KR20090043401A - Producing method for reinforcing steel - Google Patents

Abstract

The present invention relates to a method for producing an ultrahigh strength steel reinforced with high strength and high mechanical properties so as to be commercialized in a huge structure.

The method for producing an ultra-high strength steel bar according to the present invention is characterized in that the slab to which the alloy element is added is heated in a reheating furnace at a temperature of about 1000 to 1150 DEG C for about 2 to 0.5 hours and then subjected to rough rolling, After rolling, it is water-cooled at 900 to 1000 ° C. and cooled to 500 to 600 ° C. to martensite the structure. Then, the structure is air-cooled to remove internal stress to stabilize the structure of the hardened layer by making pearlite and ferrite.

According to the present invention, it is possible to produce reinforcing bars that satisfy the conditions of high strength and high tensile strength by controlling the content of alloying elements and by heat treatment and cooling methods. Therefore, there is an advantage that the reinforcing bars can be widely used for high-rise buildings and large structures at a relatively low cost.

Ultra high strength steel, high toughness, high strength, heat treatment

Description

{Producing method for reinforcing steel}

More particularly, the present invention relates to a method of manufacturing an ultrahigh strength steel reinforcing bar capable of satisfying a high strength and high tensile strength of at least 785 MPa through a heat treatment with an alloy.

Currently, it is inevitable to secure space for human activities in accordance with the population growth of the future society and to make huge structures for space utilization (for example, skyscrapers, long bridges, large space structures, huge ocean structures and huge underground structures). Reinforcing bars in these huge structures must satisfy the requirements of high strength and high toughness. Currently, reinforcing bars with a yield strength of 500 MPa are commercialized and used for 100-story high-rise structures. This trend is expected to accelerate further in the future.

In Japan, high-priced V and Nb are added in large quantities to design the alloy satisfying the mechanical properties of the SD785 ultra-high-strength steel produced by an electric furnace or a blast furnace. The SD785 super high strength steel has mechanical properties such as a yield strength of 785 MPa, a tensile strength of 932 MPa, an elongation of 10% or more and a bending test (D = 2.5,3d).

In Europe and Korea, by using water-cooling equipment (Tempcore), the structure of the outer part (hardened layer) and the center part of the reinforcing steel are made different from each other, thereby reducing expensive alloying elements (V, Nb, Mn) . However, in the case of constant alloy composition and severe water-cooling conditions, problems such as fracture occur during stretching and bending tests, and therefore new alloying design and cooling conditions are required. Therefore, it is important to study the design and fabrication methods of alloys for the development of rebars with a yield strength of 785 MPa or more, which can be widely used in large-scale structures.

Accordingly, it is an object of the present invention to provide a steel sheet which has a yield strength of 785 MPa or more, a tensile strength of 932 MPa or more, an elongation of 10% or more by controlling the V element content and heat treatment conditions and satisfies D = 2.5d / 90 ° when the diameter of the reinforcing bars is 25 mm or less And when the diameter of the reinforcing bar exceeds 25 mm, D = 3d / 90 ° can be satisfied.

In order to accomplish the above object, the present invention provides a method of manufacturing a semiconductor device, which comprises, by weight, 0.3 to 0.35 carbon (C), 0.3 to 0.36 silicon (Si), 1.3 to 1.35 manganese (Mn), 0.10 to 0.15 vanadium ) Of 0.01 to 0.012 and the remainder is formed of a reinforcing steel through a hot rolling process, wherein the steel is inevitably included in an electric furnace,

The steel containing the above-mentioned weight% of the alloying element and inevitably contained impurities is characterized in that the steel is heated at a temperature of 1000 to 1150 캜 for 2 ± 0.5 hours and subjected to cooling treatment after rough rolling, intermediate rolling and finishing rolling.

Particularly, it is preferable that the cooling treatment is carried out by water cooling at 900 to 1000 占 폚, cooling to 500 to 600 占 폚, followed by air cooling.

As described in detail above, according to the method for producing ultra-high strength steel of the present invention, it is possible to produce reinforcing bars capable of satisfying high strength and high tensile strength by adjusting the content of vanadium (V) element and heat treatment. The reinforcing bars satisfy the requirements of tensile strength, elongation and bending test which have not been satisfactorily satisfied in the past, so that they can be widely commercialized at a relatively low cost in high-rise buildings and large-scale structures.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, a method of producing an ultrahigh strength steel bar according to the present invention will be described in detail with reference to preferred embodiments.

The present invention includes alloying elements other than nickel, molybdenum, and copper which are expensive and have no effect even when added. The alloying element includes carbon, silicon, manganese, vanadium, nitrogen, and chromium. The alloying element is added to steel composed of iron (Fe) and unavoidable impurities, hot-rolled and water-cooled, Tensile strength of 932 MPa or more, elongation of 10% or more, and physical properties of the bending test.

More specifically, the content of vanadium in the alloying elements added to the steel is set to be between 0.10% and 0.15%, thereby promoting the miniaturization of the crystal grains. The hot-rolled alloy steel is subjected to water cooling and air cooling to produce martensite and pearlite ferrite So that the surface strength and the internal toughness can be adjusted to be high strength and high toughness.

The function and content of the alloying elements of the present invention are as follows.

Carbon (C): 0.30 to 0.35%

Carbon is an indispensable element for imparting high strength to the steel. When the carbon content is 0.3% or less, high strength can not be obtained. When the carbon content is 0.35% or more, the hardness of the cured layer is high according to the water-cooling condition, but the ductility is poor and the steel can be broken at the bending test. Therefore, the content of carbon is preferably between 0.30 and 0.35%.

Silicon (Si): 0.3 to 0.35%

Silicones deoxidize the steel and strengthen the ferrite. If the content of the silicon is 0.3% or less, desired strength can not be obtained, and if the content is increased, the toughness is lowered. In order to satisfy the SD785 physical property, it is preferable to adjust it to 0.3 to 0.35%.

Manganese (Mn): 1.3 to 1.35%

Manganese prevents decarburization and red-hot brittleness, improves the strength of the steel and stabilizes the cementite formed between 200 and 700 ° C. If the content of manganese is less than 0.3%, the effect can not be obtained. Therefore, it is preferable that the optimum composition of manganese is controlled within 1.3 to 1.35%.

Vanadium (V): 0.10 to 0.15%

Vanadium plays a role of increasing the strength of the steel by dispersing and precipitating carbonitride when it transforms from an austenite phase to a ferrite phase. In addition, it promotes the miniaturization of crystal grains and increases the tensile strength and the elastic limit so that excellent strength can be obtained at a low yield ratio. If the content of vanadium is less than 0.1, the steel may have a too high hardness, which may result in a decrease in elongation or a break in the bending test. Therefore, the content of vanadium is preferably set to 0.10 to 0.15%. This is because when the vanadium is added to the steel, the transformation point and the transformation speed change, so that appropriate cooling conditions for the steel satisfying the conditions of the strength increase at the center portion and the high strength and high toughness can be set.

Nitrogen (N): 0.01 to 0.012%

The nitrogen is an element that increases the strength by precipitating vanadium and nitride or carbonitride. If the content of nitrogen is too high, the nitrogen acts as an element which hinders toughness. Therefore, the content of nitrogen is preferably regulated in a range of 0.01 to 0.012% .

Chromium (Cr): 0.10 to 0.15%

Since chromium increases the strength similar to manganese, at least 0.10% should be added so that the pearlite colonies can be subdivided and ductility improved. In addition, the pearlite colony functions as an element for improving the softness. However, when 2.0% or more of the pearlite colony is added, the desired texture can not be obtained by increasing the incombustibility. Therefore, the content of Cr is preferably regulated in the range of 0.10 to 0.15%.

(O), copper (Cu), molybdenum (Mo), and molybdenum (Mo) as elements contained in the raw materials, materials, Etc. are also allowed.

The slab having the above composition is subjected to rough rolling, intermediate rolling and finishing rolling sequentially through a rolling process to form a reinforcing bar shape, and then the desired mechanical properties are obtained through water cooling and air cooling, which will be described below.

Hot rolling process;

The slab to which the alloy element is added is heated in a reheating furnace at a temperature of about 1000 to 1150 DEG C in order to reuse the segregated components during casting, and rolled to a desired thickness and width through rough rolling, intermediate rolling, . At this time, it is preferable that the slab is heated for 2 ± 0.5 hours by changing the transformation point and transformation speed by the alloying element and changing the critical cooling rate.

Cooling process of heat treatment;

Adjusting the cooling rate in order to obtain the final desired structure of the reinforcing bars, which is water-cooled at 900 to 1000 占 폚, cooled to 500 to 600 占 폚 to be martensized, air cooled to remove internal stress, and the structure of the cured layer is stabilized . At this time, the hardness value of the martensite formed on the self-templated outer surface of the reinforcing bar after water cooling is preferably 350 to 370 Hv, and the thickness thereof is preferably between 8 and 12%. It is preferable that the hardness of the center portion of the reinforcing bars of pearlite and ferrite is 300 to 320 Hv. More preferably, the hardness value according to the above experiment is preferably equal to or higher than the above-mentioned value so as to obtain a desired hardness value at the actual site.

Table 1 and Table 2 below show the results of the relationship between the alloying elements of the reinforcing bars, the cooling conditions and the mechanical properties according to the alloy design before the present invention.

C Si Mn P S Ni Cr Mo Cu V N SD785-19 0.36 0.24 1.25 0.026 0.018 0.07 0.09 0.02 0.21 0.082 0.012 SD785-22 0.35 0.24 1.27 0.025 0.018 0.07 0.09 0.02 0.20 0.083 0.011 SD785-29 0.35 0.23 1.24 0.025 0.019 0.07 0.09 0.02 0.21 0.077 0.013

Cooling water (t / hr) Rolling speed (m / s) Inlet temperature (캜) Outlet temperature (캜) Roll pass (times) Yield strength (N / nm ² ) Tensile strength (N / nm ² ) Elongation (%) Bending result SD785-19 995 8.5 922 490 18 811 898 8.7 ○ SD785-22 960 5.5 910 485 17 952 999 7.4 × SD785-29 895 5.2 950 495 18 824 919 8.9 ×

Table 1 and Table 2 show the relationship between the amount of cooling water, the rolling speed and the rolling speed in the course of cooling the steel sheet by heating at 1000 to 1150 ° C in a heating furnace while adjusting the addition amount of the ingot element in SD785-19, SD785-22 and SD785-29, , Inlet temperature, outlet temperature, yield strength, tensile strength, elongation, and bending results.

When the optimum ratio of the alloying elements is added to the slab and the reinforcing bars are formed through the heat treatment, the surface is martensized and the core structure becomes pearlite and ferrite structure to satisfy the conditions of high strength and high toughness. Particularly, in the present invention, the yield strength, tensile strength and elongation of the reinforcing bar (SD785) differ depending on the weight% of vanadium (V) and the cooling conditions.

It will be apparent to those skilled in the art that various changes, modifications, and other equivalent embodiments may be made without departing from the spirit and scope of the invention. Therefore, the true scope of the present invention should be determined by the technical idea of the appended claims.

Claims (2)

(Si) 0.3 to 0.36, manganese (Mn) 1.3 to 1.35, vanadium (V) 0.10 to 0.15, and nitrogen (N) 0.01 to 0.012 in terms of weight% A method of manufacturing an ultrahigh strength steel reinforcing bar comprising the steps of: (a) The steel containing the above-mentioned weight% of the alloying element and inevitably contained impurities is heated at a temperature of 1000 to 1150 캜 for 2 ± 0.5 hours and subjected to cooling treatment after rough rolling, intermediate rolling and finishing rolling. Method of manufacturing high strength steel bars. The method according to claim 1, Wherein the cooling treatment is carried out by water-cooling at 900 to 1000 占 폚, cooling to 500 to 600 占 폚, followed by air cooling.