CN114182080B - Production method of ultra-thin ultra-wide low-temperature steel LT-FH36 - Google Patents

Abstract

The invention relates to a production method of ultra-thin ultra-wide low-temperature steel LT-FH36, which comprises the following chemical components in percentage by weight: c=0.07 to 0.08 percent, si=0.10 to 0.50 percent, mn=1.41 to 1.50 percent, P is less than or equal to 0.012 percent, S is less than or equal to 0.003 percent, nb=0.016 to 0.020 percent, ti=0.008 to 0.02 percent, al=0.015 to 0.05 percent, and the balance of Fe and unavoidable impurities. According to the production method, an ultra-wide steel plate with the thickness of 6-10 mm and the thickness of 3800-4300 mm is produced by a controlled rolling process through a heavy plate mill, the metallographic structure of the ultra-wide steel plate is ferrite, pearlite and a small amount of bainite, and the grain size is 11-13 grades. The low-temperature steel has good toughness and low yield ratio, the strength grade is LT-FH36, the impact energy at minus 60 ℃ is more than or equal to 80J, the yield ratio is less than or equal to 0.8, the unevenness of the steel plate can be controlled within 3mm/m, the process is simple, the cost is low, the mass production is easy, the product quality is stable, and the use requirement of the low-temperature steel plate for the ultra-large liquefied gas transport ship can be better met.

Description

Production method of ultra-thin ultra-wide low-temperature steel LT-FH36

Technical Field

The invention belongs to the technical field of metallurgy, and relates to a production method of ultra-thin ultra-wide low-temperature steel LT-FH 36.

Background

The ultra-large liquefied gas carrier (Very Large Gas Carrier, VLGC) is a type of high-technology and high-added-value ship in the liquefied gas carrier market, and has high technical content and great development difficulty. The ultra-large liquefied petroleum gas ship is generally constructed by adopting low-temperature steel and is constructed by 8.4 ten thousand m ³ For example, VLGC, the required amount of the low-temperature steel plate per ship is about 1 ten thousand tons, wherein 10mm or moreThe lower ultra-thin plate exceeds 1000 tons, and the most critical liquid cargo warehouse is designed by adopting ultra-wide steel plates because the influence of welding beads is reduced. For the steel plate steckel mill with the width exceeding 3800mm, the steel plate steckel mill cannot be produced, the heavy and medium plate mill must be used for producing, and the heavy and medium plate mill for the ultrathin steel plate with the ultra-wide specification is difficult to realize controlled rolling, and a hot rolling process must be adopted to ensure that the plate shape of the rolled steel plate is good, but for low-temperature steel, the toughness of the steel plate is difficult to meet the requirement by adopting the hot rolling process.

Disclosure of Invention

The invention aims to provide a production method of ultra-thin ultra-wide low-temperature steel LT-FH36, wherein the thickness of the steel plate is 6-10 mm, the width is 3800-4300 mm, and the use requirements of the ultra-large low-temperature steel LT-FH36 for liquefied petroleum gas ships can be well met: the yield strength is more than or equal to 360MPa, the tensile strength is more than or equal to 490MPa, the elongation is more than or equal to 22%, the Charpy impact energy at minus 60 ℃ is more than or equal to 80J, and the yield ratio is less than or equal to 0.8.

The technical scheme of the invention is as follows:

the production method of the ultra-thin ultra-wide low-temperature steel LT-FH36 comprises the following chemical components in percentage by weight: c=0.07 to 0.08 percent, si=0.10 to 0.50 percent, mn=1.41 to 1.50 percent, P is less than or equal to 0.012 percent, S is less than or equal to 0.003 percent, nb=0.016 to 0.020 percent, ti=0.008 to 0.02 percent, al=0.015 to 0.05 percent, and the balance of Fe and unavoidable impurities; the metallographic structure is ferrite, pearlite and bainite, and the grain size is 11-13 grades; the key process steps comprise:

(1) Cogging: smelting by adopting a converter according to the components and casting into a slab; heating the slab and rolling the slab at a high temperature to form a fire blank with the size of 60-100 mm, and cutting the fire blank into small slabs with the length of the finished product width of +50-80 mm by flame cutting after the fire blank is cooled;

(2) Rolling: the second time of furnace feeding, rapid heating to 1220+/-20 ℃, long and wide rolling after furnace discharging, adopting two-stage rolling, wherein the first stage is not widened, and transverse rolling is performed to the bottom; the thickness of the intermediate blank is 3-5 times of the thickness of the finished product; the second stage of controlled rolling, wherein the initial rolling temperature is 850-880 ℃, and the final rolling temperature is 770-820 ℃;

(3) Pre-correction: and (3) pre-straightening after rolling, wherein the straightening speed is 0.3-0.35 m/s, and then air cooling to room temperature.

The technical principle of the invention is as follows: in the invention, the low-temperature toughness of the steel plate is improved by adopting low-carbon high-manganese and Nb-Ti-Al multi-element microalloying component design to refine grains. The low C content reduces the strength, and the high C content can improve the strength and reduce the Ar3 temperature, but increases the pearlite content and the grain size in the air cooling process, seriously worsens the low-temperature toughness of the ultrathin steel plate, and therefore controls the final carbon content to be 0.07-0.08%; manganese is a weak carbide forming element which can lower the austenite transformation temperature, refine ferrite grains, and is beneficial to improving the strength and toughness of the steel sheet, so that in the composition design, the lower limit of manganese design is 1.41%, but when the content exceeds 1.50%, center segregation is increased, causing serious deterioration of low-temperature toughness; in order to ensure the low-temperature toughness of the ultra-thin ultra-wide steel plate, the molten steel must have higher purity, P, S is used as a harmful impurity element, the lower the content is, the better the content is, and the final control endpoint P is less than or equal to 0.012 percent and S is less than or equal to 0.003 percent in consideration of manufacturing cost; nb is taken as an important microalloy element, the effect of adding a proper amount of Nb on deferring the occurrence of recrystallization, refining grains and strengthening precipitation in the rolling process is quite obvious, and in order to reasonably match the toughness and yield ratio, the experiment of the influence of NbC precipitation on the grain size is carried out through the thermodynamic calculation of NbC in austenite, so that the niobium content needs to be strictly controlled to be 0.016-0.020%; the steel is subjected to micro Ti treatment, and Ti with the content of more than 0.008 is added to be beneficial to forming TiN pinning grain boundary in the heating process of the blank, inhibiting the overgrowth of austenite grains, refining the grains in the rolling process and improving the toughness of the steel plate, but when the Ti content is more than 0.02%, part of Ti composite inclusion can be formed, and becomes a fracture source in the steel instead, and is very harmful to the low-temperature toughness of the steel.

In the process design, the austenite grain size after rolling and the uniformity and flattening degree thereof are controlled by two-fire forming, heating temperature, rolling temperature and pass reduction rate, in particular setting lower two-stage rolling temperature; the rolling pass can be effectively reduced through two-fire length and width rolling, the rolling pass reduction rate of one-stage rolling is increased, the influence of the widening pass on the temperature drop and the plate shape of the ultra-thin ultra-wide plate is avoided, the two-stage rolling is ensured to be controlled in a complete non-recrystallization temperature range, the influence of mixed crystals on the low-temperature toughness of the steel plate is eliminated, and the plate shape after rolling is effectively improved; the uniformity of the residual stress of the steel plate is improved through the constant high Wen Yujiao of the low speed of multiple passes, and the plate shape and the unevenness requirements of the steel plate are ensured.

The invention has the beneficial effects that: aiming at the ultra-thin ultra-wide plate with the thickness of more than 3800mm which cannot be produced by a steckel mill, the invention utilizes two-fire forming materials and long-width rolling, is matched with proper chemical components, breaks through the characteristic of insufficient rolling control capability of producing the ultra-thin ultra-wide steel plate by the heavy and medium plate mill, and produces the steel plate with good comprehensive mechanical property: the yield strength is more than or equal to 360MPa, the tensile strength is more than or equal to 490MPa, the elongation is more than or equal to 22%, the Charpy impact energy at minus 60 ℃ is more than or equal to 80J, and the yield ratio is less than or equal to 0.8. The steel plate has good plate shape, the unevenness is less than or equal to 3mm/m, and meanwhile, the steel plate has uniform structure, uniform performance, small residual stress and excellent low-temperature toughness. The ultra-thin ultra-wide LT-FH36 low-temperature steel plate produced by the invention has high added value, can be supplied in batches, and can well meet the use requirement of the low-temperature steel plate for ultra-large liquefied petroleum gas (VLGC).

Drawings

FIG. 1 is a photograph showing the metallographic structure of a steel sheet according to example 1 of the present invention.

FIG. 2 is a photograph showing the metallographic structure of the steel sheet of comparative example 1 of the present invention.

Detailed Description

According to the production method, a continuous casting blank is cast through 120t converter smelting, LF furnace refining and RH furnace vacuum treatment, and then a finished product is rolled on a 5000mm double-rack medium plate production line. The present invention is further illustrated by the following examples and comparative examples. The chemical compositions of the steel sheets in the examples and comparative examples are shown in Table 1.

Table 1 chemical compositions and Ceq (weight%) of the steel sheets of examples and comparative examples

The key process parameters for the example and comparative steel sheets are as follows.

Example 1

The thickness of the steel plate is 8mm, and the width of the steel plate is 4200mm. The thickness of the continuous casting billet is 300mm, the continuous casting billet is rolled by two fires, and the thickness of the first fire billet is 80mm; and (3) feeding the steel into the furnace for the second time, wherein the heating temperature is 1230 ℃, rolling the steel in a long and wide way after discharging, adopting two-stage rolling, wherein the intermediate billet is 30mm, the initial rolling temperature is 862 ℃ in the second stage, the final rolling temperature is 781 ℃, pre-straightening after rolling, and the straightening speed is 0.35m/s, and then air cooling to room temperature.

Comparative example 1

The thickness of the steel plate is 8mm, and the width of the steel plate is 3600mm. The thickness of the continuous casting billet is 300mm, the continuous casting billet is rolled by two fires, and the thickness of the first fire billet is 80mm; and (3) feeding the steel into the furnace for the second time, heating to 1234 ℃, carrying out normal stretching rolling after discharging, adopting two-stage rolling, wherein the intermediate billet is 30mm, and carrying out pre-straightening after rolling at the initial rolling temperature of 865 ℃ and the final rolling temperature of 785 ℃ for ensuring the rolled plate shape, and then carrying out air cooling to room temperature at the straightening speed of 0.35 m/s.

Example 2

The thickness of the steel plate is 10mm, and the width of the steel plate is 4280mm. The thickness of the continuous casting billet is 300mm, the continuous casting billet is rolled by two fires, and the thickness of the first fire billet is 100mm; and (3) feeding the steel into the furnace for the second time, heating to 1232 ℃, rolling the steel in a long and wide mode after discharging, adopting two-stage rolling, wherein the initial rolling temperature of the middle billet is 40mm, the final rolling temperature of the middle billet is 779 ℃, pre-straightening after rolling, straightening at the speed of 0.30m/s, and then air cooling to room temperature.

Comparative example 2

The thickness of the steel plate is 10mm, and the width of the steel plate is 4280mm. The thickness of the continuous casting billet is 300mm, the continuous casting billet is rolled by two fires, and the thickness of the first fire billet is 100mm; and (3) feeding the steel into the furnace for the second time, wherein the heating temperature is 1235 ℃, rolling the steel in a long and wide way after discharging, adopting two-stage rolling, wherein the intermediate billet is 40mm, the initial rolling temperature in the second stage is 856 ℃, the final rolling temperature is 780 ℃, pre-straightening after rolling, and the straightening speed is 0.30m/s, and then air cooling to room temperature.

Example 3

The thickness of the steel plate is 6mm, and the width of the steel plate is 4050mm. The thickness of the continuous casting billet is 260mm, the continuous casting billet is rolled by two fires, and the thickness of the first fire billet is 60mm; and (3) feeding the steel into the furnace for the second time, heating to 1238 ℃, rolling the steel in a long and wide mode after discharging, adopting two-stage rolling, wherein the intermediate billet is 20mm, the initial rolling temperature of the second stage is 877 ℃, the final rolling temperature is 772 ℃, pre-straightening after rolling, straightening at the speed of 0.35m/s, and then air cooling to room temperature.

Comparative example 3

The thickness of the steel plate is 6mm, and the width of the steel plate is 3400mm. The thickness of the continuous casting billet is 180mm, the heating temperature is 1235 ℃, the rolling is normally stretched after the billet is discharged from the furnace, the rolling is carried out in two stages, the thickness of the intermediate billet is 50mm, in order to ensure the plate shape after rolling, the initial rolling temperature in the second stage is 951 ℃, the final rolling temperature is 784 ℃, the pre-straightening is carried out after the rolling, the straightening speed is 0.35m/s, and then the air cooling is carried out to the room temperature.

The comprehensive mechanical properties of the steel plates of the examples and the comparative examples are shown in Table 2.

Table 2 comprehensive mechanical properties of example and comparative example steel sheets

Wherein the impact test pieces of example 1, comparative example 1, example 2 and comparative example 2 have dimensions of 10mm×7.5mm×55mm, and the test result should be not less than 75% of the prescribed value; the impact test pieces of example 2 and comparative example 2 were 10mm by 5mm by 55mm in size, and the test results should be not less than 50% of the prescribed values.

The comprehensive mechanical properties of the steel plates of the embodiment 1, the embodiment 2 and the embodiment 3 of the invention meet the requirements. The steel sheets of the examples were subjected to metallographic structure observation, and the microstructure consisted of fine-grained ferrite + pearlite + a small amount of bainitic structure, and the grain size was 11-13 grade, as shown in fig. 1. The comprehensive mechanical properties of the steel plate of the comparative example do not completely meet the requirements, the component designs of the steel plate of the comparative example 1 are the same as those of the steel plate of the example 1, the conventional two-fire stretching rolling is adopted, the rolling maximum width is 3600mm, the compression ratio of the first stage is insufficient after stretching, the starting rolling temperature of the second stage is higher, the grain size of the steel plate is relatively larger and uneven, the impact power of the steel plate at minus 60 ℃ is poorer, and the metallographic structure is shown in figure 2; the steel of the comparative example 2 is a bridge plate Q370qE, and the strength and the low-temperature toughness of the steel meet the requirements by adopting the rolling mode of the invention, but the yield ratio of the steel is higher than the standard due to the higher content of alloy elements Nb and the like; in comparative example 3, ship plate EH36 was prepared by adding 0.25% Ni to the composition design to improve toughness, and conventional one-fire widening rolling was used to obtain a gauge shape with a maximum rolling width of 3400mm, and it was found that although Ni element was added, low-temperature impact toughness was not improved by the conventional hot rolling process. Therefore, the ultra-wide and ultra-thin steel plate is difficult to stably control the low-temperature toughness and the yield ratio in industrial production by adopting a conventional rolling mode, and simultaneously meets the requirements, so that the invention is fully shown to be ingenious in composition and process design and compatible with uniqueness.

Claims (1)

1. The production method of the ultra-thin ultra-wide low-temperature steel LT-FH36 is characterized in that the thickness of the steel plate is 6-10 mm, and the width is 3800-4300 mm: the steel comprises the following chemical components in percentage by weight: c=0.07 to 0.08 percent, si=0.10 to 0.50 percent, mn=1.41 to 1.50 percent, P is less than or equal to 0.012 percent, S is less than or equal to 0.003 percent, nb=0.016 to 0.020 percent, ti=0.008 to 0.02 percent, al=0.015 to 0.05 percent, and the balance of Fe and unavoidable impurities; the metallographic structure is ferrite, pearlite and bainite, and the grain size is 11-13 grades; the key process steps comprise:

(1) Cogging: smelting by adopting a converter according to the components and casting into a slab; heating the slab and rolling the slab at a high temperature to form a fire blank with the size of 60-100 mm, and cutting the fire blank into small slabs with the length of the finished product width of +50-80 mm by flame cutting after the fire blank is cooled;

(2) Rolling: the second time of furnace feeding, rapid heating to 1220+/-20 ℃, long and wide rolling after furnace discharging, adopting two-stage rolling, wherein the first stage is not widened, and transverse rolling is performed to the bottom; the thickness of the intermediate blank is 3-5 times of the thickness of the finished product; the second stage of controlled rolling, wherein the initial rolling temperature is 850-880 ℃, and the final rolling temperature is 770-820 ℃;

(3) Pre-correction: and (3) pre-straightening after rolling, wherein the straightening speed is 0.3-0.35 m/s, and then air cooling to room temperature.