WO2021017521A1

WO2021017521A1 - Low yield-to-tensile ratio thin specification pipeline steel manufacturing method

Info

Publication number: WO2021017521A1
Application number: PCT/CN2020/084022
Authority: WO
Inventors: 孙宪进; 李国忠; 许晓红; 白云; 苗丕峰; 许峰; 诸建阳; 石艾来; 林涛; 王小双
Original assignee: 江阴兴澄特种钢铁有限公司
Priority date: 2019-08-01
Filing date: 2020-04-09
Publication date: 2021-02-04
Also published as: CN110453157A

Abstract

A low yield-to-tensile ratio thin specification pipeline steel manufacturing method. The method comprises steps such as convert smelting, LF refining, RH vacuum degassing, 150 mm slab continuous casting, inspecting and cleaning up the continuous cast slabs, cast slab heating, high pressure water descaling, coiling, rolling, and ACC cooling. The chemical composition contents by percent of the pipeline steel are: C 0.075-0.095, Si 0.2-0.3, Mn 1.65-1.75, Al 0.02-0.04, Nb 0.045-0.055, Ti 0.01-0.02, Cr 0.20-0.30, and S ≤ 0.0008, with Ca/S ≥ 2 and the balance being Fe and unavoidable impurities; the thickness specification of the product is 5-8 mm, yield strength ≥ 485 MPa, tensile strength ≥ 570 MPa, yield-to-tensile ratio ≤ 0.85, elongation ≥ 30%, and -40°C impact energy ≥ 100 J.

Description

Method for manufacturing thin gauge pipeline steel with low yield ratio

Technical field

The invention belongs to the field of metallurgy and relates to a method for manufacturing thin-gauge pipeline steel with a low yield ratio.

Background technique

In the design process of pipeline projects, the strength and thickness of the required materials are generally designed according to the requirements of the project. In order to reduce the total cost of the project, combined with the current situation of material development, the method of improving the strength of the material and reducing the thickness of the material will be adopted to reduce the comprehensive investment cost. Therefore, there is a certain market demand for thin-gauge pipeline steel plates. However, as the strength increases, the yield ratio of steel also increases. At the same time, due to the large temperature drop in the final rolling process of the thin-gauge steel plate, the large temperature difference between the plate and the plate results in poor strength uniformity of the steel plate. At the same time, the thinner the steel plate, the higher the yield ratio, which is difficult to meet the needs of engineering projects.

Patent Announcement Number CN 104328356A proposes a method for producing thin-gauge high-strength steel sheets on a Steckel mill. The thin-gauge high-strength steel sheets are produced by coil rolling TMCP process and offline tempering. The microalloying elements are Ni, Nb, V, and Ti. , Cr and other elements, the specification is 8-12mm.

Patent announcement number CN 107326261A proposes a low yield ratio, thin gauge, high strength, acid-resistant pipeline steel hot-rolled coil and its manufacturing method; the manufacturing method uses a hot continuous rolling production line to produce steel coils, and the yield ratio is also It is significantly higher than 0.85.

Judging from the existing patents and documents of thin gauge pipeline steel with low yield ratio, there is no method for manufacturing pipeline steel plate with a thickness of less than 8mm and a yield ratio of less than 0.85.

Summary of the invention

The purpose of the present invention is to provide a method for manufacturing a thin-gauge pipeline steel sheet with a low yield ratio. According to the method, the yield ratio of the 5-8mm thin-gauge pipeline steel produced is lower than 0.85, and the strength uniformity of the entire plate is good.

The specific technical solution of the present invention is: a low-yield-strength thin-gauge pipeline steel plate, the percentage of chemical composition %: C 0.075-0.095, Si 0.2-0.3, Mn 1.65-1.75, Al 0.02-0.04, Nb 0.045 -0.055, Ti 0.01-0.02, Cr 0.20-0.30, S≤0.0008%, Ca/S≥2, the balance is Fe and inevitable impurities, the product thickness specification is 5-8mm.

The chemical composition of the steel plate is the key technology of this application. The function of each element and the principle of determining its content are explained as follows:

C: The key factor that affects phase transformation in the controlled rolling and controlled cooling process. Too high will lead to serious center segregation and obvious decrease in toughness; too low will result in low strength and high yield ratio; in order to reduce the yield ratio, the selection range of C content is 0.075 -0.095%.

Si: the main deoxidizing element, can play a solid solution strengthening effect, if it is too high, it will deteriorate the ductility and toughness of the pipeline steel; the Si content of the present invention is selected in the range of 0.2-0.3%.

Mn: The main solid solution strengthening element, which improves the strength and toughness of steel in the form of solid solution strengthening. If it is too high, it will cause segregation of Mn, forming MnS and impairing low-temperature impact toughness. In the present invention, the solid solution strengthening effect of Mn is used to achieve To lower the yield ratio of steel, the range of Mn is 1.65-1.75%. In order to exert this effect, S must be strictly limited.

Al: The main deoxidizing element, and plays the role of grain refinement by forming AlN. The selection range of the Al content of the present invention is 0.02-0.04%.

Nb: The most important fine-grain element in pipeline steel. Nb can significantly increase the austenite recrystallization temperature of steel and expand the range of unrecrystallized areas. This patent combines the temperature limit of the coiling furnace in the coil rolling process to give full play to Nb The effect of expanding the range of the unrecrystallized zone, significantly refines the original austenite grain size, improves the strength, and makes up for the lack of strength due to the low carbon content. Considering comprehensively, the selection range of the Nb content of the present invention is 0.045-0.055%.

Cr: Chromium has a strong solid solution strengthening effect. If the content is low, the strengthening effect cannot be achieved. If the content is too high, the welding performance may be deteriorated. Combining the two, the selection range of the Cr content of the present invention is 0.20-0.30%.

Ti: Plays the role of precipitation strengthening by forming TiN and effectively refines crystal grains. At the same time, the compound addition of Nb/Ti can reduce the probability of microcracks. The selection range of the Ti content of the present invention is 0.01-0.02%.

S: The main impurity element in steel, interacts with Mn to form MnS inclusions, which are distributed in the center and significantly reduce toughness. The present invention takes into account the high content of Mn element, so the S content is strictly limited, S≤0.0008%. At the same time, the calcium treatment process is adopted to spheroidize the MnS that has inevitably been produced, so the limit requirement of Ca/S≥2 is further increased.

The manufacturing method of the low-yield ratio thin-gauge pipeline steel plate of the present invention includes the following process steps:

(1) In the steelmaking process, high-purity molten steel is produced by converter smelting, LF+RH vacuum degassing treatment, S≤0.0008%, Ca/S≥2, and then produced through thin slab continuous casting slab production process 150mm thick continuous casting slab.

(2) After the continuous casting billet is checked, it is loaded into the furnace and reheated. The heating furnace is in the furnace for 170-200min, the soaking section temperature is 1150-1220℃, and the holding time is ≥40min. Properly extend the total time and holding time to obtain a more uniform grain size of the original austenite.

(3) In the rolling stage, the coiling rolling process is adopted. First, flat rolling (rough rolling) is used to roll the steel plate to the thickness of the furnace. The thickness of the coiling rolling furnace is 20-30mm, and the temperature range of the coiling furnace is 1000 -1050℃, the temperature of the coiling furnace is set to 850-900℃; the last pass of the coil rolling is set to the empty aisle pass under light reduction, the purpose is to extend the coiling holding time. This stage is the key process setting of the present invention. Firstly, the heat preservation rolling in the unrecrystallized zone is realized through the coil rolling process, which overcomes the problem of uneven strength caused by excessive temperature drop in the final rolling stage of the thin gauge steel sheet; Press down the empty passage to extend the holding time of the coiling furnace to homogenize the flat austenite and provide good conditions for the next phase transformation.

(4) After the steel plate is rolled, it enters the ACC cooling equipment for cooling, the opening cooling temperature is 780-820℃, the final cooling temperature is 500-550℃, and the cooling speed is 8-15℃/S, followed by hot straightening, shearing, and online flaw detection. , Online surface and size inspection, storage.

The purpose of the present invention is to provide a method for producing thin-gauge pipeline steel sheets with low yield ratio. Compared with the prior art, the advantages of the present invention are: through reasonable composition design, special coiling and rolling methods are used to achieve 5 Industrial production of low yield ratio of -8mm thin gauge pipeline steel. The yield strength of the steel plate produced is ≥485MPa, tensile strength ≥570MPa, yield ratio ≤0.85, elongation ≥30%, -40℃ impact energy ≥100J, Under the premise of reducing the carbon content and ensuring the yield ratio, through Mn element and production process, a thin gauge pipeline steel with high strength and good uniformity of the overall plate strength can be obtained. The product also has good welding performance.

Description of the drawings

Figure 1 is the microstructure of the 6mm thick steel plate of the present invention.

Detailed ways

The present invention will be further described in detail below in conjunction with the embodiments.

Example 1-3:

A method for manufacturing low-yield-strength thin-gauge pipeline steel plate. The process is as follows: converter smelting-LF refining-RH vacuum degassing-150mm continuous casting billet-continuous casting billet inspection and cleaning-casting billet heating-high pressure water descaling-coiling Rolling-ACC cooling and other process steps to manufacture 5-8mm low yield ratio pipeline steel plate.

The specific process of the above heating, rolling and cooling stage is: heating the 150mm thick continuous casting slab to 1150-1220℃, and the total heating time is 185min (Example 1)/183min (Example 2) and 180min (Example 3) , The holding time of the soaking section is 48min (Example 1), 45min (Example 2) and 46min (Example 3). After the continuous casting slab is released from the furnace, high-pressure water is used for descaling; first, the continuous casting slab is flat rolled to the thickness of the furnace. Then coiling and rolling, the thickness of the coiling rolling into the furnace is 20mm (Example 1), 23mm (Example 2) and 26mm (Example 3), and the temperature of the coiling furnace is controlled within the range of 1000-1050°C ( Example 1-3), the temperature of the coiling furnace is set to 880°C (Example 1-3); after the steel plate is rolled, it enters the ACC cooling device for cooling, and the cooling temperature is 785°C (Example 1) 789°C (implementation Example 2) and 806°C (Example 3), final cooling temperature 542°C (Example 1) 537°C (Example 2) and 521°C (Example 3), cooling rate 12°C/s (Example 1) 11.8 °C/S (Example 2) and 10.5 °C/S (Example 3).

The chemical composition corresponding to each embodiment is shown in Table 1, the mechanical properties of the steel plate is shown in Table 2, and the microstructure of the steel plate is shown in Fig. 1, and the grain size is extremely fine and uniform.

Table 1 Chemical composition of X70 steel plate in the example (wt.%)

Table 2 Mechanical properties of X70 steel plate in Examples

The size of the impact sample in this embodiment is 5*5*55mm;

Based on the above embodiments, the present application provides a method for manufacturing a pipeline steel plate with a thickness of 5-8mm with a yield ratio lower than 0.85, which is successful and effective. It is a domestic initiative, ingeniously carried out effective technical upgrades, and industrialized the product without increasing the difficulty of production.

In addition to the foregoing embodiments, the present invention also includes other implementation modes, and all technical solutions formed by equivalent transformations or equivalent substitutions should fall within the protection scope of the claims of the present invention.

Claims

A method for manufacturing thin-gauge pipeline steel with low yield ratio, characterized in that it comprises the following steps

(1) Molten steel smelting, using converter smelting and LF+RH vacuum degassing to produce high-purity molten steel, controlling S≤0.0008%, Ca/S≥2, and then pouring the molten steel into thin slab continuous casting billets;

(2) The continuous casting billet is loaded into the furnace and reheated, the furnace time is 170-200min, the heat preservation temperature of the soaking section is 1150-1220℃, and the heat preservation time is ≥40min;

(3) The continuous casting billet after the furnace is coiled and rolled. The steel plate is first flat rolled to the thickness of the furnace. The thickness of the coiling and rolling furnace is 20-30mm. The temperature range of the coiling furnace is 1000-1050℃. The temperature of the coiling furnace is 850-900℃, and then the rolling is controlled to be the holding rolling in the non-recrystallization zone. The last pass of the coiling is set to the light reduction empty pass to extend the holding time of the coiling;

(4) ACC cooling is carried out after the steel plate is rolled, the cooling temperature is 780-820℃, the final cooling temperature is 500-550℃, and the cooling speed is 8-15℃/s, followed by hot straightening.
The method for manufacturing a thin gauge pipeline steel with a low yield ratio according to claim 1, wherein the thickness of the continuous casting billet in step (1) is 150 mm.
The method for manufacturing thin gauge pipeline steel with low yield ratio according to claim 1, wherein the final thickness of the steel plate is 5-8 mm.
The manufacturing method of low-yield-strength thin-gauge pipeline steel according to claim 1, characterized in that: in step (1) the percentage of chemical composition in molten steel smelting is: C 0.075-0.095; Si 0.2-0.3; Mn 1.65-1.75; Al 0.02-0.04; Nb 0.045-0.055; Ti 0.01-0.02; Cr 0.20-0.30; S≤0.0008%; Ca/S≥2; the balance is Fe and unavoidable impurities.
The method for manufacturing thin gauge pipeline steel with low yield ratio according to claim 4, characterized in that: the yield strength of the product is ≥485MPa, the tensile strength is ≥570MPa, the yield ratio is ≤0.85, and the elongation is ≥30%, -40°C Impact energy ≥100J.