JPS5942743B2 - hot work tool steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a hot work tool steel that has excellent wear resistance, heat check resistance, heat resistance, and toughness and is suitable for large molds with a mold weight of 50 kl7 or more for hot forging.

Conventionally, SKD61, SKD62, etc. have been generally used as copper for hot forging molds having wear resistance and heat check resistance. However, as forging operations have become faster in recent years, the effects of heat and load on the mold have become greater. There has been a need for hot work tool steel with In response to such demands, precipitation hardening type 3% C and -3% Mo steels are used as steels for high-speed forging dies.

This steel has better heat resistance than SKD6162, which improves mold life. However, at 3% C-
Although %Mo steel is a precipitation hardening type, it has a high quenching hardness, so it is difficult to die-sink it in a low-temperature tempered state before precipitation hardening. Therefore, this steel must be used for round molds that are easy to machine even with high hardness materials, or it must be tempered at a high temperature beyond the peak of precipitation hardening to lower its hardness. However, it was not possible to extend the life of the mold by utilizing the precipitation hardening function.
Furthermore, molds using this steel have low toughness at the center of the mold.
This tendency becomes more pronounced as the mold becomes larger, and when used in a large mold with a mold weight of 50 kg or more, there is a drawback that toughness is insufficient and cracks occur in the mold. The present invention solves the drawbacks of the conventional steel (3% C to -% Mo steel), and simply increases the Cr content of the conventional steel in order to increase the toughness at the center of the mold, resulting in a decrease in hardness in the quenched state. Therefore, by lowering the C content, it is possible to increase the amount of Cr, which lowers the hardness in the quenched state and in the tempered state before precipitation hardening to HRC4.
This work succeeded in achieving a hardness of around 2, which allows easy cutting, and improving the toughness of the center of a large mold.

Therefore, even when the steel of the present invention is used at a high temperature of 600 to 650°C, the hardness of the conventional steel decreases due to tempering at a high temperature exceeding the precipitation hardening temperature, whereas the steel of the present invention It is a steel with excellent wear resistance and heat resistance that does not soften even if it undergoes precipitation hardening during use and becomes hard.

In addition, the toughness of the steel of the present invention is greatly improved due to the combined effect of C, Cr, MO, V, etc., and sufficient toughness can be obtained in both the surface layer and the center of large molds, making it suitable for high-speed forging, etc. Even during processing under severe conditions, the mold does not crack and has a stable mold life. The steel of the present invention will be explained in detail below. 1st
The invention steel is CO. iO~0.22%, SiO. l5~1.
50%, Mnl. OO% or less, Cr4. OO~6.00
%, MO2.6O~4.00%, VO. 3O~0.80
%, and the second invention steel is the first invention steel containing Nb,
The third invention steel further improves the toughness of the first invention steel by containing one type of Ti in a range of 0.30% or less, and the third invention steel has a Cr content of 4.30 to 6.00 in the first invention steel. %, VO
．． 3O to 0.70%, and further WO. 5O~2.00
%, COO. Contains one of 5O to 3.00% and is the first
The fourth invention steel further improves the wear resistance of the invention steel, and the fourth invention steel has a Cr content of 4.30 to 6.00% and a V of 0.
．． 30 to 0.70%, and further NiO. 3O~2.0
It contains 0% Cr, which improves the toughness of the first invention steel and prevents the formation of ferrite.
, is a steel containing 3% MO, but as a result of various experiments conducted by the inventors, the steel of the present invention lowers the quenching hardness to around HRC42 and improves toughness due to the combined effect of C, Cr, MO, and V. It has wear resistance, heat check resistance, and sufficient toughness in both the surface layer and center of large molds weighing 50 kg or more, and shows stable mold life even when used under harsh conditions. It is a hot work tool steel that exhibits excellent heat resistance and toughness.

Next, the reasons for limiting the composition of the steel of the present invention will be explained.

C is an element necessary to ensure hardness, and is 0.10
% or less, the strength and wear resistance required for a hot forging die cannot be obtained, so the lower limit is set to 0. i0% closed.

On the other hand, as the amount of C increases, the quenching hardness increases significantly. In other words, the quenching hardness of test steels A, B, and G, which have different amounts of C shown in Table 1 below, is as shown in the figure.
The higher the content, the higher the value. If the amount of C is higher than this, it is clear that tempering must be performed at a high temperature in order to obtain a hardness of about HR, C42 suitable for use, and the softening resistance will decrease and the heat resistance will decrease. decreases. In addition, the toughness of the mold decreases both in the center of the surface layer, so in the present invention, the upper limit is set to 0.22%. Si is required to be 0.15% or more because it forms a solid solution in the base material and has the effect of increasing the strength at room temperature and high temperature. However, since Si is the second most powerful ferrite-forming element after V, the upper limit should be set at 1.50.
%. Although Mn has the effect of improving hardenability,
If the content increases, the toughness and machinability will decrease significantly, so the content was set to 1.00% or less. In the present invention, Cr is an element that greatly influences wear resistance and toughness, which are its main properties, in relation to C. Heat check resistance, toughness, and hardenability improve as the Cr content increases.

In order to improve the toughness of the center of the mold and to prevent the mold from cracking during hot use, the content must be at least 4.00%. In addition, when containing WCONi,
In order to avoid deterioration of heat check resistance, it is necessary to set the lower limit of the Cr content to 4.30%. However, as the content increases, the quenched hardness increases, making tempering at low temperatures impossible. Furthermore, in the coexistence of MO and V, the precipitation hardening temperature shifts to the lower temperature side, resulting in lower heat resistance and shorter mold life. Therefore, in order to lower the quench hardness and enable cutting in a low-temperature tempered state before precipitation hardening, and to utilize the precipitation hardening mechanism most effectively and obtain the features of the present invention, the upper limit should be 6.00%. There is a need. MO is an element that raises the transformation point, improves heat check resistance, participates in precipitation hardening, improves heat resistance, and greatly contributes to extending mold life, and must be contained in an amount of 2.60% or more. However, even if a large amount is added, the tempering resistance does not improve much, and since it is an expensive element, the upper limit was set at 4.00%.

Like MO, V is an element that participates in precipitation hardening and improves heat resistance, and also improves the balance of C and Cr content, further improving toughness and contributing to extending mold life. .30% or more content is required. However, if too much is added, ferrite tends to form and the toughness, which is a feature of the steel of the present invention, is inhibited, so the upper limit was set at 0.80%. In addition,
When W, CO, and Ni are contained, since W is a ferrite-forming element, the tendency to form ferrite increases, and when a large amount of Ni is contained, the annealing hardness becomes difficult to decrease, so the upper limit of V is set. needs to be 0.70%. Nb,T
i is an element that is effective in refining crystal grains and improving toughness.

In the steel of the present invention, the toughness of the center of the mold is improved and the toughness is further improved**. However, even if a large amount is added, there is little improvement in the effect, so each content is set at 0.3% or less.
W and CO are elements that increase hardness at high temperatures and reduce mold wear, and both WCO must be contained in an amount of 0.50% or more.

However, if too much is added, the heat check resistance will deteriorate for W, so the upper limit is set at 2.00%, and for CO, the improvement in effectiveness is small and it is expensive, so the upper limit is set at 3.00%.
%. Ni is an element that improves toughness and prevents the formation of ferrite, and must be contained in an amount of 0.30% or more. However, even if the content exceeds 2,00%, the improvement in the effect is small, so the upper limit was set at 2.00%. Next, the characteristics of the steel of the present invention will be clarified by comparing it with comparative steel and conventional steel through examples.

Table 1 shows the chemical components of the invention steel, comparative steel, and conventional steel.

In Table 1, steels A and B are conventional steels, steel A is SKD6l,
B steel is 3% Cr-3% MO steel, CD steel is comparative steel,
E to I steels are the first invention steels, J and K steels are the second invention steels, L,
M steel is the third invention steel, and N steel is the fourth invention steel.

Table 2 shows the steels A to N in Table 1, which were held at 1050°C for 4 hours, then quenched by air cooling, and then subjected to H? The results are for molds tempered at a temperature that provides a hardness that does not interfere with die engraving around C42.

Note that the number of die strikes in this table is based on the case where a ring gear with a die weight of about 100 mm is used in a hot forging die.
A No. I83 test piece was cut out and tested at room temperature.
As shown in Table 2, in order to obtain a hardness suitable for die engraving of around HRC42, both conventional steels A and B have high hardness in the quenched state of HRC48 to 55, as explained in the figure. 620-640°C which exceeds the precipitation hardening temperature
However, since the E-N steel, which is the steel of the present invention, has a low hardness of around HRC42 in the quenched state, it can be tempered at a low temperature of 400°C.

In addition, steel D, which is a comparison steel, has 4.65% Cr, but C
Since the content is as high as 0.28%, high-temperature tempering had to be performed in the same way as conventional steel. Regarding the impact value, both the conventional steel A steel and the comparison steel D steel have a slightly lower impact value of 2.6 to 3.0 #Cg・W in both the surface layer and the center, but their high Cr content makes them hardenable. , so there is little difference between the surface layer and the center.

Steel B, which is a conventional steel, and Steel C, which is a comparison steel, have a small impact value at the center of 1.4, 1, and 9k9 m due to their small Cr content.
It can be seen that /d is significantly lower than that of the surface layer.
On the other hand, the steels E to N, which are the steels of the present invention, have an impact value of 4.0 kg・m/Crl or more in both the surface layer and the center.
Moreover, it can be seen that the difference between the surface layer and the center portion is small, and it has excellent toughness. Therefore, as seen in the number of die strikes, conventional steels A and B and comparative steel D were tempered at a high temperature exceeding the precipitation hardening temperature, which caused the die surface layer to soften during the forging process. However, due to the deterioration of wear resistance, steel A was used 5,500 times and steel D was used 8,000 times.
Regarding steel B, the impact value of the center part was much lower than that of the surface layer, so cracks occurred in the mold during stamping.
This is an extremely low result of 100 times. Also,
Steel C, which is a comparison steel, had a lower impact value in the center compared to the surface layer, similar to steel B, and cracks occurred in the mold during stamping, so the number of cycles was 8,400. In contrast, the steel of the present invention, E
All -N steels have low hardness in the quenched state, and can be die carved by low-temperature tempering, and because they can effectively utilize their precipitation hardening function, they exhibit high tempering resistance. It has excellent heat resistance due to (W, CO) and heat check resistance due to low C, and also has a sufficient impact value in both the surface layer and the center.
It showed an excellent number of mold strikes of 0.00 to 15,700 times, which was 2 to 3 times longer than conventional steel.

As mentioned above, the steel of the present invention also contains Cr,
By appropriately containing MO, V, etc., the hardness in the quenched state can be made suitable for die carving, and sufficient impact values can be obtained for both the surface layer and the center, and it can also be used for large molds. It is also a hot work tool steel that has excellent wear resistance, toughness, heat check resistance, and heat resistance, and the impact value of the center part does not decrease compared to the surface layer, making it a great contribution to industry. .

[Brief explanation of the drawing]

The figure is a diagram showing the relationship between C content and tempering hardness.

Claims (1)

[Claims] 1. C0.10-0.22% by weight, Si0.1
5-1.50%, Mn 1.00% or less, Cr 4.00-
6.00%, Mo2.60~4.00%, V0.30~
A hot work tool steel characterized by containing 0.80% Fe and the remainder consisting of Fe and impurity elements. 2 C0.10-0.22% by weight, Si0.1
5-1.50%, Mn 1.00% or less, Cr 4.00-
6.00%, Mo2.60~4.00%, V0.30~
A hot work tool steel characterized by containing 0.80% of Nb and Ti in an amount of 0.30% or less each, and the remainder being Fe and impurity elements. 3 C0.10-0.22% by weight, Si0.1
5-1.50%, Mn 1.00% or less, Cr4.30-
6.00%, Mo2.60~4.00%, V0.30~
Contains 0.70%, further W0.50 to 2.00%,
Contains one type of Co from 0.50 to 3.00% and the balance is Fe.
and hot work tool steel characterized by comprising impurity elements. 4 C0.10-0.22% by weight, Si0.1
5-1.50%, Mn 1.00% or less, Cr4.30-
6.00%, Mo2.60~4.00%, V0.30~
Contains 0.70% and further contains 0.30 to 2.00% Ni.
A hot work tool steel characterized in that it contains Fe and the remainder consists of Fe and impurity elements.