JP2007100195A - Method for producing cold tool steel - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for producing a cold tool steel with which the segregation of components in the cold tool steel is reduced and the characteristics of the cold tool steel can be improved. <P>SOLUTION: After introducing a strain by applying one series of hot forging to at least any one-direction in three directions mutually crossing at the right angle to an ingot 10 of the cold tool steel containing 7-13 mass% Cr, a soaking is applied at the temperature condition of 1,100-1,200°C for 6hr or more. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

  The present invention relates to a method for producing a cold tool steel containing 7 to 13% by mass of Cr, and more particularly to a method for producing a cold tool steel characterized by technical means for reducing component segregation.

In the conventional manufacturing method of cold tool steel, as shown in FIG. 4, after hot-forging is repeatedly performed after ingot 200 is soaked, and heating is performed in the meantime, the objective steel piece 204 is finally obtained. Was getting.
In the figure, 202-1 and 202-2 represent intermediate forged products during hot forging.

Here, the heating performed in the middle of the hot forging process is for raising the temperature of the intermediate forged product in the middle so that the temperature of the intermediate forged product does not decrease during processing and the processing crack is generated. Heating is performed for a time required to reach a target temperature and a uniform temperature as a whole.
That is, the heating in this intermediate stage is performed only for the purpose of raising the temperature of the entire intermediate forged product.

On the other hand, the soaking performed on the ingot 200 is for diffusing steel components. In this soaking, the ingot 200 is heated until the entire temperature reaches a uniform temperature, and then the temperature is kept at the heating temperature for a predetermined time. Hold and allow the components to diffuse during the hold.
Here, the ingot 200 is soaked at a temperature of 1240 ° C. for 35 hours.

By the way, the cold tool steel is a high alloy steel, and when the ingot 200 is formed by casting, there is a problem that component segregation is likely to occur during the solidification process.
Thus, for example, when segregation of Cr components contained in steel occurs, the structure becomes non-uniform, and properties such as toughness deteriorate.
In addition, anisotropy occurs in the structure in each direction of the steel slab, and accordingly, anisotropy also occurs in the toughness value.
In addition, although there exists what was disclosed by following patent document 1 as a prior art with respect to this application, what is disclosed by this patent document 1 is related with hot tool steel, and performs soaking in the stage of an ingot, and this invention The problem cannot be solved.

JP 2003-286545 A

  With the background as described above, the present invention aims to provide a method for producing cold tool steel that can reduce segregation of components in cold tool steel and improve the properties of cold tool steel. It was made.

  Thus, according to the first aspect, a series of hot forging processes are performed in at least one of the three directions orthogonal to the ingot of the cold tool steel containing 7 to 13% by mass of Cr. After introducing the applied strain, soaking is performed for 6 hours or more under a temperature condition of 1100 to 1200 ° C.

  According to a second aspect of the present invention, in the first aspect, the hot forging process is performed in each of the three orthogonal directions to introduce the strain, and then the soaking is performed.

  According to a third aspect of the present invention, in the second aspect, the series of hot forging processes performed before the soaking is perpendicular to the upsetting forging process in which the ingot is compressed in the axial direction and thereafter in the direction perpendicular to the axis. It includes a process of compressing at a reduction rate of 30% or more as a whole in two directions and forging in the axial direction.

  A fourth aspect of the present invention is characterized in that, in any one of the first to third aspects, the cold tool steel further contains 1.5 to 5% by mass of Mo.

Effects and effects of the invention

  As described above, the present invention performs a series of hot forging processes in at least one of three directions orthogonal to the ingot, regardless of whether or not soaking is performed at the ingot stage. After introduction, soaking is performed for 6 hours or longer (preferably 12 hours or longer) at a temperature of 1100 to 1200 ° C.

  According to the present invention, even when component segregation such as Cr occurs in the solidification process during ingot casting, the component segregation is favorably reduced by subsequent hot forging and soaking, and the structure is uniform and tough. It is possible to obtain a cold tool steel having excellent properties such as the above.

The reason why the component segregation is reduced by the present invention is considered to be as follows.
That is, when hot forging processing is performed on the ingot, the concentration gradient becomes large at the location where the component segregation occurs due to the introduction of strain, and then the diffusion of the component is promoted by the increase of the concentration gradient when soaking is performed, As a result, it is considered that the segregated component is well dispersed throughout.

In the present invention, it is desirable to perform processing in each of three orthogonal directions as hot forging prior to soaking.
By doing so, each of the three orthogonal directions is distorted, that is, the concentration gradient of the segregation component is increased in each of the three orthogonal directions. The segregation component can be well dispersed throughout, the steel structure can be made uniform, and the anisotropy problem can be improved more advantageously.

In the present invention, the soaking performed after the series of hot forging processes is for diffusing the components. Therefore, it is not sufficient to simply perform heating until the intermediate forged product has a uniform temperature as a whole. After that, it is necessary to hold the intermediate forged product at the ultimate temperature for a predetermined time.
Therefore, in the present invention, it is necessary to perform soaking at a temperature of 1100 to 1200 ° C. over 6 hours.
If the temperature is lower than this temperature condition and the time is short, the components cannot be sufficiently diffused.
In this case, the soaking is preferably performed over 12 hours, more preferably over 15 hours, and even more preferably over 20 hours.

In the present invention, the series of hot forging processes performed before soaking is performed by upsetting forging process in which the ingot is compressed in the axial direction, and thereafter 30% or more as a whole in two orthogonal directions perpendicular to the axis. And a process of forging in the axial direction by compressing with a reduction in area (claim 3).
By performing such hot forging before soaking, the segregation component can be diffused satisfactorily during the subsequent soaking.
Here, it is desirable that the upset forging process in which the ingot is compressed in the axial direction is performed at a compression rate of 30% or more.

The present invention can also be suitably applied to cold tool steel containing 1.5 to 5% by mass of Mo in addition to Cr (7 to 13%) (Claim 4).
In addition, when the Cr content is 7 to 9%, the cold tool steel can contain Mo: 1.5 to 5% as an essential component.

Next, embodiments of the present invention will be specifically described below.
Cold tool steel No. 1 with the components shown in Table 1. 1-No. Each of 3 was cast into an ingot 10 of φ1000 mm (diameter) × 1500 mm (height) as shown in FIG. 1, and this was subjected to soaking (I) under the conditions shown in the soaking (I) of Table 1.

Subsequently, after performing an upset forging process that compresses this in the axial direction (Z direction in the figure) at a compression rate of 50% to obtain an intermediate forged product 12-1, each of the X direction and Y direction orthogonal to the Z direction is obtained. The intermediate forged product 12-2 is obtained by compression at a surface reduction rate of 45%, and then each corner portion of the intermediate forged product 12-2 is compressed in the P direction and the Q direction respectively (the surface reduction rate is respectively 5%) and forged into an octagonal column intermediate forging 12-3 with a width of 750 mm.
In addition, the area reduction rate of the processing from the intermediate forged product 12-1 to the intermediate forged product 12-3 was set to 50% as a whole.

  Thereafter, after performing soaking (II) on the intermediate forged product 12-3 under the conditions of soaking (II) in Table 1, intermediate forged products having the same shape as the intermediate forged products 12-1 and 12-2 again. After forging into 14-1 and 14-2, and further heating the intermediate forged product 14-2 under the heating conditions shown in Table 1, the forging process was continued to obtain the final steel slab 16 of interest. (The size is 300 × 740 × 5300 mm).

  Samples taken from each of the intermediate forged product 12-3 before soaking (II) and the intermediate forged product 12-3 after soaking (II) and the segregation state of the Cr component were observed, and the prior art shown in FIG. Sample is taken before and after the soaking in the manufacturing method in which the soaking is not performed after hot forging, specifically, the ingot 200 before soaking and the ingot 200 after soaking. As a result of observing the segregation state of the Cr component, segregation was reduced in the product produced in this embodiment.

As described above, when the ingot 10 is first subjected to hot forging and then subjected to soaking (II), the segregation of Cr component is favorably reduced for the following reason. It is considered a thing.
Fig. 2 (b) schematically shows the Cr component concentration distribution without hot forging, with the distance on the horizontal axis and the concentration on the vertical axis. The gradient is expressed as ΔC ₀ / Δd ₀ = K ₀ .
On the other hand, FIG. 2 (a) shows the Cr concentration distribution after hot forging, and the concentration distribution ΔC / Δd = K in this case is the concentration gradient ΔC ₀ / Δd _{0 in} FIG. 2 (b). = K is greater than ₀ .
As described above, the concentration gradient K of the Cr component is increased by the hot forging process before soaking as compared with the concentration gradient K ₀ before the hot forging process, so that the diffusion of the Cr component is promoted by the subsequent soaking. This is considered to reduce the segregation of Cr components.

Table 2 shows No. 1 in Table 1. 1-No. 3, the segregation level of Cr when manufactured in the present embodiment shown in FIG. 1 is compared with the segregation level when manufactured by the conventional manufacturing method shown in FIG. 4. However, the numerical values in the table represent the segregation level of Cr as an index, and the smaller the numerical value, the smaller the degree of segregation.
As shown in Table 2, it can be seen that according to this embodiment, the segregation of Cr can be satisfactorily reduced.

Next, Table 3 shows the result of Charpy impact test by collecting Charpy test pieces (JIS No. 3 test pieces) from the L direction, T direction and H direction in FIG. 3 of the steel piece 16 obtained in this embodiment. FIG. 4 shows a comparison with the result of a Charpy impact test on a steel piece 204 obtained by the conventional manufacturing method shown in FIG.
The Charpy test piece was sampled such that the longitudinal direction of the test piece was the L direction, the T direction, and the H direction.

As apparent from Table 3, in the manufacturing method of the present embodiment, the Charpy impact value is almost constant in any of the L direction, the T direction, and the H direction as compared with the conventional manufacturing method. There is no particular anisotropy in the impact value, and overall, the value is higher than the value in the conventional manufacturing method.
That is, according to this embodiment, it can be seen that the Charpy impact value is higher than that of the conventional manufacturing method of FIG. 4 and the anisotropy is also improved.
In the above embodiment, the overall reduction in area from the intermediate forged product 12-1 to the intermediate forged product 12-3 is 50%. However, even when the reduction is performed at an area reduction of 30%, it is good. Results were obtained.

  Although the embodiment of the present invention has been described in detail above, this is merely an example, and the present invention can be implemented in variously modified forms without departing from the spirit of the present invention.

It is process explanatory drawing of the manufacturing method of the cold tool steel of embodiment of this invention. It is the figure which showed the change of the density | concentration gradient of Cr component by a hot forging process. It is the figure which showed the sampling direction of the Charpy impact test piece. It is process explanatory drawing of the manufacturing method of the conventional cold tool steel.

Explanation of symbols

10 Ingot 12-1, 12-2, 12-3, 14-1, 14-2 Intermediate forged product 16 Steel slab

Claims (4)

  After introducing a series of hot forging processes in at least one of the three directions orthogonal to the ingot of the cold tool steel containing 7 to 13% by mass of Cr, 1100 A method for producing cold tool steel, wherein soaking is performed at a temperature condition of 1200 ° C. over 6 hours or more.   The method of manufacturing cold tool steel according to claim 1, wherein the soaking is performed after the hot forging process is performed in each of the three orthogonal directions to introduce strain.   In Claim 2, the series of hot forging performed before the soaking includes upsetting forging for compressing the ingot in its axial direction, and thereafter 30% as a whole in two orthogonal directions perpendicular to the axis. The manufacturing method of the cold tool steel characterized by including the process for which it compresses by the above-mentioned area reduction rate and forges in an axial direction.   The method for producing cold tool steel according to any one of claims 1 to 3, wherein the cold tool steel further contains 1.5 to 5% by mass of Mo.

Images