CN108950388A - L485M pipeline steel with excellent low-temperature toughness and manufacturing method thereof - Google Patents

Abstract

The invention discloses L485M pipeline steel with excellent low-temperature toughness and a manufacturing method thereof. Steel contains C: 0.045% to 0.065%, Si: 0.10% to 0.25%, Mn: 1.55% to 1.65%, Nb: 0.055% to 0.075%, Ti: 0.010% to 0.025%, Mo: 0.10% to 0.15% , Cr: 0.20%-0.30%, Als: 0.015%-0.045%, P≤0.015%, S≤0.003%, N≤0.006%, the rest is Fe and unavoidable impurities, and Pcm≤0.18%. Continuous casting slab thickness ≤ 170mm, hot charging at 500-850°C, heating to 1160-1200°C, finishing rolling temperature of rough rolling ≥980°C, finishing rolling temperature of 780-820°C, total rolling reduction of finishing rolling ≥65%, Then it is firstly cooled to 550-600°C at a speed of 40-60°C/s, and finally cooled to 400-500°C at a rate of 10-25°C/s for coiling. Coils have excellent low temperature toughness and are used in extremely cold regions and areas with severe conditions.

Description

A kind of L485M pipeline steel with excellent low temperature toughness and its manufacturing method

technical field

The invention belongs to the field of high-strength low-alloy steel for metal materials, and in particular relates to a method for manufacturing pipeline steel for oil and gas transportation pipelines, in particular to a method for manufacturing L485M grade pipeline steel with excellent low-temperature toughness and its hot-rolled coil.

Background technique

In recent years, the construction of energy transmission pipelines has developed rapidly. In order to improve transmission efficiency and operational safety, the requirements for the strength and toughness of the selected steel raw materials are getting higher and higher. At present, major domestic and foreign main pipelines basically use L485M and L555M steel grades.

When the pipeline is located in an extremely cold region or the transport medium is low-temperature liquefied natural gas, from the perspective of operational stability and safety, higher requirements are put forward for the low-temperature toughness index of raw materials, such as low-temperature drop hammer and ductile-brittle transition temperature requirements for inspection The temperature is lower, the Charpy impact energy requirements are higher, etc., so these projects have put forward higher individual requirements, and it is not enough to just meet the API SPEC 5L specification.

Although there are many domestic and foreign literatures on L485M steel grade pipeline steel hot-rolled coils, few of them can meet the low-temperature fracture toughness requirements below -30°C. The following briefly introduces several similar literatures:

Chinese patent, application number 201310001803.4, discloses a controlled rolling and controlled cooling process for hot-rolled coils of Cr-containing economical X70 pipeline steel. In the patented composition, C 0.05%-0.075%, Si 0.10%-0.35%, Mn 1.00%-1.65%, Nb 0.050-0.080%, Ti 0.010%-0.025%, Cr 0.30%-0.40%, and 8 ～10 passes rough rolling, coiling at 300～400℃. There is no Mo in the patent composition, which is not conducive to the hardenability control of the product, and it is difficult to ensure lower low-temperature fracture toughness. In addition, there are many rough rolling passes and a small reduction in a single pass, which not only affects the refinement of the original austenite, but also It affects the production efficiency, the coiling temperature is low, the difficulty of process control is increased, and it is easy to cause poor coil shape.

Chinese patent, application number 200410066297.8, discloses acicular ferritic X70 pipeline steel with high crack arrest toughness and its manufacturing method. The patented ingredients C 0.02%~0.06%, Si 0.10%~0.50%, Mn 1.45%~1.75%, S≤0.002%, P 0.004%~0.012%, Nb 0.050%~0.080%, V 0.010%~0.060%, Ti 0.005%-0.025%, Mo 0.10%-0.30%, Cu≤0.30%, Ni≤0.30%, N≤0.008%, Al0.015%-0.045%. In this patent, the alloy system is complicated, the addition of precious elements is large, and the cost is high.

Chinese patent, application number 200910237313.8, discloses a hot-rolled coil of steel X70 for pipelines. The patent composition C 0.04%～0.08%, Si 0.10%～0.30%, Mn 1.40%～1.75%, S≤0.003%, P≤0.018%, Nb 0.03%～0.06%, V 0.02%～0.05%, Ti 0.010 % to 0.020%, Mo 0.10% to 0.15%, Cr 0.20% to 0.30%, Cu 0.15% to 0.20%, Ni 0.10% to 0.15%, N≤0.006%. In this patent, the alloy system is complicated, the addition of precious elements is large, and the cost is high.

Korean patent KR20140130324(A), HOT-ROLLED STEEL SHEET FOR PIPE AND METHODOF MANUFACTURING THE SAME. The patent composition C 0.03%～0.12%, Si 0.2%～0.4%, Mn 1.4%～1.8%, S≤0.01%, P≤0.03%, Nb 0.02%～0.06%, Ti 0.01%～0.04%, Mo≤ 0.10%, Cr0.1%-0.5%, Cu 0.15%-0.20%, Ni 0.05%-0.5%, N≤0.01%. The composition requirements are broad and contain Ni and Cu, the addition of alloys is large, and the cost is high. In addition, the requirements for the content of impurity elements are too broad, which is not conducive to the control of low temperature fracture toughness.

Japanese patent JP2015054983(A), HIGH TOUGHNESS, HIGH DUCTILITY AND HIGHSTRENGTH HOT ROLLED STEEL SHEET AND PRODUCTION METHOD THEREOF. The patented ingredients C0.04%～0.15%, Si 0.01%～0.55%, Mn 1.0%～3.0%, S≤0.01%, P≤0.03%, Nb0.001%～0.035%, V 0.001%～0.1%, Ti 0.001% ~ 0.035%, the patent composition requirements are broad, containing V, without Mo, Cr hardenability elements, which is not conducive to the uniformity control of the structure, and it is difficult to ensure low temperature fracture toughness.

US Patent US2012247625 (A1), LOW YIELD RATIO, HIGH STRENGTH AND HIGH UNIFORM ELONGATION STEEL PLATE AND METHOD FOR MANUFACTURING THE SAME. The patent composition C 0.06%～0.12%, Si 0.01%～1.0%, Mn 1.2%～3.0%, S≤0.005%, P≤0.015%, Nb0.005%～0.07%, Ti 0.005%～0.025%, N ≤0.01% etc. The composition requirements of this patent are broad, and it does not contain Mo and Cr hardenability elements, which is not conducive to the uniformity control of the structure, and it is difficult to ensure low temperature fracture toughness.

Niu Tao, An Chenggang and others published the paper "Study on Microstructure and Properties of Hot-Rolled Coil of Ultra-thick Specification X70 Pipeline Steel" in the Tenth China Iron and Steel Annual Conference. The disclosed chemical composition is: C 0.055%, Si 0.20% , Mn 1.60%, P ≤ 0.001%, S ≤ 0.0012%, Al _t 0.035%, Nb 0.07% ~ 0.09%, Ti 0.015%, while containing Mo, Cr, Ni, Cu. In terms of technology, converter + refining and vacuum degassing process are used for smelting. The thickness of continuous casting slab is 250mm, and it is rolled by 2500mm hot continuous rolling. After final rolling, it enters ultra-fast cooling and laminar cooling device. The alloy design of this type of steel has high Nb content and contains Ni and Cu, which leads to high cost, large thickness of slab, large overall rolling reduction, and low control difficulty.

Zheng Lei and Gao Shan published "Development of Large-diameter X70 Gas Pipeline Coils for West-East Gas Transmission Project" in "Journal of Iron and Steel Research", 2006, 18(3), in which only C, Mn Composition, other elements include Mo, Ni, Cu and other precious elements, it can be inferred that the cost is high.

The L485M grade pipeline steel disclosed in the above documents can meet the strength index requirements, but it is basically difficult to meet the low-temperature fracture toughness requirements below -30°C. Conventional laminar flow cooling cannot fully and effectively refine the grains and control the uniformity of the structure, and some components are added with precious alloying elements such as Ni and Cu, and the cost is relatively high.

Contents of the invention

The object of the present invention is to design a simple and economical alloy system, match accurate two-stage controlled rolling and ultra-fast cooling + laminar cooling process, and provide a hot-rolled coil of L485M steel grade pipeline steel with excellent low-temperature toughness and its Manufacturing method.

The specific technical solutions are:

The present invention proposes a L485M pipeline steel with excellent low temperature toughness, its chemical composition weight percent is: C: 0.045%-0.065%, Si: 0.10%-0.25%, Mn: 1.55%-1.65%, Nb: 0.055%- 0.075%, Ti: 0.010%～0.025%, Mo: 0.10%～0.15%, Cr: 0.20%～0.30%, Als: 0.015%～0.045%, P≤0.015%, S≤0.003%, N≤0.006%, The balance is iron and unavoidable impurities, and the cold crack sensitivity coefficient Pcm≤0.18%, wherein, Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/ 10+5B.

The composition design idea of L485M pipeline steel is to adopt C-Mn-Nb-Mo micro-alloy design, add a small amount of Cr, Ti and other elements, and combine two-stage controlled rolling and ultra-fast cooling + laminar cooling process to obtain fine needles. ferrite structure to ensure that the pipeline steel has high strength, high toughness and excellent low temperature fracture toughness. Its main basic elements and functions are as follows:

C: It is the most economical and basic strengthening element in steel. It has a significant effect on improving the strength of steel through solid solution strengthening and precipitation strengthening, but increasing the C content has a negative impact on the ductility, toughness and weldability of steel. Low carbon guarantee Pipeline steel has good low temperature toughness, good weldability and resistance to hydrogen-induced cracking (HIC). Therefore, the present invention controls the C content to 0.045%-0.065%.

Si: It can play a certain role in solid solution strengthening, but if the content is too high, the plasticity and toughness will be reduced. Therefore, the content of Si in the present invention is 0.10%-0.25%.

Mn: Improve the strength of steel through solid solution strengthening. It is the most important and economical strengthening element in pipeline steel to compensate for the loss of strength caused by the reduction of C content. Mn is also an element that expands the γ phase region, which can reduce the γ-α phase transition temperature of steel, help to obtain fine phase transition products, improve the toughness of steel, and reduce the ductile-brittle transition temperature. However, manganese is an element that is easy to segregate, and if it is too high, it is easy to form a banded structure. The Mn content of the present invention is 1.55% to 1.65%.

Nb: It is one of the most important elements in modern microalloyed pipeline steel, and it has a very obvious effect on grain refinement. The strain-induced precipitation of NbC in the hot rolling process hinders the recovery and recrystallization of deformed austenite, and through controlled rolling and controlled cooling, the deformed austenite structure in the non-recrystallized area of the finishing rolling stage is transformed into a fine phase during phase transformation Change products to make steel with higher strength and high toughness. Nb also improves the strength of steel through precipitation strengthening, and reducing the carbon content can increase the solid solubility of Nb in high-temperature austenite when the slab is reheated, and can give full play to the grain refinement and precipitation strengthening of Nb in the subsequent controlled rolling process role. Nb is a precious element, and its cost is relatively high. Therefore, the present invention controls the Nb content to be 0.055%-0.075%.

Mo: It can effectively improve the hardenability of steel, expand the γ phase region, reduce the γ-α phase transition temperature, inhibit the formation of polygonal ferrite (PF), promote the transformation of acicular ferrite (AF), and improve The precipitation strengthening effect of Nb (C, N) improves the strength and fracture toughness of steel. However, molybdenum is a precious element, and its cost is high. Therefore, the Mo content in the present invention is 0.10% to 0.15%.

Ti: It is a strong N-fixing element. The stoichiometric ratio of Ti/N is 3.42. Using about 0.02% Ti can fix the N below 60ppm in the steel, and form fine high-temperature stable TiN precipitation during slab continuous casting Mutually. This kind of fine TiN particles can effectively hinder the growth of austenite grains when the slab is reheated, help to increase the solid solubility of Nb in austenite, and at the same time improve the impact toughness of the welded heat-affected zone. Obvious effect. Therefore, the Ti content in the present invention is 0.010% to 0.025%.

Cr: It can effectively improve the hardenability, inhibit the generation of polygonal ferrite and pearlite, and promote the formation of ferrite or bainite with a large number of dislocations in the grain in the medium and low temperature regions. When used in combination with Mo, the effect is More notably, compared with Mo, the price is low. Therefore, the Cr content of this patent is 0.20% to 0.30%.

Als: a deoxidizing element. Adding an appropriate amount of aluminum can form fine and dispersed AlN particles, which is beneficial to refine grains and improve the toughness of steel. Therefore, the content of Als in the present invention is 0.015% to 0.045%.

P, S, N: These are unavoidable impurity elements in steel. It is hoped that the lower the better, but too low will increase the production cost. In the present invention, P≤0.015%, S≤0.003%, and N≤0.006%.

Pcm: controlling the cold crack sensitivity coefficient is beneficial to guarantee the welding performance of the product, and the Pcm of the present invention is controlled at ≤0.18%.

The invention also proposes a method for manufacturing hot-rolled coils of L485M pipeline steel with good low-temperature toughness by adopting thin slab continuous casting and rolling process. Its production process involves: molten iron pretreatment - converter smelting - out-of-furnace refining (RH + LF + calcium treatment) - continuous casting - slab heating - rolling - ultra-fast cooling + laminar cooling - coiling. Its characteristics are:

(1) Smelting and continuous casting process: pretreatment of molten iron, converter smelting - through top blowing or top-bottom compound blowing, refining outside the furnace - through RH vacuum treatment, LF furnace light desulfurization treatment and calcium treatment to control the shape of inclusions And to improve the ductility, toughness and cold bending performance of steel, slabs are continuously cast to make continuous casting slabs - full protection pouring, and put into dynamic light reduction to reduce the center segregation of continuous casting slabs, the thickness of casting slabs is ≤ 170mm, It is obviously thinner than the 200mm thick continuous casting slab currently mainly used, its solidification cooling rate is much higher than that of the traditional thick slab, and the secondary dendrite spacing is greatly reduced.

(2) Rolling process: the continuous casting slab is directly hot-charged at 500-850°C, and the continuous-casting slab is heated to 1160-1200°C by a walking heating furnace, followed by two-stage controlled rolling by rough rolling and finishing rolling units , the finishing rolling temperature of rough rolling is ≥980°C, the rough rolling is 3~5 passes, the starting rolling temperature of finishing rolling is ≤960°C, the finishing rolling temperature is 780~820°C, the total rolling reduction of finishing rolling is guaranteed to be ≥65%, Then adopt two-stage controlled cooling, first use ultra-fast cooling method to cool to 550-600°C at a speed of 40-60°C/s, and then use laminar flow cooling method to finally cool to 400-500°C at a speed of 10-25°C/s ℃ Coiling.

The L485M steel grade hot-rolled coil of the present invention has excellent comprehensive properties: the yield strength is 500-600 MPa, the tensile strength is 600-700 MPa, the elongation after fracture is ≥30%, and the yield ratio is ≤0.90; Charpy impact energy at -40°C (3 samples) Akv≥300J, -40°C average drop weight tear test shear area (2 samples) SA≥95%; hardness HV10≤230, qualified for cold bending.

The special features of the technical solution of the present invention are: (1) the alloy design is simple, adopts the C-Mn-Nb-Mo alloy design, and adds a small amount of microalloying elements such as Cr and Ti, and does not contain Ni, Cu, V, etc. Alloy cost is low. (2) The thickness of the continuous casting slab is less than or equal to 170mm, and the short-flow continuous casting and rolling production process is adopted, which has high efficiency and saves energy. (3) Adopt ultra-fast cooling + laminar flow cooling method to control cooling in two stages to ensure fine grain and uniform structure. (4) The product has excellent comprehensive properties, especially low temperature toughness. At -40°C, the average shear area of the drop weight tear test is ≥95%.

Beneficial effect:

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

(1) The alloy design is simple, using C-Mn-Nb-Mo alloy design, and adding a small amount of Cr, Ti and other microalloying elements, without Ni, Cu, V, etc., and the alloy cost is low.

(2) The thickness of the continuous casting slab is less than or equal to 170mm, and the short-flow continuous casting and rolling production process is adopted, which has high efficiency and saves energy.

(3) Adopt ultra-fast cooling + laminar flow cooling method to control cooling in two stages to ensure fine grain and uniform structure.

(4) The product has excellent comprehensive properties, especially low temperature toughness. At -40°C, the average shear area of the drop weight tear test is ≥95%.

(5) The L485M pipeline steel produced by the above method can be widely used in cold regions close to the polar regions or regions with harsh natural environments such as deserts and deep seas.

Description of drawings

Fig. 1 is the optical microstructure of embodiment 1; Microstructure is ferrite;

Detailed ways

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

Table 1 is the chemical composition of the steel of the embodiment. Its technological process is molten iron pretreatment—converter smelting—extra-furnace refining (RH+LF+calcium treatment)—continuous casting—slab heating—rolling—ultrafast cooling+laminar cooling—coiling. Table 2. Table 3 is the main mechanical properties of the steels of the examples.

Table 1 embodiment steel chemical composition (wt, %)

Example C Si mn P S Nb Ti Mo Cr als N Pcm 1 0.052 0.13 1.61 0.012 0.002 0.055 0.017 0.10 0.30 0.028 0.005 0.16 2 0.045 0.25 1.65 0.013 0.003 0.066 0.019 0.13 0.28 0.033 0.006 0.16 3 0.057 0.17 1.58 0.014 0.001 0.060 0.010 0.14 0.24 0.045 0.004 0.16 4 0.065 0.10 1.62 0.015 0.002 0.072 0.025 0.12 0.26 0.022 0.004 0.17 5 0.060 0.14 1.55 0.011 0.002 0.068 0.022 0.11 0.20 0.015 0.005 0.16 6 0.048 0.22 1.49 0.013 0.003 0.075 0.014 0.15 0.22 0.020 0.004 0.15

Note: Pcm=C+Si/30+(Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B.

Table 2 Example steel process system

The main mechanical properties of table 3 embodiment steel

It can be seen from Tables 1 to 3 that the L485M pipeline steel hot-rolled coil produced by the technical solution of the present invention has good comprehensive mechanical properties, especially excellent low-temperature toughness, and can be widely used in extremely cold regions and areas with severe natural conditions .

Claims (2)

1. a kind of L485M pipe line steel of excellent in low temperature toughness, which is characterized in that chemical component is by mass percentage in steel are as follows: C: 0.045%~0.065%, Si:0.10%~0.25%, Mn:1.55%~1.65%, Nb:0.055%~0.075%, Ti: 0.010%~0.025%, Mo:0.10%~0.15%, Cr:0.20%~0.30%, Als:0.015%~0.045%, P≤ 0.015%, S≤0.003%, N≤0.006%, remaining is Fe and inevitable impurity, and cold crack sensitivity coefficient Pcm≤ 0.18%, wherein Pcm=C+Si/30+ (Mn+Cu+Cr)/20+Ni/60+Mo/15+V/10+5B.

2. a kind of manufacturing method of the L485M pipe line steel of excellent in low temperature toughness as described in claim 1, the production technology of steel plate Are as follows: the molten iron pretreatment-converter smelting-ultrafast cold+section cooling-volume of external refining-continuous casting-heating of plate blank-rolling- It takes, which is characterized in that

(1) it smelts continuous casting process: top blast or top bottom blowing is used, through RH application of vacuum, the frivolous sulphuring treatment of LF furnace and progress Calcium treatment, whole process protection casting, Mobile state slighter compress of going forward side by side, thickness of strand≤170mm；

(2) rolling mill practice: continuous casting steel billet is heated to 1160~1200 DEG C in 500~850 DEG C of direct hot chargings, after through roughing and The rolling of mm finishing mill unit two-stage control, roughing finishing temperature >=980 DEG C, 3~5 passage of roughing, finish rolling start rolling temperature are≤960 DEG C, finish rolling finishing temperature is 780~820 DEG C, and finish rolling total reduction >=65% is then cooling using two-stage control, is first used Ultrafast cold mode is cooled to 550~600 DEG C with the speed of 40~60 DEG C/s, then uses section cooling mode with 10~25 DEG C/s Speed be cooled to 400~500 DEG C eventually and batch.