JP7305483B2 - Hot work tool steel with excellent toughness - Google Patents

Description

The present invention relates to hot work tool steel suitable for hot work die steel used in hot forging, hot extrusion, die casting and other castings.

Since hot tools are used while being in contact with high-temperature workpieces or hard workpieces, they must have both strength and toughness to withstand thermal fatigue and impact. Therefore, in the field of hot work tools, for example, SKD61 series alloy tool steel, which is a steel grade of JIS G4404, has been used.
However, since the amount of V is large, coarse carbonitrides are formed, which tends to deteriorate toughness and workability.

Further, a high-toughness and high-strength hot work tool steel has been proposed by reconsidering the balance of constituent elements of SKD61 (see, for example, Patent Document 1).

Further, in addition to reviewing the amount of addition, hot work tool steel with excellent toughness has been proposed in which the content of various impurities is regulated (see, for example, Patent Document 2).

Further, a hot work tool steel with excellent toughness has been proposed by limiting the range of alloying elements and limiting the degree of segregation of elements such as Cr, Mo, W, and V (see, for example, Patent Document 3).

However, even with the hot work tool steels proposed in these patent documents, sufficient improvement in toughness cannot always be obtained, and there have been cases where variations in properties occur even within the specified range. .

JP 2013-087322 A JP-A-1988-203744 Japanese Patent Application Laid-Open No. 2005-314788

The problem to be solved by the present invention is to provide a hot work tool steel with stable properties and high toughness.

Therefore, in order to solve the above-mentioned problems, the inventors of the present application have made intensive research and development. In addition to this, the inventors have found that the segregation of elements involved in solid-solution strengthening, the width of the segregation of the elements, and the like also affect the decrease in toughness, leading to the present invention.

The means of the present invention for solving the above problems is, in the first means, C: 0.30 to 0.42%, Si: 0.15 to 1.00%, Mn: 0.00% by mass %. 20-0.50%, Cr: 4.50-6.00%, Mo: 1.00-2.20%, V: 0.30-0.60%, P: 0.03% or less, S: A steel containing 0.01% or less, N: 130 ppm or less, O: 20 ppm or less, and the balance being Fe and inevitable impurities,
When the index representing the ease of crystallization of coarse carbides in this steel is S',
S' = ([C] - 0.2) × (2 [Cr] + 3 [Mo] + 6 [V]) S' value is 3.1 or less,
The difference between the maximum and minimum amounts of the elements C, Mn, Cr, Mo, and V in 160 μm×1000 μm of this steel is
_Cmax - _Cmin : 0.25% or less,
_Mnmax - _Mnmin : 0.25% or less,
_Crmax - _Crmin : 1.25% or less,
_Momax - _Momin : 1.20% or less,
V _max −V _min : 0.30% or less,
Furthermore, the value of the degree of change of each element component of C, Mn, Cr, Mo, and V per 1 μm of the segregation part of this steel is
C _change : within 0.0025%/μm,
Mn _change : within 0.0030%/μm,
Cr _change : within 0.125%/μm,
Mo _change : within 0.105%/μm,
V _change : within 0.0025%/μm,
It is a hot work tool steel with excellent toughness characterized by

When SKD61, which is a steel grade described in JIS G4404, has a tempering hardness of 45 HRC, the Charpy impact value is 20/cm ^{2 .} According to the means of the invention of the present application, hot work tool steel excellent in toughness can be stably obtained.

It is a photograph showing an example of segregation bands seen on the observed surface after picral corrosion. FIG. 3 is an image diagram illustrating a method of obtaining the degree of change of each elemental component of C, Mn, Cr, Mo, and V per 1 μm of a segregation part by graphing it.

Prior to the description of the mode for carrying out the present invention, first, the reason for limiting the chemical composition of the invention of the hot work tool steel according to the claims of the present application, the value of S' of this steel, C, Mn, Cr, The reason why the difference between the maximum and minimum values of the component amounts of each element of Mo and V and the degree of change of each element of C, Mn, Cr, Mo and V per 1 μm of the segregation part of the steel are specified. I will explain in order.
In addition, % in each chemical component means % by mass.

C: 0.30-0.42%
C is an element that forms hard carbides, improves hardness and wear resistance, and enhances hardenability. For this purpose, C needs to be 0.30% or more. However, if the C content exceeds 0.42%, it forms coarse carbides in the steel, degrading toughness and workability. Therefore, C is 0.30 to 0.42%, preferably 0.32 to 0.40%.

Si: 0.15-1.00%
Si is an element that is added as a deoxidizing agent and increases the hardness and hardenability of the matrix. For this purpose, Si needs to be 0.15% or more. However, if Si is included in an amount exceeding 1.00%, the steel matrix becomes embrittled, and toughness and workability deteriorate. Therefore, Si should be 0.15 to 1.00%.

Mn: 0.20-0.50%
Mn is an element added as a deoxidizing agent to improve hardenability. For this purpose, Mn must be 0.20% or more. However, when the Mn content exceeds 0.50%, it embrittles the steel matrix and deteriorates toughness. Therefore, Mn is set to 0.20 to 0.50%.

Cr: 4.50-6.00%
Cr is an element that forms hard carbides, improves hardness and wear resistance, and enhances hardenability. For this purpose, Cr must be 4.50% or more. However, if the Cr content exceeds 6.00%, it forms coarse carbides in the steel, degrading toughness and workability. Therefore, Cr is set to 4.50 to 6.00%.

Mo: 1.00-2.20%
Mo is an element that forms hard carbides, improves hardness and wear resistance, and enhances hardenability and temper softening resistance. For this purpose, Mo needs to be 1.00% or more. However, when the Mo content exceeds 2.20%, coarse carbides are formed in the steel to deteriorate toughness and workability. Therefore, Mo is set to 1.00 to 2.20%.

V: 0.30-0.60%
V is an element that forms hard carbides, improves hardness, wear resistance, and hardenability, has the effect of suppressing coarsening of crystal grains during hardening, and contributes to the improvement of toughness. For this purpose, V needs to be 0.30% or more. However, if the V content is more than 0.60%, it forms coarse carbonitrides and deteriorates toughness and workability. Therefore, V is set to 0.30 to 0.60%.

P: ≤ 0.03%
P is an element that is generally desirably reduced because it lowers the toughness, but it is an element that is unavoidably contained. Therefore, the allowable limit for P is 0.03%.

S: ≤ 0.01%
S is an element that should be reduced because it forms MnS together with Mn and lowers the toughness. Therefore, S is set to 0.01% or less.

N: ≤ 130ppm
N is an element that forms coarse nitrides and carbonitrides and deteriorates toughness and workability. Therefore, N is set to 130 ppm or less.

O: ≤20ppm
O is an element that forms oxide-based inclusions in steel and lowers toughness. Therefore, O is set to 20 ppm or less.

The index S' value representing the ease of crystallization of coarse carbide: 3.1 or less, that is, S' = ([C] - 0.2) x (2 [Cr] + 3 [Mo] + 6 [V]) Yes, S': ≤ 3.1
S' is an index representing the easiness of crystallization of coarse carbides. By adjusting C, Cr, Mo, and V, which are the alloying components of the steel, so that the value of S' is 3.1 or less, the formation of coarse carbides becomes difficult, so a steel with high toughness can be obtained. can.
Therefore, S′=([C]−0.2)×(2[Cr]+3[Mo]+6[V])≦3.1.

Regarding the difference between the maximum and minimum values of the component amounts of C, Mn, Cr, Mo, and V in 160 μm×1000 μm of steel C, Mn, Cr, Mo, and V in 160 μm×1000 μm of steel of the present invention The difference between the maximum and minimum component amounts of each element in is mass%,
_Cmax - _Cmin : 0.25% or less,
_Mnmax - _Mnmin : 0.25% or less,
_Crmax - _Crmin : 1.25% or less,
_Momax - _Momin : 1.20% or less,
V _max -V _min : 0.30% or less.
In a region of 160 μm × 1000 μm, if the difference between the maximum and minimum values of the component amounts of each element of C, Mn, Cr, Mo, and V in the steel is large, coarse carbides are likely to crystallize at locations where the components are rich. Differences in hardenability and differences in the degree of strengthening due to solute elements occur between areas with high concentrations and areas with low concentrations. Therefore, the structure and strength after heat treatment tend to be non-uniform, which causes deterioration in toughness and other properties.
However, by limiting the segregation degree (component width) of each element of C, Mn, Cr, Mo, and V to a small value, coarse carbides and non-uniformity of the structure are eliminated, and the deterioration of toughness does not occur.
Therefore, the difference between the maximum and minimum amounts of the elements C, Mn, Cr, Mo, and V in the steel of 160 μm × 1000 μm is, in mass%, C _max −C _min : 0.25% or less. , Mn _max -Mn _min : 0.25% or less, Cr _max -Cr _min : 1.25% or less, Mo _max -Mo _min : 1.20% or less, V _max -V _min : 0.30% or less .

By the way, when molten steel of alloy tool steel with an adjusted chemical composition is cast as it is, segregation such as center segregation and inverse V segregation occurs in the casting structure. It is difficult to
However, in the hot work tool steel of the present invention, for example, after the above ingot making, remelting by ESR (electroslag remelting method) is applied, steel ingots and/or steel slabs obtained by hot working the steel ingots. However, by performing homogenization heat treatment under suitable conditions of 1150 ° C. or higher, preferably 1200 ° C. or higher, it is possible to limit the difference between the maximum and minimum values of the amount of each element to a small value. .

The degree of change in each elemental component of C, Mn, Cr, Mo, and V per 1 μm of the segregation part of the steel is C _change : within 0.0025%/μm, Mn _change : within 0.0030%/μm, Cr _change : within 0.125%/μm, Mo _change : within 0.105%/μm, V _change : within 0.0025%/μm A sudden change in the amount causes sudden non-uniformity in strength at the microscopic level in the steel material, resulting in a decrease in toughness. Therefore, the value of the degree of change in the composition of each element of C, Mn, Cr, Mo, and V in the steel per 1 μm of the segregation part is mass%, and the C _change , which is the degree of change of C, is 0.0025%/ Mn change, which is the degree of change in Mn, is within 0.0030%/µm, Cr _change , which is _the degree of change in Cr, is within 0.125%/µm, Mo change degree _{The change} in Mo is within 0.105%/μm, and _{the change} in V, which is the degree of change in V, is within 0.0025%/μm.

Then, the form for implementing this invention is demonstrated below.
Each No. shown in Table 1. and the remaining Fe and unavoidable impurities constitute 100% of the steel composition. 100 kg each of the test materials of the invention steel and the comparative steel were melted in a vacuum induction melting furnace, then soaked at 1200 ° C. for 48 hours, and then forged to a square of 50 mm at a forging ratio of 4 s. stretched. Further, this was held at 870° C. for 2 hours and annealed, and then cooled to room temperature at a cooling rate of 10° C./hr.

In addition, No. of comparative steel in Table 1. 34 to 38 were produced by omitting the soaking treatment step among the steps shown above.

Here, the forging ratio is expressed by A/a, which is the ratio of the cross-sectional area A of the steel ingot to the cross-sectional area a of the cross-sectional area reduced by hot working the steel ingot. value.

Table 1 below shows the chemical composition of each test material and the S' value representing the susceptibility to crystallization of coarse carbides.

The difference between the maximum and minimum amounts of the elements C, Mn, Cr, Mo, and V in the steel of 160 μm×1000 μm is obtained as follows. First, a 15 mm × 15 mm × 15 mm sample for structure observation was taken from the central part of the forged material, and the surface in the rolling direction was subjected to picral corrosion to reveal the structure. The positions of the segregation bands as shown were confirmed. Then, the thickest segregation zone was selected from among the segregation zones in the observation plane, and a plane in the range of 160 μm×1000 μm was determined and marked so as to cross the segregation zone.
Note that the range of 160 μm×1000 μm was defined as a portion that did not contain carbides with a diameter of 5 μm or more and MnS. After polishing the marked surface to a mirror surface, an EPMA (electron probe microanalyzer) was used to measure the maximum and minimum elemental amounts (C, Mn, Cr, Mo, and V) within an area of 160 μm×1000 μm. mass %) was obtained. In the case of C, the difference in the elemental amount of C can be obtained from the difference between the maximum value _Cmax and the minimum value _Cmin , so this is defined as _Cmax - _Cmin . The difference in the amount of each element was obtained and shown in Table 2 as the difference between the maximum and minimum amounts of each element of C, Mn, Cr, Mo and V in 160 μm×1000 μm.

The degree of change of each element component of C, Mn, Cr, Mo, and V per 1 μm of the segregation portion is obtained as follows. First, a sample was prepared by measuring the difference between the maximum value and the minimum value of the component amount, and within a measurement range of 160 μm × 1000 μm, the central part of the 160 μm width was measured over 1000 μm using EPMA, and C, Mn, Cr, Mo , and V were analyzed. From the measurement results, for each element, the locations where the composition changes the most were determined as shown in FIG.

Toughness was measured as follows. First, the forged material is cut to 70 mm, and after being quenched by heating to 1030 ° C. and air cooling, it is then tempered twice or more by heating to 550 to 650 ° C. and air cooling to 45 HRC. tempered. A 10 mm × 10 mm × 55 mm test piece was indexed from the center of the tempered material so that the longitudinal direction of the test piece was aligned with the rolling direction of the steel material to prepare a 2 mm U-notch Charpy impact test piece. A Charpy impact test was performed at room temperature using the 2 mm U notch Charpy impact test piece thus produced.
When this Charpy impact test is performed on SKD61, which is a JIS steel type, an impact value of 20 J/mm ² is obtained when the temper hardness is 45 HRC, so it can be used as a standard for comparison with general steel. In the present invention, if an impact value higher than 30 J/mm ² which is superior to SKD61 is obtained, it can be evaluated as having the excellent toughness of the present invention, so it is evaluated as ○, and if it is lower than 30 J/mm ² , toughness The evaluation results are shown in Table 2, with x indicating inferior to the above.

As shown in Table 2, the present invention steel No. All of Nos. 1 to 21 had a Charpy impact value of 30 J/mm ² or more, excellent toughness, and were evaluated as ◯.
On the other hand, comparative steel No. In Nos. 22 to 29, the underlined elements in Table 1 were contained in a larger amount than the range defined by the present invention, and as shown in Table 2, the Charpy impact value was low and the toughness was poor.
Comparative steel no. In No. 30, although the V content was small, coarsening of the crystal grains was caused, so the Charpy impact value in Table 2 was low and the toughness was poor.
Comparative steel no. In Nos. 31 and 32, there were many inclusions due to the inclusion of N and O, the Charpy impact value in Table 2 was low, and the toughness was poor.
Comparative example no. In No. 33, the value of the index S′ representing the ease of crystallization of coarse carbides was high, and the crystallization of coarse carbides resulted in a low Charpy impact value in Table 2 and poor toughness.
Comparative example no. In Nos. 34 to 38, the difference between the maximum value and the minimum value of the component amount of each element is large, coarse carbides are likely to crystallize at locations with high concentrations, and there is a difference in hardenability between locations with high concentrations and locations with low concentrations. , there is a difference in the degree of strengthening due to solid solution elements. Therefore, the structure and strength after the heat treatment became uneven, the Charpy impact value in Table 2 was low, and the toughness was poor.
Comparative example no. In 39 to 43, the degree of change in each elemental component per 1 μm of the segregation part was large, and as a result of sudden non-uniformity in strength occurred in the steel material, the Charpy impact value in Table 2 was low and toughness was poor. became a thing.

1 Segregation zone 2 Location with large change in composition

Claims (1)

in % by mass,
C: 0.30 to 0.42%,
Si: 0.15 to 1.00%,
Mn: 0.20-0.50%,
Cr: 4.50-6.00%,
Mo: 1.00-2.20%,
V: 0.30 to 0.60%,
P: 0.03% or less,
S: 0.01% or less,
N: 130 ppm or less,
O: contains 20 ppm or less,
A steel consisting of the balance Fe and inevitable impurities,
When the index representing the ease of crystallization of coarse carbides in this steel is S',
S' = ([C] - 0.2) × (2 [Cr] + 3 [Mo] + 6 [V]) S' value is 3.1 or less,
The difference between the maximum and minimum amounts of the elements C, Mn, Cr, Mo, and V in 160 μm×1000 μm of this steel is C _max −C _min : 0.25% or less in mass %,
_Mnmax - _Mnmin : 0.25% or less,
_Crmax - _Crmin : 1.25% or less,
_Momax - _Momin : 1.20% or less,
V _max −V _min : 0.30% or less,
Furthermore, the degree of change of each element component of C, Mn, Cr, Mo, and V per 1 μm of the segregation part of this steel is C _change : within 0.0025%/μm in mass %,
Mn _change : within 0.0030%/μm,
Cr _change : within 0.125%/μm,
Mo _change : within 0.105%/μm,
V _change : within 0.0025%/μm;
Hot work tool steel with excellent toughness characterized by

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