JP4811637B2 - Steel for die casting mold and die casting mold - Google Patents

Description

  The present invention relates to a die casting mold steel and a die casting mold that have excellent machinability and toughness and also have heat crack resistance.

  In a die-cast mold, heating by contact with a molten metal such as an aluminum alloy and cooling by a water-soluble mold release agent are repeatedly performed, so that the mold surface is subjected to compressive and tensile thermal stress. In actual operation, since this thermal stress is repeatedly applied, thermal fatigue cracks are generated on the mold surface and transferred to the workpiece. The transfer of this crack gradually becomes violent and is discarded when the mold cannot be used.

  Under such a thermal stress load, many die casting molds must withstand the use of tens of thousands to several tens of thousands of shots, and materials that are excellent in high-temperature strength and toughness are generally required. JIS SKD61 is used. In SKD61, sufficient machinability cannot be obtained in the quenching and tempering state. Therefore, the mold production is a process in which the roughing process is performed in the annealed state, the quenching and tempering process is performed, and finally the finishing process is performed. .

On the other hand, there are some die casting molds that do not need the service life as long as they can be obtained with SKD61, such as prototype molds and small production molds. Such a mold is required to shorten the production lead time in order to correspond to the speed of its use cycle. In other words, it is necessary to perform a process from roughing to finishing after hardness tempering by quenching and tempering, and a mold material having excellent machinability is used (see Patent Document 1).
JP 2003-138342 A

  Although the technique disclosed in Patent Document 1 described above as a mold material having excellent machinability is advantageous in terms of improving machinability, heat crack resistance, which is another important characteristic as a mold material. In terms of the nature, there is a concern that the target life cannot be achieved because the hardness is set low. On the other hand, generally, a mold material having excellent machinability, which includes many free-cutting elements such as S based on SKD61 or the like and is tempered around 40 HRC, such as MnS, is used. It is an organization in which many inclusions are distributed. However, since these inclusions are elongated in the forging direction due to forging at the time of manufacturing the material, the machinability is good, but heat cracks are easily generated from the inclusions, and the toughness is also good. There is a disadvantage that it is low. For this reason, heat cracks occurred early and progressed deeply along the inclusions, and there were many cases where the target life could not be achieved even for trial molds and molds for small volume production. .

  In such a case, in order to improve the service life, measures have been taken to perform nitriding treatment on the mold surface of the mold, but if the inclusions are still elongated, the generated heat cracks Since it rapidly progresses to the base material, it is difficult to obtain a great effect by nitriding. In addition, performing the nitriding treatment increases the die production lead time and production cost.

Accordingly, an object of the present invention is to provide a die casting mold steel and a die casting mold that have excellent machinability and toughness and also have heat crack resistance.

  The present inventor suppresses toughness deterioration due to inclusions by suppressing the addition of free-cutting elements to the minimum necessary, and improves the machinability of the base without adding a large amount of free-cutting elements. As a result of earnest research on components capable of giving good machinability and further obtaining good high temperature strength and room temperature ductility for improving heat crack resistance, the present invention has been achieved.

That is, the die-cast die steel of the present invention is in mass%, C: 0.02 to less than 0.1%, Si: 1.0% or less, Mn: 0.1 to 3.0%, P: 0 0.005 to 0.05%, S: 0.01 to 0.05%, Ni: 2.0% or less, Cr: 1.0 to 3.0%, Mo and W are used alone or in combination (Mo + 1 / 2W) ): 2.0% or less, V: 0.01 to 1.0%, Cu: more than 0.5% and 3.0% or less, balance Fe and unavoidable impurities, excellent cutting It is a steel for die casting molds that can achieve heat crack resistance as a mold.

Further, in these die casting steels of the present invention, part of Fe is replaced with B: 0.003% or less, or the hardness is tempered to 35 HRC or less.

  And if it is the die-casting die manufactured using the steel for die-casting die | dyes of the above this invention, sufficient heat crack resistance can be achieved.

  According to the present invention, it is possible to produce a die casting mold for trial production or small production using a steel tempered by quenching and tempering, and the obtained mold is made of Al or the like during use in die casting. Since the surface is hardened by heat from the work material and the heat crack resistance is improved, it is not necessary to perform a special treatment such as nitriding, and the lead time and production cost of mold production can be reduced. In addition, since the amount of alloying elements is small and the thermal conductivity is good, the heat from the work material such as Al can be diffused quickly, and the mold temperature can be prevented from becoming too high, so the seizure resistance is also improved. It is good.

  The feature of the present invention lies in finding a mold steel and a mold which are most suitable for application to a die casting mold and can shorten the production lead time.

  In order to achieve these problems, it is necessary to slow down the generation and progress of heat cracks by suppressing toughness reduction due to inclusions and increasing the hardness and increasing the high-temperature strength. However, if the inclusion such as MnS is reduced or the hardness is increased in order to suppress the decrease in toughness, the machinability deteriorates. We considered a means to maintain excellent machinability even if the addition of elements was kept to a minimum, and to increase the hardness without applying any special treatment after the mold was created.

  In other words, by forming an upper bainite structure by quenching and precipitating and agglomerating Cr, Mo or W, V carbides by tempering to give strength and moderate embrittlement, a good coverage can be applied to the base. It is to give machinability. Furthermore, when the mold is completed and used as a die-casting mold, heat from the work material such as Al is applied, so that the surface of the mold cavity surface or the entire mold is age-hardened and has good heat crack resistance. The present inventor has reached the present invention as a result of intensive studies on components capable of achieving these actions. Hereinafter, the reason for limiting the components of the steel of the present invention will be described.

  C is a basic additive element necessary for maintaining the hardened structure of the steel of the present invention in an upper bainite structure having good machinability and providing strengthening by precipitation of Cr, Mo or W, V carbides during tempering. If the amount is too large, the base is martensite-organized, and excessive carbides are formed, resulting in an increase in hardness and reducing the machinability. Therefore, if the amount is too low, ferrite is formed. % Or more. Preferably they are 0.04% or more and 0.08% or less.

  Si is a deoxidizer during steelmaking and is an element that enhances machinability, but if it is too much, it will lead to the formation of ferrite, so 1.0% or less.

  Mn has the effect of improving hardenability, suppressing the formation of ferrite, and obtaining appropriate quenching and tempering hardness. Moreover, it exists as nonmetallic inclusion MnS and has a great effect in improving machinability. In order to obtain this effect, addition of 0.1% or more is necessary. However, if too much, the bainite structure is excessively refined, and the viscosity of the base is increased to lower the machinability. . More preferably, it is 0.3% or more.

  S exists as a non-metallic inclusion MnS and has a great effect on improving machinability. In order to obtain this effect, addition of 0.01% or more is necessary, but if it is contained in a large amount, it becomes a structure in which a large amount of elongated MnS is present, tends to be a starting point of heat cracks as described above, and further cracks develop due to a decrease in toughness. Resulting in. For this reason, the heat cracking resistance is remarkably lowered, and the improvement effect by the surface treatment such as nitriding treatment cannot be obtained.

  Ni is an element that enhances hardenability and suppresses the formation of ferrite. If the amount is too large, the bainite structure is excessively refined, the base viscosity is increased, and the machinability is lowered. More preferably, it is 1.0% or less.

  Cr is added for precipitating and agglomerating fine carbides by tempering to give strength, and when nitriding is performed, it is added to obtain appropriate hardening. In order to obtain this effect, 1.0% or more is necessary. However, when the amount is too large, the bainite structure is refined to increase the viscosity of the base, and the machinability is decreased. Since it hardens and deteriorates the toughness of the nitrided layer and lowers the heat crack resistance, it is set to 3.0% or less.

  Mo and W are added singly or in combination to give strength by precipitating and agglomerating fine carbides by tempering and to improve softening resistance. In the case of this application, a large amount of addition is unnecessary, and if it is too much, machinability and ductility are reduced, so (Mo + 1 / 2W) was set to 2.0% or less. In order to obtain the above effect, the content is preferably 0.03% or more.

  V increases resistance to temper softening and suppresses coarsening of crystal grains, thereby contributing to improvement of toughness. In order to obtain this effect, 0.01% or more is required. However, if it is too much, machinability and ductility are lowered, so 1.0% or less was set. More preferably, it is 0.5% or less.

  Cu is an important additive element for the present invention. When heat is applied from a workpiece such as Al during use as a mold, Fe-Cu solid solution and Cu particles are precipitated, and good heat crack resistance. It is added to give the hardness necessary to obtain If the amount is too large, the bainite structure is refined and the machinability is lowered, and the hot workability is lowered. Therefore, the amount is set to 3.0% or less. . More preferably, it is 1.0% to 2.0%.

When P is contained, it is possible to moderately reduce toughness by temper embrittlement and improve machinability. To obtain this effect , 0 . 005% or more is added, and preferably 0.01% or more is added, but if it is too much, the toughness and ductility are remarkably deteriorated and the heat crack resistance is lowered, so the content is made 0.05% or less.

Hereinafter, other effective component elements constituting the present invention will be described.
B is an element that increases the hardenability and gives the hardness necessary to improve heat crack resistance by binding to nitrogen and forming boron nitride, and is added as necessary. However, if the content is too large, weld cracking tends to occur, so the content is made 0.003% or less.

  Note that Ca, Te, Zr, Se, Ce, Pb, Bi, Sn, and Sb are all elements for imparting machinability, and one or more of these may be added as necessary. It may be added. If the amount is too large, the toughness is reduced, so that Ca: 0.01% or less, Te: 0.2% or less, Zr: 0.2% or less, Se: 0.05% or less, Ce: 0.1% or less, Pb: 0.2% or less, Bi: 0.2% or less, Sn: 0.05% or less, Sb: 0.05% or less are preferable.

  The above is the reason for limiting the components of the steel of the present invention, but when producing a die-casting mold using the steel of the present invention, it is preferable to temper to 35 HRC or less in order to give further machinability to the base. . In this, the hardness can be imparted by tempering after quenching. And it is preferably 30 HRC or more to obtain the excellent machinability and sufficient hardness as a mold.

  Table 1 shows chemical components of the steels of the present invention and comparative steels. The comparative steel H is JIS SKD61 generally used as a die casting mold.

  These inventive steels and comparative steels were quenched and tempered by tempering at a low temperature (400 ° C.) to obtain die cast steels for evaluation samples. It is confirmed that all the steels of the present invention have an upper bainite structure, and the structure has good machinability. And in the die-casting die formed by cutting these evaluation samples, the change in the hardness of the sample subjected to aging treatment at 480 ° C. for 2 hours, assuming the state in use. Table 2 shows.

  The steels A, B, C and D of the present invention are excellent in machinability in the steel material state after tempering, while a sufficient age hardening amount is obtained during use as a mold produced therefrom. In particular, the steels B, C and D of the present invention, in which the amount of Cu is preferable, have a large amount of hardening.

  On the other hand, in the comparative steels G and H having a C amount larger than the range of the present invention, the hardenability is too good and the carbon amount is large, so that the quenching hardness is high, and the low temperature tempering is adjusted to a range of 30 to 35 HRC. Since the quality cannot be achieved, the machinability is poor as it is. In addition, even if the temperature is tempered in the range of 30 to 35 HRC by high temperature tempering, almost all elements contributing to age hardening are precipitated during tempering. Loses age hardening. The comparative steels E and F having a Cu content less than the range of the present invention hardly undergo age hardening during use as a mold and do not have sufficient hardness for heat crack resistance.

  Next, 0.2% proof stress value and room temperature (23 ± 2) obtained from a high temperature tensile test at 600 ° C. of samples obtained by refining the inventive steels B and C and the comparative steel H in Table 1 to 32HRC and 37HRC. Table 3 shows the drawing value obtained from the tensile test at [° C.] and the thermal conductivity of the sample tempered to 37 HRC. Here, in the present invention steels B and C, the hardness of 32HRC assumes an initial state in use as a mold, and the hardness of 37HRC has improved in use. It assumes a state. In the comparative steel H, the improvement in hardness during use as a mold cannot be expected, but the two types of hardness samples are prepared in order to show the effect of the present invention.

  When the hardness of the comparative steel H, which is SKD61, is increased, the proof stress value is improved and the squeezing value is decreased accordingly. The steels B and C of the present invention are in any hardness state. However, since the high temperature strength is higher than that of the comparative steel H of 37HRC and the room temperature ductility is higher than that of the comparative steel H of 32HRC, it can be seen that excellent heat crack resistance is exhibited. It can also be seen that the inventive steels B and C having a small amount of alloy elements have higher thermal conductivity than the comparative steel H and have excellent seizure resistance.

  Next, Table 4 shows the results of a 2 mm U-notch Charpy impact test on samples prepared by tempering steels B and C of the present invention to 32 HRC and 37 HRC at room temperature. In this case as well, the reason for preparing two types of hardness is as in Example 2.

The steel of the present invention shows a high Charpy impact value of around 60 J / cm ² even in any hardness state, and is age-hardened (37 HRC) during use even in the initial state of use of the mold (32 HRC). Even if the hardness is increased), it can be seen that it has sufficient toughness.

Claims (4)

In mass%, C: 0.02 to less than 0.1%, Si: 1.0% or less, Mn: 0.1 to 3.0%, P: 0.005 to 0.05 %, S: 0.00. 01 to 0.05%, Ni: 2.0% or less, Cr: 1.0 to 3.0%, Mo and W alone or in combination (Mo + 1 / 2W): 2.0% or less, V: 0.0. A die casting steel characterized by comprising 01 to 1.0%, Cu: more than 0.5% and 3.0% or less, the remainder Fe and inevitable impurities. The die casting steel according to claim 1, wherein a part of Fe is replaced by B: 0.003% or less in mass%. The die casting steel according to claim 1 or 2 , wherein the hardness is tempered to 35HRC or less. A die casting mold manufactured using the steel for a die casting mold according to any one of claims 1 to 3 .