JPS5925026B2 - mold steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high hardness and toughness mold steel used for cold, warm and hot working molds.

Conventionally, high-speed tool steels SKH9, 55, and 57 or low-alloyed steels are used as steel materials for manufacturing cold and warm molds that have a hardness of about HRC 50 to 63. However, such steels tend to crack prematurely due to their lack of toughness, making it difficult to improve mold life.

Moreover, such steel has poor hardenability and generally requires quenching in a salt bath furnace, which has the disadvantage that local deformation tends to occur. Therefore, in order to develop a mold steel with excellent hardenability that can be bright hardened in a vacuum furnace and also has high hardness and toughness, the present inventors investigated in detail the effects of various alloying elements, and Based on the results, the plow tip was designated as "high hardness and toughness steel for molds" (Japanese Patent Application No. 54-814760;
See No. 14.

) was filed. However, they subsequently discovered that by adding a small amount of rare earth elements to the steel of the prior application, the hardness and toughness could be further improved without impairing other properties, leading to the present invention. That is, the gist of the present invention is as follows.

(1) C■ 0.4-1.0%, 5i0.4-1.5%
, Mn: 0.4-1.5%, Ni■ 0.3-2.0
%, Cr: 2.0-6.0%, 2Mo+W ■ 2.
0 to 15.0%, V: 0.5 to 205%, and one or more rare earth elements in a total amount of 0.005%.
A high-hardness, strong-toughness steel for molds, characterized in that it contains Fe in an amount of 0.0960%, with the remainder essentially consisting of Fe.

(2) C: 0.4-1.0%, Si: 0.4-15%,
Mn: 0.4-1.5%, Ni: 0.3-2.0%
, Cr: 2. C) ~6.0%, 2M0+W: 2.

0-15.0%, V: 0.5-2.5%, B: 0.00
1 to 0.050%, and one or two rare earth elements
A high-hardness, strong-toughness steel for molds, characterized in that it contains 0.005 to 0.60% in total of at least one of the following elements, with the remainder essentially consisting of Fe.

(3) C: 0.4-1.0%, Si: 0.4
~1.5%, Mn: 0.4~1.5%, Ni: 0.3
~2.0%, Cr:20~6.0%, 2M0+W:2.

0-15.0%, v: 0.5-'2.5%, B: 0
．． 001-0.050%, CO: 01-12.0%,
A high-hardness, strong-toughness steel for molds, further comprising a total amount of 0.005 to 0.60% of one or more elements selected from rare earth elements, with the remainder essentially consisting of Fe.

Note that the rare earth elements in the present invention include La, Ce, and Nd.
, Sc, Y, Sm and other rare earth elements.

Next, the reason for limiting the chemical composition range of the steel of the present invention will be described below.

C: 0.4 to 1.0% C combines with carbide-forming elements such as Cr, MO, W, V, Nb, and Zr to form a hard composite carbide, and at the same time dissolves in the base to achieve the required hardness. It is a component element necessary for imparting strength and improving strength and wear resistance.

In order to ensure the hardness required for a forging or press mold, it is necessary to contain at least 0.40% or more. However, if C is contained in an amount exceeding 1.0%, the toughness and thermal shock resistance properties will be significantly deteriorated, causing premature failure of the mold, so the content is limited to 1.0% or less. Si: 0.4~
1.5% It is mainly added as a deoxidizing agent, but if added in an amount of 0.4% or more, it is an element that dissolves solidly in carbides and matrix and contributes to improving hardenability and increasing yield strength.

However, if added in a large amount, the life of the mold would be deteriorated due to a decrease in thermal conductivity, so it was limited to 1.5% or less. Mn
: 0.4 to 1.5% It is added as a deoxidizing agent like Si, but if it is added at 0.4% or more, it is effective in improving hardenability, but if it is added in a large amount, it will reduce tempering hardness and softening resistance. The content was limited to 1.5% or less since resistance or rigidity would decrease.

Ni: 0.3 to 2.0% This is an element that significantly contributes to improving hardenability and improving toughness through grain refinement, and must be contained at least 0.30%.

However, if it is contained in a large amount, the amount of retained austenite will increase rapidly, resulting in a decrease in tempering softening resistance and toughness, and at the same time, machinability during mold machining will deteriorate, so it should not exceed 2.0%. limited to. Cr: 2. C)-
6.0% It is an element that combines with C to form fine composite carbides and greatly contributes to improving wear resistance.

It is also an element that dissolves in large amounts in the matrix, improving hardenability and greatly contributing to improving oxidation resistance.
The effect cannot be achieved unless the content is 20% or more. On the other hand, if it is contained in a large amount, it will form huge eutectic carbides and cause significant deterioration of the toughness, reducing temper softening resistance, so the content is limited to 6.0% or less. 2M0+W:
2.0 to 15.0% MO and W combine with C to form fine M2C type or M6C type composite carbides, which greatly improves wear resistance and high temperature hardness, as well as tempering softening resistance. It is a contributing element.

Because forging or press dies have a large mass, they must have excellent hardenability. Maximum 100
Considering the effects of Si, Mn, Ni, Cr, B, and rare earth elements, the amount of MO and W to obtain the same hardness up to the center of the mm-thick mold material is 2M0 + W is 2.0 ~ 1
A range of 50% has been found to be preferred. In other words, if 2M0+W is less than 2.0%, hardenability and high-temperature hardness will be insufficient, and if it is contained in a large amount exceeding 15.0%, the amount and size of carbides will be excessive, resulting in poor toughness and thermal shock properties. 2M0+W is 2.0 because
It was limited to ~15.0%. Note that one of the features of the present invention is that by setting 2M0+W within the above range, the balance of hardenability, strength, toughness, high temperature properties, etc. is optimized.

V: 0.5-2.5% Combines with C to form MC-type carbides that are extremely hard and difficult to form solid solutions, greatly contributing to improving wear resistance and improving toughness by making crystal grains finer. It has the effect of

However, if it is contained in a large amount exceeding 2.5%, huge MC-type carbides are produced, which reduces the grindability or mirror finish properties and reduces the mold release property when used as a mold. On the other hand, if it is less than 0.5%, wear resistance deteriorates, so V
The amount added was limited to 0.05 to 2.5%. Rare earth elements: 1
0.005 to 0.60% in total amount of a species or two or more rare earth elements have the effect of suppressing the precipitation of pro-eutectoid carbides at austenite grain boundaries, pearlite transformation, and paynite transformation during the cooling process of quenching treatment. It is the most important additive element that significantly improves the hardenability and toughness of the steel of the present invention.

That is, the rare earth element combines with the carbide-forming element and has the effect of reducing the solid solubility of the carbide-forming element in the matrix. Therefore, during the quenching and cooling process, precipitation of carbides (especially MC type carbides) at austenite grain boundaries is suppressed, strengthening the grain boundaries, and pearlite transformation and paynite transformation are delayed, resulting in a decrease in impact value and hardness. can be significantly prevented. In order to effectively exhibit the above effects, it is necessary to contain one or more rare earth elements in a total amount of at least 0.005%.

However, if a large amount is added, a large amount of MC-type huge carbides will be formed during solidification, and the forgeability will be significantly deteriorated, so the total amount of the above elements was limited to 0.60% or less. Further, since deoxidizing and denitrifying elements such as AI, Nb, TilZr, and Hf as impurity elements have the effect of reducing the effect of rare earth elements, they each need to be kept at 0.5% or less. Furthermore, if N is contained in a large amount, it will form nitrides with rare earth elements and inhibit the effects of rare earth elements.
It is desirable to make it 0.6% or less. With the above alloy composition, a high hardness, strong toughness mold steel with good hardenability can be obtained.
In cases where even higher hardenability and excellent mold life are required, it is desirable to add appropriate amounts of the following elements.

B: 0.001-0.050% An element that significantly improves hardenability and strength when added in a very small amount, and suppresses the precipitation of pro-eutectoid carbides at austenite grain boundaries during the quenching and cooling process. , has the effect of preventing deterioration of toughness.

In order to effectively exhibit the above effect, at least 0.0
It is necessary to contain 0.01% or more. However, if it is contained in a large amount, a large amount of borides will be formed and the forgeability will be significantly deteriorated, so the content was limited to 0.050% or less.

CO: 0.1 to 12.0% This is an element that forms a solid solution in the matrix, strengthens the matrix, delays precipitation and aggregation of carbides, and significantly improves hardness and yield strength at high temperatures.

Also, when the temperature rises during use of the mold, a dense and smooth oxide film is formed on the surface to protect the mold surface from oxidation.
Furthermore, it has the effect of improving heat check resistance. However, if the content is less than 0.1%, these effects cannot be expected, and if the content is too large, internal strain due to solid solution will occur, reducing toughness, and retained austenite remains stable up to high temperatures, which may cause early failure. Become. Therefore, CO is 0
．． It was limited to 1-12.0%. As explained above, the steel of the present invention has high hardenability, high strength, and high toughness, and is extremely excellent as a mold material for cold and warm working. It also has excellent high-temperature properties and is useful as a mold material for hot processing.

Next, the characteristics of the steel of the present invention will be explained in detail using examples.

Examples Various mold steels having the compositions shown in Table 1 were melted.

Test materials /461-5, 9-11 are high hardness and tough mold steels of the present invention, and test materials //66, 7, 12, 13 are conventional high alloy mold steels. It is steel. Various characteristic values were tested using the sample materials shown in Table 1.

(1) Hardenability Test pieces were taken from the test materials shown in Table 1, and isothermal transformation curves were determined. From the curves, the temperature and time at the nose of the start line of pearlite transformation and paynite transformation were investigated.

The results are shown in Table 2. As seen in the same table, the steel of the present invention has higher pearlite and bainis content than the comparative steel.
・The time it takes for metamorphosis to occur is long, indicating that metamorphosis does not occur easily.

This result shows that even if the cooling rate from the austenitizing temperature is slow in the steel of the present invention, pearlite or paynite structures are less likely to form during cooling, and at the same time, the carbide precipitation line shifts to the long-term side, resulting in poor hardenability. It shows that it is good. (2) Impact value after quenching and tempering After heating the test materials in Table 1 to their respective quenching temperatures,
Quenching was performed at a cooling rate of 800°C/Hr (corresponding to the cooling rate of the center when a round bar with a diameter of 70 mm was air cooled from the quenching temperature), and then tempered to a hardness of HRC6O. Ta.

Charpy impact test piece (notch depth: 2mm, 1 notch bottom R: 10m) made from the above-mentioned hardened and tempered material.
m) was collected and used for testing. The results are shown in Table 3. As seen in the same table, the steels of the present invention all exhibit superior impact properties compared to the comparative steels.

In other words, when the comparison steel is quenched at a slow cooling rate, a large amount of thin film-like carbides precipitates at the austenite grain boundaries during the cooling process, which is a major cause of deterioration of impact properties. Even if the cooling rate from the input heating temperature is slow, there is almost no precipitation of carbides at grain boundaries, so it is thought that no deterioration in impact properties is observed. (3
) Compressive properties ) Among the test materials in Table 1, the /W of the steel of the present invention; 1, /161
Compression test pieces (6 mm in diameter x 10 mm in parallel length) were taken from Comparative Steel/I67 and A6l2, and hardened and tempered at the respective temperatures to adjust the hardness to HRC6O. The cooling rate after quenching heating was set to 800°C/Hr, which corresponds to the cooling rate at the center when a round bar with a diameter of 70 mm was air-cooled from the quenching temperature. Table 4 shows the quenching and tempering treatment conditions and compression characteristics.

As seen in the same table, it can be seen that the steels of the present invention exhibit superior compressive properties compared to the comparative steels.

1) Fatigue properties Among the test materials in Table 1, the AI of the steel of the present invention was /16r. 10
Sieck type and Ono type fatigue test pieces were manufactured using comparative steels 7467 and A6l2, and the Sieck type was tested at room temperature and the Ono type was tested at a high temperature of 700°C.

The results are shown in FIG. 1 (Sienck type fatigue characteristics) and FIG. 2 (Ono type fatigue characteristics). The test pieces were heated at 800°C/Hr (diameter 70°C) after being heated to the quenching temperature shown in Table 5.
The material was quenched at a cooling rate of (corresponding to the cooling rate of the center when air-cooling a mm material) and tempered to a hardness of HRC6O. As can be seen in the figure, the fatigue properties of the steel of the present invention are clearly superior to those of comparative steels subjected to similar heat treatment, and the fatigue properties are particularly improved at high temperatures.

It has been confirmed that similar excellent effects can be obtained with rare earth elements other than those shown in the Examples of the present application.

As explained above, the steel of the present invention is a high-hardness, strong mold steel with an appropriate balance of 2M0+W and the addition of trace amounts of B, La, Ce, Nd, and other rare earth elements. Compared to mold steel, it has superior toughness, yield strength, and fatigue properties at room and high temperatures, and is found to be suitable as a mold material for cold, warm, and hot forging.

In particular, the mold material of the present invention has high hardenability, so it is most suitable as a mold material for large-sized forging.

[Brief explanation of drawings]

Figures 1 and 2 are diagrams showing the fatigue properties of the steel of the present invention and comparative steel.

Claims (1)

[Claims] 1 C: 0.4 to 1.0%, Si: 0.4 to 1.5%,
Mn: 0.4-1.5%, Ni: 0.3-2.0%, C
r: 2.0-6.0%, 2Mo+W: 2.0-15.0
%, V: 0.5 to 2.5%, and further contains one or more rare earth elements in a total amount of 0.005 to 0.60%, with the remainder substantially consisting of Fe. High hardness and toughness steel for molds. 2C: 0.4-1.0%, Si: 0.4-1.5%,
Mn: 0.4-1.5%, Ni: 0.3-2.0%, C
r: 2.0-6.0%, 2Mo+W: 2.0-15.0
%, V: 0.5-2.5%, B: 0.001-0.05
0% and further contains one or more rare earth elements in a total amount of 0.005 to 0.60%, with the remainder being substantially Fe.
High hardness and toughness steel for molds. 3C: 0.4-1.0%, Si: 0.4-1.5%,
Mn: 0.4-1.5%, Ni: 0.3-2.0%, C
r: 2.0-6.0%, 2Mo+W: 2.0-15.0
%, V: 0.5-2.5%, B: 0.001-0.05
0%, Co: 0.1 to 12.0%, and one or more rare earth elements in a total amount of 0.005 to 0.60.
%, with the remainder essentially consisting of Fe.