CN110656288A - Steel for ultra-wide, ultra-thick and high-toughness X80 straight welded pipe and production method thereof - Google Patents

Abstract

The invention discloses a steel for X80 straight seam welded pipe with ultra-wide, extra-thickness and high toughness and a production method thereof. In steel, C: 0.035% to 0.055%, Si: 0.12% to 0.22%, Mn: 1.60% to 1.75%, P≤0.010%, S≤0.0015%, Nb: 0.06% to 0.10%, Ti: 0.008% ～0.020%, Ni: 0.10%～0.24%, Mo: 0.15%～0.24%, Cr≤0.25%, (Ni+Mo+Cr): 0.40%～0.65%, Al: 0.010%～0.030%, N: 0.0010 %～0.0040%, Ca: 0.0015%～0.0045%, Ca/S≥1.8, the balance is iron and inevitable impurities. The slab heating temperature is 1200～1230℃, the soaking temperature is 1180～1210℃; the rough rolling temperature is 1100～1140℃, the finishing rolling temperature is 810～840℃, and the cooling temperature is 730～760℃. The thickness of the finished steel plate is ≥31mm, the width is ≥4330mm, and the low temperature toughness is good.

Description

Steel for ultra-wide, extra-thick, high-toughness X80 straight seam welded pipe and its production method

technical field

The invention belongs to the technical field of low-carbon and low-alloy steel, and in particular relates to an X80-level ultra-wide, extra-thick and high-temperature-toughness hot-rolled steel plate with a thickness of ≥ 31 mm and a width of ≥ 4330 mm for a low-temperature environment large-capacity oil and gas transportation straight seam welded pipe and its production method.

Background technique

For a long time, increasing the efficiency of oil and gas pipeline transportation has been the goal pursued in the field of pipeline transportation. With the increasing consumption of oil and natural gas, the need to improve the efficiency of pipeline transportation has become more urgent. Increasing the diameter of the pipeline and the transmission pressure is an important measure to improve the efficiency of oil and gas transportation; however, it must be premised on ensuring the safety of the pipeline. In order to increase the diameter of the pipeline, the width of the steel used for the straight seam welded pipe must be increased; in order to ensure safety and increase the conveying pressure, the steel plate must have higher strength, toughness and greater thickness; moreover, the pipeline serving in the low temperature environment must also have good quality. Therefore, the development of steel for oil and gas transportation longitudinally welded pipes with comprehensive technical characteristics such as ultra-wide, extra-thick, high strength and good low-temperature toughness has become an urgent need for oil and gas pipeline construction.

The complexity and diversity of technical features and indicators have significantly increased the difficulty of research and development of ultra-wide, extra-thick, and high-toughness X80 longitudinally welded steel pipes. First of all, the increase of the width of the steel plate increases the resistance to rolling deformation, the deformation rate of the pass is limited, and the difficulty of grain refinement and control is doubled. And the uniformity of rapid cooling has put forward higher requirements; and the increase in the thickness of the steel plate will lead to an increase in the temperature gradient of the thickness section, which will deteriorate the cooling in the thickness direction and the uniformity of the structure; secondly, the width and thickness of the steel plate will also intensify the strength-toughness contradiction. Increase the difficulty of performance control.

At present, there are some researches on wide and thick pipeline steel plates for oil and gas transportation at home and abroad, and some patents and literatures have been found through search, but the contents of the records are the same as the components, production methods, properties, product categories and size specifications described in the technical solution of the present invention. There are obvious differences among others.

CN107502836A discloses a thick-walled, large-diameter, high-grade pipeline steel with improved low-temperature toughness and a manufacturing method thereof, and provides a wide and thick plate for X80 level large-diameter longitudinally welded pipe and a manufacturing method thereof. Ni (0.25%~0.30%) design, high cost; manufacturing process requires the final pass reduction ratio of rough rolling to be greater than 25%, which requires too much equipment capacity and is not suitable for the production of high-strength and wide-specification pipeline steel.

CN108396299A discloses a production method of X80 pipeline steel wide and thick plate, and provides an X80 pipeline steel wide and thick plate and its production method. High temperature rough rolling is used, which is not conducive to the strengthening and grain refinement of the steel plate.

CN107385326A discloses a production process of ultra-fine grain wide and thick pipeline steel plate. Provided is a production process for wide and thick pipeline steel. The casting billet heating temperature is low (1120-1140° C.), which is not conducive to the solid solution of elements, especially Nb; The requirements for equipment capabilities are too high.

RU2270873 (C1) discloses a production method of a steel plate for high-performance submarine gas transmission welded pipelines, and provides a submarine pipeline steel plate and a production method, in which more elements such as Ni and Cu are added to the components, and the cost is high; the production method requires The cooling rate after rolling is 35～55℃/s, and the process is difficult to realize.

Hong Liang, Zuo Xiurong, Ji Yinglun and others mainly introduced the low temperature fracture toughness and microstructure of 27.2mmX80 thickness in "Research on Low Temperature Fracture Behavior of Thick X80 Pipeline Steel" published in "Journal of Materials Research", Vol. 32, No. 1, 2018 It emphasizes the effect of acicular ferrite on improving low temperature toughness, and the product thickness is small, and the specific production process is not involved in the text.

To sum up, the research on the steel for X80 straight seam welded pipe with extra wide, extra thick and high toughness is still insufficient in the prior art.

SUMMARY OF THE INVENTION

The purpose of the present invention is to overcome the deficiencies of the prior art, solve the technical index matching problems such as the size, strength, low temperature toughness and the like of the ultra-wide and extra-thick pipeline steel, and provide a straight seam welded pipe for oil and gas transportation in a low temperature environment with a thickness of ≥31 mm and a width of ≥ A 4330mm X80-grade hot-rolled steel sheet with ultra-wide, extra-thick, high-strength, good low-temperature toughness and a production method thereof.

The thickness of the ultra-wide, extra-thick and high-toughness X80 longitudinally welded pipe steel of the invention is ≥ 31 mm and width ≥ 4330 mm; the composition design improves the toughness of the material through low C and low Mn, and uses Nb and Ti elements to suppress the growth of austenite grains And in the process of austenite transformation, it promotes nucleation to refine the grains, control the thickness of the core structure of the steel plate, and improve the uniformity of the structure. Increasing the rolling temperature can effectively reduce the rolling resistance and increase the deformation rate of the rolling pass; increase the solid solution strengthening effect by Ni element and use Ni element to improve low temperature toughness; with corresponding smelting, heating, rolling, Cooling, straightening and other production processes can obtain comprehensive properties such as ultra-wide, extra-thick, high-strength, good low-temperature toughness and ideal microstructure.

The specific technical solutions are:

The chemical composition weight percentages of the ultra-wide, extra-thick and high-toughness X80 longitudinally welded pipe steel according to the present invention are C: 0.035%-0.055%, Si: 0.12%-0.22%, Mn: 1.60%-1.75%, P≤0.010% , S≤0.0015%, Nb: 0.06%～0.10%, Ti: 0.008%～0.020%, Ni: 0.10%～0.24%, Mo: 0.15%～0.24%, Cr≤0.25%, (Ni+Mo+Cr) : 0.40% to 0.65%, Al: 0.010% to 0.030%, N: 0.0010% to 0.0040%, Ca: 0.0015% to 0.0045%, Ca/S≥1.8, and the balance is iron and inevitable impurities.

The super-wide, extra-thick, high-toughness X80 straight seam welded pipe steel CE _IIW of the present invention is controlled at 0.39% to 0.43%, and CE _IIW =C+Mn/6+(Cr+Mo)/5+(Ni+Cu)/15 ; CE _Pcm is controlled at 0.15% to 0.18%, CE _Pcm =C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B.

The reasons for the composition design of the present invention are as follows:

C: C can play a strengthening role through interstitial solid solution, and can also act with alloying elements such as niobium to form fine carbide precipitation, which precipitates before rolling deformation or austenite transformation, hinders grain growth and improves nucleation rate , refines the structure; at the same time, it can also hinder the movement of dislocations and effectively improve the strength, therefore, the carbon content should not be too low; however, the increase of carbon is unfavorable to the toughness, especially for the low temperature toughness; moreover, the increase of carbon When the billet is reheated, the solid solution temperature of carbide is increased to promote the growth of crystal grains, so the carbon content cannot be too high, and the present invention considers that it is appropriate to control carbon at 0.035% to 0.055%.

Si: Si plays a strengthening role and is also beneficial to improve hardenability; however, if its content is too high, M/A in the structure will increase, bainite grain size will increase, resulting in reduced toughness and easy to cause cold brittleness. It is 0.12% to 0.22%.

Mn: Mn can effectively improve the strength and hardenability; reduce the austenite transformation temperature, inhibit the growth of the transformation grains before the accelerated cooling of the steel plate, and play the role of grain refinement; however, too high manganese content is easy to induce segregation, It deteriorates the microstructure uniformity and thickness center toughness of the thick-walled steel plate and is not conducive to welding. The present invention considers that it is more appropriate to control the manganese content to 1.60% to 1.75%.

P, S: P and S are harmful impurity elements in the present invention, and the lower the content, the better; among them, P has obvious adverse effects on low temperature toughness, the present invention controls P to be less than or equal to 0.010%, and the increase of S content will promote inclusions The formation and growth of the material will deteriorate the performance, therefore, S≤0.0015%.

Nb: The role of niobium in the present invention includes (1) solid solution strengthening; (2) precipitation during rolling and before accelerated cooling, pinning grain boundaries, promoting nucleation, and effectively refining grains, thereby improving strength and Improve toughness; (3) reduce the austenite transformation temperature, which can refine grains; (4) increase the recrystallization rolling temperature, reduce the rolling force, and improve the rolling of the ultra-wide and extra-thick pipeline steel plate of the present invention. The pass deformation rate and grain refinement effect are obvious; however, excessive niobium content will deteriorate the toughness of the weld and the heat affected zone, and also increase the cost. The present invention considers that it is appropriate to control the niobium content to 0.06% to 0.10%.

Ti: Ti can exert a nitrogen fixation effect, form a precipitation phase mainly composed of TiN, can inhibit the grain growth of austenite under high temperature conditions, and can also improve the toughness of the heat-affected zone after welding. The present invention considers that the titanium content should be controlled to 0.008% ~0.020% is suitable.

Ni: Ni can increase the strength, improve the low temperature toughness, and is conducive to a good match of strength and toughness; Ni can reduce the critical cooling rate, delay the transformation of pearlite, and control the microstructure, grain refinement and uniformity of the extra-thick pipeline steel plate according to the present invention. However, the price of nickel is relatively high, so the present invention controls its content to 0.10% to 0.24%.

Mo: Mo is an effective strengthening element, which can improve the hardenability, which is beneficial to improving the cooling effect of thick-walled pipeline steel plates; it can also promote the transformation of medium and low temperature microstructures, and also has a certain effect of grain refinement. However, the higher price of molybdenum, the Increase the cost, therefore, the present invention controls its content at 0.15% to 0.24%.

Cr: Cr has a solid solution strengthening effect, increases the stability of austenite, reduces the austenite transformation temperature, and can improve the uniformity of the thickness direction; but too high chromium content is not good for plasticity and weldability, so the chromium content is controlled to 0.25 %the following.

Too low Ni+Mo+Cr is not conducive to performance and microstructure control, and too high content affects weldability and economy. Therefore, the present invention controls (Ni+Mo+Cr) to be 0.40% to 0.65%.

Al: Al is a deoxidizing element, and an excessively high content is detrimental to weldability. The present invention considers that the aluminum content should be controlled at 0.010% to 0.030%.

N:N can form fine precipitates with niobium and titanium at high temperature, and play the role of fine grain and precipitation strengthening, thereby improving the strength and toughness, but the content is too high to deteriorate the toughness.

Ca: Ca mainly promotes the denaturation of inclusions and reduces the damage of inclusions to performance. The present invention considers that Ca: 0.0015% to 0.0045% and Ca/S≥1.8 can achieve an ideal control effect of inclusions.

The invention controls CE _IIW at 0.39%-0.43% and CE _Pcm at 0.15%-0.18%, which can not only ensure the strength and toughness of the steel plate, but also make the steel plate have suitable weldability.

The production method of the steel for X80 straight seam welded pipe with ultra-wide, extra-thick and high toughness according to the present invention comprises molten iron pretreatment, converter smelting, out-of-furnace refining, continuous casting, slab heating, rolling, cooling and straightening; wherein, converter smelting Tapping temperature ≤ 1640 ℃, C ≤ 0.035%, the use of lower tapping temperature can ensure the phosphorus control effect of the converter, and the lower carbon content of the tapping can ensure that the carbon content of the final product is effectively controlled. The casting superheat degree of continuous casting billet is 10～25℃, and the thickness of continuous casting billet/finished steel plate thickness is ≥8; the control of casting superheat degree can effectively reduce the quality defects of casting billet; increasing the compression ratio of continuous casting billet to finished steel plate can effectively control the crystallinity. grain size.

The temperature of the heating section of the continuous casting billet is 1200-1230 °C, the temperature of the soaking section is 1180-1210 °C, and the soaking section time is not less than 50 minutes; the heating process can meet the solid solution of the alloy, especially the niobium element, and at the same time, prevent the austenite grains. Excessive growth; heating time can ensure the uniformity of billet temperature.

The starting temperature of rough rolling is 1100~1140℃, and the final rolling temperature of rough rolling is 1020~1050℃. The horizontal and vertical rolling method is adopted. After the horizontal rolling of the cast slab, the spray + air cooling method can be used to cool down to the starting temperature of vertical rolling. The starting temperature of longitudinal rolling is lower than 1070℃, and the rolling speed is 1.0～2.0m/s. In the longitudinal rolling stage, the deformation rate of each pass of at least the last 2 passes is guaranteed to be greater than 15% and the time interval between passes does not exceed 15s; in the rough rolling stage The high rolling temperature and deformation process recrystallize the austenite grains and inhibit the grain growth. The accelerated cooling + air cooling after rough rolling and cross rolling is beneficial to inhibit the grain growth. The rolling deformation penetrates into the thickness center of the slab, and the structure near the thickness center is refined, which is beneficial to improve the performance of extra-thick pipeline steel; the use of large pressure reduction and short interval process at the end of longitudinal rolling can reduce the equipment load of ultra-wide pipeline steel, and use more The superposition effect of the pass deformation promotes the recrystallization of austenite and achieves the goal of grain refinement, which is suitable for the production of the ultra-wide and extra-thick pipeline steel plate of the present invention.

The thickness of the intermediate billet to be warmed is 2.5t~3.4t, where t is the thickness of the finished steel plate, the starting temperature of finishing rolling is 810~840℃, and the final rolling temperature of finishing rolling is 740~770℃, to ensure the cumulative deformation rate below 790℃ Not less than 20%; the appropriate intermediate thickness of the hot billet can not only meet the accumulation of austenite deformation and deformation energy in the unrecrystallized area, but also ensure that the rough rolling stage can obtain a sufficient deformation rate under the condition that the thickness of the original billet is constant. To achieve the purpose of grain refinement; low finishing rolling temperature promotes the accumulation of austenite deformation energy and the induced precipitation of Nb and Ti fine precipitation phases, increasing the nucleation position; sufficient deformation near the transformation point temperature at the end of finishing rolling has It is conducive to the formation of fine ferrite, which can reduce the effective grain size and significantly improve the low temperature toughness.

After rolling, the steel plate shall be accelerated by water cooling, the interval between the completion of rolling and the start of water cooling shall not exceed 25s, the starting temperature of water cooling shall be 730-760°C, the final cooling temperature shall be 380-460°C, and the cooling time of water cooling shall not be less than 15s; then, heat straightening shall be carried out. And air cooling, then, after secondary straightening at 100 ~ 250 ℃, air cooling to room temperature. Controlling the starting water cooling temperature and interval time of the steel plate can inhibit the growth of pre-eductive ferrite, reduce the effective grain size, and improve the low temperature toughness; the final cooling temperature can promote the formation of bainite and refine the M/A; longer water cooling time It can improve the cooling uniformity and improve the plate shape while ensuring the water cooling effect; the appropriate secondary straightening temperature can effectively release the internal stress of the steel plate, and the steel plate will not be deformed after straightening, which is useful for ensuring the shape of the ultra-wide and extra-thick steel plate. important role.

The final microstructure of the steel plate is mainly composed of bainite + fine polygonal ferrite, in which the volume percentage of polygonal ferrite is 5% to 25%, and the grain size reaches grade 11 or finer. The average size of the bulk grain is less than 7μm; the steel plate has ultra-wide, ultra-thick, high strength and good low-temperature toughness, which meets the requirements of making Φ1422mm large-diameter thick-walled X80 oil and gas pipelines for low-temperature environments.

Beneficial effects:

Compared with the prior art, the present invention has the following beneficial effects:

(1) The composition of the present invention improves the toughness of the material through low C and low Mn design, uses Nb and Ti elements to suppress the growth of austenite grains and promotes nucleation in the austenite transformation process to refine the grains, control At the same time, according to the characteristics of ultra-wide and high-strength, the effect of Nb to inhibit the recrystallization of austenite is used to increase the rolling temperature, effectively reduce the rolling resistance, and also help to increase the rolling pass Deformation rate; increase the solid solution strengthening effect by Ni element and improve low temperature toughness by using Ni element; with the corresponding unique production process, it solves the matching problem of technical indicators such as size, strength and low temperature toughness of ultra-wide and extra-thick pipeline steel.

(2) The carbon equivalents CE _IIW and CE _Pcm of the present invention are suitable to ensure that the material has good strength and weldability.

(3) The smelting and continuous casting process scheme of the present invention realizes low phosphorus and low carbon control, improves the quality of the casting billet, and thus improves the performance of the final product.

(4) The present invention adopts a unique production process, fully refines the grains, obtains the microstructure of bainite + fine polygonal ferrite, exerts the strengthening and fine-graining effect of fine precipitation, and effectively improves product performance, especially at low temperature. Toughness, at the same time, increase cooling uniformity and warm straightening, effectively control the shape of the ultra-wide steel plate.

(5) The thickness of the ultra-wide, extra-thick, high-toughness X80 longitudinally welded pipe of the present invention is ≥ 31 mm, width ≥ 4330 mm, the transverse yield strength can reach 530-590 MPa, the transverse tensile strength can reach 650-710 MPa, and the transverse yield-strength ratio can reach 530-590 MPa. Not more than 0.88, the average transverse impact energy at -60°C is ≥360J, the transverse DWTT shear area at -20°C is ≥85%, and the microstructure is mainly composed of bainite + fine polygonal ferrite. Among them, polygonal ferrite The volume percentage is between 5% and 25%, the grain size is 11 or finer, and the average size of polygonal ferrite grains is less than 7μm, which meets the requirements of making Φ1422mm large-diameter thick-walled X80 oil and gas pipelines for low temperature environments.

Description of drawings

FIG. 1 is the metallographic structure of the steel sheet in Example 2 of the present invention.

Detailed ways

The following embodiments are used to specifically illustrate the content of the present invention, and these embodiments are only general descriptions of the content of the present invention, and do not limit the content of the present invention.

The chemical composition of the embodiment of the present invention is shown in Table 1; the smelting and continuous casting process of the corresponding embodiment is shown in Table 2; the slab heating process of the corresponding embodiment is shown in Table 3; the rough rolling process of the corresponding embodiment is shown in Table 4; The finishing rolling process is shown in Table 5; the cooling and straightening process of the corresponding embodiment is shown in Table 6; the microstructure of the corresponding embodiment is shown in Table 7. The properties of the corresponding examples are shown in Table 8.

Table 1 The chemical composition wt% of the steel plate of the embodiment of the present invention

In the table: CE _IIW =C+Mn/6+(Cr+Mo)/5+(Ni+Cu)/15;

CE _Pcm =C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B

The smelting and continuous casting process of table 2 embodiment of the present invention

Example Smelting tapping temperature/℃ Smelting tap C content/% Pouring superheat/℃ Slab thickness/finished thickness 1 1627 0.030 16 8.3 2 1639 0.032 twenty two 9.3 3 1633 0.032 20 8.3 4 1628 0.034 20 9.3 5 1630 0.030 17 9.3 6 1631 0.031 19 9.3

Table 3 Slab heating process of the embodiment of the present invention

Example Heating temperature/℃ Soaking temperature/℃ Soaking period time/min 1 1211 1184 69 2 1205 1191 73 3 1227 1202 68 4 1223 1194 62 5 1216 1182 70 6 1228 1206 60

Table 4 Rough rolling process of the embodiment of the present invention

Table 5 The finishing rolling process of the embodiment of the present invention

Table 6 Cooling and straightening process of the embodiment of the present invention

Table 7 Microstructures of the examples of the present invention

Example Steel plate thickness/mm Steel plate width/mm Ferrite Volume Percent/% Grain size class 1 35 4330 17 11.5 2 32.1 4335 19 11.5 3 35 4330 11 11.5 4 32.1 4335 13 12 5 32.1 4335 15 11.5 6 32.1 4335 7 12

Table 8 Performance of the embodiments of the present invention

Note: The tensile specimen is a full-thickness rectangular specimen, and the width of the parallel test section is 38.1mm; the DWTT specimen is a full-thickness specimen; the size of the impact specimen is 10*55*55mm.

Claims (3)

1. The steel for the ultra-wide, ultra-thick and high-toughness X80 straight welded pipe is characterized by comprising the following chemical components in percentage by mass: c: 0.035-0.055%, Si: 0.12-0.22%, Mn: 1.60-1.75%, P is less than or equal to 0.010%, S is less than or equal to 0.0015%, Nb: 0.06% -0.10%, Ti: 0.008% -0.020%, Ni: 0.10% -0.24%, Mo: 0.15-0.24%, Cr is less than or equal to 0.25%, and (Ni + Mo + Cr): 0.40-0.65%, Al: 0.010% -0.030%, N: 0.0010-0.0040%, Ca: 0.0015 to 0.0045 percent, Ca/S is more than or equal to 1.8, and the balance is Fe and inevitable impurities; CE of Steel_IIWControlled at 0.39% -0.43%, CE_PcmControlled at 0.15% -0.18%, wherein, CE_IIW＝C+Mn/6+(Cr+Mo)/5+(Ni+Cu)/15；CE_Pcm＝C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B。

2. The steel for the ultra-wide ultra-thick high-toughness X80 straight welded pipe according to claim 1, wherein the steel plate has a thickness of 31mm or more and a width of 4330mm or more.

3. A method for producing the steel for the ultra-wide ultra-thick high-toughness X80 straight welded pipe as claimed in claim 1 or 2, the production process of the steel plate is as follows: the method comprises the steps of molten iron pretreatment, converter smelting, external refining, continuous casting, slab heating, rolling, cooling and straightening, and is characterized in that,

the tapping temperature of the converter smelting is less than or equal to 1640 ℃, and the content of C in the molten steel is less than or equal to 0.035% during tapping; the casting superheat degree of the continuous casting billet is 10-25 ℃, and the thickness of the continuous casting billet/the thickness of a finished steel plate is more than or equal to 8;

the temperature of a heating section of the continuous casting billet is 1200-1230 ℃, the temperature of a soaking section is 1180-1210 ℃, and the time of the soaking section is more than or equal to 50 min;

the initial rough rolling temperature is 1100-1140 ℃, the final rough rolling temperature is 1020-1050 ℃, a transverse and longitudinal rolling mode is adopted, after the transverse rolling of the casting blank is finished, the casting blank is cooled to a longitudinal rolling starting temperature in a spraying and air cooling mode, the longitudinal rolling starting temperature is less than 1070 ℃, the rolling speed is 1.0-2.0 m/s, and the deformation rate of each pass of at least the last 2 passes is more than 15% and the time interval of the pass is less than or equal to 15s in the longitudinal rolling stage;

the thickness of the intermediate temperature-waiting blank is 2.5 t-3.4 t, wherein t is the thickness of a finished steel plate, the start rolling temperature of finish rolling is 810-840 ℃, the finish rolling temperature is 740-770 ℃, and the accumulated deformation rate below 790 ℃ is not lower than 20%;

carrying out accelerated water cooling on the rolled steel plate, wherein the time interval from the rolling completion to the beginning of water cooling is less than or equal to 25s, the beginning temperature of water cooling is 730-760 ℃, the final cooling temperature is 380-460 ℃, and the water cooling time is more than or equal to 15 s; and then, carrying out thermal straightening and air cooling, then carrying out secondary straightening at 100-250 ℃, and then carrying out air cooling to room temperature.

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