JPS6383249A - Hot working tool steel and its manufacture - Google Patents

Abstract

PURPOSE:To manufacture a Cr hot working tool steel enabling an increase in the size of dies by subjecting a steel having a specified compsn. consisting of C, Si, Mn, Ni, Cr, V, Mo, W and Fe to proper heat treatment so as to improve the suitability to bainite hardening. CONSTITUTION:An Ni-contg. Cr hot working tool steel consisting of 0.20-0.35% C, <=1.5% Si, <=1.5% Mn, 0.3-3.0% Ni, 2.0-6.0% Cr, 0.10-2.0% V, 0.5-4.0% Mo and/or 0.5-4.0% W (2Mo-W<=8.0%) and the balance essentially Fe or further contg. <=4.0% Co is heated to 1,050-1,300 deg.C to coarsen the austenite grains to grain size No.<=3.0. The steel is cooled at such a cooling rate that austenitic transformation is avoided and it is tempered at 650- the Ac1 point. The previous austenite grains are made fine to grain size No.>=5.0 during final thermal refining by hardening and tempering. Thus a tool steel having high hardenability is obtd.

Description

[Detailed description of the invention]

OBJECT OF THE INVENTION [Industrial Application Field] The present invention relates to an improvement in a Ni-containing Cr-based hot work tool steel and a method for producing the same.

[Conventional technology]

Cr-based hot work tool steel, for example, C: 0.20-0.35
%, Si: 1.5% or less, Mn: 1.5% or less, Cr
:2.0~6.0%, Mo:0.5~2.0%, W:0
．． 5 to 2.0% (however, 2Mo +W≦4.0>, V
:0.10 to 1.0%, with the balance essentially consisting of Fe, which has excellent softening resistance at high temperatures, making it suitable as mold steel that requires a long life. There is. However, because this steel has low bainitic hardenability, the size of the product is limited to relatively small pieces, such as 10 cm square blocks or equivalent, and the quenching process requires rapid cooling such as oil cooling. shall be. Recently, molds for hot working have become more and more large-sized, and vacuum heat treatment has become widespread, and as a result, there has been an urgent need to improve bainite hardenability. To improve the bainitic hardenability of CR-based hot work tool steel, Ni
It is known that the addition of However, when Ni is added to an alloy with the above composition, the austenite crystal grains become significantly coarsened when heated for quenching, and the grain size number becomes, for example, 4.0 or less, resulting in a significant decrease in toughness. This has the disadvantage of decreasing. In order to prevent this, it is possible to add small amounts of l'-i, Zr, Nb, etc. to generate poorly soluble carbides and nitrides, and to disperse them finely to prevent the growth of austenite grains. . However, as molds tend to become larger, steel ingots must also become larger, and large steel ingots tend to segregate carbides and the like, so there is a limit to the effectiveness of the above countermeasures.

[Problems to be solved by the invention] The purpose of the present invention is to overcome the current state of the art, improve the bainitic hardenability of Ni-containing Cr-based hot work tool steel, and provide a hot work tool steel that can cope with the tendency for molds to become larger. , and to establish its manufacturing method. Composition of the invention [Means to solve the problem]

The Ni-containing Cr-based hot work tool steel of the present invention that achieves the above objects has a content of C: 0.20-0.35%, Si: 1.5% or less, Mn: 1.5% or less, and Ni: 0°. 3-3.0%, C
r: 2.0-6.0% and V: 0.10-2.0%
In addition, Mo: Q, 5~4°0% and W: 0.5~
Contains 4.0% of one or two types, but 2Mo+W
: 8.0% or less, the remainder substantially consists of Fe, and the prior austenite crystal grains after final heat treatment have a grain size number of @5°0 or more. The Ni-containing Cr-based hot work tool steel may contain Co: 4.0% or less in addition to the above alloy components. The method for producing hot work tool steel of the present invention includes C: 0.20-0.
35%, Si: 1.5% or less, Mn: 1°5% or less, N
i: 0.3~3.0%, Cr: 2°0~6.0% and V: 0.10~2.0%, Mo: 0.5~
4.0% and W: 0.5 to 4.0%, but 2Mo+W: 8.0% or less,
A Ni-containing Cr-based hot work tool steel with the remainder essentially consisting of Fe and optionally containing 4.0% or less Co,
After heating to a temperature of 50 to 1300'C to make the austenite crystal grains coarsely sized with a grain size number of 3.0 or less,
By cooling at a rate that does not cause pearlite transformation and then tempering at a temperature of 650°C to AC1 point, the prior austenite crystal grains at the final tempering by quenching and tempering are made to have a grain size number of 5.0 or more. It is characterized by [Function] Conventionally, spheroidizing annealing has been common as a heat treatment performed before quenching this type of hot tool steel, but instead of this, normalizing and tempering or spheroidizing annealing have been used. Prior to annealing, it may be normalized and tempered. The normalizing temperature in these cases is around the quenching temperature (±20° C.). The inventor closely observed a practical material of Ni-containing Cr-based hot work tool steel in which the austenite crystal grains were significantly coarsened, and found that the coarsening tendency was due to the microscopic segregation tendency of carbides in the microstructure before quenching. I noticed that they were in good agreement. The segregation of carbides in the microstructure before quenching, that is, after annealing, normalizing and tempering, or normalizing and tempering and annealing, is caused by hot working such as forging and rolling, and subsequent cooling. Process (generally slow cooling)
It was found that this tendency is determined by the microstructure formed by Ni, and that this tendency becomes larger as the amount of Ni added increases. Therefore, it was concluded that in order to prevent grain coarsening of Ni-containing Cr-based hot work tool steel, it is necessary to reduce the segregation of carbides in the microstructure before quenching.
As a result of various experiments in search of a means to achieve this, we arrived at the above-mentioned prior austenite crystal grains with a grain size number of 5.0 or more after the final heat treatment, and the heat treatment conditions to obtain it. . In other words, by performing high-temperature normalization, which has not been considered in the past, for a relatively long period of time, the carbides are dissolved into solid solution and the components are homogenized, and then rapidly cooled to suppress the precipitation of carbides. We succeeded in virtually eliminating the microscopic segregation of steel and refining the structure during quenching. The high-temperature normalizing that is a feature of the present invention is carried out at a higher temperature and for a longer time, the coarser the austenite crystal grains will be after normalizing, but the solid solution and homogenization of carbides will progress. The austenite crystal grains during quenching become rather fine grains. This fact will be understood by the examples below.

[Example 1] Ni-containing Cr-based hot work tool steel having the following composition (weight %, remainder Fe) was forged to produce a block of 300 m square x 500 # length. Ω Dan↓ Mn Ni Cr MoV-0,2
70,330,680,863,180,920,63
After the block was forged, it was slowly cooled in a pit, and a 20 m square test piece was taken in the longitudinal direction from the center and subjected to three types of pre-quenching heat treatment and quenching and tempering under the following conditions. (Heat treatment before quenching) LA: Low temperature furnace annealing...720'CX 5 hours - air cooling S
A: Spheroidizing annealing... 800'CX 1 hour - slow cooling (
150'C/hr) to 600 °C with air cooling of 1 or less N-LA: Normalizing → Low temperature furnace annealing...900~'1
300°C x 1 hour - air cooling → 720'C x 5 hours - air cooling (quenching and tempering) Quenching... 970°C x 1 hour - Air cooling in a 150# diameter case Tempering... 1st time 570°C x 2 hours - air cooling 2nd time 6
20'CX 2 hours - air cooling (refined to HRC44±1) As a heat treatment before quenching, the austenite grain size number of the sample subjected to N-LA according to the present invention was investigated and plotted as shown in FIG. As shown in the figure, the normalizing heating temperature ranges from 900℃ to 1
As the temperature increases toward 300°C, the grain size number becomes smaller and the grains become coarser. All of the coarse grains were well-sized. The grain size numbers after quenching and tempering were investigated, including those subjected to LA or SA as comparative examples, and an impact test was conducted. The results are shown in FIG. From the data in the figure, the ones that underwent N-LA according to the present invention are:
The grains are finer and have a higher grain size number than those obtained by conventional LA or SA, and the degree of fineness increases as the normalizing temperature increases, and as the grains become finer, the impact strength also increases. Recognize. In this way, contrary to conventional wisdom, high-temperature normalizing is performed to coarsen the crystal grains, but after quenching and tempering, the grains become finer to a level that was previously unobtainable, resulting in high toughness. This fact is the first knowledge obtained by the present inventor.

[Example 2] Six types of Ni-containing Or-based hot work tool steels A to F shown in Table 1 were produced and forged into blocks of 300 mm square and 500 M in length. B marked with * is a comparative example. Test specimens taken longitudinally from the center of each block were subjected to various heat treatments shown in Table 2, and then quenched and tempered as shown in Table 2 to determine hardness and impact. The value was measured. The results are shown in Table 2 together with the grain size number after the heat treatment prior to quenching and tempering and the grain size number after quenching and tempering. Effects of the Invention When the manufacturing method of the present invention is used, the decrease in toughness due to the addition of Ni, which was previously unavoidable, can be avoided under specific conditions in Cr-based hot work tool steel that has increased high-temperature softening resistance by adding Ni. By employing heat treatment, a tool steel that prevents this from occurring can be obtained. This tool steel has high hardenability, so it is especially suitable for use as a product.
Large block exceeding 0# corner, 120~ as material
Items that are 130s square or more and as thick as 300s square can be effectively hardened, making it possible to cope with the tendency for forging molds to become larger.

[Brief Description of the Drawings] All the drawings show the effects of the embodiments of the present invention. Fig. 2 is a graph showing the relationship between the austenite grain size number and impact value after quenching and tempering, and the heat treatment temperature preceding the same, in comparison with the case using conventional technology. It is. Patent applicant: Daido Steel Co., Ltd., patent attorney: Souo Suga. ■

Claims (3)

[Claims] (1) C: 0.20-0.35%, Si: 1.5% or less, Mn: 1.5% or less, Ni: 0.3-3.0%, Cr
:2.0~6.0% and V:0.10~2.0%, in addition to Mo:0.5~4.0% and W:0.5~4.0%.
Contains 0% of one or two types, provided that 2Mo+W:8
．． 0% or less, the remainder is substantially composed of Fe, and the prior austenite crystal grains after final annealing have a grain size number of 5.
A Cr-based hot work tool steel containing Ni, characterized in that the Ni content is 0 or more. (2) C: 0.20-0.35%, Si: 1.5% or less, Mn: 1.5% or less, Ni: 0.3-3.0%, Cr
:2.0~6.0% and V:0.10~2.0%, in addition to Mo:0.5~4.0% and W:0.5~4.0%.
Contains 0% of one or two types, provided that 2Mo+W:8
．． 0% or less, and further contains Co: 4.0% or less, the balance substantially consists of Fe, and the prior austenite crystal grains after final tempering have a grain size number of 5.0 or more. Ni-containing Cr-based hot work tool steel. (3) C: 0.20-0.35%, Si: 1.5% or less, Mn: 1.5% or less, Ni: 0.3-3.0%, Cr
:2.0~6.0% and V:0.10~2.0%, in addition to Mo:0.5~4.0% and W:0.5~4.0%.
Contains 0% of one or two types, provided that 2Mo+W:8
．． Ni-containing Cr containing 0% or less, the remainder substantially consisting of Fe, and optionally containing 4.0% or less of Co.
After heating the system hot work tool steel to a temperature of 1050 to 1300 ° C. to make the austenite crystal grains coarsely sized with a grain size number of 3.0 or less, cooling at a rate sufficient to avoid pearlite transformation, Then, by performing tempering at a temperature of 650°C to A_C_1 point, the prior austenite crystal grains have a grain size number of 5.
A method for producing a Ni-containing Cr-based hot work tool steel, characterized in that the Ni-containing Cr-based hot work tool steel has a Ni-containing temperature of 0 or more.