KR20000042008A - Method of manufacturing molten metal with ferrite structure - Google Patents

Abstract

PURPOSE: A method of manufacturing molten metal with a ferrite structure is provided to have high strength of 65kgf/mm¬2 as well as to meet the demand of molten metal. CONSTITUTION: An alloy steel consists of, by weight %: 0.05-0.2% of C, 0-0.5% of Si, 0.9-1.8% of Mn, 0.08-0.12% of Ti, 0.03-0.08% of V, 0.005-0.01% of N, Fe and incidental impurities. The alloy steel is casts to a steel ingot, and then the steel ingot is heated at a temperature of 1100-1300°C. The steel ingot is rolled at a rolling rate of not less than 50% at an austenitic recrystallization temperature of 850-1000°C. Ferrite is formed by rolling the steel sheet at a rolling rate of not less than 50% at a temperature of 700-850°C and annealing the steel sheet. The steel sheet is rolled at a rolling rate of not less than 50% again to recrystallize the ferrite, and finally cooled down to the normal temperature of 1-10°C.

Description

Production Method of the high strength Structure steel

The present invention relates to a method for manufacturing ferritic fine grained structural steel, specifically, high strength 65 kgf / mm2 structural steel having a yield strength of 48 kgf / mm2 or more, a tensile strength of 62 kgf / mm2 or more, and an elongation of 2% or more. It relates to a manufacturing method.

With the development of industry, structural steel is used in many fields such as industry, construction, and civil engineering, and it demands high strength quality.

This is because there is an advantage that the weight of the steel can be lowered according to the strength of the steel to reduce the volume and weight of the steel and to form a structure of the required strength.

In general, the steel used for structural steel has a tensile strength of 40 to 50kgf / mm2, which is air-cooled after performing hot rolling such as normal controlled rolling or recrystallization controlled rolling. It is prepared by a method of accelerated cooling.

Recrystallized Controlled Rolling (RCR) is the suppression of grain growth during slab reheating and high-temperature rolling in steels designed with alloy elements such as Ti, V, and N, refinement of grains by static recrystallization during rolling, and carbonitride during r → α transformation. It is a technology that can increase the strength and impact impression even if the FRT (rolling end temperature) is increased to 900 ° C to 1050 ° C by using ferrite grain refinement and precipitation strengthening.

In addition, alloying elements may be added or heat treatment (quenching or tempering) may be performed to increase the strength.

However, the structural steel described above has a tensile strength of 40-50 kgf / mm 2, which does not correspond to the high-strength structural steel of the consumer. Recently, a method of manufacturing high strength 60 kgf / mm 2 structural steel has been announced.

An example of a 60 kg class structural steel manufacturing method is to improve the strength by miniaturizing the ferrite by rolling by varying the rolling method without adding alloying elements. If this is explained in more detail, rolling 50% or more directly above the austenite transformation temperature By forming ferrite and rolling again 50% or more continuously to recrystallize the already formed ferrite to refine the ferrite to improve the strength.

The rolling method described above has applied for a patent in 1998 at Pohang Works.

However, according to the development of the industry, materials required for transportation that require high strength and low weight materials, substrates or structures requiring flotation, especially structures required for off shore, are constantly demanding high strength materials. In order to achieve this, development of high strength structural steel with a high degree of strength is required.

Therefore, the technical problem to be achieved by the present invention is to provide a method for producing a structural steel having a strength of 65kgf / ㎡ higher than the strength of the existing structural steel suitable for the demand of the structural steel described above.

1 is an optical microscope microstructure photograph of the invention and comparative steel,

Figure 2 is a microstructure photograph of the precipitate of the invention steel.

In order to achieve the above-described technical problem, ferrite is refined by using a conventional rolling method and titanium is added at a level that does not impair weldability, thereby suppressing grain growth of austenite when reheating, and adding vanadium to add carbon. Precipitation is further achieved by further improving the strength and inhibiting the growth of ferrite by these precipitates, thereby making the ferrite finer.

This will be described in detail as follows.

C: 0.05 to 0.2% by weight, Si: 0 to 0.5%, Mn: 0.9 to 1.8%, Ti: 0.008 to 0.012%, V: 0.03 to 0.08%, N: 0.005 to 0.01% and a small amount of unavoidable impurities Casting the alloy steel composed of Fe into a steel ingot, and heating the above-described ingot in a range of 1100 to 1300 ° C. and then rolling it at a reduction ratio of 50% or more at an austenite recrystallization temperature (850 to 1000 ° C.) to refine the austenite. And rolling the fine austenite plated steel sheet at a degree between 700 and 850 ° C. over 50% of Ar _{3 to} Ae ₃ , followed by slow cooling to produce ferrite, and again producing ferrite. Recrystallizing the ferrite deformed by the heat of rolling during rolling by rolling at a pressure of not less than%;

The ferrite is recrystallized steel sheet is cooled to room temperature at 1 ~ 10 ℃ / S.

Hereinafter, the reason for limitation of the components and hot rolling will be described in detail.

When the content of carbon (C) is small, the fraction of the second phase structure is lowered, and the strength is lowered. When the carbon (C) is high, the strength is increased but the elongation is deteriorated and the weldability is bad.

The silicon (Si) is added as a deoxidizer during steelmaking and also has a solid solution effect, but the impact transition temperature is increased, and when the 0.5% or more is added, the weldability is deteriorated and the oxide film is severely formed on the surface of the steel sheet. It is preferable.

When the manganese (Mn) is less than 0.9%, the hardenability of the steel is lowered, so that it is difficult to form bainite, which is a second phase structure upon cooling after hot rolling, and thus it is difficult to secure strength. It is preferable to limit it to -1.8%.

The titanium (Ti) forms an TiN precipitate to prevent grain growth of austenite during reheating and to suppress grain growth after recrystallization during rolling. Depending on the stoichiometric difference, the size of the precipitates is different. For the TiN precipitates of appropriate size, the content of Ti is preferably 0.008 to 0.012%.

When the vanadium (V) is less than 0.01%, the strength improvement effect due to carbonitride formation of the steel is lowered, and when added over 0.08%, it is difficult to dissolve in austenite steel during reheating, so the content is limited to about 0.01 to 0.08%. It is preferable.

Nitrogen (N) is an important element for forming TiN or V (C, N), if the amount is too small it is difficult to expect effective precipitates, if the amount is too large because the difficulty in the manufacturing process, the content is 0.005 ~ 0.01% It is desirable to.

At the reheating temperature, when the heating temperature is 1300 ° C. or more, the austenite particles are too coarse and delta-ferrite is partially formed in the steel, thereby degrading the properties of the steel sheet. It is preferable that the ingot heating temperature be between 1100 and 1300 ° C because it is difficult to secure the strength because it is not dissolved in.

In the recrystallization reverse rolling, even if the austenite is recrystallized at a rolling temperature of 1000 ° C. or higher, the grain size is coarse, so that the ferrite refining effect is small due to the subsequent rolling. By this process, the following process is performed, and the ferrite refinement effect at that time is small.

The rolling in the non-recrystallization zone (Ar3 ~ Ae3) is to maximize the generation of strain bands by rolling just before transformation from austenite to ferrite. At this time, the rolling reduction should give more than 50% and the ferrite after rolling 30% Slow cooling until the above-mentioned production is performed, and rolling is again 50% or more to recrystallize ferrite.

If the cooling rate is too slow, the grain size of the ferrite produced is coarse, and if it is too straight, continuous yielding occurs due to the formation of martensite or lower bainite, resulting in lower yield strength and lower ductility. It is preferable to limit to.

Hereinafter, the present invention will be described with reference to the following examples.

<Example>

Ingots composed of C: 0.15%, Mn: 1.5%, Si: 0.25%, V: 0.03%, Ti: 0.01%, N: 0.008, P: 0.003, S: 0.003 and Fe were heated to 1200 ° C. After the hot rolling in the rolling process shown in Table 2 of the invention method and cooled to room temperature at a cooling rate of 5 ℃ / S to prepare a specimen of the present invention steel,

Comparative steel 1 was prepared from the steel produced by general controlled rolling and accelerated cooling (RCR) with the same composition as the main steel and slightly increased compared to the invention steel.

Comparative steel 2 is a carbon steel, the main components of C, Si, Mn components are the same as the invention steel and comparative steel, and the alloying elements are not added to prepare a specimen using the rolling process shown in Table 2.

Table 1 shows the composition of each specimen. P and S are impurities contained in the steel as impurities.

Table 1

(wt%)

Psalms C Si Mn P S Ti V N Invention steel 0.15 0.25 1.5 0.004 0.003 0.01 0.03 0.008 Comparative Steel 1 0.15 0.25 1.5 0.02 0.007 0.01 0.05 0.005 Comparative Steel 2 0.15 0.25 1.5 0.004 0.003 - - -

Table 2 is a graph showing the rolling process.

TABLE 2

The yield strength, tensile strength, and elongation ferrite grain size of the specimens prepared as described above were measured, and the measurement results are shown in Table 3.

TABLE 3

Yield strength (㎏f / ㎠) Tensile Strength (㎏f / ㎠) Elongation (%) Ferrite Particle Size (㎛) Manufacturing method Invention steel 53 65 27 3 2-stage rolling Comparative Steel 1 40 54 30 10 RCR Comparative Steel 2 51 61 28 4 2-stage rolling

As shown in Table 3, the invention steel has a yield strength of 53 kgf / mm 2, an elongation of 27%, and a ferrite grain of 3 μm. Comparative steel 1 was manufactured by RCR method, and after reheating at 1250 ° C. for 2 hours, control rolling was carried out in 6 to 7 passes and accelerated cooling at 5 ° C./S at FRT 950 ° C., yield strength of 40 kgf / Mm 2, tensile strength 54 kgf / mm 2, and elongation were 30%. The steel of the present invention had a higher tensile strength than Comparative Steel 1 having a higher content of vanadium having a greater precipitation strengthening effect. This is because austenite crystals are reduced by 1/10 (250 → 25㎛) at the reheating temperature due to rolling in the recrystallization zone of austenite in the case of the inventive steel, and in the recrystallization zone, the austenite recrystallization grain size in the recrystallization zone is 1 It is small as about / 5 (250 → 50㎛). This means that the effective grain boundary area where ferrite can be produced is larger in the case of the invention steel. In the case of the invention steel, continuous rolling at 760 ° C. as shown in FIG. 1 first increases the strain band in the tissue by 60% deformation of austenite, which was previously reduced in the case of 1-step rolling, thereby increasing the nucleation site of the ferrite and After the ferrite in the tissue was transformed by slow cooling, the ferrite had already been recrystallized by processing exotherm by continuous 2-step rolling. At this time, the coarsening of ferrite is suppressed by carbonitride as much as possible, followed by cooling to room temperature at a cooling rate of -5 ° C / S to suppress the formation of martensite or bainite which causes continuous yield without ferrite being coarse and fine ferrite and pearlite. Got. 1 shows the optical microscope microstructure of the inventive steel (a), comparative steel 1 (b), and comparative steel 2 (c). The size of the ferrite grains was 3 µm for the inventive steel, 10 µm for the comparative steel 1, and 4 µm for the comparative steel 2, which was the smallest for the invention steel. In addition, the yield strength and the tensile strength of the inventive steel were increased compared to Comparative Steel 2 manufactured under the same rolling conditions. This is due to the reinforcing effect of precipitation of vanadium carbonitride as shown in FIG. It is due to the effect.

Due to this effect, the ferritic steel produced by the 2-pass rolling in the Ar-Ae3 region has a finer structure than that of the comparative steel rolled by the multipass and the 2-pass rolling without adding the joint. Increasing strength by precipitates showed excellent mechanical properties. In this way, carbonitride was precipitated during rolling, pressed down immediately before the fate transformation, and continuous rolling again to obtain fine ferrite, thereby obtaining ferrite fine grain type 65 kgf / mm 2 structural steel.

In order to enhance the strength of the present invention, an alloying element is added to suppress the grain growth of austenite as much as possible, to precipitate carbonitride during cooling, and to perform accelerated cooling by performing two rollings continuously at a temperature between Ar3-Ae3. By obtaining fine ferrite and suppressing the grain growth of ferrite with precipitates, the structural strength of 65kgf / mm2 grade steel is produced to achieve high strength of structural steel used for off shore or transportation means requiring high strength and low weight. Cost savings, as well as manufacturing and mainteance costs can be reduced.

Claims (1)

Tensile strength 65kgf / mm2 class structural steel in the weight ratio of C: 0.05 to 0.2%, Si: 0 to 0.5%, Mn: 0.9 to 1.8%, Ti: 0.08 to 0.12%, V: 0.03 to Casting alloy steel composed of 0.08%, N: 0.005% to 0.01% and a small amount of unavoidable impurity elements and Fe into steel ingots, and heating the cast steel in the range of 1100 to 1300 ° C., followed by austenite re-correction temperature (850 to 1000 ℃) at a rolling reduction of 50% or more to refine the austenite, 50% or more of the austenitic steel sheet is rolled at a temperature (700 to 850 ° C.) between Ar3-Ae3, followed by slow cooling to produce ferrite; Re-crystallizing the ferrite deformed by the processing heat during rolling by rolling the steel sheet on which ferrite is produced by rolling over 50% again; A ferrite fine grain structure steel manufacturing method comprising the step of cooling the steel sheet recrystallized ferrite to room temperature at 1 ~ 10 ℃ / S.