CN115161551B - High-strength high-formability super-atmospheric corrosion-resistant steel and manufacturing method thereof - Google Patents

Abstract

The high-strength high-formability super-atmospheric corrosion resistant steel and the manufacturing method thereof comprise the following components in percentage by weight: c:0.04 to 0.10 percent, si is less than or equal to 0.50 percent, mn:0.20 to 1.80 percent, P is less than or equal to 0.03 percent, S is less than or equal to 0.01 percent, al is less than or equal to 0.30 percent, cu:0.10 to 0.60 percent, cr:1.5 to 4.5 percent of Ti:0.04 to 0.18 percent, N is less than or equal to 0.008 percent, and the balance of Fe and other unavoidable impurities; and at the same time, the requirements are as follows: c is more than or equal to 0.08% -Ti/3, mn+Cr is more than or equal to 2.0%. The relative corrosion rate of the steel cycle immersion test and the common structural steel Q355B is less than or equal to 30%, the weather resistance is more than 3 times of that of the common structural steel Q355B, and is more than 2 times of that of the common weathering steel Q450NQR 1; the corrosion rate is fast attenuated along with the time and the corrosion depth, and the corrosion depth of 25 years is less than or equal to 0.1mm; the yield strength is more than or equal to 700MPa, the tensile strength is more than or equal to 800MPa, the yield ratio is less than or equal to 0.90, the breaking elongation is more than or equal to 18%, the cold bending performance meets 180 DEG D=1t, and the impact energy at the low-temperature impact toughness of minus 20 ℃ reaches more than 60J.

Description

High-strength high-formability super-atmospheric corrosion-resistant steel and manufacturing method thereof

Technical Field

The invention relates to the technical field of low alloy steel, in particular to high-strength high-formability super-atmospheric corrosion-resistant steel and a manufacturing method thereof.

Background

The weather-resistant steel is widely applied to the production of outdoor steel structures with the requirement of weather resistance, such as containers, railway vehicles, bridges and the like, and the production and the application of the weather-resistant steel have been in the past decades. In recent years, with the improvement of the requirements on green, low carbon and environmental protection, the application scene of the atmospheric corrosion resistant steel is also expanding. The weather resistance of the atmospheric corrosion resistant steel is utilized to replace some steel surface corrosion prevention processes, such as pre-galvanization or zinc-aluminum-magnesium, post-galvanization and the like, so that the bare use of the material is realized, the problems of energy consumption and pollution brought by the plating process are reduced, and the maintenance cost of post-corrosion prevention maintenance and renovation is also reduced. For example, a photovoltaic bracket for the photovoltaic power generation industry, which is conventionally made of a post-galvanization process or a pre-galvanization aluminum magnesium material, has the problems of high production cost, energy consumption in the production process, high pollution, poor damage corrosion resistance of a notch and the like, high post-maintenance cost and the like, and a solution for bare weathering steel is also sought. Because the material is used for a photovoltaic bracket in a bare mode and is guaranteed to be in service for 25 years in a maintenance-free environment, the material is required to have very high atmospheric corrosion resistance, the relative corrosion rate of the material is generally required to be doubled in a C1-C3 environment compared with that of the conventional common weathering steel, the relative corrosion rate of the material and the common structural steel Q355B is less than or equal to 30%, and the corrosion depth of the material in 25 years is less than or equal to 0.1mm. At present, no suitable weather-resistant steel can be widely applied to the photovoltaic bracket.

Also, in order to reduce the material consumption, the photovoltaic support steel needs to have higher strength to replace the current low-strength steel with the strength below 500MPa, for example, the high-strength steel with the strength above 700MPa is adopted, so that the consumption of steel can be remarkably reduced, the cost of transporting and installing the photovoltaic support to the photovoltaic power generation field in the remote areas is also reduced, and the photovoltaic support steel has important significance for the construction of the photovoltaic power station.

The high-strength steel is used for realizing photovoltaic brackets and the like, and the high-strength steel is required to be processed into various profile structural members by cold forming, so that the material is required to have good extensibility and cold bending performance, which is a challenge for materials with high strength, and therefore, the photovoltaic bracket material also has good cold forming performance while having ultrahigh weather resistance and high strength.

The prior high-strength weathering steel also relates to some invention patents, but the product of the weathering steel still has difficulty in meeting the use requirements of high weather resistance, high strength and high forming of the photovoltaic bracket. The method comprises the following steps:

chinese patent CN201910243121.1 discloses a weather-resistant steel plate for high strength building and its production method, the yield strength of the weather-resistant steel is only 420MPa, and the tensile strength is 540-700 MPa. The component Ni:0.35 to 0.65 percent, cr:0.15 to 0.30 percent, cu:0.20 to 0.40 percent, the corrosion resistant alloy content is low, and only the general weather resistance level can be achieved; the higher Ni content is adopted, so that the economy is not high; nb:0.020 to 0.035 percent, ti:0.010 to 0.030 percent, the content of the microalloy Nb and Ti elements is not high, and the strength cannot be effectively improved.

Chinese patent CN202010116991.5 discloses a high strength weathering steel applied to railway vehicles, and also aims to provide a high strength weathering steel to solve the technical problem that the steel in the prior art cannot achieve both high strength and high weather resistance. The components are as follows: 0.06 to 0.07 percent of C, 0.23 to 0.26 percent of Si, 1.40 to 1.50 percent of Mn, 0.0 to 0.19 percent of Ni, 0.0 to 0.51 percent of Cr, 0.31 to 0.33 percent of Cu, 0.110 to 0.12 percent of Ti, 0.030 to 0.036 percent of Nb and 0.0 to 0.09 percent of Sb. The steel adopts a near-full ferrite structure, and the content of pearlite is below 2%; the yield strength is 636-710 MPa, and the tensile strength is 698-775 MPa. From the viewpoint of component design, the main principle is to add weathering resistance elements such as Ni, cr, cu, sb and the like simultaneously to improve weathering resistance, and to improve strength through Nb and Ti composite precipitation strengthening. The technical disadvantage is that Cr content is low, so weather resistance is still a level of general weathering steel. The invention also mentions that Sb is used for improving the corrosion resistance, but Sb is a harmful element in steel and deteriorates the performance of the steel, especially the low-temperature toughness; meanwhile, the smelting point of Sb is low, so that the steelmaking yield is difficult to control, and the invention has low practicability.

Chinese patent CN201510031313.8 discloses a hot-rolled high-strength low-nickel-copper ratio thick weathering steel and its production method, the yield strength of the weathering steel is not less than 700MPa, the tensile strength is not less than 750MPa, the elongation is not less than 18%, the impact absorption power at-40 ℃ is not less than 60j, and 180°d=2t is qualified by bending. The patent is still oriented to the application requirement of general weathering steel, and the Cr content is controlled to be less than 1.25%, so that adverse effects of high Cr content on smelting cost and difficulty and on steel forming property are avoided. And the chemical composition adopts higher Si: 0.56-0.64%, si is an element which seriously affects the surface quality, and high Si content can cause serious red iron scale on the surface of the strip steel. In the weathering steel composition, nb:0.052 to 0.058 percent; ti: 0.125-0.135%, the refined crystal grain and precipitation strengthening effect of the niobium-titanium microalloy are utilized, but the Nb content is 0.052-0.058%, the Nb content is higher, and the production cost is higher.

The Chinese patent CN200710045329.X discloses a high corrosion-resistant high-strength weathering steel and a manufacturing method thereof, the yield strength is more than 700MPa, the elongation is more than 20%, the atmospheric corrosion resistance is higher than that of the traditional high-strength weathering steel, and the requirements of the railway freight car, container and bridge construction field are met. But in its composition design, C:0.002-0.005%, wherein the ultra-low C design is adopted to inhibit the formation of pearlite structure and other carbides, ensure the microstructure of the steel to be a homogeneous structure, avoid the corrosion of primary cells caused by the potential difference between heterogeneous phases, and improve the corrosion resistance of the steel; mn:0.01 to 0.05 percent, is very low and cannot play a role in strengthening; ti is less than or equal to 0.03 percent, which is added to inhibit the growth of austenite grains, inhibit the growth of ferrite grains, improve the toughness of steel and hardly play a role in precipitation strengthening. The patent does not use the strengthening contribution of C-Mn-Ti, but its Cr content: 4.5-5.5%, the design is very high, and the atmospheric corrosion resistance is doubled compared with the traditional weathering steel through the high Cr. Therefore, the realization principle is that high corrosion resistance and high strength are realized by using high Cr alloy, which is very uneconomical.

As can be seen from comparison with existing patents, current weathering steels are mainly aimed at railway vehicles, containers, bridges and other uses, and some patents consider high strength designs, but their weathering requirements and designs remain at a general level of weathering performance.

Disclosure of Invention

The invention aims to provide high-strength high-formability super-atmospheric corrosion-resistant steel and a manufacturing method thereof, wherein the steel has very high atmospheric corrosion resistance, the relative corrosion rate of a cycle immersion test and common structural steel Q355B is less than or equal to 30%, the weather resistance is more than 3 times of that of the common structural steel Q355B, and the weather resistance is more than 2 times of that of common weathering steel Q450NQR 1; the corrosion rate is fast attenuated along with time and corrosion depth, the corrosion depth of 25 years is less than or equal to 0.1mm, the bare coating-free surface of structural members such as a photovoltaic bracket and the like can be met, and the super-atmospheric corrosion resistance requirement of service for more than 25 years can be met; meanwhile, the steel has higher strength and forming performance, the yield strength is more than or equal to 700MPa, the tensile strength is more than or equal to 800MPa, the yield ratio is less than or equal to 0.90, the breaking elongation is more than or equal to 18%, the cold bending performance can meet the requirement that the bending (D is the bending diameter and t is the thickness of the steel plate) is 180 DEG D=1t, and the impact energy at the low-temperature impact toughness of-20 ℃ reaches more than 60J. Therefore, the steel has high strength, high forming performance, economy and applicability, and can meet the light design requirements of structural members such as photovoltaic brackets and the like and the processing requirements such as rolling forming and the like.

In order to achieve the above purpose, the technical scheme of the invention is as follows:

the high-strength high-formability super-atmospheric corrosion-resistant steel disclosed by the invention has the advantages that the component design utilizes the high Cr content of 1.5-4.5% to promote the formation of a uniform and compact rust layer, and the rust layer can be rapidly enriched to the concentration of more than 12% at the interface position with a matrix in a thinner rust layer, so that the corrosion potential and electrochemical impedance are obviously improved, the corrosion is interrupted to continue to occur, and the super-high atmospheric corrosion-resistant performance is obtained; meanwhile, the multiphase structure of ferrite+bainite (MA is the main material) +pearlite (or carbide) +a large number of nano precipitated particles with the diameter less than or equal to 10nm is realized by utilizing the component designs of C, ti, mn and high Cr, and the high strength and the high forming performance of the steel are obtained by utilizing the mechanisms of high plasticity of ferrite, tiC precipitation strengthening, bainitic structure strengthening and the like.

Specifically, the high-strength high-formability super-atmospheric corrosion-resistant steel comprises the following components in percentage by weight: c:0.04 to 0.10 percent, si is less than or equal to 0.50 percent, mn:0.20 to 1.80 percent, P is less than or equal to 0.03 percent, S is less than or equal to 0.01 percent, al is less than or equal to 0.30 percent, cu:0.10 to 0.60 percent, cr:1.5 to 4.5 percent of Ti:0.04 to 0.18 percent, N is less than or equal to 0.008 percent, and the balance of Fe and other unavoidable impurities; and at the same time, the requirements are as follows:

c is more than or equal to 0.08% -Ti/3, and can exert the tissue strengthening effect of C;

Mn+Cr is more than or equal to 2.0 percent, so that the tissue transformation of the material is controlled.

Further, it is also required to satisfy: si+2Ni is more than or equal to 0.10%, and the influence of copper brittleness can be reduced.

Further, nb is less than or equal to 0.06%, V is less than or equal to 0.15%, mo is less than or equal to 0.40% or B is less than or equal to 0.002%, and strength can be improved.

In addition, the alloy also contains less than or equal to 0.40% of Ni, less than or equal to 0.15% of Sb, less than or equal to 0.15% of Re, less than or equal to 0.015% of Ca or less than or equal to 0.015% of Mg, so that the corrosion resistance can be further improved.

The microstructure of the super-atmospheric corrosion-resistant steel is ferrite, pearlite or carbide and MA-based bainite, wherein the ferrite content is 55-90%, the bainite content is 5-35%, and a large number of TiC precipitation particles with the diameter less than or equal to 10nm are dispersed in the microstructure.

The super-atmospheric corrosion resistant steel has super-atmospheric corrosion resistant performance, the relative corrosion rate between a peri-immersion test and the common structural steel Q355B is less than or equal to 30%, the weather resistance is more than 3 times of that of the common structural steel Q355B, and is more than 2 times of that of the common weathering steel Q450NQR 1; the corrosion rate can be rapidly attenuated along with the time and the corrosion depth, and the corrosion depth of 25 years is less than or equal to 0.1mm.

The yield strength of the super-atmospheric corrosion resistant steel is more than or equal to 700MPa, the tensile strength is more than or equal to 800MPa, the yield ratio is less than or equal to 0.90, the breaking elongation is more than or equal to 18%, the cold bending performance meets 180 DEG D=1t bending (D bending diameter, t is the thickness of a steel plate), and the low-temperature impact toughness and impact energy at-20 ℃ reach more than 60J.

In the composition design of the super-atmospheric corrosion resistant steel, the following components are adopted:

and C, the effective strengthening elements in the steel can form nano second phase precipitation particles with microalloy elements such as Ti, nb and the like besides solid solution strengthening, and play roles in precipitation strengthening and tissue refining. Meanwhile, under a high Cr component system, C can be converted into a bainite or martensite hard phase at a lower critical cooling speed, and the tensile strength of the material can be obviously improved. As the most economical strengthening element, the content of C is more than or equal to 0.04 percent; however, excessive C can form more carbide in the steel, so that on one hand, the toughness and the forming property of the material can be reduced, the primary cell effect can be formed to reduce the corrosion resistance of the steel, and meanwhile, the welding property of the steel is also reduced, so that the C content is less than or equal to 0.10 percent.

Si, the deoxidizing element commonly used in steel has solid solution strengthening effect on steel, can improve the corrosion resistance of the material, and has a certain effect on alleviating copper embrittlement, but the higher Si content can cause serious red iron sheet defect on the surface of steel, influence the surface quality of strip steel, and simultaneously can reduce the welding performance of the material and cause the deterioration of toughness of a welding heat affected zone, so the Si content is less than or equal to 0.50 percent;

mn is an important strengthening element in steel, has the function of solid solution strengthening, can reduce the supercooled austenite transformation temperature, reduce the ferrite transformation temperature, is beneficial to tissue refinement, and improves the strength and toughness of the material. However, excessive Mn content suppresses ferrite transformation, and transformation of structure into bainite, and decreases the plasticity and cold formability of the material. Therefore, the Mn content is designed to be 0.2-1.8%;

cr is an important element for improving the weather resistance of the steel sheet. The main mechanism of weather resistance improvement of weathering steel has two points: firstly, corrosion resistance elements are added to improve the corrosion potential of a matrix, and electrochemical impedance is improved to reduce and etch the rate; secondly, the surface is promoted to form a compact rust layer to realize physical barrier to corrosive medium, the corrosive environment of the substrate position is changed, and the corrosion is gradually slowed down along with the increase of the corrosion depth. Cr has such an effect. When the Cr content in the steel exceeds 1.5%, under the combined action of elements such as Cr and Cu, a uniform and compact rust layer can be formed on the surface of the substrate, the higher Cr content is matched with the action of Cu element and the like, and the alpha-FeOOH in the rust layer is very fine, so that the rust layer is very favorable for blocking the further penetration of electrochemical corrosive mediums such as water and the like, and the electrochemical impedance is improved. Meanwhile, as the concentration of Cr is higher, the concentration of Cr enrichment at the front edge of the rust layer is also rapidly increased along with the increase of the rust layer, and the concentration of the interface position of the rust layer and the matrix reaches more than 12 percent before the rust layer reaches 0.1mm. When the concentration of Cr enrichment reaches more than 12%, the effect similar to stainless steel occurs, the corrosion potential of the corrosion front interface is very high, and the barrier electrochemical impedance of a dense rust layer low corrosion medium is very high, so that the corrosion reaction is basically interrupted. However, as the Cr content of the matrix increases, the corrosion potential of the matrix increases, for example, if the Cr content exceeds 4.5%, the selective corrosion of the rust layer at the initial stage of formation is enhanced, and the uniformity of the thickness of the rust layer is deteriorated, that is, the Cr enrichment concentration at the front edge of the rust layer and the corrosion environment cause chemical resistance to become inconsistent, which instead increases the corrosion potential difference at the front edge of the corrosion, aggravates the occurrence of the galvanic effect, and deteriorates the corrosion inhibition effect. In this case, although the relative corrosion rate under the condition of the limited number of dip tests is lowered, the depth of corrosion is not lowered in the view of the actual long period, and the meaning of improving the weather resistance is lost. Therefore, the Cr content of 1.5 to 4.5% is required in the present invention.

Referring to fig. 1 and 2, fig. 1 shows the effect of Cr content on the relative corrosion rate in the dip-cycle test, and fig. 2 shows the effect of Cr content on the corrosion depth in the long-cycle condition. As can be seen, the corrosion rate of the steel of the present invention decays rapidly with time and corrosion depth, the 25-year corrosion depth simulated presumption value is less than or equal to 0.1mm, and the 25-year corrosion depth simulated presumption value of the comparative example with the higher Cr content of 5% is about 0.12mm, but is reduced.

Cr is also an element that increases hardenability of steel, and it is considered that too high Cr causes damage to the performance of steel. The invention discovers that the higher Cr content and Mn matching can lead the steel to form air-cooled bainite or air-cooled martensite under the lower cooling rate, thereby obviously improving the tensile strength of the material, reducing the yield ratio of the material, and being beneficial to reducing the forming rebound and improving the stability of the forming size. The invention combines the design of higher Cr content required by meeting the corrosion resistance requirement, fully utilizes the effect of Cr, and further improves the strength by matching with the design of C, mn and other elements, and reduces the addition of other strengthening elements.

Cu, one of the important corrosion-resistant elements, has more remarkable effect when added together with Cr. Cu can promote the formation of a compact rust layer on the surface of steel, and the atmospheric corrosion resistance of the steel can be obviously improved by adding more than 0.10% of Cu. However, cu is a metal with a low melting point, and when the strip steel is heated, copper-rich compatibility with a low melting point is easy to enrich on the surface of a substrate due to the problem of selective oxidation, and copper-brittle network cracks and skin-tilting defects are easy to form on the surface of the strip steel when steel is rolled, so that the surface quality is deteriorated. Meanwhile, cu is also a noble element, and the Cu content is 0.10-0.60%.

P is often used as an added corrosion resistant element in the traditional atmospheric corrosion resistant steel, can promote the formation of a surface protective rust layer, and effectively improves the atmospheric corrosion resistant performance of the steel, but P is also a harmful impurity element in the steel and is easy to segregate in the thickness center during billet continuous casting. Meanwhile, P is easy to generate segregation at the grain boundary, and the bonding energy of the grain boundary is reduced, so that the toughness and the plasticity of the steel are reduced. In the same principle, P is very unfavorable for welding performance, so that the invention does not adopt the action of P on the atmospheric corrosion resistant steel, and the content of P in the steel is required to be reduced as much as possible, and the content of P is controlled to be less than or equal to 0.03 percent.

S, a common harmful impurity element in steel has adverse effects on weather resistance, low-temperature toughness, welding performance, cold forming performance and the like, so that the S content is required to be less than or equal to 0.01%.

Al is an effective deoxidizing element, and is favorable for refining grains and improving the toughness of steel, but higher Al is unfavorable for casting during continuous casting and is easy to block a water gap, so that the Al content is required to be less than or equal to 0.30 percent.

Ti, a strong carbonitride forming element, can be precipitated in the form of extremely fine TiC or Ti (C, N) second phase particles, and remarkably improves the strength of the material. Ti is relatively inexpensive compared with microalloy elements such as Nb, and therefore is added as an important strengthening element in the present invention. Meanwhile, tiC is separated out, carbide or pearlite with larger size formed by free C is obviously reduced, so that the primary cell effect of heterogeneous phase in the corrosion process is reduced, the intergranular corrosion resistance of the material is improved, uniform corrosion is formed, the quality of a rust layer is improved, and the corrosion resistance of the material is improved while the strength is improved. However, when the content of Ti is too high, the precipitation strengthening effect of Ti gradually decreases, and the low-temperature toughness of the steel is remarkably affected. Therefore, the Ti content is designed to be 0.04-0.16%.

N, impurity element in steel, N and Ti can combine in molten steel to form larger size TiN inclusion, which reduces effective content of Ti on one hand, and on the other hand, tiN inclusion also obviously damages toughness of steel, so the content of N is controlled as low as possible, and the content of N is required to be less than or equal to 0.008%.

Nb, a strong nitrogen carbide forming element, can also form NbC and Nb (CN) carbide particles, producing precipitation strengthening. But the cost of Nb is far higher than Ti, so that the strength improvement economy is not better than that of Ti, meanwhile, the quality of a casting blank in the continuous casting and cooling process of strip steel is influenced by the excessive content of Nb, and the defects of surface cracks, angle cracks and the like are generated, so that the Nb content is designed to be less than or equal to 0.06 percent.

V, a strong carbide forming element, can produce a strong precipitation strengthening effect. Compared with TiC and VC, the precipitation temperature is lower, and a better precipitation strengthening effect can be generated at 500-550 ℃ generally, so that V can be adopted to improve the strength when the strip steel is coiled at a lower temperature, but the economy of improving the strength by V is not as good as that of Ti, and meanwhile, the welding toughness of steel can be reduced by too high content of V, so that the V content is less than or equal to 0.15%.

Mo, an alloying element commonly used in steel, has the effects of increasing hardenability, inhibiting ferrite transformation, refining structure, improving TiC precipitation strengthening contribution and the like, and a small amount of addition is beneficial to the improvement of steel performance, but Mo is a noble metal element, and the content is too high, so that the economical efficiency of the steel is not more than 0.40 percent, and the invention is limited.

And B, the material has strong hardenability, and can obviously inhibit ferrite transformation, so that the bainitic structure is obtained to improve the strength, but the plasticity and the processability of the material are reduced. Therefore, the content of B is limited to less than or equal to 0.002 percent.

Ni can improve the corrosion resistance of steel and improve the brittleness quality of copper on the surface caused by Cu by improving the melting point of a Cu-rich phase. However, ni is an important strategic material, and in recent years, the price is very high, and excessive addition greatly increases the alloy cost of the material, so that the Ni content is limited to be less than or equal to 0.40 percent.

Sb may be added as an element for improving corrosion resistance. But Sb is also a harmful element in steel, and deteriorates the properties of steel, especially low temperature toughness, so the present invention limits the Sb content to 0.15%.

Re rare earth element has the functions of improving corrosion resistance and toughness. However, the yield of Re element in the steelmaking process is difficult to control, and excessive addition can reduce the economical efficiency of steel, so that the Re content is limited to be less than or equal to 0.15.

Ca and S can form spherical dispersed CaS, so that the distribution of sulfide inclusions in steel is improved, the uniform corrosion of materials is improved, the toughness of the materials is improved, and the content of Ca is generally limited to be less than or equal to 0.015 percent.

Mg can be combined with O to form finer MgO and can be used as nucleation core of other inclusions, thereby being beneficial to refining the size of the inclusions and improving the dispersion distribution of the inclusions, and the uniform corrosion of the material and the toughness of the material can be improved. The Mg content is generally limited to be less than or equal to 0.015.

In addition, the composition design of the steel provided by the invention also needs to satisfy the following conditions:

c is more than or equal to 0.08% -Ti/3, and the strength of the steel can be ensured more flexibly. The main strengthening elements of the invention are C-Ti-Mn-Cr, mn and Cr mainly play a role in regulating tissue transformation, so the main strengthening elements are C and Ti. The C-Ti combination can form TiC precipitate phase to be separated out, has strong strengthening contribution, and can realize the strengthening effect of 100-300MPa according to different Ti contents. The invention requires C not less than 0.08% -Ti/3, and aims to ensure that enough allowance C is left for realizing reinforcement in addition to TiC reinforcement. After TiC formation, free C is reduced by Ti/3, the remainder of C being able to form a certain amount of bainite (MA) phase in the later structural transformation. If the Ti content is lower, in order to achieve higher strength, more C supplement strength is needed, and the lower limit of the C content is required by 0.08% -Ti/3, so that the strength design of the steel can be well met.

Mn+Cr is more than or equal to 2.0%, mn and Cr have the function of improving the hardenability of the material, so that the material forms bainite or martensite at a lower critical cooling rate, and higher strength and lower yield ratio are obtained. The invention requires Mn+Cr to be more than or equal to 2.0 percent, and aims to ensure the strength of the steel and obtain more stable strength performance under a larger process window. Although TiC precipitation has a higher contribution to the strength, the TiC precipitation requires more process conditions, such as higher heating temperature and longer heating time to ensure sufficient solid solution of Ti element, and the strip steel needs to ensure sufficient slow cooling process after coiling to ensure slow precipitation of TiC. These fluctuations in production conditions cause fluctuations in the properties of the steel, resulting in large differences in the strength of the material. Through thermal simulation and CCT analysis, under the condition that Mn+Cr is more than or equal to 2.0%, the balance C after TiC precipitation is formed by combining with Ti can be converted into MA-based bainite instead of carbide in the cooling process of strip steel after strip steel coiling, so that the ideal multiphase structure of the steel is realized, and higher tensile strength and relatively low yield ratio are generated. Therefore, the design principle that Mn+Cr is more than or equal to 2.0% and strength performance can be stabilized is that when Ti is insufficient in solid solution or slow cooling condition after coiling is insufficient, more C is reserved, and Mn+Cr is more than or equal to 2.0% and more C is converted into MA-based bainite to complement strength.

Si+2Ni is more than or equal to 0.10%, because Cu is easy to form copper embrittlement defects on the surface of the strip steel, si and Ni have the effect of improving the copper embrittlement defects, and the Si and the Ni can complement each other. Ni has better effect on improving copper brittleness, but the cost is higher. When the content of Si+2Ni is more than 0.10%, the method plays a role, so that the upper limit content of Si and Ni is limited, one or two of Si and Ni elements are added according to the requirement that Si+2Ni is more than or equal to 0.10%, and the balance among the design economy of the material, the surface quality of the material and the copper brittleness problem can be adjusted by utilizing the complementary relation of the Si and the Ni elements, so that the copper brittleness problem is more economically in a controllable state.

The invention relates to a manufacturing method of high-strength high-formability super-atmospheric corrosion resistant steel, which comprises the following steps:

1) Smelting and casting

Smelting and casting the components into a plate blank;

2) Slab heating

The billet is controlled to be heated up rapidly, and the heating time at the temperature of 1050-1150 ℃ is lower than 15min; heating time above 1230 ℃ is required to be 30-60 min, and tapping temperature is 1230-1280 ℃;

3) Hot rolling

The slab is discharged out of the heating furnace for width setting and then enters rough rolling, and the pressure of the descaling high-pressure water in the rough rolling stage is more than 20 MPa; the rough rolling outlet temperature is lower than 1080 ℃; then the strip steel enters a finish rolling process, the finish rolling adopts a multi-frame continuous rolling process, and the final rolling temperature is controlled to be 820-900 ℃;

4) Cooling and coiling

Laminar cooling is adopted for cooling, and the cooling mode is controlled to be front-stage cooling, wherein the temperature of the outlet water is required to be higher than 650 ℃; cooling and coiling the strip steel, wherein the coiling temperature is 550-660 ℃; after the layer cooling is finished, the strip steel is required to be cooled by hot coil stacking, slow cooling walls or heat preservation covers.

Similarly, the steel of the invention can also be produced by adopting a thin slab continuous casting and rolling production line, and the heating of the slab adopts soaking furnace soaking of the thin slab continuous casting and rolling production line.

In the manufacturing method of the present invention:

and (3) heating the slab obtained by smelting and continuous casting in a heating furnace of a conventional hot rolling production line or soaking in a soaking furnace of a thin slab continuous casting and rolling production line. In order to reduce the influence of copper embrittlement, the heating curve of a steel billet in a heating furnace is optimized, and a high-temperature quick-firing process is adopted to coordinate contradiction between the copper embrittlement on the surface and the full solid solution strengthening of Ti. The heating time of the billet temperature in the range of 1050-1150 ℃ is controlled to be lower than 15min, so that the billet rapidly passes through a sensitive temperature range where copper embrittlement occurs. The heating time of the billet above 1230 ℃ is required to be 30-60 min, and the tapping temperature is 1230-1280 ℃.

The billet is discharged from the heating furnace and enters rough rolling after being fixed in width, the rough rolling stage should ensure high enough descaling pressure to obtain good descaling effect, the production proves that the high-pressure water above 20MPa has good crushing and removing effects on the denser primary oxide skin on the surface of the billet with higher Cr content, and the removal of the oxide skin has remarkable benefit on reducing the copper embrittlement problem caused by Cu enriched on the surface layer, so the pressure of the descaling high-pressure water is required to be above 20 MPa.

The subsequent strip steel enters a finish rolling process, the finish rolling adopts a multi-frame continuous rolling process, and the final rolling temperature is controlled to be 820-900 ℃ according to different thickness specifications of the rolled strip steel. Because of the design that the C is more than or equal to 0.8% -Ti/3 and the Mn+Cr is more than or equal to 2.0%, the steel can obtain the required performance under a wider process window. When the coiling temperature is changed within a larger range of 550-660 ℃, the strength of the strip steel can realize relatively small change through the balance of precipitation strengthening and tissue strengthening.

The steel produced by the process obtains a multiphase structure of ferrite and bainite (MA is the main material) +pearlite (or carbide), the ferrite content is 55-90%, the bainite content is 5-35%, and a large number of TiC precipitation particles less than or equal to 10nm are dispersed in the structure. The steel has high ferrite content, so that the material has high plasticity, the elongation at break is more than or equal to 18 percent, and the cold bending test meets 180 DEG D=1t cold bending, so that the steel has high cold forming performance. TiC precipitation particles have very obvious contribution to strength, and when the TiC precipitation contribution is insufficient, the balance C can supplement strength through a bainitic structure which mainly comprises MA in the structure, so that the yield strength is more than or equal to 700MPa, and the tensile strength is more than or equal to 800MPa; in addition, the multiphase structure with Ti precipitation strengthening and more MA ensures that the material has high strength and lower yield ratio which is less than or equal to 0.90, thereby being very beneficial to improving the forming dimensional stability of the material. Because of Cr and Ti addition and control of rolling process, the material structure is fine, so the low-temperature toughness is also good, and the impact energy at minus 20 ℃ can reach more than 60J.

Compared with the prior art, the invention has the following advantages:

the invention adopts the economical component design, utilizes the components such as C-Mn-Ti-Cu and the like and adopts the design of high Cr content of 1.5 to 4.5 percent, promotes the formation of a uniform and compact rust layer, and can rapidly enrich the Cr concentration to more than 12 percent in the interface position of a matrix in a thinner rust layer less than or equal to 0.1mm; meanwhile, by means of enrichment of uniform and compact rust layers and high-concentration Cr at the front edges of the rust layers, remarkable improvement of corrosion potential and electrochemical impedance is achieved, and corrosion interruption continues to occur, so that ultra-high atmospheric corrosion resistance is achieved. The relative corrosion rate of the dip test and the common structural steel Q355B is less than or equal to 30%, the weather resistance is more than 3 times of that of the common structural steel Q355B, and is more than 2 times of that of the common weathering steel Q450NQR 1; the corrosion rate is fast attenuated along with the formation of a stable rust layer and along with the time and the corrosion depth, so that the surface of structural members such as a photovoltaic bracket and the like can be free of coating and bare use, and the corrosion depth is less than or equal to 0.1mm in a service period of more than 25 years.

The invention also utilizes the strong precipitation strengthening effect of Ti, and the material strength is economically improved. The MA-based bainite after coiling is obtained by utilizing the residual C design with the C more than or equal to 0.8% -Ti/3 and the influence of Mn+Cr more than or equal to 2.0% on the critical cooling speed of phase change, the contribution of TiC precipitation strengthening and MA-based bainite structure strengthening can be balanced, and the required high-strength performance can be obtained under a wider process window. And forming a multiphase structure of 55-90% ferrite+5-35% bainite (MA is the main material) +pearlite (or carbide) with microstructure of the steel, and a large amount of TiC precipitation particles with diameter less than or equal to 10 nm. TiC precipitation and the improvement of the MA relative strength of fine dispersion are greatly contributed, and the ferrite-based structure also ensures the plasticity of the material, so that the high strength and the high forming performance of the steel are realized, namely, the yield strength is more than or equal to 700MPa, the tensile strength is more than or equal to 800MPa, the yield ratio is less than or equal to 0.90, the breaking elongation is more than or equal to 18%, the cold bending performance can meet the bending of 180 DEG D=1t (D is the bending diameter, t is the thickness of a steel plate), the low-temperature impact toughness and the impact energy of minus 20 ℃ can reach more than 60J, and the lightweight design requirements of structural members such as photovoltaic brackets and the processing requirements of rolling forming and the like can be met.

According to the manufacturing method, the balance among the design economy of the material, the surface quality of the material and the copper embrittlement problem is adjusted by combining Si+2Ni which is more than or equal to 0.10%, the heating control of the heating furnace billet is optimized, the contradiction between the copper embrittlement on the surface and the full solid solution strengthening of Ti is coordinated by adopting a high-temperature quick-burning process, and the copper embrittlement problem in the steel production is reduced by combining rough rolling section descaling and adopting high-pressure water descaling with the pressure of more than 20 MPa.

Drawings

FIG. 1 is a schematic representation of the effect of different Cr content on the relative corrosion rate of the dip test;

FIG. 2 is a schematic representation of corrosion depths of steels of different Cr content in long-period atmospheric exposure;

FIG. 3 is a metallographic view of a typical structure 1 of the steel according to the embodiment of the present invention;

FIG. 4 is a metallographic view of a typical structure 2 of the steel according to the embodiment of the present invention;

FIG. 5 is a metallographic photograph showing a large number of precipitated phases (thin film TEM dark field morphology) having diameters of 10nm or less in the steel according to the example of the present invention.

Detailed Description

The invention is further described below with reference to examples and figures.

In the embodiment of the invention, converter smelting, refining and continuous casting are adopted to obtain the chemical compositions of billets, and the chemical compositions are shown in Table 1. The steel billet is heated by a heating furnace, and is coiled to obtain a hot rolled steel strip after rough rolling descaling, side pressure width fixing, rough rolling, flying shear, finish rolling descaling, finish rolling and layer cooling control cooling, and the corresponding technological parameters are shown in table 2. The example steels were subjected to a performance test and the relevant properties are shown in table 3.

The steel test of the examples and the relative corrosion to Q355B are shown in Table 3 according to TB/T2375, "periodic immersion corrosion test method for weathering steel for railway".

The ingredients, preparation processes and properties related to comparative example 1 (Q355B), comparative example 2 (Q460 NQR 1), comparative example 3 (CN 201910243121.1), comparative example 4 (CN 201510031313.8), and comparative example 5 (CN 200710045329. X) are shown in tables 1 to 3.

Referring to fig. 3 and 4, metallographic photographs of the microstructure of the steel according to the embodiment of the present invention are shown. FIG. 5 shows a typical TEM phase of precipitated particles below 10nm in the steel according to the example of the invention.

Comparative example 1 is the composition and properties of Q355B, and comparative example 2 shows the properties and relative corrosion rates of general corrosion resistant steels as a comparative reference for the dip-cycle test. Comparative examples 3-5 are realistic ranges of composition, process, and properties from the examples data in the various inventive patents.

Comparison of the inventive examples with comparative examples 1, 2 shows that the invention has a lower relative corrosion rate.

Compared with comparative examples 3-5, the composition design of the present invention is greatly different from that of the prior patent, and the performance difference is also great.

Comparative example 3 is still a general weathering steel having low strength properties, and comparative example 4 also uses a high Ti design to achieve high strength of 700MPa, but has a higher Si content design and a lower Cr content design, so the patent specification does not suggest a high weathering design.

Comparative example 5 is a 700MPa strength high weathering steel, the weathering resistance of which is also twice that of the common corrosion resistant steel, but the composition design is quite different from the design of the invention, the conventional strengthening elements are quite low, the contents of C, mn and Ti are quite low, which is an order of magnitude lower than the design of the invention, and the strengthening mechanism of the invention is difficult to be clarified; the design principle disclosed in the specification is also quite different from that of the invention, for example, the design of ultra-low C is adopted to inhibit the formation of pearlite structure and other carbides, ensure the microstructure of steel to be a homogeneous structure, avoid the corrosion of primary batteries caused by the potential difference between heterogeneous phases and improve the corrosion resistance of the steel; it is considered that Mn content exceeding 0.05% results in an increase in hardenability resulting in deterioration of weldability and toughness of weld heat affected zone; the purpose of adopting Ti is to grow austenite grains in the slab reheating process, inhibit ferrite grains in the recrystallization rolling control process, and improve the toughness of steel. These are all significantly different from the C-Mn-Ti design strengthening purposes of the present invention. In addition, the invention employs a higher Cr content of 4.5-5.5%, although lower relative corrosion rates are also achieved. However, in the present invention, when Cr exceeds 4.5%, the effect is not as good as that of Cr content of 4.5% or less in the case of corrosion under long-term atmospheric exposure (as shown in FIG. 2), and the mechanism is also clarified in the above-mentioned effect of Cr. Therefore, the technical scheme of the invention can meet the application requirements of long-period atmospheric corrosion resistance and high strength and light weight.

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Claims (7)

1. The high-strength high-formability super-atmospheric corrosion-resistant steel comprises the following components in percentage by weight: c: 0.04-0.10%, si is less than or equal to 0.50%, mn: 0.20-1.80%, P is less than or equal to 0.03%, S is less than or equal to 0.01%, al is less than or equal to 0.30%, cu: 0.10-0.60%, cr: 1.5-4.5%, ti: 0.04-0.18%, N is less than or equal to 0.008%, and the balance comprises Fe and other unavoidable impurities; and at the same time, the requirements are as follows: c is more than or equal to 0.08% -Ti/3, mn+Cr is more than or equal to 2.0%;

the microstructure of the super-atmospheric corrosion resistant steel is ferrite, pearlite or bainite with carbide and MA as main components, wherein the ferrite content is 55-90%, the bainite content is 5-35%, and TiC precipitation particles with the diameter less than or equal to 10nm are dispersed in the microstructure;

the relative corrosion rate of the super atmospheric corrosion resistant steel pericycle test and the common structural steel Q355B is less than or equal to 30%, the weather resistance is more than three times that of the common structural steel Q355B, the yield strength is more than or equal to 700MPa, the tensile strength is more than or equal to 800MPa, the yield ratio is less than or equal to 0.90, the breaking elongation is more than or equal to 18%, the cold bending performance meets 180 DEG D=1t bending, D is the bending diameter, and t is the thickness of the steel plate; the impact energy at the low temperature impact toughness of minus 20 ℃ reaches more than 60J.

2. The high strength, high formability super atmospheric corrosion resistant steel of claim 1 further satisfying: si+2Ni is more than or equal to 0.10 percent.

3. The high strength, high formability, super atmospheric corrosion resistant steel according to claim 1 or 2 further comprising at least one of Nb 0.06%, V0.15%, mo 0.40% or B0.002%.

4. The high strength, high formability, super atmospheric corrosion resistant steel of claim 1 or 2 further comprising at least one of Ni equal to or less than 0.40%, sb equal to or less than 0.15%, re equal to or less than 0.15%, ca equal to or less than 0.015%, or Mg equal to or less than 0.015%.

5. The high strength, high formability, super atmospheric corrosion resistant steel of claim 3 further comprising at least one of Ni equal to or less than 0.40%, sb equal to or less than 0.15%, re equal to or less than 0.15%, ca equal to or less than 0.015%, or Mg equal to or less than 0.015%.

6. The method for manufacturing high-strength high-formability super atmospheric corrosion resistant steel according to any one of claims 1 to 5, comprising the steps of:

1) Smelting and casting

Smelting and casting into slabs according to claim 1 or 2 or 3 or 4 or 5;

2) Slab heating

The billet is controlled to be heated up rapidly, and the heating time when the temperature passes through the range of 1050-1150 ℃ is lower than 15min; heating time above 1230 ℃ is required to be 30-60 min, and tapping temperature is 1230-1280 ℃;

3) Hot rolling

The slab is discharged out of the heating furnace for width setting and then enters rough rolling, and the pressure of the descaling high-pressure water in the rough rolling stage is more than 20 MPa; the rough rolling outlet temperature is lower than 1080 ℃; then the strip steel enters a finish rolling process, the finish rolling process adopts a multi-frame continuous rolling process, and the final rolling temperature is controlled to be 820-900 ℃;

4) Cooling and coiling

Laminar cooling is adopted for cooling, and the cooling mode is controlled to be front-stage cooling, wherein the temperature of the outlet water is required to be higher than 650 ℃; cooling and coiling the strip steel, wherein the coiling temperature is 550-660 ℃; after the layer cooling is finished, the strip steel is cooled by hot coil stacking, slow cooling wall or heat preservation cover.

7. The method for producing high strength and high formability super atmospheric corrosion resistant steel according to claim 6, wherein the sheet billet is heated by soaking in a soaking furnace of a sheet billet continuous casting and rolling line.

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