CN110284066B

CN110284066B - Thin-gauge low-yield-ratio pipeline steel and manufacturing method thereof

Info

Publication number: CN110284066B
Application number: CN201910672394.8A
Authority: CN
Inventors: 吴长柏; 刘鹏; 杨佳威; 陈建恩; 严印通
Original assignee: Baosteel Zhanjiang Iron and Steel Co Ltd
Current assignee: Baosteel Zhanjiang Iron and Steel Co Ltd
Priority date: 2019-07-24
Filing date: 2019-07-24
Publication date: 2021-04-16
Anticipated expiration: 2039-07-24
Also published as: CN110284066A

Abstract

The invention relates to thin-gauge low-yield-ratio pipeline steel and a manufacturing method thereof, wherein the steel comprises the following chemical components in parts by weight: c: 0.04-0.06%, Si: 0.2-0.3%, Mn: 1.66-1.74%, P: 0-0.012%, S: 0-0.02%, Cr: 0.13-0.19%, Nb: 0.035 to 0.045%, Ti: 0.0115-0.0175%, Ca: 0.001-0.0035%, Al: 0.02-0.04%, and the balance of Fe and inevitable trace impurity elements, wherein the elements simultaneously satisfy the following relations: v + Nb + Ti is less than or equal to 0.15 percent; the production method of the pipeline steel comprises the following steps: s1, smelting and casting; s2, heating; s3, rolling and cooling; and S4, straightening. The invention combines the thermo-mechanical rolling process of controlled rolling and controlled cooling to obtain the steel plate in TMCP state, the whole plate thickness obtains a microstructure mainly comprising bainite to improve the low-temperature toughness of the steel, and the steel plate has the mechanical property characteristics of high strength and low yield ratio, particularly the yield ratio of a full-thickness tensile test is less than or equal to 90 percent, the integral yield ratio is less than or equal to 87 percent, the steel plate has good comprehensive mechanical property and lower carbon equivalent, and is beneficial to improving the forming welding and field girth welding performances of the steel pipe.

Description

Thin-gauge low-yield-ratio pipeline steel and manufacturing method thereof

Technical Field

The invention belongs to the technical field of steel plate production, and particularly relates to thin-gauge low-yield-ratio pipeline steel and a manufacturing method thereof.

Background

The key of the domestic prior art is to reduce the yield ratio of the pipeline steel in thin specifications by adjusting components and a cooling process, for example, steel mills such as saddle steel, horse steel and the like reduce the yield ratio by reducing C and increasing Mn and reducing the finish rolling temperature.

In the domestic enterprises, the yield ratio is reduced by taking the angle of the optimization process as a starting point, the low yield ratio requirements are met mainly in the aspects of increasing the proportion of proeutectoid ferrite, increasing the grain size of the ferrite and the like, but the yield strength and the tensile strength of the steel plate are influenced along with the reduction of the yield ratio, so that the thin-gauge and low-yield-ratio steel plate cannot be provided in the environments of petroleum and natural gas transportation, pipeline construction in various regions and the like.

Disclosure of Invention

The invention aims to provide thin-gauge low-yield-ratio pipeline steel and a manufacturing method thereof, the pipeline steel is the pipeline steel with the thin gauge low-yield-ratio yield strength of 485MPa, the thickness of a finished steel plate is 12-15mm, the microstructure of the pipeline steel mainly comprises bainite, the yield strength is more than or equal to 485MPa, the tensile strength is more than or equal to 580MPa, the transformation temperature of full-size Charpy impact work is-60 ℃, the transformation temperature of the shearing area of a full-plate-thickness DWTT fracture is-30 ℃, and the pipeline steel has manufacturability and can be used for manufacturing thin-wall straight-seam submerged arc welded pipes.

In order to achieve the purpose of the invention, the technical scheme adopted by the invention is as follows:

a thin-gauge low-yield-ratio pipeline steel and a manufacturing method thereof are disclosed, wherein the pipeline steel comprises the following chemical components in percentage by weight: c: c is the most basic strengthening element, and C is dissolved in steel to form an interstitial solid solution and plays a role of solid solution strengthening, and forms carbide precipitation with a strong carbide forming element to play a role of precipitation strengthening. However, too high C is detrimental to the toughness and weldability of the steel; too low C in turn reduces the strength of the steel. Therefore, the content of C is controlled to be 0.04-0.06%;

si: si is a solid solution strengthening element and is also a deoxidizing element in steel, but the welding performance of steel is deteriorated due to the excessively high content of Si, and hot-rolled iron scales are not removed in the rolling process, so that the content of Si is controlled to be 0.2-0.3%;

mn: mn improves the strength of steel by solid solution strengthening, and is the most important and economical strengthening element in steel to compensate for strength loss caused by a decrease in C content. Mn is also an element for expanding a gamma phase region, can reduce the gamma → alpha phase transition temperature of steel, is beneficial to obtaining a fine phase transition product, and can improve the toughness of the steel; however, Mn is an easily segregated element, and when the content of Mn is high, Mn is easily segregated in the center of the plate thickness in the casting process, a hard-phase martensite structure is generated after rolling is finished, and the low-temperature toughness and the dynamic tearing resistance of the material are reduced. Therefore, the Mn content is controlled to be 1.66-1.74 percent;

cr: cr is an important element for improving the hardenability of steel, the structure and performance uniformity of the full thickness of a thick steel plate are ensured, and the corrosion resistance of the steel can be effectively improved when the content of Cr is more than 0.10%; however, the addition of too high chromium and manganese to the steel at the same time results in the formation of low melting point Cr-Mn complex oxides, surface cracks during hot working, and severe deterioration of weldability. Therefore, the Cr content is limited to 0.13-0.19% in the present invention;

nb: nb is one of important elements of the low-carbon microalloyed steel, Nb (N, C) particles are formed by the strain-induced precipitation of the Nb which is dissolved in the hot rolling process, the growth of the deformed austenite and the recrystallization are inhibited by pinning the grain boundary, and the deformed austenite is transformed into a fine product with high dislocation density by controlled rolling and controlled cooling. For thick pipeline steel, too low Nb content has no obvious dispersion precipitation effect, and does not play a role in refining grains and strengthening a matrix; too high Nb content is disadvantageous for grain refinement because it suppresses the occurrence of recrystallization of the steel sheet core. The solid solution of Nb is related to the content of C, and the content of C is too high, so that the Nb has small solid solution amount and cannot play roles in precipitation strengthening and grain gliding; if the content of C is too low, the grain boundary is weakened, and if the content of Nb is too low, the precipitation strengthening effect is not obvious. Therefore, the Nb content in the present invention should be limited to 0.035 to 0.045%;

ti: ti is a strong carbonitride forming element, undissolved carbonitride of Ti prevents growth of austenite grains when the steel is heated, and TiN precipitated during rough rolling in the high-temperature austenite region effectively suppresses growth of austenite grains. In addition, in the welding process, TiN particles in the steel can obviously prevent the crystal grains in the heat affected zone from growing, thereby improving the welding performance of the steel plate and having obvious effect on improving the impact toughness of the welding heat affected zone. Therefore, the Ti content is controlled to be 0.0115-0.0175 percent in the invention;

s, P: s, P are inevitable impurity elements in steel, and it is desirable that the lower the content, the better. The inclusion morphology of the sulfide is controlled by ultra-low sulfur (less than 20ppm) and Ca treatment, and meanwhile, the P content is controlled to be less than 0.012 percent, so that the invention steel can be ensured to have good low-temperature impact toughness;

ca: the form of sulfide can be controlled through Ca treatment, the anisotropy of the steel plate is improved, the low-temperature toughness is improved, and the control range of Ca for ensuring the best effect is 0.0010-0.0035%;

al: al is an element added into steel for deoxidation, and the addition of a proper amount of Al is beneficial to refining grains and improving the toughness of steel, wherein the control range of the Al content is 0.02-0.04%;

the balance of Fe and inevitable impurities, and the following relationship is satisfied: v + Nb + Ti is less than or equal to 0.15 percent.

The manufacturing method of the steel plate comprises the following steps:

s1, smelting and casting: pouring according to the components, smelting the balance of Fe and inevitable impurities in a converter, then carrying out LF + RH double refining desulfurization and dehydrogenation, and then carrying out continuous casting;

s2, heating: heating the plate blank at the temperature of 1150-1190 ℃;

s3, rolling and cooling: the rolling comprises rough rolling and finish rolling, the steel plate needs to finish the whole rough rolling process in an austenite complete recrystallization area, then the temperature is waited in an intermediate roller way with the thickness of 4.2-5.5T, the finish rolling is carried out after the temperature is reduced to an austenite non-recrystallization area, the initial rolling temperature is set at 980 and 1020 ℃, and the final rolling temperature is set at 840 and 880 ℃; in order to ensure that the steel plate has enough strength and toughness, a water spray cooling process is added after the controlled rolling stage, so that the structure grains after rolling are further refined and uniformly distributed, the start cooling temperature is controlled at 770-810 ℃, the cooling speed is 30-45 ℃/S, and the final cooling temperature is controlled at 500-560 ℃;

s4, straightening: pre-straightening for 1 pass, setting the roll gap to be 11-13mm, and thermally straightening for 1 or 3 passes, and setting the roll gap to be 9-11 mm.

Further, bainite is adopted as the microstructure of the pipeline steel, and the effective average grain size is 11 grades.

Compared with the prior art:

1. the invention effectively controls the drop hammer shearing section rate, the one-time performance qualification rate exceeds 99 percent, and the alloy elements only contain Mn, Al and a small amount of Cr and Nb elements, thereby replacing the metal Mo element in the prior art to improve segregation and toughness and greatly reducing the alloy cost.

2. The steel plate in TMCP state is obtained by combining the thermo-mechanical rolling process of controlled rolling and controlled cooling, a microstructure mainly comprising bainite is obtained through the whole plate thickness so as to improve the low-temperature toughness of the steel, and the steel plate has the mechanical property characteristics of high strength and low yield ratio, particularly the yield ratio of a full-thickness tensile test is less than or equal to 90%, and the overall yield ratio is less than or equal to 87%.

3. The manufactured pipeline steel with the thickness of 12-15mm meets the following requirements in performance: yield strength rt 0.5: 485 and 630 MPa; tensile strength Rm: 580-720 MPa; -15 ℃ full-size charpy ballistic work AKv: more than or equal to 350J; -15 ℃ full plate thickness DWTT (Drop Weight TearTest, Drop tear test) Performance: fracture shear area ratio SA: not less than 85 percent.

Drawings

FIG. 1 is a metallographic structure of a steel plate prepared according to example 1 of the present invention;

FIG. 2 is a metallographic structure of a steel plate prepared in example 2 of the present invention.

Detailed Description

The following detailed description of embodiments of the present invention is provided in connection with the accompanying drawings and examples. The following examples are intended to illustrate the invention but are not intended to limit the scope of the invention.

Example 1

Designing chemical components: the chemical components are designed according to mass percent as follows: c: 0.04-0.06%, Si: 0.2-0.3%, Mn: 1.66-1.74%, P: 0-0.012%, S: 0-0.02%, Cr: 0.13-0.19%, Nb: 0.035 to 0.045%, Ti: 0.0115-0.0175%, Ca: 0.001-0.0035%, Al: 0.02-0.04%, and the balance of Fe and inevitable impurities.

Combining the steps 1, smelting and casting as shown in tables 1 and 2: casting the designed chemical components, firstly carrying out KR stirring desulfurization treatment, then blowing in a converter, then carrying out LF + RH double refining desulfurization and dehydrogenation, and finally carrying out continuous casting to obtain a plate blank; the calculated slab thickness was 227 mm.

Step 2, heating: the slab is heated, the heating temperature is set to 1130-1170 ℃, and the heating time is more than 160min (under the heating condition, the sufficient solid solution and precipitation strengthening of the Nb element can be ensured).

Step 3, rolling and cooling: the rolling process comprises rough rolling and finish rolling, the steel plate needs to finish the whole rough rolling process in an austenite complete recrystallization area, the rough rolling is finished, the temperature is kept in a middle roller way at the temperature-keeping thickness of 4.2-5.5T (T is the thickness of a finished steel plate), the finish rolling is carried out after the temperature is reduced to an austenite non-recrystallization area, the initial rolling temperature is set at 980-.

Step 4, straightening: pre-straightening for 1 pass, setting the roll gap to be 13-15mm, and thermally straightening for 1 or 3 passes, and setting the roll gap to be 11-13 mm.

Finally obtaining the 12-15mm thick pipeline with the tensile property: yield strength rt 0.5: 622 MPa; tensile strength Rm: 690 MPa; elongation percentage A: 32 percent; yield ratio YR: 90 percent;

-15 ℃ full-size charpy ballistic work AKv: 433J; fracture shear area ratio SA of the full plate thickness DWTT (drop weight tear test) performance at 15 ℃: 97 percent.

Example 2

Step 4, straightening: pre-straightening for 1 pass, setting the roll gap to be 11-13mm, and thermally straightening for 1 or 3 passes, and setting the roll gap to be 9-11 mm.

Finally obtaining the 12-15mm thick pipeline with the tensile property: yield strength rt 0.5: 527 MPa; tensile strength Rm: 634 MPa; elongation percentage A: 38 percent; yield ratio YR: 83 percent;

-15 ℃ full-size charpy ballistic work AKv: 446J; fracture shear area ratio SA of the full plate thickness DWTT (drop weight tear test) performance at 15 ℃: 98 percent.

In conclusion, the rolled steel plate with low-temperature impact toughness and the manufacturing method thereof can obtain the pipeline steel meeting the target performance requirements, have good comprehensive mechanical properties, have low carbon equivalent, are beneficial to improving the forming welding and field girth welding performances of the steel pipe, have simple components and wider process window, and have stronger manufacturability.

Table 1: comparison of specific parameters of example 1 and example 2

Table 2: comparison of test Properties of example 1 and example 2

Claims

1. The thin-gauge low-yield-ratio pipeline steel is characterized by comprising the following chemical components in percentage by weight: c: 0.04-0.06%, Si: 0.2-0.3%, Mn: 1.66-1.74%, P: 0-0.012%, S: 0-0.02%, Cr: 0.13-0.19%, Nb: 0.035 to 0.045%, Ti: 0.0115-0.0175%, Ca: 0.001-0.0035%, Al: 0.02 to 0.04%, and the balance of Fe and inevitable impurities, and satisfies the following relationship: v + Nb + Ti is less than or equal to 0.15 percent; the manufacturing method of the pipeline steel comprises the following steps:

s1, smelting and casting: pouring according to the chemical components of the pipeline steel, smelting the balance of Fe and inevitable impurities in a converter, then performing LF + RH double refining desulfurization and dehydrogenation, and then performing continuous casting to obtain a plate blank;

s2, heating: the heating temperature of the plate blank is set to 1150-1190 ℃;

s3, rolling and cooling: the rolling comprises rough rolling and finish rolling, the steel plate finishes the whole rough rolling process in an austenite complete recrystallization area, then the temperature is waited in an intermediate roller way with the thickness of 4.2-5.5T to be waited, the finish rolling is carried out after the temperature is reduced to an austenite non-recrystallization area, the initial rolling temperature is set to be 980-; the cooling is carried out after the rolling control stage, a water spraying cooling process is arranged, the start cooling temperature is 770-;

2. The thin gauge, low yield ratio pipeline steel of claim 1, wherein the microstructure of the pipeline steel is bainite, and the effective average grain size is 11 grades.