KR100345704B1 - A method of manufacturing high strength hot rolled steel strip with low susceptibility of SSCC - Google Patents

Abstract

PURPOSE: A method of manufacturing high strength hot rolled steel strip with low susceptibility of sulfide stress corrosion cracking (SSCC) suitable for use as pipelines transporting sour oil is provided. CONSTITUTION: The method includes the steps of reheating a steel slab comprising C 0.03 to 0.12 wt.%, 0.50 wt.% or less of Si, Mn 0.5 to 1.50 wt.%, 0.020 wt.% or less of P, 0.005 wt.% or less of S, Ca 0.0010 to 0.0050 wt.%, Cu 0.2 to 2.0 wt.%, Ti 0.01 to 0.07, Nb 0.01 to 0.07 wt.%, one element selected from the group consisting of V 0.01 to 0.07 wt.%, 1.0 wt.% or less of Ni, 1.0 wt.% or less of Cr and 1.0 wt.% or less of Mo, a balance of Fe and incidental impurities at 1100 to 1250°C; hot rolling the steel slab, wherein cumulative reduction ratio at higher than 1000°C is greater than 60 % and finish hot rolling temperature is 900 to 950°C; cooling the hot rolled steel sheet to the temperature range of 550 to 450 at a cooling rate of 6 to 13°C/sec, followed by air cooling; and heat treatment at 550 to 650°C.

Description

A method of manufacturing high strength hot rolled steel strip with low susceptibility of SSCC

The present invention relates to a method for producing a hot rolled steel sheet used in an oil well pipe or a pressure vessel for oil transportation and storage, and more particularly, to a method for producing a high strength hot rolled steel sheet having excellent resistance to corrosion of hydrogen hydrogen stress.

In the case of oil transportation and storage oil wells or steel for pressure vessels, the accident of cracking caused by the oil causes hydrogen cracking in the base material, especially in the welding part, causing large-scale equipment accidents, causing economic damage. As a countermeasure against this, it is at a time when it is absolutely necessary to derive the optimum manufacturing conditions for steels having excellent resistance to hydrogen organic stress corrosion corrosion by hydrogen sulfide.

Until now, steels with excellent resistance to cracking and corrosion of organic stresses were manufactured by cold rolling with a rolling finish temperature of 800 ° C or less and accelerated cooling at a cooling rate of 10 ° C / sec, but the tensile strength is only about 60㎏ / ㎠ grade steel. The steel plate had the drawback that the thickness should be thick. In addition, in order to perform low temperature rolling, air cooling was carried out to 800 ° C. or lower, and during the hot rolling, the yield per unit time was decreased due to the low temperature reverse rolling.

Therefore, there is an increasing need for a method capable of producing cheaply by increasing the strength to make the thickness of the steel sheet thinner and a steel material capable of increasing the rolling productivity.

Accordingly, the present invention has been made in order to solve the above-mentioned conventional problems, to provide a method for producing a hot rolled steel sheet having improved rolling productivity, high strength and excellent corrosion resistance to hydrogen-containing organic stresses, and an object thereof.

The present invention for achieving the above object, by weight% C: 0.03-0.12%, Si: 0.50% or less, Mn: 0.5-1.50%, P: 0.020% or less, S: 0.005% or less, Ca: 0.0010-0.0050 %, Cu: 0.2-2.0%, Ti: 0.01-0.07, Nb: 0.01-0.07%, V: 0.01-0.07%, Ni: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less Slabs containing species and composed of the remaining Fe and other unavoidable impurities are heated to a temperature range of 1100-1250 ° C., and the cumulative pressure reduction rate at 1000 ° C. or higher is 60% or more, and the rolling finish temperature condition is 900-950 ° C. After hot rolling, it is cooled to 550-450 ° C at a rate of 6-13 ° C / sec, followed by air cooling, and then heat-treated at a temperature of 550-650 ° C.

Hereinafter, the present invention will be described in detail.

The present invention is to control the alloying elements, second, to control the hot rolling method, and third, to control the heat treatment method to improve the above-described rolling productivity and hydrogen stress stress corrosion cracking at the same time. Same as

First, hydrogen-organic stress corrosion cracking is caused by the central segregation caused by the non-metallic inclusions and the elongated pearlite structure in the high-strength steel.

That is, 1) the alloying elements reduce the content of Si and Mn that promote P, S and co-segregation, and also reduce the carbon content, and additionally add Ca to reduce the central segregation caused by non-metallic inclusions and the cleanliness of steel. To increase.

2) Then, 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, an appropriate amount of Cu, Ni, Cr, and Mo is added for high strength.

3) In addition, the crystal grains of the base metal and the welded part are refined to suppress the elongated pearlite structure, and an appropriate amount of Ti, Nb and V is added.

Second, to secure the rolling production by hot rolling instead of the conventional low-temperature rolling, hot rolling in the rolling finish temperature range of 900-950 ℃ and then accelerated cooling.

Third, heat treatment at 500-650 ℃ to remove the processing thermal stress to improve the corrosion resistance and strength of the hydrogen-organic organic stress. According to the characteristic conditions of the present invention, it is possible to produce a high strength hot rolled steel sheet having excellent resistance to hydrogen organic stress corrosion cracking.

In more detail, when the steel component is described, first, C is added to secure the strength. For this, 0.03% or more is added, and when the amount is increased by 0.15%, the pearlite structure is increased to decrease the hydrogen stress corrosion cracking resistance. In order to increase the hardness and structure of the welded part and the base metal, it is desirable to add 0.03-0.15%.

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.

The Mn is an effective element that deoxidizes in steel and improves weldability, hot workability, and strength. However, when Mn is added, the Mn forms a non-metallic inclusion such as MnS to increase its length in hot rolling. Since cracking property is inhibited, it is preferable to add 0.5-1.50%.

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.

Ca is an element that reduces hydrogen organic stress corrosion cracking by converting and spheroidizing MnS into CaS, and when the amount thereof is less than 0.0010%, it is ineffective, and the content of more than 0.0050% is saturated, and in particular, as a nonmetallic inclusion It is preferable to add it at 0.0010-0.0050% because it remains in and impairs cleanliness.

The Cu is an element that improves the hydrogen stress stress cracking resistance in the welded portion and the base material by strengthening the matrix structure to suppress the absolute amount of hydrogen in the steel, less than 0.20% has a small effect of improving the resistance to hydrogen stress corrosion cracking If it exceeds 2.0%, it is preferable to add it in the range of 0.2-2.0% because surface bonding occurs easily during hot rolling and damages weldability during steelmaking.

Ti, Nb, and V are added to the microstructure by the micro addition of microstructures to increase the strength without increasing the carbon equivalent and the hydrogen organic cracking sensitivity. However, when the addition amount is 0.01% or less, the effect of grain refinement is achieved. If the addition is more than 0.07%, the micronization effect is saturated and no further effect is observed.

The Ni has an effect of suppressing high temperature cracking of the steel sheet due to the addition of Cu in the steel, and greatly improves hydrogen stress corrosion cracking resistance and low temperature toughness without significantly increasing the strength and hardness of the welded portion and the base portion after hot rolling. Although it is an element, the addition of 1.0% or more not only increases the hydrogen stress corrosion cracking susceptibility in the corrosion atmosphere, but is preferably added below 1.0% because it is expensive.

The Cr is an element that increases the strength, the strength is continuously increased at the addition of up to 1.0%, but since the effect is not only moderate but also economically disadvantageous, it is preferable to add Cr below 1.0%.

Mo is an element that can increase the softening resistance to heat treatment after hot rolling to improve the strength, and is added as an element that suppresses the formula that is the starting point of hydrogen organic stress corrosion cracking cracking, hardening the matrix structure at 1.0% or more. Since it is severe, it is desirable to add 1.0% or less.

The slab formed as described above is heated, but if the heating temperature is less than 1100 ° C., the slab is not sufficiently heated and the temperature is reduced during hot rolling, which 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 the heating as described above and hot rolling, it is preferable that the rolling at this time is a cumulative reduction of 60% or more at 1000 ° C. or more to ensure productivity. The rolling finish temperature is preferably 900 to 950 ° C. 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.

After the air-cooled as described above, the heat treatment to remove the processing thermal stress, if the heat treatment temperature is less than 550 ℃ processing heat stress is not completely removed, hydrogen organic stress corrosion cracking is easy to occur, if the above 650 ℃ grain coarsening phenomenon Since this occurs and the strength decreases, the heat treatment is preferably performed at 550-650 ° C. The heat treatment time does not limit the heat treatment time because it does not have a significant influence on the hydrogen stress resistance and strength compared to the heat treatment temperature as well as limiting the heat treatment time according to the thickness of the steel sheet.

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

EXAMPLE

In the vacuum melting furnace, steel grades having the components shown in Table 1 below were dissolved in a weight of 25 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 obtain a final slab having a thickness of 80 mm.

Chemical composition (% by weight) C Si Mn P S Ca Cu Ti Nb V Ni Cr Mo Comparative steel One 0.16 0.30 1.41 0.015 0.003 - - 0.015 0.04 0.05 - - - 2 0.09 0.29 2.00 0.014 0.003 0.0023 0.29 - - - 0.15 0.13 - Invention steel One 0.09 0.31 0.70 0.013 0.003 0.0020 0.28 0.014 0.04 0.05 0.16 - - 2 0.09 0.32 0.70 0.015 0.003 0.0024 0.35 0.014 0.04 0.05 - 0.25 - 3 0.09 0.30 0.70 0.018 0.002 0.0020 0.24 0.013 0.02 0.02 - - 0.30 4 0.09 0.32 0.90 0.015 0.002 0.0024 0.70 0.013 0.04 0.03 0.16 0.24 - 5 0.09 0.29 0.90 0.015 0.002 0.0023 0.67 0.012 0.02 0.03 0.15 - 0.29 6 0.09 0.25 0.90 0.015 0.003 0.0024 1.40 0.017 0.04 0.05 - 0.24 0.50 7 0.09 0.25 0.90 0.014 0.002 0.0024 1.44 0.014 0.03 0.05 0.15 0.60 0.40 8 0.05 0.25 1.40 0.009 0.003 0.0024 0.26 0.015 0.04 0.05 0.16 0.0024 0.50

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 17 mm.

Rolling pass One 2 3 4 5 6 Rolling Start Temperature (℃) 1150 1110 1070 1020 970 925 Cumulative reduction rate (%) 18.6 37.5 53.8 65.0 73.8 78.8 Thickness after rolling (mm) 65 50 37 28 21 17

As can be seen in Table 2, the cumulative reduction rate at 1000 ℃ or more was set to 65% (in the case of 1020 ℃), it was oil-cooled immediately after hot rolling. As described above, the heating temperature at the time of hot rolling, the rolling finish temperature, the accelerated cooling method after rolling, and the post-heat treatment 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 tensile strength and hydrogen organic stress corrosion cracking of the hot rolled sheet thus obtained were tested and the results are shown in Table 3 below.

At this time, in order to quantitatively express the value of hydrogen-hydrogen organic corrosion cracking, NACE Standard TM-02-84 solution (hydrogen sulfide) after first bending a specimen having a width of 15 mm and a length of 100 mm to 50% of the length from the origin to the maximum strength It was immersed in saturated 5% NaCl-0.5% acetic acid (CH ₃ COOH), pH = 3 solution) for 96 hours. Then, the non-corroded specimen is again limited to 50% of the distance from the maximum strength point to the break, and the corroded specimen is secondarily bent to obtain a stress-strain curve and stretched to a certain length before bending. Was measured and compared with quantitative values.

Steel grade Manufacture conditions Experiment result Conclusion Heating temperature (℃) Rolling Finish Temperature (℃) Accelerated cooling start temperature (℃) Accelerated cooling end temperature (℃) Accelerated Cooling Speed (℃ / sec) Heat treatment temperature (℃) Tensile Strength (kg / ㎡) Flexural Load ^* (kgf) Comparative material One Comparative Steel 1 1150 925 905 500 10 620 64.5 820 × 2 1150 905 500 10 - 63.3 104 × 3 Comparative Steel 2 1200 905 500 10 620 62.2 213 × 4 1200 905 500 16 620 64.7 0 × Invention One Inventive Steel 1 1150 910 890 500 10 620 65.2 3124 ○ 2 Inventive Steel 2 1150 925 905 500 10 620 65.9 3099 ○ 3 Invention Steel 3 1150 940 920 500 10 620 66.8 2984 ○ 4 Inventive Steel 4 1150 925 905 540 10 620 67.3 3511 ○ 5 Inventive Steel 5 1150 905 500 10 620 68.4 3694 ○ 6 Inventive Steel 6 1150 905 465 10 620 71.1 3820 ○ 7 Inventive Steel 7 1200 905 500 7 620 77.4 3219 ○ 8 Inventive Steel 7 1200 905 500 9 620 75.2 3420 ○ 9 Inventive Steel 7 1200 905 500 11 620 72.4 3128 ○ 10 Inventive Steel 8 1200 905 500 10 570 71.9 2751 ○ 11 Inventive Steel 8 1200 905 500 10 620 72.1 2639 ○ 12 Inventive Steel 8 1200 905 500 10 645 75.2 2869 ○ * The flexural load is a quantitative value for the hydrogen organic stress corrosion cracking

As shown in Table 3, only the comparative material (2) and the manufacturing conditions not subjected to heat treatment after hot rolling the comparative steel (1-2) that is excessive or insufficient content of alloying elements outside the steel component system of the present invention In case of the comparative materials (1, 3-4) prepared by the low flexural load value by the hydrogen organic stress corrosion cracking test was insufficient to prevent the hydrogen organic stress corrosion cracking.

On the contrary, the invention steel (1-3) produced by the invention steel (1-8) while the rolling finish temperature and the accelerated cooling start temperature were changed, and the invention material produced by changing the accelerated cooling end temperature (4-). 6) Invention material (7-9) manufactured by changing the accelerated cooling rate, and invention material (10-12) manufactured by changing the heat treatment temperature after hot rolling have good tensile strength value and excellent resistance to organic stress stress corrosion. Sex. At this time, the tensile strength value was over 65kg / mm2, and the hydrogen organic stress corrosion cracking test value was over 2700kgf.

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 that can improve the rolling productivity and the corrosion resistance of hydrogen hydrogen stress. Therefore, the steel provided by the present invention is applied to the storage tank or the transportation pipe of petroleum, natural gas and petroleum refinery, and can prevent the long service life of the facility and large accidents in advance.

Claims (1)

By weight%, C: 0.03-0.12%, Si: 0.50% or less, Mn: 0.5-1.50%, P: 0.020% or less, S: 0.005% or less, Ca: 0.0010-0.0050%, Cu: 0.2-2.0%, Ti: 0.01-0.07, Nb: 0.01-0.07%, V: 0.01-0.07% and Ni: 1.0% or less, Cr: 1.0% or less, Mo: 1.0% or less, and the other Fe and other The slab composed of unavoidable impurities is heated to a temperature range of 1100-1250 ° C, and the cumulative reduction ratio at 1000 ° C or higher is 60% or more and hot-rolled at a rolling finishing temperature condition of 900-950 ° C, then 6-13 ° C /. Cooling down to 550-450 ℃ at the rate of sec and then air-cooled, heat-resistant at a temperature of 550-650 ℃ characterized in that the hydrogen-resistant organic stress corrosion cracking excellent manufacturing method.