A cold-rolled low alloy steel strip and a method for producing the same

Abstract

The application belongs to the technical field of steel and iron, and particularly relates to a cold-rolled low-alloy steel strip and a preparation method thereof. The cold-rolled low-alloy steel strip contains the following chemical components in percentage by weight: C: 0.06%-0.09%, Si: 0.10%-0.30%, Mn: 1.20%-1.50%, S: <=0.010%, P: <=0.020%, Nb: 0.004-0.010%, Alt: 0.020%-0.050%, and the rest is iron and inevitable impurities. The preparation method comprises the following steps: hot-rolling raw material, pickling, cold-rolling, cover annealing, flattening, inspection, and warehousing. The cold-rolled low-alloy steel strip obtained by the application has a yield strength of >=305 MPa, a tensile strength of >=440 MPa, and an elongation of >=29%.

Description

Technical Field

[0001] This invention belongs to the field of steel technology. Specifically, this invention relates to a cold-rolled low-alloy steel strip with a tensile strength of 440MPa produced by a single-stand reversible rolling mill and a bell-type annealing process, and its preparation method. Background Technology

[0002] As the requirements for reducing vehicle emissions and weight gradually increase, automobile manufacturing is adopting higher-strength cold-rolled thin-gauge products to replace low-strength hot-rolled thick-gauge products, which is conducive to achieving the development requirements of reducing vehicle weight and saving energy and reducing emissions.

[0003] There are two main production methods for cold-rolled high-strength steel. One method uses microalloying to improve grain size and two-phase particle precipitation strengthening to increase material strength. The other method achieves high strength through phase transformation strengthening. Although both types of cold-rolled high-strength steel have ultra-high strength, their elongation is generally low, which cannot meet the requirements of subsequent deep processing such as stretching and bending.

[0004] Chinese patent application CN104294145A discloses a cold-rolled steel with a tensile strength of 600 MPa based on a bell-type annealing treatment and its preparation method. The chemical composition is: C: 0.10-0.17%; Si ≤ 0.3%; Mn: 1.5-2.0%; P ≤ 0.02%; S ≤ 0.01%; N ≤ 0.008%; Al: 0.02-0.05%; O: ≤ 0.004%; the remainder being iron and unavoidable impurities. The high-strength cold-rolled steel sheet prepared has a yield strength exceeding 600 MPa and an elongation greater than 15%. However, this patent application has a high manganese content, does not add elements such as grain refiners, and involves a high number of cold rolling passes and a long holding time, resulting in a still low elongation that cannot meet the requirements for tensile and bending.

[0005] Chinese patent application CN113186459A discloses a cold-rolled low-alloy steel strip with a yield strength of 355 MPa based on a bell-type annealing treatment and its preparation method. The chemical composition is: C: 0.17%–0.22%, Si: 0.10%–0.50%, Mn: 1.20%–1.50%, S: ≤0.0100%, P: ≤0.020%, Ti: 0.025%–0.035%; the remainder is iron and unavoidable impurities. The prepared high-strength cold-rolled steel strip has a yield strength ≥355 MPa, a tensile strength ≥470 MPa, and an elongation ≥27%. This steel strip has high processing performance, but its composition system design and performance indicators differ from those of this invention. Summary of the Invention

[0006] The purpose of this invention is to provide a cold-rolled low-alloy steel strip, which is produced using a single-stand reversible rolling mill and a bell-type annealing process. The production method involves adding microalloying elements, establishing a reference system for the thickness of hot-rolled raw materials, controlling the cold-rolling reduction rate, employing a 6-roll single-stand reversible cold rolling mill, and combining it with medium-temperature annealing to shorten the holding time. This achieves the effects of promoting uniform diffusion of carbides, improving the plasticity of the steel strip, adjusting the grain size, and softening the steel strip, while simultaneously reducing energy consumption and achieving energy conservation and emission reduction. The resulting steel strip product can meet the requirements of subsequent deep processing such as stretching, bending, and quenching.

[0007] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0008] A cold-rolled low-alloy steel, the chemical composition of which, by weight percentage, comprises: C: 0.06%–0.09%, Si: 0.10%–0.30%, Mn: 1.20%–1.50%, S: ≤0.010%, P: ≤0.020%, Nb: 0.004%–0.010%, Alt: 0.020%–0.050%, with the remainder being iron and unavoidable impurities.

[0009] The yield strength of cold-rolled low alloy steel strip is ≥305MPa, the tensile strength is ≥440MPa, and the elongation is ≥29%.

[0010] The cold rolling production process of a cold-rolled low alloy steel with a tensile strength of 440MPa or above is as follows: hot-rolled raw material—pickling—cold rolling—baffle annealing—leveling—inspection—warehousing.

[0011] Specifically, the cold rolling production process of the above-mentioned cold-rolled low-alloy steel of the present invention includes the following steps:

[0012] 1) Select the appropriate hot-rolled raw material specifications according to the raw material reference system (see Table 1) based on the thickness specifications of the cold-rolled final product.

[0013] Table 1

[0014] ＜1.2 2.75～3.0 1.2～1.5 3.0～3.75 1.5～1.75 3.5～4.0 ＞1.75 4.5

[0015] 2) Pickle the hot-rolled raw materials according to the pickling operation procedure. After pickling, there should be no iron oxide scale residue on the surface of the steel strip.

[0016] During steel strip pickling, the pickling solution temperature should be controlled at 70-95℃ (preferably 85℃), the pickling solution concentration at 10-30% (preferably 18%), and the pickling speed at 50-150m / min (preferably 110m / min).

[0017] The pickling tension control is specified in Table 2. After pickling, there should be no iron oxide scale residue on the surface.

[0018] Table 2

[0019] 2.5～4.5 16～25 55～62

[0020] 3) The pickled steel strip is cold rolled on a 6-roll reversible rolling mill in 7 passes, and the total cold rolling reduction rate shall not be less than 55%.

[0021] 4) The cold-rolled intermediate products undergo medium-temperature annealing. The annealing process employs a full hydrogen protection process to prevent decarburization of the steel strip during heating. The annealing process is as follows: Stage 1 heating temperature range: 0–470℃, heating time: 2–4 hours; Holding temperature: 470℃, holding time: 1–2 hours; Stage 2 heating temperature range: 470–650℃, heating time: 5–6 hours; Holding temperature: 650℃, holding time: 6–8 hours; Stage 1 cooling temperature range: 650–380℃, cooling method: 2 hours of cooling with a cover followed by air cooling; Stage 2 cooling temperature range: 380–85℃, cooling method: water cooling; Stage 3 cooling temperature range: 85℃ to room temperature, cooling method: air cooling.

[0022] 5) To ensure the flatness of the sheet, the annealed product is leveled. The surface roughness Ra of the leveling machine rolls is controlled between 3.0 and 4.5 μm. Wet leveling is used, with a leveling solution concentration of 1.0 to 3.0% and an elongation controlled between 0.5 and 1.5%. Alternatively, a constant rolling force mode can be used, with the rolling force selected between 3000 and 6000 kN depending on the product thickness.

[0023] 6) Inspection: The yield strength of this product must be ≥305MPa, the tensile strength must be ≥440MPa, and the elongation must be ≥29%.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] This invention, by adding microalloying elements, establishing a reference system for selecting the thickness of hot-rolled raw materials, controlling the cold-rolling reduction rate, employing a 6-roll single-stand reversible cold rolling mill, and using medium-temperature annealing to shorten the holding time, produces qualified products that can meet the downstream users' needs for subsequent deep processing such as stretching, bending, and heat treatment. The products of this invention have wide applications and high processing performance. Attached Figure Description

[0026] Figure 1 This is a longitudinal metallographic image of the rolled steel strip in Example 1 of the present invention, with a scale of 50 μm.

[0027] Figure 2 This is a longitudinal metallographic image of the rolled steel strip in Example 1 of the present invention, with a scale of 10 μm.

[0028] Figure 3 This is a transverse metallographic image of the rolled steel strip in Example 1 of the present invention, with a scale of 20 μm.

[0029] Figure 4 This is a transverse metallographic image of the rolled steel strip in Example 1 of the present invention, with a scale of 10 μm. Detailed Implementation

[0030] The technical solution of the present invention will be described in detail below with reference to the accompanying drawings and embodiments.

[0031] Example 1

[0032] 1. The percentage of chemical composition of the hot-rolled raw materials is as follows:

[0033] C: 0.08%, Si: 0.23%, Mn: 1.40%, S: 0.005%, P: 0.011%, Nb: 0.008%, Alt: 0.030%, with the remainder being Fe and unavoidable impurities.

[0034] 2. Cold rolling production process:

[0035] (1) Hot-rolled raw material thickness 4.5mm; cold-rolled steel strip product thickness 1.97mm;

[0036] (2) Production process flow: pickling—cold rolling—blind annealing—leveling—inspection—warehousing.

[0037] (3) Preparation process;

[0038] During pickling of the steel strip, the pickling solution temperature was controlled at 84℃, the pickling solution concentration at 19%, and the pickling speed at 110 m / min. After pickling, no iron oxide scale residue remained on the surface of the steel strip. The pickled steel strip was then cold-rolled on a 6-roll reversible mill with a total rolling pass of 7 passes and a total reduction of 55%. The cold-rolled steel strip underwent a 6-stage annealing heat treatment using a full hydrogen protection process; the annealing process is shown in Table 3. The annealed steel strip was leveled using a constant rolling force mode, with a rolling force of 4000 kN. Simultaneously, a light surface oiling treatment was applied to protect the steel strip from rust.

[0039] Table 3 Annealing process

[0040] Heating section 1 0～470 2h Insulation Section 1 470 1h Heating section 2 470～650 5.5h Insulation section 650 7h Cooling section 1 650～380 After cooling with a cover for 2 hours, air cooling is applied. Cooling section 2 380～85 Water cooling Cooling section 3 85°C to room temperature air cooling

[0041] 3. Inspection:

[0042] Performance testing showed that the product fully meets all performance requirements; specific data are shown in Table 4. The metallographic structure consists of ferrite and dot-shaped cementite. The cementite is primarily dot-shaped with a small amount of spherical cementite, and the precipitation is uniform. The metallographic structure is shown in [Table 4]. Figures 1-4 .

[0043] Table 4

[0044] Example 1 1.97*1000mm 367MPa 473MPa 29％

[0045] Example 2

[0046] 1. The percentage of chemical composition of the hot-rolled raw materials is as follows:

[0047] C: 0.07%, Si: 0.24%, Mn: 1.38%, P: 0.009%, S: 0.007%, Nb: 0.006%, Alt: 0.030%, with the remainder being Fe and unavoidable impurities.

[0048] 2. Cold rolling production process:

[0049] (1) Hot-rolled raw material thickness 3.5mm; cold-rolled steel strip product thickness 1.47mm;

[0050] (2) Production process flow: pickling—cold rolling—blind annealing—leveling—inspection—warehousing.

[0051] (3) Preparation process;

[0052] During pickling of the steel strip, the pickling solution temperature was controlled at 85℃, the pickling solution concentration at 18%, and the pickling speed at 110 m / min. After pickling, no iron oxide scale residue remained on the surface of the steel strip. The pickled steel strip was then cold-rolled on a 6-roll reversible mill with a total rolling pass of 7 passes and a total reduction of 60%. The cold-rolled steel strip underwent a 6-stage annealing heat treatment using a full hydrogen protection process; the annealing process is shown in Table 5. The annealed steel strip was leveled using a 1.5% elongation leveling method, and a light surface oiling treatment was applied to protect the steel strip from corrosion.

[0053] Table 5 Annealing Process

[0054] Heating section 1 0～470 2.5h Insulation Section 1 470 1h Heating section 2 470～650 5h Insulation section 650 7.5h Cooling section 1 650～380 After cooling with a cover for 2 hours, air cooling is applied. Cooling section 2 380～85 Water cooling Cooling section 3 85°C to room temperature air cooling

[0055] 3. Inspection:

[0056] After performance testing, the product fully meets all performance requirements. Specific data are shown in Table 6. The metallographic structure of this embodiment is basically the same as that of Embodiment 1.

[0057] Table 6

[0058] Example 2 1.47*1000mm 353MPa 460MPa 30％

[0059] The upper and lower limits of the process parameters (such as temperature, time, etc.) and the range values ​​of the present invention can all achieve the method, and examples are not listed here.

[0060] All aspects not described in detail in this invention can be covered using conventional technical knowledge in the field.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to the embodiments, those skilled in the art should understand that modifications or equivalent substitutions to the technical solutions of the present invention do not depart from the spirit and scope of the technical solutions of the present invention, and all such modifications or substitutions should be covered within the scope of the claims of the present invention.

Claims

1. A cold-rolled low-alloy steel strip, characterized in that, The chemical composition of the cold-rolled low-alloy steel strip, by weight percentage, includes: C: 0.06%–0.09%, Si: 0.10%–0.30%, Mn: 1.20%–1.50%, S: ≤0.010%, P: ≤0.020%, Nb: 0.004%–0.010%, Alt: 0.020%–0.050%, with the remainder being iron and unavoidable impurities; The method for preparing the cold-rolled low-alloy steel strip includes: Hot-rolled raw materials—pickling—cold rolling—baffle annealing—leveling—inspection—warehousing; Select the appropriate hot-rolled raw material specifications according to the following hot-rolled raw material reference system: For finished product thickness < 1.2 mm, the hot-rolled raw material thickness is 2.75–3.0 mm; for finished product thickness 1.2–1.5 mm, the hot-rolled raw material thickness is 3.0–3.75 mm; for finished product thickness 1.5–1.75 mm, the hot-rolled raw material thickness is 3.5–4.0 mm; for finished product thickness > 1.75 mm, the hot-rolled raw material thickness is 4.5 mm. The pickled steel strip is cold rolled on a 6-roll reversible rolling mill in 7 passes, and the total cold rolling reduction rate shall not be less than 55%. The cold-rolled intermediate products are subjected to medium-temperature annealing; the annealing process adopts a full hydrogen protection process to prevent decarburization of the steel strip during heating. The annealing process is as follows: Stage 1 heating temperature range: 0–470℃, heating time: 2–4 hours; holding temperature: 470℃, holding time: 1–2 hours; Stage 2 heating temperature range: 470–650℃, heating time: 5–6 hours; holding temperature: 650℃, holding time: 6–8 hours; Stage 1 cooling temperature range: 650–380℃, cooling method: cooling with a cover for 2 hours followed by air cooling; Stage 2 cooling temperature range: 380–85℃, cooling method: water cooling; Stage 3 cooling temperature range: 85℃ to room temperature, cooling method: air cooling.

2. The cold-rolled low-alloy steel strip according to claim 1, characterized in that, The cold-rolled low-alloy steel strip has a yield strength ≥305MPa, a tensile strength ≥440MPa, and an elongation ≥29%.

3. The cold-rolled low-alloy steel strip according to claim 1, characterized in that, During pickling, the temperature of the pickling solution should be controlled at 70-95℃, the concentration of the pickling solution at 10-30%, and the pickling speed at 50-150 m / min.

4. The cold-rolled low-alloy steel strip according to claim 3, characterized in that, During pickling, the pickling solution temperature is controlled at 85℃, the pickling solution concentration is 18%, and the pickling speed is 110 m / min.

5. The cold-rolled low-alloy steel strip according to claim 1, characterized in that, Pickling tension control is as follows: thickness 2.5~4.5mm, unwinding tension 16~25KN, winding tension 55~62KN.

6. The cold-rolled low-alloy steel strip according to claim 1, characterized in that, The surface roughness Ra of the leveling mill rolls is controlled at 3.0 to 4.5 μm; wet leveling is adopted, with a leveling liquid concentration of 1.0 to 3.0% and an elongation controlled at 0.5 to 1.5%, or a constant rolling force mode is adopted, with the rolling force selected from 3000 to 6000 kN according to the product thickness.

Images