Low-cost normalized Q355NE low-alloy structural steel and production process

Abstract

The invention discloses low-cost normalized Q355NE low-alloy structural steel, which relates to the technical field of steel production and comprises the following chemical components in percentage by mass: 0.11 to 0.14 percent of C, 1.30 to 1.55 percent of Mn, 0.35 to 0.45 percent of Si, 0.015 to 0.035 percent of Nb, 0.020 to 0.035 percent of Ti, 0.020 to 0.050 percent of Alt, less than or equal to 0.020 percent of P, less than or equal to 0.010 percent of S and the balance of iron and inevitable impurities. According to the invention, the component design with medium and low C content is adopted, only by adding Nb and Ti for compounding, a fine and uniform structure is obtained in a special rolling mode in the rolling stage, and a special heating system is adopted in the heat treatment normalizing stage, so that the rolled fine structure meets the requirements of inheritance, strength guarantee and-40 DEG C low-temperature impact; and the low-temperature impact toughness at-40 DEG C is 150 J or above, and the ductility is 27% or above.

Description

Technical Field

[0001] The invention relates to the technical field of steel production, and in particular to a low-cost normalized Q355NE low-alloy structural steel and a production process. Background Art

[0002] Low alloy structural steel Q355 grade is widely used in many fields such as construction, bridges, automobiles and machinery manufacturing. Low temperature environment generally uses normalized state -40℃ impact toughness Q355NE low alloy structural steel. Compared with the delivery state of controlled rolling and controlled cooling, Q355NE has more uniform performance after normalizing heat treatment and higher product stability.

[0003] At present, there are few patents for Q355NE published in China. The patent "A method for producing thin-gauge low-cost normalized rolled steel plate Q355NE" (patent number CN202311019140.9) is mainly for Q355NE produced by normalizing rolling with a thickness of 10-35mm. Therefore, it is necessary to design a low-cost method for manufacturing Q355NE steel plates with normalizing heat treatment. Summary of the invention

[0004] The technical problem to be solved by the present invention is to overcome the shortcomings of the prior art and provide a low-cost normalized Q355NE low-alloy structural steel and a production process.

[0005] In order to solve the above technical problems, the technical solution of the present invention is as follows: A low-cost normalized Q355NE low-alloy structural steel, whose chemical composition and mass percentage are: C: 0.11-0.14%, Mn: 1.30-1.55%, Si: 0.35-0.45%, Nb: 0.015-0.035%, Ti: 0.020-0.035%, Alt: 0.020-0.050%, P≤0.020%, S≤0.010%, and the balance is iron and unavoidable impurities.

[0006] The present invention also provides a low-cost production process of normalized Q355NE low-alloy structural steel, which comprises the following steps in sequence: slab heating, rolling, controlled cooling and heat treatment.

[0007] As a preferred solution of the production process of low-cost normalized Q355NE low-alloy structural steel described in the present invention, wherein: in the slab heating process, the steel tapping temperature is 1100-1130°C, the soaking time is ≥0.12min / mm*continuous casting slab thickness mm, the total time in the furnace is 8-11 min / mm*continuous casting slab thickness mm, and the temperature uniformity is ≤10°C.

[0008] As a preferred solution of the production process of the low-cost normalized Q355NE low-alloy structural steel described in the present invention, in the rolling process, the starting rolling temperature is 880-850°C, and the final rolling temperature is 780-820°C.

[0009] As a preferred solution of the production process of the low-cost normalized Q355NE low-alloy structural steel described in the present invention, in the controlled cooling process, the steel plate adopts a weak cooling method and the red-returning temperature is controlled at 700-740°C.

[0010] As a preferred solution of the production process of low-cost normalized Q355NE low-alloy structural steel described in the present invention, wherein: in the heat treatment process, the normalizing temperature in the heating section is 910-920°C, the normalizing temperature in the soaking section is 890-900°C, and the total time in the furnace is 40-80 minutes.

[0011] As a preferred solution of the production process of the low-cost normalized Q355NE low-alloy structural steel described in the present invention, in the heat treatment process, the steel plate is air-cooled after being taken out of the furnace.

[0012] The beneficial effects of the present invention are: (1) The chemical composition of the steel plate in the present invention is reasonably designed, and only Nb and Ti microalloying elements are used. The cost of alloy raw materials is low and it has strong market competitiveness.

[0013] (2) The present invention adopts a low-temperature heating system during the rolling stage to avoid the influence of high silicon content on the surface quality, and adopts weak cooling during rolling cooling to obtain a fine and uniform structure.

[0014] (3) The present invention adopts a two-stage gradient temperature design in the normalizing stage of heat treatment. The heating section is heated quickly to avoid grain growth and provide more nucleation points for austenitization. The obtained steel plate has a grain size of 8.5 or above, an elongation of ≥27%, and an impact strength of ≥150J at -40°C. It can meet the technical requirements for low-temperature impact toughness of key projects in the fields of steel structure, construction, engineering machinery, etc. DETAILED DESCRIPTION

[0015] In order to make the content of the present invention more clearly understood, the present invention is further described in detail below according to specific implementation methods.

[0016] Example 1: This example provides a low-cost normalized Q355NE low-alloy structural steel, the chemical composition and mass percentage of which are shown in Table 1.

[0017] The steel plate of this embodiment adopts LF and RH refining technology during smelting to ensure that the gas H content of the steel plate is 1.6ppm, the N content is 40ppm, and the continuous casting billet thickness is 260mm.

[0018] This embodiment also provides a low-cost production process of normalized Q355NE low-alloy structural steel, which includes the following steps in sequence: slab heating, rolling, controlled cooling and heat treatment. The specific process steps are as follows: (1) Slab heating process: steel tapping temperature 1125°C, soaking time, total time in furnace and continuous casting slab thickness are determined, soaking time 35 min / mm, total time in furnace 258 min / mm, temperature uniformity 5°C.

[0019] (2) Rolling process: The starting rolling temperature of the steel plate is 880°C and the final rolling temperature is 816°C.

[0020] (3) Controlled cooling process: The steel plate returns to red temperature at 740°C.

[0021] (4) Heat treatment process: Different normalizing temperatures are set for the heating section and the soaking section, 915°C for the heating section and 898°C for the soaking section. The total time in the furnace is 40 min, and air cooling is used after the furnace is taken out.

[0022] The thickness of the low-cost normalized Q355NE low-alloy structural steel obtained in this embodiment is 20 mm, and the mechanical properties are shown in Table 2.

[0023] Example 2: This example provides a low-cost normalized Q355NE low-alloy structural steel, the chemical composition and mass percentage of which are shown in Table 1.

[0024] The steel plate of this embodiment adopts LF and RH refining technology during smelting to ensure that the gas H content of the steel plate is 1.8ppm, the N content is 36ppm, and the continuous casting billet thickness is 260mm.

[0025] This embodiment also provides a low-cost production process of normalized Q355NE low-alloy structural steel, which includes the following steps in sequence: slab heating, rolling, controlled cooling and heat treatment. The specific process steps are as follows: (1) Slab heating process: The tapping temperature is 1108°C. The soaking time and the total time in the furnace are determined by the thickness of the continuous casting slab. The soaking time is 40 min / mm, the total time in the furnace is 282 min / mm, and the temperature uniformity is 7°C.

[0026] (2) Rolling process: The starting rolling temperature of the steel plate is 872°C and the final rolling temperature is 820°C.

[0027] (3) Controlled cooling process: The steel plate returns to red temperature at 716°C.

[0028] (4) Heat treatment process: Different normalizing temperatures are set for the heating section and the soaking section. The heating section temperature is 915°C, the soaking section temperature is 900°C, the total furnace time is 56 min, and air cooling is used after the furnace is taken out.

[0029] The thickness of the low-cost normalized Q355NE low-alloy structural steel obtained in this embodiment is 25 mm, and the mechanical properties are shown in Table 2.

[0030] Example 3: This example provides a low-cost normalized Q355NE low-alloy structural steel, the chemical composition and mass percentage of which are shown in Table 1.

[0031] The steel plate of this embodiment adopts LF and RH refining technology during smelting to ensure that the gas H content of the steel plate is 2ppm, the N content is 39ppm, and the continuous casting billet thickness is 370mm.

[0032] This embodiment also provides a low-cost production process of normalized Q355NE low-alloy structural steel, which includes the following steps in sequence: slab heating, rolling, controlled cooling and heat treatment. The specific process steps are as follows: (1) Slab heating process: The tapping temperature is 1130°C. The soaking time and total time in the furnace are determined by the thickness of the continuous casting slab. The soaking time is 48 min / mm, the total time in the furnace is 388 min / mm, and the temperature uniformity is 5°C.

[0033] (2) Rolling process: The starting rolling temperature of the steel plate is 860°C and the final rolling temperature is 796°C.

[0034] (3) Controlled cooling process: The steel plate returns to red temperature at 707°C.

[0035] (4) Heat treatment process: Different normalizing temperatures are set for the heating section and the soaking section, 920°C for the heating section and 892°C for the soaking section. The total time in the furnace is 68 min, and air cooling is used after the furnace is taken out.

[0036] The thickness of the low-cost normalized Q355NE low-alloy structural steel obtained in this embodiment is 38 mm, and the mechanical properties are shown in Table 2.

[0037] Example 4: This example provides a low-cost normalized Q355NE low-alloy structural steel, the chemical composition and mass percentage of which are shown in Table 1.

[0038] The steel plate of this embodiment adopts LF and RH refining technology during smelting to ensure that the gas H content of the steel plate is 1.5ppm, the N content is 35ppm, and the continuous casting billet thickness is 370mm.

[0039] This embodiment also provides a low-cost production process of normalized Q355NE low-alloy structural steel, which includes the following steps in sequence: slab heating, rolling, controlled cooling and heat treatment. The specific process steps are as follows: (1) Slab heating process: The tapping temperature is 1100°C. The soaking time and the total time in the furnace are determined by the thickness of the continuous casting slab. The soaking time is 55 min / mm, the total time in the furnace is 397 min / mm, and the temperature uniformity is 8°C.

[0040] (2) Rolling process: The starting rolling temperature of the steel plate is 865°C and the final rolling temperature is 797°C.

[0041] (3) Controlled cooling process: The steel plate re-red temperature is 700℃.

[0042] (4) Heat treatment process: Different normalizing temperatures are set for the heating section and the soaking section. The heating section temperature is 912°C, the soaking section temperature is 890°C, the total furnace time is 71 minutes, and air cooling is used after the furnace is taken out.

[0043] The thickness of the low-cost normalized Q355NE low-alloy structural steel obtained in this embodiment is 46 mm, and the mechanical properties are shown in Table 2.

[0044] Example 5: This example provides a low-cost normalized Q355NE low-alloy structural steel, the chemical composition and mass percentage of which are shown in Table 1.

[0045] The steel plate of this embodiment adopts LF and RH refining technology during smelting to ensure that the gas H content of the steel plate is 1.7ppm, the N content is 39ppm, and the continuous casting billet thickness is 370mm.

[0046] This embodiment also provides a low-cost production process of normalized Q355NE low-alloy structural steel, which includes the following steps in sequence: slab heating, rolling, controlled cooling and heat treatment. The specific process steps are as follows: (1) Slab heating process: The tapping temperature is 1124°C. The soaking time and the total time in the furnace are determined by the thickness of the continuous casting slab. The soaking time is 64 min / mm, the total time in the furnace is 403 min / mm, and the temperature uniformity is 9°C.

[0047] (2) Rolling process: The starting rolling temperature of the steel plate is 850°C and the final rolling temperature is 780°C.

[0048] (3) Controlled cooling process: The steel plate returns to red temperature at 705°C.

[0049] (4) Heat treatment process: Different normalizing temperatures are set for the heating section and the soaking section, 914°C for the heating section and 892°C for the soaking section. The total time in the furnace is 80 min, and air cooling is used after the furnace is taken out.

[0050] The thickness of the low-cost normalized Q355NE low-alloy structural steel obtained in this embodiment is 46 mm, and the mechanical properties are shown in Table 2.

[0051]

[0052] Table 1

[0053] Table 2 Therefore, the technical solution of this application adopts a composition design with medium and low C content, and only by adding Nb and Ti composite, a fine and uniform structure is obtained through a special rolling mode in the rolling stage. In the heat treatment normalizing stage, a special heating system is adopted to inherit the fine structure after rolling, ensuring the requirements of strength and -40℃ low-temperature impact, -40℃ low-temperature impact toughness of more than 150J, and elongation of more than 27%. At the same time, the Q500qD steel plate produced by ordinary normalizing or quenching and tempering has the characteristics of high yield strength ratio and small impact toughness surplus.

[0054] In addition to the above embodiments, the present invention may also have other implementation modes; any technical solutions formed by equivalent replacement or equivalent transformation shall fall within the protection scope required by the present invention.

Claims

1. A low-cost normalized Q355NE low-alloy structural steel, characterized by: Its chemical composition and mass percentage are: C: 0.11~ 0.14%, Mn: 1.30~ 1.55%, Si: 0.35 ~ 0.45%, Nb: 0.015 ~ 0.035%, Ti: 0.020 ~0.035%, Alt: 0.020 ~ 0.050%, P≤0.020%, S≤0.010%, and the balance is iron and unavoidable impurities.

2. A production process of low-cost normalized Q355NE low-alloy structural steel according to claim 1, characterized in that: The process includes the following steps in sequence: slab heating, rolling, controlled cooling and heat treatment.

3. The production process of low-cost normalized Q355NE low-alloy structural steel according to claim 2, characterized in that: In the slab heating process, the tapping temperature is 1100-1130°C, the soaking time is ≥0.12 min / mm*thickness of the continuous casting slab (mm), the total time in the furnace is 8-11 min / mm*thickness of the continuous casting slab (mm), and the temperature uniformity is ≤10°C.

4. The production process of low-cost normalized Q355NE low-alloy structural steel according to claim 2, characterized in that: In the rolling process, the starting rolling temperature is 880-850°C, and the final rolling temperature is 780-820°C.

5. The production process of low-cost normalized Q355NE low-alloy structural steel according to claim 2, characterized in that: In the controlled cooling process, the steel plate is cooled in a weak cooling mode, and the red-returning temperature is controlled at 700-740°C.

6. The production process of low-cost normalized Q355NE low-alloy structural steel according to claim 2, characterized in that: In the heat treatment process, the normalizing temperature in the heating section is 910-920° C., the normalizing temperature in the soaking section is 890-900° C., and the total time in the furnace is 40-80 minutes.

7. The production process of low-cost normalized Q355NE low-alloy structural steel according to claim 2, characterized in that: In the heat treatment process, the steel plate is air-cooled after being taken out of the furnace.

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