CN115161551A - High-strength high-formability super-atmospheric corrosion resistant steel and manufacturing method thereof - Google Patents

Abstract

A high-strength high-formability super-atmospheric corrosion resistant steel and a manufacturing method thereof are disclosed, which comprises the following components by weight percent: c: 0.04-0.10%, si is less than or equal to 0.50%, mn:0.20 to 1.80 percent, less than or equal to 0.03 percent of P, less than or equal to 0.01 percent of S, less than or equal to 0.30 percent of Al, cu:0.10 to 0.60%, cr:1.5 to 4.5%, ti: 0.04-0.18 percent of the total weight of the alloy, less than or equal to 0.008 percent of N, and the balance of Fe and other inevitable impurities; and simultaneously the following requirements are met: c is more than or equal to 0.08 percent and Ti/3, and Mn + Cr is more than or equal to 2.0 percent. The relative corrosion rate of the steel immersion test and the common structural steel Q355B is less than or equal to 30 percent, and the weather resistance is more than 3 times of that of the common structural steel Q355B and more than 2 times of that of the common weathering steel Q450NQR 1; the corrosion rate is rapidly reduced 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 fracture elongation is more than or equal to 18 percent, the cold bending property meets 180-degree D =1t, and the low-temperature impact toughness-20 ℃ impact energy 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 steel structures with the requirement of atmospheric corrosion resistance outdoors, such as containers, railway vehicles, bridges and the like, and the production and the application of the weather-resistant steel have already been in the history of 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 expanded. People replace some steel surface corrosion prevention processes, such as pre-plating zinc or zinc-aluminum-magnesium, post-plating zinc and the like, by using the weather resistance of the atmospheric corrosion resistant steel, so that the naked use of materials is realized, the problems of energy consumption and pollution caused by the plating process are reduced, and the maintenance cost of later-stage corrosion prevention, maintenance and renovation is also reduced. For example, a photovoltaic bracket used in the photovoltaic power generation industry traditionally adopts a post-galvanization process or pre-galvanization of aluminum magnesium, and has the problems of high production cost, energy consumption and pollution in the production process, poor damage and corrosion resistance of notches and the like, high later maintenance cost and the like, and people are seeking a solution for the naked use of weathering resistant steel. Because the material is naked for the photovoltaic bracket and ensures that the material is in service in a maintenance-free environment for 25 years, the material requires very high atmospheric corrosion resistance, the C1-C3 environment generally requires that the relative corrosion rate of the material is doubled compared with that of the existing common weathering steel, the relative corrosion rate with the common structural steel Q355B is less than or equal to 30 percent, and the corrosion depth in 25 years is less than or equal to 0.1mm. At present, no suitable weathering steel can be widely applied to photovoltaic supports.

Similarly, based on the requirement of green and light weight, in order to reduce the material consumption, the steel for photovoltaic supports requires higher strength to replace the current low-strength steel below 500MPa, for example, if the high-strength steel above 700MPa is adopted, the steel consumption can be obviously reduced, the cost for transporting and installing the photovoltaic supports to photovoltaic power plants in remote areas can also be reduced, and the steel has important significance for the construction of photovoltaic power plants.

The high-strength steel is used for realizing photovoltaic brackets and other applications, and the high-strength steel needs to be cold-formed into various sectional structural members, so that the material is required to have good elongation and cold bending performance, which is a challenge for the material with high strength, and therefore, the photovoltaic bracket material also needs to have good cold forming performance while having ultrahigh weather resistance and high strength.

At present, high-strength weathering steel also relates to a plurality of invention patents, but the product 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 specific steps:

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

Chinese patent CN202010116991.5 discloses a high-strength weathering steel applied to railway vehicles, and its purpose is to provide a high-strength weathering steel to solve the technical problem that steel materials in the prior art cannot achieve high strength and high weathering resistance at the same time. 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 percent; the yield strength is 636 MPa-710 MPa, and the tensile strength is 698 MPa-775 MPa. From the viewpoint of the composition design, the main principle is to simultaneously add weather-resistant elements such as Ni, cr, cu, and Sb to improve weather resistance, and to enhance strength by Nb and Ti composite precipitation strengthening. The technical disadvantage is that the Cr content is low, so the weather resistance is still the level of the common weathering steel. The invention also mentions that Sb is adopted to improve the corrosion resistance, but is a harmful element in the steel, and the performance of the steel is deteriorated, especially the low-temperature toughness; meanwhile, the melting point of Sb is low, so that the yield of steel making is difficult to control, and the practicability of the invention is not high.

Chinese patent CN201510031313.8 discloses a hot-rolled high-strength low-nickel-copper ratio thick weathering steel and a production method thereof, wherein the yield strength of the weathering steel is more than or equal to 700MPa, the tensile strength is more than or equal to 750MPa, the elongation is more than or equal to 18%, and the impact absorption power at minus 40 ℃ is more than or equal to 60J,180 degrees D =2t bending pass. The patent still faces to the application requirement of general weathering steel, the Cr content is intentionally controlled to be less than 1.25%, and the adverse effects of high Cr content on smelting cost and difficulty and steel forming performance are avoided. Moreover, the chemical composition adopts higher Si: 0.56-0.64%, but Si is an element which seriously affects the surface quality, and the high content of Si can cause serious red iron sheet on the surface of the strip steel. In the weathering steel, nb:0.052 to 0.058 percent; ti: 0.125-0.135%, utilizes the grain refining and precipitation strengthening effects of niobium-titanium microalloying, but has the Nb content of 0.052-0.058%, higher Nb content and higher production cost.

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 in the field of railway freight cars, containers and bridge construction are met. However, in the composition design, C:0.002-0.005%, the ultra-low C design is adopted to inhibit the formation of pearlite structures and other carbides, the microstructure of the steel is ensured to be a homogeneous structure, the corrosion of primary batteries caused by potential difference between heterogeneous phases is avoided, and the corrosion resistance of the steel is improved; mn:0.01-0.05%, very low, can't play a reinforcing role; ti is less than or equal to 0.03 percent, and the addition of the Ti is to inhibit the growth of austenite grains, inhibit the growth of ferrite grains, improve the toughness of the steel and hardly play a role in precipitation strengthening. Therefore, the patent does not apply the strengthening contribution of C-Mn-Ti, and the Cr content: 4.5-5.5%, the design is very high, and the atmospheric corrosion resistance is doubled compared with the traditional weathering steel by the high Cr. Therefore, the realization principle is that high corrosion resistance and high strength are realized by using the high Cr alloy, which is very uneconomical.

Compared with the prior patents, the current weathering steel is mainly used for railway vehicles, containers, bridges and the like, and part of the patents consider high-strength design, but the weathering resistance requirement and design of the weathering steel are still general weather resistance.

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 weekly immersion test and common structural steel Q355B is less than or equal to 30%, and the weather resistance is more than 3 times of that of the common structural steel Q355B and more than 2 times of that of the common weather resistant steel Q450NQR 1; the corrosion rate is rapidly attenuated along with time and corrosion depth, the corrosion depth of 25 years is less than or equal to 0.1mm, the surface of structural members such as photovoltaic brackets and the like can be free of coating and naked, and the requirement of over 25 years of service on super atmospheric corrosion resistance can be met; meanwhile, the steel has high 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 fracture elongation is more than or equal to 18%, the cold bending performance can meet 180-degree D =1t bending (D is the bending diameter, t is the thickness of a steel plate), and the low-temperature impact toughness-20 ℃ impact energy reaches more than 60J. Therefore, the steel has high strength and high forming performance, is economical and applicable, and can meet the light-weight design requirements of structural members such as photovoltaic brackets and the like and the processing requirements of rolling forming and the like of the structural members.

In order to achieve the 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 1.5-4.5% of high Cr content to promote the formation of a uniform and compact rust layer, the uniform and compact rust layer can be rapidly enriched in a thinner rust layer to reach the concentration of the interface position with a matrix of more than 12%, the corrosion potential and the electrochemical impedance are remarkably improved, the continuous occurrence of corrosion is interrupted, and the super-high atmospheric corrosion resistance is obtained; meanwhile, by utilizing the design of C, ti, mn and high Cr components, the multiphase structure of ferrite, bainite (MA) as a main component, pearlite (or carbide), a large number of nano precipitated particles with the diameter less than or equal to 10nm is realized, and the high strength and the high forming performance of the steel are obtained by utilizing the mechanisms of high plasticity of the ferrite, tiC precipitation strengthening, bainite 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: 0.04-0.10%, 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 to 4.5%, ti: 0.04-0.18 percent of the total weight of the alloy, less than or equal to 0.008 percent of N and the balance of Fe and other inevitable impurities; and simultaneously the following requirements are met:

c is more than or equal to 0.08-Ti/3, and the structure strengthening effect of C can be exerted;

mn + Cr is more than or equal to 2.0 percent, thereby controlling the structure transformation of the material.

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

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

And also contains Ni less than or equal to 0.40 percent, sb less than or equal to 0.15 percent, re less than or equal to 0.15 percent, ca less than or equal to 0.015 percent or Mg less than or equal to 0.015 percent, thereby further improving the corrosion resistance.

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

The super atmospheric corrosion resistant steel has super high atmospheric corrosion resistance, the relative corrosion rate of a periimmersion test and common structural steel Q355B is less than or equal to 30%, and the weather resistance is more than 3 times of that of the common structural steel Q355B and more than 2 times of that of the common weathering steel Q450NQR 1; the corrosion rate can be quickly reduced along with time and 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 fracture elongation is more than or equal to 18%, the cold bending performance meets 180-degree D =1t bending (D bending diameter, t is the thickness of a steel plate), and the low-temperature impact toughness-20 ℃ impact energy reaches more than 60J.

In the component design of the super atmospheric corrosion resistant steel of the invention:

c, effective strengthening elements in the steel can form nano-scale second-phase precipitated particles with micro-alloy elements such as Ti, nb and the like besides solid solution strengthening, and play roles of precipitation strengthening and structure 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 economic strengthening element, the content of C designed by the invention is more than or equal to 0.04 percent; however, excessive C forms more carbide in steel, which reduces the toughness and the forming performance of the material, reduces the corrosion resistance of the steel under the action of a galvanic cell and reduces the welding performance of the steel, so that the content of C is designed to be less than or equal to 0.10 percent.

Si, a common deoxidizing element in steel, has a solid solution strengthening effect on the steel, can also improve the corrosion resistance of the material, and has a certain effect on reducing copper brittleness, but the high Si content can cause the surface of the steel to generate serious red iron sheet defects, influence the surface quality of strip steel, reduce the welding performance of the material and cause the toughness deterioration of a welding heat affected zone, so the Si content is designed to be less than or equal to 0.50 percent;

mn is an important strengthening and toughening element in steel, has the effect of solid solution strengthening, can also reduce the transformation temperature of super-cooled austenite, reduce the transformation temperature of ferrite, is beneficial to structure refinement, and improves the strength and toughness of the material. However, an excessive Mn content suppresses ferrite transformation and makes the structure transformed into bainite, which deteriorates 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 improving the weather resistance of the weathering steel is two points: firstly, corrosion resistance elements are added to improve the corrosion potential of a matrix, and electrochemical impedance reduction and corrosion rate are improved; and secondly, a compact rust layer is formed on the surface to realize physical barrier to a corrosive medium, the corrosive environment of the position of the substrate 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 percent, a uniform and compact rust layer can be formed on the surface of the matrix under the comprehensive action of elements such as Cr, cu and the like, the higher Cr content is matched with the action of the elements such as Cu and the like, and alpha-FeOOH in the rust layer is very fine, so that the further permeation of electrochemical corrosion media such as water and the like is favorably blocked, and the electrochemical impedance is improved. Meanwhile, 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 substrate reaches more than 12 percent before the rust layer reaches 0.1mm. When the enrichment concentration of Cr reaches more than 12%, the effect similar to that of stainless steel occurs, the corrosion potential of the corrosion front interface is very high, the barrier electrochemical impedance of a compact rust layer low corrosion medium is very high, and the corrosion reaction is basically interrupted. However, as the Cr content of the substrate increases, the corrosion potential of the substrate increases, and if the corrosion potential exceeds 4.5%, selective corrosion at the initial generation stage of the rust layer increases, and the uniformity of the thickness of the rust layer deteriorates, that is, the chemical impedance becomes inconsistent due to the Cr enrichment concentration at the front edge of the rust layer and the corrosion environment, which conversely increases the corrosion potential difference at the front edge of corrosion, and thus aggravates the occurrence of the galvanic cell effect, and deteriorates the corrosion inhibition effect. In this case, although the relative corrosion rate under the limited number of the immersion test conditions is lowered, the depth of corrosion is not lowered in a practical long-term view, and the significance of improving the weather resistance is lost. Therefore, the present invention requires that the Cr content be 1.5 to 4.5%.

Referring to fig. 1 and 2, fig. 1 shows the effect of Cr content on the relative corrosion rate in the immersion test, and fig. 2 shows the effect of Cr content on the corrosion depth under long cycle conditions. As can be seen, the corrosion rate of the steel according to the invention decays rapidly with time and corrosion depth, the 25-year simulated corrosion depth estimate is less than or equal to 0.1mm, whereas the 25-year simulated corrosion depth estimate for the comparative example with a higher Cr content of 5% is about 0.12mm, but instead decreases.

Further, cr is also an element that increases the hardenability of steel, and it is considered that excessively high Cr impairs the performance of steel. The invention discovers that the high Cr content, in combination with Mn, can enable the steel to form air-cooled bainite or air-cooled martensite at a low cooling rate, thereby obviously improving the tensile strength of the material, simultaneously reducing the yield ratio of the material, 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 function of Cr, and is matched with the design of elements such as C, mn and the like, thereby further improving the strength and reducing the addition of other strengthening elements.

Cu, one of important corrosion resistance elements, is added together with Cr to give a more remarkable effect. Cu can promote the formation of a compact rust layer on the surface of the steel, and the atmospheric corrosion resistance of the steel can be obviously improved by adding more than 0.10 percent of Cu. However, cu is a metal with a lower melting point, and a copper-rich phase with a lower melting point is easy to enrich on the surface of a matrix due to the problem of selective oxidation when strip steel is heated, and copper brittle network cracks and warping 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 designed to be 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 improve the atmospheric corrosion resistance of the steel, but 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. On the same principle, P is also very unfavorable for welding performance, so the invention does not adopt the atmospheric corrosion resistant steel function of P, requires reducing the content of P in steel as much as possible, and controls the content of P to be less than or equal to 0.03 percent.

S, common harmful impurity elements in steel have adverse effects on weather resistance, low-temperature toughness, welding performance, cold formability and the like, so that the content of S is required to be less than or equal to 0.01 percent.

Al is an effective deoxidizing element, and is favorable for refining grains and improving the toughness of steel, but higher Al is not favorable for casting blank in continuous casting and is easy to block a water gap, so the Al content is required to be less than or equal to 0.30 percent.

Ti, a strong carbonitride forming element, can be precipitated as very fine TiC or Ti (C, N) second phase particles, significantly improving the strength of the material. Since Ti is inexpensive relative to microalloy elements such as Nb, it is added as an important reinforcing element in the present invention. Meanwhile, tiC is separated out, so that the formation of carbide or pearlite with larger size by free C is remarkably reduced, the galvanic effect of a 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 improvement of the corrosion resistance of the material is facilitated while the strength is improved. However, if the content is too high, the precipitation strengthening effect of Ti gradually weakens, and the low-temperature toughness of the steel begins to be significantly affected. Therefore, the Ti content is designed to be 0.04-0.16%.

N, impurity elements in steel, N and Ti can be combined in molten steel to form TiN inclusions with larger sizes, which can reduce the effective content of Ti on one hand, and the TiN inclusions can also obviously damage the toughness of the steel on the other hand, 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 percent.

Nb, a strong element forming nitride, also forms NbC and Nb (CN) carbide particles, and exerts a precipitation strengthening effect. But the cost of Nb is far higher than that of Ti, so the economical efficiency of improving the strength is not better than that of Ti, and meanwhile, the too high content of Nb can influence the casting blank quality in the continuous casting and cooling process of strip steel and generate defects of surface cracks, angular cracks and the like, so the designed Nb content is less than or equal to 0.06 percent.

V, a strong carbide former, 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 ℃, so that V can be adopted to improve the strength of the strip steel by adopting lower-temperature coiling, but the economy of improving the strength by V is inferior to that of Ti, and meanwhile, the welding toughness of steel can be reduced by excessively high content of V, so that the content of V is designed to be less than or equal to 0.15 percent.

Mo, a common alloy element 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 Mo is beneficial to improving the performance of steel, but Mo is a noble metal element, and the content of Mo is too high, so that the economical efficiency of steel is not facilitated, and the content of Mo is limited to be less than or equal to 0.40 percent.

B has strong hardenability, can obviously inhibit ferrite transformation, thereby obtaining a bainite structure to improve the strength, but reduces the plasticity and the processing performance of the material. Therefore, the invention limits B to be less than or equal to 0.002 percent.

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

Sb may be added as an element for improving corrosion resistance. However, sb is also a harmful element in steel and deteriorates the properties of steel, particularly the low-temperature toughness, so that the content of Sb is limited to be less than or equal to 0.15 percent.

Re rare earth element, which has the functions of improving corrosion resistance and improving material toughness. However, the yield of the Re element in the steelmaking process is difficult to control, and the economic efficiency of steel can be reduced by excessively adding the Re element, so that the Re content is limited to be less than or equal to 0.15.

Ca and S can form CaS in spherical dispersed distribution, 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%.

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

In addition, the composition design of the steel of the invention also needs to meet the following requirements:

c is more than or equal to 0.08-Ti/3, and the strength of the steel can be flexibly ensured. The main strengthening elements of the invention are C-Ti-Mn-Cr, and Mn and Cr mainly play a role in regulating the structure 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 the different Ti contents. The invention requires that C is more than or equal to 0.08% -Ti/3, and aims to ensure that C with enough allowance is used for realizing the supplement of the reinforcement besides the TiC reinforcement. After TiC formation, the free C decreases by Ti/3, the remainder being C capable of forming a certain amount of bainite (MA) phase in the following structure transformation. If the Ti content is lower, which means more C-supplementary strength in order to achieve higher strength, the lower limit of the C content is required to be 0.08% -Ti/3, which proves to be well satisfactory for the strength design of the steel of the present invention.

Mn + Cr is more than or equal to 2.0 percent, and both 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, thereby obtaining higher strength and lower yield ratio. The invention requires that Mn + Cr is more than or equal to 2.0 percent, and aims to ensure the strength of the invented steel and obtain more stable strength performance under a larger process window. Although TiC precipitation has high contribution to strength, the TiC precipitation needs more process conditions, such as higher heating temperature and longer heating time to ensure the sufficient solid solution of Ti element, and the steel strip needs to ensure the sufficient slow cooling process to ensure the slow precipitation of TiC after being coiled. These fluctuations in production conditions cause fluctuations in the properties of the steel, resulting in large differences in the strength of the material. Through hot mold simulation and CCT analysis, under the condition that Mn + Cr is more than or equal to 2.0%, the C which is combined with Ti to form TiC and is remained after precipitation can be converted into MA-based bainite instead of carbide in the cooling process of the strip steel after the strip steel is coiled, so that the ideal multi-phase 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 can stabilize the strength performance is that when Ti is in insufficient solid solution or the slow cooling condition after coiling is insufficient, more C is reserved, and Mn + Cr is more than or equal to 2.0% to ensure that more C is converted into MA-based bainite to complement the strength.

Si +2Ni is more than or equal to 0.10 percent, and because Cu is easy to form copper brittleness defects on the surface of strip steel, si and Ni both have the function of improving the copper brittleness defects and can complement each other. Ni has a better effect on improving copper brittleness, but the cost is higher. When the content of Si +2Ni is more than 0.10 percent, the effect can be achieved, so that when 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 percent, and the balance between the material design economy, the material surface quality and the copper brittleness problem can be adjusted by utilizing the complementary relation between the Si +2Ni and the Ni, so that the copper brittleness problem is economically controlled.

The invention relates to a method for manufacturing 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) Heating of slabs

Controlling the billet to rapidly heat up, wherein the heating time of the billet at 1050-1150 ℃ is less than 15min; the heating time is required to be 30-60 min at the temperature of more than 1230 ℃, and the tapping temperature is 1230-1280 ℃;

3) Hot rolling

The plate blank is taken out of the heating furnace, subjected to rough rolling after being subjected to width setting, and subjected to descaling high-pressure water at the rough rolling stage, wherein the pressure is more than 20 MPa; the outlet temperature of rough rolling is lower than 1080 ℃; then the strip steel is subjected to finish rolling, a multi-frame continuous rolling process is adopted for the finish rolling, and the finish rolling temperature is controlled to be 820-900 ℃;

4) Cooling and coiling

Laminar cooling is adopted for cooling, the cooling mode is controlled to be front-stage cooling, and the temperature of outlet water is required to be above 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 in a hot coil stacking mode, a slow cooling wall mode or a heat preservation cover mode.

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

In the manufacturing method of the present invention:

and (3) heating the plate blank obtained by smelting and continuous casting in a heating furnace of a conventional hot rolling production line or soaking in a soaking pit of a thin plate blank continuous casting and continuous rolling production line. In order to reduce the influence of copper brittleness, the temperature rise curve of the steel billet in the heating furnace is optimized, and the contradiction between the surface copper brittleness and the sufficient solid solution strengthening of Ti is coordinated by adopting a high-temperature quick-firing process. And controlling the heating time of the billet steel at 1050-1150 ℃ for less than 15min, so as to quickly pass through the sensitive temperature range generating copper brittleness. The heating time of the billet steel is required to be 30-60 min at the heating temperature of 1230 ℃ or above, and the tapping temperature is 1230-1280 ℃.

The steel billet is taken out of a heating furnace and subjected to rough rolling after being subjected to width setting, the rough rolling stage needs to ensure high enough descaling pressure to obtain a good descaling effect, the production proves that high-pressure water with the pressure of more than 20MPa has good crushing and removing effects on primary oxide scales with high Cr content on the surface of the steel billet, and the removal of the oxide scales has obvious benefits on reducing the copper brittleness problem caused by Cu enriched on the surface layer, so that the pressure of the descaling high-pressure water is required to be more than 20 MPa.

And (3) finishing the subsequent strip steel, wherein the finishing rolling adopts a multi-frame continuous rolling process, and the finishing temperature is controlled to be 820-900 ℃ according to different thickness specifications of the rolled strip steel. Due to the design that the C of the steel is more than or equal to 0.8-Ti/3 and the Mn + Cr is more than or equal to 2.0 percent, the required performance can be obtained under a wider process window. When the coiling temperature is changed within a large range of 550-660 ℃, the strength of the strip steel can realize relatively small change through the balance of precipitation strengthening and structure strengthening.

The steel produced by the process obtains a multi-phase structure of ferrite, bainite (MA) and pearlite (or carbide), wherein the ferrite content is 55-90%, the bainite content is 5-35%, and a large amount of TiC precipitated particles which are less than or equal to 10nm are dispersed and distributed in the structure. The steel has high ferrite content, so that the material has high plasticity, the fracture elongation is more than or equal to 18%, and the cold bending test meets the requirement of cold bending at 180 degrees D =1t, thereby having 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 the strength through a bainite structure taking MA as a main component 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; moreover, 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, and is very favorable for improving the forming dimensional stability of the material. Due to the addition of Cr and Ti and the control of the rolling process, the material structure is fine, so the low-temperature toughness is also good, and the impact energy at the temperature of minus 20 ℃ can reach more than 60J.

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

the invention adopts the economic component design, utilizes the components of C-Mn-Ti-Cu and the like and adopts the design of 1.5-4.5 percent of high Cr content to promote the formation of a uniform and compact rust layer, and can quickly enrich the Cr concentration to more than 12 percent at the interface position of a matrix in a thinner rust layer with the thickness of less than or equal to 0.1mm; meanwhile, the corrosion potential and the electrochemical impedance are obviously improved by uniformly compacting the rust layer and enriching the front edge of the rust layer with high concentration of Cr, and the continuous occurrence of corrosion is interrupted, so that the ultrahigh atmospheric corrosion resistance is obtained. The relative corrosion rate of the immersion test and the common structural steel Q355B is less than or equal to 30 percent, and the weather resistance is more than 3 times of that of the common structural steel Q355B and more than 2 times of that of the common weathering steel Q450NQR 1; and the corrosion rate is rapidly reduced along with the formation of the 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 from coating and naked, 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 to economically improve the strength of the material. The bainite with MA as the main part after coiling is obtained by utilizing the design of the residual C with C being more than or equal to 0.8-Ti/3 and the influence of Mn + Cr being more than or equal to 2.0% on the critical cooling speed of phase transformation, the contribution of TiC precipitation strengthening and the structure strengthening of the bainite with MA as the main part can be balanced, and the required high strength performance can be obtained under a wider process window. The steel of the invention has a multi-phase structure of 55-90% ferrite, 5-35% bainite (MA is main) and pearlite (or carbide) as a microstructure and a large number of TiC precipitation particles with the diameter less than or equal to 10 nm. The TiC precipitation and the fine dispersed MA relative strength are greatly improved, the ferrite-based structure 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 fracture elongation is more than or equal to 18%, the cold bending performance can meet 180-degree D =1t bending (D is the bending diameter, and t is the thickness of a steel plate), the low-temperature impact toughness-20 ℃ impact power can reach more than 60J, and the lightweight design requirements of structural members such as photovoltaic supports and the like and the processing requirements of roll forming and the like of the structural members can be met.

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

Drawings

FIG. 1 is a graphical representation of the effect of different Cr contents on the relative corrosion rate of a weekly leaching test;

FIG. 2 is a schematic illustration of corrosion depths for steels of different Cr contents during extended periods of atmospheric exposure;

FIG. 3 is a metallographic photograph of a typical structure 1 of a steel according to an example of the invention;

FIG. 4 is a metallographic photograph of a typical structure 2 of a steel according to an example of the invention;

FIG. 5 is a metallographic photograph of a large number of precipitates (thin film TEM dark field morphology) having diameters of less than 10nm in steel according to an embodiment of the present invention.

Detailed Description

The invention is further illustrated by the following examples and figures.

In the embodiment of the invention, the chemical components of the obtained steel billet are shown in table 1 by adopting converter smelting, refining and continuous casting. The steel billet is heated by a heating furnace, and then is subjected to rough rolling descaling, side pressure width setting, rough rolling, flying shear, finish rolling descaling, finish rolling, layer cooling control cooling and then is coiled to obtain a hot rolled steel strip, and corresponding process parameters are shown in table 2. The example steels were subjected to property tests and the relevant properties are shown in Table 3.

The corrosion of the steel of the examples, as measured by TB/T2375 "periodic infiltration Corrosion test method for weathering steel for railways", relative to Q355B is shown in Table 3.

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), comparative example 5 (cn200710045329. X) are shown in tables 1 to 3.

Referring to fig. 3 and 4, metallographic images of the microstructure of the steel according to the example of the present invention are shown. FIG. 5 shows typical TEM appearances of precipitated particles of 10nm or less in steels according to examples of the present invention.

Comparative example 1 is the composition, properties of Q355B, and comparative example 2 shows the properties and relative corrosion rate of general corrosion-resistant steel as a comparative reference for the immersion test. Comparative examples 3-5 are realistic composition, process, performance ranges from the example data in the different invention patents.

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

Compared with comparative examples 3-5, the invention has larger difference of component design and performance compared with the prior patent.

Comparative example 3 is still a general weathering steel of low strength property, and comparative example 4 is also designed to achieve high strength of 700MPa using high Ti, but Si content is designed to be high and Cr content is designed to be low, so that design of high weather resistance is not proposed in the patent specification.

Comparative example 5 is a high weathering steel of 700MPa strength, the weathering resistance is also one time of that of the ordinary corrosion-resistant steel, but the difference from the design of the invention is very large in view of the component design, the conventional strengthening elements are very low, the contents of C, mn and Ti are very low, and are all one 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 greatly different from the invention, for example, the design of adopting ultra-low C is characterized in that the formation of pearlite structure and other carbides is inhibited, the microstructure of the steel is ensured to be homogeneous structure, the corrosion of a galvanic cell caused by potential difference between different phases is avoided, and the corrosion resistance of the steel is improved; it is considered that the Mn content exceeding 0.05% causes the increase of hardenability to deteriorate weldability and weld heat-affected zone toughness; the purpose of adopting Ti is to grow austenite grains in the reheating process of the plate blank, inhibit the growth of ferrite grains in the recrystallization controlled rolling process and improve the toughness of steel. These are all clearly different from the C-Mn-Ti design reinforcement objectives of the present invention. In addition, the invention uses a higher Cr content, 4.5-5.5%, although a lower relative corrosion rate is also obtained. However, in the present invention, the effect is not as good as the effect of the Cr content of 4.5% or less in the case of corrosion under long-term atmospheric exposure (see FIG. 2) when Cr exceeds 4.5% or more, and the mechanism is clarified by the action of Cr as described above. Therefore, the technical scheme of the invention can better meet the application requirements of atmospheric corrosion resistance in a long period and high strength and light weight.

Claims (8)

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 to 0.60%, cr:1.5 to 4.5%, ti: 0.04-0.18 percent of the total weight of the alloy, less than or equal to 0.008 percent of N and the balance of Fe and other inevitable impurities; and simultaneously needs to satisfy: c is more than or equal to 0.08 percent to Ti/3 percent, and Mn + Cr is more than or equal to 2.0 percent.

2. The high strength, high formability and superatmospheric corrosion steel as recited in claim 1, further satisfying: si +2Ni is more than or equal to 0.10 percent.

3. The high-strength high-formability superatmospheric corrosion steel as set forth in claim 1 or 2, further comprising at least one of Nb of 0.06% or less, V of 0.15% or less, mo of 0.40% or B of 0.002% or less.

4. The high-strength high-formability superatmospheric corrosion steel as set forth in claim 1, 2 or 3, further comprising at least one of Ni 0.40% or less, sb 0.15% or less, re 0.15% or less, ca 0.015% or Mg 0.015% or less.

5. The high-strength high-formability superatmospheric corrosion steel as claimed in any one of claims 1 to 4, wherein the microstructure of the superatmospheric corrosion steel is bainite mainly composed of ferrite + pearlite or carbide + MA, wherein the ferrite content is 55 to 90%, the bainite content is 5 to 35%, and a large number of TiC precipitated particles with a diameter of 10nm or less are dispersed in the microstructure.

6. The high-strength high-formability superatmospheric corrosion steel as set forth in any one of claims 1 to 5, wherein the relative corrosion rate of the superatmospheric corrosion resistant steel in a soaking test with a general structural steel Q355B is not more than 30%, the weather resistance is more than three times that of the general structural steel Q355B, the yield strength is not less than 700MPa, the tensile strength is not less than 800MPa, the yield ratio is not more than 0.90, the elongation at break is not less than 18%, the cold bending property satisfies 180 degrees D =1t bending, D is the bending diameter, and t is the steel plate thickness; the low-temperature impact toughness reaches more than 60J at the temperature of minus 20 ℃.

7. The method for producing a high-strength high-formability superatmospheric corrosion steel as recited in any one of claims 1 to 6, comprising the steps of:

1) Smelting and casting

Smelting and casting a slab from the composition of claim 1 or 2 or 3 or 4;

2) Heating of slabs

Controlling the steel billet to rapidly heat up, wherein the heating time when the temperature passes through the range of 1050-1150 ℃ is less than 15min; the heating time is required to be 30-60 min at the temperature of above 1230 ℃, and the tapping temperature is 1230-1280 ℃;

3) Hot rolling

The slab is taken out of the heating furnace and enters rough rolling after being subjected to width setting, and the pressure of the descaling high-pressure water in the rough rolling stage is more than 20 MPa; the outlet temperature of rough rolling 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 finish rolling temperature is controlled to be 820-900 ℃;

4) Cooling and coiling

Laminar cooling is adopted for cooling, the cooling mode is controlled to be front-stage cooling, and the temperature of outlet water is required to be above 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 in a hot coil stacking mode, a slow cooling wall mode or a heat preservation cover mode.

8. The method for manufacturing the high-strength high-formability superatmospheric corrosion steel according to claim 7, wherein the thin slab continuous casting and rolling process is adopted in the steps 1) and 2), and the soaking in a soaking pit of a thin slab continuous casting and rolling production line is adopted for heating the slab.

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