345mpa grade hot-rolled strip for building structures resistant to corrosion in splash zone and method of manufacturing the same

Abstract

345MPa grade hot-rolled strip for building structure resisting splash zone corrosion of sea waves and its manufacturing method, the hot-rolled strip includes base layer, corrosion resistant layer and interface transition layer between the base layer and the corrosion resistant layer; the base layer component mass percentage is: C 0.03~0.10%, Si 0.1~0.3%, Mn 1.00~1.65%, P 0.0005~0.003%, S 0.0005~0.01%, Cr 0.02~0.2%, Ni 0.05~0.15%, Cu 0.002~0.020%, Al 0.015~0.03%, Ti 0.008~0.018%, Nb 0.02~0.065%, N 0.0005~0.005%, the balance contains Fe and inevitable impurities; the corrosion resistant layer uses industrial pure titanium.The yield strength of the hot-rolled strip for building structure is greater than or equal to 350MPa, the tensile strength is greater than or equal to 490MPa, the yield strength ratio is 0.71~0.80, the impact energy at-40 DEG C is greater than or equal to 190J, the corrosion rate of sea wave splash is less than or equal to 0.006mm / year, the interface transition layer thickness is less than or equal to 8μm, and the interface shear strength is greater than or equal to 256MPa.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of building structure steel, and particularly relates to a hot-rolled strip steel for building structure resistant to corrosion in splash zone and a manufacturing method thereof. BACKGROUND

[0002] The ocean is a very harsh and complex corrosion environment. Seawater is a strong electrolyte solution containing high concentrations of chloride ions. Steel facilities as the main structure of marine facilities are prone to electrochemical reactions with the surrounding medium and are severely corroded, greatly reducing the service life of these facilities. Especially in the splash zone, which is the most severe marine environment corrosion area, various facilities are subjected to a series of external factors such as dry-wet alternating, sea spray, sunlight, corrosive components in the atmosphere and oxygen, and the corrosion of materials is particularly serious.

[0003] Investigations have shown that the steel piles of facilities such as marine wharfs and offshore oil platforms in China are severely corroded in this area, which is 3-10 times that of the seawater immersion area. Once severe localized corrosion damage occurs in this area, the carrying capacity of the entire facility will be greatly reduced, the service life will be shortened, production safety will be affected, and even the facility will be prematurely scrapped.

[0004] In the splash zone, due to being in the dry-wet alternating zone, oxygen supply is sufficient, and the corrosion products produced have no protective effect; due to the splashing of seawater, the spray can directly hit the metal surface, causing severe corrosion. Corrosion tests and investigation results show that, under normal circumstances, the average corrosion rate of ordinary carbon steel, low alloy steel and the like in the marine atmosphere is about 0.03-0.08 mm / year, and in the splash zone, it is 0.3-0.5 mm / year. Severe corrosion damage easily occurs in the splash zone, greatly reducing the carrying capacity of the entire steel structure and affecting safe production, shortening the service life and prematurely scrapping the facility.

[0005] According to the above working conditions, industrial pure titanium is selected as the corrosion-resistant layer. Titanium has high chemical activity and is easily reacted with oxygen in the air to form an oxide. The oxide on the surface of titanium metal is dense, stable and has strong self-healing ability. The self-healing ability of titanium oxide mainly refers to the fact that after the titanium oxide film at a certain place on the surface of titanium material is damaged, a new titanium oxide film is rapidly generated to prevent the corrosive medium from further contacting titanium.

[0006] For marine construction steel, in addition to the requirement of corrosion resistance, the whole plate also needs to have good and uniform mechanical properties, among which the yield ratio and low temperature impact toughness are becoming the focus of attention. The yield ratio is the ratio of yield strength to tensile strength of steel, which reflects the ability of steel to not produce strain concentration during plastic deformation. The lower the yield ratio, the more uniform the plastic deformation of steel can be distributed to a wider range. The plastic deformation of steel structure system made of steel with low yield ratio can be uniformly distributed to a wider range under the action of earthquake force; while the material with high yield ratio may have strain concentration, which reduces the overall plastic deformation of steel, thereby leading to brittle failure of the structure and sudden collapse of the structure at room temperature. Steel will undergo brittle transition at low temperature, and the fracture mode of steel will change from ductile fracture to brittle fracture. The engineering significance is that the structure steel usually needs to meet the corresponding requirements of low temperature impact performance of the steel according to the service environment of the material, because the temperature difference of different latitudes of the ocean is large. For example, the winter sea temperature near the Bohai Bay in China can reach below-20℃, which requires that the building material meet the impact performance of-40℃ to ensure that brittle fracture does not occur. If the tensile strength of the material improves while the plasticity and toughness improve, the yield ratio will increase significantly, and it will be difficult to control the low yield ratio.

[0007] Chinese patent CN201210260231.7 discloses a preparation method of titanium-steel-titanium double-sided composite plate. Four titanium plates and three steel plates are stacked in a closed frame formed by welding the outermost two steel plates in a certain order, an isolating agent mixed by 1 part by weight of active α-Al2O3 and 1.5 parts by weight of 4% polyvinyl alcohol aqueous solution is added between the titanium plates, and nickel-based alloy is used as the transition layer between the titanium plate and the steel plate. The assembly is heated to 500-630℃ and vacuumized, and the vacuum degree reaches 20-200 Pa. The characteristics are that the assembly is first welded and then vacuumized, and ordinary electric arc welding and submerged arc welding can meet the conditions during welding. Compared with welding under vacuum conditions, the welding conditions are low and the cost is low, and a vacuum chamber does not need to be additionally built. Then the combined blank is rolled in a conventional heating furnace at a rolling temperature of 700-900℃. The outermost steel plate is sealed and welded, and vacuumized to block C in coal gas, and a nickel-based alloy isolation layer is added to prevent the generation of interface TiC, thereby obtaining a titanium-steel composite plate with a shear strength of 230-260 MPa and an interface bonding rate of 99.6%-100%.

[0008] Chinese patent CN201710769999.X discloses a method for preparing a titanium-steel composite plate, selecting the surfaces of titanium-steel combination blanks in contact with each other, coating a high-temperature-resistant anti-carburization and nitriding isolation coating on the contact titanium material surface, and drying at room temperature; after drying treatment is completed, the titanium blanks are aligned and stacked two by two, with a steel blank in between, to obtain a composite blank, wherein the thickness of the titanium plate is greater than 2 mm, and the thickness of the steel plate is greater than 5 mm, then the composite blank is sealed and welded around and a certain size of un-welded area is left, the blank is vacuumized to 10-2~10-3Pa, and then welded; the plate blank is heated to 500~700℃ and rolled, the first pass reduction is more than 25%, the final pass reduction is not more than 15%, the total reduction is 60~70%, and the rolling speed is 0.1~1.0mm / s. The coating used in this patent has high-temperature anti-permeation protection effect, preventing diffusion and oxidation of other impurity elements at high temperature, and blocking the diffusion of elements such as C and N. The examples of this patent use Q235 combined with TA1, and the shear strength of the produced steel plate reaches 176MPa, 181MPa, and 182MPa.

[0009] The above two patents mainly avoid the generation of brittle Ti compounds by adding an additional nickel-based alloy isolation layer between titanium and carbon steel.

[0010] Chinese patent CN201811327623.4 discloses a titanium-steel-titanium composite plate and a preparation method thereof, by fixing a carbon steel between two titanium plates of the same size, warm composite rolling is performed by an irreversible large rolling force warm rolling mill to make the three-layer strip composite into one body, after rolling is completed, the rolled composite plate is subjected to heat treatment operation, including initial annealing at 500~600℃ for 20~60 minutes and recrystallization annealing at 680~700℃ for 30~120 minutes, finally the product is obtained through straightening, leveling, shearing and shaping, etc. This patent mainly describes a preparation method of a non-heat-rolled composite titanium-steel plate, since an irreversible rolling mill is used for rolling, only single-pass rolling production can be performed, and heat treatment is also required. The examples mainly relate to a steel strip production method, and the performance of the composite plate is not mentioned.

[0011] Chinese patent CN201510543767.3 discloses a method for preparing titanium-steel composite plate. The titanium-steel composite plate prepared by the method has high bonding strength. The patent fixes a titanium plate between two universal carbon steel plates or steel blanks, welds the blanks around in a vacuum environment, heats the combined blanks to 850-900℃ for 120-360 min, controls the open rolling temperature to be higher than 800℃, the final rolling temperature to be lower than 700℃, and the single pass deformation to be 20-30%, and the total deformation to be greater than 90%, and performs large reduction rolling to break the brittle phase compounds generated at the interface to reduce their influence on the bonding surface. The titanium-steel composite plate prepared by the method has a bonding strength greater than 240 MPa. The reduction and total deformation required by the patent are both high, which is easy to cause edge weld cracking and destroy the vacuum degree during rolling, and is not suitable for interface bonding.

[0012] Chinese patent CN201610994234.1 discloses a production method of titanium-steel composite plate, and relates to an annealing technology production method of titanium-steel plate. First, titanium plates and steel plates are combined to form a symmetric multi-layer combined blank of steel plate-titanium plate-separator-titanium plate-steel plate, and then the combined blank is combined by rolling or explosion, and is annealed and pickled by a continuous annealing and pickling line. The blank is first heated to 500-750℃ to recrystallize the titanium plate, and then heated to 950-1050℃ to recrystallize the steel plate. The patent aims to obtain the properties of the composite material and the base material by two-stage heat treatment. However, the two-stage heat treatment will cause excessive diffusion of titanium, iron and carbon elements, produce brittle phases such as iron-titanium intermetallic compounds and titanium carbide, and deteriorate the interface shear strength.

[0013] Chinese patent CN201710996925.X discloses a thin composite layer double-sided titanium steel composite plate and its preparation method. Through large thickness assembly and large reduction rolling technology, good composite between titanium and steel is realized. The patent is a double-sided titanium composite plate composed of a titanium composite layer, a base layer and a titanium composite layer. The material of the titanium composite layer is TA2, the thickness of the titanium composite layer is 0.2-1 mm, the assembly is placed in the middle according to the order of cover plate, titanium composite material, carbon steel base material, titanium composite material and cover plate from top to bottom, and the vacuum chamber is extracted after vacuumizing. The vacuum degree is 1.0x10-2-4.5x10-2Pa. The composite blank after sealing and welding treatment is heated to 900-920℃ and kept for 1 min / mm x total thickness of the composite blank. The rolling temperature is 880-900℃, the final rolling temperature is above 800℃, and the air cooling is cooled to room temperature. The single pass reduction rate is ≥15%, and the first three pass reduction rates are ≥20%. The total reduction rate is ≥80%. The composite plate obtained after rolling is edge cut, divided and surface ground to obtain a double-sided titanium steel composite plate. The patent controls the rolling temperature and adopts large reduction to make titanium iron and titanium carbide generated at the composite interface broken, refined and dispersedly distributed in the composite interface, improve the distribution state of the compound, further ensure the composite quality and performance stability, and the shear strength reaches 241MPa.

[0014] Chinese patent CN201710983322.6 discloses a thin composite layer titanium steel composite plate and its preparation method. A double-layer structure of titanium and carbon steel is adopted. The assembly method and heating process are similar to those of Chinese patent CN201710996925.X. The rolling temperature is 880-900℃, the single pass reduction rate is 25-30%, the total reduction rate is ≥85%, the thickness of the titanium steel composite plate is limited to 3-16mm, the final rolling temperature is above 800℃, and the air cooling is cooled to room temperature. The titanium steel composite plate is obtained through surface treatment. The thickness of the titanium composite layer is ≤1mm. The patent improves the composite quality by symmetric assembly and sealing of titanium into the carbon steel plate. The shear strength of the steel plate after rolling reaches above 238MPa, and the composite interface bonding rate is 100%. The carbon steel layer meets the national standard requirements of Q345 grade carbon steel.

[0015] The above two patents do not mention the detailed design of the composite layer and the base layer, only describe the tensile properties and shear strength, and the pass reduction rate and total reduction rate required by the composite layer are high, and the corrosion resistance of the material, the low temperature impact performance of the base material, the yield ratio and other performance indicators are not controlled, which does not meet the requirements of building structure steel.

[0016] In summary, the above patents mainly describe the preparation method of the composite steel plate, and the specific embodiments mainly briefly describe the interfacial shear strength and tensile properties, etc. in the performance aspect. The steel structure steel in the sea-spray area needs to not only resist the corrosion of the sea-spray area, but also needs to ensure the necessary structural steel performance requirements, such as the low yield ratio and the corresponding low-temperature impact performance to ensure the structural safety. However, the above patents do not perform the related component and process design on the corrosion rate of the corrosion-resistant layer, the yield ratio, the low-temperature impact, etc., and thus cannot guarantee that the high corrosion-resistant steel structure steel plate in the sea-spray area can meet the use requirements. SUMMARY

[0017] The present application aims to provide a 345MPa grade hot-rolled steel strip for building steel structure resisting sea-spray area corrosion and a manufacturing method thereof. The mechanical properties of the base layer (carbon steel) can meet the corresponding strength grade requirements without reducing the corrosion resistance of the corrosion-resistant layer. The base layer has excellent yield ratio and low-temperature impact toughness, and the overall performance of the hot-rolled steel strip is uniform. The yield strength of the hot-rolled steel strip for building structure is ≥350MPa, the tensile strength is ≥490MPa, the yield ratio is 0.71-0.80, the impact energy at -40℃ is ≥190J, the corrosion resistance to sea-spray is ≤0.006mm / year, the interface transition layer thickness is ≤8μm, and the interfacial shear strength is ≥256MPa. The hot-rolled steel strip can meet the corrosion resistance requirements in the sea-spray area environment, has good mechanical properties and high economic efficiency, and can be applied to steel structural members such as steel piles of facilities such as seaports and offshore oil platforms.

[0018] To achieve the above-mentioned purpose, the technical scheme of the present application is:

[0019] The present application adopts low-carbon micro-alloying component design, realizes the excellent combination of titanium and carbon steel without adding a metal isolation layer, controls the interface transition layer thickness, and meets the corresponding strength grade requirements of the mechanical properties of the base layer (carbon steel) without reducing the corrosion resistance of the corrosion-resistant layer. The base layer has excellent yield ratio and low-temperature impact toughness, and the overall performance of the hot-rolled steel strip is uniform.

[0020] Specifically, the 345MPa grade hot-rolled steel strip for building structure resisting sea-spray area corrosion comprises a base layer, a corrosion-resistant layer, and an interface transition layer between the base layer and the corrosion-resistant layer.

[0021] The base layer has the following chemical composition: C 0.03-0.10%, Si 0.1-0.3%, Mn 1.00-1.65%, P 0.0005-0.003%, S 0.0005-0.01%, Cr 0.02-0.2%, Ni 0.05-0.15%, Cu 0.002-0.020%, Al 0.015-0.03%, Ti 0.008-0.018%, Nb 0.02-0.065%, N 0.0005-0.005%, and the balance of Fe and other inevitable impurities.

[0022] The corrosion-resistant layer is made of industrial pure titanium.

[0023] The hot-rolled strip steel for building structure has a yield strength of ≥350 MPa, a tensile strength of ≥490 MPa, a yield strength ratio of 0.71-0.80, an impact energy at -40 ℃ of ≥190 J, a corrosion rate of sea wave splashing of ≤0.006 mm / year, an interface transition layer thickness of ≤8 μm, and an interface shear strength of ≥256 MPa.

[0024] Preferably, the base layer has the following chemical composition:

[0025] 0.02%≤Cu+Ni≤0.16%;

[0026] 2(C+N)≤Ti+Nb+Cr≤0.25%.

[0027] Preferably, the balance of the base layer is Fe and other inevitable impurities.

[0028] Preferably, the base layer has a microstructure of ferrite+pearlite and / or a small amount of bainite structure, a pearlite and / or bainite content of ≥5%, and an average grain size of ≥8.5 grade.

[0029] The base layer has a yield strength of ≥350 MPa, a tensile strength of ≥490 MPa, a yield strength ratio of 0.71-0.80, and an impact energy at -40 ℃ of ≥190 J.

[0030] Preferably, the corrosion-resistant layer is made of TA1, TA2, TA3 or TA4.

[0031] The corrosion-resistant layer has a single, equiaxed α-Ti microstructure.

[0032] The corrosion-resistant layer has a sea wave splashing corrosion rate of ≤0.006 mm / year.

[0033] Preferably, the interface transition layer has 100% metallurgical bonding, atomic height coherent, an interface transition layer thickness of ≤8 μm, and an interface shear strength of ≥256 MPa.

[0034] Preferably, the interface transition layer has fine grain size, with average grain size of 15-40 μm, and contains less than 120 nm (Ti, Nb)C precipitated particles.

[0035] Preferably, the hot-rolled strip steel for building structure has a thickness of 1.0-20 mm.

[0036] In the base layer component design of the hot-rolled strip steel for building steel structure according to the present application:

[0037] C: C plays a role of solid solution strengthening in steel, and can significantly improve the strength of the steel, but too high C content is not conducive to the welding performance and toughness, and more importantly, too high C content will diffuse to the composite interface, forming a large number of TiC hard phases in the interface transition layer, reducing the strength of the composite interface. In order to ensure the shear strength of the interface, low C content is used. The change of C content has less effect on the yield strength of the steel than on the tensile strength. On the premise of ensuring the forming and welding performance of the product, appropriately increasing the C content is beneficial to reducing the yield strength ratio of the steel. Therefore, in the base layer component according to the present application, the C content is controlled to be 0.03-0.10%.

[0038] Si: The addition of Si element in steel can effectively deoxidize and improve the purity of the steel. In addition, Si element can play a role of solid solution strengthening in steel, and can improve the strength and hardness of the steel, but Si element is not conducive to the welding performance of the material. Therefore, in the base layer component according to the present application, the Si content is controlled to be 0.10-0.30%.

[0039] Mn: Mn is the cheapest strengthening matrix element, which can reduce the austenite transformation temperature, expand the austenite phase region, delay the pearlite transformation, refine the ferrite grain, and improve the strength of the steel. Meanwhile, Mn can also eliminate the effect of S on the steel. However, too high Mn content is prone to segregation and martensite organization, which is not conducive to the toughness of the steel. Therefore, in the base layer component according to the present application, the Mn content is controlled to be 1.0%-1.65%.

[0040] Al: Al is mainly added in excess as a deoxidizing element to ensure that the O content in the steel is as low as possible. After deoxidization, the excess Al combines with N element in the steel to form AlN precipitates. AlN hinders the growth of austenite grains during the heating process, refines the austenite grains, and improves the strength and toughness of the matrix. At the same time, the formation of AlN fixes part of the N in the matrix, reduces the diffusion of interstitial atoms N in the carbon steel base layer to the composite interface to form hard TiN in the interface transition layer, and deteriorates the shear strength of the composite plate interface. At the same time, the addition amount of Ti and Nb can be reduced, and the total cost can be reduced. Therefore, in the base layer component according to the present application, the Al content is controlled to be 0.015-0.03%.

[0041] Ti: Ti forms stable TiN or Ti(N,C) at high temperature, which plays a role of fixing C and N, prevents the diffusion of gap C and N atoms in the carbon steel base layer to the interface, forms hard TiN or Ti(N,C) precipitates in the interface transition layer, and obtains a high interface shear strength composite plate. At the same time, TiN hinders the growth of austenite during the heating process, refines the austenite grains, and can improve the strength and toughness of the matrix. In the subsequent welding, especially in the heat affected zone (HAZ) close to the weld fusion boundary, the growth of austenite grains is inhibited, thereby improving the toughness of the welding HAZ, which can meet the needs of large welding heat input process. In order to improve the strength of the low-carbon matrix, reduce the diffusion of C and N to the interface, and obtain a high interface shear strength composite plate, the Ti content in the base layer composition described in the present application is controlled at 0.008-0.018%.

[0042] Nb: Nb exists in the form of solid solution Nb and Nb(C,N) in the steel, which plays a role of solid solution drag and precipitated pinning during recrystallization. The addition of a small amount of Nb in the base layer carbon steel is mainly to increase the recrystallization temperature, so that the base layer carbon steel is rolled in the recrystallization and unrecrystallization zone, which is beneficial to improve the low temperature impact toughness of the base layer carbon steel. Due to the effect of Nb(C,N) precipitates, the original austenite grains will be finer, thereby promoting the formation of finer recrystallized grains, obtaining the ideal combination of high strength and high toughness, and at the same time, Nb can fix the gap C and N atoms in the matrix, reduce the diffusion of C and N to the interface, and obtain a high interface shear strength composite plate. Based on this, the Nb content in the base layer described in the present application is controlled at 0.02-0.065%.

[0043] Cu: Cu plays a role of solid solution strengthening, and with the increase of Cu content, the room temperature impact toughness of the steel is slightly improved, therefore, the Cu content in the base layer composition described in the present application is controlled at 0.002-0.02%.

[0044] N: N element can generate second phase particles with Ti and Al, refine austenite grains, and improve the strength and toughness of the matrix. However, when the N content is too high, the amount of TiN generated is too large and the particles are too coarse, which will affect the plasticity and toughness of the base layer carbon steel of the present application. Based on this, the N content in the base layer described in the present application is controlled at 0.0005-0.005%.

[0045] Ni: Ni is an element that expands and stabilizes austenite, which lowers the phase transition point. At the same time, it has a certain effect on improving the strength. The addition of Ni in the steel can greatly improve the low temperature impact toughness of the steel. However, nickel is expensive, and excessive addition will increase the cost of the composite plate. Based on this, an appropriate amount of Ni is added in the base layer composition described in the present application, and the Ni content is controlled at 0.05-0.15%.

[0046] Cr: Cr is a strong carbide forming element, with a small diffusion rate in austenite, and hinders the diffusion of C elements. Fine carbides are formed during low-temperature processes, which play a role in precipitation strengthening, and at the same time, can fix interstitial C, N atoms in the matrix, reduce the diffusion of C, N to the interface, and obtain high interface shear strength composite plate. Cr can increase the strength of the matrix in the steel while reducing the toughness, and in order to obtain the best match of strength and toughness, the content of Cr in the base layer composition described in the application is controlled at 0.02-0.20%.

[0047] S, P, both are unavoidable impurity elements, and the lower the content is, the better. Considering the actual steelmaking level of the steel plant, the content of S and P is controlled as follows: S≤0.010%; P≤0.003%;

[0048] The corrosion-resistant layer described in the application uses industrial pure titanium, preferably TA1, TA2, TA3 and TA4, and the composition meets the GB / T3620.1-2016 "Titanium and Titanium Alloy Grade and Chemical Composition" standard.

[0049] In addition, in the base layer chemical composition design of the hot-rolled strip steel for building steel structure described in the application:

[0050] Because Cu and Ni can both improve the toughness of the base layer, and the combined addition effect is particularly significant, and at the same time, the addition of Ni element can reduce the diffusion rate of C in the steel and reduce the diffusion of C to the interface, therefore, the application controls 0.02%≤Cu+Ni≤0.12%, and the interface transition layer can be controlled within 10 μm.

[0051] Because Ti, Nb and Cr are all strong carbonitride forming elements, corresponding carbonitrides are formed in the base layer carbon steel, which can fix the interstitial atoms in the base layer, hinder the diffusion of C, N interstitial atoms to the interface, form large particles and aggregated carbonitrides in the interface transition layer, and the interface transition layer can be controlled within 10 μm, thereby improving the interface shear strength. At the same time, Ti, Nb and Cr can refine the grains of the base layer carbon steel at different stages of hot rolling and improve the toughness. Therefore, the application controls: 2(C+N)≤Ti+Nb+Cr≤0.25%.

[0052] The manufacturing method of the 345MPa grade hot-rolled strip steel for building structure resistant to corrosion in sea wave splash zone described in the application comprises the following steps:

[0053] 1) Smelting and casting

[0054] According to the compositions of the base layer and the corrosion-resistant layer described above, the blanks are smelted and cast respectively;

[0055] 2) Grouping

[0056] The surface of the base layer and the corrosion-resistant layer blank is polished, and the surrounding of the blank is welded to form a composite blank; the combined surface after welding is sealed and vacuumized;

[0057] 3) heating

[0058] The composite blank is heated to 900-1000℃;

[0059] 4) rolling

[0060] The rough rolling temperature is controlled above 860℃; the intermediate blank is inductively heated within 250mm from the edge after rough rolling, and the heating temperature is 830-860℃; the finish rolling temperature is controlled at 760-850℃, the pass reduction rate is controlled at 5-20%, and the cumulative reduction rate is ≥88%;

[0061] 5) cooling

[0062] After leaving the rolling stand, the strip is cooled to 300-450℃ at a cooling rate of ≤10℃ / s and coiled.

[0063] Preferably, the thickness of the corrosion-resistant layer in step 1) is 0.5-20% of the total thickness of the composite blank.

[0064] Preferably, the pass reduction rate in step 4) is 10-20%.

[0065] In the manufacturing method described in the present application:

[0066] 1) smelting: P and S elements can deteriorate the fracture toughness of the steel, so low P and low S control should be performed during smelting to improve the quality of the blank. Clean steel production technology is adopted to reduce the gas and inclusion content in the steel and improve the comprehensive performance of the steel, especially the resistance to lamellar tearing.

[0067] 2) blanking; the thickness of the corrosion-resistant layer is prepared according to 0.5-20% of the total thickness of the composite blank. The corrosion-resistant layer and the base layer carbon steel blank are pretreated, and the surrounding of the blank is welded to form a composite blank; the combined surface after welding is sealed and vacuumized. Vacuum treatment protects the surface of the corrosion-resistant layer from oxidation, and is also an important condition for ensuring the corrosion resistance of the corrosion-resistant layer in the splash area.

[0068] 3) Heating: for single carbon steel, the slab heating temperature is generally controlled at 1000-1250°C, which is beneficial to the dissolution and diffusion of precipitates in the steel, promotes the slab element homogenization, and plays a role in strengthening the steel micro-alloy elements; for single industrial pure titanium plate, the heating temperature is generally controlled at 850-1000°C, and a high heating temperature will cause β phase transformation, and the β phase will grow rapidly, which will deteriorate the performance of the industrial pure titanium. Because a high heating temperature will also make the elements diffuse fully, promoting the subsequent realization of 100% metallurgical bonding of the interface; but a higher heating temperature will increase the tendency of austenite grain coarsening, increase the difficulty of subsequent controlled rolling, and most importantly, it will accelerate the diffusion of C, N, Ti, Fe to the interface, forming a thick brittle precipitate and intermetallic compound at the interface, forming a thick interface transition layer, which will deteriorate the interface shear strength. Preferably, the heating temperature is set to 900-1000°C.

[0069] 4) Rolling: the rough rolling temperature is controlled above 860°C. In the high-temperature rough rolling area, a large reduction rate is applied to make the structure fully recrystallize, so that the grain is refined, and the strength and toughness of the material are improved; the pass reduction rate is ensured to be 5-20%, and the cumulative reduction rate is ≥88%. In addition, during the production of hot-rolled strips, the thickness is rapidly thinned, the temperature drops rapidly, especially the edge part, because the heat dissipation area is larger than the middle part, the temperature drop is more obvious. This makes the edge part prone to phase transformation, and the edge part has poor mechanical properties and various edge defects. Especially for titanium composite strips, the low-carbon micro-alloyed base layer has a relatively high phase transition point, and the composite blank has a low heating temperature. During the rolling process, the edge part is more prone to phase transformation, which deteriorates the edge performance of the strip, and edge cracks and other defects occur during the rolling process. Therefore, after rough rolling, the intermediate blank is inductively heated within a range of 250 mm from the edge, and the heating temperature is 830-860°C.

[0070] Controlled rolling is carried out in the non-recrystallization zone of finish rolling. At this stage, austenite recrystallization no longer occurs, and through reasonable reduction rate and finish rolling temperature, the deformation energy and dislocation are accumulated, so that high-density deformation bands are formed inside the austenite grains, the ferrite phase nucleation points are increased, the grain size after the base phase transformation is further refined, and the strength and toughness of the material are improved. At the same time, during this stage, the deformation induces the precipitation of Nb, Ti, and Cr carbonitride, which improves the strength of the matrix, suppresses the diffusion of C to the interface, and avoids the formation of a thick TiC at the interface to deteriorate the interface shear strength. Preferably, the finish rolling temperature is controlled at 760-850°C, which ensures the corrosion performance of titanium while avoiding the rolling of the base layer in the two-phase zone, and obtaining ferrite pearlite and / or a small amount of bainite structure with an average grain size greater than 8.5.

[0071] 5) Cooling: through the control of cooling by open cooling, final cooling and cooling speed, the control of the type and size of the rolled structure is realized. Too fast cooling speed will form bainite and martensite structure, which is low toughness and high yield ratio, and is not good for the performance of the steel plate. Too slow cooling speed will lead to the formation of a large amount of coarse ferrite structure, which is good for the crack propagation and causes the impact performance to decrease, so the cooling speed should be reasonably controlled. The control of the final rolling temperature can avoid the abnormal coarse structure caused by rolling in the two-phase region. At the same time, the rolled steel can be quickly cooled to the phase transition temperature after rolling, further inhibiting the growth of the structure, and the strength and low temperature impact toughness of the material are improved by refining the grain size. Preferably, the cooling speed is controlled at 5-10 ℃ / s, and the cooling is performed to 300-450 ℃ for coiling.

[0072] Preferably, when the corrosion-resistant layer is too thick, the mechanical properties of the material and the production cost will be affected. When the corrosion-resistant layer is too thin, the corrosion resistance and service life of the material will be reduced. Therefore, the ratio of the corrosion-resistant layer to the total thickness of the composite blank in the above blanking process is preferably 0.5-20%.

[0073] By combining the corrosion-resistant layer with the base layer, designing the components and the thickness ratio of the two, and forming the corrosion-resistant layer on the surface of the base layer, i.e. the carbon steel plate, through the rolling process, a steel strip with good corrosion resistance in the sea wave splashing area, good mechanical properties and high economic efficiency is finally formed. The steel strip can be effectively applied to the steel structure used in the sea wave splashing area after being processed into a structural member.

[0074] Compared with the prior art, the present application has the following beneficial effects:

[0075] The present application adopts low-carbon micro-alloying component design, realizes the excellent combination of titanium and carbon steel under the condition of no additional metal isolation layer, controls the thickness of the interface transition layer, and meets the corresponding strength level requirements of the mechanical properties of the base layer (carbon steel) without reducing the corrosion resistance of the corrosion-resistant layer itself. The base layer has excellent yield ratio and low temperature impact toughness, and ensures the uniformity of the overall performance of the hot-rolled steel strip.

[0076] In addition, the present application adopts low-C micro-alloy design, reduces the formation of TiC compounds in the interface transition layer and the formation of carbonitride in the base layer by reducing C, hinders the grain growth, improves the low temperature impact toughness of the base layer, and solves the problem of low material strength under low carbon condition by adding micro-alloying elements. The yield strength is ≥350 MPa, the tensile strength is ≥490 MPa, the material yield ratio is 0.71-0.80, and the impact energy at-40 ℃ is ≥190 J, all of which are higher than the performance requirements in the national standard GB / T19879-2015 "Steel Plate for Building Structure".

[0077] Chinese patent CN201210260231.7 does not specify the heating temperature, by adding a layer of nickel plate as an isolation layer between the composite layer and the base layer to prevent the formation of interface TiC, the interface bonding rate of the titanium steel composite plate obtained is 99.6-100%. The present application specifies that the composite blank heating temperature is 900-1000℃, through low-carbon and micro-alloy design, the addition of Ni is reduced, the production cost is reduced, and at the same time, through heating, rolling and other process optimization, a certain thickness of transition layer organization can be formed, and the formation of brittle phase TiC in the interface transition layer is reduced, and the interface bonding rate of 100% is realized.

[0078] Chinese patent CN201710769999.X slab heating temperature is 500-700℃, the total reduction is 60-70%, and the interface shear strength of the produced strip steel is at most 182MPa. The present application fully considers the influence of high-temperature phase transition of the corrosion-resistant layer of industrial pure titanium on corrosion resistance and the strength and toughness control of the base layer carbon steel. Combined with low-carbon micro-alloy design, through global design of processing technology, the heating temperature of the composite blank is set to 900-1000℃, at which temperature the corrosion-resistant layer does not undergo phase transition, and the precipitates in the base layer carbon steel are fully dissolved, which plays a role in refining the base layer grains and improving the strength and toughness of the base layer in the controlled rolling process, combined with the cumulative reduction ratio≥70%, the brittle phase of the interface transition layer is broken, and the interface shear strength is improved.

[0079] The above two patents mainly avoid the formation of brittle Ti compounds by adding an additional nickel-based alloy isolation layer between titanium and carbon steel, while the present application does not add an isolation layer through composition and process design, and the composite blank method and the base layer carbon steel material are also significantly different from the above two patents.

[0080] Under the process conditions of the present application, the original corrosion resistance of the corrosion-resistant layer of industrial pure titanium is ensured, and the mechanical properties of the base layer are also ensured, solving the problem that the traditional titanium and carbon steel two processing process windows differ too much to be considered together; at the same time, the base layer and the corrosion-resistant layer elements are fully diffused to form an interface transition layer not greater than 8μm, the layer has fine grains with an average grain size of 15-40μm, and contains less than 120nm(Ti,Nb)C precipitated particles, which strengthens the interface bonding performance and ensures that the interface shear strength is≥256MPa, which is higher than the interface shear strength of 182MPa of the patent.

[0081] Chinese patent CN201811327623.4 realizes compounding by warm rolling, and then needs to be subjected to two-stage heat treatment operation, including 500-600℃ for 20-60 minutes of primary annealing and 680-700℃ for 30-120 minutes of recrystallization annealing, which is a preparation method of a non-hot-rolled composite titanium steel plate, which is completely different from the manufacturing method of the present application.

[0082] The heating temperature of Chinese patent CN201510543767.3 is 850-900℃, the finish rolling temperature is below 700℃, and the single pass deformation is controlled in 20-30%, the total deformation is ≥90%, the shear strength of titanium steel composite plate is greater than 240MPa. The pass reduction and total deformation of the patent are both high, which is easy to cause edge weld cracking, destroy the vacuum degree, and is not easy to compound and has poor rolling stability. The single pass reduction rate of the present application is controlled in 5-20%, which can effectively control the weld from cracking in the rolling process, ensure the vacuum degree inside the slab, and improve the interface shear strength, rolling stability and success rate.

[0083] Chinese patent CN201610994234.1 is a kind of annealing technology production method of titanium steel plate. First, titanium plate and steel plate are formed into a symmetrical multi-layer combined blank of steel plate-titanium plate-separator-titanium plate-steel plate. The combined blank is annealed and pickled by continuous annealing and pickling line after being combined by rolling or explosion. It is heated to 500-750℃ first to make the core titanium plate recrystallize, and then heated to 950-1050℃ to make the base steel plate recrystallize. The rolling process and the corrosion and structural performance of the obtained steel plate are not specifically clear. The present application is obviously different from the present application in manufacturing process. The present application does not need two-stage heat treatment, and the two-stage heat treatment will cause excessive diffusion of titanium, iron and carbon elements, produce brittle intermetallic compounds of iron and titanium and titanium carbide, and deteriorate the interface shear strength.

[0084] The corrosion-resistant layer material of Chinese patent CN201710996925.X is TA2, the titanium composite layer thickness is 0.2-1mm, the heating temperature is 900-920℃, the opening rolling temperature is 880-900℃, the finish rolling temperature is 800℃ or above, and the air cooling is to room temperature. The shear strength reaches 241MPa. The heating temperature of the present application is 900-1000℃, the finish rolling temperature is 760-850℃, water cooling is used, and the cooling speed is controlled in 5-10℃ / s. Under this process, TA1, TA2, TA3 and TA4 can be used as the corrosion-resistant layer, the corrosion-resistant layer accounts for 0.5-20% of the total thickness of the composite blank, and the interface shear strength is ≥256MPa.

[0085] The blanking method and heating process of Chinese patent CN201710983322.6 are similar to those of Chinese patent CN201710996925.X. The single pass reduction rate is 25-30%, and the total reduction rate is ≥85%. While controlling the single pass reduction rate and the total reduction rate, the thickness of the titanium steel composite plate is limited to 3-16mm, the finish rolling temperature is 800℃ or above, and the air cooling is to room temperature. The titanium steel composite plate is obtained by surface treatment, and the titanium composite layer thickness is ≤1mm. The single pass reduction rate of the present application is 5-20%, and the rolling stability is controlled. There are obvious differences in the thickness of the corrosion-resistant layer and the total thickness of the composite strip steel.

[0086] In summary, the 345MPa grade hot-rolled strip steel for building structure with resistance to corrosion in sea-spray area according to the present application can solve the essential problems of stainless steel or carbon steel used in sea-spray area environment; the 345MPa grade hot-rolled strip steel for building structure with resistance to corrosion in sea-spray area can be effectively applied in the manufacture of steel structural parts used in sea-spray area environment, such as steel structural parts of facilities such as seaport wharf, offshore oil platform, etc. in sea-spray area, which can meet the requirements of these components for resistance to corrosion in sea-spray area and mechanical properties, greatly improving the applicability, safety and durability of these components, and having great economic and social benefits. BRIEF DESCRIPTION OF DRAWINGS

[0087] Figure 1 Fig. 1 is a schematic diagram of an interlayer structure of the 345MPa grade hot-rolled strip steel for building structure with resistance to corrosion in sea-spray area according to the present application.

[0088] Figure 2 Fig. 2 is another schematic diagram of an interlayer structure of the 345MPa grade hot-rolled strip steel for building structure with resistance to corrosion in sea-spray area according to the present application.

[0089] Figure 3 Fig. 3 is a microstructure photo of the corrosion-resistant layer of Example 3 of the present application.

[0090] Figure 4 Fig. 4 is a scanning image of the interface transition layer of the combination of the base layer and the corrosion-resistant layer of Example 3 of the present application.

[0091] Figure 5 Fig. 5 is a microstructure photo of the base layer of Example 3 of the present application. DETAILED DESCRIPTION

[0092] The technical solutions of the present application will be further described in detail below in combination with the embodiments and the drawings. It should be clear that the following embodiments are only used to describe the specific embodiments of the present application and do not constitute any limitation on the protection scope of the present application.

[0093] Reference should be made to Figure 1 , Figure 2 , Figure 4 , which shows the schematic diagrams of two interlayer structures of the hot-rolled strip steel for building structure according to the present application, wherein 1 is the base layer, 2 is the corrosion-resistant layer, and 3 is the interface transition layer.

[0094] The composition of the base layer of the hot-rolled strip steel (composite strip steel) embodiment according to the present application is shown in Table 1, and the remaining amount of the composition is Fe and unavoidable impurities. Table 2 shows the manufacturing process parameters of the composite strip steel embodiment according to the present application. Table 3 shows the metallographic structure and mechanical properties of the base layer and the corrosion-resistant layer in the composite strip steel of the embodiment and the comparative example, and the thickness of the interface transition layer.

[0095] The yield strength and tensile strength of the composite strip are measured according to GB / T 6396-2008 "Mechanical and Technological Properties of Clad Steel Plates" and GB / T 228-2010 "Metallic Materials - Tensile Testing at Ambient Temperature".

[0096] The impact energy KV2 / J (longitudinal) of the base carbon steel at -40℃ is measured according to GB / T 6396-2008 "Mechanical and Technological Properties of Clad Steel Plates" and GB / T 229-2020 "Metallic Materials Charpy Pendulum Impact Test Method".

[0097] The grain size rating is performed as follows: the ferrite structure in the stainless steel and carbon steel is rated according to GB / T 6394-2017 "Metallic Materials - Determination of Average Grain Size" using the intercept method.

[0098] The comparative examples are prepared using the above steps substantially the same as the examples of the present application, except that the composition of the base carbon steel and certain process parameters used in the rolling or cooling steps do not meet the requirements of the present application.

[0099] The metallographic structure of the corrosion resistant layer of Example 3 is shown in Figure 3 , which shows single, equiaxed α-Ti with an average grain size of 104.2 um.

[0100] The interface transition layer of Example 3 is shown in Figure 4 , the interface transition layer has a thickness of 7.5 um, and the discontinuous fine particles are TiC with a size of less than 120 nm.

[0101] The metallographic structure of the base layer of Example 3 is shown in Figure 5 , which shows the metallographic structure of the base carbon steel as ferrite + pearlite or a small amount of bainite, with a pearlite volume fraction of 6.6% and a grain size rating of ≥8.5.

[0102] Table 4 shows the corrosion of the composite strip hanging samples of Examples 1-8 and Comparative Examples 1-4 after 6 months in the splash zone of the South China Sea. The observation results show that the corrosion rate of the other examples and comparative examples is ≤0.006 mm / year, except for Comparative Example 4.

[0103] Comparative Examples 1-4 do not meet the requirements of the composition design, hot working process conditions, resulting in certain properties of the composite strip not meeting the use requirements (performance parameters not within the scope of the invention). Among them:

[0104] Comparative Example 1 does not add Ti, Nb and Cr, and the rolling reduction rate is not within the scope of the present application, so the yield strength, yield strength ratio, impact performance and interface shear strength cannot meet the requirements.

[0105] Comparative Example 2 cannot meet the requirements of yield strength and shear strength because the chemical composition does not add Ni, the finish rolling temperature, the cooling rate and the finish cooling temperature are not within the range defined in the present application.

[0106] Comparative Example 3 cannot meet the requirements of mechanical properties, yield strength to tensile strength ratio and impact property because the microstructure is bainite, the amount of Cu+Ni is less than 0.02% and the cooling rate and the finish cooling temperature are not within the range defined in the present application.

[0107] Comparative Example 4 cannot meet the requirements of shear strength because the interface transition layer is too thick due to the heating temperature and the finish rolling temperature not being within the range defined in the present application, and the corrosion rate is high because β-Ti cannot be completely eliminated in the subsequent processing and cooling process due to the heating temperature being too high.

[0108] By the preparation method of the present application, especially the control of the heating, rolling and cooling processes, the base layer in the strip steel exhibits excellent yield strength to tensile strength ratio and low temperature impact toughness, and the cladding layer has excellent corrosion resistance and high bonding strength, the yield strength is 352-464 MPa, the tensile strength is 489-606 MPa, the yield strength to tensile strength ratio is 0.71-0.79, the impact energy at -40℃ is more than 190 J, and the interface shear strength is more than 256 MPa.

[0109] It should be noted that all the technical features described in the present application can be freely combined or integrated in any way, unless contradictory to each other. Various modifications and changes can be made to the present application without departing from the scope of the present application, which will be apparent to those skilled in the art. For example, the features shown or described as part of one embodiment can be used with another embodiment to produce yet another embodiment. Therefore, the present application is intended to cover these modifications and changes falling within the scope of the appended claims and their equivalents.

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Claims

1. A hot-rolled strip for building structures with a 345 MPa grade of resistance to corrosion in the splash zone of sea waves, comprising a base layer, a corrosion-resistant layer and an interface transition layer between the base layer and the corrosion-resistant layer; the base layer has the following chemical composition by mass percentage: C 0.03-0.10%, Si 0.1-0.3%, Mn 1.00-1.65%, P 0.0005-0.003%, S 0.0005-0.01%, Cr 0.02-0.2%, Ni 0.05-0.15%, Cu 0.002-0.020%, Al 0.015-0.03%, Ti 0.008-0.018%, Nb 0.02-0.065%, N 0.0005-0.005%, and the balance being Fe and other inevitable impurities; the corrosion-resistant layer is made of industrial pure titanium; the hot-rolled strip for building structures has a yield strength of ≥ 350 MPa, a tensile strength of ≥ 490 MPa, a yield strength / tensile strength ratio of 0.71-0.80, an impact energy at -40°C of ≥ 190 J, a resistance to corrosion in the splash zone of sea waves of ≤ 0.006 mm / year, an interface transition layer thickness of ≤ 8 μm, and an interface shear strength of ≥ 256 MPa.

2. The hot-rolled steel strip for a building structure according to claim 1, characterized by, the base layer chemical composition also satisfies the following relationships: 0.02%≤ Cu+Ni≤ 0.16%; 2(C+N)≤ Ti+Nb+Cr≤ 0.25%.

3. The hot-rolled strip for a building structure according to claim 1 or 2, characterized by, the industrial pure titanium is TA1, TA2, TA3 or TA4.

4. The hot-rolled strip for a building structure according to claim 1 or 2, characterized by the base layer has a microstructure of ferrite+pearlite, or ferrite+pearlite+ bainite, with a pearlite and / or bainite content of ≥ 5% and an average grain size of ≥ 8.5 grade.

5. The hot-rolled strip for a building structure according to claim 1 or 2, characterized by the base layer has a yield strength of ≥ 350 MPa, a tensile strength of ≥ 490 MPa, a yield strength / tensile strength ratio of 0.71-0.80, and an impact energy at -40°C of ≥ 190 J.

6. The hot-rolled steel strip for a building structure according to claim 4, characterized by, the base layer has a yield strength of ≥ 350 MPa, a tensile strength of ≥ 490 MPa, a yield strength / tensile strength ratio of 0.71-0.80, and an impact energy at -40°C of ≥ 190 J.

7. The hot-rolled strip steel for a building structure according to claim 1, wherein the corrosion-resistant layer has a microstructure of single, equiaxed α-Ti.

8. The hot-rolled steel strip for a building structure according to claim 1 or 7, characterized in that, the corrosion-resistant layer has a resistance to corrosion in the splash zone of sea waves of ≤ 0.006 mm / year.

9. The hot-rolled strip steel for a building structure according to claim 1, wherein the interface transition layer is 100% metallurgically bonded, has atomic high coherence, a thickness of ≤ 8 μm, a fine-grained microstructure with an average grain size of 15-40 μm, contains less than 120 nm (Ti, Nb)C precipitates, and has an interface shear strength of ≥ 256 MPa.

10. The hot-rolled steel strip for a building structure according to claim 1, characterized by, the hot-rolled strip for building structures has a thickness of 1.0-20 mm.

11. The method of producing a hot-rolled strip steel for building structures of the 345 MPa grade resistant to corrosion from sea spray in the splash zone according to any one of claims 1 to 10, characterized in that, comprising the following steps: 1) smelting and casting smelting and casting the base layer and the corrosion-resistant layer according to claim 1 or 2 into blanks, respectively; 2) assembling blanks surface grinding and polishing the base layer and the corrosion-resistant layer blanks, and welding and sealing the bonding surfaces of the blanks to form a composite blank, and performing vacuum treatment on the welded and sealed bonding surfaces; 3) heating heating the composite blank to 900-1000°C; 4) rolling The rough rolling temperature is controlled above 860℃; the intermediate billet is inductively heated in the range of 250mm from the edge after rough rolling, and the heating temperature is 830~860℃; the finish rolling temperature is controlled at 760~850℃, and the pass reduction rate is controlled at 5~20%; the cumulative reduction rate is ≥88%; 5) Cooling After leaving the rolling stand, the strip is cooled at a cooling rate of 5~10℃ / s to 300~450℃ for coiling.

12. The production method according to claim 11, wherein In step 1), the corrosion-resistant layer thickness is 0.5~20% of the composite billet thickness.

13. The production method according to claim 11, wherein In step 4), the pass reduction rate is 10~20%.

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