KR100340543B1 - A manufacturing method of ferritic steel with low susceptibility of corrosion - Google Patents

Abstract

PURPOSE: A manufacturing method of ferrite steel with low susceptibility of corrosion is provided to ensure the strength and the rolling productivity by the hot rolling while ensuring the good hydrogen resistant organic stress corrosion cracking property and flexing processability. CONSTITUTION: The manufacturing method of ferrite steel with low susceptibility of corrosion and flexing processability comprises: heating the slab composed of not more than 0.05 wt.% of C, not more than 0.050 wt.% of Si, 0.10 to 0.50 wt.% of Mn, not more than 0.020 wt.% of P, not more than 0.005 wt.% of S, 0.01 to 0.05 wt.% of Al, 0.0020 to 0.015 wt.% of N, 0.0010 to 0.0050 wt.% of Ca, 0.01 to 0.05 wt.% of Ti, 0.01 to 0.05 wt.% of Nb, 0.01 to 0.05 wt.% of V, the rest of Fe, and the other inevitable impurities at the temperature of 1100 to 1250 deg.C, followed by hot rolling using the condition having at least 60% of the cumulative pressing proportion at the temperature of not more than 1000 deg.C and the rolling finishing temperature of 900 to 950 deg.C; cooling in the rate of 6 to 13 deg.C/sec to 550 to 450 deg.C; and then air cooling.

Description

A manufacturing method of ferritic steel with low susceptibility of corrosion}

The present invention relates to a method for producing a hot rolled steel sheet used in oil well pipes or pressure vessels for oil transportation and storage, and more particularly to a method for producing ferritic hot rolled steel sheet having excellent corrosion resistance and cracking resistance of hydrogen organic stress. It is about.

In the case of oil transportation and storage oil wells or steel for pressure vessels, the cracking caused by the emulsion is caused by hydrogen sulfide cracking at the bending point of the base metal and the welding part. The situation is doing damage. As a countermeasure against this, there is an urgent need for supplying steel materials having excellent resistance to hydrogen stress, organic stress cracking and bending by hydrogen sulfide, and at the time of developing a manufacturing technology for such steels.

Until now, steel materials with excellent resistance to cracking of hydrogen-containing organic stresses were generally manufactured by hot rolling at 900 ° C. or higher and air cooling, or by cold cooling at a cooling rate of 10 ° C./sec and cold rolling having a rolling finishing temperature of 800 ° C. or lower. High temperature rolling and air cooling in such conventional steels have a disadvantage in that the strength is reduced by coarsening of grains. In addition, low-temperature rolling and accelerated-cooling steels in conventional steels are air-cooled up to 800 ° C or lower for low-temperature rolling at the time of manufacture. Had a downside.

Accordingly, the present invention has been made in order to solve the above-described conventional problems, the method of manufacturing a hot rolled steel sheet which can ensure the hydrogen resistance stress cracking and bending processability while ensuring strength and rolling productivity by high temperature rolling at the same time To provide, for that purpose.

The present invention for achieving the above object by weight, C: 0.06% or less, Si: 0.50% or less, Mn: 0.10-0.50%, P: 0.020% or less, S: 0.005% or less Al: 0.01-0.05%, Slabs composed of N: 0.0020-0.015%, Ca: 0.0010-0.0050%, Ti: 0.01-0.05%, Nb: 0.01-0.07%, V: 0.01-0.07% and the remaining Fe and other unavoidable impurities are 1100-1250 ° C. After heating to the temperature range of and then the cumulative reduction rate below 1000 ℃ 60% or more, hot-rolled under the rolling finish temperature conditions of 900-950 ℃ and then to 550-450 ℃ at a rate of 6-13 ℃ / sec It is cooled and then air cooled.

Hereinafter, the present invention will be described in detail.

The present invention is a ferrite single-phase structure to make most of the structure of the organic stress stress corrosion cracking and bendability at the same time, to refine the structure and at the same time to increase the cleanliness of the steel, and to improve the rolling productivity, high temperature zone Although rolling is carried out, it has the characteristics.

According to the present invention, it has been found that hydrogen organic stress corrosion cracking is caused by the central segregation caused by nonmetallic inclusions in the elongated pearlite structure and the high strength steel.

Therefore, 1) by lowering the carbon content to suppress the amount of the pearlite structure as much as possible to make most of the structure as a ferrite single-phase structure.

2) And, as the alloying element to reduce the content of Si and Mn to promote P, S and co-segregation (Ca) and to add additional Ca to reduce the central segregation by non-metallic inclusions and to increase the cleanliness of the steel.

3) In addition, the steel component system is designed to reduce the hydrogen absorption by increasing the strength of the matrix structure inside the steel. Hydrogen organic stress corrosion cracking occurs when the amount of hydrogen absorbed is greater than the threshold amount of hydrogen organic stress corrosion cracking. A way to prevent this should be to reduce the amount of hydrogen absorbed and increase the amount of critical hydrogen that can generate hydrogen organic stress corrosion cracking. As a specific method, the most effective method to refine the grains of the base material and the welded part is to add an appropriate amount of Al, N, Ca, Ti, Nb, V.

In addition, in order to refine the ferrite crystal grains, the cumulative reduction ratio at 1000 ° C. or lower is 60% or more, and the rolling is not performed at low temperature as in the conventional method, and hot rolling is carried out at a rolling finishing temperature range of 900-950 ° C. After rolling, accelerated cooling to 450-550 ° C., and then air-cooled, it is possible to produce a steel having a ferrite single-phase structure excellent in corrosion resistance and cracking resistance of the organic hydrogen stress.

In more detail, when the steel component system is described, first, the C increases the amount of pearlite when its content is increased, thereby increasing the pearlite structure which is elongated, thereby reducing the hydrogen stress cracking resistance of the organic hydrogen and the difference between the hardness and texture of the welded part and the base metal. It is preferable to suppress it to 0.06% or less, because it acts as a cause of increasing.

The Si deoxidizes in steel and improves corrosion resistance, but if it is added more than 0.50%, it is preferable to add 0.50% or less because it acts as a non-metallic inclusion in the steel and impairs hydrogen organic cracking.

Mn is an effective element which deoxidizes in steel and improves weldability, hot workability and strength, but it is not effective at 0.10% or less, and when it is added more than 0.50%, Mn forms a non-metallic inclusion such as MnS and elongates during hot rolling. It is preferable to add 0.10-0.50% because it inhibits organic stress cracking.

When P is a high content, it is a harmful element that segregates at grain boundaries and causes brittle cracks and high temperature cracks in the base metal and welds. Therefore, P is required to be controlled at 0.020% or less.

S is an element that is very detrimental to corrosion resistance, so the smaller the content, the better, but it is impossible to completely remove it, and it is necessary to suppress the content as much as possible. When it exceeds 0.005%, non-metallic inclusions such as MnS are present in a large amount in hydrogen It is necessary to suppress the content to 0.005% or less because it impairs organic stress corrosion cracking.

The Al is added as a deoxidizer and forms a nitride of AlN to effect grain refinement. If the addition amount is less than 0.01%, there is no effect of adding it, and if it is more than 0.05%, it is present as a non-metallic inclusion to lower the cleanliness of the steel. Al is preferably added at 0.01-0.05%.

N is an element added to form nitrides with Nb, Ti, V, and Al to make crystal grains fine. If N is less than 0.0020%, N is not effective, and if it is 0.015% or more, N is not dissolved in solid solution. N is preferably added at 0.0020-0.015% since present.

The Ca is an element that reduces hydrogen organic stress corrosion cracking by converting and spheroidizing MnS, which is a non-metallic inclusion elongated in the rolling direction, into a CaS. It is preferable to add Ca at 0.0010-0.0050% because it remains in the solution and impairs cleanliness of the steel.

The Ti, Nb, V is a component capable of miniaturizing the crystal grains of the matrix structure by a small amount of addition, thereby increasing the strength without increasing the carbon equivalent and the hydrogen organic cracking sensitivity, and when the addition amount is 0.01% or less, there is no grain refinement effect. If it is added more than 0.05%, it is preferable to add it at 0.01-0.05% because the micronization effect is saturated and no further effect is shown.

When the slab formed as described above is heated, the heating temperature is less than 1100 ℃ not only is not sufficiently heated, but also the temperature is reduced during the hot rolling, adversely affects the quality characteristics. In addition, the precipitates distributed in the steel should be re-dissolved only when heated to more than 1100 ℃ to bring about the effect of increasing the strength by increasing the capacity and to reduce the hydrogen organic stress corrosion cracking susceptibility. However, when the heating temperature exceeds 1250 ℃, it is preferable to heat to 1100-1250 ℃ because defects are likely to occur due to grain coarsening and over-production of the surface scale.

After heating as described above and hot rolling, it is preferable that the cumulative reduction rate at 1000 ° C. or less is 60% or more, and the rolling finishing temperature is 900-950 ° C. to make the ferrite crystal grains fine at this time. . The reason is that when the finish temperature of rolling finish is over 950 ℃, the coarsening of grains cannot be expected, and it is impossible to bring about the synergistic effect of strength due to the grain refinement. This is because the grain refining effect is less because it cannot be imported.

Accelerated cooling after hot rolling as described above, in which the accelerated cooling start temperature is directly related to the hot rolling temperature and is automatically determined, so it is not meaningful to limit the temperature. Accelerated cooling will begin at temperatures below 15-30 ° C. In addition, the accelerated cooling rate is the biggest factor affecting the stressed weave, the strength is not sufficiently hardened when the accelerated cooling rate is less than 6 ℃ / sec, the strength decreases, and martensite hardened structure is generated when the accelerated cooling rate is more than 13 ℃ / sec hydrogen hydrogen Accelerated cooling at a rate of 6-13 ° C./sec is desirable because it impairs stress corrosion cracking. As described above, the accelerated cooling end temperature at the time of accelerated cooling should be such that a complete ferrite structure with little austenite structure can be shown. That is, above 550 ° C, austenite structure remains, which is likely to result in incomplete quenching, and when it reaches 450 ° C, sufficient accelerated cooling effect is achieved. However, when it is below 450 ° C., since excess water and time are consumed, it is preferable to terminate accelerated cooling at a temperature of 550-450 ° C. After completion of the accelerated cooling at the temperature described above, air cooling may be performed at room temperature.

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

EXAMPLE

In the vacuum melting furnace, steel grades prepared as shown in Table 1 below were melted in a vacuum induction furnace so that the weight was 50 kg, to prepare a steel ingot having a thickness of 120 mm. The steel ingot was heated at 1250 ° C. for 2 hours, and then rough-rolled to a final thickness of 80 mm. Got a slab.

Steel grade Chemical composition (% by weight) C Si Mn P S Al N Ca Ti Nb V Comparative steel One 0.09 0.29 1.52 0.014 0.003 0.035 0.0056 0.0023 - - - 2 0.02 0.25 1.41 0.018 0.003 0.015 0.015 - 0.015 0.04 0.05 Invention steel One 0.01 0.01 0.22 0.013 0.003 0.014 0.0030 0.0020 0.014 0.01 0.05 2 0.02 0.01 0.32 0.018 0.002 0.013 0.0052 0.0020 0.013 0.01 0.02 3 0.03 0.03 0.40 0.015 0.003 0.017 0.0077 0.0024 0.017 0.02 0.05 4 0.03 0.04 0.44 0.009 0.003 0.015 0.013 0.0024 0.015 0.03 0.05

The slab having a thickness of 80 mm thus obtained was hot rolled in six passes as shown in Table 2 to obtain a hot rolled sheet having a thickness of 14 mm.

Rolling pass One 2 3 4 5 6 Rolling Start Temperature (℃) 1150 1110 1070 985 955 925 Cumulative reduction rate (%) 18.6 37.5 53.8 63.8 72.5 78.8 Thickness after rolling (mm) 65 50 37 29 22 17

As can be seen in Table 2, the cumulative reduction rate at 1000 ° C. or less was 63.8% (when 985 ° C.), and oil-cooled immediately after hot rolling. As described above, the heating temperature, the rolling finish temperature, and the accelerated cooling after rolling in hot rolling were the conditions shown in Table 3 below. For reference, the accelerated cooling start temperature was lowered by 20 ° C from the rolling finish temperature. The hydrogen-organic stress corrosion cracking test and the bendability test of the test piece thus prepared are shown in Table 3 below.

At this time, the hydrogen organic stress corrosion cracking test was conducted by bending a specimen of width 15mm and length 100mm to the strength of 50% of the maximum strength point and then NACE Standard TM-02-84 solution (hydrogen saturation 5% NaCl-0.5% acetic acid (CH ₃ COOH), pH = 3 solution) was immersed for 96 hours to determine the presence of cracks.

In the bending workability test, the test piece was bent at 180 ° from the center to observe cracks.

Steel grade Manufacture conditions Experimental crack crack Conclusion Heating temperature (℃) Rolling Finish Temperature (℃) Accelerated cooling start temperature (℃) Accelerated cooling end temperature (℃) Accelerated Cooling Speed (℃ / sec) SSCC ^* Flexibility test Comparative material One Comparative Steel 1 1150 925 905 500 1.5 Occur Occur × 2 1150 925 905 500 10 Occur Occur × 3 Comparative Steel 2 1200 925 905 500 10 Occur Occur × Invention One Inventive Steel 1 1150 910 890 500 10 none none ○ 2 1150 925 905 500 10 none none ○ 3 1150 940 920 500 10 none none ○ 4 Inventive Steel 2 1150 925 905 540 10 none none ○ 5 1150 925 905 500 10 none none ○ 6 1150 925 905 465 10 none none ○ 7 Invention Steel 3 1200 925 905 500 7 none none ○ 8 1200 925 905 500 12 none none ○ 9 Inventive Steel 4 1200 925 905 500 7 none none ○ * SSCC is hydrogen organic stress corrosion cracking

As shown in Table 3, the comparative material (1) and the manufacturing conditions produced by slowing the cooling rate of the comparative steel (1-2) that is excessive or insufficient content of alloying elements outside the steel component system of the present invention is In the case of the comparative material (2-3) prepared under the conditions of the invention, hydrogen organic stress corrosion cracking occurred, and cracking occurred even after the bending test.

On the other hand, the invention material (1-3) which manufactured the invention steel (1) while changing rolling finishing temperature and the accelerated cooling start temperature, and the invention material which produced the invention steel (2) while changing the acceleration cooling end temperature (4- 6) Invention material (7-8) and invention material (9) produced by varying the accelerated cooling rate of the inventive steel (3) did not generate hydrogen organic stress corrosion cracking, and cracks did not occur even after bending test. Did.

As described above, the present invention, unlike the conventional low-temperature-accelerated cooling type high tensile steel sheet, can be refined in the grain size by repeated recrystallization of austenite by high temperature zone cold rolling of more than 900 ℃ and rolling in a short time without air cooling atmosphere By doing so, there is an effect of improving rolling productivity. Therefore, the steel sheet produced by the present invention can be used in the bent portion of the storage tank or transport pipe of petroleum, natural gas and petroleum refinery, and can also be applied to the bent portion of the galvanizing tank to prevent prolonged service life of the equipment and large accidents. It is.

Claims (1)

By weight%, C: 0.06% or less, Si: 0.50% or less, Mn: 0.10-0.50%, P: 0.020% or less, S: 0.005% or less Al: 0.01-0.05%, N: 0.0020-0.015%, Ca: A slab consisting of 0.0010-0.0050%, Ti: 0.01-0.05%, Nb: 0.01-0.07%, V: 0.01-0.07% and the remaining Fe and other unavoidable impurities is heated to a temperature range of 1100-1250 ° C., followed by 1000 Cumulative pressure reduction rate of less than 60 ℃, hot rolling at 900-950 ℃ rolling finishing temperature conditions, and then cooled to 550-450 ℃ at a rate of 6-13 ℃ / sec, and then cooled by air Method for producing ferritic hot rolled steel sheet with excellent organic stress corrosion cracking and bending processability.