JPH09324212A - Production of hot rolled high carbon steel strip excellent in hardenability and cold workability - Google Patents

Abstract

PROBLEM TO BE SOLVED: To produce a hot rolled high carbon steel strip having fine spheroidal carbide and excellent in hardenability and cold workability by heating a steel stock of specific C content to a specific temp., applying respectively specified roughing, cooling treatment, and finish rolling to the steel stock, and coiling the resultant steel strip at high temp. SOLUTION: A steel stock, which has a composition containing, by weight, 0.2-1.3% C, preferably containing 0.1-1.0% Si, 0.05-2.0% Mn, <=0.05% P, <=0.05% S, 0.001-0.2% Al, and <=0.05% Ni, further containing, if necessary, prescribed amounts of Cr, Ni, Mo, and B, and having the balance Fe with inevitable impurities, is used. This steel stock is heated to 1000-1300 deg.C and then roughed at a temp. between 950 deg.C and the Ar3 point or a temp. not lower than the Arcm point at >=50% draft to increase the number of pearlite nucleation sites. Within 10sec after roughing, cooling is performed down to a temp. not higher than the Ar1 point at a rate of >=1 deg.C/s to prevent the formation of coarse lamellar carbide. The resultant rolled stock is finish-rolled at a temp. between the Ar1 point and 500 deg.C at >=30% draft to decompose and refine pearlite, followed by coiling at 700-450 deg.C to form spheroidal carbide by self-retained heat.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for manufacturing a high carbon hot-rolled steel strip used for machine structures and tools,
In particular, it relates to a method for producing a high carbon hot rolled steel strip having fine spherical carbides.

[0002]

2. Description of the Related Art Carbides contained in the structure of hot-worked steel produced under normal production conditions are layered carbides called pearlite. Such a layered carbide deteriorates the workability of the steel material, and also deteriorates the quenching failure and the toughness after heat treatment such as quenching and tempering. It is common to let them do it. As this spheroidizing treatment, heating at a temperature just below A ₁ point for a long time or
After heating an additional period immediately above _a point, or slow cooling, across _a point A, repeatedly heating and cooling to a temperature directly above and below them, there is a method and the like. However, since spheroidizing requires a long time in any of the methods, it has been desired to shorten the spheroidizing treatment time.

In view of this, Japanese Patent Publication No. 63-14045 discloses that after the processing of 10% or more and 60% or less at a temperature lower than the A ₁ point of the dynamic recrystallization temperature of ferrite, 630 ° C.
Above, a spheroidizing treatment has been proposed in which the temperature is kept lower than the A ₁ point for 5 seconds or more and 5 hours or less. However, according to this method, there remain problems such as the need for reheating after processing and the difficulty in controlling the temperature.

In Japanese Patent Laid-Open No. 63-86814, the ferrite-pearlite transformation is completed by cooling in the course of hot rolling, followed by rapid heating to a temperature range of less than A _C3 point.
A method of adding processing of 0% or more and 70% or less has been proposed. Furthermore, in JP-A-63-86815, cooling is performed during hot rolling to complete the martensitic transformation, or in JP-A-63-89617, cooling is performed during hot rolling. After completing the bainite transformation, there is proposed a method of rapid heating and adding 10% or more and 70% or less of processing in a temperature range of A _C3 or less. However, even with these methods, the spheroidizing treatment time could not be significantly shortened.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to propose a method for producing a high carbon hot-rolled steel strip having fine spheroidal carbide, which makes it possible to greatly reduce the spheroidizing treatment time. .

[0006]

Means for Solving the Problems The inventors of the present invention have conducted extensive studies in order to achieve the above object, and as a result, after completing the transformation from austenite to layered pearlite before or during finish rolling. It was newly found that the carbide is finely divided by the rolling process and further spheroidized by being wound at a high temperature.

Next, an experiment conducted by the present inventors will be described. Rough rolling of a steel material having a composition equivalent to SK5 of 0.79% C is completed at A _r3 point or more and 950 ° C. or less, and immediately after completion of the rough rolling, it is rapidly cooled to A _r1 or less and finish rolling (a reduction ratio of 63
%), And was wound at 570 ° C. to obtain a hot rolled coil (material of the present invention) having a thickness of 3.5 mm. Using a test piece collected from this coil, after heating at 750 ° C. while changing the holding time, quenching was performed and the surface hardness was investigated. As a comparison,
The same experiment was conducted for the conventional process material (rough / finish rolling at high temperature) and the spheroidized annealed material (700 ° C. × 24 hr). The result is shown in FIG.

With the conventional process material, the quenching temperature is 1000 s.
Quenching hardness is low even if held for ec. Further, the spheroidized annealed material reaches a high quenching hardness only after quenching after holding for 1000 seconds. On the other hand, the material of the present invention is 20 sec
A sufficiently high quenching hardness can be obtained by heating for a short time. This can be explained by the difference in the average grain size of the carbides shown in FIG. The average carbide particle size of the material (a) of the present invention is 0.03 μm.
The average carbide grain size of the conventional process material (b) is 2 μm, while m is 2. Thus, by setting the rolling reduction of the rough rolling and the rolling reduction of the finish rolling to be a certain value or more within a predetermined temperature range, the carbide is finely divided, and further, by winding at a high temperature, the carbide becomes a fine spherical shape. Become. The present invention is configured based on such knowledge.

That is, according to the present invention, C: 0.
Steel material containing 2 to 1.3%, 1000 ℃ ~ 13
Heated to a temperature of 00 ° C, below 950 ° C, A _r3 point or
subjected to rough rolling reduction rate of 50% or above _rcm point, to start the rough rolling termination cooled within 10 sec, and cooled at 1 ° C. / s or more cooling rate to below A _r1 point, following subsequent A _r1 point High carbon hot rolling with excellent hardenability and cold workability, characterized in that after finish rolling with a rolling reduction of 30% or more at a temperature of 500 ° C or more, it is wound up at a temperature of 700 ° C or less and 450 ° C or more. It is a method of manufacturing a steel strip.

In the present invention, the weight percentage of the steel material is
And C: 0.2-1.3%, Si: 0.1-1.0%,
Mn: 0.05 to 2.0%, P: 0.05% or less, S:
0.05% or less, Al: 0.001-0.2%, N:
It is preferable to use a steel material containing 0.05% or less and the balance Fe and unavoidable impurities. Further, in the present invention, the steel material further comprises C:
0.2-1.3%, Si: 0.1-1.0%, Mn:
0.05-2.0%, P: 0.05% or less, S: 0.0
5% or less, Al: 0.001-0.2%, N: 0.05
% Or less, Cr: 0.1 to 5.0%, and it is preferable to use a steel material containing the balance Fe and unavoidable impurities.

The present invention also provides, as the steel material, by weight, C: 0.2 to 1.3%, Si: 0.1 to 1.0.
%, Mn: 0.05 to 2.0%, P: 0.05% or less,
S: 0.05% or less, Al: 0.001-0.2%,
N: Contains less than 0.05%, Ni: 0.1-5.0
%, Mo: 0.1 to 1.0%, B: 0.0005 to 0.
It is preferable to use a steel material containing one kind or two or more kinds selected from 0100% and the balance Fe and unavoidable impurities.

Further, in the present invention, as said steel material, C: 0.2-1.3%, Si: 0.1-% by weight.
1.0%, Mn: 0.05 to 2.0%, P: 0.05%
Hereinafter, S: 0.05% or less, Al: 0.001 to 0.2
%, N: 0.05% or less, Cr: 0.1 to 5.0%, Ni: 0.1 to 5.0%, Mo: 0.1 to 1.0
%, B: 0.0005 to 0.0100%, and it is preferable to use a steel material containing one or more selected from the balance Fe and unavoidable impurities.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention is a method for producing a steel strip which is hot-rolled and does not require a special spheroidizing heat treatment. The gist of the present invention is that before or during finish rolling. The purpose is to complete the pearlite transformation and finish rolling to divide the pearlite into fine pieces, wind them into coils, and make them spherical by self-contained heat.

The heating of the steel material for hot rolling is sufficient if it can be completely solutionized, and 1000 ° C to 1300 ° C is optimal. First, it is important to uniformly generate layered carbides having a small layer spacing. For that purpose, it is important to uniformly increase the pearlite nucleation sites before the transformation. In order to uniformly generate pearlite, rough rolling needs to be performed at a temperature of A _r3 point or higher, and in order to increase the nucleation sites of pearlite, it is necessary to lower the rolling temperature and increase the working amount. Therefore, the rough rolling is 950 ℃
Hereafter, it is necessary that the temperature is A _r3 point or A _rcm point or higher and the _rolling reduction is 50% or higher.

After the rough rolling, the steel strip is
Cooling starts within 0 sec and 1 ° C / s until A _r1 point or less
It is cooled at the above cooling rate. As a result, it is possible to prevent the formation of layered carbide having a coarse layer spacing due to the low transformation rate. The steel strip cooled to A _r1 point or less is subjected to finish rolling. The finish rolling requires a temperature of 500 ° C. or higher at A _r1 point or lower and a reduction rate of 30% or higher. If the finish rolling temperature is less than 500 ° C, the rolling load becomes excessive, so 50
The lower limit was 0 ° C. Further, if the rolling reduction is less than 30%, pearlite is not divided, so 30% was made the lower limit. Since the rolling load becomes excessive when the rolling reduction exceeds 95%, the rolling reduction is preferably 95% or less.

The steel strip after finishing rolling is wound up. The winding temperature was 700 ° C or lower and 450 ° C or higher. In the present invention, spheroidization is achieved by self-contained heat after winding. If the temperature is lower than 450 ° C, sufficient spheroidization cannot be achieved, so the lower limit was 450 ° C. Also, above 700 ° C,
Decarburization occurs, and a special device is required for heating, so the upper limit was 700 ° C.

Next, a suitable steel composition applicable to the present invention will be described. The steel applicable to the present invention may be any steel that can form basically uniform layered carbides, but the carbon content is particularly important in order to form uniform layered carbides. Other alloy component ranges are acceptable as long as they form uniform layered carbides, and are not particularly limited, but they can be added as required depending on the requirements such as steel strip characteristics.

C: 0.2-1.3% C is particularly important for producing a uniform layered carbide. If it is less than 0.2%, uniform layered carbide is not formed in the cooling process. On the other hand, if it exceeds 1.3%, coarse coarse carbides are formed and uniform layered carbides cannot be obtained. Therefore, C is limited to 0.2 to 1.3%.

Si: 0.1 to 1.0% In addition to acting as a deoxidizing agent, Si is an element effective in securing strength and improving hardenability, but if it is less than 0.1%, its effect can be seen. I can't. Further, if it exceeds 1.0%, it becomes brittle, so the range was made 0.1 to 1.0%. Further, from the viewpoint of forming a layered carbide, 0.05 to 0.35% is preferable.

Mn: 0.05 to 2.0% Mn is also an element effective in securing strength and improving hardenability,
If it is less than 0.05%, the effect is not observed, and the solid solution S increases during hot working to cause embrittlement. Mn is
It is not preferable to add a large amount because it forms a solid solution in cementite and inhibits spheroidization of carbides. Further, if over 2.0%, the toughness deteriorates, so Mn was made 0.05 to 2.0% in range. Furthermore, from the viewpoint of layered carbide formation, 0.0
5 to 0.90% is preferable.

P: 0.05% or less P is an element that deteriorates toughness and crack resistance and is reduced as much as possible. If it exceeds 0.05%, grain boundary embrittlement is likely to occur, so the upper limit was made. Furthermore, considering the toughness,
It is preferably 0.03% or less. S: 0.05% or less S forms an inclusion and causes a decrease in ductility and toughness and an increase in anisotropy of mechanical properties, so it should be reduced as much as possible, and the upper limit is 0.05%. And Furthermore, 0.0
It is preferably 1% or less.

Al: 0.001 to 0.2% Al is added as a deoxidizing agent, and is further combined with N in steel to contribute to the refinement of crystal grains as AlN. For that purpose, 0.001% or more is required. But,
If it exceeds 0.2%, the toughness is deteriorated and free carbon is generated, so 0.2% is made the upper limit. From the cost, suppression of free carbon, and toughness, 0.001 to 0.
A range of 1% is preferred.

N: 0.05% or less N combines with Al and contributes to refinement of crystal grains. Further, it is an element effective for improving the strength and may be added positively. However, if it exceeds 0.05%, the toughness deteriorates, so the upper limit was made. When it is not necessary to improve the strength, 0.
01% or less is preferable.

Cr: 0.1 to 5.0% Cr is an element that improves strength and hardenability and corrosion resistance, and also has an effect of suppressing graphitization of carbides. If it is less than 0.1%, the effect is not recognized. On the other hand, if added in excess of 5.0%, the strength becomes high and embrittlement occurs. Therefore, Cr is set to the range of 0.1 to 5.0%.

Ni: 0.1 to 5.0%, Mo: 0.1
1.0%, and B: 0.0005 to 0.0100%, one or more of Ni, Mo, and B are all elements that improve the hardenability and increase the strength, and if necessary, the strength. It can be selected and added according to the characteristics other than. Ni is an element that improves hardenability and toughness. Below 0.01%,
The effect is not recognized, and even if added over 5.0%, the effect is saturated and the cost increases, so Ni is 0.1 to 0.1%.
The range was 5.0%. Furthermore, from the point of spherical carbide,
1 to 0.3% is preferable.

Mo is an element that improves strength and hardenability, improves wear resistance, and also improves temper embrittlement resistance. If it is less than 0.1%, the effect is not recognized. On the other hand, even if added over 5.0%, the effect is saturated and the cost is increased. Therefore, Mo is 0.1 to
The range was 1.0%. B is an element that improves hardenability, but if it is less than 0.0005%, its effect is not recognized. On the other hand, if added in excess of 0.0100%, the hardenability is impaired and the grain boundary becomes brittle. For this reason,
B was made into the range of 0.0005-0.0100%.

[0027]

【Example】

(Example 1) A slab having a thickness of 200 mm was obtained by continuous casting of steel melted according to a normal process. Table 1 shows the chemical composition of steel. These slabs, heating temperature, rough rolling conditions,
A hot rolled coil was prepared by changing the cooling conditions from the rough rolling to the finish rolling, the finish rolling conditions, and the winding temperature. Table 2 for hot rolling conditions
It shows in Table 4.

Each test piece was sampled from the hot-rolled coil thus obtained, quenched by changing the heating time, and the surface hardness was measured. From the relationship between the quenching heating time and the surface hardness, Table 3 shows the quenching hardness reaching heating time that is 95% or more of the standard hardness. The standard hardness is, for each steel type,
(A _r3 point + 30 ° C.) × 1000 sec The surface hardness when quenched after heating.

Further, the material as hot-rolled was cold-worked at a cold working rate of 35%, and the occurrence of edge cracks and sheet breaks was observed. The heating time for reaching the quenching hardness of the example of the present invention is shorter than that of the conventional example and the comparative example. In addition, it can be seen that in the examples of the present invention, no cracks in the edge cracking plate were generated, and the cold workability was excellent.

[0030]

[Table 1]

[0031]

[Table 2]

[0032]

[Table 3]

[0033]

[Table 4]

(Example 2) Steel manufactured by a conventional process was cast into a slab having a thickness of 200 mm by continuous casting. Table 5 shows the chemical composition of steel. These slabs were subjected to rough rolling and finish rolling to obtain hot rolled coils. Table 6 shows hot rolling conditions. Each test piece was sampled from the hot-rolled coil obtained in this manner, the heating time was changed, and quenching was performed to measure the surface hardness. From the relationship between the quenching heating time and the surface hardness, the quenching hardness reaching heating time, which is 95% or more of the standard hardness, was determined. Further, using the material as hot-rolled, 40% cold working was performed and the working condition was investigated. Table 6 shows the results.

In comparison with the conventional example and the comparative example, the quenching hardness reaching heating time of the present invention example is shorter. Further, the example of the present invention can be cold-worked without causing edge cracking and plate breakage as compared with the conventional example.

[0036]

[Table 5]

[0037]

[Table 6]

[0038]

According to the present invention, it is possible to produce a high carbon hot-rolled steel strip having fine spherical carbides and having excellent hardenability and cold workability only in the hot rolling step. Further, the steel strip according to the present invention has fine spherical carbides as hot-rolled, and the heating time for quenching and the spheroidizing treatment time may be short, resulting in a significant improvement in energy saving and productivity as compared with the conventional one. Can greatly contribute to

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the heat retention time in quenching and the surface hardness after quenching.

FIG. 2 shows a photograph of a metallographic structure of a cross section of (a) the material of the present invention and (b) a conventional process material.

Claims (5)

[Claims] 1. A weight%, C: a steel material containing 0.2 to 1.3%, reduction at 1000 ° C. was heated to a temperature of to 1300 ° C., 950 ° C. or less A _r3 point or A _rcm point higher subjecting the rate of 50% or more of the rough rolling, the rough rolling after completion within starts cooling 10 sec, and cooled at 1 ° C. / s or more cooling rate to below a _r1 point, followed by a _r1 point below 500 ° C. or higher temperature After finishing rolling with a reduction rate of 30% or more, 700
A method for producing a high-carbon hot-rolled steel strip having excellent hardenability and cold workability, which comprises winding at a temperature of not higher than 450 ° C and not lower than 450 ° C. 2. The steel material, in% by weight, C: 0.2 to 1.3%, Si: 0.1 to 1.0%, Mn: 0.05 to 2.0%, P: 0. 0.05% or less, S: 0.05% or less, Al: 0.001 to 0.2%, N: 0.05% or less, and a balance Fe and unavoidable impurities. 1. A method for producing a high carbon hot-rolled steel strip excellent in hardenability and cold workability according to 1. 3. The steel material, in% by weight, C: 0.2 to 1.3%, Si: 0.1 to 1.0%, Mn: 0.05 to 2.0%, P: 0. 0.05% or less, S: 0.05% or less, Al: 0.001 to 0.2%, N: 0.05% or less, Cr: 0.1 to 5.0%, and the balance Fe and unavoidable. The method for producing a high-carbon hot-rolled steel strip excellent in hardenability and cold workability according to claim 1, which is a steel material made of mechanical impurities. 4. The steel material, in% by weight, C: 0.2-1.3%, Si: 0.1-1.0%, Mn: 0.05-2.0%, P: 0. 0.05% or less, S: 0.05% or less, Al: 0.001 to 0.2%, N: 0.05% or less, and Ni: 0.1 to 5.0%, Mo. : 0.1 to 1.0%, B: 0.0005 to 0.0100%, a steel material containing one or more selected from the following, with the balance being Fe and inevitable impurities. Item 1. A method for producing a high carbon hot-rolled steel strip having excellent hardenability and cold workability according to Item 1. 5. The steel material, in% by weight, C: 0.2 to 1.3%, Si: 0.1 to 1.0%, Mn: 0.05 to 2.0%, P: 0. 0.05% or less, S: 0.05% or less, Al: 0.001 to 0.2%, N: 0.05% or less, Cr: 0.1 to 5.0%, and Ni. : 0.1 to 5.0%, Mo: 0.1 to 1.0%, B: 0.0005 to 0.0100%, and the balance Fe. The method for producing a high carbon hot-rolled steel strip excellent in hardenability and cold workability according to claim 1, which is a steel material comprising inevitable impurities.