JPS6227517A - Manufacture of high tensile cast steel - Google Patents

Abstract

PURPOSE:To manufacture high tensile cast steel with a low cost, by holding cast steel having a specified compsn. composed of C, Si Mn, Cr, Ni, Mo, Al and Fe at austenitizing temp., then applying suitable quenching and tempering thereto. CONSTITUTION:Cast steel having a compsn. composed of, by weight 0.20-0.5% C, 0.30-0.60% Si, 0.50-1.60% Mn, 0.80-1.50% Cr, 1.50-2.50% Ni, 0.60-1.00% Mo. <=0.060% Al and the balance Fe with inevitable impurity is held at 800-1,000 deg.C austenitizing temp. Thereafter, quenching or normalizing treatment in which 800-400 deg.C range is cooled at >=20 deg.C/min average rate is applied to cast steel at least >= one time, then it is tempered in 540-620 deg.C range. In this way, high tensile cast steel having >= about 100kgf/mm<2> strength is obtd. inexpensively with a low alloying element quantity, without accompanying deterioration of low temp. toughness.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) High tensile strength cast steel or especially high tensile strength steel having a tensile strength of 100 kgf/- or more and used mainly for structures advantageously used in offshore development [Related to 11] This specification describes the results of research and development aimed at advantageously increasing the tensile strength by suppressing alloy components.

(Prior Art) The present invention relates to high-strength cast steel having a tensile strength of 100 kgfz"m or more.
GF/D class high tensile strength cast steel, for example,
Although disclosed in Japanese Patent No. 2885, a high tensile strength cast steel that satisfies a tensile strength of 100 bf/i has not yet been developed.

The inventors first developed a thick-walled, high-strength cast steel with a tensile strength of 101051 C/m or more and a low-temperature toughness of 2.8 kgfrn or more at -20°C (Patent No. 58-200528 and Japanese Patent Application No. 59-122910). However, these are so-called high-alloy cast steels, and the grade is high, so the manufacturing cost is also high.

(Problem to be solved by the invention) Even though the thermal toughness is not required, only the strength is required (,! at least iook).
The aim of the present invention is to reduce the alloying element (2) as much as possible to reduce the cost of high-strength cast steel that requires gf/m.

<Means to solve the problem) The five main alloying elements are <C, Si, Mn, and P.

Besides S), Cr is commonly added.

Adding Nl, MO, etc., without adding special trace additive elements such as B, Nb, etc. In other words, reducing the types and addition values of alloying elements, and instead setting appropriate heat treatment conditions. 100kg f/, without significant deterioration of toughness! ! 1 or more high tension HS74
To achieve this, the following configuration is essential.

C: Q, 20-0.50 wt%.

Si: 0,30~o, eowt%.

Mn: 0.50 to 1.60 wt%.

Cr: 0.80 to 1.50 wt%.

\i: 1.50 ~ 2.50wt%Mo
: 0.60 ~ 1.00wt%△, Q:
Cast steel containing 0.060 vt% or less and the remainder consisting of Fe and unavoidable impurities is held at an austenitizing temperature of 850 °C to 1 ooo °C, and then heated at a temperature range of 800 °C to 400 °C at 20 °C/min. Cool with the above average cooling rate! A method for producing high-strength cast steel, which comprises subjecting the steel to at least one quenching or normalizing treatment, and then tempering within a temperature range of 5110°C to 620°C.

(Function) C: 0.20 to 0.50wt% If less than 0.20wt%, tensile strength is 100kgf/m
It is not possible to secure strength greater than f, while o, sow
If it exceeds t%, the toughness will be impaired.

S i O, 30-0.60wt% To improve castability and strength, 0.30wt% or more is required, but 0.6
If it exceeds 0 wt%, the toughness will deteriorate.

Mn: 0.50 to 1.60wt% Known as an element that improves hardenability, it is necessary to take full advantage of this effect and to take into account that if too much Mn is present, segregation to grain boundaries will deteriorate toughness. The content was set at 0.50 to 1.60 wt%.

Cr: 0.80 to 1.50 wt% hardenability improving element, it is particularly effective in increasing strength, so it is one of the main components, but if it is less than 0.80 wt%, it is not effective.
Even if it exceeds 1.50 wt%, the strength improvement effect does not become large, so the ophthalmic power is set in the range of 0.8 to 1.50 wt%.

Nl: 1.50 to 2.50wt% An alloying element that effectively affects strength and toughness, and is also one of the main components.
If it exceeds wt%, manufacturing costs will increase and overgrade will occur, so it is limited to a range of 1.50 to 2.50 wt%.

Mo: 0,60. ~ +, oowt% It is an element that improves hardenability and is known to increase resistance to temper softening, but in this invention, 0.60wt% or more is required, but if it exceeds 1.00wt%, the manufacturing cost increases. Since it is an overgrade, 0.60 to 1.00w
It was limited to a range of t%.

82≦0.060% It has a grain refining effect, but if it exceeds 0.060 wt%, it increases nonmetallic inclusions and impairs soundness.
060 wt% or less.

Next, as impurities in steel, P≦0.020%, S≦0.0
10% is harmful to mechanical properties, so the lower the better, but considering product cost, P≦0.020%, S≦o
, oio% is acceptable.

Cast steel with the above composition is maintained at an austenitizing temperature of 850°C to 100°C. Figure 1 shows the relationship between the austenitizing temperature and tensile strength of 800°C to 40°C during quenching.
The results are shown when the average cooling rate over 0°C is 30'C/min and the tempering temperature is 580°C (1-stage quenching and tempering). From this figure, 100 has a tensile strength of 8f/xi or more. In order to obtain this, an austenitizing temperature of 850°C or higher is required, while it is clear that even if the temperature exceeds 1000°C, no increase in strength can be expected considering the energy consumption for heating. Therefore, the temperature range maintained at the austenitizing temperature was limited to 850°C to 1000°C.

On the other hand, Figure 2 shows the relationship between the average cooling rate and tensile strength between 800°C and 400°C, when the austenitizing temperature during quenching is set to 50°C, and the tempering temperature is 580°C for one-stage quenching and tempering. This is the result.

From the figure, in order to obtain a strength of iokgf/Bi or higher, an average cooling rate of 20° C./min or higher is required.

Figure 3 roughly shows the relationship between tempering temperature and tensile strength in the case of one-stage quenching and tempering and two-stage quenching and tempering. The austenitizing temperature during the first stage quenching is 950°C
The austenitizing temperature during 12-stage quenching is 950℃, 90
0℃, and the average cooling rate between 800℃ and 400℃ is 3
The temperature is 0°C and 7 ml.

From the figure, in order to obtain a tensile strength of 100 kgf/- or more, the tempering temperature should be 620°C or less, but if the tempering temperature is less than 540°C, the residual stress removal of the casting will be insufficient. Therefore, the range was set at 540°C to 620°C (I).

By performing J5. two-stage quenching and tempering, the strength can be slightly improved even at the same tempering temperature.

(Example) Molten steel having the composition shown in sample number 1.2 in Table 1 was melted in a 50-meter high-frequency induction melting furnace, and melted at a speed of 60 m in a Daikal sand mold.
A Y block with a height of 170 mm and a length of 250 mm is cast, and from this cast steel material a Y block with a thickness of 25 mm and a height of 160 mm is cast.
mm.

Cut out silkworm material with a length of 125 mm and heat it at 1000℃ x 1
After diffusion annealing from 0 Hr to FC, heat treatment was performed under the heat treatment conditions shown in Table 1.

After 98 such heat treatments, the mechanical properties were investigated and the results are summarized in Table 1.

Comparative cast steel samples Nos. 3 and 4 have a tensile strength of 80i, gf because their composition is different from that of the invention cast steel and the heat treatment conditions are inappropriate.
It has stopped at /- grade.

In addition, when comparing these 80kgf/w class high tensile strength cast steels with the cast steel of this invention, although the toughness is slightly lower,
Its industrial advantage is that it has a high tensile strength of 100 kg f/vj or more.

Comparative cast steel with the same composition as this invention, sample number 1-3.
2-3 has too low austenitization temperature, same as 1-4
The cooling rate of 800~400゛ is too low and the same
-5 is because the tempering temperature is too high, and the tensile strength is 100
kgf/-i cannot be obtained.

On the other hand, the tempering temperature of materials No. 1-6.2-4 is too low, so even though the tensile strength is satisfactory, the absorbed energy is extremely degraded.

(Effects of the Invention) By advantageously suppressing the addition of alloying elements and, rather, by appropriate heat treatment, the present invention has achieved a high tensile strength that secures a tensile strength of 100 kg f / 114 or more without significant deterioration of toughness. Cast steel can be easily and stably obtained.

Therefore, the high tensile strength cast steel according to this invention has the following characteristics: (1) Large size l! If used in 8II cars, etc., it can be made into carcasses due to its high tension.

(2) Can be used for holding buoys such as ships and guide towers that are subject to large tensile forces.

(3) Castings can be made lighter, thinner, shorter and smaller.

It is useful in various points such as.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between austenitizing temperature and tensile strength, Figure 2 is a graph showing the relationship between average cooling rate and tensile strength, and Figure 3 is a graph showing the relationship between tempering temperature and tensile strength. be. Figure 1 Figure 2

Claims (1)

[Claims] 1. C: 0.20 to 0.50 wt%, Si: 0.30 to 0.60 wt%, Mn: 0.50 to 1.60 wt%, Cr: 0.80 to 1.50 wt% %, Ni: 1.50 to 2.50 wt%, Mo: 0.60 to 1.00 wt%, Al: 0.060 wt% or less, and the balance is Fe and unavoidable impurities. After being maintained at an austenitizing temperature of 1000°C, quenching or normalizing treatment is performed at least once in the temperature range of 800°C to 400°C at an average cooling rate of 20°C/min or more, and then the temperature is reduced to 540°C to A method for producing high-strength cast steel, characterized by tempering within a temperature range of 620°C.