CN109252107B - Production method of high-flatness ultrahigh-strength steel - Google Patents

Abstract

A production method of high-flatness ultrahigh-strength steel comprises the following steps of molten iron pretreatment → converter smelting → LF external refining → VD vacuum treatment → continuous casting → heating → rolling → straightening → quenching → tempering → finishing → performance inspection → flaw detection, and the steps of: the steel comprises, by weight, C = 0.15-0.20, Si = 0.15-0.35, Mn = 1.0-1.50, P ≤ 0.012, S ≤ 0.003, Nb = 0.010-0.030, V = 0.015-0.040, Ti = 0.010-0.025, Als = 0.020-0.045, Cr = 0.40-0.60, Mo = 0.10-0.30, Ni = 0.20-0.50, B = 0.0008-0.0025, CEV ≤ 0.60, and the balance Fe and inevitable impurities. The method is used for producing the ultra-high strength steel with the thickness of 6-25mm and the width of 1500-3800mm, the yield strength of the steel plate is more than or equal to 1100MPa, the tensile strength is more than or equal to 1200MPa, the elongation is more than or equal to 11 percent, the impact energy at minus 40 ℃ is more than or equal to 50J, and the straightness of the steel plate in any direction is less than or equal to 3 mm/m.

Description

Production method of high-flatness ultrahigh-strength steel

Technical Field

The invention belongs to the technical field of steel making, and particularly relates to a production method of high-flatness yield strength 1100 MPa-level ultrahigh-strength steel.

Background

In order to meet the manufacturing requirements of large-scale and large-span components, save resources and meet the requirements of environmental protection, engineering machinery equipment is developed towards light weight, and ultrahigh-strength steel with yield strength of more than 1100MPa is widely applied to key parts (such as crane booms and the like) of engineering machinery. At present, the thickness specification of the application is mainly 4-25mm, the production process is mainly TMCP + quenching + low temperature, the structure of the tempered material is mainly tempered martensite, and the residual stress is large. Because of the steel sheet intensity is high, the plate-type control degree of difficulty is big, and the performance is inhomogeneous, can produce a series of problems in the use, if cutting deformation, bending deformation is inconsistent, leads to machining precision poor etc. need increase extra levelling process, produces great influence and increase manufacturing cost to the production efficiency of enterprise.

Chinese patent 201110096170.0 discloses a 'yield strength 1100MPa-1200MPa grade ultrahigh strength steel and a production method thereof', the method is a coil plate production process, and the width of the produced steel plate is narrower. Chinese patent 201210491489.8 discloses a non-quenched and tempered hot-rolled strip steel for engineering machinery with yield strength of 1100MPa and a preparation method thereof, the production thickness specification is 2-6mm, two-stage rolling is adopted, the strip steel is cooled to 200-400 ℃ after being rolled and then is curled, no heat treatment is carried out after the rolling, the dislocation density of the material structure is high, and the residual stress is large. Chinese patent 201610587785.6 discloses a steel plate with excellent cold bending performance and yield strength greater than 1100MPa and a preparation method thereof, wherein the rolling is divided into two stages, the rough rolling temperature is 1050-. Chinese patent 201210223610.9 discloses a high-strength steel plate with yield strength of 1100MPa and its manufacturing method, wherein the alloy element Ni is added by 0.60-2.00%, and the alloy content is higher, so that the product cost is higher.

Disclosure of Invention

The patent aims at providing a production method of the ultra-high strength steel with high flatness yield strength of 1100MPa, the thickness specification of the produced steel plate is 6-25mm, the width is 1500-3800mm, through reasonable chemical composition, rolling process design and straightening treatment, the flatness of the steel plate before heat treatment is ensured to be less than or equal to 5mm/m in any direction, and the unevenness after heat treatment is less than or equal to 3 mm/m.

The technical scheme of the invention is as follows:

the production process of high-flatness ultrahigh-strength steel includes molten iron pre-treatment → converter smelting → LF external refining → VD vacuum treatment → continuous casting → heating → rolling → straightening → quenching → tempering → finishing → performance test → fault detection, and the steel has the chemical composition as follows: c = 0.15-0.20, Si = 0.15-0.35, Mn = 1.0-1.50, P ≤ 0.012, S ≤ 0.003, Nb = 0.010-0.030, V = 0.015-0.040, Ti = 0.010-0.025, Als = 0.020-0.045, Cr = 0.40-0.60, Mo = 0.10-0.30, Ni = 0.20-0.50, B = 0.0008-0.0025, CEV ≤ 0.60, and the balance of Fe and inevitable impurities; the key process steps comprise:

(1) the rolling process comprises the following steps: the slab heating tapping temperature is 1200-1250 ℃, and the soaking time is 30-50 minutes; rough rolling three-pass reduction rate is more than or equal to 18%, at least one-pass reduction rate is more than or equal to 22%, the temperature is not required to be kept after rough rolling, and the plate blank is rolled to the target thickness in a finishing mill; after rolling is finished, the steel plate is rapidly cooled to Ar at the outlet of the rolling mill₃Plus (0-40) DEG C, straightening the steel plate by using a pre-straightening machine; and then quickly passing through Mulpic without boiling water, carrying out hot straightening on the steel plate at a Mulpic outlet, conveying the steel plate to a cooling bed for air cooling, carrying out cold straightening before stacking, and then stacking and slowly cooling.

(2) And (3) quenching and tempering: the quenching temperature is 890-930 ℃, and the heating time is the plate thickness mm multiplied by 3 min/mm; the tempering temperature is 250 ℃ and 450 ℃, and the tempering time is the plate thickness mm multiplied by 6 min/mm; and (5) taking out the furnace and then cooling the furnace to room temperature.

Finally obtaining a steel plate with a structure mainly comprising tempered martensite, wherein the yield strength is more than 1100MPa, the tensile strength is 1200-1500 MPa, the elongation is more than 11%, the longitudinal and transverse Charpy impact energy (-40 ℃) is more than 30J, and the unevenness of the steel plate in any direction is less than 3 mm/m.

The invention principle is as follows:

designing chemical components: the content of C is increased, so that the hardenability and strength of the steel are improved, the weldability is greatly reduced when the carbon content is more than or equal to 0.30, and the adverse effect on the weldability can be eliminated by controlling the carbon equivalent under the condition that the carbon content is less than or equal to 0.20. The method has the advantages that the carbon content is low, the hardenability of the steel is influenced, a soft phase structure is easily generated in the cooling process, the strength of the steel plate is influenced, the required strength performance is obtained, the weldability of the material is considered, and the content of the C element is controlled to be 0.15-0.20. The Mn element can improve the strength, the hardness and the hardenability of the steel, but the Mn content in the steel is too high, and a segregation area is easy to generate brittle tissues and microcracks during cooling; the concentration product of manganese and sulfur is high, and the transverse and thickness direction performance of the steel is deteriorated due to the precipitation of manganese sulfide; in order to avoid the harm caused by high manganese, the Mn content is designed to be 1.0-1.50. In order to meet the requirement of steel performance indexes on hardenability, Cr, Mo and Ni are added to improve the hardenability and strength, but the high addition amount can cause high carbon equivalent and influence weldability, and in order to avoid the adverse effect, a small amount of B is required to be added to improve the hardenability. BN generated by the reaction of nitrogen and boron in the steel can be partially gathered at a crystal boundary to promote the generation of ferrite and cause adverse effect on hardenability, corresponding addition amounts of Ti and Al are calculated according to the control level of the nitrogen content in the steel at present, the condition that the excessive N causes the reduction of effective boron is avoided, and fine TiN and AlN particles are utilized to refine grains. Because the structure and the performance are mainly controlled by the heat treatment process, the rolling does not need to adopt controlled rolling and does not need to add too much Nb and V.

The rolling process comprises the following steps: the heating temperature of the blank is low, the alloy components can not be completely dissolved in the solution, the heating temperature is too high, the abnormal growth of crystal grains can be caused, and the NbCN₂And calculating the upper and lower limits of the heating temperature of the plate blank to be 1250 ℃ and 1200 ℃ by an NbC solid solution formula. The temperature uniformity of the blank affects the deformation uniformity in the rolling process, so that the thickness is uneven, various plate-shaped problems are caused, and the excessive heating time can cause overburning, so that the reasonable heating time needs to be determined; the rolling temperature is high, the deformation resistance is small, the control of the plate shape is facilitated, but the austenite grains are recovered and recrystallized during rolling in a recrystallization region, the austenite grains are quickly cooled to be above a phase transformation point after rolling, the austenite hardened state after rolling is reserved, more nucleation positions and driving force are provided for subsequent phase transformation, and the grains are refined.

The heat treatment process comprises the following steps: the austenitizing temperature and the heat preservation time influence the solid solution of alloy elements in the material, the hardenability and the grain size, and simultaneously influence the temperature uniformity of the steel plate, and the temperature uniformity influences the phase transformation and stress distribution of the steel plate after cooling, thereby influencing the shape of the quenched plate. The steel plate has larger internal stress after quenching, and the tempering needs to be carried out for enough time to remove the stress, so that the final stress is small and is uniformly distributed.

According to the invention, through reasonable chemical composition, rolling process and heat treatment process design, the ultrahigh-strength steel plate with the thickness of 6-25mm and the width of 1500-3800mm is obtained, the performance index meets the standard requirement, and the steel plate straightness meets the high-standard requirement of downstream enterprises. The invention has the advantages that: (1) the low-carbon equivalent design is adopted in the component design, the hardenability is improved through Cr and B, the addition of noble alloys Ni, Nb and V is reduced, and the alloy cost is low. (2) The rolling adopts high-temperature hot rolling, the rolling deformation resistance is small, the shape control is facilitated, the rolled steel plate is quickly cooled to be near a phase change point, the rolling hardening is reserved, and the crystal grains are refined. (3) High-temperature pre-straightening, hot straightening and normal-temperature cold straightening are performed for multiple times, the flatness of the steel plate before quenching is ensured, the uniform cooling of quenching is guaranteed, the uniform performance and uniform stress control are facilitated, the flatness of the steel plate is high, and the unevenness of a final product is less than 3 mm/m. (4) And the steel plate is tempered for a long time at a low temperature, so that the internal stress is removed to the maximum extent while the performance of the steel plate is ensured, the rebound deformation of the steel plate during blanking processing is small, and the bending precision is high.

Detailed Description

The present invention will be further described with reference to the following examples.

The first embodiment is as follows: production of 8mmQ1100E steel plate

The chemical composition of the steel comprises, by mass, C =0.17, Si =0.28, Mn =1.25, P =0.011, S =0.0021, Nb =0.015, V =0.022, Ti =0.018, Als =0.030, Cr =0.53, Mo =0.20, Ni =0.21, B =0.0018, CEV =0.55, and the balance Fe and other trace elements. The production process comprises the following steps:

the rolling process comprises the following steps: the method comprises the steps of heating a plate blank by a heating furnace, wherein the tapping temperature is 1230 ℃, the soaking time is 33 minutes, the thickness of an intermediate blank is 55mm after 7 rough rolling passes, the rolling reduction rate of the final three passes of the rough rolling is over 22 percent, the finish rolling temperature is 841-863 ℃, the plate blank is rolled into a steel plate with the thickness of 8mm, then the steel plate is rapidly cooled to 732-755 ℃, and the steel plate is straightened by a pre-straightening machine and a hot straightening machine, then is air-cooled in a cooling bed, and is cold-straightened again before the steel plate is off-line and stacked.

And (3) quenching and tempering: and (3) quenching and tempering the steel plate straightened for three times, wherein the quenching heating temperature is 920 ℃, the quenching time is 24min, the tempering temperature is 300 ℃, the tempering time is 48min, and the steel plate is air-cooled to the room temperature after tempering.

The yield strength of the quenched and tempered steel plate is 1261MPa, the tensile strength is 1374MPa, the elongation is 14%, the Charpy impact values of V-shaped notches of 1/2 size at minus 40 ℃ are 51J, 51J and 54J, the strength and toughness matching is good, and the unevenness of the steel plate is less than 3 mm/m.

Example two: production of 11mmQ1100E steel plate

The steel comprises the following chemical components in percentage by mass: c =0.18, Si =0.28, Mn =1.24, P =0.010, S =0.0018, Nb =0.016, V =0.021%, Ti =0.017, Als =0.030, Cr =0.55, Mo =0.21, Ni =0.20, B =0.0018, CEV =0.56, and the balance being Fe and other trace elements. The production process comprises the following steps:

the rolling process comprises the following steps: after the plate blank is heated by a heating furnace, the tapping temperature is 1225 ℃, the soaking time is 36 minutes, the thickness of the intermediate blank is 60mm after 7 rough rolling passes, wherein the rolling reduction rate of the final three passes of rough rolling is over 22 percent, the finish rolling temperature is 845-870 ℃, the intermediate blank is rolled into a steel plate with the thickness of 11mm, then the steel plate is rapidly cooled to 738-762 ℃, the steel plate is straightened by a pre-straightening machine and a hot straightening machine and then is air-cooled in a cooling bed, and the cold straightening is carried out again before the steel plate is off-line and stacked.

And (3) quenching and tempering: and (3) quenching and tempering the steel plate straightened for three times, wherein the quenching heating temperature is 920 ℃, the quenching time is 33min, the tempering temperature is 300 ℃, the tempering time is 66min, and the steel plate is air-cooled to room temperature after tempering.

The yield strength of the quenched and tempered steel plate is 1174MPa, the tensile strength is 1367MPa, the elongation is 16.5%, the Charpy impact values of 3/4V-shaped notches at the temperature of minus 40 ℃ are 75J, 58J and 70J, the toughness and the matching are good, and the roughness of the steel plate is less than 2.5 mm/m.

Claims (1)

1. A production method of high-flatness ultrahigh-strength steel comprises the process route of molten iron pretreatment → converter smelting → LF external refining → VD vacuum treatment → continuous casting → heating → rolling → straightening → quenching → tempering → finishing → performance inspection → flaw detection, and is characterized in that: the steel comprises, by weight, C = 0.15-0.20, Si = 0.15-0.35, Mn = 1.0-1.50, P ≤ 0.012, S ≤ 0.003, Nb = 0.010-0.030, V = 0.015-0.040, Ti = 0.010-0.025, Als = 0.020-0.045, Cr = 0.40-0.60, Mo = 0.10-0.30, Ni = 0.20-0.50, B = 0.0008-0.0025, CEV ≤ 0.60, and the balance Fe and inevitable impurities; the key process steps comprise:

(1) the rolling process comprises the following steps: the slab heating tapping temperature is 1200-1250 ℃, and the soaking time is 30-50 minutes; rough rolling three-pass reduction rate is more than or equal to 18%, at least one-pass reduction rate is more than or equal to 22%, the temperature is not required to be kept after rough rolling, and the plate blank is rolled to the target thickness in a finishing mill; after rolling is finished, the steel plate is rapidly cooled to Ar at the outlet of the rolling mill₃Plus (0-40) DEG C, straightening the steel plate by using a pre-straightening machine; then quickly passing through Mulpic without boiling water, carrying out hot straightening on the steel plate at a Mulpic outlet, then conveying the steel plate to a cooling bed for air cooling, carrying out cold straightening before stacking, and then stacking and slowly cooling;

(2) and (3) quenching and tempering: the quenching temperature is 890-930 ℃, and the heating time is the plate thickness mm multiplied by 3 min/mm; the tempering temperature is 250 ℃ and 450 ℃, and the tempering time is the plate thickness mm multiplied by 6 min/mm; and (5) taking out the furnace and then cooling the furnace to room temperature.