JP2000169932A - Steel for plastic molding die excellent in machinability and weldability and its production - Google Patents

Abstract

PROBLEM TO BE SOLVED: To impart excellent machinability and weldability to the steel by preparing steel for a die having a specified componential compsn. and a specified structure, and in which 6 hardness calculated from the average cooling rate at the time of quenching or the like and temp. and time at the time of tempering is specified. SOLUTION: Steel having a compsn. contg., by weight, 0.07 to O.20% C, 0.5 to 2.0% Mn, 1.5 to 2.5% Cr, 0.01 to 1.0% Mo, 0.01 to 0.2% V and <=0.1% S (including 0%) and 0.38 to 0.90% Si and having a structure of ferrite + bainite or ferrite + pearlite + bainite is prepd. At this time, in the case the average cooling rate to 500 deg.C at the time of quenching or normalizing is defined as R ( deg.C/min), and P=T (20 + logt)×10-3 [T denotes tempering temp. (K), and (t) denotes tempering time (hr)], its hardness calculated from the formula (in the formula, [element] denotes the containing ratio % of each element) satisfies (HB)<=240 in the case P=20 and R=10 and satisfies (HB)>=160 in the case P=17 and R=0.3.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plastic molding die steel excellent in machinability and weldability and a method for producing the same.

[0002]

2. Description of the Related Art Plastic molding dies are made by machining steel for plastic molding dies into a predetermined shape by cutting.
It is produced by forming a grain pattern and performing mirror finishing. Conventionally, carbon steel such as S55C and medium-low carbon steel such as SCM have been used as such plastic molding dies, especially large dies. In the manufacturing process of the molding die,
First, it is desired to improve machinability in order to increase the efficiency of machining, but it is difficult to say that conventionally used mold steel such as S55C has sufficient machinability. In addition, JP
Japanese Unexamined Patent Publication No. 202050 proposes a technique of adding a specific amount of S in order to improve the machinability, but such technique does not provide sufficient machinability.

In the manufacture of a mold, repair by welding is sometimes performed due to processing error or design change. In order to prevent welding cracks, the repair requires considerably high preheating and postheating. However, there is a problem that a dedicated heating furnace is required for uniform heating, and that a larger mold requires a longer time for preheating.

Recently, as the design of plastic products has been improved, it has been required to improve the texture of mold steel. For this reason, homogeneous steel produced by forging is desired especially for large dies.

[0005]

SUMMARY OF THE INVENTION The present invention has been made in view of the above problems, and provides a steel for a plastic molding die which has excellent machinability and weldability and does not require preheating and postheating, and a method for producing the same. It is intended to do so.

[0006]

According to the present invention, the weight%
And C: 0.07 to 0.20%, Mn: 0.5 to 2.0
%, Cr: 1.5 to 2.5%, Mo: 0.01 to 1.0
%, V: 0.01 to 0.2%, S: 0.1% or less (0%
And Si: a steel containing 0.38 to 0.90%, having a structure of ferrite + bainite or ferrite + pearlite + bainite, and averaged up to 500 ° C. during quenching or normalizing. Set the cooling rate to R (° C / mi
n), where P is the value calculated from the following equation: P = T (20 + logt) × 10 ⁻³ (where T is the tempering temperature (K) and t is the tempering time (hr)) The calculated hardness (HB) calculated from (1) is P =
It is desirable that (HB) ≦ 240 is satisfied when R = 10 and R = 10, and (HB) ≧ 160 when P = 17 and R = 0.3. (HB) = 793.5 [C] +9.7 [Si] +188.5 [Mo] +39. 7 [Cr] +44.8 (logR) -19.4P + 330.7 (1) (where [element] represents the content (%) of each element in the steel)

Further, according to the present invention, C: 0.
07 to 0.20%, Mn: 0.5 to 2.0%, Cr:
1.5-2.5%, Mo: 0.01-1.0%, V:
A steel containing 0.01 to 0.2%, S: 0.1% or less (including 0%), and Si: 0.38 to 0.90%,
In the hot working process, forging is performed so that the forging ratio becomes 2 or more, then, at 200 to 350 ° C. for preventing cracking, then reheating and holding at 800 to 950 ° C., and normalizing, The present invention provides a method for producing steel for a plastic molding die, which is excellent in machinability and weldability, in which it is air-cooled and then reheated to 550 to 680 ° C. and tempered.

At this time, the balance of the steel is preferably substantially Fe and unavoidable impurities.

[0009]

BEST MODE FOR CARRYING OUT THE INVENTION The present inventors have conducted intensive studies as to whether or not steel for plastic molding dies having excellent machinability and weldability can be obtained. As a result, the C content was kept low and the Si content was reduced. The machinability and weldability are improved by making the chemical composition larger than before, the structure to be ferrite + bainite or ferrite + pearlite + bainite, and the calculated hardness (HB) under specific production conditions to a specific range. The present inventors have found that a steel for plastic molding dies having excellent properties can be obtained, and have accomplished the present invention.

In the steel for a plastic molding die of the present invention, the function of each alloy element and the reason for limiting the composition range are as follows.

Si: 0.38 to 0.90% Si acts as a deacidifying element at the time of melting and is an important element for increasing the strength.
An oxide film having a low melting point mainly composed of O ₂ is generated, and this oxide film acts as a lubricant between the cutting tool and the workpiece to reduce wear of the cutting tool. It also has the effect of slightly reducing toughness, thereby improving the processing of chips during cutting. In order to exert such an effect effectively, it is necessary to contain 0.38% or more. On the other hand, if it is contained in a large amount, there is a problem that the hardness of the steel increases and the cutting resistance increases, so it is necessary to make it 0.90% or less. A more preferred range is 0.38 to 0.80%.

C: 0.07% to 0.20% C is an indispensable element as a strengthening element for steel to secure strength and enhance hardenability. When the amount of C is less than 0.07%, C is sufficient. High strength cannot be obtained. On the other hand, if the C content is excessively large, the weldability is impaired, segregation in the steel ingot is promoted, and further, carbides are formed and the machinability deteriorates, so the C content is suppressed to 0.20% or less. It is necessary. In order to satisfy all of hardness, strength, weldability and machinability, the C content is more preferably in the range of 0.10 to 0.17%.

Mn: 0.5 to 2.0% Mn is an element which effectively acts on the deoxidation and hardenability of steel, and fixes S unavoidably mixed into steel. It also works effectively to improve machinability. In order to effectively exert such an effect, the content must be 0.5% or more. On the other hand, if the content is large, the steel structure is hardened and the machinability deteriorates, so it is necessary to suppress the content to 2.0% or less. The more preferred content of Mn is 0.8 to 1.9%.

Cr: 1.5 to 2.5% Cr is an element effective for improving the hardenability and, in addition, in the present invention, is an element that complements the lack of strength due to the reduced C content. , 1.5% or more. On the other hand, if the Cr content is too large, the steel becomes hard and the machinability decreases, so it is necessary to suppress the content to 2.5% or less. To satisfy all of the hardenability, reinforcing effect, machinability, etc.,
The range of 1.7 to 2.1% is more preferable.

Mo: 0.01% to 1.0% Mo is an element which contributes to high strength by enhancing hardenability similarly to Cr, and therefore it is necessary to contain Mo in an amount of 0.01% or more. On the other hand, when Mo is contained in a large amount, a large amount of carbide is generated and the machinability is deteriorated.
%. A more preferred range is 0.05 to 0.5%.

V: 0.01% to 0.2% V is effective in improving the resistance to temper softening and ensuring necessary hardness. There is also an effect of making crystal grains fine. In order to exert such an effect, it is necessary to contain 0.01% or more. On the other hand, if it is contained in a large amount, carbides are generated in a large amount and the machinability is deteriorated. A more preferred range is from 0.03 to 0.1.
1%.

S: 0.1% or less (including 0%) As described above, MnS reacts with Mn to generate MnS and contributes to improvement of machinability. It is necessary to keep it as small as possible, and it is necessary to make it 0.1% or less. More preferably, it is 0.07% or less.

It is also important that the mold steel of the present invention has a structure of ferrite + bainite or ferrite + pearlite + bainite under the condition that the above requirements for the component composition are satisfied. Machinability can be improved by using a two-phase structure or a three-phase structure including a soft ferrite structure. In order to further improve machinability, the area ratio of ferrite is preferably in the range of 10 to 50%.

The calculated hardness (HB) calculated from the above equation (1) is 240 or less when P = 20 and R = 10,
It is desirable to adjust the chemical composition within a specified range so that when P = 17 and R = 0.3, it becomes 160 or more.

The above equation (1) is an experimentally obtained relation between the hardness and the composition of the steel obtained by variously changing the heat treatment conditions in the steels of various compositions. It is an index to show. According to this equation, the optimum range of chemical components can be determined from the average cooling rate R up to 500 ° C. during quenching or normalizing, and P calculated from the temperature and time during tempering. The reason for calculating the hardness when P is 20 or 17 is that the range of 17 to 20 is a range that can be actually manufactured industrially. When the tempering condition is the hardest, and when P is 20, the tempering condition is the softest. In addition, when the average cooling rate R is 10, 0.3, the hardness is calculated in the case of forging steel mold steel, the thickness is generally in the range of 100 to 1,000 mm. This is because the corresponding average cooling rate is in the range of 10 to 0.3 ° C./min. When R is 0.3, the cooling rate is the softest, and when R is 10, the cooling rate is the hardest. . As understood from the above, P = 20, R = 10 and P =
17, when R = 0.3, a thickness of 100 to 1,000
These are the conditions under which the hardness of the steel in the range of mm is the hardest and the softest.

In the composition of the first aspect, the structure of ferrite + bainite or ferrite + pearlite + bainite is desirably obtained by the following production method. That is, in the hot working step, forging is performed so that the forging ratio becomes 2 or more, then, after holding at 200 to 350 ° C., normalizing by heating and holding again at 800 to 950 ° C., and then air cooling. A method in which tempering is performed by reheating to 550 to 680 ° C is desirable.

First, in the hot working step, the forging is performed until the forging ratio becomes 2 or more, whereby the voids inside the steel ingot are pressed and sufficient strength and toughness can be obtained.

Next, by maintaining the temperature in the range of 200 to 350 ° C., cracks and the like caused by the untransformed structure (hard portion) are prevented. At this time, if the holding temperature is less than 200 ° C., there is a possibility that low-temperature cracking may occur, while the holding temperature is 35
If the temperature exceeds 0 ° C., there arises a problem that bainite transformation is not completed. The holding time is not particularly limited, but is preferably 20 to 50 hours.

Further, by normalizing while reheating and holding the temperature to 800 to 950 ° C., the structure becomes fine and a predetermined strength can be obtained. If the normalizing temperature is lower than 800 ° C., there arises a problem that the distribution of crystal grains becomes non-uniform,
On the other hand, if the normalizing temperature exceeds 950 ° C., there arises a problem that the crystal grains become coarse. The normalizing time is not particularly limited, but is preferably 2 to 30 hours.

Further, after air cooling, the hardness is reduced by reheating to 550 to 680 ° C. and tempering, whereby desired machinability can be obtained. If the tempering temperature is lower than 550 ° C., there is a problem that the hardness is high and the machinability is deteriorated, and the residual stress is increased. On the other hand, the tempering temperature is 680 ° C.
If it exceeds 2,000, the required hardness cannot be obtained. The tempering time is not particularly limited, but is preferably 2 to 30 hours.

[0026]

EXAMPLES The present invention will be described below with reference to experimental examples.
The present invention is not limited by these experimental examples.

Experimental Examples 1 to 11 A steel ingot having the chemical composition shown in Table 1 was melted in an electric furnace, and then hot forged to a forging ratio of 2. Next 2
After holding at 70 ° C. for 24 hours, the sample was heated and held at 880 ° C. to perform normalizing, air-cooled, and then reheated to 600 ° C. and tempered to prepare a test material.

The measured hardness (HB), calculated hardness (HB), machinability, maximum weld hardness, and the presence or absence of weld cracks in the case of no preheating / post-heating are shown below. Measured and investigated. Table 2 shows the results.

(Measured Hardness (Brinell Hardness)) JIS Z
It measured according to -2243.

(Machinability test) Using a high-speed end mill with a diameter of 10 mm, the test material was cut at a cutting speed of 21 m / min and a feed speed of 260 mm / min, and the flank wear (mm) was measured. 2. Using a carbide end mill having a diameter of 30 mm, the test material was cut at a cutting speed of 99 m / min and a feed speed of 260 mm / min, and the flank wear (mm) was measured.

(Highest welding hardness (HV)) JIS Z31
The highest welding hardness was measured in accordance with No. 01.

(Weld Cracking) Welding was performed on the corners of the L-shaped material, and the presence or absence of cracks was investigated.

EXPERIMENTAL EXAMPLE 12 A large ingot corresponding to an actual product having the chemical composition shown in Table 1 was melted in an electric furnace, and then hot forged to a forging ratio of 2, and then heated at 200 to 350 ° C. for 24 hours. After holding for 8 hours,
It heats to 00-950 degreeC, and normalizes. Then 55
Tempering was performed at 0 to 680 ° C. Physical properties were measured and evaluated as in Experimental Examples 1 to 11. Table 2 shows the results.

[0034]

[Table 1]

[0035]

[Table 2]

The mold steels of Experimental Examples 1 to 5 having the elemental composition and structure specified by the present invention and having a predetermined hardness have a flank wear amount of 0 in a machinability test using a high-speed end mill. 0.07 mm or less, showing a good result. Even in a cutting test using a carbide end mill, the flank wear was 0.18 mm.
The following results were excellent. The maximum welding hardness is 505
The results were as low as below, and no weld cracking occurred.

On the other hand, the die steels of Experimental Examples 6 to 11 in which the content of Si was smaller than the specified range of the present invention had a flank wear of 0.09 mm in the machinability test using a high-speed end mill. As described above, even in the cutting test using the carbide end mill, in Experimental Examples 6, 9, and 10, the wear amount of the flank was 0.20.
mm or more, and when the Si content ratio is small, the amount of wear increases. Further, the mold steels of Experimental Examples 10 and 11 in which the C content ratio was larger than the specified range of the present invention had a maximum weld hardness of 70.
It was as high as 0,850 and weld cracking also occurred.

In Experimental Example 12 in which a mold steel was actually manufactured using a large ingot corresponding to a product, U
No poor T or cracking due to heat treatment after training was observed.
Various properties such as hardness, abrasion amount, and maximum welding hardness were also good. Thereby, the graining and the electric discharge machining applied to the mold were also good. Further, the hardness distribution of the cross section was uniform.

[0039]

According to the present invention, the steel for plastic molding dies is
It excels in machinability and weldability, and does not cause welding cracks without preheating and post-heating.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Yasumasa Yoshida 2-3-1 Shinhama, Araimachi, Takasago City, Hyogo Prefecture Inside Kobe Steel, Ltd. Takasago Works (72) Inventor Toshiyuki Minade 2, Araimachi Shinama, Takasago City, Hyogo Prefecture No.3-1 F-term in Kobe Steel, Ltd. Takasago Works (reference) 4F202 AJ07 AJ11 CA30 CB01 4K032 AA04 AA05 AA12 AA16 AA19 AA23 AA29 AA31 AA36 BA00 CF03

Claims (2)

[Claims] C: 0.07 to 0.20%, Mn: 0.5 to 2.0%, Cr: 1.5 to 2.5%, Mo: 0.01 to 1. 0%, V: 0.01 to 0.2%, S: 0.1% or less (including 0%) Si: 0.38 to 0.90%, ferrite + bainite or ferrite + Perlite +
It has a structure of bainite, and the average cooling rate to 500 ° C. during quenching or normalizing is R (° C./min),
When the value calculated from the following equation is P, P = T (20 + logt) × 10 ⁻³ (where T is the tempering temperature (K) and t is the tempering time (hr)) The following equation (1) The calculated hardness (HB) calculated from the following equation satisfies (HB) ≦ 240 when P = 20 and R = 10,
A steel for a plastic molding die excellent in machinability and weldability that satisfies (HB) ≧ 160 when P = 17 and R = 0.3. (HB) = 793.5 [C] +9.7 [Si] +188.5 [Mo] +39. 7 [Cr] +44.8 (logR) -19.4P + 330.7 (1) (where [element] represents the content (%) of each element in the steel) 2. In% by weight, C: 0.07 to 0.20%, Mn: 0.5 to 2.0%, Cr: 1.5 to 2.5%, Mo: 0.01 to 1. 0%, V: 0.01 to 0.2%, S: 0.1% or less (including 0%) Si: 0.38 to 0.90% Forging to 2 or more, then holding at 200-350 ° C, heating and holding again at 800-950 ° C, normalizing, air cooling, and reheating to 550-680 ° C A method for producing steel for plastic molding dies which is excellent in machinability and weldability to be returned.