JP4605695B2 - Pre-hardened steel for die casting molds - Google Patents

Description

  The present invention relates to pre-hardened steel for die casting molds.

Conventionally, 5Cr hot die steel (for example, JIS-SKD61) has been used as a material for die casting molds. In this case, the annealed steel is roughly processed and then heat treated to a desired hardness. Finished and finished into a mold. For this reason, the 5Cr type hot die steel mold has a problem that it takes time and cost to manufacture.
Pre-hardened steel is known as a material for die casting molds that solves this problem (for example, Patent Document 1). Pre-hardened steel is a pre-heated steel material that does not need to be heat-treated between roughing and finishing. Compared with 5Cr hot die steel, the production time and cost of the mold are reduced. Are better.
JP 2003-138342 A

However, conventional pre-hardened steel die-casting dies are not at a satisfactory level in terms of life as compared with die-casting dies of 5Cr hot die steel.
The present invention has been made based on the above-described circumstances, and an object of the present invention is to provide a pre-hardened steel for a die-casting die that can extend the life of the die-casting die.

In order to achieve the above-mentioned object, the inventors of the present invention have come up with the present invention by paying attention to the correlation between the grain boundary oxidation and the heat check resistance during various studies.
According to the present invention of claim 1, by mass content, C of 0.15% or more and 0.35% or less, Si of 0.05% or more and less than 0.20%, 0.05% or more and 1. 50% or less of Mn, 0.020% or less of P, 0.013% or less of S, 0.10% or less of Cu, 0.20% or less of Ni, and 0.20% or more. 50% or less of Cr, Mo of 0.50% or more and 3.00% or less, V and Nb of 0.05% or more and 0.30% or less, and 0.020% or more and 0.040% or less Rockwell C hardness of 30 or more and 40 or less, including Al, 0.003% or less of O, and 0.010% or more and 0.020% or less of N, the balance being Fe and inevitable impurities A pre-hardened steel for die-casting molds is provided.

In this configuration, by restricting the upper limit of the Si, Cr and Mn contents, in particular, by making the upper limit of the Si content 0.20%, embrittlement of the grain boundaries due to grain boundary oxidation is prevented, and the resistance is improved. Heat check performance is improved.
According to a second aspect of the present invention, as a preferred embodiment, 0.0002% or more and 0.0020% or less of B is further contained. In this configuration, the hardenability is improved by B.

In a preferred embodiment of the present invention of claim 3, 0.0005% or more and 0.0100% or less of Ca, 0.01% or more and 0.15% or less of Se, and 0.01% or more and 0.15% or less are preferable. It contains any one or more of the following Te and 0.003% or more and 0.20% or less of Zr.
In this configuration, the machinability is improved by Ca, Se, Te and Zr, and the toughness becomes isotropic because these Ca, Se, Te and Zr spheroidize the sulfide (MnS) in the steel, Furthermore, heat check resistance is improved.

As described above, according to the die-casting die for Prin Hadon steel of claims 1 to 3, generation of surface flaws in the mold is prevented by improving the heat check resistance, to achieve a long life of the mold be able to.
Furthermore, die-casting die for Prin Hadon steel of claim 2 suitable for the production of large die-casting die so hardenability is improved.

Then, according to according to claim 3 of the die-casting die for Prin Hadon steel, it is easy roughing and finishing by improving the machinability, improved further heat check resistance by isotropic of toughness Therefore, it is possible to achieve a longer life of the mold.

Hereinafter, the prehardened steel for die casting molds according to the first embodiment of the present invention will be described.
This prehardened steel is obtained by forging or rolling an ingot having a predetermined composition into a steel material, and giving the obtained steel material a Rockwell hardness of 30 HRC or more and 40 HRC or less by heat treatment.
The reason why the lower limit of the HRC hardness is set to 30 is that when the prehardened steel is applied to a die casting mold, the strength required for the die casting mold is imparted. On the other hand, the upper limit of the HRC hardness is set to 40 in order to ensure the machinability of the prehardened steel in roughing and finishing. For this reason, according to this prehardened steel, the efficiency of a cutting process is ensured and the manufacturing time and cost of a metal mold | die can be reduced.

Hereinafter, the composition of the pre-hardened steel will be described.
This pre-hardened steel has a mass content of 0.15% to 0.35% C (carbon), 0.05% to less than 0.20% Si (silicon), 0.05% to 1.50. % Mn (manganese), 0.020% or less P (phosphorus), 0.013% or less S (sulfur), 0.10% or less Cu (copper), and 0.20% or less Ni (nickel), 0.20% or more and 2.50% or less Cr (chromium), and 0.50% or more and 3.00% or less Mo (molybdenum) in total, 0.05% or more and 0.005% or more. 30% or less of V (vanadium) and Nb (niobium), 0.020% or more and 0.040% or less of Al (aluminum), 0.003% or less of O (oxygen), and 0.010% or more of 0 0.020% or less of N (nitrogen) and the balance is substantially made of Fe (iron).

The reason why the content of each element in the pre-hardened steel, that is, the content is set in the above range is as follows.
C: 0.15% to 0.35% The reason why the lower limit of the C content is set to 0.15% is to impart hardness to the prehardened steel. On the other hand, the upper limit of the C content is set to 0.35% in order to prevent the weld cracking sensitivity of the prehardened steel from becoming high and to prevent the prehardened steel from becoming too hard and reducing the machinability. Because. In addition, while the suitable minimum of C content is 0.20%, the suitable upper limit of C content is 0.30%, The suitable range of C content is 0.20% or more and 0.30% It is as follows.

Si: 0.05% or more and less than 0.20% The lower limit of the Si content is 0.05% because of deoxidation during ingot melting, hardenability of steel materials and machinability of prehardened steel. It is for improvement. On the other hand, the upper limit of Si content is set to 0.20% because when the Si content is 0.20% or more, grain boundary oxidation becomes prominent and the grain boundaries tend to become brittle. This is because the sex is lowered.

Mn: 0.05% or more and 1.50% or less The lower limit of the Mn content is set to 0.05% for the purpose of deoxidation during ingot melting and improvement of the hardenability of the steel material. On the other hand, the upper limit of the Mn content is 1.50% because when the Mn content exceeds 1.50%, the grain boundary oxidation becomes intense and the grain boundary is easily embrittled, and the heat check resistance of the prehardened steel. This is because of a decrease. In addition, while the suitable minimum of Mn content is 0.10%, the suitable upper limit of Mn content is 1.00%, The suitable range of Mn content is 0.10% or more and 1.00 % Or less.

P: 0.020% or less Since P reduces the toughness of the prehardened steel, the smaller the P content, the better. For this reason, there is no lower limit for the P content. On the other hand, the upper limit of the P content is set to 0.020% because the influence on the toughness of the prehardened steel is small if the P content is 0.020% or less.

S: 0.013% or less S generates sulfides, and these sulfides serve as starting points for heat cracks and promote crack extension to lower the heat check resistance of prehardened steel, so the upper limit of the S content Is 0.013%. On the other hand, the lower limit of the S content may be 0, but if a small amount of S is contained, the machinability of the prehardened steel is improved, so the preferred range of the S content is 0.003% or more and 0.00. 013%.

Cu: 0.10% or less, Ni: 0.20% or less Each of Cu and Ni is an impurity mixed from raw materials and the like during ingot melting and is not an essential component of pre-hardened steel. For this reason, the upper limit of Cu content is 0.10%, and the upper limit of Ni content is 0.20%.
Cr: 0.20% or more and 2.50% or less The lower limit of the Cr content is set to 0.20% in order to increase the deoxidation at the time of ingot melting and the hardenability of the steel material. On the other hand, the upper limit of the Cr content is 2.50% because when the Cr content exceeds 2.50%, the grain boundary oxidation becomes prominent and the grain boundaries tend to become brittle, and the heat resistance of the prehardened steel is reduced. This is because the checkability is lowered. Further, when the Cr content exceeds 2.50%, the hot workability of the ingot is lowered, the softening resistance of the prehardened steel at 600 ° C. or more is lowered, and the decrease in the high temperature strength becomes remarkable, This is because machinability, weldability, and thermal conductivity are lowered, and raw material costs are increased. In addition, while the suitable minimum of Cr content is 0.50%, the suitable upper limit of Cr content is 2.00%, The suitable range of Cr content is 0.50% or more and 2.00 % Or less.

Mo: 0.50% or more and 3.00% or less The lower limit of the Mo content is 0.50%, which increases the hardenability of the steel material and secures the temper softening resistance of pre-hardened steel at 600 ° C or higher. It is to do. On the other hand, the upper limit of the Mo content is set to 3.00% because when the Mo content exceeds 3.00%, the machinability and the thermal conductivity of the prehardened steel are lowered and the raw material cost is increased. . In addition, while the suitable minimum of Mo content is 0.80%, the suitable upper limit of Mo content is 2.50%, The suitable range of Mo content is 0.80% or more and 2.50%. That's it.

V and Nb: 0.05% or more and 0.30% or less in total V and Nb are both elements for ensuring tempering softening resistance and refining of crystal grains in the pre-hardened steel. And the content of Nb is set as the sum of the contents of V and Nb. Therefore, the reason why the lower limit of the sum of the contents is set to 0.05% is to secure the temper softening resistance in the pre-hardened steel and to refine the crystal grains. On the other hand, the upper limit of the sum of the contents is set to 0.30% because when the sum exceeds 0.30%, the machinability, toughness and thermal conductivity of the prehardened steel are lowered and the raw material cost is increased. Because. The preferred lower limit of the sum of the contents is 0.08%, while the preferred upper limit of the sum of the contents is 0.15%, and the preferred range of the sum of the contents is 0.005%. It is 08% or more and 0.15% or less.

Al: 0.020% or more and 0.040% or less The lower limit of the Al content is set to 0.02% because of deoxidation during ingot melting and generation of nitride (AlN). This is because the heat check resistance of the pre-hardened steel is improved. On the other hand, the upper limit of the Al content is set to 0.040% because when the Al content exceeds 0.040%, the machinability of the prehardened steel decreases due to the deoxidation product (Al ₂ O ₃ ) and nitride. Because it does. In addition, the suitable upper limit of Al content is 0.030%, and the suitable range of Al content is 0.020% or more and 0.030% or less.

O: 0.003% or less The upper limit of the O content is set to 0.003%. O is an impurity and combines with Al added as a deoxidizer during ingot melting to form Al ₂ O _3. This is because the machinability is reduced by Al ₂ O ₃ .
N: 0.010% or more and 0.020% or less The lower limit of the N content is set to 0.010% because N combines with Al to form a nitride as described above. This is because the heat check resistance of the prehardened steel is improved. On the other hand, the upper limit of the N content is set to 0.020% because when the N content exceeds 0.020%, carbonitrides are generated and the machinability of the prehardened steel is lowered.

The above-mentioned pre-hardened steel has an upper limit of Si, Cr and Mn contents, especially the upper limit of Si content, which prevents brittleness of grain boundaries due to grain boundary oxidation and improves heat check resistance. is doing.
Moreover, in this prehardened steel, the ratio of the thermal conductivity to the thermal expansion coefficient is high because the C, Cr, Mo, and V contents are within the above-described content ranges. For this reason, this pre-hardened steel has improved thermal shock characteristics and improved heat check resistance.

Thus, in the die-casting mold made of this pre-hardened steel, even if heating and cooling are repeated by repeated casting, the pre-hardened steel has excellent heat check resistance, so that the mold surface has cracks such as cracks. Is less likely to occur and has a longer life compared to conventional pre-hardened steel die casting molds.
Moreover, in the die-casting die made of this pre-hardened steel, since the thermal conductivity is high, the thickness of the chill layer on the casting surface cast using this die becomes uniform and thick. In a casting having such a chill layer, the hardness of the surface layer is increased and the strength is improved, and the cooling efficiency inside the casting is also improved through the chill layer at the time of casting, so the internal structure of the casting is also refined. Quality is improved.

  The pre-hardened steel mentioned above can be manufactured by a normal method. For example, first, raw materials blended so as to have the above composition are melted by an electric furnace or ladle refining to form an ingot. This ingot is forged or rolled into a rectangular steel material having a thickness of 100 to 400 mm, a width of 300 to 1000 mm, and a length of 4000 mm. And this steel material is heat-processed (quenching at 950 degreeC and tempering at 650 degreeC), and it becomes the prehardened steel of HRC hardness 30-40.

The above-mentioned pre-hardened steel is formed into a die-casting die having a desired shape by sequentially performing roughing and finishing using a four-face mill, for example. This die casting mold is applied to casting of a casting made of an Al alloy, for example.
The present invention is not limited to the above-described embodiment and can be variously modified. For example, pre-hardened steel has a range in which the heat check resistance does not deteriorate, in other words, other than those described above at the impurity level. These elements may be included.

  Moreover, it is preferable that the pre-hardened steel of the above-described embodiment further contains 0.0002% or more and 0.0020% or less of B. Since B increases the hardenability of the steel material, if the B content is 0.002% or more, even if the steel material to be heat-treated is large when producing pre-hardened steel for a large die-casting die, the steel material Is fully quenched. On the other hand, the upper limit of the B content is set to 0.0020% because when the B content exceeds 0.0020%, the effect of increasing the hardenability of the steel material is saturated and the weld cracking sensitivity of the prehardened steel is increased. Because.

The pre-hardened steel according to the above-described embodiment further includes 0.0005% to 0.0100% Ca, 0.01% to 0.15% Se, 0.01% to 0.15%. It is preferable to contain at least one of Te and 0.003% to 0.20% of Zr .
Ca, Se, Te and Zr all improve the machinability of the pre-hardened steel and spheroidize sulfide (MnS) to make the toughness isotropic. For this reason, Ca, Se, The lower limits of the Te and Zr contents are 0.0005%, 0.01%, 0.01% and 0.003%, respectively. The upper limits of Ca, Se, Te and Zr contents were 0.010%, 0.15%, 0.15% and 0.20%, respectively. This is because the hardness and heat check resistance are lowered, and the hot workability of the ingot is lowered.

Examples 1-20, Comparative Examples 1-3
1. Production of pre-hardened steel Ingots having the composition shown in Table 1 were melted, and each ingot was forged into a steel material. These steel materials were heat-treated (quenched and tempered) to give the HRC hardness shown in Table 2. Then, each steel material was machined and the test piece for the evaluation test mentioned later was produced. Comparative example 1 corresponds to SKD61, and comparative examples 2 and 3 correspond to conventional pre-hardened steel.

2. Evaluation Test (1) Impact Test The impact test was performed by taking a JIS No. 3 test piece from the center of the steel cross section (thickness 300 mm, width 700 mm) and testing each test piece with a 30 J (Joule) Charpy impact tester. . The results are shown in Table 2.
(2) Grain boundary oxide layer depth measurement A 20 mm square test piece was quenched in an air atmosphere at a temperature of 900 ° C., and the cross-sectional structure of the quenched surface layer was observed with a metal microscope, whereby the depth of the grain boundary oxide layer ( Thickness) was measured. The results are shown in Table 2.
(3) Heat check test Using a high-frequency heating / water cooling type heat check tester, each test piece was subjected to a heating / cooling cycle of heating from room temperature to 700 ° C. and then cooling to room temperature again 1000 times, and then the test piece. The maximum length of the cracks was measured in the cross section. These results are shown in Table 2.
The test piece has an annular shape with an outer diameter of 15 mm, an inner diameter of 5 mm, and a thickness of 5 mm. As shown in FIG. 1, the heat check tester includes a rod 12 that supports the test piece 10 through its central hole. Yes. The outer periphery of the test piece 10 is surrounded by a coil 14 for high-frequency heating, and a spray 16 for water cooling the test piece 10 is provided on the inner peripheral surface of the coil 14. Further, a thermocouple 18 for temperature measurement is arranged in the vicinity of the test piece 10.

(4) Thermal fatigue test The thermal fatigue test was performed by heating from 200 ° C to 600 ° C with both ends constrained to each test piece using a high-frequency heating / gas blow cooling type thermal fatigue tester, and again at 200 ° C. The number of cycles until the test piece broke was measured. The results are shown in Table 2. The test piece has the same shape as the tensile test piece, and has a parallel portion of φ10 mm and a length of 20 mm. The restraint of the test piece was the complete end restraint (constraint rate: 100%) at the maximum temperature. As shown in FIG. 2, the thermal fatigue testing machine includes a chuck 22 that restrains both ends of the test piece 20, a high-frequency heating coil 24 that surrounds the test piece 20, and an air blow unit for air-cooling the test piece 20. (Not shown).
(5) Thermal shock characteristic evaluation The thermal conductivity K of each test piece is measured at room temperature (20 ° C), and the thermal expansion coefficient α of each test piece is measured from room temperature (20 ° C) to a temperature of 100 ° C. These results are shown in Table 2. Further, the Young's modulus E of the test piece was measured, and the strength σ _B was estimated from the HRC hardness. From these results, the thermal shock characteristics Phs (= σ _B · K / E · α) was determined and shown in Table 2.

From Table 2, the following is clear.
(1) Examples 1-20 (prehardened steel) have a larger impact value than Comparative Examples 1-3.
(2) In Examples 1 to 20, the grain boundary oxide layer has a depth of 15 μm or less, and the grain boundary oxide layer is thinner than those in Comparative Examples 1 to 3, particularly compared to Comparative Example 1 corresponding to SKD61. thin.
(3) Examples 1 to 20 have a shorter maximum crack length in the heat check test than Comparative Examples 1 to 3, and are particularly shorter than Comparative Examples 2 and 3 corresponding to conventional pre-hardened steel. Thus, Examples 1 to 20 have higher heat check resistance than Comparative Examples 1 to 3.
(4) In Examples 1 to 20, compared to Comparative Examples 2 and 3, the number of cycles until the test piece breaks in the thermal fatigue test is twice or more, and the heat check resistance is high. Moreover, Examples 1-20 (except Example 13) have many cycle number until it fractures | ruptures compared with the comparative example 1, and heat check resistance is high.
(5) The steel materials (prehardened steel) of Examples 1 to 20 have larger thermal conductivity K and thermal expansion coefficient α than Comparative Examples 1 to 3. For this reason, the steel materials (pre-hardened steel) of Examples 1 to 20 have large thermal shock characteristics Phs and high heat check resistance.
(6) When the results in Table 2 are analyzed in more detail, the thermal conductivity K can be expressed as follows when the C content, Cr content, Mo content and V content are κ, λ, μ and ν, respectively. :
K ≒ 31.3 + 0.01κ-1.93λ-0.22μ-1.82ν) (1)
Is approximated by On the other hand, the coefficient of thermal expansion α is given by the formula:
α ≒ 13.3 + 0.06κ-0.33λ-0.07μ-0.14ν (2)
Is approximated by

  That is, according to these approximate expressions (1) and (2), the thermal conductivity K and the thermal expansion coefficient α depend on the C content, the Cr content, the Mo content, and the V content. The pre-hardened steel of 1 to 20 has a high thermal conductivity K and a thermal expansion coefficient α depending on the C content, Cr content, Mo content and V content. For this reason, Examples 1-20 whose HRC hardness is 33-34 have a thermal shock characteristic exceeding 10,000, and have the outstanding heat check characteristic.

It is a schematic block diagram of the heat check test machine of prehardened steel. It is a schematic block diagram of the thermal fatigue testing machine of prehardened steel.

Explanation of symbols

10 Test piece 12 Rod 14 Coil 16 Spray 18 Thermocouple

Claims (3)

By mass content,
0.15% or more and 0.35% or less of C,
0.05% or more and less than 0.20% Si;
0.05% or more and 1.50% or less of Mn,
0.020% or less of P,
0.013% or less S,
Up to 0.10% Cu;
Up to 0.20% Ni,
0.20% or more and 2.50% or less of Cr;
0.50% to 3.00% Mo,
In total, 0.05% or more and 0.30% or less of V and Nb,
0.020% or more and 0.040% or less of Al,
0.003% or less of O,
Containing 0.010% or more and 0.020% or less of N, with the balance being Fe and inevitable impurities ,
A pre-hardened steel for die casting molds having a Rockwell hardness of 30 HRC or more and 40 HRC or less.   The pre-hardened steel for die-casting molds according to claim 1, further comprising B in an amount of 0.0002% to 0.0020%. 0.0005% or more and 0.0100% or less of Ca;
0.01% or more and 0.15% or less Se;
Te of 0.01% or more and 0.15% or less;
0.003% or more and 0.20% or less of Zr
The pre-hardened steel for die-casting molds according to claim 1 or 2, wherein at least one of them is contained.

Images