JPS63162811A - Manufacture of precipitation-hardening steel - Google Patents

Abstract

PURPOSE:To manufacture a precipitation hardening-type structural steel improved in crimping workability, by heating a steel containing specific percentages of C, Si, Mn, Ni, Mo, Cu, and Al and by subjecting the above steel to hot working and aging treatment under specific conditions and then to cooling. CONSTITUTION:The steel containing, by weight, 0.05-0.20% C, 0.1-2.0% Si, 0.5-2.1% Mn, 0.5-3.5% Ni, 0.05-0.6% Mo, 0.5-2.0% Cu, and 0.4-1.5% Al is heated up to about 950-1,300 deg.C. Subsequently, without being hot-rolled or after hot-rolled, the above steel is hot-worked at 700-900 deg.C at 10-40% draft, held at 450-550 deg.C for 1-100hr to undergo aging treatment, and then cooled. In this way, the precipitation-hardening steel improved in crimping workability and used for metal mold for plastic molding can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for manufacturing precipitation hardening structural steel, which is mainly used as a mold, particularly a plastic mold.

(Prior art) Conventionally, molds for plastic molding are made of carbon steel for shell structure (JIS-3C, SS, etc.), low alloy steel (JIS-S0
M improved steel), high alloy @ (JIS-SKD, SO5, etc.) have been used. In particular, carbon steel for shell structures and low alloy steel (SCM type) have been the mainstream as mold materials for large-sized plastic molding, mainly from the viewpoint of cost.

In recent years, as plastic products have become larger, the molds used have inevitably become thicker. However, although the above-mentioned carbon steel for shell structures can be manufactured to a wall thickness of several hundred centimeters, the hardness is as low as about 200 Hv, so the mold life is short.

Therefore, when a longer life than before is required, Hv
Low alloy @ (SCM type) that can secure around 300 was used. However, SCM steel has a wall thickness of 200 mm.
If the thickness exceeds mm, a ferrite phase will begin to form, making it difficult to ensure hardness when the wall becomes thicker, and problems will also arise in terms of uniformity since hardness is largely dependent on the cooling rate. was. Furthermore, since SCM steel is a high C steel, weldability is poor. Molds are frequently welded for design changes, repairs, etc., so heating work before and after welding,
A mold material that can reduce weld cracking is desired.

In addition, in the case of precipitation hardening steels that have been proposed so far, a softening region is generated in the heat affected zone (HAZ) when welding, and this HAZ
The Vickers hardness of AZ is more than 100 lower than that of the base material. Therefore, normally, these softened regions are re-hardened by post-heat treatment (which also serves as aging treatment) for several hours, and the hardness is adjusted to approximately that of the base material. however,
When considering only large molds, the above-mentioned post-heat treatment is difficult.

Therefore, recently, we have improved the hardness and weldability, and in particular, the hardness of the HAZ part shows almost the same characteristics as the base metal even as welded, and it is especially suitable for thick mold materials as plastic casting products increase in size. It is desired to develop a steel material that can solve the above-mentioned difficulties even when used. In this regard, in the case of conventional age-hardening plastic mold steels with improved weldability and electrical discharge machinability, Cu, AI! , and Ni are added to ensure strength through aging precipitation. However, Cu and Ni are also elements that improve hardenability, so when reducing costs by reducing these elements, ferrite tends to form in the structure before aging, which inhibits aging properties. . This is a fatal flaw in steel for large plastic molds because the cooling rate after heating is extremely slow. Furthermore, since the hardness of the welded part becomes uneven, post-heat treatment must be performed, which is a major drawback when used as steel for large plastic molds.

(Problems to be solved by the invention) In order to solve the above problems, the addition of a trace amount of -〇
Even when , Ni, and Al21 are low, the bainite structure is secured even at a sufficiently slow cooling rate as a steel for large molds,
Based on the knowledge that it contributes to age hardening, C: 0.05~
0.20% by weight, Si: 0.1-2.0% by weight,
Mn: 0.5-2.1% by weight, Ni: 0.5-1
．． 4-layered view% weight o: 0.05-0.6% by weight, Cu
: 0.5-2.0% by weight, Af: 0.4-1.5
We have developed an age-hardening structural steel containing %wt.

The above-mentioned precipitation hardening structural steel utilizes the synergistic effect of the precipitation of the ε-Cu phase and the NiAl phase, and is usually hot-worked, solution-treated, and then subjected to aging treatment. However, in the above-mentioned heat treatment step, in order to perform an aging treatment after the solution treatment, heating is required again, which increases the cost. In addition, if the cooling rate after processing is faster),
This results in a two-phase structure of martensite and bainite, which leads to a decrease in texturability when the precipitation hardening steel is used, for example, as a plastic mold.

An object of the present invention is to reduce manufacturing costs by omitting reheating in the heat treatment process, and to improve texturing property by suppressing the formation of martensite and creating a bainite phase structure.

(Means for solving the problems) The present invention provides C: 0.05 to 0.20% by weight, Si: 0
．． 1 to 2.0% by weight, Mn: 0.5 to 2.1% by weight, Ni: 0.5 to 3.5% by weight, Mo: 0
．． Steel containing 0.05 to 0.6% by weight, Cu: 0.5 to 2.0% by weight, and Affi: 0.4 to 1.5% by weight was heated and hot rolled or not hot rolled. After, 700~9
A method for producing precipitation hardened steel, characterized in that the steel is processed by 10 to 40% in a temperature range of 0.00"C, then held in a temperature range of 450 to 550C for 1 to 100 hours, and then cooled. solved the problems mentioned above.

(Function) The present inventors have determined that C: 0.09% by weight, Si: 0
．． 41% by weight, Mn: 1.3% by weight, Ni: 1.3% by weight, Mo: 0.41% by weight, Cu: 1.4% by weight
, Affi: After vacuum melting steel containing 0.7% by weight,
Following the pattern shown in Figure 1, after heating to 1200℃,'
Pre-rolling to a thickness of 100ffllll, holding for 1 hour, applying a 50% reduction in a temperature range of 1050'C or higher, and then applying a reduction of 3% at various temperatures ('r+).

One-pass rolling was performed at 7%, 13%, and 20%, and heat treatment was performed by holding at various holding temperatures (T2) for 3 hours and then cooling.

The steel obtained by the above heat treatment process and the HRC = 30° bainyl obtained by the normal solution temporary treatment shown in Fig. 2.
The results of comparison with -ff190% steel are shown in Figures 38 to 3d. Figure 3a shows the results when the rolling reduction rate at T is 3%, and Figure 3b shows the results when the rolling reduction rate at T1 is 7%.
Figure 3c shows the results when the rolling reduction rate at T1 is 13%, and
Figure d shows the results when the rolling reduction rate at T1 is 20%, and the O mark in each figure indicates the normal solution temporary treatment (hereinafter referred to as
This indicates that the amount of bainite is the same as that of the normal heat-treated material, and the Δ mark indicates that the amount of bainite is greater than that of the normal heat-treated material, and the black mark indicates that the hardness has increased. This indicates that the hardness is equivalent to or higher than that of normal heat treatment, and the 0 mark indicates that the hardness is lower than that of normal heat treatment. As can be seen from each figure, 10% or more rolling is performed at a rolling temperature (T, ) of 700 to 900°C, and the holding temperature (T2) is 450 to 550°C.
compared to steel obtained by normal heat treatment,
It can be seen that a structure close to a bainite phase was obtained, and the mechanical properties were also the same.

That is, the present invention achieves bainite transformation, ε-Cu phase, and N1Aj! by appropriate strain during hot working after hot rolling.
By promoting phase precipitation, it is possible to reduce martensite and create a bainite phase structure. For this purpose, it is necessary to perform 10 to 40% hot working in a temperature range of 700 to 900°C, immediately maintain the temperature in a temperature range of 450 to 550°C, and then cool it.

The reasons for limiting the components and manufacturing conditions of the present invention will be described below.

CTC is an element necessary to ensure hardenability and hardness of the hardened phase, and for this purpose, it is required in an amount of 0.05% by weight or more. However, if it is too large, weldability, electrical discharge machinability, etc. will be impaired, so the upper limit is set to 0.20% by weight.

Si: St is used as a deoxidizing element for steel and is required in an amount of 0.1% by weight or more. However, if the amount is too high, the ductility and toughness will deteriorate, so the upper limit is 2.0% by weight.
shall be.

Mn: Mn is added in an amount of 0.5% by weight or more in order to increase the aging hardness after solution treatment, but if it is too large, weldability and hot workability are inhibited, so the upper limit is 2.1%.
Weight%.

Ni: Ni improves hardenability, prevents reduction in hot workability due to grain boundary precipitation of Cu, and contributes to age hardening by precipitating N1Affi phase coexisting with A2 in the bainite phase. For this purpose, it is necessary to add 0.5% by weight or more, but if added in excess, the hardness will increase and the difference in hardness of the weld will become too large, so the upper limit is set to 3.5% by weight.

Mo: Mo is an element that contributes to hardenability and precipitation hardening, and for this purpose it is necessary to add 0.05% or more. On the other hand, if it is too large, the cost will increase, so the upper limit is set to 0.6% by weight.

Cu: Cu precipitates as an ε-Cu phase in the bainite structure and contributes to age hardening, so it is necessary to add 0.5% by weight or more. However, if the content is too large, hot workability will be inhibited, so the upper limit is set to 2.0% by weight.

8l:^2 coexists with Ni and precipitates the N1Af phase, contributing to age hardening. For this purpose, it is necessary to add 0.4% by weight or more, but if it is too large, hot workability will be inhibited, so the upper limit is set at 1.5% by weight.

Furthermore, even if the following components are added, the effects of the present invention will not be impaired.

S, Pb, Se, Te, Bi: S, Pb
, Se, Ti, and Bi are all elements that improve machinability, and for this purpose S≦0.3% by weight, p
b≦0.5% by weight, Se≦0.5% by weight, Te≦0
．． 5% by weight, any one of Bi≦0.5% by weight
A species or two or more species may be added in an amount of each species as an upper limit.

Ti+ L Nb+ Tan Zr: Ti, L
Elements promoting grain refinement such as Nb+Ta and Zr may be added to improve toughness. Each additive element is Ti≦0.5
Weight%, ■≦0.5% by weight, Zr≦0.3% by weight,
One or more types of Nb+Ta≦0.5% by weight may be added.

Cr: Cr is an element necessary to improve corrosion resistance, and is preferably added in an amount of 5.0% by weight or less.

When heating steel with the above-mentioned composition, the heating temperature should be set to 9.
The range is 50″C to 1300″C. That is, 950℃
If it is less than 1,300°C, it is hot working, and if it exceeds 1,300°C, there is a risk of deterioration in hot workability due to scale formation or grain boundary melting, so the heating temperature range is set in the range of 950°C to 1,300°C.

In addition, if the hot working temperature is less than 700°C, residual stress (strain will increase), and if it exceeds 900°C, the hardness will decrease and the mechanical properties will deteriorate, so the hot working temperature range is The temperature should be in the range of 700 to 900°C.

Next, if the hot working rate at the above hot working temperature is less than 10%, the strain to promote bainite transformation will be insufficient, and if the hot working rate exceeds 40%, it will cause deformation of the steel plate. Therefore, the hot working rate is set in a range of 10 to 40%.

Next, as for the aging treatment time, it is sufficient if it is 1 hour or more, but if it exceeds 100 hours, it will be overaged, so 1 to 10 hours or more is sufficient.
Let it be 0 hours.

(Example) C shown in Table 1: 0.12% overlapping, St: 0.6
5 weight, Mn: 1.3% by weight, Ni: 2.11% by weight, Mo: 0.25% by weight, Cu: 1.1% by weight, Aj! : Steel containing 1.2% by weight, C: 0.07
Weight %, Si: 0.41 weight %, Mn: 1.5
Weight%, Ni: 1.4% by weight, Mo: 0.43
After vacuum melting, steel containing Cu: 1.0 wt% and Al: 0.8 wt% was heated to 1200"C, and the thickness was 10% by weight.
Preliminary rolling was carried out so that it became 0 tm. Thereafter, as shown in Figure 1, after holding at 1200°C for 1 hour and rolling 50% to 1050°C, each hot rolling temperature shown in Table 1 (
After hot working at T, ) at each rolling reduction rate shown in the same table, the samples were held at the temperature (T2) shown in the same table for each holding time, and then cooled.

For comparison, the steel shown in Table 1 was heated to 860°C, held for 2 hours, cooled at a cooling rate of 30"C/min, heated again to 500°C, held for 3 hours, and then cooled. did.(
Shown in Figure 2. ) Table 2 shows a comparison of the properties of the materials obtained by each heat treatment. As can be seen from the same table, Nαl-3 and N(L 10-12, which satisfy the manufacturing conditions of the present invention, have the same hardness and more bainite than the treated materials that have undergone normal heat treatment. ing.

(Effects of the Invention) As explained above, according to the present invention, the amount of bainite phase is increased, so the texturing property is improved, and the heat treatment step is shortened, so the delivery time of the product is also shortened.

[Brief explanation of the drawing]

FIG. 1 is an example showing a heat treatment pattern of the present invention, FIG. 2 is a diagram showing a normal heat treatment pattern, and FIGS. It is a figure showing the relationship between temperature (T, ) and aging holding temperature (T2), and the amount of bainite and hardness. Figure 1 hU work 11ftWtY'l-7 Figure 2 (a) 600 9QQ 1000 1: Rolling 1 degree ('c) (Reduction 1!3%) 600 goo 1000η: Rolling 1 degree ('c)
'C) (Pressure 1'$7%) Figure 13 (C) soo boo wo. Rolling Jilt ('c) (pressure 7

Claims (1)

[Claims] 1. C: 0.05-0.20% by weight, Si: 0.1-2.0% by weight, Mn: 0.5-2.1% by weight, Ni: 0.5-0.5% by weight. 3.5% by weight, Mo: 0.05-0.6% by weight, Cu: 0.5-2.0% by weight, and Al: 0.4-1.5% by weight. After hot rolling or without inter-rolling, 10-40% processing is performed in a temperature range of 700-900°C, and then 450-400°C
A method for producing precipitation hardened steel, which comprises maintaining the temperature in a temperature range of 550°C for 1 to 100 hours, and then cooling.