A method for straightening low-temperature extra-thick steel plates

Abstract

The present invention discloses a method for straightening the shape of low-temperature, extra-thick steel plates. The method comprises: first determining the billet thickness to be 320 mm, then heating the slab according to conventional processes, removing phosphorus, and then entering a hot rolling mill for rolling; the rolled steel plate is cooled in stages, first entering an ACC for a first-stage cooling process, and then entering a hot straightening machine for straightening at a straightening temperature of 600-650°C; the straightened steel plate then enters an ACC for a second-stage cooling process, with a final cooling temperature below 300°C, for online quenching; the cooled steel plate then undergoes a subsequent cooling bed and flame cutting process to complete the operation. The present invention employs a straightening-first, cooling-later method, breaking with conventional processes and successfully resolving the shape issues of extra-thick, low-temperature, hot-rolled, online-quenched steel plates, a type of high-strength, high-quality steel, while simultaneously ensuring the high strength and high toughness standards of extra-thick, high-strength steel.

Description

Technical Field

[0001] The invention belongs to the technical field of steel production, and in particular relates to a straightening method for low-temperature and extra-thick steel plates. Background Art

[0002] Currently, there is a growing demand for low-temperature, extra-thick plate production, such as crack arrest steel EH40 and EH47, high-strength bridge steel Q500qE, and high-strength shipbuilding plate EH550, with thicknesses ranging from 50 to 100 mm. These plates require hot rolling and in-line quenching to replace heat treatment, or in-line quenching to increase strength before heat treatment and tempering. These plates are characterized by their exceptional thickness (50 to 100 mm), requiring in-line quenching to save on heat treatment costs, or to reduce alloy costs while ensuring post-heat treatment strength and toughness. The in-line quenching temperature for hot rolling must be below 300°C. At this temperature, the steel plate's strength is very high, and due to its thickness, the plate's shape cannot be straightened at all, even with a 5,000-ton straightening force. It is understood that domestic steel mills currently have the problem of being unable to straighten the plate shape after in-line quenching of extra-thick hot-rolled steel plates. The general requirement is to keep the plate shape uneven within 3mm / 2m, but conventional technology can only guarantee a limit of about 10mm / 2m. Therefore, it is very necessary to solve the flatness problem of extra-thick low-temperature hot-rolled in-line quenched steel plates. Summary of the Invention

[0003] In view of the shortcomings of the existing technology, the present invention provides a low-temperature extra-thick steel plate straightening method, which successfully solves the major quality problem of extra-thick plate shape, so that the plate shape unevenness is controlled within 3mm / 2m.

[0004] The present invention is achieved through the following technical solutions:

[0005] A method for straightening a low-temperature, extra-thick steel plate comprises the following steps:

[0006] Step 1) determining the billet thickness to be 320 mm, heating the slab according to conventional processes, dephosphorizing, and then entering a hot rolling mill for rolling;

[0007] Step 2) The rolled steel plate is cooled in stages, first entering the post-rolling accelerated cooling control system for the first stage of cooling, and then entering the hot straightening machine for straightening at a straightening temperature of 600-650°C;

[0008] Step 3) The straightened steel plate then enters the post-rolling accelerated cooling control system for the second stage of cooling, with the final cooling temperature below 300°C, and undergoes online quenching;

[0009] Step 4) The cooled steel plate then undergoes subsequent cooling bed and fire cutting processes to complete the operation.

[0010] Preferably, the hot rolling mill in step 1) performs rolling using a thermomechanical control process.

[0011] Preferably, in step 2), the first stage cooling needs to cool the steel plate to 600-650°C.

[0012] Preferably, the straightening in step 2) controls the unevenness of the steel plate to 1-2 mm / 2 m.

[0013] Preferably, the low-temperature extra-thick steel plates include crack arrest steel, bridge steel, hydropower steel, and ship plate steel.

[0014] Preferably, the thickness of the low-temperature extra-thick steel plate is 50 to 100 mm.

[0015] The beneficial effects of the present invention are as follows:

[0016] The present invention's low-temperature (low cooling temperature) straightening method for extra-thick steel plate employs a staged cooling process: warm straightening after the first stage of cooling, followed by a second stage of cooling. This breaks with conventional procedures and successfully addresses the shape issues associated with extra-thick, high-strength, high-toughness, hot-rolled, low-temperature, in-line quenched steel plate, while maintaining the high strength and toughness standards of extra-thick, high-strength steel. Extensive practical experience demonstrates that the present method can achieve plate shape control within 3 mm / 2 m. DETAILED DESCRIPTION

[0017] The present invention is further described in detail below with reference to specific embodiments.

[0018] Example 1

[0019] A method for straightening low-temperature extra-thick steel plates, the specific steps are as follows:

[0020] (1) The billet thickness is determined to be about 320 mm. The slab is heated and (primarily) dephosphorized according to the conventional process, and then enters the hot rolling mill and is rolled using the TMCP (thermomechanical control process).

[0021] (2) After rolling, the steel plate is cooled in stages. It first enters the ACC (accelerated cooling control system after rolling) for the first stage of cooling. After cooling, the temperature is controlled at 600-650℃, which precipitates some ferrite and improves the toughness of the high-strength steel. Then it enters the hot straightening machine for straightening. The straightening temperature is 600-650℃. The purpose of this operation is to ensure sufficient temperature for straightening and avoid the problem that the steel plate temperature is too low and the straightening machine cannot level it. After this operation, the unevenness of the steel plate can be controlled to 1-3mm / 2m.

[0022] (3) The straightened steel plate then enters the ACC for the second stage of cooling, with the final cooling temperature below 300°C. After this operation, the hot-rolled steel plate is quenched in the ACC to generate some lamellar bainite and improve the strength of the steel plate. This operation can improve the strength of the hot-rolled steel plate or the heat treatment tempering.

[0023] (4) After cooling, the steel plate undergoes subsequent cooling bed and fire cutting processes to complete the operation.

[0024] Example 2

[0025] A bridge steel, grade Q500qD, with a billet thickness of 319 mm and a target thickness of 78 mm, was delivered using online quenching and heat treatment and tempering, with the technical requirement that the unevenness be controlled within 3 mm / 2 m. The production method described in Example 1 was used, specifically as follows:

[0026] (1) The slab is heated in a heating furnace with a preheating section temperature of 1034°C, a first heating section temperature of 1125°C, a second heating section temperature of 1147°C, and a soaking section temperature of 1151°C. The slab is then (primarily) dephosphorized and then enters a hot rolling mill, where TMCP controlled rolling is employed, with a final rolling temperature of 783°C.

[0027] (2) After rolling, the steel plate enters the ACC for the first stage of cooling, cooling to 645℃, and then enters the hot straightening machine for straightening, with the straightening temperature at 634℃.

[0028] (3) The straightened steel plate enters the ACC for the second stage of cooling, with the final cooling temperature at 260℃.

[0029] (4) After cooling, the steel plate undergoes subsequent cooling bed and fire cutting processes to complete the operation.

[0030] The final steel plate has an unevenness of 1mm / 2m, a tensile strength of 679MPa, a yield strength of 579MPa, an elongation of 24%, a yield strength ratio of 0.85, an impact test temperature of -20°C, an average impact strength of 320J, passes the impact test, and has the characteristics of high strength and high toughness.

[0031] Example 3

[0032] A bridge steel, grade Q500qE, with a billet thickness of 318 mm and a target thickness of 56 mm, was delivered using online quenching and heat treatment and tempering, with the technical requirement that the unevenness be controlled within 3 mm / 2 m. The production method described in Example 1 was used, specifically as follows:

[0033] (1) The slab is heated in a heating furnace with a preheating section temperature of 1034°C, a first heating section temperature of 1120°C, a second heating section temperature of 1153°C, and a soaking section temperature of 1147°C. The slab is then (primarily) dephosphorized and then enters a hot rolling mill, where TMCP controlled rolling is employed, with a final rolling temperature of 780°C.

[0034] (2) After rolling, the steel plate enters the ACC for the first stage of cooling, cooling to 642℃, and then enters the hot straightening machine for straightening, with a straightening temperature of 627℃.

[0035] (3) The straightened steel plate enters ACC for the second stage of cooling, with the final cooling temperature at 261°C.

[0036] (4) After cooling, the steel plate undergoes subsequent cooling bed and fire cutting processes to complete the operation.

[0037] The final steel plate has an unevenness of 2mm / 2m, a tensile strength of 638MPa, a yield strength of 541MPa, an elongation of 24%, a yield strength ratio of 0.85, an impact test temperature of -40°C, an average impact strength of 394J, passes the impact test, and has the characteristics of high strength and high toughness.

[0038] Example 4

[0039] A crack arrest steel, grade EH40, with a billet thickness of 320 mm and a target thickness of 85 mm, is delivered using online quenching and heat treatment and tempering, with the technical requirement that the unevenness be controlled within 3 mm / 2 m. The method described in Example 1 is used for production, specifically as follows:

[0040] (1) The slab is heated in a heating furnace with a preheating section temperature of 1017°C, a first heating section temperature of 1120°C, a second heating section temperature of 1154°C, and a soaking section temperature of 1148°C. The slab is then (primarily) dephosphorized and then enters a hot rolling mill, where TMCP controlled rolling is employed, with a final rolling temperature of 767°C.

[0041] (2) After rolling, the steel plate enters the ACC for the first stage of cooling, cooling to 647℃, and then enters the hot straightening machine for straightening, with a straightening temperature of 632℃.

[0042] (3) The straightened steel plate enters the ACC for the second stage of cooling, with the final cooling temperature at 268°C.

[0043] (4) After cooling, the steel plate undergoes subsequent cooling bed and fire cutting processes to complete the operation.

[0044] The final steel plate has an unevenness of 2mm / 2m, a tensile strength of 568MPa, a yield strength of 369MPa, an elongation of 28%, a yield strength ratio of 0.65, an impact test temperature of -40°C, an average impact strength of 386J, passes the impact test, and has the characteristics of high strength and high toughness.

[0045] Example 5

[0046] A crack arrest steel, brand EH47, with a billet thickness of 320 mm and a target thickness of 80 mm, is delivered using online quenching and heat treatment and tempering, with the technical requirement that the unevenness be controlled within 3 mm / 2 m. The method described in Example 1 is used for production, specifically as follows:

[0047] (1) The slab is heated in a heating furnace with a preheating section temperature of 1025°C, a first heating section temperature of 1130°C, a second heating section temperature of 1152°C, and a soaking section temperature of 1150°C. The slab is then (primarily) dephosphorized and then enters a hot rolling mill, where TMCP controlled rolling is employed, with a final rolling temperature of 777°C.

[0048] (2) After rolling, the steel plate enters the ACC for the first stage of cooling, cooling to 638℃, and then enters the hot straightening machine for straightening, with the straightening temperature at 624℃.

[0049] (3) The straightened steel plate enters the ACC for the second stage of cooling, with the final cooling temperature at 296°C.

[0050] (4) After cooling, the steel plate undergoes subsequent cooling bed and fire cutting processes to complete the operation.

[0051] The final steel plate has an unevenness of 1mm / 2m, a tensile strength of 627MPa, a yield strength of 496MPa, an elongation of 25.5%, a yield strength ratio of 0.79, an impact test temperature of -40°C, an average impact strength of 316J, passes the impact test, and has the characteristics of high strength and high toughness.

[0052] Example 6

[0053] A hydropower steel, grade S980CF, with a billet thickness of 320 mm and a target thickness of 60 mm, is delivered using online quenching and heat treatment and tempering, with the technical requirement that the roughness be controlled within 3 mm / 2 m. The production method described in Example 1 is as follows:

[0054] (1) The slab is heated in a heating furnace with a preheating section temperature of 1019°C, a first heating section temperature of 1126°C, a second heating section temperature of 1157°C, and a soaking section temperature of 1145°C. The slab is then (primarily) dephosphorized and then enters a hot rolling mill, where TMCP controlled rolling is employed, with a final rolling temperature of 794°C.

[0055] (2) After rolling, the steel plate enters the ACC for the first stage of cooling, cooling to 640℃, and then enters the hot straightening machine for straightening, with a straightening temperature of 625℃.

[0056] (3) The straightened steel plate enters the ACC for the second stage of cooling, with the final cooling temperature at 285℃.

[0057] (4) After cooling, the steel plate undergoes subsequent cooling bed and fire cutting processes to complete the operation.

[0058] The final steel plate has an unevenness of 2mm / 2m, a tensile strength of 1054MPa, a yield strength of 987MPa, an elongation of 17%, a yield strength ratio of 0.94, an impact test temperature of -60°C, an average impact strength of 143J, passes the impact test, and has the characteristics of high strength and high toughness.

[0059] The embodiments described above are only some of the embodiments of the present invention, rather than all of the embodiments. The detailed description of the embodiments of the present invention is not intended to limit the scope of the invention as claimed, but merely represents selected embodiments of the present invention. The scope of protection of the present invention shall be based on the scope required by the claims. All other embodiments obtained by ordinary technicians in this field based on the embodiments of the present invention without making creative efforts shall fall within the scope of protection of the present invention.

Claims

1. A method for straightening low-temperature extra-thick steel plates, characterized in that: The following steps are involved: Step 1) The billet thickness is determined to be 320 mm, and the slab is heated and dephosphorized according to conventional processes before being rolled in a hot rolling mill. Step 2) The rolled steel plate is cooled in stages. It first enters the post-rolling accelerated cooling control system for the first stage cooling to 600-650°C, and then enters the hot straightening machine for straightening. The straightening temperature is 600-650°C, and the unevenness of the steel plate is controlled to 1-3 mm / 2m. Step 3) The straightened steel plate enters the post-rolling accelerated cooling control system for the second stage of cooling, with the final cooling temperature below 300°C, and undergoes online quenching; Step 4) The cooled steel plate then undergoes the subsequent cooling bed and fire cutting process to complete the operation; Wherein, the thickness of the low-temperature extra-thick steel plate is 50-100 mm.

2. A method for straightening a low-temperature, extra-thick steel plate according to claim 1, characterized in that: The low-temperature extra-thick steel plates include crack arrest steel, bridge steel, hydropower steel, and ship plate steel.