KR950004678B1 - Making method of high tensile strength steel - Google Patents

Abstract

The high tensile steel is made by heating a steel slab comprising (by wt.) 0.100˜0.20 % C, 0.1˜0.4% Mn, 0.15˜0.35% Si, 1.0˜1.8% Cr, 2.25˜3.50% Ni, 0.20˜0.60% Mo, up to 0.05% V, up to 250 ppm P, up to 150 ppm S, balance Fe and inevitable impurities, to 1150˜1300 deg.C, hot-rolling it at finishing temp. of 750˜900 deg.C with 30% reduction, water-cooling the hot-rolled steet up to room temp. with 10˜15 deg.C/sec. rate for below 30 secs., and tempering at 600˜660 deg.C of below Ac1 transformation point.

Description

Yield strength 90kgf / mm2 high tensile steel manufacturing method

1 is a process schematic diagram showing a conventional method and a process for producing high tensile steel according to the present invention.

The present invention relates to a method of manufacturing high tensile steel used for deep sea structural steel, hull hulls of deep sea probes, pressure vessels, and more particularly, by introducing a hardening process directly into the general yield strength 70kgf / mm ² high tensile steel The present invention relates to a method of producing a yield strength of 90kgf / mm ² high tensile strength steel with excellent impact toughness.

Yield strength 70kgf / mm ² high tensile strength steel is used for deep sea structural steel, hull shell of deep sea probe and pressure vessel. The manufacturing method is as follows.

That is, the yield strength of the above 70kgf / mm class ² high tensile strength steel by weight, C: 0.2% or less, Mn: 0.1-0.4%, Si: 0.15-0.35%, P: 0.025% or less, S: 0.015% or less, Ni : 2.25-3.50%, Cr: 1.0-1.8%, Mo: 0.20-0.60%, steel slab composed of the balance Fe and other unavoidable impurities is sufficiently heated at 1150-1300 ° C., 10- per each rolling pass After the end of the pressurization at 900 ℃ or more at a reduction rate of 30% and air-cooled again and then re-heated and water cooled to 900 ℃, it is produced by tempering at 600-650 ℃.

However, when the above-mentioned yield strength of 70kgf / mm ² high tensile strength steel is used for hull materials of deep sea probes, the diving depth is up to about 3,000m, and the product is lighter and smaller by improving yield strength without lowering low-temperature toughness. Is required.

The present invention is to improve the yield strength of about 20kgf / mm ² without lowering the low-temperature toughness by strictly controlling the composition and composition of the conventional yield strength 70kgf / mm ² high tensile strength steel and by introducing a direct quenching process, the object is.

EMBODIMENT OF THE INVENTION Hereinafter, this invention is demonstrated.

The present invention, by weight, C: 0.100-0.20%, Mn: 0.1-0.4%, Si: 0.15-0.35%, Cr: 1.0-1.8%, N: 2.25-3.50%, Mo: 0.20-0.60%, Steel slabs composed of V: 0.05% or less, P: 250 ppm or less, S: 150 ppm or less, balance Fe and other unavoidable impurities are sufficiently heated in a temperature range of 1150-1300 ° C, and then 10- per each rolling pass. Finish hot rolling in the unrecrystallized temperature range of 750-900 ° C with a 30% reduction rate, and then water-cooled to room temperature at a cooling rate of 10-50 ° C / sec within 30 seconds, and then at 600-660 ° C below Ac ₁ point temperature. It relates to a method of manufacturing a yield strength 90kgf / mm class ² high tensile strength steel by tempering.

Hereinafter, the reason for limitation of the said component range and manufacturing conditions is demonstrated.

When the content of C is increased, the hardenability can be increased to increase the strength, but when the weldability is impaired, on the contrary, when the C content is reduced, the hardenability is low and the strength cannot be guaranteed, so the C content is 0.10-0.20. It is desirable to limit to%.

The Mn may also improve the strength by increasing the hardenability, but since the element impairs the weldability, the Mn content is preferably limited to 0.1-0.4%.

The Si can increase the yield strength, but increases the impact transition temperature, the content is preferably limited to 0.15-0.35%.

The Cr is an alloying element effective for quenching, but impairs weldability, so the Cr content is preferably limited to 1.0-1.8%.

Ni is an alloying element capable of improving low temperature toughness, but is preferably limited to 2.25-3.50% due to its high price.

Mo improves the hardenability, but if the amount exceeds 1%, it is likely to cause severe brittleness and is expensive, so the content is preferably limited to 0.20-0.60%.

Although V is an element effective for quenching property, it impairs the weldability, so the content thereof is preferably limited to 0.05% or less.

Since P is an element that impairs impact toughness and causes brittleness, the content thereof is preferably limited to 250 ppm or less.

Since S is an element that impairs impact toughness and weldability, the content thereof is preferably limited to 150 ppm or less.

When the slab heating temperature is 1150 ℃ or less, it is difficult to match the end temperature of rolling, and when the slab heating temperature is 1300 ℃ or more, it leads to abnormal growth of the grain, it is preferable to limit the heating temperature of the slab to 1150-1300 ℃.

The higher the reduction rate during hot rolling, the better the strength and toughness, but more than 30% of the rolling causes overloading of the rolling equipment in the low temperature zone, so it is desirable to continuously roll to the end of rolling temperature at 10-30% per rolling pass. Do.

Rolling of the unrecrystallized zone below 750 ° C. may cause overloading of the rolling equipment, and the rolling temperature is preferably limited to 750-900 ° C. since the recrystallization temperature zone is 900 ° C. or higher.

The shorter the holding time after the end of hot rolling can increase the effect of reducing the recrystallization of the recrystallization, and if it is too long, the effect of the recrystallization of the recrystallization can be reduced.

The higher the cooling rate, the more advantageous in terms of securing strength, but the upper limit is 50 ° C / sec, which is the maximum range of water cooling for thick plate cooling, and at the cooling rate of 10 ° C / sec or less, the upper baitite structure forms strength and toughness. Since it cannot be ensured, the cooling rate is preferably limited to 10-50 ° C / sec.

The tempering temperature is a very important factor in securing strength and toughness, and is generally made at Ac ₁ or lower temperature. If it is 600 ° C. or lower, it is difficult to secure impact toughness, and if it is 660 ° C. or lower, it is difficult to secure strength. It is desirable to limit to 660 ° C.

Hereinafter, the present invention will be described in detail through examples.

EXAMPLE

To a continuous hot rolled in the range of 10 to 30% under the conditions as shown in Table 1 and a composition of a conventional yield strength of 70kgf / mm ² grade steel invention steel slab to heat sufficiently in ℃ 1150-1300, and Table 2 having the same After that, the specimens were prepared by directly quenching and tempering, and the mechanical properties of the specimens were evaluated. The results are shown in Table 2 below.

TABLE 1

TABLE 2

* Note HT: Holding time after finish rolling, CR: Cooling speed, FRT: Finishing rolling temperature

vE-18: -18 ℃ Impact Energy vE-85: -85 ℃ Impact Energy

As shown in Table 2, the present invention produced the present invention steel according to the manufacturing conditions of the present invention exhibits almost the same elongation and low temperature impact toughness of the conventional steel produced according to the conditions outside the scope of the present invention However, it can be seen that the yield strength and tensile strength of the present invention are increased by about 20 kgf / mm ² compared to the conventional materials.

Therefore, the present invention has much superior physical properties in terms of strength and toughness as compared with the conventional materials.

In general, the relationship between the strength and toughness of the steel is inversely related to each other, but in the case of the present invention steel, due to the increase of the solubility of the alloying element by direct hardening, the increase of dislocation density due to hardening immediately after rolling and the increase of internal defects such as vacancy. It brought about an increase in strength, and also has good strength-toughness by securing low-temperature toughness by miniaturization of effective crystal grains by well-known recrystallization reduction.

As described above, the present invention improves the yield strength by about 20kgf / mm ² without lowering the low temperature toughness to secure the diving depth up to 5000m, as well as the miniaturization and lightening of the product, but also by the conventional reheat quenching method. Rather, it is possible to reduce manufacturing costs by simplifying the process due to the elimination of the reheating process.

Claims (1)

By weight, C: 0.100-0.20%, Mn: 0.1-0.4%, Si: 0.15-0.35%, Cr: 1.0-1.8%, Ni: 2.25-3.50%, Mo: 0.20-0.60%, V: 0.05% Below, P: 250ppm or less, S: 150ppm or less, steel slab composed of balance Fe and other unavoidable impurities is sufficiently heated in the temperature range of 1150-1300 ° C, and then reduced by 10-30% per each rolling pass. In the unrecrystallized temperature range of 750-900 ℃ at the rate of finishing hot rolling, and then cooled to room temperature at a cooling rate of 10-50 ℃ / sec within 30 seconds, then tempering at 600-660 ℃ below Ac ₁ point temperature Yield strength 90kgf / mm ² high tensile strength steel manufacturing method characterized by.