Weathering resistant steel for structure and preparation method of weathering resistant steel

Abstract

The invention discloses weathering resistant steel for a structure, and the weathering resistant steel for the structure comprises the following chemical components in percentage by mass: 0.09 wt%-0.11 wt% of C, 0.08 wt%-0.20 wt% of Si, 0.85 wt%-0.95 wt% of Mn, 0.075 wt%-0.085 wt% of P, less than or equal to 0.012 wt% of S, 0.025 wt%-0.050 wt% of Alt, 0.016 wt%-0.026 wt% of Ti, 0.26 wt%-0.33 wt% of Cu, 0.43 wt%-0.53 wt% of Cr, less than or equal to 70 ppm of N, and the balance of Fe and inevitable impurities. Oxidation generated on the surface of the casting blank in the heating and rolling process is inhibited by controlling the content of Si, gathering of copper on the surface of the casting blank is inhibited, the surface quality of a steel coil is improved, the corrosion resistance is improved through the design of low Cu and high P, and good strength and toughness are achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of weathering steel manufacturing, in particular to a structural weathering steel and a preparation method thereof, and particularly relates to an economic 355MPa-grade structural weathering steel and a preparation method thereof. BACKGROUND

[0002] Weathering steel has been widely used in steel structures such as railway vehicles, containers, outdoor buildings, and towers due to its good atmospheric corrosion resistance, which improves the service period, and its good strength and toughness, weldability, and cold forming performance.

[0003] In the prior art, there are two design ideas for weathering steel. One is to add more valuable alloy elements, especially Ni, to obtain high strength and high corrosion resistance, which is costly, mainly Cu, Cr, P, and Ni series. The other is to add only an appropriate amount of Cu and Cr alloy elements without valuable alloy Ni, which is less costly, mainly Cu, Cr, and P series.

[0004] CN117026087 discloses a preparation method of a Ni-free container steel. The Si content (0.35-0.55%) is relatively high, which makes it difficult to remove the iron oxide scale on the surface of the casting blank, increases the difficulty of descaling, and deteriorates the surface quality of the coil. The Mn content (0.4-0.55) is relatively low, which results in low strength of the finished product, making it difficult to meet the user's requirements for strength.

[0005] CN109234617A discloses a preparation method of a Ni-free weathering steel coil, but it is a thin gauge, with a thickness range of only 1.2-2.5mm, which is difficult to meet the requirements of weathering steel customers and has a small application field. The steel coil with a rolling thickness of 1.2-2.5mm requires a thin slab continuous casting and rolling device, which is less used in domestic steel plants and is difficult to promote.

[0006] Currently, the Ni-free weathering steel has the following problems: poor surface quality, poor cold forming performance, and weak atmospheric corrosion resistance. SUMMARY

[0007] To overcome the defects of the prior art, the present application provides a 355MPa-grade structural weathering steel and a preparation method thereof, which solves the technical problems of poor surface quality, poor cold forming performance, and weak atmospheric corrosion resistance of the Ni-free weathering steel in the prior art.

[0008] The first aspect of the present application provides a structural weathering steel, the chemical composition of which comprises, in mass fraction:

[0009] C: 0.09wt%-0.11wt%, Si: 0.08wt%-0.20wt%, Mn: 0.85wt%-0.95wt%, P: 0.075wt%-0.085wt%, S: ≤0.012wt%, Alt: 0.025wt%-0.050wt%, Ti: 0.016wt%-0.026wt%, Cu: 0.26wt%-0.33wt%, Cr: 0.43wt%-0.53wt%, N: ≤70ppm, the balance being Fe and unavoidable impurities.

[0010] The structural weathering steel according to the first aspect, wherein the yield strength R eL ≥355MPa; the tensile strength 630MPa≥R m ≥490MPa; and / or the elongation A≥22%.

[0011] The second aspect of the present application provides a method for preparing the structural weathering steel of the first aspect, comprising the following steps:

[0012] converter smelting, LF refining, continuous casting, heating, rough rolling, finishing rolling, laminar cooling, coiling.

[0013] The method according to the second aspect, wherein in the converter smelting step, top and bottom combined blowing is used;

[0014] Before tapping in the converter smelting step, silicon-manganese alloy and manganese iron are added into the ladle to control the Si and Mn contents in the molten steel; preferably, the Si content in the molten steel is controlled to be 0.08% and the Mn content is controlled to be 0.80% by mass fraction; and / or

[0015] No slag is added during the tapping process in the converter smelting step.

[0016] The method according to the second aspect, wherein the LF refining step comprises white slag making, and after white slag making, Si, Mn and Ti alloys are added.

[0017] The method according to the second aspect, wherein in the continuous casting step, the casting speed is 1.25-1.40m / min.

[0018] The method according to the second aspect, wherein in the heating step, the temperature of the first heating section of the casting blank in the heating furnace is ≤1100℃, the temperature of the second heating section is 1060-1160℃, the temperature of the high-temperature section is 1200±20℃, and the temperature of the soaking section is 1190±20℃.

[0019] The air-fuel ratio of the casting blank in the heating furnace is 0.9;

[0020] The overall time of the casting blank in the furnace is 120-240min; and / or

[0021] The total length of the slab in the high-temperature section and the soaking section of the heating furnace is greater than 60 min and less than 90 min.

[0022] According to the method of the second aspect, wherein the rough rolling step comprises 3-4 descaling passes, and the descaling water pressure is 19-23 MPa.

[0023] Preferably, the slab is descaled when it leaves the heating furnace, and the descaling water pressure is 19-23 MPa.

[0024] More preferably, the slab is descaled twice before the finish rolling, and the descaling water pressure is 19-23 MPa.

[0025] According to the method of the second aspect, wherein in the finish rolling step, the finish rolling finishing temperature is 820-890℃.

[0026] According to the method of the second aspect, wherein in the coiling step, the coiling temperature is 605-645℃.

[0027] The technical solution of the present application has the following beneficial effects, but is not limited to:

[0028] The weather-resistant steel for hot-rolled structures provided by the embodiments of the present application does not use high-priced Ni to inhibit the generation of copper brittleness, but controls the appropriate Si content to inhibit the oxidation generated on the surface of the cast slab during heating and rolling, and inhibits the aggregation of copper on the surface of the cast slab, thereby improving the surface quality of the steel coil, and the production cost is lower than that of the weather-resistant steel with Ni. The P content of the weather-resistant steel provided by the embodiments of the present application is 0.075wt%-0.085wt%, so the atmospheric corrosion resistance is relatively strong. In order to improve the cold brittleness of high-P steel, the weather-resistant steel provided by the embodiments of the present application adds Ti elements to refine the grains and improve the strength and toughness. DETAILED DESCRIPTION

[0029] The present application will be further described in detail by the following examples. Through these descriptions, the features and advantages of the present application will become more apparent.

[0030] The special word "exemplary" here means "serving as an example, embodiment or illustration". Any embodiment described as "exemplary" here is not necessarily to be construed as superior or better than other embodiments.

[0031] In addition, the technical features involved in different embodiments of the present application described below can be combined with each other as long as there is no conflict.

[0032] White slag refers to the slag formed during the diffusion deoxidization of carbon powder and ferrosilicon powder as reducing agents in the reduction period operation of the electric arc furnace.

[0033] The application provides a structural economic weathering steel plate, the chemical composition of the structural economic weathering steel plate comprises the following components in mass fraction: C: 0.09wt%-0.11wt%, Si: 0.08wt%-0.20wt%, Mn: 0.85wt%-0.95wt%, P: 0.075wt%-0.085wt%, S: ≤0.012wt%, Alt: 0.025wt%-0.050wt%, Ti: 0.016wt%-0.026wt%, Cu: 0.26wt%-0.33wt%, Cr: 0.43wt%-0.53wt%, N: ≤70ppm, and the balance is Fe and inevitable impurities.

[0034] The yield strength R eL ≥355MPa; the tensile strength 630MPa≥R m ≥490MPa; and / or the elongation A≥22%.

[0035] Mn not only can increase the strength and hardness of the weathering steel, but also can improve the toughness of the weathering steel, so that the weathering steel has high strength and hardness and also has good impact resistance. Al can remove O elements in the steel, purify the molten steel and reduce the generation of inclusions. Al can also refine the grain, fix nitrogen in the steel, thereby significantly improving the impact toughness of the steel, reducing the cold brittleness tendency and aging tendency. Al and Cr elements are used in combination, and the corrosion resistance of the steel can be improved. Cr can combine with oxygen to form a dense chromium oxide film, effectively preventing further oxidation of the steel. Cr forms intermetallic compounds in the steel, such as FeCr, slows down the decomposition rate of austenite, thereby improving the oxidation resistance and heat resistance of the steel.

[0036] The application provides a method for preparing a structural economic weathering steel plate, and the method comprises the following steps:

[0037] The production process of the 355MPa structural economic weathering steel plate comprises the following steps: converter smelting, LF refining, continuous casting, heating, rough rolling, finish rolling, laminar cooling and coiling.

[0038] Top and bottom combined blowing is carried out on a 150-ton converter in a steel mill, the Si content is controlled to 0.08% and the Mn content is controlled to 0.80% at the converter outlet; and the Si and Mn contents in the molten steel are accurately controlled after LF white slag making;

[0039] The continuous casting blank drawing speed is stabilized at 1.25-1.40 m / min, and the thickness of the continuous casting slab is 220 mm;

[0040] The air-fuel ratio of the casting blank in the heating furnace is 0.9;

[0041] The billet is at a first adding section temperature of equal to or less than 1100 DEG C, a second adding section temperature of 1060-1160 DEG C, a high temperature section temperature of 1200 DEG C plus or minus 20 DEG C, and a soaking section temperature of 1190 DEG C plus or minus 20 DEG C.

[0042] The billet is at a first adding section temperature of equal to or less than 1100 DEG C, a second adding section temperature of 1060-1160 DEG C, a high temperature section temperature of 1200 DEG C plus or minus 20 DEG C, and a soaking section temperature of 1190 DEG C plus or minus 20 DEG C.

[0043] The total time length of the high temperature section and the soaking section of the heating furnace is greater than 60 min and less than 90 min.

[0044] The rough rolling is 3-4 passes of descaling, and the water pressure is set to 19-23 MPa.

[0045] The finished product is a slab with a thickness of 2.5 mm<=h<5.0 mm, the finish rolling final rolling temperature is 870 DEG C plus or minus 20 DEG C, and the coiling temperature is 630 DEG C plus or minus 15 DEG C.

[0046] The finished product is a slab with a thickness of 5.0 mm<=h<=12.0 mm, the finish rolling final rolling temperature is 840 DEG C plus or minus 20 DEG C, and the coiling temperature is 620 DEG C plus or minus 15 DEG C; and the laminar flow cooling mode is front section encryption main cooling.

[0047] Preferably, the slab is descaled after leaving the heating furnace, and the descaling water pressure is set to 19-23 MPa.

[0048] Preferably, the slab is double descaled before finish rolling, and the descaling water pressure is set to 19-23 MPa.

[0049] The mechanism involved in the present application is as follows:

[0050] Cu, P and Cr can all improve the atmospheric corrosion resistance of the steel, and the atmospheric corrosion resistance of P is stronger than that of Cu. P helps to form uniform alpha FeOOH on the surface of the steel, promotes the formation of a dense protective film Fe x (OH) 3-2x , prevents the corrosion medium from entering the inside of the steel, thereby protecting the steel from corrosion and improving the weather resistance of the steel. Cu can enrich between the steel matrix and the rust layer, making the rust layer dense and preventing O from entering, which leads to the difficulty of continuing corrosion into the steel, but Cu can cause the occurrence of Cu brittleness, so the high corrosion resistance of the weathering steel provided in the embodiments of the present application is realized by low Cu and high P.

[0051] Generally, Cu-containing steel is added with Ni to inhibit the occurrence of cracks on the surface of the steel during rolling, thereby improving the surface quality. The weathering steel provided in the embodiments of the present application does not inhibit the occurrence of cracks by means of Ni, but by means of Si, and therefore has a lower cost. The oxides of iron in the rust layer of steel are FeO, Fe3O4 and Fe2O3. The Si in the steel forms a dense film of Fe2SiO4 between FeO and the Fe matrix, thereby avoiding further corrosion, and Cu is not enriched, thereby improving the surface quality of the steel coil. However, an increase in the content of Si in the steel makes the adhesion of the iron oxide scale on the cast slab stronger, and the iron oxide scale is difficult to remove by means of descaling phosphorous water. Through a large amount of production practice, the content of Si is determined to be 0.08wt%-0.20wt%, which can meet the two requirements at the same time.

[0052] In the heating furnace, the cast slab is heated by mixed combustion of air and gas. If the air-fuel ratio is too high, too much air will increase the iron oxide scale on the surface of the cast slab when the steel is tapped. The use of a lower air-fuel ratio can reduce the iron oxide scale on the surface of the cast slab when the steel is tapped, thereby reducing the enrichment of Cu on the metal matrix and alleviating the copper brittleness phenomenon.

[0053] When the heating temperature exceeds 1083.4°C, the copper-rich phase begins to liquefy, and copper will penetrate into the matrix along the austenite grain boundaries. Reducing the temperature of the cast slab in the high-temperature section + soaking section and reducing the time of the cast slab in the high-temperature section + soaking section can reduce the liquefaction of copper and weaken the enrichment of copper at the grain boundaries. In the present application, the temperature of the cast slab in the high-temperature section is 1200±20°C, and the temperature of the cast slab in the soaking section is 1190±20°C, both of which are relatively low. The total time of the cast slab in the high-temperature section + soaking section of the heating furnace must be less than 90 min, which is relatively short, and therefore the enrichment of copper at the grain boundaries can be reduced, and the copper brittleness phenomenon can be inhibited.

[0054] Embodiments

[0055] The 355MPa grade structural economical weathering steel plates of Examples 1-5 are all prepared by the following steps, and the specific chemical compositions and process parameters are shown in Tables 1-3 below:

[0056] The production process of the 355MPa grade structural economical weathering steel plate is as follows:

[0057] Converter smelting→LF refining→continuous casting→heating→rough rolling→finish rolling→laminar cooling→coiling.

[0058] Specifically, it includes:

[0059] (1) Converter smelting:

[0060] Top and bottom combined blowing is carried out on a 150-ton converter in the steelmaking plant, and silicon-manganese alloy and manganese iron are added into the ladle before the converter is tapped, so that the Si in the molten steel is controlled to 0.08%, and the Mn is controlled to 0.80%. The tapping process strictly prohibits slagging.

[0061] (2) LF refining:

[0062] The molten steel feeds aluminum wire (or adds aluminum particles) to deoxidize the slag; adds lime, heats up and blows argon, and quickly calcines the lime. When FeO≤1%, the slag becomes white and has good reducibility.

[0063] After the LF white slag is formed, Si, Mn, and Ti alloys are added to precisely control the Si, Mn, and Ti in the molten steel.

[0064] The chemical composition of the molten steel obtained by smelting contains, by weight percent, C: 0.09wt%-0.11wt%, Si: 0.08wt%-0.20wt%, Mn: 0.85wt%-0.95wt%, P: 0.075wt%-0.085wt%, S:≤0.012wt%, Alt: 0.025wt%-0.050wt%, Ti: 0.016wt%-0.026wt%, Cu: 0.26wt%-0.33wt%, Cr: 0.43wt%-0.53wt%, N: 50ppm, and the balance is Fe and unavoidable impurities.

[0065] (3) Continuous casting:

[0066] The casting speed of the continuous casting billet is stabilized at 1.25-1.40m / min to ensure the surface quality of the casting billet.

[0067] (4) Heating:

[0068] The temperature of the first heating furnace section is ≤1100℃, the temperature of the second heating furnace section is 1060-1160℃, the temperature of the high temperature section is 1200±20℃, and the temperature of the soaking section is 1190±20℃.

[0069] The air-fuel ratio of the billet in the heating furnace is 0.9.

[0070] The total time of the billet in the furnace is 120-240min, and the total time of the high temperature section + the soaking section must be between 60-90min to allow the alloy to fully solid solution.

[0071] (5) Rough rolling:

[0072] The slab is descaled after exiting the heating furnace, and the descaling water pressure is set to 19-23MPa; the slab is descaled 3-4 times before rough rolling, and the water pressure is set to 19-23MPa; the slab is descaled twice before finish rolling, and the water pressure is set to 19-23MPa. The purpose of multiple descaling is to completely remove the Cu-rich iron oxide scale, thereby preventing the iron oxide scale from being pressed into and surface cracks from occurring during rolling.

[0073] (6) Finish rolling:

[0074] The finished plate slab with a thickness of 2.5mm≤h<5.0mm, the finishing rolling final rolling temperature is 870±20℃, and the coiling temperature is 630±15℃.

[0075] The finished plate slab with a thickness of 5.0mm≤h≤12.0mm, the finishing rolling final rolling temperature is 840±20℃, and the coiling temperature is 620±15℃.

[0076] Comparative examples

[0077] Comparative examples 1 and 2 are prepared by the same method as the examples, and the difference is that the chemical compositions are different, and the specific compositions and process parameters are shown in Tables 1-3.

[0078] In the present application, the chemical composition design of the 355MPa grade structural economic weathering steel plate of examples 1-5 and the related process all meet the design specification requirements of the present application.

[0079] Table 1 below lists the mass percentage of each chemical element of the weathering steel plate of examples 1-5; Tables 2 and 3 list the main process parameters of the rolling process of the weathering steel plate in the examples and comparative examples; and Table 4 lists the mechanical properties of the weathering steel plate in the examples and comparative examples.

[0080] Table 1 Chemical composition of each example and comparative example

[0081]

[0082] Table 2 Rolling temperature of each example and comparative example

[0083]

[0084]

[0085] Table 3 Rolling time of each example and comparative example

[0086]

[0087] Table 4 Mechanical properties of the weathering steel plate prepared in each example and comparative example

[0088]

[0089] The tensile property test of the present application example is carried out according to GB / T 228.1, the bending property test is carried out according to GB / T 228.1, and the impact property test is carried out according to GB / T 229.

[0090] In Table 4, in the absence of nickel, the yield strength and tensile strength of each example meet the requirements, and the mechanical properties are qualified.

[0091] Comparing the mechanical properties of the example with those of Comparative Example 1, it can be obviously seen that the yield strength and tensile strength of the example are higher, and the surface quality is better, which is caused by the moderate Si content of the weathering steel plate, the low heating temperature in the high-temperature section + soaking section, and the short heating time in the high-temperature section + soaking section.

[0092] Comparing the example with Comparative Example 2, it can be obviously seen that the tensile rate of Comparative Example 2 is the lowest, the cold bending experiment has unqualified phenomenon, and there is a phenomenon of iron oxide scale being pressed in.

[0093] The above describes the present application in combination with preferred embodiments, but these embodiments are only exemplary and serve only to illustrate. On this basis, various substitutions and improvements can be made to the present application, and these all fall within the protection scope of the present application.

Claims

1. A type of weathering steel for structural use, characterized in that, The chemical composition of the weathering steel for the structure, by mass fraction, includes: C: 0.09wt%-0.11wt%, Si: 0.08wt%-0.20wt%, Mn: 0.85wt%-0.95wt%, P: 0.075wt%-0.085wt%, S: ≤0.012wt%, Alt: 0.025wt%-0.050wt%, Ti: 0.016wt%-0.026wt%, Cu: 0.26wt%-0.33wt%, Cr: 0.43wt%-0.53wt%, N: ≤70ppm, balance Fe and unavoidable impurities.

2. The weathering steel for structural use according to claim 1, characterized in that, The yield strength R of the weathering steel used in the structure eL ≥355MPa; tensile strength ≥630MPa m ≥490MPa; and / or elongation at break A≥22%.

3. The method for preparing structural weathering steel according to claim 1 or 2, characterized in that, The method includes the following steps: Converter smelting, LF refining, continuous casting, heating, rough rolling, finish rolling, laminar flow cooling, coiling.

4. The method according to claim 3, characterized in that, In the converter smelting step, top and bottom combined blowing is adopted; Before tapping in the converter smelting step, silicon-manganese alloy and ferromanganese are added to the ladle to control the Si and Mn content in the molten steel; preferably, the Si content in the molten steel is controlled to be 0.08% and the Mn content to be 0.80% by mass fraction; and / or No slag is discharged during the tapping process of the converter smelting step.

5. The method according to claim 3, characterized in that, The LF refining step includes the production of white slag, followed by the addition of a Si, Mn, and Ti alloy.

6. The method according to claim 3, characterized in that, In the continuous casting step, the billet casting speed is 1.25 to 1.40 m / min.

7. The method according to claim 3, characterized in that, In the heating step, the temperature of the billet in the first heating section of the heating furnace is ≤1100℃, the temperature in the second heating section is 1060~1160℃, the temperature in the high temperature section is 1200±20℃, and the temperature in the soaking section is 1190±20℃. The air-fuel ratio of the cast billet in the heating furnace is 0.9; The overall furnace time for the cast billet is 120–240 min; and / or The total time for the billet to be in the high-temperature section and soaking section of the heating furnace is greater than 60 minutes and less than 90 minutes.

8. The method according to claim 3, characterized in that, The roughing step includes 3 to 4 descaling passes, with a descaling water pressure of 19 to 23 MPa. Preferably, descaling is performed when the slab exits the heating furnace, and the descaling water pressure is 19-23 MPa; More preferably, the slab is descaled twice before finishing rolling, and the descaling water pressure is 19-23 MPa.

9. The method according to claim 3, characterized in that, In the finishing rolling step, the finishing rolling temperature is 820–890℃.

10. The method according to claim 3, characterized in that, In the winding step, the winding temperature is 605–645°C.

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