KR890002612B1 - Process for manufacturing steel of structure having a good tungsten - Google Patents

Abstract

The method includes the steps of (i) heating a steel comprising (in wt.%) 0.38-0.43% C, 0.15-0.35% Si, 0.6-1.2% Mn, not more than 0.03% P, not more than 0.03% S, 0.90-1.20% Cr, 0.20-0.45% W, balance Fe and unavoidable impurities to 1,150-1,250 deg.C; (ii) hot-finish-rolling at 850-900 deg.C; (iii) rapidcooling to 600-700 deg.C at the rate of 15-20 deg.C/sec; (iv) aircooling to room temperature; (v) reheating to 830-860 deg.C; (vi) oil-cooling; (vii) reheating to 550-630 deg.C; and (viii) air-cooling.

Description

Manufacturing method of tungsten (W) -containing mechanical structural steel with excellent impact toughness

The present invention relates to the production of high strength, high toughness tungsten (W) -containing mechanical structural steel. Mechanical structural alloy steel is mainly used in industrial machinery and construction machinery, and there are various kinds of Ni-Cr-Mo steel, Cr-Mo steel, and Ni-Cr steel depending on the alloying elements added.

Double Cr-Mo steel is mainly used. After hot working, it is quenched and tempered to obtain the required high strength and toughness. Cr-Mo steels are classified into various types according to their carbon content, and the material properties are different depending on the type.

At 0.3% C (JIS standard: SCM430), tensile strength is over 85㎏ / ㎠, impact value is over 108J / ㎠, and at 0.35% C (JIS standard: SCM435), tensile strength is over 95㎏ / ㎠ and impact destination is 78J / Cm2 or more, and 0.04% C (JIS standard: SCM440) has a material quality of more than 100kg / ㎜ tensile strength, 59J / ㎠ impact value, the higher the carbon content, the higher the tensile strength but the impact value is significantly lower. As the conditions for use of mechanical structural alloy steels are severe, demand for alloy steels with higher strength and toughness is required. However, high strength Cr-Mo steels with high carbon content have relatively high impact but relatively high impact values compared to Cr-Mo steels with low carbon content. It is low enough to satisfy this requirement. In addition, the Cr-Mo steel contains 0.15-0.30% of Mo, a strategic element, and the price has soared due to the recent expansion of resource nationalism due to the depletion of underground resources.

The object of the present invention is much higher than that for use in harsher environments while having the same level of strength as Cr-Mo steel containing 0.4% C of the prior art without increasing the manufacturing cost, and the strategy in the conventional Cr-Mo steel The present invention provides a method for manufacturing a mechanical structural alloy steel having a new alloy composition capable of coping with resource nationalism by removing the element Mo.

Attempts have been made to remove Mo, a strategic element, from Cr-Mo steels (Special Steel No. 28, 11). B) was added by 0.001%, and Mn content was increased to increase the strength, and thus, B-steel steel similar in material properties to the conventional Cr-Mo steel was developed.

However, in order to make use of B effect, Ti must be added about 0.03% as a nitrogen fixing element in order to make use of the B effect. Since the amount of Ti required depends on the amount of nitrogen in the molten steel, it is difficult to precisely control the amount of nitrogen during steelmaking. On the side, the impact toughness is not high enough compared with the strength like Cr-Mo steel.

Looking at the configuration of the present invention in order to solve the problems with the conventional Cr-Mo steel and B toughness as follows.

The present invention is C: 0.38-0.43% (wt%, the same below)

Si: 0.15-0.35%, Mn: 0.6-1.2%, P: 0.03% or less S: 0.03% or less, Cr: 0.90-1.20%, W: 0.20-0.45% Remainder: Starting steel with Fe and unavoidable impurities The subsequent hot rolling and heat treatment such as quenching and thinning are carried out in the condition range of 0.4% C Cr-Mo mechanical structural steel.

The effect of the present invention will be described in detail by dividing the component range, rolling conditions and heat treatment conditions of the additive element as follows. First, the description will be given of the component range of the additive element component of the present invention.

1. Carbon (C)

Since carbon increases the strength but decreases the ductility and toughness, the component range was 0.38-0.43% in order to obtain a strength of a little more than 100 kg / mm 2.

2. Silicon (Si)

Silicon is required for deoxidation and strength increase, but in many cases the ductility is lowered, so the component range is set to 0.15-0.35%.

3. Manganese (Mn)

Mn improves strength, ductility and quenchability, but in many cases the weldability is deteriorated, so the component range is 0.6-1.2%. If it is less than 0.6%, the required strength cannot be obtained. It is difficult.

4. Phosphorus

Since phosphorus lowers toughness, the upper limit was set to 0.030% in order to obtain required high toughness.

5. Sulfur (S)

Sulfur lowered the toughness and formed an emulsion, adversely affecting the material, so the upper limit was 0.030%.

6. Chrome (Cr)

Chromium improves quenchability and strength and increases softening resistance at high temperatures, but in many cases damages weldability, so the composition range is 0.90-1.20%.

7. Tungsten (W)

W quenchs the quenching sensitization during quenching to stabilize the quenching structure, facilitating heat treatment compared to conventional Cr-Mo steel or B-added steel and reducing the variation of final structure and material. . In addition, W forms fine carbides in the tempering process and distributes them uniformly in the matrix structure, thereby simultaneously improving impact toughness and strength.

In Cr-Mo steels, Mo also forms fine carbides during tempering, but the components of carbides are different from those of W-added steels, and the effect on strength and impact toughness is much less than that of W. In the B-added steel, B does not form carbides, but only plays a significant role in improving hardenability during heat treatment. Therefore, the contribution to impact toughness is very small compared to W.

Since W has a very small affinity with nitrogen compared to B, it is not necessary to add Ti, which is essential in steel B, to fix nitrogen during steelmaking. Therefore, steelmaking is much easier than steel.

In order to make W fully exhibit the above-mentioned effect in mechanical structural steel, the additive component range was 0.2-0.45%.

When the W content is less than 0.2%, the required strength and toughness cannot be obtained. When the W content is more than 0.45%, the specific gravity of the material is increased, so the practical value is low in terms of sex energy.

The following describes the rolling conditions according to the practice of the present invention.

The heating temperature for hot working is given the hot workability of the material allows to 1200 ± 50 ℃. Finish rolling temperature of the heated material to the temperature has to be not less than 850-900 ℃ Ar ₃ transformation temperature, and finish hot rolling temperature of 850 If the temperature is lower than ℃, rolling is performed in the ferritic + austenite (α + r) or higher region, so that not only the rolling force increases but also the shape of the rolled material becomes poor. If the finish rolling temperature exceeds 900 ℃, the rolling structure becomes coarse and the material becomes bad.

The rolled material is forcedly cooled to the temperature range of 700-600 ℃, and the reason for limiting the forced cooling end temperature to 700-600 ℃ is that ferrite and the inside of the tissue are reduced when the temperature of the material is lowered to 600 ℃. In addition to pearlite, bainite is produced, which makes the material brittle and increases the hardness of the material, thus degrading workability. If the forced cooling finish temperature exceeds 700 ℃, the tissue becomes too coarse, so the material is bad.

After the forced cooling, the material is sufficiently slow cooled so that unaffected austenite tissue does not turn into bainite tissue.

Next, the heat treatment conditions of the present invention will be described.

The finished material should be quenched and tempered to increase its strength. Quenching is heated to a temperature range of 830-880 ° C. and maintained for a normal solution treatment time, followed by oil cooling. quenching to transform the tissue into martensite and make it strong.

After quenching, the material is heated to a temperature range of 530-630 ° C and treated with an annealing process. In this process, the weak martensite tissue is changed to tempered martensite to increase the ductility and cementite to the base tissue. The cementite and tungsten carbides are uniformly deposited to strengthen the structure and then quench them.

Through the quenching and tempering processes, the present invention steel, which strengthens the matrix structure by precipitating fine tungsten carbide, has not only high strength of 100 kg / mm2 or more but also the highest compared to the conventional Cr-Mo steel. Excellent impact toughness increased by 43%.

When explaining the effects of the present invention based on the above alloy element component range, rolling conditions and heat treatment conditions through the examples as follows.

EXAMPLE

Samples of steel as shown in Table 1 are sampled and each sample is heated at 1200 ℃ for 1 hour, and then hot rolled to finish hot rolling temperature of 900 ℃ and 850 ℃, and then cooled to 700 ℃ and 600 ℃. After forced cooling at ℃ / sec, the material was tested for one hour in a furnace heated to the same temperature as the end of forced cooling and after furnace cooling. The results are shown in Table 2.

Since this steel is used after heat treatment, the absolute values of strength, hardness and elongation in the rolling state have little meaning, and the workability is the most significant material property in the rolling state.

Looking at the workability in the rolled state of the steel and the comparative steel of the present invention, as shown in Table 2, in the case of SCW1, the strength, hardness, elongation shows a lower value than the comparative steel, the lower the strength, the hardness is better workability However, the lower the elongation, the worse the workability, so the two effects cancel each other, so that the workability of SCW1 is almost the same as that of the comparative steel. In the case of SCW2, the strength, hardness, and elongation all show almost the same values as those of the comparative steel, indicating that the workability is almost the same.

Rolled and cooled material is heat-treated according to the heat treatment conditions of conventional Cr-Mo steel. That is, the material was maintained at 855 ° C. for 30 minutes and then cooled in oil to obtain a quench structure. After quenching, the material was heated to 580 ° C and treated with an annealing process. In this process, the martensite structure was changed to a soft tampered martensite and fine tungsten carbide was deposited in the matrix structure. Let's do it. Hold at 580 ° C for 30 minutes and then quench. Table 3 shows the material test results of the specimens after the above series of heat treatments.

The results of the impact test shown in Table 3 were taken from the test specimens in the direction parallel to the rolling direction and tested with a charpy impact tester.The specimens used were 5mm thick, 10mm wide, and 55mm long U-notch subs. Sub-size specimens.

As shown in Table 3, the inventive steel showed almost the same elongation as compared with the comparative steel, and showed a slightly lower tensile strength and hardness. However, the tensile strength far exceeded the lower limit of 100 kg / mm2, the lower limit of SCM440. It satisfies the standard range of 301-372Hv.

On the other hand, in terms of impact value, SCW1, the present invention steel, shows a high impact value of about 12-23% compared to the comparative steel, and SCW2 shows a high impact value of about 25-43%. Taken together, the present invention steel has almost the same workability in the rolled state as compared with the conventional Cr-Mo steel, and the material properties after heat treatment are similar in strength, hardness and elongation, but the impact toughness is much better than the ball invention steel. Do.

As described above, the present invention can be seen that the present invention relates to a method for producing mechanical structural steel which has a high strength of 100 kg / mm 2 or more and has much higher impact toughness than conventional Cr-Mo steel of the same strength.

[Figure 1]

[Figure 2]

[Chart 3]

유지후 유냉 850℃에서

유지후 수냉* Heat treatment condition: at 855 ℃

After holding oil-cooled at 850 ℃

Water cooling after maintenance

Claims (1)

By weight%, C: 0.38-0.43%, Si: 0.15-0.35%, Mn: 0.6-1.2%, P: 0.03% or less, S: 0.03% or less, Cr: 0.90-1.20%, W: 0.20-0.45% , Remainder: 700-600 ℃ at 15-20 ℃ / sec cooling rate after hot rolling by the usual method of heating the steel composed of Fe and unavoidable impurities to 1150-1250 ℃ to finish rolling temperature 850-900 ℃ After cooling to a temperature of about to produce a hot-rolled steel sheet by air-cooled to room temperature, the same as the heat treatment conditions of the conventional structural steel for heating to 830-860 ℃ to cool the oil and again to 550-630 ℃ characterized in that the air-cooled Method for manufacturing W-containing mechanical structural steel with excellent impact properties.