JP2005082814A - Prehardened steel for plastic molding die - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide prehardened steel for a plastic molding die to which excellent Charpy impact value, mirror finishing properties and welding repairableness are imparted while its excellent machinability is maintained or further improved. <P>SOLUTION: The prehardened steel for a plastic molding die has a composition comprising, by mass, 0.01 to 0.2% C, 0.1 to 2% Si, 0.1 to 2% Mn, 0.3 to 2% Cu, 1.5 to 3.5% Ni, 0.01 to 2% Cr, 0.01 to 2% Mo, 0.2 to 1.5% Al, ≤0.015% N and ≤0.01% O, and in which Ni(%)/Al(%) is 1.8 to 6, and also, C(%)+Si(%)/15+Mn(%)/7+Cr(%)/6+Mo(%)/7 is 0.2 to 0.7, and the balance Fe with inevitable impurities. In the prehardened steel, grain size number prescribed by JIS G 0551 is ≥4, and Rockwell C scale hardness (HRC) is 26 to 36. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

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

  In recent years, plastic molds have been used for manufacturing a wide variety of parts such as automobile parts, office equipment parts, precision machine parts, electrical parts, and optical equipment parts. Such molds are required to have strict conditions such as simplification of mold manufacture, cost reduction, and high precision.

Various proposals have been made for steel for plastic molds, and for example, age-hardened steel processed in a pre-hardened state as in Patent Documents 1 to 4 is used. Age-hardened steel is a material with excellent machinability. However, in the mold, (a) an acute angle portion and (b) a portion having a small corner radius as shown in FIG. 4 are unavoidable in the internal shape. For example, these portions have a Charpy impact value of 20 J / cm ² or more. However, the age-hardened steel has a problem that the Charpy impact value does not reach the required level.

JP 2001-152278 A JP 2002-309341 A JP 2001-152246 A JP-A-5-70887

  The present invention has been made in view of the above-mentioned problems, and prehardened steel for plastic molding dies having excellent Charpy impact value, specularity and weld repairability while maintaining or further improving excellent machinability. The purpose is to provide.

Means for Solving the Problems and Effects of the Invention

In order to solve the above problems, in the pre-hardened steel for plastic molds of the present invention (hereinafter also referred to as the present invention steel),
In mass%, C: 0.01-0.2%, Si: 0.1-2%, Mn: 0.1-2%, Cu: 0.3-2%, Ni: 1.5-3. 5%, Cr: 0.01-2%, Mo: 0.01-2%, Al: 0.2-1.5%, N: 0.015% or less, O: 0.01% or less With
Ni (%) / Al (%) is 1.8 or more and 6 or less, and C (%) + Si (%) / 15 + Mn (%) / 7 + Cr (%) / 6 + Mo (%) / 7 is 0.2% or more 0.7% or less,
The crystal grain size number defined by JIS G 0551 is 4 or more and the Rockwell C scale hardness (HRC) is 26 or more and 36 or less.

In the steel of the present invention, the hardness of the age-hardened steel is lower than usual (for example, about 26 to 36 in Rockwell C scale hardness (HRC)), and the grain size number is 4 or more. It is possible to obtain characteristics necessary for a mold (for example, Charpy impact value of 20 J / cm ² or more), and excellent machinability, specularity, and weld repairability.

  Hardness HRC26-36 can be achieved by adjusting the components of age hardened steel. As an adjustment method, it is necessary that Ni / Al (formula (1)) is 1.8 to 6 and C + Si / 15 + Mn / 7 + Cr / 6 + Mo / 7 (formula (2)) is 0.2 to 0.7%. There is. For example, if it falls outside the range of the formula (2), as shown in FIG. 3, the required hardness is not reached (HRC 25 or less), or the hardness is more than necessary (HRC 37 or more). Even in the case of such components that are more than necessary, it is possible to achieve the hardness of HRC26-36 by changing the heat treatment conditions (aging conditions), but this is due to overaging or unaging. In this case, a sufficient Charpy impact value cannot be obtained. The most excellent balance of properties can be obtained with age-hardened steel in the case of almost peak aging. For this purpose, the above-described component adjustment is indispensable.

  Further, since C, Si, Mn, Cr, and Mo are main elements for adjusting hardenability, the structure obtained according to the amount of these components can be martensite, bainite, ferrite, and pearlite structure. The martensite structure and the lower bainite structure can obtain a high Charpy impact value, but the machinability is slightly inferior. The upper bainite structure, ferrite and pearlite structure have the opposite characteristics. Therefore, it is necessary to adjust the structure according to the required mold characteristics. However, in any structure, the machinability as age hardened steel is sufficiently ensured.

The steel of the present invention has a Charpy impact value of 20 J / cm ² or more by setting the crystal grain size number to 4 or more. When the hardness used is as low as HRC26-36, it is easy to obtain a certain Charpy impact value, but the steel of the present invention further achieves a high Charpy impact value by setting the grain size number to 4 or more. doing.

  The specularity is determined mainly by hardness and inclusions. The hardness should be uniform and hard, and the fewer inclusions, the better. Since the age-hardened steel increases in hardness due to the aging treatment, the central portion of the material does not have a lower hardness like a general quenching and tempering material. That is, since uniform hardness can be obtained up to the inside, the mirror surface property is excellent. Moreover, since machinability is sufficiently ensured without adding a free cutting element, there are no inclusions formed by adding the free cutting element. However, in order to improve the machinability within the range in which the specular characteristic is allowed, it is possible to add some free-cutting element.

  The weld repairability is mainly determined by the amount of C (for example, it is known that the higher the weld crack sensitivity Pcm = C + Si / 30 + Mn / 20 + Ni / 60 + Cr / 20 + Mo / 15, the easier it is to crack). Since age-hardened steel is age-hardened by the addition of Ni and Al, the amount of C sensitive to weld repairability can be reduced to the minimum necessary. Further, since the weld repaired portion is in a heated state by welding, for example, in the case of a conventional SCM-based material, the hardened state, that is, the weld peripheral portion tends to become hard. FIG. 1 is a diagram showing the hardness in the welding peripheral portion, and FIG. 2 is an observation view of the welding peripheral portion. In these drawings, (a) represents the inventive steel of the present application, and (b) represents a conventional SCM material. Hardened weld repair locations have lower Charpy impact values than other locations, leading to degradation of mold characteristics. In contrast, age hardened steel is excellent in weld repairability. Although the weld repaired portion is heated by welding, it corresponds to a solution treatment, not quenching, and therefore has a lower hardness than the surroundings. Alternatively, age hardening occurs by being heated and held at a low temperature after welding, and it is possible to obtain a hardness almost equal to that of the surroundings.

The steel of the present invention can be obtained, for example, by the following production method.
After the steel containing the above components is heated to the hot working temperature, the temperature is lowered to the warm working temperature, the final processing is performed at the warm working temperature, and then the age hardening heat treatment is performed. Then, the pre-hardened state having a predetermined hardness is adjusted. Note that at the hot working temperature, the steel may or may not be worked.
Further, a solution heat treatment can be performed after the final processing and before the age hardening heat treatment.

  The said manufacturing method makes steel once austenitize by hold | maintaining to normal hot processing temperature (for example, about 900-1200 degreeC). Thereafter, final processing is performed at a warm processing temperature (for example, about 600 to 900 ° C.). At this time, the processing is cooled from the hot processing temperature to the warm processing temperature. For this reason, it becomes easy to obtain an austenite state at a warm processing temperature, and it is easy to process. Moreover, it can process continuously and is time-saving compared with the past. And after final processing, it is possible to perform age hardening heat processing and to adjust to predetermined hardness. Desirably, it is better to perform an age hardening heat treatment after performing a solution heat treatment after processing. This is because a uniform structure can be easily obtained by performing the solution heat treatment. On the other hand, when the temperature of the solution heat treatment is relatively low, fine crystal grains obtained by processing are maintained, but as the temperature becomes relatively high, the crystal grains become coarse. For this reason, when performing the solution heat treatment, it is necessary to lower the temperature or to shorten the holding time. For example, the solution heat treatment can be performed at a temperature of 850 to 970 ° C. and a holding time of 10 to 40 minutes. In the case of a mold shape that requires a higher Charpy impact value or a mold application that requires high specularity (reducing the surface roughness), the crystal grain size number is preferably 7 or more.

Hereinafter, the reasons for limiting each numerical range in the present invention will be described.
The reasons for limiting the composition of the steel of the present invention are as follows. In addition, content of an additive element shall be mass%.
C (carbon): 0.01 to 0.2%
C needs to be added in an amount of 0.01% or more in order to ensure the hardness used as tool steel. However, excessive addition causes a decrease in machinability, so 0.2% is made the upper limit.

Si (silicon): 0.1 to 2%
As a deoxidizer, 0.1% or more is added. If the amount is too large, the Charpy impact value decreases or segregation of Cr, Mo, etc. is promoted, so 2% is made the upper limit. It is also possible to intentionally add 0.5% or more for the purpose of improving machinability.

Mn (manganese): 0.1 to 2%
In order to ensure the required hardness, it is added for the purpose of improving hardenability. In order to ensure sufficient hardenability, addition of 0.1% or more is desirable, but hardenability needs to be adjusted according to required hardness and material size. In other words, by cooling after solution heat treatment, adjustment to upper bainite structure excellent in machinability, ferrite and pearlite structure, or martensitic structure excellent in Charpy impact value, adjustment to lower bainite structure should be adjusted by Mn amount. Is possible. The upper limit is 2%.

Cu (copper): 0.3-2%
Age hardening is performed by age hardening heat treatment in the same manner as Ni and Al. Addition of Cu is known to cause age hardening even when the amount of Ni is small, and addition of Cu is desirable for reducing raw material costs. In order to obtain an effect from the viewpoint of hardness, addition of 0.3% or more is desirable. However, excessive addition impairs hot workability, so the upper limit is 2%. It is also possible to intentionally add 1% or more for the purpose of improving machinability.

Ni (nickel): 1.5-3.5%
It is an essential element for age-hardening steel, and the addition of Al is also essential. In terms of hardness, addition of 1.5% or more is necessary. Excessive addition causes a decrease in workability, so 3.5% is made the upper limit.

Cr (chromium): 0.01-2%
Like Mn, it can be added for necessary hardness and structure adjustment. Forming carbides has the effect of strengthening the base (improving hardness) and improving wear resistance. In order to obtain such an effect, addition of 0.01% or more is necessary. However, if the amount is too large, the machinability deteriorates, so the upper limit is made 2%.

Mo (molybdenum): 0.01-2%
Like Mn, it can be added for necessary hardness and structure adjustment. In order to obtain such an effect, addition of 0.01% or more is necessary. However, excessive addition promotes the formation of carbides and lowers the machinability, so 2% is made the upper limit.

Al (aluminum): 0.2 to 1.5%
It is an element having a deoxidizing action and is contained in a trace amount in steel. An element that is essential since age hardening can be obtained by adding Ni simultaneously. In order to obtain such an effect, addition of 0.2% or more is necessary. However, excessive addition causes a decrease in Charpy impact value, so the upper limit is made 1.5%. Particularly in the age-hardened steel with a large amount of Al, when the amount of N is high, Al nitride is formed and the mirror finish is reduced.

N (nitrogen): 0.015% or less, O (oxygen): 0.01% or less An element inevitably contained in steel. Combines with other elements to form oxides and nitrides. In particular, Al oxide and Al nitride are often formed. These compounds exist as inclusions in the steel and cause various properties such as a decrease in Charpy impact value, a decrease in machinability, and a decrease in specularity. In order to reduce these compounds, it is necessary to reduce the amount contained in steel, so N is 0.015% or less, and O is 0.01% or less. Although it is a balance with manufacturing cost, it is desirable that N: 0.01% or less and O: 0.005% or less be desirable.

Ni (%) / Al (%): 1.8-6
In order to obtain the effect of age hardening, it is necessary to be within the above range. When Ni / Al is smaller than 1.8, Al becomes excessive, so that Al oxide or Al nitride is formed as inclusions, resulting in a decrease in specularity and a decrease in Charpy impact value. On the contrary, when Ni / Al is larger than 6, since Ni becomes excessive, formation of retained austenite is promoted and hardness is lowered, so that the required hardness cannot be achieved.

C (%) + Si (%) / 15 + Mn (%) / 7 + Cr (%) / 6 + Mo (%) / 7: 0.2 to 0.7%
Both are elements that improve hardenability. When the total addition amount is smaller than 0.2, the hardness after the solution treatment becomes insufficient, and the required hardness cannot be obtained even with the highest hardness. On the contrary, when the total addition amount is greater than 0.7, the hardness after the solution treatment becomes higher than necessary, so that when the aging treatment is performed, the target Rockwell C scale hardness (HRC) is 26. The maximum hardness greatly exceeds the hardness of ~ 36. Therefore, it is necessary to adjust hardenability and adjust the hardness after the solution treatment. After solution treatment by component adjustment, the hardness is preferably 95 or more in terms of Rockwell B scale hardness (HRB) and 25 or less in terms of Rockwell C scale hardness (HRC).

  Next, the pre-hardened steel for plastic molds of the present invention is in mass%, S (sulfur): 0.15% or less, Pb (lead): 0.2% or less, Ca (calcium): 0.1. %, Mg (magnesium): 0.1% or less, Bi (bismuth): 0.3% or less, Se (selenium): 0.3% or less, Te (tellurium): 0.3% or less, Sn (tin) ): One or more of 0.05% or less can be contained.

  Any of them is an element that enhances machinability, and can be added when it is necessary to suppress degradation of other characteristics to some extent and enhance machinability. If each component is added in excess of the upper limit, the deterioration of properties other than machinability increases. In S, Se, and Te, the machinability is improved by combining with Mn to form inclusions. Ca and Mg form oxides and the like, and a machinability is improved by forming a tool protection film during cutting. Pb, Bi, and Sn exist in a dispersed manner in the steel, and the machinability is improved by forming a tool protective film.

  Next, the pre-hardened steel for plastic molds of the present invention is in mass%, W (tungsten): 0.5% or less, V (vanadium): 0.5% or less, Co (cobalt): 0.5 % Or less, Nb (niobium): 0.3% or less, Zr (zirconium): 0.5% or less, Ta (tantalum): 0.3% or less, Ti (titanium): 0.1% or less, B (boron) ): 0.01% or less, P (phosphorus): 0.2% or less, H (hydrogen): 0.01% or less, REM (rare earth element): 0.1% or less. be able to.

  W, V, Co, Nb, Zr, Ta, and Ti form carbides, which can prevent crystal grain coarsening during heat treatment due to the dispersion of the carbides and suppress a decrease in Charpy impact value. If the amount added is too large, the effect of suppressing reduction in machinability and Charpy impact value due to carbide is saturated, so it is necessary to keep it below each upper limit. P and H are elements that are inevitably included. However, in order to embrittle the crystal grain boundaries and lower the Charpy impact value, it is necessary to set the upper limit or less. Although it is a balance with manufacturing cost, it is desirable to set P ≦ 0.03%. B and REM can be added for the purpose of fixing the impurity elements such as O and P, increasing the cleanliness of the base, and improving the Charpy impact value. If it is added in a large amount, it becomes easy to generate ground, so it is necessary to make it lower than the upper limit.

(1) Melting Ingots of invention steels 1 to 15, comparative steels 1 to 8, and conventional materials 1 and 2 having the composition shown in Table 1 were produced. Note that a vacuum induction furnace is used for the inventive steel 15, and an atmospheric induction furnace is used for the other examples. In the following examples, secondary melting is further performed to produce ingots.
Invention steels 9, 10: Atmospheric induction furnace + VAR (vacuum arc remelting)
Invention steels 7 and 8: Atmospheric induction furnace + ESR (electroslag remelting)

(2) Forging In the state where the ingot was held at 1200 ° C. (hot working temperature), it was forged to a cross section of 250 × 250 mm. Thereafter, the product was cooled and forged to a cross section of 100 × 100 mm while being kept at 800 ° C. (warm temperature).

(3) Roughening of test piece Each test piece was roughly processed into the following shapes and dimensions.
Charpy test piece (JIS: Z2202: No. 3 test piece): Sampled from material T direction Machinability test piece (60 x 60 x 250 mm)
Specularity evaluation test piece (80 x 45 x 10 mm)
Welding repair specimen (80 x 80 x 30 mm)

(4) Heat treatment The following heat treatment was performed in order on the test piece.
(1) Solution heat treatment (ST): After holding at 850 ° C. to 1080 ° C. × 60 min, gas cooling (cooling rate: 1 ° C./s to 0.01 ° C./s)
(2) Age hardening heat treatment (AG): 200 ° C. to 640 ° C. × 1 hr to 8 hr, air cooling

(5) Precise processing of test pieces Each test piece subjected to the above heat treatment was precisely processed.

(6) Crystal grain size measurement The crystal grain size number was measured by the method defined in JIS G 0551 to confirm whether or not the crystal grain size was adjusted to a predetermined crystal grain size.

(7) Hardness measurement Rockwell C scale hardness (HRC) was measured by the method prescribed | regulated to JISZ2245, and it was confirmed whether it was adjusted to predetermined | prescribed hard hardness (HRC26-36). Various characteristic evaluations were performed in a state adjusted to a hardness within this range (indicated as “inspection hardness” in Table 2 described later).

(8) Machinability test The machinability was evaluated by the amount of tool wear during machining. The cutting tool uses a carbide end mill: UTi20T (tool diameter φ32, throw away, down cut), groove cutting with a depth of 3.5 mm, cutting width of 0.9 mm, cutting speed of 135 m / min, feed speed of 0.021 mm. The tool life (m) was evaluated with the tool life when the maximum wear width of the tool reached 400 μm under the / blade and dry conditions.

(9) Specularity Evaluation Test Specularity Evaluation By mechanical polishing with a diamond rotating grindstone, the grinding wheel count is finely divided in order of # 150 → # 400 → # 800 → # 1500 → # 3000 → # 8000, and mirror polishing is performed.
The surface roughness was measured at a reference length of 10 mm at 5 locations arbitrarily selected on the polished surface by the method defined in B 0601, and the arithmetic average roughness Ra was determined as the average value of the 5 locations.

(10) Charpy test A Charpy impact test was performed at room temperature on the Charpy test piece by the method defined in JIS Z 2242.

(11) Welding repair test A groove with a width of 30 x height of 15 x length of 80 mm was carried out at the center of the test piece, and multilayer overlay welding was carried out at this part. After overlay welding, photograph the appearance and measure the number of cracks. Moreover, the cross-sectional hardness of the welding peripheral part was measured, and it was judged whether the difference of the hardness of a periphery and a welding part was less than HRC5.

Table 2 shows the measurement and test results of (6) to (11).
(Comparative Examples 1 and 2)
These satisfy the composition requirements of the present invention, but have a grain size number of less than 4. Charpy's impact value is inferior to that of invention steel.
(Comparative Examples 3 to 8)
These have a crystal grain size number of 4 or more, but do not satisfy any of the composition requirements of the present invention. Any one of machinability, Charpy impact value, and specularity is inferior to that of the inventive steel, or the hardness is too high.

  As described above, the pre-hardened steel for plastic molds of the present invention has a Rockwell C scale hardness HRC of 26 or more and 36 or less, excellent machinability, and Charpy impact value, specularity, welding Since it has sufficient characteristics from the viewpoint of repairability, it is possible to obtain merits such as shortening the mold production and delivery time, reducing the processing cost, and improving the mold life.

Diagram showing the hardness distribution around the weld Appearance observation around the weld Diagram showing the relationship between hardness and elements that improve hardenability Schematic representation of the mold edge

Explanation of symbols

A Sharp corner R Corner

Claims (4)

In mass%, C: 0.01-0.2%, Si: 0.1-2%, Mn: 0.1-2%, Cu: 0.3-2%, Ni: 1.5-3. 5%, Cr: 0.01-2%, Mo: 0.01-2%, Al: 0.2-1.5%, N: 0.015% or less, O: 0.01% or less With
Ni (%) / Al (%) is 1.8 or more and 6 or less, and C (%) + Si (%) / 15 + Mn (%) / 7 + Cr (%) / 6 + Mo (%) / 7 is 0.2% or more 0.7% or less,
A plastic molding die comprising a balance Fe and inevitable impurities, having a crystal grain size number of 4 or more as defined in JIS G 0551, and a Rockwell C scale hardness (HRC) of 26 or more and 36 or less. For pre-hardened steel. The pre-hardened steel for plastic molds according to claim 1, wherein the Charpy impact value is 20 J / cm ² or more.   In mass%, S: 0.15% or less, Pb: 0.2% or less, Ca: 0.1% or less, Mg: 0.1% or less, Bi: 0.3% or less, Se: 0.3% The pre-hardened steel for plastic molds according to claim 1 or 2, wherein one or more of Te: 0.3% or less and Sn: 0.05% or less are contained.   In mass%, W: 0.5% or less, V: 0.5% or less, Co: 0.5% or less, Nb: 0.3% or less, Zr: 0.5% or less, Ta: 0.3% Hereinafter, Ti: 0.1% or less, B: 0.01% or less, P: 0.2% or less, H: 0.01% or less, REM: containing one or more of 0.1% or less The pre-hardened steel for plastic molds according to any one of claims 1 to 3.