Preparation method of cold-rolled steel for pipe making, cold-rolled steel and galvanized pipe

Abstract

The present application discloses a preparation method of cold rolled steel for pipe making, cold rolled steel and galvanized pipe. The method includes: converting molten iron in a converter and refining it in LF to obtain refined molten steel. The chemical composition of the molten iron includes: Cu ≤ 0.100%, S ≤ 0.060%, As ≤ 0.035%, Sn ≤ 0.050%, and Cu + 5As + 8Sn ≤ 0.350%; Cu + 5As + 8Sn represents the sum of the mass percentage of Cu in the molten iron, five times the mass percentage of As in the molten iron, and eight times the mass percentage of Sn in the molten iron; subjecting the refined molten steel to continuous casting and hot rolling to obtain hot rolled steel; cooling, descaling, and pickling the hot rolled steel to obtain pickled slab; continuously rolling the pickled slab using a matte roll to obtain cold rolled steel, and the surface roughness of the matte roll is 0.8 - 1.2 μm. The cold rolled coil obtained by the method of the present application has a good shape, and at the same time, the surface texturing state is better than that of a bright roll, improving the adhesion between the surface of the galvanized pipe steel and the zinc layer, and avoiding zinc layer peeling and delamination.

Description

Technical Field

[0001] This application belongs to the technical field of steelmaking, and specifically relates to a preparation method of cold-rolled hard steel for pipe making, cold-rolled hard steel, and galvanized pipe. Background Art

[0002] Steel products such as pipes and bars are manufactured through hot or cold processing, but bending will occur during processing or during heat treatment in the process. In the finishing process, this bending is usually corrected by a roller-type straightening machine (straight tube) that combines multiple drum-shaped rollers or other bending correction machines using rollers. It is required that the steel used for pipes and bars has good hardness and toughness.

[0003] In the related art, products such as pipes and bars need to be galvanized on the surface to prevent the internal base material from corroding and thus extend the service life of the protected metal components, and it can also enhance its aesthetic effect. However, when the steel that has been bent by heat treatment is subjected to the galvanizing process, its performance cannot meet the requirements and needs to be improved. Summary of the Invention

[0004] In view of this, this application provides a preparation method of cold-rolled hard steel for pipe making, cold-rolled hard steel, and galvanized pipe, aiming to provide a preparation method to improve the galvanizing effect of cold-rolled hard steel in the subsequent galvanizing process.

[0005] In a first aspect, an embodiment of this application provides a preparation method of cold-rolled hard steel for pipe making, including:

[0006] Carry out converter smelting and LF refining on hot metal to obtain refined molten steel. Among them, the chemical components of the hot metal include: Cu≤0.100%, S≤0.060%, As≤0.035%, Sn≤0.050%, and Cu + 5As + 8Sn≤0.350%; Cu + 5As + 8Sn represents the sum of the mass percentage of Cu in the hot metal, 5 times the mass percentage of As in the hot metal, and 8 times the mass percentage of Sn in the hot metal;

[0007] Carry out continuous casting and hot rolling on the refined molten steel to obtain hot-rolled steel;

[0008] Cool, descale, and pickling the hot-rolled steel to obtain a pickled slab;

[0009] Carry out tandem rolling on the pickled slab using a felt roll to obtain cold-rolled hard steel, where the surface roughness of the felt roll is 0.8 - 1.2 μm.

[0010] According to an embodiment of one aspect of this application, the surface roughness of the felt roll is 0.9 - 1.0 μm.

[0011] According to an embodiment of one aspect of the present application, the chemical composition of the refined molten steel, in mass percentage, includes: 0.170% ≤ C ≤ 0.200%, Si ≤ 0.100%, 0.150% ≤ Mn ≤ 0.400%, P ≤ 0.035%, S ≤ 0.006%, As ≤ 0.035%, 0.020% ≤ Alt ≤ 0.070%, Cu ≤ 0.100%, Sn ≤ 0.050%, and Cu + 5As + 8Sn ≤ 0.350%, where Cu + 5As + 8Sn represents the sum of the mass percentage of Cu, five times the mass percentage of As, and eight times the mass percentage of Sn in the refined molten steel.

[0012] According to an embodiment of one aspect of the present application, the mass ratio of manganese content to sulfur content in the refined molten steel is (25 - 40):1.

[0013] According to an embodiment of one aspect of the present application, the reduction ratio of continuous rolling is 55% - 80%.

[0014] According to an embodiment of one aspect of the present application, after continuous casting, the tapping temperature of the slab is 1060 - 1200 °C; the thickness of the slab is 55 - 70 mm.

[0015] According to an embodiment of one aspect of the present application, hot rolling includes rolling using a CSP hot tandem mill, and the finishing rolling temperature of hot rolling is 865 - 895 °C.

[0016] In a second aspect, an embodiment of the present application provides a cold rolled steel, including: the cold rolled steel obtained by the preparation method of the first aspect.

[0017] According to an embodiment of one aspect of the present application, the surface roughness of the cold rolled steel is 0.3 - 0.4 μm.

[0018] According to an embodiment of one aspect of the present application, the thickness of the cold rolled steel is 0.50 - 1.00 mm.

[0019] In a second aspect, an embodiment of the present application provides a galvanized pipe, including: the cold rolled steel obtained by the preparation method of the first aspect or the cold rolled steel of the second aspect; and a galvanized layer provided on the surface of the cold rolled steel.

[0020] Compared with the prior art, the present application has at least the following beneficial effects:

[0021] According to the method of the embodiments of the present application, considering the use of the cold rolled steel in the later stage, that is, considering the production process of producing hot-dip galvanized pipes, medium carbon steel composition design and steel plate production process are adopted. In continuous rolling, low-roughness hair rollers are used for rolling. The produced cold rolled coils have good shape, and at the same time, the surface texturing state is better than that of smooth rollers, which improves the adhesion between the surface of the galvanized pipe steel and the zinc layer, and avoids zinc layer peeling and delamination. On the other hand, after using hair rollers for treatment, the shape of the steel plate can be improved and the edge of the steel plate can be thinned, which is beneficial to obtaining a suitable thickness for the cold rolled steel and having better surface quality, comprehensively improving the comprehensive quality of the cold rolled steel. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the drawings required to be used in the embodiments of the present application will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present application. For those of ordinary skill in the art, without creative efforts, other drawings can be obtained according to the drawings.

[0023] Figure 1 FIG. shows a partial preparation process flow chart of cold rolled steel according to an embodiment of the present application;

[0024] Figure 2 FIG. shows a partial preparation process flow chart of cold rolled steel according to an embodiment of the present application;

[0025] Figure 3 FIG. shows a morphology diagram of cold rolled steel according to an embodiment of the present application;

[0026] Figure 4 FIG. shows the surface roughness of cold rolled steel according to an embodiment of the present application;

[0027] Figure 5 FIG. shows a morphology diagram of a galvanized pipe according to an embodiment of the present application. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0028] In order to make the application purpose, technical solutions and beneficial technical effects of the present application clearer, the present application will be further described in detail below in conjunction with embodiments. It should be understood that the embodiments described in this specification are only for explaining the present application and are not intended to limit the present application.

[0029] For simplicity, this application only explicitly discloses some numerical ranges. However, any lower limit can be combined with any upper limit to form a range not explicitly recited; and any lower limit can be combined with other lower limits to form a range not explicitly recited, and similarly any upper limit can be combined with any other upper limit to form a range not explicitly recited. In addition, although not explicitly recited, each point or single value between the range endpoints is included in the range. Thus, each point or single value can be used as its own lower or upper limit and combined with any other point or single value or combined with other lower or upper limits to form a range not explicitly recited.

[0030] In the description of this application, it should be noted that unless otherwise specified, "above" and "below" include the recited number, and the meaning of "several" in "one or several" is two or more than two.

[0031] The above application content of this application does not intend to describe every disclosed embodiment or every implementation manner in this application. The following description more specifically gives examples of exemplary embodiments. Throughout the application, guidance is provided through a series of embodiments, which can be used in various combinations. In each instance, the listing is only a representative group and should not be construed as exhaustive.

[0032] Steel products such as pipes and bars are manufactured by hot or cold processing, but they will bend during processing or when heat treatment is carried out during the process. Products such as pipes and bars need to be galvanized on the surface to prevent internal base materials from corroding and thus extend the service life of the protected metal components, and it can also enhance their aesthetic effects. However, when the steel bent by heat treatment is galvanized, its performance cannot meet the requirements and needs to be improved.

[0033] In the related art, the steel that needs to be galvanized includes the steel for hot-dip galvanized pipes, and carbon structural steel such as Q235B can be used.

[0034] To achieve the goals of improving quality, reducing costs, and increasing efficiency, pipe manufacturers put forward the need to improve Q235B grade cold rolled steel. The steel for hot-dip galvanized pipes needs to meet requirements such as slitting, straight seam welding, zinc plating, and square pipe forming. After research, it is found that compared with the hot-rolled steel in the related art, for the steel that meets the above requirements, that is, the cold rolled steel, the requirements for flatness, edge quality, and surface roughness of the plate are higher. The existing preparation methods cannot meet the requirements.

[0035] In view of this, the embodiments of this application provide a preparation method for cold rolled steel, aiming to improve the comprehensive performance of cold rolled steel including zinc plating and square pipe forming. The embodiments of this application also provide a cold rolled steel and a galvanized pipe having the above effects.

[0036] Preparation method of cold rolled steel for pipe making

[0037] In a first aspect, an embodiment of the present application provides a method for preparing cold-rolled steel for pipe making, including:

[0038] The hot metal is subjected to converter smelting and LF refining to obtain refined molten steel. The chemical composition of the hot metal includes: Cu≤0.100%, S≤0.060%, As≤0.035%, Sn≤0.050%, and Cu + 5As + 8Sn≤0.350%; Cu + 5As + 8Sn represents the sum of the mass percentage of Cu in the hot metal, five times the mass percentage of As in the hot metal, and five times the mass percentage of Sn in the hot metal.

[0039] The refined molten steel is continuously cast and hot-rolled to obtain hot-rolled steel.

[0040] The hot-rolled steel is cooled, descaled, and pickled to obtain a pickled slab.

[0041] The pickled slab is continuously rolled using a felt roll to obtain cold-rolled steel, where the surface roughness of the felt roll is 0.8 - 1.2 μm.

[0042] According to the embodiment of the present application, the chemical composition of the hot metal within the above range is beneficial to the design of medium-carbon steel composition, enabling the chemical composition of the hot metal to satisfy Cu + 5As + 8Sn≤0.350%, which has the positive effects of facilitating the control of chemical composition and the adjustment of scrap steel structure during the subsequent smelting process, and also facilitating the stability of the mechanical properties of the product.

[0043] According to the embodiment of the present application, making the surface of the felt roll have a specific roughness, on the one hand, is beneficial to making the surface of the cold-rolled steel have a suitable roughness. The suitable roughness will not reduce the surface quality of the subsequent galvanized pipe, and can also improve the adhesion between the surface of the galvanized pipe steel and the zinc layer, avoiding zinc layer peeling and delamination; on the other hand, after treatment with the felt roll, the sheet shape of the steel plate can be improved and the edge of the steel plate can be thinned, which is beneficial to making the cold-rolled steel obtain a suitable thickness and have better surface quality.

[0044] According to the embodiment of the present application, considering the use of the cold-rolled steel in the later stage, that is, considering the production process of producing hot-dip galvanized pipes, a medium-carbon steel composition design and a steel plate production process are adopted. The continuous rolling uses a felt roll with low roughness for rolling. The produced cold-rolled coil has a good shape, and at the same time, the surface texturing state is better than that of a smooth roll. The galvanized layer is easily adhered and not easily peeled off, comprehensively improving the comprehensive quality of the cold-rolled steel.

[0045] In some embodiments, the chemical composition of the hot metal includes: Cu≤0.100%, S≤0.060%, As≤0.035%, Sn≤0.050%, and Cu + 5As + 8Sn≤0.350%.

[0046] In some embodiments, the chemical composition of the hot metal, by mass percentage, includes: 0.070% ≤ C ≤ 0.500%, 0.010% ≤ P ≤ 0.300%, 0.30 ≤ Si ≤ 1.25%; Cu ≤ 0.100%, S ≤ 0.060%, As ≤ 0.035%, Sn ≤ 0.050%, with the balance being iron and inevitable impurity elements, and Cu + 5As + 8Sn ≤ 0.350%.

[0047] In some alternative embodiments, the surface roughness of the hair roll is 0.9 - 1.0 μm. According to the embodiments of the present application, the surface roughness of the hair roll being 0.9 - 1.0 μm can enable the cold rolled steel to obtain a suitable roughness, thereby improving the galvanizing quality of the cold rolled steel in subsequent galvanizing processes.

[0048] In some alternative embodiments, the mass ratio of manganese content to sulfur content in the refined molten steel is (25 - 40):1. By controlling the mass ratio of manganese content to sulfur content in the refined molten steel and using the control of the minimum Mn / S to ensure the edge quality and hot and cold working properties of the raw material coil and avoid the generation of cracks at the corners of the continuous casting billet. At the same time, manganese is a good deoxidizer and desulfurizer with a high Mn content, which is beneficial to improving the smelting quality of the molten steel. In addition, controlling the mass ratio of manganese content to sulfur content in the refined molten steel is beneficial to controlling the MnS inclusions in the cold rolled steel, and the two aspects cooperate to improve the edge quality and hot and cold working properties of the cold rolled steel.

[0049] In some alternative embodiments, the chemical composition of the refined molten steel, by mass percentage, includes: 0.170% ≤ C ≤ 0.200%, Si ≤ 0.100%, 0.150% ≤ Mn ≤ 0.400%, P ≤ 0.035%, S ≤ 0.006%, As ≤ 0.035%, 0.020% ≤ Alt ≤ 0.070%, Cu ≤ 0.100%, Sn ≤ 0.050%, and Cu + 5As + 8Sn ≤ 0.350%. Cu + 5As + 8Sn represents the sum of the mass percentage of Cu, 5 times the mass percentage of As, and 8 times the mass percentage of Sn in the refined molten steel.

[0050] According to the embodiments of the present application, the refined molten steel can be understood as the molten steel out of the refining station. By controlling the contents of manganese element and sulfur element in the refined molten steel and controlling the content of Cu + 5As + 8Sn within a suitable range, it is beneficial to control the MnS inclusions in the cold rolled steel, beneficial to improving the edge quality and hot and cold working properties of the cold rolled steel, and also beneficial to improving the order immediate delivery rate.

[0051] In some alternative embodiments, the reduction ratio of tandem rolling is 55%-80%. In the related art, the reduction ratio used in tandem rolling is 65%-75%, and there are often situations where order deliveries are not timely. Controlling the reduction ratio of tandem rolling within the above range in this application expands the thickness conversion range value between cold rolling and hot rolling to increase the thickness of the substrate, expands the range of thicknesses of cold rolled coils that can be achieved for the same substrate specification, and is beneficial for the timely fulfillment of customer orders while also improving the production efficiency of cold rolling and hot rolling.

[0052] As an example, when the thickness of the substrate for steel used in galvanized pipes increases by about 0.2 mm, the maximum rolling force of acid rolling will not exceed the rated rolling force. Using the above reduction ratio, the range of substrate thicknesses that can be achieved for the same target thickness of cold rolled coil is appropriately enlarged by 0.1 - 0.2 mm, expanding the range of thicknesses of cold rolled coils that can be achieved for the same substrate specification, which is beneficial for ensuring the timely fulfillment of customer orders while also improving the production efficiency of cold rolling and hot rolling. Specifically, as shown in Table 1.

[0053] The following table lists the comparison of the substrate thickness before and after the optimization of the reduction ratio of cold rolled coils for pipe making in the thickness range of 0.47 - 1.07 mm.

[0054] Table 1

[0055]

[0056]

[0057] In Table 1, when using the original tandem rolling reduction ratio process with a reduction ratio of 65%-75%, for a steel plate with a thickness of 3.0 mm before tandem rolling (original CQ grade smooth surface B), the reduction ratio can only fulfill the order for finished coils with a thickness range of 0.77 - 0.8699 mm. By improving the chemical composition of the molten iron, controlling the relevant preparation processes, and coordinating with the annealing process in the subsequent process of preparing cold rolled steel into galvanized pipes, the reduction ratio of this process can be widened. Using the reduction ratio of this application, the new (LGZG1 grade matte surface D) reduction ratio can fulfill the order for finished coils with a thickness range of 0.67 - 1.0699 mm for this 3.0 mm substrate. This not only greatly expands the range of thicknesses of finished coils that can be achieved for the same substrate thickness but also is beneficial for the centralized production of substrates of the same specification.

[0058] In some alternative embodiments, after continuous casting, the tapping temperature of the billet is 1060 - 1200 °C; the thickness of the billet is 55 - 70 mm. Controlling the tapping temperature of the billet is beneficial for the subsequent hot rolling process to control the thickness of the billet. Controlling the thickness of the billet within the above range is beneficial for the subsequent processes such as hot rolling and tandem rolling, is beneficial for controlling the thickness range of cold rolled coils, and improves the timely fulfillment of customer orders while also improving the production efficiency of cold rolling and hot rolling.

[0059] According to the embodiments of the present application, medium carbon steel mold powder is used as the steel grade, and it can be poured together with medium carbon steel grades such as Q235B and S235JR. Before starting the casting, leak detection is carried out on each argon sealing system, and the argon blowing in the tundish is ≥2 min; after the ladle starts casting, the superheat is controlled at 15°C to 35°C; the casting speed is ≥3.8 m / min, and the tundish composition is controlled with C at 0.170% to 0.200%, Si at 0 to 0.150%, Mn at 0.15% to 0.40%, P at 0 to 0.035%, and S at 0 to 0.006%. The cold rolled steel of the present application has high requirements for surface quality. For slabs with abnormal conditions such as rephosphorization ≥0.005%, splicing, diversion of molten steel, and the stopper rising or falling by 5 mm, the base plate of this steel grade can be not rolled.

[0060] In addition, to prevent uneven secondary cooling in continuous casting, the nozzles in the grid section and segment are inspected offline before production, and phenomena such as nozzle blockage, nozzle drop, and nozzle deviation are not allowed. The water-proof conditions of the water baffle and water collecting tank are inspected during the casting interval.

[0061] In some alternative embodiments, hot rolling includes rolling using a CSP hot tandem mill, and the finishing rolling temperature of hot rolling is 865 - 895°C. The CSP hot tandem mill can be used for rolling to effectively improve the control accuracy of the coiling temperature of this steel, so as to reduce the performance fluctuation of the base plate. Controlling the finishing rolling temperature of hot rolling is beneficial to controlling the internal quality and hot and cold processing performance of hot rolled steel, and improving the comprehensive performance of cold rolled steel.

[0062] In some alternative embodiments, the properties of hot rolled steel include: thickness h < 3.0 mm, convexity C40 is 25 - 55 μm; wedge |W40| < C40; the thickness difference of hot rolled steel ≤ 0.1 mm.

[0063] According to the embodiments of the present application, W40 and C40 are shape control parameters, and their measured values are the values at 40 mm away from the edge of the steel plate in the transverse direction, which are internationally common values. |W40| < C40 applies to the control requirements of all steel coils. Only by ensuring |W40| < C40 is it beneficial to shape control. The thickness difference of the hot rolled coil = actual thickness - target thickness. The thickness difference of hot rolled steel ≤ 0.1 mm is beneficial to the stability of the rolling force in the continuous annealing and pickling process and reduces the thickness fluctuation of the cold rolled (finished) coil. In summary, controlling the properties of hot rolled steel is beneficial to improving the comprehensive performance of cold rolled steel and making it better meet the relevant processes for subsequent production of galvanized pipes.

[0064] As an example, as Figure 1 and Figure 2 shown, the process for preparing cold rolled coils according to the embodiments of the present application includes blast furnace smelting, converter smelting, LF refining, continuous casting, heating furnace heating, hot rolling, cooling, coiling, uncoiling, descaling and pickling and trimming, and tandem rolling.

[0065] Second aspect, an embodiment of the present application provides a cold-rolled steel, comprising: the cold-rolled steel obtained by the preparation method of the first aspect.

[0066] In some alternative embodiments, the roughness of the cold-rolled steel is 0.3 - 0.4 μm. Controlling the roughness of the cold-rolled steel within the above range is beneficial to improving the adhesion between the galvanized layer and the steel substrate during the subsequent preparation of galvanized pipes.

[0067] In some alternative embodiments, the thickness of the cold-rolled steel is 0.50 - 1.00 mm. Controlling the thickness of the cold-rolled steel within the above range is beneficial to the subsequent preparation into tubular parts.

[0068] Third aspect, an embodiment of the present application provides a galvanized pipe, comprising: the cold-rolled steel obtained by the preparation method of the first aspect or the cold-rolled steel of the second aspect; and a galvanized layer provided on the surface of the cold-rolled steel.

[0069] Example

[0070] The following embodiments more specifically describe the content disclosed in the present application. These embodiments are only for illustrative purposes, because various modifications and changes within the scope of the present application disclosure are obvious to those skilled in the art. Unless otherwise stated, all parts, percentages, and ratios reported in the following embodiments are based on weight, and all reagents used in the embodiments are commercially available or synthesized according to conventional methods and can be used directly without further treatment, and the instruments used in the embodiments are all commercially available.

[0071] Example 1

[0072] An embodiment of the present application provides a preparation method of cold-rolled steel for pipe making, comprising:

[0073] Under the conditions of Cu ≤ 0.100%, S ≤ 0.060%, As ≤ 0.035%, Sn ≤ 0.050% and Cu + 5As + 8Sn ≤ 0.350%, the molten iron is subjected to quality control operations in a converter, an argon station and an LF refining furnace to obtain qualified molten steel that meets the internal control of the steel grade composition. Then, it undergoes CSP continuous casting and rolling → pickling and cold tandem rolling → inspection, packaging and warehousing.

[0074] Converter steel smelting: The C content at tapping is controlled at 0.100% - 0.155%, the carbon content at the argon station is controlled at 0.100% - 0.160%, and the tapping temperature of the converter is ≥ 1600 °C;

[0075] 1) Molten iron conditions

[0076] The molten iron conditions are as follows: P ≤ 0.150%, S ≤ 0.06%. If the S content in the molten iron > 0.06%, it enters the desulfurization station for desulfurization, and the S content at the exit ≤ 0.03%. The requirements for residual elements in the molten iron are as follows:

[0077] Table 2

[0078] Element Cu As Sn S Cu + 5As + 8Sn Max, % ≤0.1 ≤0.035 ≤0.05 ≤0.05 ≤0.35

[0079] Entering the converter for smelting, the molten steel includes by mass percentage: C: 0.1925 Wt%; Si: 0.0347 Wt%; Mn: 0.1796 Wt%; P: 0.0221 Wt%; S: 0.0027 Wt%; Al: 0.0294 Wt%; the rest is iron and inevitable trace elements. The tapped C is controlled at 0.1058%, the C at the argon station is controlled at 0.1034%, and the tapping temperature of the converter is 1617 °C;

[0080] 2) Control Table of End-point Composition and Temperature of Converter Molten Steel

[0081] Table 3

[0082] C，％ P，％ S，％ Tapping temperature Liquidus temperature, °C 0.100-0.155 ≤0.025 ≤0.060 ≥1600℃ 1515

[0083] 3) Composition and Temperature at the Handover Point of the Argon Station

[0084] Table 4

[0085]

[0086] LF refining treatment: The smelting time is controlled within 45 - 120 min; during desulfurization, the temperature is controlled at 1560 - 1580 °C; according to the slag condition and [S] content, lime, slag melting agent, pre-melted slag and calcium carbide are added to adjust the slag, and the four-component basicity of the slag is controlled at about 2.0, (FeO)+(MnO) in the slag < 1.0%, the slag is slightly thin and white. After the molten steel enters the station, the bottom blowing is turned on immediately for slag melting. Side stirring is not allowed throughout the process to prevent serious nitrogen increase. The alloy composition is controlled according to the target. Temperature measurement must be carried out before calcium treatment to ensure the tapping temperature. After calcium treatment, power supply, large argon stirring and feeding are prohibited.

[0087] The composition of the molten steel tapped from LF is shown in the following table (internal control).

[0088] Table 5

[0089]

[0090] For the first tapping furnace, appropriately increase Als: 30 - 50 ppm; the tapping temperature of non-transfer ladles can be increased by 5 - 10 °C; ensure that the calming time ≥ 12 min. LF refining treatment: The smelting time is 68 min; the temperature during desulfurization is 1566 - 1573 °C; the composition of the molten steel tapped from LF is controlled with C at 0.1896%, Si at 0.0386%, Mn at 0.1832%, P at 0.0223%, S at 0.0036%, and the soft blowing time is 10 min.

[0091] CSP continuous casting: Argon blowing in the tundish for 2 minutes before starting casting; after starting casting from the ladle, the actual superheat is controlled at 18 - 25 °C; the casting speed is increased to 4.2 m / min and kept constant after starting casting. The tundish composition is controlled as follows: C at 0.1925%, Si at 0.0347%, Mn at 0.196%, P at 0.0221%, and S at 0.0027%.

[0092] CSP continuous rolling: Control the slab tapping temperature at 1060 - 1200 °C; roll with a CSP hot continuous rolling mill, control the finishing rolling temperature at 880 ± 15 °C, and the coiling temperature at 680 ± 18 °C.

[0093] To effectively improve the control accuracy of the coiling temperature of this steel, which is beneficial to reducing the performance fluctuations of the substrate, the hot rolling CT is increased by 10 - 20 °C compared with the ordinary medium carbon steel substrate for external sales.

[0094] 1) Setting of hot rolling descaling water

[0095] Table 6

[0096] Spraying pressure before descaling, bar Spraying pressure after descaling, bar ≥200 ≥300

[0097] 2) Setting of hot rolling temperature

[0098] Table 7

[0099]

[0100]

[0101] 3) Requirements for substrate shape and size

[0102] Table 8

[0103] Thickness range, mm Crown C40, µm Wedge W40, µm Thickness deviation at head and tail, mm h＜3.0 40±15 |W40| < C40 ±0.1 h≥3.0 45±15 |W40| < C40 ±0.1

[0104] 4) Technical requirements for hot rolling

[0105] (1) Descaling pressure: It can be appropriately increased to ensure the descaling effect.

[0106] (2) The control of finishing mill load distribution, tension system, work roll cooling water, speed system and side guide all adopt the L2 AUTO mode.

[0107] (3) The multi-functional instrument, flatness meter, width, thickness and shape control systems are normal and in use, and problems should be dealt with in a timely manner.

[0108] Pickling cold tandem rolling: To meet the requirements of high shape and surface quality of the pipe-making raw material coil during customer use, this steel grade adopts the special curve for No. 9 carriage plate optimized with reference to the shape curve of automotive high-strength steel pickling and rolling. In addition, for the convenience of rolling and order production of the substrate of the raw material coil, a special reduction rate parameter table for this steel grade is compiled and maintained.

[0109] The average hit rate of strip shape at 8 points of the exported strip steel is not less than 98%.

[0110] Example 2

[0111] The difference between this example and Example 1 is that: for the converter smelting, the molten steel includes, by mass percentage: C: 0.1861 Wt%; Si: 0.0441 Wt%; Mn: 0.1743 Wt%; P: 0.0234 Wt%; S: 0.0026 Wt%; Al: 0.0276 Wt%; the rest is iron and inevitable trace elements.

[0112] Converter smelting of molten steel: the C content at tapping is controlled at 0.1194%, the C content at the argon station is controlled at 0.1206%, and the tapping temperature of the converter is 1620 °C;

[0113] LF refining treatment: the smelting time is 76 min; the temperature during desulfurization is 1568 - 1575 °C; the composition control at the LF tapping is C at 0.1834%, Si at 0.0438%, Mn at 0.1737%, P at 0.0234%, S at 0.0026%, and the soft blowing time is 8 min.

[0114] CSP continuous casting: this furnace is the second furnace of continuous casting molten steel; after the tundish is opened for pouring, the actual superheat is controlled at 20 - 25 °C; the drawing speed is kept constant at 4.2 m / min, and the composition control in the tundish is C at 0.1861%, Si at 0.0441%, Mn at 0.1743%, P at 0.0234%, S at 0.0026%.

[0115] Hot rolling: the thickness of the hot-rolled substrate is automatically matched with the order target thickness for production according to the optimized reduction ratio of the LGZG1 grade special.

[0116] Comparative Example 1

[0117] The difference between this comparative example and Example 1 is that: the composition of the molten steel at the LF tapping is different, and the type of the cold-rolled steel prepared is Q235B, and the content of Mn and S components is as shown in the following table.

[0118] Table 9

[0119] Steel grade Mn%, S％ Mn (Min)%, S (Max)%, Mn / S ratio (Min) Q235B 0.08～0.40 ≦0.008 0.08 0.008 10.0

[0120] Comparative Example 2

[0121] The difference between this comparative example and Example 1 is that: the pickled slab is continuously rolled by a bright roll.

[0122] Comparative Example 3

[0123] The difference between this comparative example and Example 1 is that the substrate thickness range in which the target thickness of the cold-rolled coil can be achieved is not appropriately enlarged, which limits the cold-rolled coil thickness range that can be achieved with the substrate specifications, resulting in untimely order delivery.

[0124] Test section

[0125] 1) Detect the thickness and width of the cold-rolled coil

[0126] The hot-rolled coils of Example 1 were grouped according to Y1 - Y4, and the hot-rolled coils of Example 2 were grouped according to Y5 - Y8. By using different finishing rolling temperatures and coiling temperatures for the steel plates of Example 1 and Example 2, their thicknesses were measured, as shown in Table 10.

[0127] Table 10

[0128]

[0129]

[0130] 2) After subjecting the hot-rolled steels of Y1 - Y8 in Example 1 and Example 1 to acid and continuous rolling with a certain reduction ratio, measure the thickness of the cold-rolled coil.

[0131] Table 11

[0132]

[0133] 3) Final product coil quality inspection

[0134] The shape and surface quality of the final product coils (cold-rolled coils) of A1 - A8 meet the requirements of the Q / OHAC935 - 2020 standard range; on a WAW - 300 micro-electro-hydraulic servo testing machine, samples with a specification of 260mm * 20mm * 0.5mm were used for performance testing of the final product (cold-rolled coil), and the specific test data are shown in Table 12. It meets the performance requirements in the customer's delivery conditions.

[0135] According to GB / T228.1 - 2010 "Metallic materials - Tensile testing - Part 1: Method of test at room temperature", its mechanical properties were tested and the comprehensive judgment results are shown in the following table.

[0136] Table 12

[0137]

[0138] As can be seen from the above table, the yield strength of the cold-rolled steel produced in Example 1 and Example 2 is between 830 - 900 Mpa, the tensile strength is between 930 - 960 Mpa, and the elongation is between 5.0% - 6.5%, meeting the requirements of cold-rolled steel.

[0139] 4) Detection of the quality of the opposite sides of the examples and Comparative Example 1 by using different mass ratio ranges of manganese content to sulfur content in refined molten steel.

[0140] Table 13

[0141]

[0142] According to the above table, the mass ratio of manganese content to sulfur content in the refined molten steel at the outlet of the refining station was measured, and the yield rate of the good quality of the edge part of the cold rolled steel produced was statistically analyzed. It was found that when the mass ratio of manganese content to sulfur content was within a suitable range, the yield rate of the good quality of the edge part of the cold rolled steel was relatively high.

[0143] 5) The cold rolled steels prepared in Examples 1-2 and Comparative Example 2 were hot dip galvanized, and the adhesion between the zinc layer and the steel base and the quality of the galvanized steel pipes produced from the cold rolled coils with different surface roughnesses of the rolling rolls were detected. The adhesion between the zinc layer and the steel base was classified by the ISO 2409 cross cut method, and the results are shown in Table 14. The cracking rate of the galvanized pipes after hot dip galvanizing was statistically analyzed, and the results are shown in the following table.

[0144] Table 14

[0145]

[0146] According to the surface roughness of the cold rolled steel in the above table, it was found that when the surface roughness of the cold rolled steel was within a suitable range, it was beneficial to improve the adhesion between the zinc layer and the steel base of the galvanized pipe. The adhesion level in the examples was relatively high, being 0-1 level, while that in the comparative examples was poor, with easy peeling and cracking, etc. Moreover, when the surface roughness of the cold rolled steel was within a suitable range, it was beneficial to reduce the cracking rate of the galvanized pipe.

[0147] Figure 3 The cold rolled steel of Example 1 is shown. Figure 4 The rough condition of the surface of the cold rolled steel of Example 1 is shown. Figure 5 The galvanized pipe made of the cold rolled steel of the embodiment of the present application is shown, indicating that the cold rolled steel prepared by the method of the application is suitable for producing galvanized pipes.

[0148] The above is only the specific implementation manner of the present application, but the protection scope of the present application is not limited thereto. Any person skilled in the art within the technical scope disclosed by the present application can easily think of various equivalent modifications or substitutions, and these modifications or substitutions should all be covered within the protection scope of the present application. Therefore, the protection scope of the present application shall be subject to the protection scope of the claims.

Claims

1. A preparation method of cold-rolled steel for pipe making, characterized in that, Comprising: The hot metal is subjected to converter smelting and LF refining to obtain refined molten steel. Among them, the chemical composition of the hot metal includes: Cu ≤ 0.100%, S ≤ 0.060%, As ≤ 0.035%, Sn ≤ 0.050%, and Cu + 5As + 8Sn ≤ 0.350%. Cu + 5As + 8Sn represents the sum of the mass percentage of Cu in the hot metal, 5 times the mass percentage of As in the hot metal, and 5 times the mass percentage of Sn in the hot metal; The chemical composition of the refined molten steel in mass percentage includes: 0.170% ≤ C ≤ 0.200%, Si ≤ 0.100%, 0.150% ≤ Mn ≤ 0.400%, P ≤ 0.035%, S ≤ 0.006%, As ≤ 0.035%, 0.020% ≤ Alt ≤ 0.070%, Cu ≤ 0.100%, Sn ≤ 0.050%, and Cu + 5As + 8Sn ≤ 0.350%. Cu + 5As + 8Sn represents the sum of the mass percentage of Cu in the refined molten steel, 5 times the mass percentage of As in the refined molten steel, and 5 times the mass percentage of Sn in the refined molten steel; The mass ratio of manganese content to sulfur content in the refined molten steel is (25 - 40):1; The refined molten steel is subjected to continuous casting and hot rolling to obtain hot-rolled steel; The hot-rolled steel is cooled, dephosphorized, and pickled to obtain a pickled slab; The pickled slab is continuously rolled using a felt roll to obtain cold-rolled steel; Among them, the surface roughness of the felt roll is 0.8 - 1.2 μm; The thickness of the cold-rolled steel is 0.50 - 1.00 mm.

2. The preparation method according to claim 1, characterized in that, The surface roughness of the felt roll is 0.9 - 1.0 μm.

3. The preparation method according to any one of claims 1-2, characterized in that, The reduction ratio of the continuous rolling is 55% - 80%.

4. The preparation method according to any one of claims 1-2, characterized in that, After continuous casting, the tapping temperature of the slab is 1060 - 1200 °C; The thickness of the slab is 55 - 70 mm.

5. The preparation method according to any one of claims 1-2, characterized in that, The hot rolling includes rolling using a CSP hot tandem mill, and the finishing rolling temperature of the hot rolling is 865 - 895 °C.

6. A cold-rolled steel, characterized in that, Comprising: The cold-rolled steel prepared by the preparation method according to any one of claims 1 to 5.

7. The cold-rolled steel according to claim 6, characterized in that, The surface roughness of the cold-rolled steel is 0.3 - 0.4 μm; and / or, The thickness of the cold-rolled steel is 0.50 - 1.00 mm.

8. A galvanized pipe, characterized in that, Comprising: The cold-rolled steel prepared by the preparation method according to any one of claims 1 to 5 or the cold-rolled steel according to claim 6 or 7; And a galvanized layer provided on the surface of the cold-rolled steel.

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