Low-cost q690 steel plate

Abstract

The application provides a low-cost Q690 steel plate and a production method thereof, and the chemical component content percentage of the Q690 structural steel plate is as follows: C: 0.12-0.18%, Si: 0.45%, Mn: 1.45-1.60%, P: 0.020%, S: 0.010%, Alt: 0.02-0.040%, Nb: 0.045-0.06%, Ti: 0.015-0.025%, B: 0.0025-0.0040%, N: 0.0040-0.0065%, carbon equivalent CEV: 0.44%, and the balance is Cu: 0.80%, Ni: 0.80%, and As: 0.080%. The tempering sorbite proportion of the surface layer structure of the low-cost Q690 steel plate is more than 85%, and the 1 / 4 and center parts are tempered sorbite. The application effectively reduces the production cost of the Q690 steel plate, is low in energy consumption, and is high in production efficiency.

Description

[0001] The application is a divisional application, the parent application number is: 2022108435167, the invention name is: a low-cost Q690 steel plate and its production method, the application date is: July 18, 2022. TECHNICAL FIELD

[0002] The application relates to the field of metallurgy, in particular to a Q690 steel plate and a production method thereof, in particular to a low-cost Q690 steel plate. BACKGROUND

[0003] At present, with the light-weighting and large-scale of products in the engineering machinery industry, more and more high-strength thick plates such as Q690 are used, and most of the high-strength thick plates in the industry are produced by adding a large amount of alloy elements such as Ni, Cr, Mo and V and quenching and tempering treatment, which is high in cost and long in production cycle.

[0004] In summary, the prior art has the following problems: the production cost of the Q690 steel plate is high and the production cycle is long. SUMMARY

[0005] The application provides a Q690 steel plate and a production method thereof, in particular to a low-cost Q690 steel plate and a production method thereof, to solve the problems of high production cost and long production cycle of the Q690 steel plate.

[0006] Therefore, the application provides a production method of a low-cost Q690 steel plate, the chemical component content ratio of the low-cost Q690 steel plate is C: 0.12-0.18%, Si≤0.45%, Mn: 1.45-1.60%, P≤0.020%, S≤0.010%, Alt: 0.02-0.040%, Nb: 0.045-0.06%, Ti: 0.015-0.025%, B: 0.0025-0.0040%, N: 0.0040-0.0065%, carbon equivalent CEV≤0.44%, and the balance is Cu≤0.80%, Ni≤0.80%, As≤0.080%.

[0007] The production method of the low-cost Q690 steel plate comprises the following process steps in sequence:

[0008] The heating furnace heating, high-pressure water phosphorus removal, rough rolling, cooling, finish rolling, pre-straightening machine straightening, electromagnetic induction heating, quenching, hot straightening, cooling bed and tempering heat treatment.

[0009] In the rough rolling stage, the single pass reduction rate is increased, and the reduction rate of the last 2-3 passes is >20%; in the finishing rolling stage, the single pass reduction rate is increased, and the pass reduction rate is >10%; the steel plate is discharged from the finishing rolling unit, the surface temperature of the steel plate is measured by a pre-straightening machine to form a temperature curve, and the power of heating is adjusted according to the temperature curve of the surface of the steel plate to control the uniformity of the surface temperature.

[0010] Further, the thickness of the steel plate is 6-50mm.

[0011] Further, the heating time of the heating furnace is 230-250min, and the soaking time is >45min.

[0012] Further, the rough rolling control opening temperature is 1100-1180℃, and the longitudinal-horizontal-longitudinal rolling mode is adopted.

[0013] Further, the cooling controls the temperature of the steel plate to 960-1060℃ to start the finishing rolling.

[0014] Further, the finishing rolling control finish rolling temperature is 900-960℃, and the upper limit is controlled for thin specifications and the lower limit is controlled for thick specifications.

[0015] Further, the quenching temperature control is 770-830℃, and the upper limit is controlled for thin specifications and the lower limit is controlled for thick specifications.

[0016] Further, the tempering temperature is 690-700℃, and the tempering time is the thickness of the steel plate + 50min.

[0017] The application also provides a low-cost Q690 steel plate, and the chemical component content ratio of the low-cost Q690 steel plate is C: 0.12-0.18%, Si: ≤0.45%, Mn: 1.45-1.60%, P: ≤0.020%, S: ≤0.010%, Alt: 0.02-0.040%, Nb: 0.045-0.06%, Ti: 0.015-0.025%, B: 0.0025-0.0040%, N: 0.0040-0.0065%, carbon equivalent CEV: ≤0.44%, and the balance is Cu: ≤0.80%, Ni: ≤0.80%, and As: ≤0.080%.

[0018] Further, the thickness of the low-cost Q690 steel plate is 6-50mm.

[0019] The principle of the present application is: (1) using cheap alloying elements to replace expensive elements to improve hardenability and austenite recrystallization temperature; (2) using high temperature finish rolling and temperature control after rolling to meet the temperature requirement during on-line quenching, and reducing the off-line heating quenching process; (3) using reasonable heat treatment process to realize the production of Q690 steel plate with thickness of 6-50 mm. The measures include pending composition design, production line layout, reasonable rolling process and heat treatment process.

[0020] Based on the above principle, the present application "a low-cost Q690 steel plate and its production method" has the following specific measures:

[0021] 1. Specific composition design: using low-cost B element to replace expensive Ni, Cr and Mo, thereby improving hardenability, determining that the steel plate can be fully quenched after rolling to form uniform martensite structure; designing the C content at the upper limit of the standard, reducing the A3 temperature, reducing the quenching temperature requirement of the steel plate after rolling, promoting the formation of martensite structure, and replacing the low-carbon high-purity alloy (metallic manganese, low-carbon manganese, ferrosilicon, etc.) that must be added when designing low-C with low-cost silicon-manganese alloy to reduce cost; adding Nb, Ti and other micro-alloying elements to improve the austenite recrystallization temperature, so that the billet is still in the austenite recrystallization zone during high-temperature finish rolling; Ti can fix N in the steel to form nitrides, improve strength and avoid N and B forming nitrides, reducing the effect of B on improving hardenability, as shown in Tables 1 and 2.

[0022] Table 1 Chemical composition of low-cost Q690 steel plate (content percentage: %)

[0023]

[0024] Table 2 Specification of low-cost Q690 steel plate

[0025]

[0026]

[0027] (1) C content: controlled at the upper limit of the standard, since C element affects the A3 temperature of the steel, the increase of C content lowers the A3 temperature, and quenching needs to be carried out above the A3 temperature; therefore, higher C content can ensure that the steel plate is quenched in a wider temperature range after rolling, enlarging the process window before quenching and reducing the control difficulty.

[0028] (2) Si and Mn: higher Si content is beneficial to strength, but not to plasticity, so it should be controlled in a lower range; Mn can improve the strength, plasticity and hardenability of the steel, and should be controlled in a higher range.

[0029] (3) P and S are both harmful elements and should be controlled at a lower level.

[0030] (4) Alt is a strong deoxidizing element, and can refine the grain and improve the plasticity, and should be controlled within a certain range.

[0031] (5) Nb can increase the austenite recrystallization temperature, so that the billet is still in the austenite recrystallization zone when finishing rolling at a higher temperature, and can improve the temperature of the steel plate after rolling, and ensure that the steel plate is still in the austenite zone when quenching after rolling. Nb has precipitation strengthening effect, and can refine the grain, and improve the strength and plasticity.

[0032] (6) Ti, N, B: Ti element increases the austenite recrystallization temperature, so that the billet is still in the austenite recrystallization zone when finishing rolling at a higher temperature, and Ti can fix N in the steel to form nitride to improve the strength and welding performance; if the N content is too low and the matching Ti content is too high, coarse TiN will be formed, which is not conducive to plasticity; if the N content is too high and the matching Ti content is too low, the solid solution N will increase, which is not conducive to plasticity and will reduce the hardenability improvement brought by B, therefore the N content should be matched with the Ti content; B element can improve the hardenability, and after the addition of Ti, the solid solution N is reduced, which can avoid the formation of nitride between N and B to reduce the effect of B on improving the hardenability, and the thicker the rolling specification, the higher the B content.

[0033] (7) Ni, Cr, Mo, V: Ni, Cr, Mo can improve the hardenability and plasticity, but are expensive and not added in production; V element has strong precipitation strengthening effect and weak fine-grain strengthening effect, and too strong precipitation strengthening is not conducive to plasticity and is expensive, therefore V is not added.

[0034] (8) Production organization: Q690 can be produced in the same tundish as Q345, Q355, etc. during steelmaking production, and the mixed pouring part of the two steel grades can be determined as Q345 or Q355, thereby reducing the loss of mixed pouring and reducing production cost.

[0035] 2. Production line layout and production process

[0036] Heating furnace heating→high-pressure water descaling→roughing mill→cooling device→finishing rolling mill→pre-straightening machine→electromagnetic induction heating→rapid cooling device(quenching)→hot straightening→cooling bed→tempering heat treatment.

[0037] The electromagnetic induction heating can use the existing slab induction heating device. When producing thin specifications (for example, 6mm-10mm), the temperature of the steel plate surface decreases rapidly after the finishing rolling mill is completed, and the temperature before quenching will be lower than the Ar3 temperature, and ferrite organization has been generated in the two-phase zone. Therefore, an electromagnetic induction heating device is arranged after the pre-straightening machine, the skin effect of electromagnetic induction is used to heat the steel plate, so that the steel plate is in the austenite zone and the temperature is uniform, and the temperature after heating is controlled to be 790-830℃.

[0038] Previously, there is no heating or temperature raising process after finish rolling and before quenching. Therefore, it is impossible to ensure that the steel plate is in austenite region when producing thin specifications, and the thin specification steel plate generally uses offline heating before quenching process.

[0039] The present application introduces an induction heating device into the steel rolling production line, changes the previous offline heating before quenching to online real-time heating, avoids the interruption of the steel rolling process, and can produce in real time and continuously.

[0040] The present application introduces an induction heating device into the steel rolling production line, changes the previous offline heating before quenching to online real-time heating, avoids the interruption of the steel rolling process, and can produce in real time and continuously.

[0041] A cooling device is arranged between the rough rolling mill set and the finish rolling mill set: after austenite recrystallization rolling, the steel plate is still in the austenite recrystallization region, and the austenite grains will grow again to affect the plasticity. At this time, the cooling device is used to immediately cool the steel plate to the austenite recrystallization region, so as to control the growth of austenite grains, improve the strength and plasticity of the finished steel plate, and improve the production efficiency.

[0042] 3. Reasonable steel rolling process

[0043] (1) Heating requirement: heating time 230-250 min, and soaking time ≥45 min.

[0044] (2) Rough rolling opening temperature 1100-1180℃, longitudinal-horizontal-longitudinal rolling mode is adopted, and attention is paid to eliminating the same plate difference of water cooling black mark.

[0045] (3) Two-stage rolling is adopted, the single pass reduction rate is increased in the rough rolling stage, and the reduction rate of the last 2-3 passes is >20%; the single pass reduction rate is increased in the finish rolling stage, and the pass reduction rate is >10%.

[0046] (4) High-temperature finish rolling: after rough rolling, the cooling device behind it is immediately controlled to control the steel plate temperature at 960-1060℃ to start finish rolling, and the finish rolling final rolling temperature is controlled at 900-960℃. When rolling thin specifications, the steel plate dissipates heat quickly, and the temperature is controlled at the upper limit. When rolling thick specifications, the steel plate dissipates heat slowly, and the temperature is controlled at the lower limit; the thicker the rolled specification, the lower the temperature.

[0047] 4. Heat treatment after rolling

[0048] (1) Quenching temperature (start-up cooling temperature): Controlled at 770~830℃, with the upper limit for thin specifications and the lower limit for thick specifications. After the steel plate exits the finishing mill and passes through the pre-straightening machine, the surface temperature of the steel plate is measured and a temperature curve is generated. When the temperature is lower than the set temperature, the electromagnetic induction heating device is activated to raise the temperature, and the heating power is adjusted according to the surface temperature curve of the steel plate to uniformly heat the surface. The surface temperature of the steel plate after cooling (redness temperature) should be less than 100℃.

[0049] (2) Tempering: Tempering temperature 690℃~700℃, tempering time: steel plate thickness + 50min.

[0050] (3) Metallographic requirements after tempering: The proportion of tempered sorbite in the surface layer should be above 85%, and the structure in the 1 / 4 and central parts should be tempered sorbite, and ferrite structure should not appear.

[0051] The beneficial effects of this invention are: this invention avoids the addition of precious alloying elements such as Ni, Cr, Mo, and V; Q690 can be produced in the same ladle as Q345 and Q355 during steelmaking; the mixed casting portion of the two steel grades can be directly identified as Q345 or Q355, thereby improving production efficiency and reducing costs; this invention utilizes the residual heat of the steel plate after finishing rolling for direct quenching, reducing the heating process of offline quenching, resulting in low energy consumption and high production efficiency. Detailed Implementation

[0052] To provide a clearer understanding of the technical features, objectives, and effects of this invention, the invention is now described.

[0053] 1. This invention first employs a rational composition design: low-cost boron (B) is used to replace expensive Ni, Cr, and Mo to improve hardenability; the carbon (C) content is designed to be within the standard medium to upper limits; the A3 temperature is lowered, reducing the quenching temperature requirement after steel plate rolling; simultaneously, low-cost silicon-manganese alloys are used to replace low-carbon high-purity alloys (metallic manganese, low-carbon manganese, ferrosilicon, etc.) to reduce costs; and microalloying elements such as Nb and Ti are added to increase the austenite recrystallization temperature. Ti can fix N in the steel to form nitrides, improving strength and preventing N from forming nitrides with B, thus reducing the hardenability-improving effect of B. Table 3 shows the chemical composition (by percentage) of the steel plates in various embodiments of this invention. As shown in Table 3, the composition control of this invention is as follows:

[0054] Table 3: Chemical composition (%) of steel plates in each embodiment

[0055]

[0056]

[0057] 2. For detailed process parameters of Q690 steel plates with a thickness of 6-50mm, please refer to Table 4.

[0058] Table 4: Specific process parameters of each example

[0059]

[0060] 3, The example of the application, the metallographic structure is all tempered sorbite, the yield strength of the steel plate is 740-820Mpa, the tensile strength is 800-870Mpa, the elongation after fracture is 12%-20%, the impact average is 100-160J, and the detailed mechanical properties are shown in Table 5.

[0061] Table 5: Specific mechanical properties after heat treatment of each example

[0062]

Claims

1. A low-cost Q690 steel plate, characterized in that, The low-cost Q690 steel plate has a chemical composition ratio of C: 0.12-0.18%, Si≤0.45%, Mn: 1.45-1.60%, P≤0.020%, S≤0.010%, Alt: 0.02-0.040%, Nb: 0.045-0.06%, Ti: 0.015-0.025%, B: 0.0025-0.0040%, N: 0.0040-0.0065%, carbon equivalent CEV≤0.44%, and the balance of Cu≤0.80%, Ni≤0.80%, and As≤0.080%, and the low-cost Q690 steel plate is produced by a low-cost Q690 steel plate production method, The low-cost Q690 steel plate production method comprises the following process steps in sequence: heating in a heating furnace, high-pressure water phosphorus removal, rough rolling, cooling, finish rolling, straightening in a pre-straightening machine, electromagnetic induction heating, quenching, hot straightening, a cooling bed, and tempering heat treatment; An electromagnetic induction heating device is arranged after the pre-straightening machine, the skin effect of electromagnetic induction is used to heat the steel plate, the steel plate is in the austenite region and has a uniform temperature, and the temperature after heating is controlled to be 790-830 DEG C; The induction heating device is introduced into a rolling production line, and offline heating before quenching is changed to online real-time heating, The interruption of the rolling process is avoided, and the production is continuously carried out in real time; A multi-point temperature measuring device is arranged before the induction heating device, temperature data are transmitted to a computer to form a temperature curve, the power of the induction heating device is dynamically adjusted by the computer according to the temperature curve when the steel plate reaches the induction heating device, and the surface temperature of the steel plate is ensured to be 790-830 DEG C; High-temperature finish rolling and temperature control after rolling are adopted to meet the temperature requirement during online quenching and reduce the offline temperature rising and quenching process; The proportion of the tempered sorbite of the surface layer structure of the low-cost Q690 steel plate is more than 85%, the structure at 1 / 4 and the center is tempered sorbite, and no ferrite structure appears; High-temperature finish rolling: after rough rolling, the cooling device is controlled to control the temperature of the steel plate to be 960-1060 DEG C to start finish rolling, and the finish rolling final rolling temperature is controlled to be 900-960 DEG C; Quenching temperature: controlled to be 770-830 DEG C; Tempering temperature: 690 DEG C-700 DEG C; The thickness of the steel plate is 6-10 mm; The yield strength of the steel plate is 766-812 MPa, the tensile strength is 813-854 MPa, the elongation after fracture is 15%-16%, the average value of the impact at -20 DEG C is 146.4-180.6 J. The thickness of the steel plate is 6 or 10 mm.

2. The low-cost Q690 steel plate of claim 1, wherein, The thickness of the steel plate is 6 mm, the yield strength is 812 MPa, and the tensile strength is 854 MPa.

3. The low-cost Q690 steel plate of claim 1, wherein, The thickness of the steel plate is 10 mm, the yield strength is 766 MPa, and the tensile strength is 813 MPa.

4. The low-cost Q690 steel plate of claim 1, wherein, The proportion of the tempered sorbite of the surface layer structure of the low-cost Q690 steel plate is 90%.

5. The low-cost Q690 steel plate of claim 3, wherein, The proportion of the tempered sorbite of the surface layer structure of the low-cost Q690 steel plate is 92%.

6. The low-cost Q690 steel plate of claim 4, wherein, ​

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