KR100415657B1 - METHOD FOR MANUFACTURING ULTRA-HIGH TENSILE STRENGTH STEEL HAVING 220 ksi CLASS YIELD STRENGTH - Google Patents

Abstract

PURPOSE: A method for manufacturing ultra-high tensile strength steel having 220 ksi class yield strength is provided to lower the minimum addition amount of expensive Co to 9.5% level by applying direct quenching right after hot rolling and simplify manufacturing facility and remove environment polluting elements by skipping solution treatment. CONSTITUTION: The method comprises a step of heating a steel slab comprising 0.13 to 0.19 wt.% of C, 0.1 wt.% or less of Mn, 0.1 wt.% or less of Si, 9.5 to 10.5 wt.% of Ni, 9.5 to 14.5 wt.% of Co, 1.8 to 2.2 wt.% of Cr, 0.9 to 1.1 wt.% of Mo, 0.008 wt.% or less of P, 0.005 wt.% or less of S, 0.0007 to 0.0020 wt.% of B and a balance of Fe and other inevitable impurities to the temperature range of 1,150 to 1,300 deg.C; a step of hot rolling the heated steel slab at a reduction ratio of 10 to 30% and a finish rolling temperature of 800 to 950 deg.C per each rolling pass; a step of water cooling the hot rolled steel to an ordinary temperature in a cooling rate of 5 to 50 deg.C/sec by starting cooling within 30 seconds right after finishing the hot rolling step; and a step of aging the water cooled steel in the temperature range of 500 to 530 deg.C for 5 hours or more.

Description

Yield strength 220ksi grade ultra high strength steel manufacturing method

The present invention relates to a method for producing a yield strength of 220ksi ultra high tensile strength steel mainly used in armor plate, fabric material, rocket, motor case, landing gear, weapon system, etc., in more detail by adding a small amount of boron (B) The present invention relates to a method of manufacturing high tensile strength steel having excellent strength and toughness while significantly reducing expensive Co content by applying direct quenching.

Conventional yield strength of the 220ksi grade super high strength steel is represented by weight percent, C: 0.13-0.19%, Mn: 0.1% or less, Si: 0.1% or less, Ni: 9.5-10.5%, Co: 13.5-14.5%, Cr: 1.8-2.2%, Mo: 0.9-1.1%, P: 0.008% or less, S: 0.005% or less, Steel slab composed of the balance Fe and other unavoidable impurities is sufficiently heated above 1,150 ° C, and then hot rolled. The solution is cooled to room temperature at an appropriate cooling rate, and then solution-treated at 750-900 ° C., cooled to room temperature by oil cooling, and then aged at 500-530 ° C. for at least 5 hours.

The conventional steel has a disadvantage in that the yield cost is high because the yield strength is about 220 ksi and has a high Co content of about 14% while having excellent strength and toughness. In addition, since the oil cooling must be performed after the solution treatment during the manufacturing process, not only the oil cooling equipment is required, but also causing the environmental pollution due to the generation of waste oil.

Accordingly, the present inventors have conducted research and experiments to solve the above-mentioned problems and proposed the present invention on the basis of the present invention. The present invention applies a direct quenching method immediately after hot rolling to reduce the minimum amount of expensive Co content to 9.5. The purpose of the present invention is to provide a method of manufacturing high strength steel of yield strength of 200ksi, which can reduce the level to%, while simplifying the manufacturing equipment and eliminating environmental pollution by eliminating the solution treatment.

1 is a graph comparing the strength and impact toughness of the invention steel and the comparative steel

In order to achieve the above object, the present invention provides a yield strength of 220ksi grade ultra high strength steel, in weight%, C: 0.13-0.19%, Mn: 0.1% or less, Si: 0.1% or less, Ni: 9.5-10.5% , Co: 9.5-14.5%, Cr: 1.8-2.2%, Mo: 0.9-1.1%, P: 0.008% or less, S: 0.005% or less, B: 0.0007-0.0020%, balance Fe and other unavoidable impurities The steel slab is heated to a temperature range of 1,150-1,300 ° C, hot rolled at a rolling reduction of 10-30% for each rolling pass and a finish rolling temperature of 800-950 ° C, and within 30 seconds immediately after the end of the rolling. A method of producing a yield strength of 220 ksi class ultra high tensile strength steel comprising starting cooling, cooling to room temperature at a cooling rate of 5-50 ° C./sec, and then aging at a temperature range of 500-530 ° C. for at least 5 hours. will be.

Hereinafter, the present invention will be described in detail.

The present invention supplements the hardenability by adding an appropriate amount of B, an element that can increase the hardenability by precipitating the grain at the grain boundary to the basic composition system of yield strength 220ksi ultra high tensile strength steel, and also by applying direct hardening, Promote the design of steel components that can drastically reduce the conventional Co content, improve the raw heat treatment unit by eliminating the solution treatment, and secure the physical properties without performing oil cooling, which was essential in the conventional solution heat treatment process. In addition, it is possible to prevent the environmental pollution caused by the generation of waste oil, it is characterized.

To this end, the C must be added at least 0.13% to secure a predetermined strength, and when added above 0.19%, the weldability is deteriorated and toughness is degraded by solid solution C other than carbide formation, so it is added in the range of 0.13-0.19%. It is preferable.

Although Mn is effective in increasing the strength, it is preferable to limit the toughness to 0.1% or less since the toughness is reduced by the formation of MnS.

Although Si has some effect on improving strength like Mn, it is preferable to limit it to 0.1% or less because it may damage the toughness by forming an oxide such as SiO ₂ .

Ni is an alloying element for improving low temperature toughness, but is preferably limited to 9.5-10.5% due to its high cost.

Co is a very important alloying element in the secondary hardening alloy, delays the potential recovery of the needle martensite base, promotes the precipitation of fine M ₂ C carbide, and plays a significant role in improving the strength. However, because it is too expensive, it is desirable to limit it to 9.5-14.5%.

The Cr is an alloying element directly related to the M ₂ C carbide should be added more than 1.8% and too much addition is detrimental to weldability, so it is preferable to limit it to 1.8-2.2%.

Mo is an alloying element directly related to the precipitation of M ₂ C carbides, such as Cr, should be added at least 0.9% and is preferably limited to 0.9-1.1% because it is expensive.

Since P is an element that impairs impact toughness and induces brittle brittleness, it is effective to limit the content to 0.008% or less.

Since S is an element that impairs impact toughness and weldability, S is preferably limited to 0.005% or less.

The B is an alloying element which precipitates at the grain boundary and reduces the grain boundary energy as a key element of the present invention, thereby increasing the hardenability to allow air cooling instead of conventional oil cooling after solution treatment. If the content is 0.0007% or less, the effect cannot be expected, and if it is added 0.0020% or more, the weldability and hot workability are deteriorated, so it is preferable to limit the content to 0.0007-0.0020%.

The steel slab formed as described above is difficult to match the finish temperature when the heating temperature is 1,150 ℃ or less, and when heated to 1,300 ℃ or more it causes abnormal growth of grains to reduce the toughness of the steel slab heating temperature is 1,150-1,300 ℃ It is preferable to set it as the range of.

After heating the slab under the above conditions, hot rolling has the effect of improving the strength and toughness as the reduction ratio of each rolling pass increases, but over 30% of the rolling causes overload of the rolling equipment in the low temperature zone. The rolling reduction per rolling pass is preferably 10-30% and is continuously rolled until the end of rolling.

The slab is pressed under the above conditions. At this time, when the rolling end temperature is 750 ° C. or lower, the rolling equipment may be overloaded. The rolling end temperature is higher than 950 ° C. as the recrystallization rolling zone may reduce the strength. It is desirable to limit to 750-950 ° C.

Water cooling is performed after the end of hot rolling as described above, wherein the shorter the holding time until the start of the water cooling can increase the microcrystalline reduction effect. On the other hand, if the time is too long, the microcrystalline reduction effect is reduced. It is effective to limit the time to 30 seconds or less. After the end of rolling, the cooling rate is more favorable in terms of securing strength after the end of rolling. However, the cooling speed of thick plate cooling at 50 ° C / sec or more or less than 5 ° C / sec or less in manufacturing is disadvantageous for securing strength. It is preferable to limit the cooling rate to 5-50 ° C./sec since the formation of a tissue such as knight is concerned.

After cooling as above, the aging temperature is an important factor to secure the strength and toughness. At temperatures below 500 ℃, precipitation of M ₂ C, which is a major carbide for securing strength, is difficult, whereas when aging at temperatures above 530 ℃, Since it is difficult to secure the strength due to the aging treatment at 500-530 ℃ more than 5 hours is preferable.

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

Example

To prepare a steel slab as shown in Table 1 under the conditions of Table 2 and then to measure the mechanical properties, the results are shown in Table 2 below. In Table 2, for Inventive Materials 1, 2 and 3, cooling was started within 15 seconds immediately after the end of rolling.

TABLE 1

TABLE 2

※ DQ: Direct quenching, FRT: Rolling end temperature, CR: Cooling speed, HT: Holding time after rolling end, YS: Yield strength, TS: Tensile strength, vE Room _temperature : Room temperature impact energy

As shown in Tables 1 and 2, in the case of conventional steel, oil cooling was performed after the solution treatment to produce a yield strength of 220 ksi class ultra high tensile strength steel (prior material 4). In the case of the present invention, when a small amount of B is added and the direct quenching method is applied, the yield strength of the same Co content (invention steel A) is slightly different depending on the manufacturing conditions, but is generally increased with the strength of about 7-15 ksi and room temperature. Impact toughness is also improved by about 20 Joules (Invention material (1)). In the case of the inventive steel (C) having about 10% Co content, although the Co content, which is expensive compared to the conventional steel, was reduced by about 4%, the room temperature impact toughness was improved by about 30 Joules while having almost the same strength as the conventional steel. (Invention material (3)). Therefore, it was found that the present invention material (1-3) has much better physical properties in terms of strength and toughness than the conventional material (4).

On the other hand, the mechanical properties of the mechanical properties of Table 2 are compared graphically in Figure 1, the relationship between the strength and toughness of the steel is generally inversely proportional to each other, but as shown in Figure 1 in the case of the present invention As a result, the solid solution of the alloying element was increased and immediately after rolling, an increase in dislocation density and porosity resulted in an increase in strength, and a good strength-toughness was ensured by securing toughness by miniaturization of effective grains by microcrystalline rolling. It can be seen that the relationship with.

As described above, the present invention steel can reduce the minimum amount of Co to 9.5% level compared to conventional steels by applying a small amount of B and direct quenching method, thereby reducing the expensive Co content by up to about 4%. In addition, the elimination of the solution treatment can simplify the manufacturing equipment and reduce the manufacturing cost. In addition, the present invention has the effect of having excellent strength-toughness while having the advantage that can significantly improve the problem of environmental pollution by leakage of cooling oil by performing oil cooling after the conventional solution treatment.

Claims (1)

Yield strength In the manufacturing method of the 220 ksi class ultra high tensile strength steel, in weight%, C: 0.13-0.19%, Mn: 0.1% or less, Si: 0.1% or less, Ni: 9.5-10.5%, Co: 9.5-14.5%, Cr : 1.8-2.2%, Mo: 0.9-1.1%, P: 0.008% or less, S: 0.005% or less, B: 0.0007-0.0020%, steel slab composed of balance Fe and other unavoidable impurities, at 1,150-1,300 ° C. After heating to the temperature range, hot rolling was carried out at a rolling rate of 10-30% and finishing rolling temperature of 800-950 ° C. for each rolling pass, and then cooling was started within 30 seconds immediately after the end of rolling. After cooling to room temperature at a cooling rate of 5, the method of producing a yield strength of 220ksi ultra high strength steel, characterized in that it comprises aging treatment for at least 5 hours in the temperature range of 500-530 ℃.