Punching cold-rolled hot-dip galvanized steel plate with tensile strength of 440 MPa

Abstract

The invention discloses a 440MPa-tensile-strength cold-rolled hot-dip galvanized steel sheet for stamping. The technical problems that an existing cold-rolled hot-dip galvanized steel sheet for a photovoltaic support cracks in stamping and is poor in formability are mainly solved. According to the technical scheme, a base plate of the punching cold-rolled hot-dip galvanized steel plate with the tensile strength of 440 MPa comprises chemical components in percentage by weight as follows: 0.06%-0.08% of C, 0.001%-0.050% of Si, 0.80%-0.90% of Mn, smaller than or equal to 0.020% of P, smaller than or equal to 0.008% of S, 0.010%-0.020% of Nb, 0.015%-0.030% of Ti, smaller than or equal to 0.0060% of N, 0.015%-0.060% of Al and the balance of iron and inevitable impurities. And the percentage elongation after fracture A50mm of the cold-rolled hot-dip galvanized steel plate with the thickness of 1.0-2.0 mm is 32-38%. The method is mainly used for photovoltaic supports and other parts.

Description

Technical Field

[0001] The present invention relates to a cold-rolled hot-dip galvanized steel sheet, and in particular to a cold-rolled hot-dip galvanized steel sheet for stamping with a tensile strength of 440 MPa and a manufacturing method thereof; specifically, it relates to a micro-alloy-reinforced cold-rolled hot-dip galvanized steel sheet with a tensile strength of 440-470 MPa and a manufacturing method thereof. The cold-rolled hot-dip galvanized steel sheet is suitable for preparing photovoltaic brackets by stamping processing technology, and belongs to the technical field of iron-based alloys. Background Art

[0002] In recent years, with the development of photovoltaic energy, new demands have been put forward for photovoltaic steel. Due to the outdoor working conditions, photovoltaic brackets require that the steel used for photovoltaic brackets must have high strength, good stamping formability, good surface quality and strong corrosion resistance.

[0003] A Chinese patent application with application publication number CN104975226A discloses an alloyed hot-dip galvanized steel for automobiles with a tensile strength of 440 MPa and a production method. The composition is mainly designed by carbon-manganese strengthening and phosphorus solid solution strengthening. The carbon content of the steel is 0.08-0.11%, the manganese content is 1.10-1.40%, and the phosphorus content is 0.015-0.030%. However, since the C content is in the peritectic region, it is easy to produce longitudinal cracks during continuous casting, which is a quality risk.

[0004] A Chinese patent application with publication number CN108823498A discloses a hot-dip galvanized low-alloy high-strength steel for automobiles and a production method thereof. The steel contains 0.7-0.8% Mn and 0.03-0.04% Nb. A relatively high niobium element is added, resulting in high production costs.

[0005] A Chinese patent application with application publication number CN103509997A discloses a 440MPa grade cold-rolled high-strength automotive structural steel and a method for manufacturing the same. The design features of the patent are mainly achieved by adding Mn content (1.0-1.4%) and increasing carbon content and / or moderately adding Si content, thereby achieving the effect of strengthening the steel. A high Si content in the steel is detrimental to the surface quality.

[0006] Existing cold-rolled hot-dip galvanized steel sheets for photovoltaic brackets have technical problems such as stamping cracking and poor formability. Summary of the invention

[0007] The purpose of the present invention is to provide a cold-rolled hot-dip galvanized steel sheet for stamping with a tensile strength of 440 MPa and a manufacturing method thereof, mainly to solve the technical problems of stamping cracking and poor formability of the existing cold-rolled hot-dip galvanized steel sheets for photovoltaic brackets.

[0008] The technical idea of ​​the present invention is to design a composition with a moderate carbon and manganese content by adding titanium and micro-niobium alloys for cold-rolled hot-dip galvanized steel sheets with a thickness of 1.0-2.0 mm under a low cold rolling reduction ratio, and to regulate the intrinsic structure of the strip through hot rolling temperature, cooling and annealing processes to obtain ferrite and free cementite structure with better uniformity of the grain structure, thereby meeting the requirement of good formability of 440MPa grade cold-rolled hot-dip galvanized steel sheets when stamping to prepare photovoltaic brackets.

[0009] The technical solution adopted by the present invention is a cold-rolled hot-dip galvanized steel plate for stamping with a tensile strength of 440MPa, wherein the chemical composition weight percentage of the substrate is: C: 0.06%-0.08%, Si: 0.001%-0.050%, Mn: 0.80%-0.90%, P≤0.020%, S≤0.008%, Nb: 0.010%-0.020%, Ti: 0.015%-0.030%, N≤0.0060%, Al: 0.015%-0.060%, and the rest is iron and unavoidable inclusions.

[0010] The metallographic structure of the cold-rolled hot-dip galvanized steel sheet of the present invention is ferrite + free cementite, and the ferrite grain size grade is I9.5 to I10.5; the yield strength R of the 1.0 to 2.0 mm thick cold-rolled hot-dip galvanized steel sheet is P0.2 340~370MPa, tensile strength R m 440~470MPa, elongation after fracture A 50 mm It is 32% to 38%.

[0011] The cold-rolled hot-dip galvanized steel sheet of the present invention is mainly used for parts with high stamping forming requirements and for stamping processing of photovoltaic brackets.

[0012] The reasons why the chemical composition of the substrate of the cold-rolled hot-dip galvanized steel sheet for stamping with a tensile strength of 440 MPa described in the present invention is limited to the above range are as follows:

[0013] Carbon: Carbon is a solid solution strengthening element that can increase the strength of steel. If the carbon content in steel is high, it will not only increase the strength, but also form more coarse and brittle carbide particles in the steel, which is not conducive to forming; if the carbon content is too low, the strength of the steel plate will inevitably decrease. In order to prevent the longitudinal crack defects during continuous casting from causing defects on the hot-rolled plate and causing cold bending cracking, the peritectic region should be avoided as much as possible, and the C content target value should be set at 0.07%. Taking all factors into consideration, the carbon content range is set to 0.06% to 0.08%;

[0014] Silicon: Si in steel affects the quality of coating. When the silicon content is high, heavier Fe2O3 will be formed on the surface of the hot-rolled plate, which is difficult to remove during the pickling process. In the subsequent hot-dip galvanizing, the surface quality of the galvanized layer will be affected. Moreover, when the SI content is high, the iron atoms in the steel matrix will quickly migrate to the zinc liquid on the surface of the plated part, and the growth rate of the ξ phase will also be accelerated. It will penetrate into the thinner pure zinc layer and damage it, and the surface of the coating will become rough and dull. Therefore, the silicon content is controlled to be 0.001%~0.050%.

[0015] Manganese: As the main element for solid solution strengthening, adding appropriate amount of manganese to steel is beneficial to improving strength. However, high Mn content not only increases strength, but also increases the cost of steel accordingly. At the same time, in order to reduce the fluctuation of strength, it is necessary to limit the range of manganese content. Therefore, the Mn content is limited to 0.80% to 0.90%.

[0016] Phosphorus: Phosphorus is a cheap element with strong solid solution strengthening ability. As the phosphorus content increases, the solid solution strengthening effect of phosphorus on steel is enhanced, the banded structure becomes heavier, the product strength increases, while the plasticity and toughness decrease sharply, which is not conducive to the stamping formability of the product. Therefore, the present invention limits P to ≤ 0.020%.

[0017] Sulfur: Sulfur is a harmful element in steel. Sulfur will form sulfide inclusions in steel, reducing its ductility and toughness. Therefore, it should be controlled at a low level as much as possible, with a limit of S≤0.008%.

[0018] Niobium: Due to the thick thickness of the final product and the low cold rolling reduction ratio, in order to ensure product strength, fine grains and uniform structure, it is necessary to add trace alloying elements Nb for grain refinement strengthening, thereby further improving strength and product formability; therefore, the Nb content is limited to 0.010% to 0.020%.

[0019] Titanium: In order to ensure strength and production cost, a certain amount of Ti content of alloy elements needs to be added for precipitation strengthening; therefore, the Ti content is limited to 0.015% to 0.030%.

[0020] Nitrogen: Nitrogen is a harmful element in the steel of the present invention. Nitrogen will seriously reduce the plasticity and toughness of the material and is very sensitive to aging, which is not conducive to the subsequent processing formability of the steel plate. Therefore, the nitrogen content needs to be limited, and the limit is N≤0.0060%.

[0021] Aluminum: Aluminum is a strong oxidizing element, and forms Al2O3 with oxygen in steel, which is removed during steelmaking; too much aluminum will form too many Al2O3 inclusions, and it is easy to block the pouring nozzle during continuous casting. Therefore, the Al content is limited to 0.015% to 0.060%.

[0022] A method for manufacturing a cold-rolled hot-dip galvanized steel sheet for stamping with a tensile strength of 440 MPa, the method comprising:

[0023] The molten steel is continuously cast to obtain a continuous casting slab, wherein the chemical composition of the molten steel is as follows: C: 0.06% to 0.08%, Si: 0.001% to 0.050%, Mn: 0.80% to 0.90%, P≤0.020%, S≤0.008%, Nb: 0.010% to 0.020%, Ti: 0.015% to 0.030%, N≤0.0060%, Al: 0.015% to 0.060%, and the rest is iron and unavoidable inclusions;

[0024] The continuous casting slab is heated in a heating furnace and then hot rolled. The heating temperature of the continuous casting slab is 1210-1250°C. The hot rolling is a two-stage rolling process. The rough rolling is 5 passes of continuous rolling. The rolling is performed above the austenite recrystallization temperature. The end temperature of the rough rolling is 1030-1080°C. After the rough rolling, the thickness of the intermediate slab is controlled to be 38-42 mm. The finishing rolling is 7 passes of continuous rolling. The rolling is performed in the austenite single phase region. The end temperature of the finishing rolling is 840-880°C. After the finishing rolling, the thickness of the steel plate is controlled to be 3.0-4.0 mm. The laminar cooling adopts the front cooling. The coiling temperature is 605-635°C to obtain the hot rolled steel coil.

[0025] After the hot-rolled steel coil is unrolled, it is pickled, cold rolled, degreased, annealed in a horizontal continuous annealing furnace, hot-dip galvanized, and flattened to obtain a finished cold-rolled hot-dip galvanized steel sheet with a thickness of 1.0 to 2.0 mm; the cold rolling reduction rate is 50% to 75%, the annealing temperature of the hardened strip after cold rolling in the horizontal continuous annealing furnace is 780 to 810° C., the annealing time is 60 to 70 seconds, and the flattening elongation is 1.0% to 1.2%.

[0026] The reasons for the production process adopted by the present invention are as follows:

[0027] 1. Setting of heating temperature of continuous casting slab

[0028] For microalloyed steel, the heating temperature should ensure that microalloying elements such as Nb and Ti are dissolved in austenite, and play a role in precipitation strengthening and grain refinement during the subsequent deformation and cooling process, but at the same time, it is necessary to prevent the austenite grains from coarsening during heating. The heating temperature of the continuous casting slab is set at 1210℃~1250℃.

[0029] 2. Setting of rough rolling end temperature and intermediate billet thickness

[0030] The rough rolling process needs to be controlled to be rolled above the austenite recrystallization temperature. If the rough rolling end temperature is too high, it is easy to form too much secondary iron oxide scale, which is not easy to remove during the finishing rolling process. After pickling, a mountain-shaped iron oxide scale defect will be formed on the surface of the steel plate, affecting the surface of the steel plate and the use of the user; if the rough rolling end temperature is too low, the finishing rolling end temperature cannot be guaranteed, that is, rolling is performed above the austenite recrystallization temperature. The rough rolling end temperature of the present invention is set to 1030℃~1080℃.

[0031] The thickness of the intermediate billet affects the final rolling temperature of the steel plate. If the thickness of the intermediate billet is too low, the temperature drop of the strip billet is too large, resulting in a decrease in the subsequent final rolling temperature, especially a large temperature drop at the edge of the steel plate, which will deteriorate the structure and properties of the strip. If the intermediate billet is too thick, the finishing rolling load and rolling stability will increase. The present invention sets the thickness of the intermediate billet to 38mm-42mm.

[0032] 3. Finishing rolling end temperature setting

[0033] In order to refine the ferrite grains and obtain equiaxed grains, a lower final rolling temperature should be used. Low rolling temperature forms more deformation bands, reserves a large amount of deformation energy, promotes ferrite nucleation, and thus plays a role in refining ferrite grains. The target temperature for finishing rolling is set at 840℃~880℃.

[0034] 4. Setting of hot rolling coiling temperature

[0035] If the hot rolling coiling temperature is too high, not only will the recrystallized grains grow easily, but also the oxide scale will be too thick, making subsequent pickling difficult; if the hot rolling coiling temperature is too low, not only will the performance increase too much, but also affect the stability of the entire coil control; for this reason, the coiling temperature is set at 605-635℃.

[0036] 5. Setting of laminar cooling method after finishing rolling

[0037] Front-stage cooling is beneficial to obtaining grains with better uniformity, improving strength, and reducing the thickness of iron oxide scale as much as possible, which is beneficial to the surface quality of subsequent pickling. Therefore, the cooling method is designed to be front-stage cooling.

[0038] 6. Cold rolling reduction rate setting

[0039] Due to the increase in the cold rolling deformation of the cold rolling mill, the recrystallization of the subsequent annealing can be promoted to obtain a fine grain structure, thereby improving the formability of the product; but due to the limitations of the rolling capacity of the user's cold rolling mill and the finished product thickness of 1.0-2.0mm, the present invention sets the cold rolling reduction rate to 50% to 75%.

[0040] 7. Setting of annealing temperature and annealing time

[0041] In order to ensure the completion of recrystallization of the thick microalloy galvanized product and obtain the required product strength and product grain uniformity, the present invention sets the annealing temperature of the hardened strip after cold rolling in a horizontal continuous annealing furnace to 780-810°C and the annealing time to 60-70s.

[0042] 8. Setting of flattening elongation

[0043] Flattening can not only ensure a good plate shape of the strip steel, but also appropriately improve the strength of the material through the flattening rate. The flattening elongation of the present invention is 1.0% to 1.2%.

[0044] The metallographic structure of the hot-rolled steel plate produced by the present invention is ferrite + a small amount of pearlite, and the ferrite grain size level is 9.5-10.5; the yield strength R of the 3.0-4.0 mm thick hot-rolled steel plate is eH 380~460MPa, tensile strength is R m 470~500MPa, elongation after fracture A 50 mm It is 33% to 42%.

[0045] The metallographic structure of the cold-rolled hot-dip galvanized steel sheet produced by the present invention is ferrite + free cementite, and the ferrite grain size level is I9.5-I10.5; the yield strength R of the 1.0-2.0 mm thick cold-rolled hot-dip galvanized steel sheet is P0.2 340~370MPa, tensile strength R m 440~470MPa, elongation after fracture A 50 mm It is 32% to 38%.

[0046] Compared with the prior art, the present invention has the following positive effects: 1. The present invention adopts a composition design containing titanium and micro-niobium, and matches the hot rolling, acid rolling, annealing and flattening processes to obtain a cold-rolled hot-dip galvanized steel sheet with a surface and performance, especially tensile strength and elongation, that meets the processing requirements of photovoltaic brackets with high stamping requirements. At the same time, compared with the same type of steel, its manufacturing cost is lower. 2. The present invention adopts a composition containing titanium and micro-niobium, hot rolling temperature and cooling design, and through fine grain strengthening and precipitation strengthening, not only does its strength meet the requirements, but also its material has better uniformity of internal grain structure, thereby further improving the product forming performance, and completely solving the technical problem that the 1.0-2.0mm thick cold-rolled hot-dip galvanized steel sheet has poor uniformity of grain structure when the cold rolling reduction rate is 50% to 75% due to the limitation of cold rolling rolling capacity. 3. The present invention uses a low-carbon (0.06%-0.08%) and titanium-containing micro-niobium composition design, which is relatively high-carbon (0.09%-0.15%) composition design. During steelmaking production, the peritectic region can be avoided, which greatly avoids quality risks such as longitudinal cracks in the strip, thereby stabilizing product quality and performance. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 This is a metallographic photograph of the hot-rolled steel plate according to Example 1 of the present invention.

[0048] Figure 2 This is a metallographic structure photograph of the cold-rolled hot-dip galvanized steel sheet according to Example 2 of the present invention. DETAILED DESCRIPTION

[0049] The present invention is further described below in conjunction with Examples 1-6, as shown in Tables 1-4.

[0050] Table 1 shows the chemical composition (by weight percentage) of the substrate of the cold-rolled hot-dip galvanized steel sheet according to the embodiment of the present invention, and the rest is Fe and unavoidable impurities.

[0051] Table 1 Chemical composition of the substrate of the cold-rolled hot-dip galvanized steel sheet in the embodiment of the present invention, unit: weight percentage.

[0052]

[0053] According to the requirements of the material composition design of the present invention, molten steel with the required chemical composition is obtained by converter smelting. The molten steel is subjected to vacuum decarburization and degassing treatments and then continuously poured to obtain a continuous casting slab. The continuous casting slab has a thickness of 210 to 230 mm, a width of 900 to 1600 mm, and a length of 8500 to 11000 mm.

[0054] The cut-length slab produced by steelmaking is sent to the heating furnace for reheating, and then sent to the hot rolling mill for rolling after being descaled. The rolling is controlled by the rough rolling and finishing rolling mills, and the coiling is carried out after laminar cooling. The laminar flow adopts the front cooling to produce qualified hot-rolled steel coils. The thickness of the hot-rolled steel plate is 3.0-4.0 mm. The hot rolling process control parameters are shown in Table 2. Figure 1 The metallographic structure of the hot-rolled steel plate is ferrite + a small amount of pearlite, and the ferrite grain size is 9.5-10.5; the yield strength R of the 3.0-4.0 mm thick hot-rolled steel plate is eH 380~460MPa, tensile strength is R m 470~500MPa, elongation after fracture A 50 mm It is 33% to 42%.

[0055] Table 2 Hot rolling process control parameters of the embodiment of the present invention

[0056]

[0057]

[0058] The hot rolled steel coil is unrolled and then pickled, cold rolled, degreased, annealed in a horizontal continuous annealing furnace, hot dip galvanized, and flattened to obtain a finished cold rolled hot dip galvanized steel sheet with a thickness of 1.0 to 2.0 mm; the cold rolling reduction rate is 50% to 75%, the annealing temperature of the hardened strip in the horizontal continuous annealing furnace after cold rolling is 780 to 810°C, the annealing time is 60 to 70 seconds, and the flattening elongation is 1.0% to 1.2%. The process control parameters of cold rolling, annealing, and flattening are shown in Table 3.

[0059] Table 3 Cold rolling, annealing and leveling process control parameters of the embodiment of the present invention

[0060] parameter Cold rolling reduction ratio / % Soaking section temperature / ℃ Annealing time / S Flat elongation / % The present invention 50-75 780-810 60-70 1.0-1.2 Example 1 63.75 795 64 1.20 Example 2 62.50 786 62 1.15 Example 3 60.00 805 68 1.15 Example 4 57.10 790 65 1.10 Example 5 53.30 783 66 1.05 Example 6 60.00 793 68 1.10

[0061] The cold-rolled hot-dip galvanized steel sheet obtained by the above method is shown in Figure 2 The metallographic structure of cold-rolled hot-dip galvanized steel sheet is ferrite + free cementite, and the ferrite grain size is I9.5~I10.5; the yield strength R of 1.0~2.0mm thick cold-rolled hot-dip galvanized steel sheet is P0.2 340~370MPa, tensile strength R m 440~470MPa, elongation after fracture A 50 mm It is 32% to 38%.

[0062] The cold-rolled hot-dip galvanized steel sheet obtained by the present invention was subjected to a tensile test according to "GB / T228.1-2010 Tensile test of metallic materials Part 1: Room temperature test method", and the mechanical properties of the cold-rolled hot-dip galvanized steel sheet are shown in Table 4.

[0063] Table 4 Mechanical properties of cold-rolled hot-dip galvanized steel sheets according to the present invention

[0064] Performance Indicators <![CDATA[Yield strength R P0.2 / MPa]]> <![CDATA[Tensile strength R m / MPa]]> <![CDATA[Elongation after fracture A 50mm / %]]> The present invention 340～370 440～470 32-38 Example 1 354 451 36.2 Example 2 362 448 35.5 Example 3 352 462 34.5 Example 4 343 456 36.75 Example 5 367 467 33.25 Example 6 359 465 35.8

[0065] As shown in Table 4, the cold-rolled hot-dip galvanized steel sheets of Examples 1-6 have good mechanical properties and good surface quality, and fully meet the processing requirements of steel for stamping photovoltaic brackets and the like.

[0066] In addition to the above embodiments, the present invention may also have other implementation modes. Any technical solution formed by equivalent replacement or equivalent transformation falls within the protection scope required by the present invention.

Claims

1. A cold-rolled hot-dip galvanized steel sheet for stamping with a tensile strength of 440 MPa, the chemical composition weight percentage of the substrate is: C: 0.06%~0.08%, Si: 0.001%~0.050%, Mn: 0.80%~0.90%, P≤0.020%, S≤0.008%, Nb: 0.010%~0.020%, Ti: 0.015%~0.030%, N≤0.0060%, Al: 0.015%~0.060%, the rest is iron and unavoidable inclusions; 1.0~2.0mm thick cold-rolled hot-dip galvanized steel sheet yield strength R P0.2 340~370MPa, tensile strength R m 440~470MPa, elongation after fracture A 50mm It is 32% to 38%.

2. The cold-rolled hot-dip galvanized steel sheet for stamping with a tensile strength of 440 MPa as claimed in claim 1, characterized in that: The metallographic structure of the cold-rolled hot-dip galvanized steel sheet is ferrite+free cementite, and the ferrite grain size grade is I9.5-I10.

5.

3. The method for manufacturing a cold-rolled hot-dip galvanized steel sheet for stamping with a tensile strength of 440 MPa as claimed in claim 1, characterized in that: The following steps are involved: The molten steel is continuously cast to obtain a continuous casting slab, wherein the chemical composition of the molten steel is as follows: C: 0.06% to 0.08%, Si: 0.001% to 0.050%, Mn: 0.80% to 0.90%, P≤0.020%, S≤0.008%, Nb: 0.010% to 0.020%, Ti: 0.015% to 0.030%, N≤0.0060%, Al: 0.015% to 0.060%, and the rest is iron and unavoidable inclusions; The continuous casting slab is heated in a heating furnace and then hot rolled. The heating temperature of the continuous casting slab is 1210-1250°C. The hot rolling is a two-stage rolling process. The rough rolling is 5 passes of continuous rolling. The rolling is performed above the austenite recrystallization temperature. The end temperature of the rough rolling is 1030-1080°C. After the rough rolling, the thickness of the intermediate slab is controlled to be 38-42 mm. The finishing rolling is 7 passes of continuous rolling. The rolling is performed in the austenite single phase region. The end temperature of the finishing rolling is 840-880°C. After the finishing rolling, the thickness of the steel plate is controlled to be 3.0-4.0 mm. The laminar cooling adopts the front cooling. The coiling temperature is 605-635°C to obtain the hot rolled steel coil. After the hot-rolled steel coil is unrolled, it is pickled, cold rolled, degreased, annealed in a horizontal continuous annealing furnace, hot-dip galvanized, and flattened to obtain a finished cold-rolled hot-dip galvanized steel sheet with a thickness of 1.0 to 2.0 mm; the cold rolling reduction rate is 50% to 75%, the annealing temperature of the hardened strip after cold rolling in the horizontal continuous annealing furnace is 780 to 810° C., the annealing time is 60 to 70 seconds, and the flattening elongation is 1.0% to 1.2%.

4. The manufacturing method according to claim 3, characterized in that: The metallographic structure of the hot-rolled steel plate is ferrite + a small amount of pearlite, and the ferrite grain size is 9.5 to 10.5; the yield strength R of the 3.0 to 4.0 mm thick hot-rolled steel plate is eH 380~460MPa, tensile strength is R m 470~500MPa, elongation after fracture A 50mm It is 33% to 42%.

5. The manufacturing method according to claim 3, characterized in that: The metallographic structure of the cold-rolled hot-dip galvanized steel sheet is ferrite+free cementite, and the ferrite grain size grade is I9.5-I10.5.

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