A production method for improving low-temperature toughness of super-thick steel plate

Abstract

The present application relates to the technical field of super-thick steel plate, and particularly relates to a production method for improving low-temperature toughness of super-thick steel plate, which comprises the steps of water cooling and heat treatment of a rolling piece, the water cooling step cools the rolling piece to 20-40 DEG C below the bainite production temperature point, the cooling water amount and cooling speed are controlled, the structure from the near surface to the surface 10mm is mainly granular bainite + ferrite, the structure at the thickness 1 / 4 is mainly granular bainite + ferrite, and the structure at the thickness 1 / 2 is pearlite + ferrite; the heat treatment step is to heat the rolling piece after water cooling to 30-50 DEG C below the actual phase transition temperature for heat treatment, the structure from the near surface to the surface 10mm is mainly pearlite + ferrite, the structure at the thickness 1 / 4 is mainly ferrite + pearlite, and granular bainite is supplemented. The present application has simple process, high stability, short production cycle, low cost, great popularization and application prospect, and superior low-temperature toughness of the product.

Description

Technical Field

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

[0002] Thick steel plates are widely used in wind power, high-rise buildings, offshore platforms, large stadium construction, and other fields that require high performance. Due to the plate's thickness, the low-temperature toughness of thick steel plates is poor. To improve the steel's low-temperature toughness, the conventional practice is to forge or roll steel ingots into blanks, then heat and roll them into finished plates. Subsequent heat treatment processes are required to improve the steel's performance. However, the yield rate of steel ingots is low, and the metallurgical quality is not as high as that of continuous casting ingots. When using a quenching and tempering heat treatment process, a large amount of alloying elements must be added to ensure strength and toughness, resulting in high production costs. The production method also involves multiple steps, a long cycle, and high costs.

[0003] Chinese invention patent application CN102330017A uses low-temperature controlled rolling + TMCP + normalizing production. The second stage of rolling has a low starting temperature, high rolling pressure, heavy load on the rolls, rolling mills, and motors, severe wear, and reduced equipment life. The normalizing heat treatment temperature is high and the time is long, which consumes a lot of energy and has a long production cycle. Chinese patent CN106567011A has produced a high-strength, high-toughness, and extra-thick steel plate suitable for easy welding at -60°C through tempering production. The tempering process of quenching and tempering requires two uses of the heat treatment furnace, which occupies heat treatment capacity, has high heat treatment costs, and has a long production cycle. Chinese patent CN104357623A uses a walking beam car bottom furnace for six-stage normalizing treatment, which takes a long time to heat treatment.

[0004] It can be seen that the existing methods for improving the low-temperature toughness of extra-thick steel plates generally have the disadvantages of long production cycle and high cost. Summary of the Invention

[0005] In view of the problems that existing methods for improving the low-temperature toughness of extra-thick steel plates generally have long production cycles and high costs, the present invention provides a production method for improving the low-temperature toughness of extra-thick steel plates. The method has simple process, high stability, short production cycle, low cost, great prospects for promotion and application, and excellent low-temperature toughness of the product.

[0006] The technical solutions of the present invention are as follows:

[0007] A production method for improving the low-temperature toughness of extra-thick steel plates, comprising the steps of water cooling and heat treating the rolled pieces, wherein:

[0008] In the water cooling step, the rolled piece is finally cooled to 20-40°C below the bainite production temperature (Bs). The cooling water volume and cooling rate are controlled so that the structure of the rolled piece from near the surface to 10 mm from the surface is mainly granular bainite + ferrite, wherein the granular bainite content is greater than 30%. The structure of the rolled piece at 1 / 4 of the thickness is mainly granular bainite + ferrite, wherein the granular bainite content is 5%-10%. The structure of the rolled piece at 1 / 2 of the thickness is pearlite + ferrite, and the ferrite grains are uniform and fine, with a size of ≤12μm.

[0009] The heat treatment step is to heat the water-cooled rolled piece to 30~50℃ below its actual phase transformation temperature (AC3) for heat treatment, so that the structure of the rolled piece from near the surface to 10mm from the surface is mainly pearlite + ferrite, among which the pearlite is dispersed and the ferrite grains are uniform and small, with a size of ≤10μm; the structure at 1 / 4 of the thickness of the rolled piece is mainly ferrite + pearlite, supplemented by 2%~6% granular bainite.

[0010] Furthermore, the thickness of the extra thick steel plate is 100~120mm.

[0011] Furthermore, the thickness of the continuous casting slab is 300 mm.

[0012] Furthermore, the chemical composition and weight percentage of the continuous casting slab are as follows:

[0013] C 0.10%~0.17%, Si 0.20%~0.30%, Mn 0.90%~2.20%, P≤0.015%, S≤0.005%, Nb0.010%~0.025%, V 0.040%~0.055%, Ti 0.010%~0.020%, Cr≤0.25%, the balance is Fe and unavoidable impurities.

[0014] Furthermore, the chemical composition of the continuous casting billet satisfies:

[0015] CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15=0.44%~0.46%.

[0016] Furthermore, by controlling the amount of cooling water and the cooling rate, the ideal microstructure distribution can be obtained; specifically, in the water cooling step, the water flow rate of each group of lower headers is controlled to be 380~480L / s, the water flow rate of each group of upper headers is controlled to be 300~385L / s, the roller speed is 0.3~0.4m / s, the water cooling time is 100~120s, and the cooling rate is achieved at 13±3℃ / s at 10mm from the surface of the rolled piece, 9±3℃ / s at 1 / 4 of the thickness of the rolled piece, and 5±2℃ / s at 1 / 2 of the thickness of the rolled piece.

[0017] Furthermore, the yield strength of the finished steel plate is ~350MPa, and KV2 ≥ 180J at -40℃.

[0018] The beneficial effects of the present invention are:

[0019] (1) The present invention utilizes 300mm thick continuous casting billets to produce 100-120mm extra-thick high-toughness steel plates, solving the domestic problem of high-quality and efficient production of extra-thick high-toughness steel plates.

[0020] (2) The production method of the present invention adopts conventional rolling process, and the internal grain size of the steel plate reaches 12μm, which solves the problem that the uniformity and grain size of the internal grains of the extra-thick steel plate can only be improved by low-temperature and high-pressure reduction, thereby reducing the losses of the rollers, rolling mills, and motors; through innovative water cooling process and heat treatment process, the hardenability of the rolled piece is controlled, the internal grain size of the extra-thick steel plate is refined, the structure is uniform, and the low-temperature toughness of the extra-thick steel plate is improved. It is a process innovation of finely controlling the structure of the thick plate by controlled rolling + controlled cooling + heat treatment. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0022] Figure 1 This is a metallographic structure photograph of the 120 mm steel plate near the surface after heat treatment in Example 2.

[0023] Figure 2 This is a metallographic photograph of the 120 mm steel plate at 1 / 4 of the thickness after heat treatment in Example 2.

[0024] Figure 3 This is a metallographic photograph of the 120 mm steel plate at 1 / 2 of the thickness after heat treatment in Example 2.

[0025] Figure 4 This is one of the metallographic structure photos of the 110mm steel plate at 1 / 4 of the thickness after heat treatment in Example 1.

[0026] Figure 5 This is the second metallographic structure photo of the 110 mm steel plate at 1 / 4 of the thickness after heat treatment in Comparative Example 1.

[0027] Figure 6 This is a metallographic photograph of the 110 mm steel plate at 1 / 2 of the thickness after heat treatment in Comparative Example 1. DETAILED DESCRIPTION

[0028] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention are clearly and completely described below. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of them. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative work should fall within the scope of protection of the present invention.

[0029] In the specific embodiments of the present invention, the carbon equivalent (CEV) is calculated according to the following formula:

[0030] CEV (%)=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15.

[0031] Example 1

[0032] A production method for improving the low-temperature toughness of extra-thick steel plates comprises the following steps:

[0033] (1) The raw materials are smelted and refined to obtain molten steel: the molten iron is pretreated, and the final S content is 0.0025%; the converter adopts top and bottom double blowing to fully dephosphorize, and the final phosphorus content is 0.004%. Argon is blown throughout the process, weak stirring is used, the slag surface is slightly turned over without being exposed, and lime and fluorite are used to make white slag; the RH vacuum degree is 200 Pa, and the vacuum time is 12 min; the molten steel is calmed for 34 min before tapping.

[0034] (2) Continuous casting and heating: a continuous casting billet with a thickness of 300 mm is obtained, and the billet outlet temperature is 1189°C;

[0035] The chemical composition and weight percentage of the continuous casting billet are as follows:

[0036] C 0.10%, Si 0.25%, Mn 2.00%, P 0.011%, S 0.004%, Nb 0.015%, V 0.045%, Ti 0.018%, the rest are Fe and unavoidable impurities, CEV = 0.442%.

[0037] (3) Descaling and rolling: The continuous casting billet is descaled by 21MPa high-pressure water after being taken out of the furnace and then rolled. A two-stage rolling process is adopted to obtain a rolled piece with a thickness of 110mm.

[0038] (4) Water cooling: Water cooling is carried out after rolling is completed. According to the bainite production temperature point (Bs) of the rolled piece = 623 °C, the rolled piece is finally cooled to 590 °C. The water flow rate of each group of lower headers for water cooling is controlled at 413~423 L / s, and the water flow rate of each group of upper headers is controlled at 333~341 L / s. The roller speed is 0.35 m / s, and the water cooling time is 108 s. The model calculation shows that the cooling rate reaches 15 °C / s at a distance of 10 mm from the surface of the rolled piece, 11 °C / s at 1 / 4 of the thickness of the rolled piece, and 5 °C / s at 1 / 2 of the thickness of the rolled piece. After water cooling, the structure of the rolled piece from near the surface to 10 mm from the surface is mainly granular bainite + ferrite, of which the granular bainite content is 40%~42%. The structure at 1 / 4 of the thickness of the rolled piece is mainly granular bainite + ferrite, of which the granular bainite content is 8%~9%. The structure at 1 / 2 of the thickness of the rolled piece is pearlite + ferrite, and the ferrite grains are uniform and fine, with a size of 10um.

[0039] (5) Heat treatment: According to the actual phase change temperature of the rolled piece (AC3) = 863 °C, the rolled piece is heated to 830 °C for heat treatment to obtain a steel plate with a thickness of 110 mm.

[0040] After heat treatment, the structure of the rolled piece from near the surface to 10 mm from the surface is mainly pearlite + ferrite, among which pearlite is dispersed and the ferrite grains are uniform and small, with a size of 9 μm; the structure at 1 / 4 of the thickness of the rolled piece is mainly ferrite + pearlite, supplemented by a small amount of granular bainite, and the granular bainite content is 3%~4%.

[0041] Example 2

[0042] A production method for improving the low-temperature toughness of extra-thick steel plates comprises the following steps:

[0043] (1) The raw materials are smelted and refined to obtain molten steel: the molten iron is pretreated, and the final S content is 0.003%; the converter adopts top and bottom double blowing to fully dephosphorize, and the final phosphorus content is 0.003%. Argon is blown throughout the process, weak stirring is used, the slag surface is slightly turned over without being exposed, and lime and fluorite are used to make white slag; the RH vacuum degree is 190 Pa, and the vacuum time is 13 min; the molten steel is calmed for 35 min before tapping.

[0044] (2) Continuous casting and heating: a continuous casting billet with a thickness of 300 mm is obtained, and the billet outlet temperature is 1189°C;

[0045] The chemical composition and weight percentage of the continuous casting billet are as follows:

[0046] C 0.17%, Si 0.25%, Mn 1.00%, P 0.012%, S 0.004%, Nb 0.021%, V 0.043%, Ti 0.018%, Cr 0.23%, the rest are Fe and unavoidable impurities, CEV = 0.455%.

[0047] (3) Descaling and rolling: The continuous casting billet is descaled by 21MPa high-pressure water after being taken out of the furnace and then rolled. A two-stage rolling process is adopted to obtain a rolled piece with a thickness of 120mm.

[0048] (4) Water cooling: Water cooling is carried out after rolling is completed. According to the bainite production temperature point (Bs) of the rolled piece = 656 °C, the rolled piece is finally cooled to 630 °C. The water flow rate of the lower header of each water cooling group is controlled at 438~451 L / s, and the water flow rate of the upper header of each group is controlled at 353~361 L / s. The roller speed is 0.32 m / s, and the water cooling time is 118 s. The model calculation shows that the cooling rate reaches 14 °C / s at 10 mm from the surface of the rolled piece, 9 °C / s at 1 / 4 of the thickness of the rolled piece, and 4 °C / s at 1 / 2 of the thickness of the rolled piece. After water cooling, the structure of the rolled piece from near the surface to 10 mm from the surface is mainly granular bainite + ferrite, of which the granular bainite content is 39%~41%. The structure at 1 / 4 of the thickness of the rolled piece is mainly granular bainite + ferrite, of which the granular bainite content is 7%~8%. The structure at 1 / 2 of the thickness of the rolled piece is pearlite + ferrite, and the ferrite grains are uniform and fine, with a size of 11μm.

[0049] (5) Heat treatment: According to the actual phase change temperature of the rolled piece (AC3) = 853 °C, the rolled piece is heated to 815 °C for heat treatment to obtain a steel plate with a thickness of 120 mm.

[0050] like Figures 1 to 3 As shown in the figure, after heat treatment, the structure of the rolled piece from near the surface to 10 mm from the surface is mainly pearlite + ferrite, in which pearlite is dispersed and the ferrite grains are uniform and small, with a size of 10 μm; the structure of the rolled piece at 1 / 4 of the thickness is mainly ferrite + pearlite, supplemented by a small amount of granular bainite, and the granular bainite content is 3%~4%.

[0051] Comparative Example 1

[0052] The same furnace casting as in Example 1 was used, and the same heating and rolling process was used to obtain a rolled piece with a thickness of 110 mm. The rolled piece was cooled to a temperature of 650-680° C. in accordance with a conventional process using an ACC water cooling process, and then normalized at a normalizing temperature of AC3+30-50° C. in accordance with a conventional process.

[0053] as follows Figures 4 to 6 As shown in the figure, after heat treatment, the microstructure uniformity of the steel plate along the thickness direction is poor and the grain size varies greatly.

[0054] The properties of the steel plates prepared in Example 1, Example 2, and Comparative Example 1 were tested, and the results are shown in Table 1.

[0055] Table 1 Properties of the steel plates of Examples 1 and 2 of the present invention

[0056]

[0057] It can be seen from the data in Table 1 that the steel plates produced by conventional water cooling and normalizing processes have poor performance, especially poor low-temperature toughness. The steel plates produced by the method for improving the low-temperature toughness of extra-thick steel plates of the present invention have good mechanical properties and excellent low-temperature toughness.

[0058] Although the present invention has been described in detail by way of preferred embodiments, the present invention is not limited thereto. Without departing from the spirit and substance of the present invention, persons of ordinary skill in the art may make various equivalent modifications or substitutions to the embodiments of the present invention, and such modifications or substitutions shall be within the scope of the present invention. Any changes or substitutions that can be readily conceived by persons skilled in the art within the technical scope disclosed in the present invention shall be within the scope of protection of the present invention.

Claims

1. A production method for improving the low-temperature toughness of extra-thick steel plates, characterized in that: The method comprises the steps of water cooling and heat treating the rolled piece, wherein: In the water cooling step, the rolled piece is finally cooled to 20-40°C below the bainite production temperature. The cooling water volume and cooling rate are controlled so that the structure of the rolled piece from near the surface to 10 mm from the surface is mainly granular bainite + ferrite, wherein the granular bainite content is greater than 30%, the structure at 1 / 4 of the thickness of the rolled piece is mainly granular bainite + ferrite, wherein the granular bainite content is 5%-10%, and the structure at 1 / 2 of the thickness of the rolled piece is pearlite + ferrite, and the ferrite grains are uniform and fine, with a size of ≤12μm; The cooling water volume and cooling speed are specifically controlled as follows: the water flow rate of each group of lower headers is controlled to 380-480 L / s, the water flow rate of each group of upper headers is controlled to 300-385 L / s, the roller speed is controlled to 0.3-0.4 m / s, the water cooling time is controlled to 100-120 s, and the cooling speed is achieved at 13±3°C / s at a distance of 10 mm from the surface of the rolled piece, 9±3°C / s at 1 / 4 of the thickness of the rolled piece, and 5±2°C / s at 1 / 2 of the thickness of the rolled piece; The heat treatment step is to heat the water-cooled rolled piece to 30~50℃ below its actual phase transformation temperature for heat treatment, so that the structure of the rolled piece from near the surface to 10mm from the surface is mainly pearlite + ferrite, among which the pearlite is dispersed and the ferrite grains are uniform and small, with a size of ≤10μm; the structure at 1 / 4 of the thickness of the rolled piece is mainly ferrite + pearlite, supplemented by 2%~6% granular bainite.

2. The production method according to claim 1, wherein The thickness of extra thick steel plate is 100~120mm.

3. The production method according to claim 1, wherein The thickness of the continuous casting slab is 300 mm.

4. The production method according to claim 1, wherein The chemical composition and weight percentage of the continuous casting billet are as follows: C 0.10%~0.17%, Si 0.20%~0.30%, Mn 0.90%~2.20%, P≤0.015%, S≤0.005%, Nb 0.010%~0.025%, V 0.040%~0.055%, Ti 0.010%~0.020%, Cr≤0.25%, the balance is Fe and unavoidable impurities.

5. The production method according to claim 4, characterized in that The chemical composition of the continuous casting billet meets the following requirements: CEV=C+Mn / 6+(Cr+Mo+V) / 5+(Ni+Cu) / 15=0.44%~0.46%.

6. The production method according to claim 1, wherein The yield strength of the finished steel plate is 300~350MPa.

7. The production method according to claim 1, wherein Finished steel plate's -40℃ KV2≥180J.

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