CN118726834A

CN118726834A - Steel suitable for nitriding and high in surface quality and used for aluminum-clad plate strip substrate and production method thereof

Info

Publication number: CN118726834A
Application number: CN202310328897.XA
Authority: CN
Inventors: 宋凤明; 温东辉; 陆敏; 陈建梁; 苏鹏举
Original assignee: Baoshan Iron and Steel Co Ltd
Current assignee: Baoshan Iron and Steel Co Ltd
Priority date: 2023-03-30
Filing date: 2023-03-30
Publication date: 2024-10-01

Abstract

A steel suitable for nitriding high surface quality aluminum-clad plate strip substrate and a production method thereof, wherein the weight percentage of chemical components is ：C 0.01～0.07％,0＜Si≤0.005％,Mn 0.4～1.2％,P0.008～0.020％,S≤0.008％,0＜Al≤0.005％,Cr 0.03～0.60％,N0.011～0.030％,0＜O≤0.01％,Cu 0.01～0.15％,Ca 0.001～0.004％,, one or more of 0.01 to 0.06 percent of Ti, 0.01 to 0.03 percent of Nb and 0.01 to 0.03 percent of V are added, and the balance comprises Fe and unavoidable impurities; and simultaneously satisfies 0.5C+3.83N+O+0.05Cu+0.047Cr-0.89Al-1.15Si is more than or equal to 0.085;150 (C-0.01) +60P+300N+5Mn+3Cr is not less than 12.0. The obtained steel plate has high strength and excellent plasticity and aluminum coating performance, the defects of surface warping, slag inclusion and the like are eliminated, and the aluminum coating plate strip produced by the steel has excellent surface quality and high strength, can meet the subsequent high-temperature nitriding treatment process requirements, and is mainly used for producing frying pans or other aluminum coating plate strips with high strength requirements.

Description

Steel suitable for nitriding and high in surface quality and used for aluminum-clad plate strip substrate and production method thereof

Technical Field

The invention belongs to the field of low alloy steel manufacturing, and particularly relates to steel for a high-surface-quality aluminum-clad plate strip substrate suitable for nitriding and a production method thereof.

Background

The aluminum-clad plate strip is a composite strip formed by cladding an aluminum layer on the surface of strip steel through rolling deformation under the condition of room temperature, and structurally has a steel-aluminum structure as well as a steel-aluminum-steel or aluminum-steel-aluminum structure. The aluminum-clad plate has the strength of the existing steel, has the characteristics of good heat radiation, corrosion resistance, light weight, attractive appearance and the like of aluminum, and greatly reduces the cost compared with pure aluminum materials, so the aluminum-clad plate is widely applied to the fields of radiator fins, corrosion-resistant pipelines, household appliance panels and the like, and gradually expands to the fields of automobile engine shields, new energy automobile parts, atmospheric corrosion resistance, cookers, buildings and the like.

The aluminum-clad substrate is required to have not only good steel-aluminum bonding properties but also high surface and high strength. In the kitchen ware field, the novel frying pan adopting the steel-aluminum-steel structure has the strength of steel, and has the characteristic of good Al heat dissipation, so that the defects of uneven heat dissipation and easy burning of an iron frying pan are avoided, and the frying pan is lighter. To prolong the service life, the surface of the material is usually subjected to nitriding treatment to improve the hardness, which requires that the substrate can adapt to nitriding, a nitriding layer with high hardness is formed on the surface, and the substrate still maintains high strength after high-temperature treatment.

With the development of aluminum-clad base plates and the application of aluminum-clad plate strips, numerous patents for composite plate strips and steel for base plates thereof have also been formed.

Chinese patent CN102019727 discloses "aluminum coated steel strip for cooler, method for preparing same, steel strip and aluminum alloy strip used therein", which describes a method and process for producing composite strip, although referring to the substrate used, it is mainly used for producing aluminum coated strip with thickness of about 1.5mm, its yield strength is lower, and at the same time, nitriding processing requirements are not satisfied.

The substrate used for producing the aluminum-clad strip steel is deformed together with the clad aluminum layer in the production process of the composite strip material, so that the substrate is required to have strength and plasticity equivalent to those of aluminum, particularly to have excellent plasticity, and to have good steel-aluminum bonding performance, and specific requirements are imposed on the composition. The existing conventional low strength steel is difficult to use in the production of aluminum clad sheet strips.

Japanese patent JP2005281806 discloses "low yield point steel with excellent toughness" and a production method thereof, and the disclosed steel belongs to low alloy structural steel with lower yield strength and higher elongation, and the yield strength is generally about 200 MPa. The obtained product is generally a thick plate product, one or more components of chromium (Cr), molybdenum (Mo), nickel (Ni), copper (Cu), boron (B) and other alloys are added on the basis of lower carbon (C) -silicon (Si) -manganese (Mn) in component design, the strength of the obtained steel is lower, the aluminum coating performance is not mentioned, and the steel is mainly used for the production of anti-seismic dampers.

The Chinese patent CN111349869A discloses a high-strength steel for aluminum-coated substrate and a production method thereof, the yield strength of the obtained steel is 210-290MPa, the tensile strength is more than or equal to 320MPa, the elongation is more than or equal to 40%, and the strength is lower than that of the invention. The diffusion of Al is restrained by adding the P element with higher content, so that the steel-aluminum combination property is improved, but the related steel is used for the production of aluminum coated materials, an iron-aluminum compound layer with the thickness not exceeding 5 mu m still exists in a steel-aluminum interface, and the problem of cold brittleness is easily caused by the too high P content, so that the forming and welding properties of the steel plate are not good. Furthermore, the nitriding treatment is not mentioned in this patent, and the characteristic of maintaining high strength after the nitriding treatment is not mentioned.

The high-strength aluminum-clad plate strip and the manufacturing method thereof disclosed in China patent CN107881426A ensure the bonding performance of steel and aluminum by N, O and controlling Si and Al, has yield strength of 280-400MPa, is mainly used for the production of radiator fins, household appliance panels and the like, but has no nitriding characteristic and has unclear performance after high-temperature nitriding.

"A steel for a base plate of an aluminum-clad plate for kitchen ware and a production method thereof", disclosed in chinese patent CN114250411a, the steel plate, although involving a nitriding process, has yield and tensile strength not exceeding 280MPa and 380MPa, respectively; at the same time, the patent requires control of the carbon content below 0.005% and limits P, S to not more than 0.012% and 0.006% respectively, which increases the difficulty of steelmaking and the production cost. The strength in nitriding is improved mainly through precipitation strengthening of V and Mo, and nitriding adaptability of the substrate is not mentioned; functionally, the steel is only limited to processing of aluminum coated materials for frying pans, and is not suitable for other aluminum coated material production with high strength requirements.

The comparison with the prior patent shows that the prior steel for the aluminum-clad plate strip substrate has low strength, poor steel-aluminum combination property, unsatisfied nitriding treatment requirement or poor surface quality of casting blank, and increases the cleaning difficulty of the casting blank.

Disclosure of Invention

The invention aims to provide steel for a high-surface-quality aluminum-clad plate strip substrate suitable for nitriding and a production method thereof, wherein the yield strength is more than or equal to 280MPa, the tensile strength is more than or equal to 340MPa, the elongation is more than or equal to 30%, the steel has high strength, excellent plasticity and aluminum-clad performance, the surface quality is good, the defects of surface warping, slag inclusion and the like are eliminated, the aluminum-clad plate strip produced by the steel has excellent surface quality and high strength, the subsequent high-temperature nitriding treatment process requirements can be met, and the steel is mainly used for production of frying pans or other aluminum-clad plate strips with high-strength requirements.

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

A steel for a high surface quality aluminum-clad plate strip substrate suitable for nitriding comprises the following chemical components in percentage by weight of ：C：0.01～0.07％,0＜Si≤0.005％,Mn：0.4～1.2％,P：0.008～0.020％,S≤0.008％,0＜Al≤0.005％,Cr：0.03～0.60％,N：0.011～0.030％,0＜O≤0.01％,Cu：0.01～0.15％,Ca：0.001～0.004％, and Ti:0.01 to 0.06 percent, nb:0.01 to 0.03 percent, V:0.01 to 0.03%, the balance comprising Fe and other unavoidable impurities; and at the same time satisfies the requirements of,

0.5C+3.83N+O+0.05Cu+0.047Cr-0.89Al-1.15Si≥0.085；

150(C-0.01)+60P+300N+5Mn+3Cr≥12.0。

Further, the balance being Fe and other unavoidable impurities.

The microstructure of the steel for the aluminum-clad plate strip substrate is uniform ferrite and a small amount of bainite, wherein the bainite content is less than or equal to 10%.

The yield strength of the steel for the aluminum-clad plate strip substrate is more than or equal to 280MPa, the tensile strength is more than or equal to 340MPa, and the elongation is more than or equal to 30%.

The steel for the high-surface-quality aluminum-clad plate strip substrate is suitable for nitriding, has the requirements of yield strength and tensile strength of more than 280MPa and more than 340MPa respectively, has the elongation of more than 30 percent, has good steel-aluminum combination property, and has the requirement of high strength after high-temperature heat treatment such as nitriding, so that the content of specific alloy elements is limited as much as possible.

In the design of the chemical composition of the steel for the composite aluminum plate and strip substrate, the invention comprises the following steps:

c improves yield strength through solid solution strengthening and phase change strengthening, and simultaneously inhibits the diffusion of Al in steel and the formation of iron-aluminum compounds. Too high C content is unfavorable for the subsequent stamping, deep drawing and other processing performances of the material; however, higher C tends to be biased in defective positions such as dislocation, and the surface quality is not good. The C content is controlled to be 0.01 to 0.07% according to the actual steelmaking process.

Si is a deoxidizing element and also a solid solution strengthening element, so that the yield strength is increased, the elongation is reduced, and a proper amount of Si is favorable for improving the bonding performance of a steel-aluminum interface, but the inhibition effect of O (oxygen) on the formation of a brittle compound layer of the steel-aluminum interface is weakened, so that Si is controlled to be more than 0 and less than or equal to 0.005 percent.

Mn is also a common strengthening element in steel, and improves yield strength through solid solution strengthening, so that elongation is reduced; the proper amount of Mn can combine with S in steel to generate MnS, so that the hot brittleness of the steel is reduced, but too high Mn solid solution prevents the recovery of a structure, inhibits the growth of recrystallized grains, reduces the strength of gamma texture (ND <111 >), is very unfavorable for the stamping forming of the steel plate, and increases the cost, so the content of Mn is controlled to be 0.40-1.20%.

P is generally controlled as an impurity element in steel, and the addition of P is only allowed in a few special purpose steels. For example, P is added to sulfur-containing free-cutting steel in an amount of not more than 0.15% to make it solid-soluble in ferrite, thereby causing strengthening and embrittlement to improve cutting properties. In addition, P is a main corrosion resistant element in the traditional atmospheric corrosion resistant steel, and P in the steel can accelerate the uniform dissolution of the steel and the oxidation rate of Fe ²⁺, thereby being beneficial to forming a uniform FeOOH rust layer on the surface of the steel, promoting the generation of an amorphous iron oxyhydroxide FeO _x(OH)_3-2x compact protective film and effectively improving the atmospheric corrosion resistant performance of the steel. Since the corrosion resistance is generally optimal when the P content is 0.08 to 0.15%, P is added as a corrosion resistant element to the early part of the atmospheric corrosion resistant steel. However, too much P causes "cold embrittlement" of the steel, deteriorates plasticity and impact toughness, and deteriorates weldability and cold bending property of the steel, so that the addition amount of the steel grade having a high requirement for low-temperature impact toughness or forming property is definitely defined. In recent years, some atmospheric corrosion resistant steels also have reduced P content, and other substitute corrosion resistant elements are selected to ensure corrosion resistance, even if the corrosion resistant steels are controlled as impurity elements, the required content is less than or equal to 0.015%, but controlling the P content at such a low level increases the difficulty of steelmaking and increases the manufacturing cost. The invention considers that P improves the strength of steel and the strength after nitriding by means of solid solution strengthening, phase change strengthening and the like, and simultaneously improves the steel aluminum bonding performance by utilizing the P. Therefore, 0.008 to 0.020% of P is added in the present invention.

S is unfavorable for the performance of steel, is easy to cause the hot brittleness of the steel, reduces the low-temperature toughness of the steel, and simultaneously worsens the aluminum interface bonding performance of the steel. Mn can form MnS with S, so that the hot ductility can be improved by improving the Mn/S ratio, the content is required to be controlled to be as low as possible, and the steel-making difficulty and the production cost are increased in consideration of the excessively low control content, so that the S content is required to be controlled to be less than 0.008%.

Al is an important deoxidizing element in steel, but higher Al is easy to diffuse to the steel-aluminum bonding interface, and the interface bonding strength is deteriorated. Therefore, the content of Al must be controlled within a certain range, so that Al is more than 0 and less than or equal to 0.005 percent.

Cr forms a continuous solid solution with Fe in steel, has a solid solution strengthening effect, and forms various types of carbides such as M ₃C、M₇C₃ and M ₂₃C₆ with C, etc., resulting in a secondary strengthening effect. Meanwhile, the addition of Cr is beneficial to inhibiting the diffusion of Al and improving the bonding performance of steel and aluminum. In addition, cr is also an effective nitriding element, and can simultaneously improve the surface hardness and the nitriding layer depth. The conventional nitriding steel is usually added with up to 3% of Cr, the higher Cr is unfavorable for toughness, the welding difficulty is increased, and the Cr is a noble alloy element, so the content of the Cr is controlled to be 0.03-0.6%.

Cu has the functions of solid solution and precipitation strengthening, and tempering has the secondary hardening effect at proper temperature when the content is higher, so that the strength is improved, and the strength is kept during high-temperature nitriding treatment. Meanwhile, the addition of Cu is also beneficial to improving the steel-aluminum bonding performance. Too high Cu causes cracking of the slab during heating and hot rolling, deteriorating surface properties, so that the content thereof is controlled to be 0.01 to 0.15%.

N can form AlN particles with Al in the steel, so that the effect of binding the Al is achieved, and the diffusion of the Al in the steel is limited; meanwhile, N is similar to C, and is easy to form a Korotkoff gas mass by offset polymerization at a dislocation, so that strain concentration is caused, the phenomenon of uneven strain is generated in the processing process of the aluminum-clad material, the surface quality is affected, and the impact toughness is deteriorated due to the excessively high N, so that the content of the N is controlled to be 0.011-0.030%.

The oxygen (O) element can suppress the adverse effect of Al element on the aluminum-coating property in steel, so that it is required to appropriately add a certain O element. However, too high oxygen can generate defects such as subcutaneous bubbles, looseness and the like, and aggravate the thermal embrittlement effect of sulfur, so that the defects such as skin tilting and slag inclusion are easy to occur on the surface of the strip steel. During solidification of the steel, oxygen will precipitate in large amounts in the form of oxides, reducing the plasticity, impact toughness, etc. of the steel. So control 0 < O the content is less than or equal to 0.01 percent.

Ca can change the shape of sulfide when being added into steel, inhibit the hot shortness of S and improve the toughness. And titanium sulfide or titanium carbosulfide may be formed with sulfide when excess Ti is present in the steel. The effect is not obvious when the Ca content is too low, the size of Ca (O, S) formed by the Ca content exceeding 0.005% is too large, the brittleness is increased, the Ca can become a crack starting point, the purity of the steel is reduced, and the toughness of a welding heat affected zone is deteriorated. Therefore, the content is limited to 0.001 to 0.004%.

In addition to the above elements, the present invention requires the optional addition of Ti:0.01 to 0.06 percent, nb:0.01 to 0.03 percent, V:0.01 to 0.03% by weight of a metal alloy, thereby further improving room temperature strength and strength after heat treatment. Wherein:

Nb is a strong nitrogen carbide forming element, and can combine with carbon and nitrogen in steel to form intermediate phases such as NbC, nb (CN), nbN and the like in the cooling process after rolling, and the formed fine carbide particles can refine the structure, generate fine crystal strengthening and precipitation strengthening effects, and remarkably improve the strength of the steel. When the Nb content is high, coarse carbonitride particles are formed at grain boundaries, and the impact toughness is deteriorated, and the addition may be selected and controlled to be 0.01 to 0.03%.

V is a strong carbon-nitrogen compound forming element and can be separated out in the phase change process. The carbonitride V ₄C₃ formed by V has a lower precipitation temperature than that of Ti carbonitride, and has the functions of preventing dislocation movement, inhibiting grain boundary movement and grain growth. V and C, N and other microalloy elements in the steel are combined to have good precipitation strengthening effect in low-temperature medium-temperature heat treatment. Particularly, the precipitation temperature of V (C, N) is generally not higher than 700 ℃, the lower the temperature is, the finer the precipitation is, the better the strengthening effect is, the nitriding treatment temperature is generally 550-580 ℃, and the carbon nitrogen compounds of V are precipitated in a large quantity at the temperature, so that the aluminum-clad plate strip still has good strength after nitriding treatment. N forms BN and AlN with B, al in the steel preferentially, V is added to form VN with the rest N in the steel, and C, N compound of V is used for precipitation strengthening, so that high strength is ensured during nitriding treatment. Meanwhile, the addition of V can obviously improve the hardness of the nitriding layer, deepen the depth of the permeation layer, and has good toughness and impact resistance. When the V content is high, coarse carbonitride particles are formed, and the impact toughness of the weld heat affected zone is significantly deteriorated, so that the content thereof is limited to 0.01 to 0.03%.

Ti has high chemical activity, is easy to form a compound with C, N, O, S and the like, and is separated out in the rolling and cooling processes to improve the strength; meanwhile, fine precipitates can be nailed and rolled on grain boundaries, so that austenite grains are thinned, the growth of grains in a welding heat affected zone is prevented, and the welding performance of steel is improved. Ti has strong affinity with N, can increase the surface hardness and the depth of a permeation layer, improves the nitriding temperature, has great effect on shortening the nitriding time, and is an important added element of the rapid nitriding steel. When the Ti content is too high, titanium nitride particles are easy to grow up and agglomerate at high temperature, and the plasticity and toughness of steel are damaged, so that the Ti content needs to be controlled to be 0.01-0.06%.

The steel provided by the invention has the advantages of high strength, suitability for nitriding and good steel-aluminum combination property, so as to meet the use requirements of a frying pan or other products with high strength requirements, and the yield strength is more than or equal to 280MPa, and the tensile strength is more than or equal to 340MPa. In order to meet the performance requirements, the invention strictly designs components according to the influence of different alloy elements on the performance, and realizes the performance requirements through the comprehensive action of multiple elements. Wherein, the addition of C has the functions of solid solution strengthening and phase change strengthening, and the existing aluminum-coated steel patent (such as China patent CN 114250411A) is to ensure that the plasticity is generally controlled to be less than or equal to 0.005 percent, thus greatly increasing the steelmaking difficulty. The invention adopts 0.01 to 0.07 percent of C in order to ensure high strength after nitriding treatment and simultaneously requires the yield strength of the steel plate to be more than 280 MPa. On one hand, the method plays a role in solid solution strengthening, and meanwhile, forms a ferrite and a small amount of bainite structure in steel through controlled rolling and controlled cooling after rolling in the production process, plays a role in phase transformation strengthening, and improves strength.

Because the aluminum-clad plate strip has annealing heat treatment and subsequent nitriding heat treatment in the production process, the effect of the conventional fine crystal strengthening means is not obvious after the heat treatment, and the solid solution strengthening is not limited by the heat treatment. The higher the steel strength, the higher the strength of the finished aluminum clad material. For this reason, the present invention adds 0.4 to 1.2% of Mn, ensures steel strength through solid solution strengthening of Mn, and such strengthening effect exists even though it is subjected to heat treatment.

In the invention, the content of P is controlled to be 0.008-0.020%, and P is usually controlled as an impurity element in steel, so that the lower the content is, the better the content is, the lower the content of P is, the higher the steelmaking cost is, and the higher the difficulty is. Since P is an element of the same family as N, proper P is advantageous for steel-aluminum bonding properties, but the study of the present invention found that the effect of improving steel-aluminum bonding properties is not linear, and that the deterioration of steel-aluminum bonding properties starts when the P content exceeds 0.020%. Therefore, the control upper limit of P is relaxed to 0.020% by comprehensively considering the steel-making difficulty, cost and performance requirements, so that the steel-aluminum combination performance is ensured, the strength is improved, and the steel-making cost is reduced.

The steel of the present invention is required to have nitriding characteristics, and it is desirable that the steel grade is liable to form a nitrided layer during nitriding. Cr has a secondary strengthening effect in addition to solid solution strengthening in steel. However, the nitriding performance of the steel can be improved by adding 0.03-0.6% of Cr in the invention besides improving the strength. The current nitriding steels such as 35MoCrAl and 42CrMo generally have Cr content of more than 3 percent, cr is a noble alloy element, and the high content obviously increases the cost. The research of the invention shows that Cr can promote the diffusion of N in the matrix and form a nitriding layer rapidly, so that the steel has good nitriding characteristics. The steel provided by the invention is added with a proper amount of N at the same time so as to improve the aluminum binding property of the steel, and the N in the steel is matched with Cr, so that the formation of a nitriding layer is further promoted, the matrix is reinforced, and the strength of the steel is improved. The steel only needs to have the nitriding layer hardness of about 500Hv, and the thickness of the nitriding layer is not more than 100 mu m, so that the steel can be added with 0.03-0.6% of Cr to obtain the required nitriding layer thickness and hardness. Higher Cr plays a rich role and increases costs.

Cu has a solid solution strengthening effect in steel as well, and can enhance strength by precipitation strengthening during heat treatment. However, the invention mainly utilizes the addition of Cu to further improve the steel-aluminum bonding performance. Because the steel grade is subjected to annealing heat treatment and nitriding treatment in the production and use processes, the high temperature in the heat treatment process is easy to promote the formation of brittle iron-aluminum compounds, and the steel-aluminum bonding performance is deteriorated. Therefore, the steel grade has higher requirements on the steel-aluminum combination property than the existing steel for the aluminum-clad plate strip, and the addition of a proper amount of Cu is beneficial to improving the steel-aluminum combination property and improving the strength after nitriding treatment.

According to the strengthening effect of the elements on the steel, the contents of C, P, N and Mn and Cr in the steel are required to be controlled to satisfy the relation 1:150 (C-0.01) +60P+300N+5Mn+3Cr.gtoreq.12.0, and if the value of the relational expression is lower than 12.0, the problem of lower strength is liable to occur.

The formation of the iron-aluminum compound is very sensitive to temperature, and the high temperature during nitriding provides power for the formation of the iron-aluminum compound, so that the formation of the iron-aluminum compound is easier to induce, and the iron-aluminum delamination is caused. Therefore, the steel has higher requirements on the steel-aluminum bonding performance than the existing steel for the aluminum-clad plate strip, and the key of the steel-aluminum bonding performance is to control the formation of brittle iron-aluminum compounds at the steel-aluminum interface in the aluminum-clad plate. The formation of the iron-aluminum compound is closely related to the diffusion of Al in steel, and the formation of the iron-aluminum compound can be inhibited within a certain temperature range by controlling the diffusion of Al. The research of the invention shows that the combination property of steel and aluminum is further improved through the synergistic effect of a plurality of elements such as C, cr, cu, O, N and the like, and O (oxygen) and N can inhibit the diffusion of Al and reduce the formation of brittle iron aluminum compounds, thereby improving the combination property of steel and aluminum. However, too high O can cause defects of submerged bubbles, looseness and the like of a casting blank, and aggravate the hot embrittlement effect of sulfur, so that the defects of skin tilting, slag inclusion and the like are easy to occur on the surface of steel, the O content is limited to be less than or equal to 0.01%, and the bonding performance of steel and aluminum is ensured by using 0.011-0.03% of N. Si and Al as deoxidizing elements hinder the improvement of the aluminum bonding properties of steel by O, so the content thereof should be limited.

According to the invention, through the synergistic effect of multiple elements such as C, cr, cu, O, N and the like, good steel-aluminum bonding performance and good surface quality are obtained. According to C, N, O, cu, cr and the strength of the action of Al and Si on the steel-aluminum bonding performance, the content of the aluminum alloy is required to satisfy the relation 2:0.5C+3.83N+O+0.05Cu+0.047Cr-0.89Al-1.15Si is more than or equal to 0.085, thereby ensuring that the steel has excellent steel-aluminum bonding performance, and if the relational value is lower than 0.085, the aluminum-clad plate strip prepared from the steel is easy to be layered in the subsequent nitriding treatment.

In general, the invention adopts the component system required by the above, comprehensively considers the surface quality, the steel-aluminum combination property, the strength and the nitriding characteristic, and can obtain the steel grade meeting the performance requirement through the combined action of all elements.

The invention relates to a production method of steel for a high-surface aluminum-clad plate strip substrate suitable for nitriding, which comprises the following steps:

1) Smelting

Pretreating molten iron, converting by a converter, refining outside the converter, and casting into a plate blank according to the components;

2) Reheating and controlled rolling

The thickness of the rolled steel strip is more than 2.5mm, the heating temperature is more than or equal to 1230 ℃, and the rough rolling finishing temperature is more than 1050 ℃; the finish rolling starting temperature is over 980 ℃, and the finish rolling ending temperature is 860-900 ℃;

The thickness of the rolled steel strip is less than or equal to 2.5mm, the heating temperature is more than or equal to 1100 ℃, the rough rolling finishing temperature is more than 900 ℃, and the accumulated deformation of the rough rolling stage is more than or equal to 80%; the finish rolling starting temperature is 800-860 ℃ and the finish rolling ending temperature is 710-770 ℃;

3) Cooling is controlled, and the cooling speed is more than 20 ℃/s;

4) Coiling, wherein the coiling temperature is 540-600 ℃.

Preferably, the slab obtained in step 1) is mechanically cleaned.

Preferably, in the step 2) of controlling rolling, the side pressure of the rough rolling stage is controlled within 50 mm.

In the production process of the invention:

pretreatment of molten iron, and P, S removal, so as to ensure low P, S content in steel; the top and bottom of the converter are combined for converting, and controlling the content of C.

The machine cleaning after continuous casting has the functions of eliminating the surface defects of casting blanks, reducing the defects of slag inclusion, skin tilting, pits, scars and the like on the surface of strip steel, improving the surface quality of the strip steel and reducing the surface defects of finished product aluminum coated materials.

According to the phase transition temperature curve of the steel grade and combining the target performance, a rolling process is formulated, and the temperature control parameters of the steel rolling process are determined. The invention relates to a steel grade with lower alloy content, and the continuous cooling curve is shown in figure 1. As can be seen from the figure, the ferrite transformation start temperature of the steel grade is 865 ℃ and pearlite formation starts after the temperature is reduced to 709 ℃, the bainite transformation start temperature is 663 ℃, and when the cooling rate is above 3 ℃/s, only ferrite and bainite are formed.

Aiming at different thickness specifications, the invention designs two rolling processes for the steel grade. For the thickness of the steel strip being more than 2.5mm, adopting a conventional austenite rolling process; and for steel strips with a thickness below 2.5mm, the ferrite rolling process is selected to obtain good strip shape and higher strength.

According to the composition calculation of the steel grade of the invention, the Ar ₃ temperature of the steel grade is 871 ℃, the ferrite transformation temperature is 866 ℃, so that when the austenite rolling process is performed, the finish rolling finishing temperature is required to be controlled to be more than 860 ℃, when the finish rolling finishing temperature is too high, the heating temperature is required to be higher, the energy consumption and the oxidation of the steel strip are increased, and the upper limit of the finish rolling finishing temperature is set to be not more than 900 ℃. The inlet temperature of finish rolling is 980 ℃ and the finishing temperature of rough rolling is 1050 ℃ or above. In order to ensure the finishing temperature of rough rolling and consider the dissolution behavior of microalloy element carbonitride in austenite and the growth behavior of austenite grains in the heating process, the heating temperature is required to be controlled to be more than 1230 ℃.

When the thickness of the steel strip is below 2.5mm, the deformation resistance of the steel strip is low at the high temperature of 860-900 ℃, and the problems of strip breakage, local stretching and side bending are easy to occur at the outlet of a finishing mill. Therefore, the invention adopts the ferrite rolling technology to produce, on one hand, the strength of the strip steel is higher at lower temperature, and the strip steel is not easy to deform under the same coiling tension, thereby reducing the occurrence of narrowing and rolling strip breakage, and simultaneously, the higher strength also allows higher coiling tension, thereby improving the shape of the strip and reducing the occurrence of side bending. Since the entire finish rolling is required to be performed in the ferrite region, the finish rolling is controlled so that the start temperature is not higher than 860 ℃ and the end temperature is not lower than 710 ℃. When the initial temperature of finish rolling is too low, the thermal deformation resistance is high, and the rolling load of the last stand is increased, so that the energy consumption is increased; when the rolling force is too high, the steel strip easily enters an austenite phase region and a ferrite phase region, mixed crystals are easily formed in a matrix, fluctuation of rolling force is caused, and the thickness control and the plate shape of the steel strip are not good. Therefore, in the ferrite rolling process, the finish rolling start temperature is set to 800 to 860 ℃ and the finish rolling end temperature is set to 710 to 770 ℃. According to the finish rolling start temperature, taking the temperature drop of the intermediate billet on a roller way into consideration, the rough rolling end temperature is required to be more than 900 ℃, and the corresponding casting blank heating temperature is controlled to be more than 1100 ℃. In addition, in order to ensure the effect of re-crystallizing and refining grains, the accumulated deformation of the rough rolling stage is required to be more than or equal to 80 percent.

Copper has limited solid solubility in steel and decreases rapidly with decreasing temperature, and C dissolved in austenite at high temperature is precipitated and dispersed in steel in a form of fine epsilon-Cu, and its twinning structure and pinning effect on dislocation improve the strength of steel. Studies have shown that Cu precipitates form at 650 ℃, with lower temperatures resulting in finer precipitate sizes; the lower limit of the coiling temperature cannot be lower than 500 ℃ because the precipitation strengthening effect is optimal at 500 ℃. From the TTT curve of fig. 2, 566 ℃ is the temperature at which bainite is most rapidly transformed. The steel of the present invention is mainly equiaxed ferrite, and a certain amount of bainite structure is desired in the matrix to obtain higher strength, so the coiling temperature is controlled to be about 570 ℃. The added carbonitrides of Nb, V and Ti have good precipitation strengthening effect near the temperature, so that the coiling temperature is set to be 540-600 ℃ by comprehensively considering the precipitation strengthening effect and the phase change strengthening effect, and the cooling speed after rolling is controlled to be more than or equal to 5 ℃/s.

To achieve the desired properties, the structure of the steel is controlled to be a uniform equiaxed ferrite structure and to allow a small amount of bainite structure to form. From fig. 1, ferrite + a small amount of bainitic structure that can be obtained at a cooling rate of 3 ℃/s or more; considering that the rapid cooling can refine the structure and improve the phase change speed, the invention relates to the control of the cooling speed of the steel grade after rolling above 20 ℃/s.

In order to obtain good surface quality of the strip steel, the side pressure of the rough rolling stage is controlled within 50 mm. Meanwhile, the edge and corner positions of the casting blank are required to be free from defects such as air holes and scars, or the surface is required to be cleaned.

In order to inhibit abnormal growth of ferrite grains in the strip steel after finish rolling and ensure that the steel structure is uniform equiaxial ferrite and bainite structure, a front cooling mode is required to be adopted for cooling after rolling. Namely, the strip steel is immediately and densely cooled after exiting the hot continuous rolling unit. The process can obtain uniform ferrite and a small amount of bainite structure, and the specific requirement is that the bainite content is less than or equal to 10 percent, and the excessive bainite content can lead to insufficient plasticity and increase the difficulty of aluminum cladding rolling.

The invention has the following advantages:

1. According to the characteristic that the solid solution strengthening and phase change strengthening effects are not affected by subsequent annealing and nitriding heat treatment, the simple C-Mn design is adopted in the aspect of component design, and C, P, N and Mn and Cr contents are controlled to meet the relational expression: 150 (C-0.01) +60P+300N+5Mn+3Cr is more than or equal to 12.0, and higher strength is obtained. The yield strength of the steel is more than or equal to 280MPa the tensile strength is more than or equal to 340MPa. Meanwhile, the selective addition of Cu, nb, V and Ti ensures the high strength after nitriding heat treatment so as to meet the use requirements of more application fields.

2. The steel provided by the invention is required to meet the requirement of subsequent nitriding treatment, so that the steel-aluminum bonding performance of the steel is required to be higher, and the content of the steel is required to meet the relation formula according to C, N, O, cu, cr and the action of Al and Si on the steel-aluminum bonding performance in the component design: 0.5C+3.83N+O+0.05Cu+0.047Cr-0.89Al-1.15Si is more than or equal to 0.085, and the formation of brittle iron-aluminum compound at the steel-aluminum interface is inhibited, thereby ensuring that the steel has good steel-aluminum bonding performance. The finished aluminum-coated material has clean interface and no iron-aluminum compound formation in the subsequent high-temperature nitriding treatment, processing and using processes.

3. The invention adopts the low-oxygen high-nitrogen component design, limits the O content to be less than 0 and less than or equal to 0.01 percent, and compensates the steel aluminum bonding performance by using 0.011 to 0.03 percent of N. Therefore, the defects of submerged bubbles, looseness and the like of the casting blank with high oxygen components are overcome, the defects of steel surface, skin warping, slag inclusion and the like are reduced, and the surface quality is better.

4. According to the influence of Cr on nitriding and the requirement of a nitriding layer of steel, a small amount of Cr is added: 0.03-0.6%, promoting the diffusion of N in steel, forming nitriding layer rapidly, thus making steel have good nitriding characteristic, improving steel strength at the same time, the steel plate performance meets the subsequent processing use requirement, saving the use of noble alloy elements compared with the existing nitriding steel, reducing the cost, and obtaining good nitriding characteristic.

5. The invention further improves the steel-aluminum bonding performance of the steel by controlling the P content to be 0.008-0.020% and utilizing the solid solution strengthening effect of P and the influence on the steel-aluminum bonding performance, reduces the steel-making difficulty and the steel-making cost and has higher production efficiency.

6. In the invention, in process control, austenite rolling and ferrite rolling processes are respectively adopted for different casting blank thicknesses obtained after rolling, so that the steel has high strength, the shape of a thin steel strip is obviously improved, and the problems of steel strip narrowing, strip breakage, side bending and the like in the rolling process are reduced. The production process is simple, the cost is low, and the method is suitable for large-scale production of enterprises.

7. The process adopts different controlled rolling processes, combines the coiling temperature of 540-600 ℃ and the cooling speed after rolling of more than or equal to 20 ℃/s, so that the obtained steel structure is uniform ferrite and a small amount of bainite structure, wherein the bainite content is less than or equal to 10%, the obtained steel has good deformability, can meet the deformation requirement of over 80% of aluminum-clad rolling, and the stamping deep drawing processing deformation in the subsequent processing process, does not need annealing in the middle, reduces the production procedures and improves the production efficiency.

Drawings

Fig. 1 is a schematic view of static CCT curves (calculations) for the steel grades of the present invention.

Fig. 2 is a schematic view of the static TTT curve (calculation) of the steel grade of the present invention.

FIG. 3 is a photograph of a typical matrix structure of the steel of example 1 of the present invention.

Detailed Description

The invention is further described below with reference to the accompanying drawings.

The embodiment of the invention adopts a 500kg vacuum induction furnace for steelmaking, the chemical compositions of the embodiment steel are shown in table 1, the specific technological parameters are shown in table 2, and the mechanical properties of the embodiment steel are shown in table 3.

As can be seen from FIG. 3, the microstructure of the steel obtained by the composition and the process of the invention is uniform ferrite and a small amount of bainite, wherein the bainite content is less than or equal to 10%.

The composition, production process and performance of the steel are compared with those of similar steel. Wherein,

The comparative steel 1 is Chinese patent CN101514426A which is a low yield point steel for building earthquake resistance with 100MPa yield strength and a production method thereof, has higher Si and Al content and does not contain O.

The comparative steel 2 is a novel special substrate for preparing aluminum-coated steel strips in China patent CN101525720, and the alloy components are too high.

The comparative steel 3 is Chinese patent CN111349869A, which is a high-strength steel for aluminum-coated substrate and a production method thereof, wherein 0.018-0.03% of P is added to the components, an iron-aluminum compound layer with the thickness not more than 5 mu m appears on the steel-aluminum interface, the bonding performance of the steel-aluminum is reduced, and the strength is lower.

The comparative steel 4 is a high-strength aluminum-clad plate strip and a manufacturing method thereof of China patent CN107881426A, and the nitriding characteristics and the strength reduction problem during nitriding are not considered.

The steel grade of the invention has obvious difference in composition from four comparative steels. The steel grade of the invention is specially designed for a component system in order to ensure good steel-aluminum combination property and high-temperature strength after nitriding treatment.

Of the four types of comparative steels, comparative steels 1 and 2 contained high Al and extremely high P, S, N, etc., and the steel-aluminum bonding performance was not considered; p is explicitly added into the comparative steel 3, which is unfavorable for the bonding performance of steel and aluminum and has lower strength; comparative steel 4 relates to an aluminum clad sheet strip, but the problem of strength decrease upon nitriding is not considered, so that high-temperature strength is not considered in composition as in comparative steel 3. The comparative steel grade is therefore markedly different in composition from the steel grade of the invention.

As can be seen from Table 3, the steel of the present invention has stable yield strength, yield and tensile strength of various specifications of steel plates are 280MPa and 340MPa or more under different rolling processes, and has high elongation. The composition design and the function are superior to those of the comparative steel grade, so that the aluminum-clad plate strip produced by the steel has good stamping and deep drawing performance, and is suitable for processing components with high-temperature heat treatment such as nitriding process requirements and high-strength requirements.

From the performance point of view, the comparative steels 1, 2 are only required to have lower yield strength and higher elongation, while the steel grade of the invention is clearly required to have good aluminum coating performance besides the above requirements, and is suitable for the production of aluminum coated strip steel, and annealing is not required in the process, which is also not provided by the comparative steel 2. The comparative steels 3 and 4, although also requiring aluminum coating properties, are mainly used for the production of aluminum coated plate strips such as radiator fins, household appliance panels and the like, and the thickness of the aluminum layer on the surface is not more than 0.1mm; the steel grade related to the invention has wider application field, and neither of the comparative steels 3 and 4 meets the nitriding treatment requirement. Therefore, the steel of the present invention is also significantly different from the comparative steel in terms of properties.

In conclusion, the steel provided by the invention adopts a C-Mn design, and is supplemented with appropriate C, N, V, O, cu, cr and other components, so that the aluminum-coated substrate with yield and tensile strength of more than 280MPa and 340MPa is designed, the requirements of high elongation, good aluminum-coated performance, good surface quality and strong deformability, and the steel is suitable for aluminum-coated plate strip steel subjected to nitriding treatment, and can be also used for other aluminum-coated materials with high strength requirements. And the production period is short, the process is simple, and the performance of the steel meets or exceeds the requirements of the comparison steel types.

In addition, the examples are only illustrative, and the thickness of the steel grade according to the present invention is not limited thereto, and a strip steel having a thinner or thicker thickness may be produced within the allowable range of the apparatus.

Claims

1. A steel for a high surface quality aluminum-clad plate strip substrate suitable for nitriding comprises the following chemical components in percentage by weight of ：C：0.01～0.07％,0＜Si≤0.005％,Mn：0.4～1.2％,P：0.008～0.020％,S≤0.008％,0＜Al≤0.005％,Cr：0.03～0.60％,N：0.011～0.030％,0＜O≤0.01％,Cu：0.01～0.15％,Ca：0.001～0.004％, and Ti:0.01 to 0.06 percent, nb:0.01 to 0.03 percent, V:0.01 to 0.03%, the balance comprising Fe and other unavoidable impurities; and at the same time satisfies the requirements of,

0.5C+3.83N+O+0.05Cu+0.047Cr-0.89Al-1.15Si≥0.085；

150(C-0.01)+60P+300N+5Mn+3Cr≥12.0。

2. The steel for a high surface quality aluminum coated strip substrate suitable for nitriding of claim 1, wherein the balance is Fe and other unavoidable impurities.

3. The steel for a high surface quality aluminum clad sheet strip substrate suitable for nitriding according to claim 1 or 2, wherein the microstructure of the steel for an aluminum clad sheet strip substrate is a uniform ferrite+a small amount of bainite structure, wherein the bainite content is 10% or less.

4. The steel for a high surface quality aluminum-clad plate strip substrate suitable for nitriding as set forth in claim 1,2 or 3, wherein the steel for an aluminum-clad plate strip substrate has a yield strength of not less than 280MPa, a tensile strength of not less than 340MPa, and an elongation of not less than 30%.

5. A method for producing a steel for a high surface quality aluminum-clad sheet strip substrate suitable for nitriding according to any one of claims 1 to 4, comprising the steps of:

1) Smelting

The composition according to claim 1 or 2 is cast into slabs by molten iron pretreatment, converter blowing, external refining;

2) Reheating and controlled rolling

3) Cooling is controlled at a cooling speed of more than 20 ℃/s;

4) Coiling, wherein the coiling temperature is 540-600 ℃.

6. The method according to claim 5, wherein the slab obtained in step 1) is mechanically cleaned.

7. The method according to claim 5, wherein the side pressure of the rough rolling stage is controlled within 50mm in the controlled rolling of the step 2).

8. The method of claim 5, wherein the microstructure of the steel for a base plate of an aluminum-clad plate is a uniform ferrite+a small amount of bainite, and wherein the bainite content is 10% or less.

9. The method of claim 5 or 8, wherein the steel for the aluminum-clad plate and the base plate has a yield strength of 280MPa or more, a tensile strength of 340MPa or more, and an elongation of 30% or more.