CN107747042A - A kind of high reaming steel of the economical great surface quality of 690MPa levels and preparation method thereof - Google Patents

Abstract

The invention discloses a 690MPa grade economical steel with high surface quality and high hole expansion and a preparation method thereof, belonging to the technical fields of iron and steel metallurgy and rolling. 690MPa grade economical high surface quality high hole reaming steel, by weight percentage, its chemical composition is [C], [Si], [Mn], [P], [S], [Alt], [Ca], [ N] and [O], the rest are Fe and unavoidable impurities. The preparation method comprises the following steps: smelting process→continuous casting process→heating process→rough rolling process→hot coil box process→finish rolling process→laminar flow cooling process→coiling→slow cooling→pickling process→coiling and packaging. The invention controls the chemical composition and preparation method of the high-hole-expanding steel, so that the high-hole-expanding steel has the characteristics of high surface quality, low cost and excellent performance, and can meet the requirements of steel for automobile chassis and complex stamping parts.

Description

A kind of 690MPa grade economical high surface quality high hole reaming steel and its preparation method

technical field

The invention belongs to the technical field of iron and steel metallurgy and rolling, and in particular relates to a 690MPa economical steel with high surface quality and high hole expansion and a preparation method thereof.

Background technique

In order to reduce the weight of automobiles, reduce fuel consumption, and improve vehicle safety, etc., many new steel research and development plans in recent years have mainly focused on the development of Advanced High Strength Steel (AHSS) with good formability. There are many types of AHSS steels, such as ferrite/martensite dual phase (Ferrite/Martensite Dual Phase, FMDP for short) steel, ferrite/bainite dual phase (Ferrite/Bainite Dual Phase, FBDP for short) steel, phase Transformation Induced Plasticity (TRIP) steel, Twinning Induced Plasicity (TWIP) steel, Complex Phase (CP) steel, Martensite (M) steel, quenching partition (Quenching and Paritioning, referred to as Q&P) steel and hot formed (Hot Formed, referred to as HF) steel.

In addition to strengthening methods such as solid solution, grain refinement, and precipitation, the strengthening methods of AHSS steel also have phase transformation strengthening, which is not available in ordinary high-strength steels, and is one of the main strengthening methods of AHSS steel. Compared with ordinary high-strength steel, AHSS steel has the advantages of good formability, high energy absorption rate, good anti-collision performance, bake hardening performance, yield strength, high work hardening rate and good fatigue performance, etc., and is becoming more and more popular in the automotive industry. Shows its good application scenarios.

Hot-rolled high-strength steel plates are used to make automobile chassis, wheels and other automobile running parts, which is one of the important and effective ways to reduce the weight of the car body. Automobile chassis parts are not exposed, although the requirements for the surface quality of the steel plate are not very strict, but due to its complex shape, the main forming includes: stretch flanging, bending, reaming and EDM, etc. These forming methods have a great impact on the formability of the steel plate , especially the extended flange performance requirements are higher.

When traditional FMDP steels are used to form complex automobile parts (such as automobile chassis, wheel rims, and spokes, etc.), there are two-phase (F+M) interfaces with great differences in deformation ability, which are easy to crack at the opening parts during the forming process. The formability of the extended flange is not good enough, especially after flash welding, it is easy to temper and soften the martensite in the heat-affected zone (HAZ), and the fatigue strength is low at the same time, so it cannot meet the forming requirements.

FBDP steel is also called Stretch-Flangeable (SF for short) steel or High Hole-Expansion (HHE for short) steel. When the bainite content is 10% to 20%, it has very good formability. performance, especially extended flange performance. Compared with HSLA (High Strength Low Alloy Steel) and FMDP steel, the main advantage of FBDP steel is that it improves the shear edge ductility, taking into account the contradiction and balance of strength and ductility, and its welding, fatigue and expansion Pore performance is good, which has incomparable advantages of general materials; the hardness of FBDP steel in the heat-affected zone after flash welding is higher than that of the base metal, and there will be no softening phenomenon during subsequent processing. At the same time, the steel plate has good light-fatigue performance. Compared with HSLA steel of the same strength level, FBDP steel has higher strain hardening index (n value) and total elongation, so FBDP steel is more suitable for stamping parts with high requirements and good elongation performance, such as automobile chassis.

In the 1990s, the United States and Japan successively developed hot-rolled FB steel plates with high hole expansion performance of 440-780MPa. The hole expansion rate is 70% to 131%. It is mainly used in formability, especially good flanging performance. Automobile chassis, wheels and other parts, because they contain more precious alloy elements Cr, Nb, Ti, V and Mo, etc., although the ferrite/bainite dual phase structure can be obtained under the condition of low cooling rate, but Its cost is higher.

Contents of the invention

The technical problem to be solved by the present invention is to provide a 690MPa economical steel with high surface quality and high hole expansion, which does not contain precious alloy elements such as Cr, Nb, Ti, V and Mo, and at the same time It has the advantages of good hole expansion performance, formability, microstructure performance, stamping performance, isotropy and so on.

The technical solution adopted by the present invention to solve the above-mentioned technical problems is to provide a 690MPa-level economical high-surface-quality high-hole reaming steel, and its chemical composition is: [C]: 0.055% to 0.095%, [Si] : 0.85%～1.50%, [Mn]: 1.30%～1.80%, [P]: 0.010%～0.020%, [S]: ≤0.003%, [Alt]: 0.010%～0.050%, [Ca]: 0.0010 %～0.0050%, [N]: ≤0.0050%, [O]: ≤0.0040%, the rest is Fe and unavoidable impurities, and [C]×[Mn]×10 ⁴ ≤0.15, {[P]+10 ×[S]}×10 ² ≤0.04, [Ca]/[S]≥0.3.

Wherein, in the above-mentioned 690MPa grade economical high surface quality high hole reaming steel, the thickness specification of the 690MPa grade economical high surface quality high hole reaming steel is 1.4-6.0mm.

Wherein, in the above-mentioned 690MPa grade economical high surface quality high hole reaming steel, the microstructure of the 690MPa grade economical high surface quality high hole reaming steel is composed of ferrite and bainite.

Among them, the yield strength (R _el (or P _p0.2 )) of the above-mentioned 690MPa grade economical high surface quality and high hole expansion steel is 450-550MPa, the tensile strength (R _m ) is ≥690MPa, and the hole expansion rate (λ) is ≥75%, the elongation after break (A ₈₀ ) is ≥19%, and the 180° cold bending test D=0a is qualified; wherein, the parallel section gauge length L ₀ of the tensile sample = 80mm, and the parallel section width b =20mm; in the cold bending test, D is the diameter of the bending center, and a is the thickness of the steel strip.

The present invention also provides a preparation method for the above-mentioned 690MPa-level economical high-surface-quality high-hole reaming steel, comprising the following steps: smelting process→continuous casting process→heating process→rough rolling process→hot coil box process→finish rolling process→laminar flow Cooling process → coiling → slow cooling → pickling process → coiling and packaging.

Among them, in the above-mentioned preparation method of 690MPa economical high surface quality high hole reaming steel, the smelting process is carried out according to the corresponding chemical composition of 690MPa economical high surface quality high reaming steel; LF desulfurization treatment is adopted , to control the S content.

Wherein, in the preparation method of 690MPa grade economical high surface quality and high hole reaming steel mentioned above, in the continuous casting process, casting with low superheat is adopted, the superheat is controlled at 20-25°C, and a chamfering crystallizer is used. Continuous casting into slabs.

Wherein, in the above-mentioned preparation method of 690MPa grade economical steel with high surface quality and high hole expansion, the continuous casting slab temperature in the heating process is 1200±30°C.

Wherein, in the above-mentioned preparation method of 690MPa grade economical high surface quality and high hole reaming steel, descaling is carried out in the rough rolling process, the descaling water pressure is ≥20MPa, and the phosphorus removal nozzle angle is 40-50°.

Wherein, in the above-mentioned preparation method of 690MPa grade economical high surface quality high hole reaming steel, descaling is carried out in the hot coil box process, the descaling water pressure is ≥ 20MPa, and the phosphorus removal nozzle angle is 40-50°.

Wherein, in the above-mentioned preparation method of 690MPa grade economical high surface quality high hole reaming steel, descaling is carried out in the finishing rolling process, the descaling water pressure is ≥ 20MPa, and the phosphorus removal nozzle angle is 40-50°.

Among them, in the preparation method of the above-mentioned 690MPa grade economical high surface quality and high hole reaming steel, the intermediate billet is obtained through the rough rolling process, enters the hot coil box, coils and breaks the scale, and uses the descaling water to descale, and the descaling water The pressure is ≥20MPa, and the angle of the dephosphorization nozzle is 40-50°.

Wherein, in the above-mentioned preparation method of 690MPa grade economical high surface quality and high hole expansion steel, the thickness of the intermediate billet at the rough rolling exit in the rough rolling process is 40±3mm.

Wherein, in the above-mentioned preparation method of 690MPa grade economical high-surface-quality high-hole reaming steel, the finishing rolling entrance temperature in the finishing rolling process is 1080±40°C, the finish rolling temperature is 820-880°C, and the finish rolling exit The thickness is 1-6mm.

Among them, in the preparation method of 690MPa economical high surface quality and high hole reaming steel described above, the laminar flow cooling process adopts a double-stage cooling method to rapidly cool 650~ 700°C, then air-cooled for 7-12s, and then rapidly cooled to 400-470°C at a cooling rate not lower than 18°C/s; the coiling temperature of the coiling is 400-470°C; the slow cooling is coiling Then slowly cool to 280-350°C, and then naturally cool to below 40°C.

Wherein, in the above-mentioned preparation method of 690MPa grade economical high surface quality and high hole reaming steel, the tensile elongation rate in the pickling process is 0.8-1.8%, and the temperature of the HCl acid solution in the pickling tank is 55-85°C , the concentration of free acid HCl in the pickling tank is 50-160g/L, the concentration of Fe ²⁺ in the pickling tank is 50-140g/L, the pickling speed is 60-120m/min, the temperature of the rinsing tank is 50±10°C, and the drying The unit is stable at 85±5°C.

The beneficial effects of the present invention are:

The 690MPa grade economical high surface quality and high reaming steel of the present invention adopts the "low C + Si + Mn" alloying treatment technology, does not add precious alloy elements such as Cr, Nb, Ti, V and Mo, and significantly reduces the alloying properties of the steel. Cost, realized economical type from the aspect of material; The preparation cost of the economical type high surface quality high reaming steel of the present invention is low, and process is simple, and its yield strength (R _el (or P _p0.2 )) is 450～550MPa, and tensile strength (R _m ) ≥ 690MPa, hole expansion rate (λ) ≥ 75%, elongation after fracture (A ₈₀ ) ≥ 19%, 180° cold bending test D = 0a qualified; has good hole expansion performance and formability , microstructure performance, stamping performance, isotropy and other advantages, to meet the needs of steel for automobile chassis and complex stamping parts.

The present invention obtains a 690MPa-level economical high-surface-quality high-reaming steel by controlling the chemical composition of the steel and its preparation method: a reasonable chemical composition range is designed, and a thermomechanical control process (Thermo Mechanical Control Process, TMCP for short) process is adopted Technology and controlled rolling and controlled cooling technology, combined with reasonable rapid cooling rate and coiling temperature range, can effectively refine ferrite grains on the one hand, and obtain bainite with moderate proportion, ideal shape and distribution on the other hand microstructure of steel strip ferrite and bainite, the process control is simple, easy to produce, no subsequent heat treatment process is required, the production process is shortened, and the process cost is saved; through the design of chemical composition and the inclusion of steel in the smelting process Effective control of material, the use of chamfering crystallizers in the continuous casting process and low superheat casting to obtain high-quality slabs; the hot rolling process adopts the control of descaling times and descaling water pressure, as well as the descaling process and pickling in the pickling process process to obtain products of high surface quality.

Description of drawings

Fig. 1 is the pickling finished metallographic structure of the experimental group 2 of embodiment 1 of the present invention;

Fig. 2 is the metallographic structure of the pickling finished product of the experimental group 5 of Example 1 of the present invention.

Detailed ways

Specifically, an economical high-surface-quality high-hole reaming steel with a tensile strength of 690 MPa, its chemical composition is: [C]: 0.055% to 0.095%, [Si]: 0.85% to 1.50% by weight percentage , [Mn]: 1.30% to 1.80%, [P]: 0.010% to 0.020%, [S]: ≤0.003%, [Alt]: 0.010% to 0.050%, [Ca]: 0.0010% to 0.0050%, [ N]: ≤0.0050%, [O]: ≤0.0040%, the rest is Fe and unavoidable impurities, and [C]×[Mn]×10 ⁴ ≤0.15, {[P]+10×[S]}× 10 ² ≤0.04, [Ca]/[S]≥0.3.

The 690MPa grade economical high surface quality and high hole reaming steel of the present invention must have a certain content of C, so as to promote the formation of carbide strengthening phase, and then ensure the strength level of the steel strip; it is found that when the C content is too high, the precipitated carbide The increase of particles is not conducive to the hole expansion performance of the steel plate; when the C content is too low, the strength cannot meet the design requirements; the present invention controls the C content at 0.055% to 0.095%.

Si has solid solution strengthening effect in steel, can improve the strength and elongation of steel strip simultaneously, improve the formability of steel strip, the present invention controls Si content to be 0.85%～1.50%; However, when Si content is higher, steel strip The surface is easy to produce Fe ₂ SiO ₄ red rust, which is difficult to remove during pickling. In order to improve the surface quality of the steel strip, the present invention designs a descaling method and pickling process suitable for this steel type.

Mn belongs to the solid solution strengthening element. According to the strength requirements of the steel grade of the present invention, when the Mn content is too low, the strength will not meet the design requirements; if the Mn content is too high, the proportion of pearlite will be increased, and it is very easy to cause The segregation of the Mn composition of the slab is one of the main reasons for the banded structure of the steel strip, and the banded structure will sharply reduce the hole expansion performance of the steel strip. Therefore, the present invention controls the Mn content to 1.30% to 1.80%. ; At the same time, for carbon-manganese steel, the interaction between C and Mn has a certain influence on the formation and distribution of pearlite structure. The pearlite is coarse and banded, so the present invention puts forward control requirements for the matching of C and Mn contents, that is, [C]×[Mn]≤0.15.

P, as the strongest ferrite strengthening element, has strong solid solution strengthening and cold hardening effects. When used as an alloying element, it can improve its strength and the atmospheric corrosion resistance of the steel strip, but when the content is too high , the cold stamping performance of the steel strip will drop sharply. At the same time, the composition of the continuous casting slab will segregate, causing uneven structure, increasing temper brittleness, and significantly reducing the plasticity and toughness of the steel strip, so that the steel strip will appear during cold working. In addition, P is an impurity element in carbon-manganese steel, so its content should not be too high, usually the content should not exceed 0.030% to 0.040%. In order to make full use of the solid solution strengthening effect of P, reduce the fluctuation of the mechanical properties of the steel strip, and at the same time reduce the effect of P as much as possible, the content of P is controlled to be 0.010%-0.020%.

S is an impurity element in steel, and it is easy to form MnS in steel. The quantity and form of sulfide in steel directly affect the hole expansion performance of steel strip, especially strip-shaped sulfide inclusions are prone to cracks during deformation, so in the design The S content must be reduced during the chemical composition, and the present invention controls the S content to be ≤0.003%, and requires [P]+10×[S]≤0.04.

As an excellent deoxidizing element in steel, Al element can reduce oxide inclusions in steel, improve steel purity, and thus improve the formability of steel strip. After deoxidation, most of Al ₂ O ₃ floats up to The upper layer of molten steel reduces the oxygen content in the steel, and the remaining elemental Al dissolves into the steel, which is called acid-soluble aluminum (Als). At the same time, very little Al ₂ O ₃ will also remain in the steel to form Al ₂ O ₃ inclusions, reducing the formability of the steel strip, the sum of the contents of elemental Al and Al ₂ O ₃ is called total aluminum (Alt), so the present invention must contain a certain amount of Als in the steel, and at the same time reduce the inclusion of Al ₂ O ₃ The Alt content is not easy to be too high, and the Alt is controlled to be 0.010% to 0.050% in the economical high surface quality high hole reaming steel of the present invention.

Ca, as an important element of inclusions such as sulfides, oxides and silicates in modified steel, can change the shape and composition of these inclusions, and promote the transformation of massive, chain-like and strip-like sulfides into spherical shapes. Thereby improving the plasticity and toughness of the steel strip, and then improving the fatigue resistance of the steel strip; in addition, the inventor finds in practice: Ca can improve the anisotropy of the steel strip, promote the isotropic performance of the steel strip, with [Ca] As /[S] increases, the isotropic performance of the steel strip shows an upward trend; when [Ca]/[S]=0.2, the ratio of the transverse and longitudinal impact toughness values of the steel strip is 0.85; when [Ca]/[ When S]≥0.3, the ratio of impact toughness values in the transverse direction and longitudinal direction of the steel strip is close to 1, so the present invention controls the Ca content to be 0.0010%-0.0050%, and requires [Ca]/[S]≥0.3.

N can be combined with alloying elements to produce non-metallic inclusions, which reduces the effect of alloying elements and is regarded as a harmful element. Therefore, the present invention controls the N content to be ≤0.0050%; the residual oxygen in the steel strip, or the oxygen diffused to the surface of the steel strip , are easy to make grain boundary oxides form brittle oxide interlayers, causing intergranular cracks during subsequent deformation processing, thereby causing cracks in the workpiece, so the oxygen content in the steel strip should be as low as possible. The present invention controls the O content to be ≤0.0040 %.

The 690MPa grade economical high surface quality and high hole reaming steel of the present invention adopts the "low C + Si + Mn" alloying treatment technology on the one hand, does not add precious alloy elements such as Cr, Nb, Ti, V and Mo, and significantly reduces the steel quality. The cost of the alloy realizes economy from the aspect of material; on the other hand, the yield strength (R _el (or P _p0.2 )) of the economical high surface quality high hole expansion steel of the present invention is 450～550MPa, and the tensile strength ( R _m ) is ≥690MPa, hole expansion rate (λ) is ≥75%, elongation after fracture (A ₈₀ ) is ≥19%, and 180° cold bending test D=0a is qualified; it has good hole expansion performance, formability, Microstructure performance, stamping performance, isotropy and other advantages meet the needs of steel for automobile chassis and complex stamping parts.

Wherein, among the above-mentioned economical high surface quality high hole reaming steels with a tensile strength of 690 MPa, the thickness specification of the economical high surface quality high hole reaming steels is 1.4-6.0 mm.

The present invention also provides a preparation method for the above-mentioned economical high-surface-quality high-hole reaming steel with a tensile strength of 690 MPa, which includes the following steps: smelting and continuous casting process→heating process→rough rolling process→hot coil box process→finish rolling process → Laminar cooling process → Pickling process → Coiling and packaging.

In the smelting process, smelting should be carried out according to the following chemical composition range, and the weight percentage of the chemical composition is as follows, [C]: 0.055% ~ 0.095%, [Si]: 0.85% ~ 1.50%, [Mn]: 1.30% ~ 1.80% , [P]: 0.010% to 0.020%, [S]: ≤0.003%, [Alt]: 0.010% to 0.050%, [Ca]: 0.0010% to 0.0050%, [N]: ≤0.0050%, [O] : ≤0.0040%, the rest is Fe and unavoidable impurities, and [C]×[Mn]×10 ⁴ ≤0.15, {[P]+10×[S]}×10 ² ≤0.04, [Ca]/[ S]≥0.3.

Aiming at the chemical composition of the economical, high-surface-quality, and high-hole reaming steel designed by the present invention, the inventors made a lot of attempts on various process operations and parameters in its preparation method.

The invention adopts LF desulfurization treatment in the smelting process, and the finished product [S]≤0.003%, which ensures that the S content is low enough; in the continuous casting process, low superheat casting is adopted, and the superheat is controlled at 20-25°C, which can reduce the core of the slab. Partial component segregation and the use of chamfering molds can greatly reduce the risk of cracks at the corners of the slab and avoid subsequent peeling.

In the heating process, the continuous casting slab temperature is controlled to be 1200±30°C.

Descaling is carried out in the rough rolling process and the finishing rolling process, including: in the rough rolling process, the whole process of descaling is required, the descaling water pressure is ≥ 20Mpa, and the angle of the dephosphorization nozzle is 40-50°; the intermediate billet is obtained through the rough rolling process , enter the hot coil box process, the intermediate billet is coiled and descaled, and it is required to use descaling water to descale the whole process. Descaling is carried out at the exit of the finish rolling, the pressure of the descaling water is ≥20MPa, and the angle of the dephosphorization nozzle is 40-50°.

In addition, the present invention also controls the thickness of the intermediate slab at the rough rolling exit to be 40±3mm; in the finish rolling process, the finish rolling inlet temperature is 1080±40°C, the finish rolling temperature is 820～880°C, and the finish rolling exit thickness is 1～6mm .

In the laminar cooling process, the steel strip after finishing rolling adopts a double-stage cooling method, and the cooling rate is not lower than 26 ° C / s to rapidly cool 650 ~ 700 ° C, then air cooling for 7 ~ 12 s, and then Rapid cooling to 400-470°C at a cooling rate of not less than 18°C/s, coiling into rolls at this temperature range, and slowly cooling to 280-350°C for stacking after coiling, and then naturally cooling to below 40°C; The rapid cooling rate and the coiling temperature range are used. On the one hand, it can effectively refine the ferrite grains, and on the other hand, it can obtain a bainite structure with a moderate proportion, ideal shape and distribution, and realize the ferrite and bainite structure of the steel strip. The microstructure of the body, the process control is simple, easy to produce, no subsequent heat treatment process is required, the production process is shortened, and the process cost is saved.

In the pickling process, the hot-rolled steel strip below 40 °C passes through the shot peening descaling unit and the tension leveling descaling unit to break the oxide scale, and then pickling, rinsing, squeezing and drying through the shallow groove turbulent pickling unit. Drying; the present invention controls the elongation rate of tension and straightening to 0.8～1.8%, the temperature of the acid solution in the pickling tank is 55～85°C, the concentration of free acid in the pickling tank is 50～160g/L, and the ^Fe concentration in the pickling tank is 50 ~140g/L, the pickling speed is 60~120m/min, the temperature of the rinsing tank is 50±10℃, and the drying unit is stable at 85±5℃.

The hot rolling process of the present invention adopts the control of descaling times and descaling water pressure, as well as the scale-breaking process and pickling process of the pickling process. When the rolling surface is delivered, the surface color of the steel strip is uniform, and the thickness of the oxide scale is between 5 and 15 μm. In between, there are no surface defects such as inclusions, scale pressing, pitting, peeling, roll printing, and poor plate shape; when the pickling surface is delivered, in addition to the above-mentioned surface defects, there is no under-pickling or over-aciding Defects such as washing and poor roll shape; the treatment of the rolled surface and the pickling surface all meet the high-level surface quality requirements.

By trying the chemical composition and preparation method of the economical high surface quality high hole expansion steel, the present invention has obtained a 690MPa economical high surface quality high hole expansion steel whose microstructure consists of ferrite (F) Composed of bainite (B); yield strength (R _el (or P _p0.2 )) is 450-550MPa, tensile strength (R _m ) is ≥690MPa, hole expansion rate (λ) is ≥75%, after fracture The elongation (A ₈₀ ) is ≥19%, and the 180° cold bending test D = 0a is qualified; wherein, the parallel section gauge length L ₀ of the tensile sample = 80mm, and the parallel section width b = 20mm; in the cold bending test, D is The diameter of the bending center, a is the thickness of the steel strip; the economical high surface quality and high reaming steel of the present invention has the advantages of good hole reaming performance, forming performance, microstructure performance, stamping performance, isotropy, etc., and is mainly used for automobile chassis Forming of complex shaped parts.

The present invention will be further described in detail through the examples below, but the protection scope of the present invention is not limited to the scope of the examples.

In the embodiment of the present invention:

1. The evaluation method of the hole expansion rate is as follows: the symbol of the hole expansion rate is: λ, and the unit is: %; The circular pie sample preparation, and open in the center As the initial hole, mark it as d1; use _a conical punch with a top angle of 60° to expand the initial hole; measure the hole diameter when the crack generated in the punching part penetrates the thickness direction of the steel strip, mark it as _d2 ;

Calculation formula of hole expansion rate (λ): λ=[(d ₂ -d ₁ )÷d ₁ ]×100%;

In the present invention, when λ≥100%, it is regarded as qualified.

2. Microstructural evaluation, yield strength, tensile strength, elongation, hole expansion rate, and the determination methods are all determined according to national standards.

Example 1

This embodiment adopts six experimental groups, and the chemical composition of the 690MPa grade economical high surface quality and high hole reaming steel of experimental groups 1 to 6 is shown in Table 1;

The 690MPa grade economical steel with high surface quality and high hole reaming of experimental group 1-6 was prepared by the following method:

It includes the following steps: smelting process → continuous casting process → heating process → rough rolling process → hot coil box process → finishing rolling process → laminar cooling process → coiling → slow cooling → pickling process → coiling and packaging; The parameters are shown in Table 2 and Table 3;

The chemical composition of each experimental group steel of table 1 embodiment 1

In the smelting process, smelting is carried out according to the corresponding chemical composition range, and LF desulfurization treatment is used to control the S content; in the continuous casting process, low superheat casting is used, and the superheat is controlled at 20-25°C. Slab: In the laminar flow cooling process, the steel strip after finishing rolling adopts the laminar flow cooling process and adopts a double-stage cooling method, and is rapidly cooled at a cooling rate not lower than 26°C/s (that is, before the laminar cooling rate in Table 2) for 650 ~700°C, then air-cooled for 7~12s, then rapidly cooled to 400~470°C at a cooling rate not lower than 18°C/s (after the laminar cooling rate in Table 2), coiled into rolls, and stacked slowly to 280～350℃, then naturally cool to below 40℃;

The hot rolling process of experimental group 1～6 steel in this embodiment is shown in Table 2;

The hot rolling process of each experimental group of table 2 embodiment 1

The pickling process of experimental group 1-6 steel in this embodiment is shown in Table 3;

The pickling process of each experimental group of table 3 embodiment 1

The performance of experimental group 1～6 steel in this embodiment is shown in Table 4;

The performance of each experimental group steel of table 4 embodiment 1

It can be seen from this example that the economical high surface quality and high hole reaming steel of the present invention adopts the "low C+Si+Mn" alloying treatment technology, does not add precious alloy elements such as Cr, Nb, Ti, V and Mo, and significantly reduces the Low alloy cost, realizing economy in terms of material; by controlling the chemical composition of the steel and its preparation method, a 690MPa economical steel with high surface quality and high hole expansion is obtained: using TMCP technology and controlled rolling and controlled cooling technology , combined with a reasonable rapid cooling rate and coiling temperature range to achieve the microstructure of steel strip ferrite and bainite, the process control is simple, easy to produce, no subsequent heat treatment process is required, the production process is shortened, and the process is saved Cost; through chemical composition design, effective control of steel inclusions in the smelting process, use of chamfering crystallizers in the continuous casting process, and low superheat casting to obtain high-quality slabs; The water pressure, as well as the scale breaking process and pickling process of the pickling process, have obtained products with high gloss, low roughness, good finish and high surface quality.

The economical high surface quality economical high surface quality high hole reaming steel of the present embodiment gained, its microstructure is made up of ferrite (F) and bainite (B); Yield strength (R _el (or P _p0.2 )) is 360~450MPa, tensile strength (R _m ) is ≥690MPa, hole expansion rate (λ) is ≥85%, elongation after fracture (A ₈₀ ) is ≥22%, 180° cold bending test D=0a is qualified ; Among them, the gauge length of the parallel section of the tensile sample is L ₀ =80mm, and the width of the parallel section b=20mm; in the cold bending test, D is the diameter of the bending center, and a is the thickness of the steel strip; it has good hole expansion performance, formability, Microstructural performance, stamping performance, isotropy and other advantages, it can be used for the forming of parts with complex shapes of automobile chassis.

Claims (9)

1. The high reaming steel of the economical great surface quality of 1.690MPa levels, it is characterised in that：By weight percentage, its chemical composition For：[C]：0.055%~0.095%, [Si]：0.85%~1.50%, [Mn]：1.30%~1.80%, [P]：0.010%~ 0.020%, [S]：≤ 0.003%, [Alt]：0.010%~0.050%, [Ca]：0.0010%~0.0050%, [N]：≤ 0.0050%, [O]：≤ 0.0040%, remaining is Fe and inevitable impurity, and [C] × [Mn] × 10⁴≤ 0.15, [P]+ 10×[S]}×10²≤ 0.04, [Ca]/[S] >=0.3.
2. 2. the high reaming steel of the economical great surface quality of 690MPa levels according to claim 1, it is characterised in that：It is described The thickness specification of the high reaming steel of the economical great surface quality of 690MPa levels is 1.4~6.0mm.
3. 3. the high reaming steel of the economical great surface quality of 690MPa levels according to claim 1 or 2, it is characterised in that：It is described The microstructure of the high reaming steel of the economical great surface quality of 690MPa levels is made up of ferrite and bainite.
4. 4. the high reaming steel of the economical great surface quality of 690MPa levels according to claim 1 or 2, it is characterised in that：It is described The yield strength of the high reaming steel of the economical great surface quality of 690MPa levels is 450~550MPa, and tensile strength is >=690MPa, is expanded Porosity is >=75%, and elongation after fracture is >=19%, and 180 ° of cold bending test D=0a are qualified.
5. 5. the preparation method of the high reaming steel of the economical great surface quality of 690MPa levels described in any one of Claims 1 to 4, it is special Sign is：Comprise the following steps：Smelting procedure → continuous casting working procedure → heating process → roughing operation → Coil Box process → finish rolling Process → laminar flow cooling technology → batches → Slow cooling → pickling process → and batches packaging.
6. 6. the preparation method of the high reaming steel of the economical great surface quality of 690MPa levels according to claim 5, its feature exist In：De-scaling, de-scaling water pressure >=20MPa, dephosphorization nozzle angle are carried out in the roughing operation, Coil Box process and finishing rolling step For 40~50 °.
7. 7. the preparation method of the high reaming steel of the economical great surface quality of 690MPa levels according to any one of claim 5~6, It is characterized in that：The workpiece thickness that roughing exports in the roughing operation is 40 ± 3mm；Finish rolling entrance in the finishing rolling step Temperature is 1080 ± 40 DEG C, and finishing temperature is 820~880 DEG C, and finish rolling exit thickness is 1~6mm.
8. 8. the preparation method of the high reaming steel of the economical great surface quality of 690MPa levels according to claim 5, its feature exist In：Using double section types of cooling in the laminar flow cooling technology, with the cooling velocity not less than 26 DEG C/s quickly cooling 650~ 700 DEG C, then 7~12s of air cooling, is then quickly cooled to 400~470 DEG C with the cooling velocity not less than 18 DEG C/s；The volume The coiling temperature taken is 400~470 DEG C；The Slow cooling is to slowly cool to 280~350 DEG C, then natural cooling after batching To less than 40 DEG C.
9. 9. the preparation method of the high reaming steel of the economical great surface quality of 690MPa levels according to any one of claim 5~8, It is characterized in that：Straightening elongation percentage is 0.8~1.8% in the pickling process, and descaling bath HCl acid liquor temperatures are 55~85 DEG C, Descaling bath free acid HCl concentration is 50~160g/L, descaling bath Fe²⁺Concentration is 50~140g/L, and pickling speed is 60~120m/ Min, potcher temperature are 50 ± 10 DEG C, and Drying unit stabilizes to 85 ± 5 DEG C.