.

Next, the temperature range of 700 to 500 ° C. after finish rolling is cooled at an average cooling rate of 0.05 to 0.7 ° C./sec. This is because if it exceeds 7 ° C / sec, the structure after rolling and cooling becomes a structure in which 5% or more of bainite is mixed. It is for consumption. After finishing rolling, 70
Up to 0 ° C, it is desirable to cool at a cooling rate higher than air cooling. Further, for cooling at 500 ° C. or lower, any cooling rate can be selected. As a method for adjusting cooling, a method such as attaching a slow cooling cover can be considered.

The effects of the present invention will be more specifically described below with reference to examples.

[0029]

[Examples] Table 1 shows the chemical components of the test materials.

All of these were cast by continuous casting after melting in a converter. After slab-rolling into a 162 mm square steel piece, it was rolled into a round bar steel under the rolling conditions shown in Table 2. Regarding the method of the present invention,
After rolling, controlled cooling was performed by covering the cooling floor with a slow cooling cover. Further, only the material rolled in the comparative example was soft-annealed under the condition of heating and cooling at 680 ° C. for 2 hours.

Hardness was evaluated as an index of cold workability. Table 3 shows the material characteristics of each steel material in comparison with the present invention and a comparative example. As is clear from this, the as-rolled material of the present invention has a bainite fraction of 5% or less, and the hardness level is softened by 10 or more at HV as compared with the as-rolled material of the comparative example. , A softening level almost equal to that of the “comparative example (conventional method) rolled material → softening annealed” material is achieved.

[0032]

[Table 1]

[0033]

[Table 2]

[0034]

[Table 3]

[0035]

[Table 4]

[0036]

[Table 5]

[0037]

As described above, by using the method of the present invention, it is possible to manufacture a steel bar which can guarantee the excellent cold workability of the conventional softening annealing level as it is in the hot rolling. Since the previous softening anneal can be omitted, the manufacturing cost can be greatly reduced and the productivity can be improved, and the industrial effect is extremely remarkable.

[Brief description of drawings]

FIG. 1 shows a surface area reduction rate of 10% or less in a temperature range of less than 880 ° C.
Immediately after the above rolling, the steel surface temperature is once Ms point to 700
Cool to 0 ° C, and subsequently reduce the surface area by 10% or more.
Rolling is performed in a process that has one process.
The present invention in which the surface temperature on the delivery side of final rolling is 700 to 880 ° C
It is a figure which shows an example of the temperature history of the steel material surface.

[Fig. 2] In a temperature range of less than 880 ° C, "reduction rate of 10% or less
Immediately after the above rolling, the steel surface temperature is once Ms point to 700
Cool to 0 ° C, and subsequently reduce the surface area by 10% or more.
Rolling is performed in a process that has two steps of "rolling
The present invention in which the surface temperature on the delivery side of final rolling is 700 to 880 ° C
It is a figure which shows an example of the temperature history of the steel material surface.

Claims (3)

(57) [Claims] 1. A weight ratio of C: 0.10 to 0.80%, Si: 0.01 to 0.40%, Mn: 0.25 to 1.70%, S: 0.01 to 0. Steel containing 15%, Al: 0.015 to 0.05%, N: 0.003 to 0.020%, P: 0.035% or less, the balance being iron and inevitable impurities (A) heating to 900 to 1250 ° C., (B) rolling to a total area reduction of 50% or more in a temperature range of 880 ° C. or higher, and (C) after that, 880 In a temperature range of less than 0 ° C., immediately after the rolling with a surface reduction rate of 10% or more, the steel surface temperature is once cooled to Ms point to 700 ° C., and then the surface reduction rate is 10%.
% Rolling is performed at least once, and the steel surface temperature on the final rolling exit side is 700 to 880.
C., and (D) after finish rolling, cooling in a temperature range of 700 to 500 ° C. at an average cooling rate of 0.05 to 0.7 ° C./sec. A method for manufacturing a steel bar having excellent cold workability. 2. The steel further contains one or more of Cr: 1.5% or less, Ni: 3.5% or less, Mo: 1.0% or less, B: 0.005% or less. The method for producing a steel bar having excellent cold workability according to claim 1. 3. The steel further comprises one or more of Ti: 0.005 to 0.04%, Nb: 0.005 to 0.1%, V: 0.03 to 0.3%. The method for producing a steel bar having excellent cold workability according to claim 1 or 2, which contains.