CN1330786C - Strength of extension 780 MPa grade complex phase steel plate and mfg. method thereof - Google Patents

Abstract

The present invention relates to a complex phase steel plate with the tensile strength of 780MPa, which comprises the chemical constituents of the weight percent: 0.13 to 0.17% of C, 0.20 to 0.40% of Si, 1.30 to 1.50% of Mn, 0.02 to 0.03% of Nb and Fe as the rest. The complex phase steel plate has a complex phase structure of granular bainite and martensite, wherein the volume ratio of granular bainite is from 90 to 95%, and the volume ratio of martensite is from 10 to 5%. The yield strength is from 525 to 550MPa, the tensile strength is from 785 to 795MPa, and the total elongation percentage after fracture is from 20 to 25%. Rolling and cooling control technology is utilized for rolling, the initial rolling temperature is controlled to be from 1100 to 1150 DEG C, and the finish rolling temperature is from 800 to 850 DEG C. The cooling speed of the first section is controlled to be from 40 to 70 DEG C/s, and the temperature after cooling is from 550 to 610 DEG C. The cooling speed of the second section is from 220 to 360 DEG C/s, and the coiling temperature is from 20 to 250 DEG C. After being cooled, the steel plate is coiled, and the thickness of the steel plate is controlled to be from 3 to 4.5mm.

Description

A composite steel plate with a tensile strength of 780MPa and its manufacturing method

technical field

The invention belongs to the technical field of steel rolling, and relates to the manufacture of high-strength steel for automotive structural parts, in particular to a multiphase steel plate with a tensile strength of 780MPa and a manufacturing method.

Background technique

The development of high-strength hot-rolled steel sheets for automobiles has always been one of the hot spots in the research of materials work at home and abroad. Generally used ferrite + pearlite hot-rolled steel plate has a maximum strength of 500MPa. Although adding microalloying elements such as Nb or Ti can further increase the strength, the formability will deteriorate, thus limiting its application. To further increase strength, dual phase steels have been developed. The characteristic of dual-phase high-strength steel is that about 15% of the hard phase is distributed on the fine ferrite matrix, and it is further strengthened by solid solution atoms. The resistance of ferrite-bainite and dual-phase ferrite-martensitic steel The tensile strength is between 550-650MPa. Due to the limitation of formability, dual-phase steel cannot reach the 800MPa level. In order to meet the requirements of higher strength and formability, the 800MPa three-phase TRIP steel containing retained austenite was developed. During the deformation process, the residual austenite strain-induced transformation into martensite, which improves the strength while improving Formability. However, the direct hot-rolled dual-phase steel and TRIP steel that can be commercially produced at present all use complex alloying methods, which have problems such as high cost and difficult process, which is not conducive to promotion, and large-scale production is not yet possible in China.

Considering environmental issues and energy issues, the chemical composition of new high-strength automotive structural parts hot-rolled steel plates will tend to be simple C-Si-Mn series steel (or add trace alloy elements); develop controlled rolling and controlled cooling new Technology, comprehensive use of fine grain strengthening and phase transformation strengthening methods to improve the strength limit of steel.

Contents of the invention

Aiming at the problems existing in the existing high-strength automobile hot-rolled steel plate and its production process, the invention provides a multi-phase steel plate with a tensile strength of 780MPa and a manufacturing method.

The invention is based on the composition of common C-Mn series steel, adds trace alloy elements, adopts appropriate controlled cooling process, and develops hot-rolled steel plate for high-strength automobile structural parts. The tensile strength 780MPa grade granular bainite and martensitic composite steel plate, its main chemical composition is: 0.13-0.17% C, 0.20-0.40% Si, 1.30-1.50% Mn, 0.02-0.03% Nb , the balance being Fe. The structure is a composite structure of granular bainite and martensite, in which: the volume fraction of granular bainite is 90-95%, the volume fraction of martensite is 10-5%; the yield strength is 525-550MPa, and the tensile strength is 785~795MPa, the total elongation after breaking is 20~25%.

The present invention adopts steel slabs with the above-mentioned chemical composition and a thickness of 80 to 250 mm to produce a multi-phase steel plate with a tensile strength of 780 MPa through hot rolling and two-stage rapid cooling processes, specifically according to the following steps:

(1) Heat the billet to 1150-1250°C and keep it warm for 2-2.5 hours.

(2) Hot rolling is carried out after heat preservation, the temperature of starting rolling is controlled at 1100-1150°C, and the temperature of finishing rolling is 800-850°C.

(3) Two-stage controlled cooling is carried out after hot rolling, the cooling rate of the first stage is controlled at 40-70°C/s, and the temperature after cooling is 550-610°C; the cooling rate of the second stage is 220-360°C/s, and the final cooling temperature ( Namely coiling temperature) 20 ~ 250 ℃.

(4) Coil after cooling, the coiling temperature is 20-250°C, and the thickness of the steel plate is controlled to be 3-4.5mm.

The invention provides a new structure state for the hot-rolled steel plate with a tensile strength of 780MPa, that is, the composite phase structure of granular bainite and martensite. Among them: the volume fraction of granular bainite is 90-95%, and the volume fraction of martensite is 10-5%. The yield strength is 525-550MPa, the tensile strength is 785-795MPa, the total elongation after breaking is 20-25%, and has considerable strength and plastic comprehensive properties.

Description of drawings

The accompanying drawing is a photo of a typical metallographic structure of a granular bainite and martensitic composite steel plate with a tensile strength of 780 MPa in the present invention. Among them: (a) is the photo of the tissue corroded by nitric acid alcohol solution; (b) is the photo of the tissue corroded by LEPERA reagent (1% Na ₂ S ₂ O ₅ aqueous solution + 4% picric acid alcohol solution), the bright white tissue in the photo is horse body.

Detailed ways

Select the steel with the chemical composition shown in Table 1 as the raw material, and the blank size is 80mm (thickness) × 70mm (width) × 140mm (length). In the hot rolling mill, the steel plate with a thickness specification of 3-4.5 mm was rolled, and a two-stage rapid cooling process was adopted after rolling. The cooling process is shown in Table 2, and the mechanical properties and microstructure scores are shown in Table 3.

Table 1 embodiment measured chemical composition (mass fraction %, balance is Fe)

  Ingredient number C Mn Si Nb S0 0.15 1.40 0.20 0.02 S1 0.13 1.30 0.22 0.022 S2 0.17 1.47 0.36 0.024 S4 0.15 1.50 0.40 0.03

Table 2 embodiment measured cooling process parameters

serial number heating temperature Holding time Starting temperature Finishing temperature Temperature after the first stage of cooling The first cooling rate final cooling temperature The second cooling rate (℃) Hour (℃) (℃) (℃) (℃/s) (℃) (℃/s) S0-1 1150 2 1100 825 560 60 200 330 S0-2 1250 2.5 1150 830 580 65 170 345 S1-1 1160 2.2 1150 850 610 40 250 360 S1-2 1180 2.3 1140 835 590 49 150 220 S2-1 1200 2.1 1120 815 575 70 85 245 S2-2 1220 2.4 1100 800 550 50 20 265 S4-1 1230 2.5 1125 840 585 55 50 250 S4-2 1240 2.5 1130 820 570 45 120 300

Table 3 Example Mechanical Properties and Tissue Volume Fraction

serial number Steel plate thickness _Re R _m A Volume fraction GB m (mm) (MPa) (MPa) (%) % % S1-1 4.5 550 785 20 95 5 S1-2 4.0 532 795 twenty three 90 10 S2-1 3.0 525 790 twenty four 90 10 S2-2 4.2 535 795 25 95 5

Among them: _Re -yield strength; R _m -tensile strength; A-total elongation after fracture (indefinite gauge length); GB-granular bainite; M-martensite

Claims (2)

1. A composite steel plate with a tensile strength of 780MPa, the chemical composition of which is: 0.13-0.17% C, 0.20-0.40% Si, 1.30-1.50% Mn, 0.02-0.03% Nb, and the balance is Fe; The structure is a composite structure of granular bainite and martensite, in which: the volume fraction of granular bainite is 90-95%, the volume fraction of martensite is 10-5%; the thickness of the steel plate is 3-4.5mm, and the yield strength is 525~550MPa, the tensile strength is 785~795MPa, and the total elongation after breaking is 20~25%. 2. The method for manufacturing a composite steel plate with a tensile strength of 780 MPa as claimed in claim 1, characterized in that the process steps and process parameters are: (1) heating the steel slab of chemical composition described in claim 1 to 1150～1250 ℃, heat preservation 2～2.5 hours; (2) Hot rolling is carried out after heat preservation, the starting rolling temperature is controlled at 1100-1150°C, and the final rolling temperature is 800-850°C; (3) Two-stage controlled cooling is carried out after hot rolling, the cooling rate of the first stage is controlled at 40-70°C/s, and the temperature after cooling is 550-610°C; the cooling rate of the second stage is 220-360°C/s, and the final cooling temperature is Coiling temperature 20～250℃; (4) Coil after cooling, the coiling temperature is 20-250°C, and the thickness of the steel plate is controlled to be 3-4.5mm.

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