KR100431850B1 - High strength steel having low yield ratio and method for manufacturing it - Google Patents

Abstract

The present invention relates to a structural steel used in construction, offshore structures, or line pipes, and the object is to provide a high strength steel having a high strength through grain refinement and a resistive ratio using bainite structure and a method of manufacturing the same.

The present invention for achieving the above object, in weight%, C: 0.07 to 0.16%, Si: 0.5% or less, Mn: 1.2-1.6%, Al: 0.005-0.1%, Ti: 0.005-0.02%, Nb: 0.01-0.06%, V: 0.03-0.08%, P: 0.03% or less, S: 0.03% or less, N: 0.003 to 0.014%, remaining Fe and other unavoidable impurities, and the microstructure of 20 to 35% High strength steel having bainite having an ordinary fraction and remaining ferrite, and having a resistivity ratio in which the ferrite grain size is 2 to 4 µm;

By weight%, C: 0.07 to 0.16%, Si: 0.5% or less, Mn: 1.2-1.6%, Al: 0.005-0.1%, Ti: 0.005-0.02%, Nb: 0.01-0.06%, V: 0.03-0.08 %, P: 0.03% or less, S: 0.03% or less, N: 0.003 ~ 0.014%, the ingot composed of the remaining Fe and other unavoidable impurities is heated in the temperature range of 1100-1250 ° C, and then 50 in the austenite recrystallization zone. First hot rolling with a total reduction rate of more than%, then second continuous multi-stage hot rolling by processing heat so that the cumulative reduction rate is more than 90% at a reduction rate of more than 20% per pass in the temperature range of Ar ₃ ± 20 ℃. Next, accelerated cooling to room temperature at a cooling rate of 5 ~ 20 ℃ / s to produce a high strength hot rolled steel sheet having a resistivity ratio of 20 to 35% bainite and the remaining ferrite, the ferrite grain size of 2 to 4㎛ It is the technical point of the method.

Description

High strength steel having low yield ratio and method for manufacturing the same

The present invention relates to structural steels used in construction, offshore structures, line pipes, and the like, and more particularly, to a high strength steel having a high strength through grain refinement and a resistive ratio using bainite structure and a method of manufacturing the same.

In general, structural steels are urgently required to have high strength in terms of reduction of members, low alloying for ease of welding, and high efficiency of transportation. In addition, the use of steel materials that can withstand strong earthquakes caused by earthquake fluctuations and strong winds caused by extreme weather conditions is required. In this sense, structural steels with excellent anti-intrinsic properties should have low yield strength (yield strength / tensile strength).

The tensile properties of materials with a resistive yield ratio produce continuous yielding. Continuous yielding causes deformation of the ferrite already created by volume expansion when austenite is transformed into bare knight, and introduces a moving potential inside. It is a phenomenon of transition. The yield ratio is dependent on the amount of austenite transformed to ferrite and the remaining residual austenite is converted to bainite by the cooling rate and is also affected by its size. Is made possible.

In general, line pipe steel, which is one of structural steels, is classified into API X52, X60, X65, X70, etc. according to the guaranteed strength. In line pipe type steel, high-strength and high-toughness steel are used in accordance with the demand for large diameter of pipe and securing high stability. Currently, X65 steel with a yield strength of 65ksi (45.7kgf / mm ² ) is most used. . In addition, since a large diameter pipe is manufactured by welding, weldability is very important in these steel materials. Therefore, the steel used in the line pipe should have a carbon equivalent value of 0.45 or less, which is an indirect measure of weld hardness.

As such, the steel for line pipes should have high strength-toughness and high solidity weldability. The API X65 steel having a yield strength of 65ksi was normally manufactured by normalizing heat treatment at around 850-900 ° C. However, the normalizing material has a number of problems, such as the carbon equivalent is more than 0.42% poor weldability, and preheating should be performed to suppress the occurrence of low temperature cracking.

In recent years, recrystallization-controlled rolling has reduced the toughness by miniaturizing austenite and increasing the rolling temperature to obtain fine grains. For example, Korean Patent Publication No. 99-205536 adds Ti, Nb, V, etc. to suppress the growth of austenite grains upon reheating, and the reduction ratio is about 20% in the recrystallization region to 6-50 ° C up to 650-550 ° C. A technique for producing API X65 material having a yield strength of 65 ksi (45.7 kgf / mm ² ) or more by water cooling to / s and then air cooling to room temperature has been proposed.

In addition, unrecrystallized station rolling is carried out, and bending is performed in an ideal area, followed by air cooling to obtain a line pipe steel having a resistance ratio. For example, Japanese Patent Application Laid-Open No. 8-283848 discloses that molybdenum (Mo) alloy is added at about 0.05 to 0.25%, rolled in a non-recrystallized zone, heated to an ideal temperature, and bent, followed by air cooling. The yield strength was 45.7 kgf / mm ² , and the steel pipe with yield ratio of 70% or less was obtained. However, there is a disadvantage in that molybdenum is added which causes a cost increase.

On the other hand, in order to increase the strength of the lypipe, such as X70 and X80, Ni, Cu, and Mo are added to prevent the growth of austenite grains and to add Nb, V, Ti for precipitation strengthening, and to increase the solid solution without inhibiting weldability. In some cases. However, this method has a disadvantage in that cost increases due to the addition of expensive alloying elements.

Conventional techniques for refining grains in structural steels include a method of rolling in (1) austenite uncrystallized regions or (2) austenite + ferritic ideal regions by controlled rolling. According to this method, it is possible to increase the nucleation site of the ferrite by generating the austenite lip stretching and intragranular deformation zone, or to refine the ferrite lip by the recovery and recrystallization of the processed ferrite. It is effective for improvement.

(1) As a method of increasing the ferrite nucleation site in the above austenite unrecrystallized rolling, for example, as known in Proceeding of Microalloying 75 (1975), p120, the cumulative pressure drop at high temperature is increased. However, ferrite grains were coarse to 10 mu m.

(2) Rolling may be performed in the above austenite + ferrite or higher region to refine the ferrite grains. For example, as known from iron and steel 65 (1979) 9, p1400, the processed ferrite was recovered and recrystallized by reheating after rolling, and the crystals became fine. However, the process is complicated by the addition of heat treatment.

On the other hand, as a method of refining the structure by cumulative rolling at high temperature, Japanese Patent Publication No. 63-050426 can be exemplified, in this case the rolling temperature range of 150 ℃, such as Ar ₃ ~ Ar ₃ +150 ℃ It defines the scope. However, due to the high temperature, the cumulative effect was not high, and the grain refinement effect was not large. An alternative reverse rolling method in which austenite and ferrite are present at or below Ar ₃ is also proposed. The low temperature causes the ferrite to be stretched without recrystallization.

It is an object of the present invention to provide fine ferrite and a bainite structure having an appropriate phase ratio and high strength while providing a high strength steel and a method for producing the steel without the addition of a large amount of alloy or heat treatment after reheating.

High-strength steel having a resistive ratio of the present invention for achieving the above object, in weight%, C: 0.07 to 0.16%, Si: 0.5% or less, Mn: 1.2-1.6%, Al: 0.005-0.1%, Ti: 0.005-0.02%, Nb: 0.01-0.06%, V: 0.03-0.08%, P: 0.03% or less, S: 0.03% or less, N: 0.003-0.014%, remaining Fe and other unavoidable impurities. The structure consists of bainite and the remaining ferrite having an phase fraction of 20 to 35%, and the ferrite has a particle size of 2 to 4 mu m.

Furthermore, the manufacturing method of the high strength steel which has the resistivity ratio of this invention is C by 0.07 to 0.16%, Si: 0.5% or less, Mn: 1.2-1.6%, Al: 0.005-0.1%, Ti: 0.005- 0.02%, Nb: 0.01-0.06%, V: 0.03-0.08%, P: 0.03% or less, S: 0.03% or less, N: 0.003-0.014%, ingots composed of the remaining Fe and other unavoidable impurities are 1100-1250 After heating in the temperature range of ℃, the first hot rolling in the austenite recrystallization zone with a cumulative reduction rate of more than 50%, and then the cumulative reduction ratio of 90% or more per pass in the Ar ₃ ± 20 ℃ temperature range is 90 Secondary continuous multi-stage hot rolling to be more than% and accelerated cooling to room temperature at a cooling rate of 5 to 20 ℃ / s 20% to 35% of the percentage of bainite and the remaining ferrite ferrite, the ferrite grain size is 2 ~ 4㎛ It consists of what consists of.

Hereinafter, the present invention will be described in detail.

The present invention is a microstructure of the API X65 grade steel with a small amount of alloy is added to the composite structure of bainite and the remaining ferrite having a phase ratio of 20-35%, the ferrite grain size is 2 ~ 4㎛ to increase the high strength and resistance ratio There is a primary characteristic of this.

In order to have a high-strength resistive microstructure as described above, first, in the present invention, ferrite can be nucleated by recrystallization rolling to make the grain size of austenite fine by reducing the reheating temperature to decrease the grain size of the austenite. Increasing the effective grain area, and increasing the amount of processing once near Ar ₃ to prevent the temperature from dropping after rolling due to the exothermic heat to recover and recrystallize the ferrite, thereby finely refining the grains of the ferrite; Second, by controlling the cooling rate after hot rolling, the remaining austenite is transformed into bainite of an appropriate phase ratio to achieve a resistance ratio. The steel of this invention based on such a characteristic, and its manufacturing method are demonstrated separately.

[Steel Ingredients]

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. Therefore, the carbon (C) is preferably limited to 0.07 to 0.16%.

The silicon (Si) is added as a deoxidizer during steelmaking, and when the content exceeds 0.5%, toughness and weldability are deteriorated and the oxide film is severely formed on the surface of the steel sheet, so it is preferably limited to 0.5% or less.

The manganese (Mn) is a basic element of steel effective for improving the strength and toughness of the steel, and if less than 1.2% is contained, its effect is low and the hardenability of the steel is lowered. Thus, bainite, which is a second phase structure during accelerated cooling after rolling, is difficult to secure. Is difficult, and if it exceeds 1.6%, the weldability is lowered, so the content thereof is preferably limited to 1.2 to 1.6%.

The phosphorus (P) is an element particularly bad for impact toughness, the lower the content is better, but it is inevitable in the steelmaking process, so the content is preferably limited to 0.03% or less so as not to adversely affect the physical properties.

Since the sulfur (S) is present as a non-metallic inclusion of Mn and is elongated by hot rolling to promote anisotropy of the steel sheet, the content thereof is preferably limited to 0.03% or less.

The aluminum (sol-Al) is an element added for deoxidation of the steel, and when less than 0.005%, a sufficient effect is not obtained, but when it exceeds 0.1%, the effect is saturated. Therefore, Al is preferably made 0.005 to 0.1%.

The titanium (Ti) is a major element that forms a TiN precipitate during the solidification process of steel to inhibit growth during heating of the ingot, and inhibits the growth of recrystallized grains during hot rolling, thereby playing a major role in refining grains of steel. The appropriate amount of Ti is determined according to the content of N. Coarse carbonitride is formed during casting when the Ti / N ratio is about 3.42 or more, which is the stoichiometric ratio of the TiN precipitate. It will not inhibit growth. Therefore, in the present invention, the amount of Ti is preferably limited to 0.005-0.02% in consideration of aspects for maintaining the Ti / N ratio at 3.42 or less together with the amount of nitrogen.

The nitrogen (N) is a major element that forms a TiN precipitate with Ti to play a major role in the grain refinement of the steel to be contained more than 0.003% for proper TiN precipitation, the more the amount of nitrogen is better to precipitate Ti completely This causes a work load in the steelmaking process, and also makes the TiN precipitate coarse so that the grain cannot be finely refined, and N can degrade toughness as an austenite stabilizing element. In view of the above aspects, the content is more preferably 0.003-0.01%.

The niobium (Nb) is dissolved in austenite to increase the hardenability, thereby finely depositing the ferrite transformation temperature in the form of (Ti, Nb) (C, N) during casting and reheating, thereby simultaneously minimizing austenite and ferrite grain sizes. Thereby greatly increasing the strength and toughness. However, when the addition amount is too large, the weldability is lowered, and when casting, NbC coalesces with Ti, Nb, C, and N deposited at high temperature, resulting in coarsening of carbonitrides, resulting in coarsening of austenite grain size, which is obtained after transformation. Since the tissue may become coarse, the content is limited to 0.01 to 0.06%.

The vanadium (V) is an element that contributes to strength increase by forming VC during cooling to increase the strength by strengthening precipitation and suppressing ferrite grain growth, but when it contains 0.08% or more, coarse VC is formed and brittle. It is preferable to limit the content to 0.03-0.08% since it is caused and the weldability is lowered.

Steel formed as described above is a composite structure of bainite + ferrite, the phase fraction of bainite is associated with the tensile strength and elongation, the higher the percentage is higher the tensile strength and the elongation is decreased. Therefore, in consideration of this, it is preferable that the phase fraction of bainite is 20 to 35%. On the other hand, the particle size of ferrite correlates with the yield strength, and in consideration of this, the particle size is preferably 2 to 4 m. That is, when the particle size of the ferrite is less than 2 µm, the yield strength is too large to obtain a resistance yield ratio of 76% or less, and when it exceeds 4 µm, it is difficult to secure sufficient yield strength. Since the design strength of building structures is set based on the yield strength of the steel, the steel with the highest yield strength is advantageous. On the other hand, the lower the yield ratio, the greater the ability to absorb external shocks such as earthquakes. It is difficult to provide a steel grade having both a high yield strength and a low yield ratio. Accordingly, in the present invention, the yield strength is considered to be 56 kgf / It is to provide a high strength steel having a resistivity ratio of at least mm ² and a yield ratio (yield strength / tensile strength) of 76% or less.

[Manufacturing method]

It is preferable to heat the ingot (slab or powder) produced in the chemical composition in the temperature range of 1100 ~ 1250 ℃. The reason is that in the present invention, high strength and high toughness are obtained by fine carbonitride precipitate and ferrite grain size refinement and dislocation reinforcement mechanism by solid solution Nb. This is because coarse TiN precipitation occurs during reheating if the heating temperature exceeds 1250 ℃.

The ingot heated to the above temperature range is first hot rolled so that the total reduction in recrystallization reverse rolling is 50% or more. This is to make the grain size of austenite sufficiently small, because if the grain size of austenite is small, the generation of strain bands is easy and nucleation of ferrite is promoted by rolling. Primary hot rolling may be carried out by rolling one pass at 50% or more, or may be multistage rolling so that the cumulative reduction ratio is 50% or more. In the present invention, the recrystallized reverse rolling is more preferably set to 1000 to 900 占 폚. Even if the austenite is recrystallized above 1000 ° C, the grain size is coarse, so that the ferrite refining effect is small due to the subsequent rolling. Since the recrystallization area is below 900 ° C, the next process is performed by the coarse austenite deformation. The ferrite refinement effect at the time is small.

Subsequently, continuous multi-stage rolling is performed in the unrecrystallized region of Ar ₃ ± 20 ℃ by exothermic heat. This is to maximize the generation of the strain band just before the transformation from austenite to ferrite. At this time, the reduction amount per pass is added more than 20%, the rolling is progressed at the same temperature by processing heat, so that the ferrite phase transformation is promoted, and it is produced by continuous multi-stage rolling so that the cumulative reduction amount (total reduction amount) becomes 90% or more. The ferrite is recovered and recrystallized so that the grains are refined.

After the hot rolling as described above, if the cooling rate is too slow, the grain size of the ferrite produced is coarse, and if it is too fast, martensite is generated and the ductility is lowered, so the cooling rate is preferably limited to 5-20 ° C / s.

Hereinafter, the present invention will be described in more detail with reference to Examples.

Example 1

A hot rolled steel sheet was manufactured using the ingot having a steel component of Table 1 under the manufacturing conditions of Table 2.

ingredient C Si Mn P S Al Nb V Ti N Ceq A 0.10 0.251 1.43 0.015 0.004 0.028 - 0.075 0.011 0.004 0.353 B 0.09 0.248 1.54 0.017 0.003 0.030 0.025 0.03 0.013 0.006 0.353 C 0.10 0.250 1.50 0.018 0.004 0.029 0.046 0.053 0.016 0.005 0.361 D 0.04 0.30 1.63 0.015 0.003 0.031 0.04 0.08 0.019 0.005 0.32 E 0.06 0.30 1.91 0.015 0.003 0.03 0.05 0.08 - 0.005 0.43 D steel is 12ppm boron-added steel.E steel is Ni + Cu: 0.61% Ceq,% = C + Mn / 6 + (Cr + Mo + V) / 5 + (Ni + Cu) / 15

In Table 2, Inventive Materials (1-6) were air-cooled by primary rolling with 50% reduction in recrystallization temperature range using A, B, and C steels similar to API X65 ash. Subsequently, rolling was carried out at a reduction ratio of 20% or more per pass on Ar ₃ directly, and continuous multi-stage rolling was carried out at the same temperature or Ar ₃ to 20 ° C. by heat of processing.

division River ingredient Rolling Type Ar3 (℃) Rolling Start Temperature (℃) Rolling end temperature (℃) Single pressure drop (%) Total pressure drop (%) Cooling condition after rolling (℃ / s) Invention One A Continuous multi-stage 770 790 750 20 90 -10 2 B Continuous multi-stage 770 780 750 30 90 -8 3 B Continuous multi-stage 770 780 760 40 90 -15 4 C Continuous multi-stage 765 790 750 20 90 -7 5 C Continuous multi-stage 765 780 750 30 90 -5 6 C Continuous multi-stage 765 780 760 40 90 -20 Comparative material 7 A Control rolling 765 1100 950 20 75 -12 8 A Control rolling - 1100 900 20 80 -9 9 B Control rolling - 1100 850 20 85 -10 10 B Control rolling - 1100 900 20 80 -5 11 C Control rolling - 1150 800 15 90 -5 12 C Control rolling - 1150 850 12 85 Air cooling 13 D Control rolling 750 1070 720 15 87 -4 14 E Control rolling 720 1070 690 15 87 -2

In order to investigate the tensile properties of the materials prepared as described above, the tensile test piece was used as the KS standard (KS B 0801) No. 4 test piece and the tensile test was carried out at a crosshead speed of 5mm / min. Tensile properties of the inventive and comparative materials were measured and the results are shown in Table 3.

division Yield strength (kgf / mm ² ) Tensile strength (kgf / mm ² ) Elongation (%) Ferrite Particle Size (㎛) Bainite fraction (%) Yield fee Invention One 56.1 74.8 31 3.7 27 75 2 54.9 75.2 29 3 33 73 3 56.5 76.3 28 3.5 32 74 4 59.4 78.1 30 2.5 25 76 5 58.8 78.5 27 3 26 75 6 56.0 80 25 3.5 35 70 Comparative material 7 49.6 62.0 32 10 15 76 8 49.5 61.0 33 9 12 81 9 51.6 64.0 28 8 18 81 10 54.1 66.1 27 7 20 82 11 47.8 58.1 33 10 10 83 12 46.1 54.5 38 12 0 85 13 53.3 63.2 49 11 20 84 14 59.4 73.0 38 8 30 81

As shown in Table 3, the inventive material (1-6) has a high yield strength and tensile strength corresponding to API X80 grade steel as in Comparative Material 14, the yield ratio is 76% or less than the comparative steel Indicated. Moreover, the comparative material 14 has a disadvantage in that the weldability is poor because of high Ceq. Achieving high strength without adding Ni or Cu can be said to have economic advantages. In the case of the same alloy, the inventive material (4-6) and the comparative material (11-12) were compared with the comparative material (11-12), the ferrite grain size was finer than that of the comparative material and the yield strength was increased. The tensile strength was high because of the large fraction.

As described above, the present invention provides a manufacturing condition for miniaturizing ferrite grains by continuous multi-stage rolling by processing exotherm at a temperature of Ar ₃ without adding a large amount of alloying or heat treatment and generating a fraction of bainite by accelerated cooling. As the resistance ratio and high strength are secured, there is a useful effect of providing a steel with excellent weldability.

Claims (7)

By weight%, C: 0.07 to 0.16%, Si: 0.5% or less, Mn: 1.2-1.6%, Al: 0.005-0.1%, Ti: 0.005-0.02%, V: 0.03-0.08%, P: 0.03% or less , S: 0.03% or less, N: 0.003 to 0.014%, remaining Fe and other unavoidable impurities, the microstructure of which is bainite and the remaining ferrite having an phase ratio of 20 to 35%, and the ferrite particle size is 2 to A high strength steel having a resistivity ratio of 4 ㎛, yield strength of 56kgf / mm ² or more and yield ratio (yield strength / tensile strength) of less than 76%. The high strength steel having a resistivity ratio according to claim 1, wherein N is 0.003 to 0.01%. The high strength steel of claim 1, wherein the steel further contains 0.01-0.06% of Nb. delete By weight%, C: 0.07 to 0.16%, Si: 0.5% or less, Mn: 1.2-1.6%, Al: 0.005-0.1%, Ti: 0.005-0.02%, V: 0.03-0.08%, P: 0.03% or less , S: 0.03% or less, N: 0.003 ~ 0.014%, ingot composed of the remaining Fe and other unavoidable impurities is heated in the temperature range of 1100 ~ 1250 ℃, and then at a total pressure drop of 50% or more in the austenite recrystallization zone. After hot rolling, secondary continuous multi-stage hot rolling is carried out so that the cumulative reduction rate is 90% or more at a reduction rate of 20% or more per pass at a temperature range of Ar ₃ ± 20 ° C, followed by a cooling rate of 5 to 20 ° C / s. A method of producing high strength steel having a resistivity ratio of 20 to 35% by weight of bainite and the remaining ferrite, and the ferrite grain size of 2 to 4 µm. The method of claim 5, wherein the primary hot rolling is carried out at 900 to 1000 ° C. 6. The method of claim 5, wherein the steel further contains 0.01-0.06% of Nb.