JP2795873B2 - Method for spheroidizing layered carbide in eutectoid or hypoeutectoid steel - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a method for spheroidizing eutectoid or hypoeutectoid steel used for structural steel, tool steel and the like.

(Prior art) Hot working steel materials manufactured under the usual method, that is, heating temperature: 1200-1300 ° C, hot rolling finishing temperature: 800-900 ° C, winding temperature: 500-650 ° C, It has a layered carbide called pearlite in its structure. Since such a layered carbide deteriorates workability and causes poor quenching and deterioration of toughness during heat treatment such as quenching, it is general that the carbide is made spherical in the next step.

As spheronization method layered carbide as described above, the conventional (1) after prolonged heating to a temperature just below _point A using a batch furnace, a method of cooling, heating to a temperature just above (2) A ₁ point after, a method of gradually cooling the vicinity of a ₁ point, but like are known, both it takes a long processing time of up to ten hours. In this regard, as measures for resolving the above-mentioned problems, (3) after hot working, the temperature is equal to or higher than the dynamic recrystallization temperature of ferrite;
After addition of processed at temperatures below A ₁ point, 360 ° C. or higher, a method of holding 5 seconds to 5 hours to a temperature of less than ₁ point A (JP-B-63-1404
No. 5) and (4) Transformation into pearlite, bainite, martensite, etc. during hot working, then heat up rapidly, then work in a temperature range of less than Ac ₃ points, and then apply the above (1), A method of performing the treatment of (2) in a relatively short time (Japanese Patent Laid-Open No. 63-86814,
63-86815 and 63-89617).

(Problems to be Solved by the Invention) However, the method (3) not only requires heating during processing, but also has a severe temperature control, and the method (4) still requires considerable processing for subsequent processing. Left a problem where it took time.

The present invention has been developed in view of the above-mentioned situation, and not only can the processing time required for spheroidizing carbides be significantly reduced as compared with the conventional one, but also the degree of spheroidization is higher than the conventional one (carbide particle size is 1 μm or less). It is an object of the present invention to propose a sphering treatment method that can perform the sphering treatment.

(Means for Solving the Problems) The inventors of the present invention have conducted intensive studies to achieve the above object, and as a result, prior to the spheroidizing treatment, cut and refined the layered structure of the carbide by cold working. Thereafter, it was found that rapid heating, short-time holding, and rapid cooling were extremely effective in achieving the intended purpose.

 The present invention is based on the above findings.

That is, the present invention is to provide a eutectoid or hypoeutectoid steel having a pearlite structure having a C content of 0.8 wt% or less, after subjecting the eutectoid steel to cold working of 30% or more, and then re-heating at a rate of 0.5 ° C / s or more. crystal temperature or higher, and heated to a temperature below the a ₁ transformation point, then held 30 to 600 seconds in this temperature range, by cooling at 0.5 ° C. / s or faster, is the particle size of the lamellar carbide pearlite structure in A method for spheroidizing a layered carbide in a eutectoid or hypoeutectoid steel, wherein the method is a fine spherical carbide of 1 μm or less.

The method of the present invention can exert its effect as long as it has a layered carbide (pearlite) in the initial structure, and is basically not subject to restrictions on components. However, in hypereutectoid steel containing carbon exceeding 0.8 wt% (hereinafter simply referred to as%), large reticulated carbides are present in the primary structure, and it is difficult to achieve sufficient spheroidization only by following this invention method. The steel type was limited to eutectoid steel or hypoeutectoid steel having a carbon content of 0.8% or less.

 Hereinafter, the present invention will be described specifically.

In the present invention, the addition of a cold work of 30% or more is particularly important in enabling a reduction in the processing time in the subsequent heat treatment. This is because such cold working breaks down and refines carbides that are extremely difficult to form spheroids in the form of a layer, and as a result, spheroidization with a small amount of energy is possible thereafter.

The reason why the working ratio is set to 30% or more is that if the working ratio is less than 30%, the lamination of the layered carbide is insufficient, and the spheroidization requires a long time, so that the large object of the present invention of shortening the processing time cannot be achieved. is there.

Also, although the upper limit is not particularly specified because the degree of work is preferably somewhat large, if the degree of work is too large, the occurrence of cracks from carbides and work hardening in ferrite will overlap, which may lead to breakage of steel during processing. Since it becomes large, it is desirable to set it to about 70% or more. When cold working is particularly difficult because of a large amount of carbon in steel, it is advantageous to use a powerful working machine such as a lubricant or a Sendzimir rolling mill.

Next, the recrystallization temperature or higher as spheroidizing, by heating to a temperature below the A ₁ transformation point to hold 30 to 600 seconds. This heating is essentially responsible for the spheroidization of the carbide, and it is necessary to hold at least 30 seconds for sufficient spheroidization. However, if it is held for more than 600 seconds, the carbide becomes coarse and the particle size exceeds the target 1 μm diameter, so the upper limit is
It was 600 seconds. If the heating temperature is lower than the recrystallization temperature, not only the energy required for spheroidizing the carbides is not obtained, but also the recrystallization of ferrite becomes insufficient and the quality of the steel material after the treatment is deteriorated. Meanwhile A ₁ is heated to a temperature above the transformation point carbide solid solution in the austenite phase, since the re-occurring layered carbides during cooling, not reach the target.

If the heating rate and the cooling rate are too slow, not only does the processing time become unnecessarily long, but also the holding time at a high temperature cannot be ignored, and begins to affect the coarsening of the carbide. Therefore, for the purpose of the present invention such as shortening of processing time and fine spheroidization of carbide, both heating rate and cooling rate need to be 0.5 ° C./s or more.

As a specific method for performing rapid heating, short-time holding, and rapid cooling treatment, a continuous annealing furnace mainly used for recrystallization annealing of a low carbon steel sheet is advantageously applicable.

Note that the target steel types of the present invention are not limited to carbon steel, but include chromium steel (SCr415 to SCr445), chromium molybdenum steel (SCM415 to 445), and nickel chrome molybdenum steel (S
NCM240, 431, 439, 447) can provide the same effect.

(Example) Example 1 JIS standard SK7 (C: 0.66) having a layered carbide as the initial structure
%, Si: 0.24%, Mn: 0.36%, P: 0.020%, S: 0.008%) and
S35C (C: 0.35%, Si: 0.27%, Mn: 0.80%, P: 0.016%, S: 0.0
(%) (Sheet thickness: 3.0 mm), and were subjected to cold working and heat treatment using an infrared image furnace under various conditions shown in Table 1.

FIG. 1 shows the relationship between the rate of cold working and the time required for carbide spheroidization when treated under condition A. In addition, the judgment of the time was that the ratio of the carbide (the ratio of minor axis / major axis was 0.8 or more) was 8
When it became 0% or more, it was regarded as “spheroidization completed”.

As is clear from the figure, the processing time of each steel is significantly reduced by setting the working ratio in cold working to 30% or more.

FIG. 2 shows the relationship between the heating temperature and the hardness when the treatment is performed under the condition B.

As apparent from the figure, the heating temperature is not sufficient softening is Namaze is lower than the recrystallization temperature, whereas again occurs layered carbide when it exceeds A ₁ point, is rapidly cured.

3 and 4 show the relationship between the heating time, hardness, and average carbide diameter when treated under the condition C of Table 1 for each of SK7 and S35C.

As is clear from FIG.
If less than 30 seconds, sufficient softening did not occur, while if more than 600 seconds, carbides were coarsened, and the diameter exceeded 1 μm.

Example 2 JIS standard S50C (C; 0.50) having a layered carbide as a primary structure
%, Si: 0.26%, Mn: 0.73%, P: 0.016%, S: 0.007%) Using hot-rolled sheet (sheet thickness: 3.0mm), cold working and heat treatment under various conditions shown in Table 2. Was done.

Table 2 also shows the results of an examination on the hardness, carbide particle size, and total heat treatment time of the steel sheet after the treatment.

The heat treatment according to the present invention was performed by a continuous annealing furnace, and the heat treatments other than the present invention were performed by a batch furnace.

As is clear from the table, according to the present invention, not only softening and fine carbide spheroidization as in the conventional method but also higher can be achieved, and the processing time required for it is reduced by 1 /.
It was greatly reduced to less than 100.

(Effects of the Invention) According to the method of the present invention, spheroidizing treatment of a layered carbide present in steel can be realized in a very short time, which greatly contributes to energy saving and improvement in productivity.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph showing a relationship between a cold working ratio and a spheroidizing time, FIG. 2 is a graph showing a relationship between a heating temperature and hardness during spheroidizing annealing, FIG. 3 is a graph showing the relationship between heating time, hardness and carbide diameter during spheroidizing annealing of SK7, and FIG. 4 is a graph showing the relationship between heating time, hardness and carbide diameter during spheroidizing annealing of S35C. It is a graph shown.

──────────────────────────────────────────────────の Continuation of the front page (72) Inventor Hideo Abe 1 Kawasaki-cho, Chiba City, Chiba Pref. Kawasaki Steel Corp. Technical Research Division (56) References JP-A-58-207325 (JP, A) Akira Tokubo 63-14045 (JP, B2) JP-B-54-27821 (JP, B2) (58) Fields investigated (Int. Cl. ⁶ , DB name) C21D 8/02 C21D 1/32

Claims (1)

(57) [Claims] (1) A cold working of 30% or more is applied to a eutectoid or hypoeutectoid steel having a pearlite structure with a C content of 0.8% by weight or less, and then recrystallized at a rate of 0.5 ° C./s or more. Above temperature, A
Heat to a temperature below ₁ transformation point, hold in this temperature range for 30 to 600 seconds, and then cool at a rate of 0.5 ° C / s or more to reduce the layered carbide in the pearlite structure to a fine particle size of 1 μm or less. A method for spheroidizing a layered carbide in a eutectoid or hypoeutectoid steel, wherein the method is a spherical carbide.