CSP-process 420 mpa-grade low-alloy pickled steel for hot-dip galvanization and manufacturing method therefor

Abstract

The present invention belongs to the technical field of iron and steel metallurgy production. Disclosed are a CSP-process 420 MPa-grade low-alloy pickled steel for hot-dip galvanization and a manufacturing method therefor. The low-alloy pickled steel for hot-dip galvanization of the present invention comprises the chemical components in percentage by mass: 0.04-0.06% of C, 0.01-0.04% f Si, 0.55-0.75% of Mn, ≤0.010% of P, ≤0.006% of S, 0.25-0.35% of Cr, 0.025-0.035% of Ti, 0.020-0.030% of Nb, 0.02-0.05% of Als, and ≤0.006% of N, the Si equivalent value Sieq=Si+Cr / 3-P≥0.1% being satisfied, and the balance being iron and inevitable impurities. The low-alloy pickled steel for hot-dip galvanization of the present invention has excellent surface quality and structural property uniformity, reduces the defective rate caused by iron oxide scale press-in, has excellent mechanical properties, and can achieve stable batch production.

Description

Low-alloy pickling steel for 420mpa hot-dip galvanizing using CSP process and manufacturing method thereof TECHNICAL FIELD

[0001] The present application belongs to the technical field of steel metallurgical production, and particularly relates to a low-alloy pickling steel for 420MPa hot-dip galvanizing using a CSP process and a manufacturing method thereof. BACKGROUND

[0002] Under the background of green manufacturing in the steel industry, the thin slab continuous casting and rolling process represented by the CSP process has become a trend for producing thin-gauge hot-rolled pickling steel. The relevant varieties include low-carbon steel, medium-high carbon steel, low-alloy high-strength steel, and advanced high-strength steel, which can be widely used in the automobile and parts manufacturing, container, logistics, power engineering, agricultural machinery and mechanical manufacturing, hardware and electrical appliances, and tool industries. The use of the CSP production line to produce thin-gauge hot-rolled pickling steel can achieve "hot instead of cold", reducing the material procurement and processing costs of end users. Hot-dip galvanizing with pickling steel has the advantages of stable process, simple operation, good corrosion resistance of products, short production cycle, low cost, and the like. In addition, the plated layer is beautiful, uniform in thickness, firmly combined, and has a long service life. However, the hot-dip galvanizing process has an upper limit requirement on the Si content in the base material (Si≤0.04 wt%) to prevent the Sandelin effect. Under the conditions of large slab thickness and high rolling rhythm in the short process CSP production line, the low-Si-content low-alloy pickling steel for hot-dip galvanizing is prone to iron oxide scale indentation defects at the finishing stage, affecting the galvanizing appearance of the final product and causing the iron oxide scale indentation defect rate to rise. In addition, the surface supercooling during the finishing process also causes the mechanical property qualification rate to decrease. Therefore, the main difficulty in producing the 420MPa low-alloy pickling steel for hot-dip galvanizing using the CSP process lies in the comprehensive control of surface quality and mechanical properties. SUMMARY

[0003] The technical problem to be solved by the present application is to provide a low-alloy pickling steel for 420MPa hot-dip galvanizing using a CSP process and a manufacturing method thereof, which has excellent surface quality and uniformity of microstructure and properties, reduces the iron oxide scale indentation defect rate, and has excellent mechanical properties, enabling stable batch production.

[0004] To solve the technical problems proposed in the present application, the present application provides a 420MPa-grade hot-dip galvanizing low-alloy pickling steel in a CSP process, which has a chemical composition in terms of mass percentage of C: 0.04-0.06%, Si: 0.01-0.04%, Mn: 0.55-0.75%, P≤0.010%, S≤0.006%, Cr: 0.25-0.35%, Ti: 0.025-0.035%, Nb: 0.020-0.030%, Als: 0.02-0.05%, N≤0.006%, and the balance of iron and inevitable impurities. Preferably, the chemical composition in terms of mass percentage is C: 0.045-0.050%, Si: 0.020-0.030%, Mn: 0.65-0.70%, P≤0.010%, S≤0.005%, Cr: 0.30-0.35%, Ti: 0.030-0.035%, Nb: 0.024-0.028%, Als: 0.02-0.035%, N≤0.006%, and the balance of iron and inevitable impurities. Further preferably, the chemical composition in terms of mass percentage is C: 0.045-0.050%, Si: 0.022-0.025%, Mn: 0.65-0.68%, P≤0.010%, S≤0.005%, Cr: 0.30-0.32%, Ti: 0.030-0.032%, Nb: 0.024-0.026%, Als: 0.030-0.035%, N≤0.006%, and the balance of iron and inevitable impurities.

[0005] In the above scheme, the low-alloy pickling steel for hot-dip galvanizing satisfies the Si equivalent value Si eq =Si+Cr / 3-P≥0.1%, wherein Si, Cr and P represent the mass percentage of each.

[0006] The present application also provides a manufacturing method of the 420MPa-grade hot-dip galvanizing low-alloy pickling steel in a CSP process, which comprises the following steps: molten steel smelting→ thin slab continuous casting→ slab pre-heat scaling→ slab reheating furnace heating→ post-precision rolling descaling→ precision rolling→ laminar cooling→ coiling, so as to obtain a hot-rolled raw material coil, and then the low-alloy pickling steel for hot-dip galvanizing is obtained through flattening, pickling and oiling.

[0007] In the scheme, the continuous casting adopts a two-strand slab continuous casting machine, and the production of the steel grade based on the product contract plan of the production line has two modes, i.e., the same steel grade mode and the different steel grade mode, and the thickness of the two-strand slab is controlled by the liquid core pressure reduction in both modes. Further, when the two-strand slab is of the same steel grade, i.e., both are hot-dip galvanizing low-alloy pickling steel, the thickness of the two-strand slab is controlled to be 58-62 mm; when the two-strand slab is of different steel grades, one strand is hot-dip galvanizing low-alloy pickling steel and the other strand is an auxiliary steel grade, the thickness of the hot-dip galvanizing low-alloy pickling slab is controlled to be 58-62 mm, and the thickness of the auxiliary steel slab is controlled to be 62-76 mm. Specifically, the auxiliary steel grade adopts C-Mn steel or micro-alloyed steel, and the composition satisfies Si equivalent value Si eq ≥0.10%, and the yield strength of the finished product is ≤500 MPa. The auxiliary steel grade is not the target steel grade of the present application, and is based on the actual product contract plan of the production line. The optimal selection of other steel grades other than the steel grade of the present application has certain requirements for the Si equivalent and the yield strength of the auxiliary steel, which aims to reduce the deterioration of the roll and is beneficial to the roll in a favorable state during the rolling of the steel grade of the present application.

[0008] In the scheme, the pressure of the descaling before the furnace is 140-160 bar, and the flow rate is 80-90 m 3 / h.

[0009] In the scheme, low-temperature short-time heating is adopted in the slab heating stage, the heating time in the soaking furnace is 25-35 min, and the discharge temperature is 1180-1200℃.

[0010] In the scheme, the pressure of the descaling before the finishing rolling is 300-380 bar.

[0011] In the scheme, the finishing rolling adopts two-strand slab cross rolling.

[0012] Further, when the two-strand slab is of the same steel grade, any strand slab is used for roll heating in the rolling unit, and the thickness of the rolling finished product of the two strands is controlled to be 1.5-3.0 mm; when the two-strand slab is of different steel grades, the auxiliary steel slab is used for roll heating in the rolling unit, the thickness of the rolling finished product of the hot-dip galvanizing low-alloy pickling steel is 1.5-3.0 mm, and the thickness of the rolling finished product of the auxiliary steel grade is 4.0-8.0 mm. The thicker the finished product thickness of the auxiliary steel grade, the smaller the unit rolling force on the roll surface, which is beneficial to the establishment and maintenance of the roll surface oxidation film, i.e., beneficial to the roll surface state during the rolling of the target steel grade of the present application.

[0013] In the above scheme, the low-alloy pickling steel plate blank for hot-dip galvanizing in the finishing process is subjected to thickness transition by 3.0 mm→2.0 mm→1.8 mm→1.5 mm. Further, when the two-flow plate blanks are of the same steel grade, the low-alloy pickling steel for hot-dip galvanizing in the rolling unit is used for rolling of the 3.0 mm specification for transition≥3, the 2.0 mm specification for transition≥2, and the 1.8 mm specification for transition≥2. Further, when the two-flow plate blanks are of different steel grades, the low-alloy pickling steel for hot-dip galvanizing in the rolling unit is used for rolling of the 3.0 mm specification for transition≥1, the 2.0 mm specification for transition≥2, and the 1.8 mm specification for transition≥2.

[0014] In the above scheme, when the rolling sequence number in the rolling unit is≤6, the rolling gap time is 60-65 s; when the rolling sequence number is>6, the rolling gap time is 45-50 s. The rolling sequence number is the sequential number of each rolled piece in a rolling unit, and the first 6 rolled pieces belong to the initial stage of rolling, and the oxidation film of the roll is not yet stable in the initial stage, and the gap time needs to be prolonged to prevent the roll surface temperature from rising too fast and causing the oxidation film of the roll to fail prematurely.

[0015] In the above scheme, the finishing rolling adopts a seven-stand finishing rolling mill.

[0016] Further, a composite surface cooling method of roll gap cooling+descaling between stands+strip cooling between stands is used for the surface of the strip; the roll gap cooling water flow rate of each stand F1-F4 is 150-165 m 3 / h; the descaling water pressure between F1-F2 stands and F2-F3 stands is 140-160 bar, and the flow rate is 80-90 m 3 / h; the cooling water flow rate between F1-F2 stands and F2-F3 stands is 170-195 m 3 / h, and the cooling water flow rate between F3-F4 stands is 115-130 m 3 / h.

[0017] Further, a working roll cooling+lubricating oil lubrication rolling method is used for the surface of the roll, the working roll cooling water parameters of each stand F2-F4 are 0.48-0.56 m 3 / (h·mm), the lubricating rolling oil flow rates of F1-F4 are 70-75 ml 3 / h, 90-95 ml 3 / h, 90-95 ml 3 / h, 50-55 ml 3 / h.

[0018] Further, the F2-F4 stands adopt centrifugal casting high-speed steel rollers, and the unit area rolling force of the F2-F4 stands is controlled, the unit area rolling force of the F2 stand is ≤300 MPa, the unit area rolling force of the F3 stand is ≤500 MPa, and the unit area rolling force of the F4 stand is ≤700 MPa.

[0019] In the scheme, the finish rolling temperature of the low-alloy pickling steel for hot-dip galvanizing is 850-870 DEG C.

[0020] In the scheme, the laminar cooling and coiling stage is according to the effective Ti value Ti eff =Ti-3.4N-3S, wherein Ti, N and S respectively represent the mass percentage of each. eff When Ti eff ≤0.01%, the former dense fast cooling mode is adopted, the single-group water quantity of the rough adjustment section is 1450-1500 m 3 / h, 2-2.25 groups are opened, and the coiling temperature is 600-620 DEG C; when Ti eff ≥0.01%, the former fast cooling mode is adopted, the single-group water quantity of the rough adjustment section is 700-750 m 3 / h, 3.5-3.75 groups are opened, and the coiling temperature is 620-640 DEG C.

[0021] The low-alloy pickling steel for hot-dip galvanizing manufactured by the application has a yield strength of 423-471 MPa, a tensile strength of 512-570 MPa, and an elongation of 18-26%, and the microstructure is ferrite and pearlite, wherein the volume ratio of the ferrite is 96-98%, and the ferrite grain size is 10.5-12.

[0022] The chemical component design of the low-alloy pickling steel for hot-dip galvanizing is based on the following principles:

[0023] C: low-carbon component design is adopted to avoid the peritectic zone, provide certain solid solution and precipitation strengthening effect; the CO gas formed by the C element in the steel is precipitated to reduce the adhesion, which is the main factor leading to the blistering of the iron oxide scale. Therefore, the C content is controlled to be 0.04-0.06%.

[0024] Mn: provide strengthening effect and improve toughness; the Mn element will not be segregated at the interface of the iron oxide scale, and the oxidation resistance is general, and has no significant influence on the high-temperature adhesion of the iron oxide scale. Therefore, the Mn content is controlled to be 0.55-0.75%.

[0025] Si: hot galvanizing process is to prevent Sandelin effect limit the Si content in the substrate ≤0.04%; Si element will form Fe2SiO4 at the interface between the substrate and the oxide scale, reduce the mismatch between the oxide scale and the substrate, increase the high temperature adhesion; the interface between the substrate and the oxide scale forms Fe2SiO4 to hinder the diffusion of Fe ions, thins the oxide scale thickness, is beneficial to the ductility of the oxide scale; when the Si content in the steel is low, there is a very high tendency of oxide scale blistering. Therefore, the Si content of the present application is controlled to be 0.01~0.04%, and the alloy or scrap steel is added.

[0026] P: harmful element in steel, deteriorates cold formability; P will form molten Fe3(PO4)2 at the interface between the oxide scale and the substrate, which will deteriorate the high temperature adhesion, and the oxide scale is easy to blister when P is high; based on the steelmaking conditions, the P content is controlled to be ≤0.010% considering the cost.

[0027] S: harmful element in steel, deteriorates cold formability, affects the strengthening effect of Ti, based on the steelmaking conditions, the S content is controlled to be ≤0.006% considering the cost.

[0028] N: affects the strengthening effect of Ti element, based on the steelmaking conditions, the N content is controlled to be ≤0.006% considering the cost.

[0029] Als: when Al is used for deoxidation, the Als content is controlled to be 0.02~0.05% in the present application.

[0030] Nb: fine-grain strengthening and precipitation strengthening element, the Nb content is controlled to be 0.020~0.030% in the present application.

[0031] Ti: precipitation strengthening and fine-grain strengthening element, the strengthening effect is affected by the control level of N and S in the steel, the Ti content is controlled to be 0.025~0.035% in the present application.

[0032] Cr: the Cr content is controlled to be 0.25~0.35% in the present application, and the Si equivalent value Si eq=Si+Cr / 3-P≥0.1%, the purpose is to control the growth and bubbling behavior of the strip steel oxide skin under the premise of guaranteeing the mechanical properties of the strip steel.(1) The chemical elements in the steel affect the high-temperature oxidation behavior, when the growth stress of the oxide skin is greater than the adhesion stress, the oxide skin will bubble, and the bubbling oxide skin will be folded under the action of rolling and form a fish scale-shaped oxide skin pressed into the strip steel along the rolling direction. The C element in the steel is the main factor promoting the bubbling of the oxide skin. The Si element added in the steel increases the adhesion to inhibit bubbling. P promotes bubbling, but generally, the steel grade does not intentionally add P except for a small amount of P strengthened steel or weathering steel. The Si content of the present application is low, and there is a high tendency of oxide skin bubbling, and a certain amount of Cr element is added to compensate for the adhesion of the oxide skin.(2) The chemical elements in the steel affect the high-temperature oxidation kinetics, the Si element added in the steel will form Fe2SiO4 at the interface between the matrix and the oxide skin, hinder the diffusion of Fe ions, and thus thin the oxide skin thickness, which is beneficial to the ductility of the oxide skin; a certain amount of Cr element is added to compensate for the oxidation resistance.(3) The oxidation resistance and anti-oxidation skin bubbling of the Si element in the steel are stronger than those of the Cr element, so the Cr element needs to be compensated by multiple times, and the Si eq =Si+Cr / 3-P≥0.1%;(4) Increasing the Cr element can appropriately reduce the Mn element, and the mechanical properties and alloy cost are balanced through the ratio of the Mn element and the Cr element.

[0033] The design concept of the low-alloy pickling steel for hot galvanizing in the manufacturing method is as follows:

[0034] In the finishing rolling stage, the surface of the strip steel and the surface of the roller are in a complex working condition, if the rolling condition is abnormal (such as the surface of the roller is deteriorated or the oxide skin of the surface of the strip steel is abnormally grown and bubbled), the oxide skin of the surface layer of the strip steel will be pressed into the matrix to form an oxide skin pressing-in defect, and the oxide skin pressing-in defect will appear as a pitting or pit defect after pickling, which significantly affects the surface quality, and figure 5 is a schematic diagram of the oxide skin pressing-in defect of the strip steel surface caused by the bubbling of the oxide skin. The oxide skin pressing-in defect in the finishing rolling stage is directly related to the factors such as the oxidation behavior of the surface of the strip steel, the oxidation behavior of the surface of the roller, the friction condition of the roll gap, and the mechanical state of the strip steel and the roller in the roll gap. The present application controls the oxidation behavior of the surface of the strip steel, the oxidation behavior of the surface of the roller, the friction condition of the roll gap, and the mechanical state of the strip steel and the roller in the roll gap to obtain a strip steel surface with controlled oxide skin pressing-in defect:

[0035] (1) Control of the strip surface oxidation behavior: High surface temperature will increase the tendency of the iron oxide scale thickening and blistering, and the high valence oxides will increase the abrasive wear of the work roll surface, the work roll protective oxide layer is easy to wear and peel off, which will lead to the decrease of the work roll surface hardness and the increase of the heat exchange, accelerating the work roll degradation. Therefore, the present application adopts low temperature and short time heating in the slab heating stage, and in the finishing stage, the composite surface cooling mode of high pressure descaling before finishing + roll gap cooling + inter-stand descaling + inter-stand cooling is used to reduce the strip surface temperature, inhibit the thickening of the strip surface iron oxide scale, inhibit the growth of the high valence oxides (Fe2O3, hard phase) on the strip surface, inhibit the blistering of the strip surface iron oxide scale and the heat penetration of the strip to the roll.

[0036] (2) Control of the roll surface oxidation behavior: In the continuous casting and finishing stage, the strength grade and Si equivalent value Si eq of the cross-assisted steel grade for different steel grades are limited, the slab thickness and the finished product thickness for the same steel grade are limited, the slab thickness and the rolling transition strategy of the two-flow slab for the same steel grade are limited, and the roll heating and rolling gap time are specifically limited. The purpose is to use some "simple" slabs for roll heating in the early stage of rolling unit, so as to form a thin and stable oxide layer on the surface of the work roll, which acts as a protective barrier and can delay the surface degradation of the roll. If the stable thin oxide layer can be maintained as long as possible during the whole rolling process, a longer rolling plan can be realized; this requires a balance between the wear and growth of the oxide: if the oxide layer is too thick, it will be easy to peel off, and the wear of the oxide film is too fast, which will expose the roll material and have no protection, leading to rapid surface degradation. In order to generate and maintain a stable thin oxide layer, a suitable rolling plan is needed to prevent excessive wear and deterioration caused by too many continuous difficult-to-roll steel coils (thin gauge or high strength).

[0037] (3) Comprehensive control of the roll gap friction condition, the strip and the roll in the mechanical state of the roll gap: in the finishing stage, the work roll cooling + lubrication rolling mode is adopted for the roll surface, the purpose is to reduce the roll surface temperature through work roll cooling to ensure the thermal fatigue performance of the roll, and to reduce the rolling force and slow down the abrasive wear of the roll surface oxide layer and reduce the rolling mill vibration through rolling lubrication; the purpose of controlling the unit area rolling force of F2, F3 and F4 stands is to improve the service performance of the roll oxide film by reducing the unit area rolling force of the roll surface in the roll gap, to prevent its premature failure, wear and peeling.

[0038] In the laminar cooling and coiling stage, different laminar cooling modes and coiling temperatures are adopted according to the Ti eff content of the hot-dip galvanizing low alloy pickling steel smelting composition, the reasons are as follows:

[0039] (1) Ti in Ti-containing micro-alloyed steel combines with C, N and S in the steel to generate TiN, TiC, TiS and Ti4C2S2, etc., wherein TiC accounts for the main strengthening effect. Assuming that the effective Ti content is all present in the form of TiC, the effective Ti content in the steel is calculated using the formula Ti eff = Ti - 3.4N - 3S. The N and S control in the CSP production line smelting process fluctuates, so the calculated Ti eff content fluctuates;

[0040] (2) A composite surface cooling mode is used on the surface of the strip steel, and a lower finish rolling temperature induces more TiC to precipitate in the austenite region. The larger size of the precipitates contributes less to the strength, reduces the TiC dissolved in the austenite, weakens the number of TiC particles precipitated in the low temperature region, and reduces the precipitation strengthening effect, i.e., the actual Ti eff < calculated Ti eff . Therefore, according to the calculated Ti eff value of the low-alloy pickling steel for hot galvanizing in actual smelting composition, different laminar cooling modes and coiling temperatures CT are adopted, i.e., when Ti eff < 0.01%, the front dense fast cooling and the CT of 600-620 DEG C are adopted, and when Ti eff ≥ 0.01%, the front fast cooling and the CT of 620-640 DEG C are adopted, i.e., a faster cooling speed after rolling is adopted to refine the grains.

[0041] Compared with the prior art, the present application has the following beneficial effects:

[0042] The present application obtains the required microstructure and performance of the 420 MPa grade low-alloy pickling steel for hot galvanizing through Nb-Ti composite micro-alloying and corresponding rolling and cooling process control; the composition optimization design and the comprehensive control of the continuous casting-heating-precision rolling processes are adopted to regulate the surface scale layer of the strip steel, the oxidation film on the surface of the roll, the thermal fatigue, the contact fatigue and the roll surface wear behavior, reduce the surface scale indentation defect occurrence rate under the conditions of thick slab, high rolling rhythm of the short process CSP production line, and through the flexible adjustment of the laminar cooling mode and the coiling temperature CT, the problem of the performance qualified rate reduction caused by the surface supercooling during the precision rolling process is solved; and finally the low-alloy pickling steel for hot galvanizing has excellent surface quality and microstructure performance uniformity, reduces the scale indentation defect rate, and has excellent mechanical properties, so that batch stable production can be realized. BRIEF DESCRIPTION OF DRAWINGS

[0043] Fig. 1 is a manufacturing process schematic diagram of the low-alloy pickling steel for hot galvanizing in the embodiment of the present application.

[0044] Fig. 2 is a schematic diagram of the seven-stand precision rolling mill and the precision rolling water system in each stand in the embodiment of the present application.

[0045] Fig. 3 is a schematic diagram of the opening of the header of the roughing section header group when the laminar cooling adopts (a) the front fast cooling mode and (b) the front dense fast cooling mode in the embodiment of the present application, and the diagram shows one header group.

[0046] Fig. 4 is a schematic diagram of the influence of the finishing temperature FT7 on the yield strength YS in the test process of the present application.

[0047] Fig. 5 is a schematic diagram of the scale indentation defect caused by the scale blistering of the strip steel surface.

[0048] Fig. 6 is the (a) metallographic structure and (b) pickling surface diagram of the low-alloy pickling steel for hot galvanizing manufactured in the embodiment 8 of the present application.

[0049] Reference signs: 1a-two-strand slab continuous casting machine A line; 1b-two-strand slab continuous casting machine B line; 2a-continuous casting slab A line; 2b-continuous casting slab B line; 3a-furnace front medium-pressure descaler A line; 3b-furnace front medium-pressure descaler B line; 4a-pivoting shear A line; 4b-pivoting shear B line; 5a-slab A line; 5b-slab B line; 6a-soaking furnace A line; 6b-soaking furnace B line; 7-precise rolling front high-pressure descaler; 8-seven-stand precise rolling mill group; 9-strip steel; 10-laminar cooling section; 11-coiler; 8.1-roller gap cooling water, 8.2-roller gap lubrication, 8.3-inter-stand descaling water, 8.4-working roll cooling water, 8.5-inter-stand cooling water. Embodiment of the present application

[0050] In order to better understand the present application, the content of the present application is further illustrated below in combination with the embodiments, but the content of the present application is not limited to the following embodiments only.

[0051] Fig. 1 is a schematic diagram of the manufacturing process of the low-alloy pickling steel for hot galvanizing in the embodiment of the present application, and the specific process is as follows: the continuous casting is divided into a flow and a b flow, the molten steel after smelting is cast into uncut continuous casting slabs 2a, 2b through two-strand slab continuous casting machines 1a, 1b, then the scale on the surface of the slab is removed through furnace front medium-pressure decalers 3a, 3b, the slab is cut into slabs 5a, 5b of fixed size through pivoting shears 4a, 4b, and then enters soaking furnace A line 6a and soaking furnace B line 6b for heating, respectively, and then the two-strand slab enters the precise rolling front high-pressure descaler 7 for surface descaling, and the two-strand slab after descaling enters the seven-stand precise rolling mill group 8 for rolling into strip steel 9, and the strip steel 9 enters the laminar cooling section 10 for cooling and then enters the coiler 11 for coiling. The continuous casting a flow slab and the b flow slab share one set of rolling mill for rolling, and in general, the number of the two-strand slab is basically the same, and cross rolling is adopted.

[0052] Figure 2 is a schematic diagram of the seven-stand finishing mill group and the finishing water system in each stand in the embodiment of the present application. The specific process is as follows: after the slab is descaled on the surface by the high-pressure descaling machine 7, the slab enters the seven-stand finishing mill group 8 for finishing. When the strip is being finished, the roll gap cooling water 8.1 is turned on to cool the surface of the strip before entering the roll gap, the roll gap lubrication 8.2 is turned on to form a lubricating oil film on the surface of the roll, the inter-stand descaling water 8.3 is turned on to remove the iron oxide scale on the surface of the strip while reducing the surface temperature, the work roll cooling water 8.4 is turned on to cool the surface of the roll and reduce the thermal fatigue of the roll, and the inter-stand cooling water 8.5 is turned on to reduce the surface temperature of the strip.

[0053] Figure 3 is a schematic diagram of the opening of the header of the roughing section header group when the laminar cooling is in the (a) front section fast cooling mode and (b) front section dense fast cooling mode in the embodiment of the present application. Figure 3 shows one group of headers. Each group of headers is composed of 6 upper headers and 16 lower headers. The 6 upper headers and 16 lower headers are turned on at the same time for dense fast cooling, and the 4 upper headers and 8 lower headers are turned on at the same time for fast cooling.

[0054] In the following examples, the chemical composition of the 420 MPa grade hot-dip galvanizing low-alloy pickling steel in the CSP process is as follows in terms of mass percentage: C: 0.04-0.06%, Si: 0.01-0.04%, Mn: 0.55-0.75%, P≤0.010%, S≤0.006%, Cr: 0.25-0.35%, Ti: 0.025-0.035%, Nb: 0.020-0.030%, Als: 0.02-0.05%, N≤0.006%, and Si equivalent value Si eq =Si+Cr / 3-P≥0.1%, and the balance is iron and unavoidable impurities. The specific composition used in each example is shown in Table 1.

[0055] Table 1 Melting composition of the hot-dip galvanizing low-alloy pickling steel in the examples and comparative examples (wt.%)

[0056] Item C Si Mn P S Al N Nb Ti Cr Si eq Example 1-4 0.04 0.05 0.02 0.65 0.008 0.004 0.032 0.003 4 0.024 0.032 0.30 0.12 Example 5-8 0.04 0.05 0.02 0.68 0.007 0.002 0.032 0.003 4 0.024 0.032 0.30 0.12 Comparative Example 1-4 0.05 0.029 0.75 0.009 0.004 0.032 0.004 7 0.025 0.035 0.03 0.03 Comparative Example 5-8 0.04 0.021 0.63 0.008 0.003 0.033 0.005 0 0.026 0.028 0.31 0.12

[0057] In the following embodiment, the CSP process 420 for manufacturing a low alloy pickling steel for hot-dip galvanizing at 420 MPa includes the following steps:

[0058] S1, smelting of the molten steel: the molten steel is smelted by a converter steelmaking- ladle refining process according to the composition in Table 1;

[0059] S2, thin slab continuous casting: a two-strand slab caster is used, and the two strands are of the same steel grade or different steel grades. The thickness of the two strands is controlled by liquid core reduction, and specifically includes:

[0060] When the two strands are of the same steel grade, i.e., both are low alloy pickling steel for hot-dip galvanizing, the thickness of the two strands is controlled to be 58-62 mm;

[0061] When the two strands are of different steel grades, one strand is low alloy pickling steel for hot-dip galvanizing, and the other strand is an auxiliary steel grade. The auxiliary steel grade is a C-Mn steel or a micro-alloyed steel, and the composition satisfies the Si equivalent value Si eq ≥ 0.10%, and the yield strength of the finished product is ≤ 500 MPa. The thickness of the slab of the low alloy pickling steel for hot-dip galvanizing is controlled to be 58-62 mm, and the thickness of the slab of the auxiliary steel grade is controlled to be 62-76 mm;

[0062] S3, slab descaling before the furnace: the descaling water pressure is 140-160 bar, and the flow rate is 80-90 m 3 / h;

[0063] S4, slab heating in the soaking pit: the soaking time is 25-35 min, and the out-of-furnace temperature is 1180-1200°C;

[0064] S5, descaling before finish rolling: the descaling water pressure is 300-380 bar;

[0065] S6, finish rolling: a seven-stand finish rolling mill is used, and the two strands are cross-rolled. The slab is gradually transitioned to the finished product thickness. The strip and roll surface temperatures are controlled by a finish rolling water system, and the roll surface stress state in the roll gap is adjusted by a finish rolling reduction system. Specifically includes:

[0066] When the two strands are of the same steel grade, any strand is used for roll heating in the rolling unit, and the finished product thickness of the two strands is controlled to be 1.5-3.0 mm;

[0067] When the two strands are of different steel grades, the auxiliary steel grade strand is used for roll heating in the rolling unit. The finished product thickness of the low alloy pickling steel for hot-dip galvanizing is 1.5-3.0 mm, and the finished product thickness of the auxiliary steel grade is 4.0-8.0 mm;

[0068] The low-alloy pickling steel plate for hot galvanizing is subjected to thickness transition from 3.0 mm to 2.0 mm to 1.8 mm to 1.5 mm; when the two-flow plate blanks are of the same steel grade, the rolling quantity of the low-alloy pickling steel for hot galvanizing for transition of the 3.0 mm specification in the rolling unit is ≥3, the rolling quantity of the 2.0 mm specification is ≥2, and the rolling quantity of the 1.8 mm specification is ≥2; when the two-flow plate blanks are of different steel grades, the rolling quantity of the low-alloy pickling steel for hot galvanizing for transition of the 3.0 mm specification in the rolling unit is ≥1, the rolling quantity of the 2.0 mm specification is ≥2, and the rolling quantity of the 1.8 mm specification is ≥2;

[0069] When the rolling sequence number in the rolling unit is ≤6, the rolling gap time is 60-65 s, and when the rolling sequence number is >6, the rolling gap time is 45-50 s;

[0070] The composite surface cooling mode of roll gap cooling + inter-stand descaling + inter-stand strip cooling is adopted for the surface of the strip; the roll gap cooling water flow rate of each stand F1-F4 is 150-165 m 3 / h; the descaling water pressure between F1-F2 stands and between F2-F3 stands is 140-160 bar, and the flow rate is 80-90 m 3 / h; the cooling water flow rate between F1-F2 stands and between F2-F3 stands is 170-195 m 3 / h, and the cooling water flow rate between F3-F4 stands is 115-130 m 3 / h;

[0071] The working roll cooling + lubricating oil lubrication rolling mode is adopted for the surface of the roll, the working roll cooling water parameters of each stand F2-F4 are 0.48-0.56 m 3 / (h·mm), and the lubricating rolling oil flow rates of F1-F4 are 70-75 ml 3 / h, 90-95 ml 3 / h, 90-95 ml 3 / h, 50-55 ml 3 / h;

[0072] The centrifugal casting high-speed steel rolls are adopted for F2-F4 stands, the unit area rolling force of F2-F4 stands is controlled, the unit area rolling force of F2 stand is ≤300 MPa, the unit area rolling force of F3 stand is ≤500 MPa, and the unit area rolling force of F4 stand is ≤700 MPa;

[0073] The final rolling temperature FT7 of the low-alloy pickling steel for hot galvanizing is 850-870℃;

[0074] S7, laminar cooling and coiling: the effective Ti value Ti eff of the low-alloy pickling steel for hot galvanizing is calculated according to the Ti, N and S contents of the low-alloy pickling steel for hot galvanizing eff= Ti-3.4N-3S, and then according to Ti eff Different laminar cooling modes and coiling temperatures are adopted, specifically including:

[0075] When Ti eff ≤ 0.01%, the former dense fast cooling mode is adopted, the single-group water quantity of the rough adjustment section is 1450~1500 m 3 / h, 2~2.25 groups are opened, and the coiling temperature is 600~620℃;

[0076] When Ti eff ≥ 0.01%, the former fast cooling mode is adopted, the single-group water quantity of the rough adjustment section is 700~750 m 3 / h, 3.5~3.75 groups are opened, and the coiling temperature is 620~640℃;

[0077] S8, the hot-rolled raw material coil is subjected to flattening, pickling and oiling to obtain a low-alloy pickling steel for hot galvanizing.

[0078] The specific process parameters adopted in each embodiment are shown in Tables 2~7. The manufacturing processes of each comparative example and embodiment are the same, except that the individual process parameters are different.

[0079] Table 2 Process parameters of continuous casting in the examples and comparative examples

[0080]

[0081] Table 3 Process parameters of descaling before the furnace, soaking in the soaking furnace, and descaling before finish rolling in the examples and comparative examples

[0082]

[0083] Table 4 Process parameters of finish rolling in the examples and comparative examples (I)

[0084]

[0085] Table 5 Process parameters of finish rolling in the examples and comparative examples (II)

[0086]

[0087] Table 6 Process parameters of finish rolling in the examples and comparative examples (III)

[0088]

[0089] Table 7 Process parameters of laminar cooling and coiling in the examples and comparative examples

[0090]

[0091] The low-alloy pickling steel coils for hot-dip galvanizing produced in the rolling plans of examples 1-8 are 89 coils in total, the low-alloy pickling steel coils for hot-dip galvanizing produced in the rolling plans of comparative examples 1-4 are 25 coils in total, the low-alloy pickling steel coils for hot-dip galvanizing produced in the rolling plans of comparative examples 5-8 are 28 coils in total, the surface quality and performance index results of a coil of each example and comparative example are shown in table 8, and the statistical results of the surface quality and performance index results of all the steel coils are shown in table 9.

[0092] Table 8 surface quality and performance sampling detection results of the steel obtained from examples and comparative examples

[0093] Item Yield strength, MPa Tensile strength, MPa Elongation, % Performance qualified or not Microstructure Grain size Scale press-in defect Defect severity Example 1 445 282 3.3 Yes F+P 11 None 0 Example 2 445 250 5.3 Yes F+P 11.5 None 0 Example 3 345 355 5 3.6 Yes F+P 11.5 None 0 Example 4 445 632 2.5 Yes F+P 12 None 0 Example 5 435 533 2 4.1 Yes F+P 10.5 None 0 Example 6 445 251 2 5.3 Yes F+P 11 None 0 Example 7 445 566 2 4.0 Yes F+P 11 None 0 Example 8 435 343 2 3.0 Yes F+P 11 None 0 Comparative example 1 425 521 2.9 Yes F+P 10.5 None 0 Comparative example 2 445 656 2 4.9 Yes F+P 10.5 1 Comparative example 3 425 321 2.9 Yes F+P 11 2 Comparative example 4 445 656 2 4.9 Yes F+P 11 3 Comparative example 5 425 510 2 4.1 Yes F+P 10.5 None 0 Comparative example 6 430 533 2 6.2 Yes F+P 10.5 None 0 Comparative example 7 415 515 2 5.1 No F+P 11 None 0 Comparative example 8 383 849 2 4.0 No F+P 11 None 0

[0094] Table 9 statistical results of the surface quality and performance of the steel obtained from examples and comparative examples

[0095] Item Number of low-alloy pickling steel for hot-dip galvanizing produced in rolling plan, coil Scale press-in defect rejudgment rate Performance qualified rate Example 1-8 89 100% 100% Comparative example 1-4 25 20% 100% Comparative example 5-8 28 80% 89.2%

[0096] The specific embodiments of the above Comparative Examples 1 to 4, Comparative Examples 5 to 8 are compared as a whole with the Examples 1 to 4, Examples 5 to 8, respectively. The overall rate of improvement of the scale indentation defect of the steel obtained in Comparative Examples 1 to 4 is high, and the overall performance qualification rate of the steel obtained in Comparative Examples 5 to 8 is low. Although the surface quality or the mechanical properties of the steel strip can be selected in the Comparative Examples, the overall scale indentation defect rate and the performance qualification rate are low in this case, and the production line cannot be accepted. The problem to be solved by the present application is to provide a product having good surface quality and good mechanical properties by taking into account the surface quality and the mechanical properties, and to provide a production method for the steel product with low defect improvement rate and high performance qualification rate.

[0097] The present application controls the scale layer on the surface of the steel strip, the oxide film on the surface of the roll, the thermal fatigue, the contact fatigue and the roll surface wear behavior by comprehensive control of the CSP process of smelting-casting-heating-finishing rolling, solves the scale indentation defect problem of the 420 MPa grade hot-dip galvanized low alloy pickling steel under the conditions of short process CSP production line, large billet thickness and high rolling speed, reduces the rate of defective products caused by scale indentation defect, and realizes batch stable production of the steel. At the same time, the present application solves the problem of performance qualification rate reduction caused by surface supercooling in the finishing rolling process by flexible adjustment of the laminar cooling mode and the coiling temperature CT. In Comparative Examples 1 to 4, the Si equivalent of the hot-dip galvanized low alloy pickling steel and the auxiliary steel is too low, the furnace time is too long and the discharge temperature is too high during the casting billet heating stage, and the related parameters in the finishing rolling do not meet the requirements of the present application, and the scale indentation defect rate is significantly increased. In Comparative Examples 5 to 8, although the surface quality problem is solved by using the composite surface cooling mode of roll gap cooling + inter-stand descaling + inter-stand steel cooling, the performance qualification rate is significantly reduced when the laminar cooling mode and the coiling temperature are not adjusted accordingly when Ti eff <0.01%. The above Comparative Examples are difficult to realize batch stable production taking into account the surface quality and the mechanical properties.

[0098] The CSP process 420 MPa grade hot-dip galvanizing low-alloy pickling steel provided by the present application has the characteristics of excellent surface quality, microstructure and performance. Figure 6 is the (a) metallographic structure and (b) pickling surface diagram of the hot-dip galvanizing low-alloy pickling steel manufactured by the embodiment 8 of the present application. As can be seen from figure 6 (a), the microstructure of the hot-dip galvanizing low-alloy pickling steel is ferrite F and pearlite P, wherein the volume ratio of ferrite F is 97.5%, and the grain size of ferrite F is 11 grade, which provides support for good cold forming performance. As can be seen from figure 6 (b), the 420 MPa grade hot-dip galvanizing low-alloy pickling surface is white, without color difference, iron oxide skin indentation and pitting and other surface defects, showing good surface quality. The microstructure and performance of the hot-dip galvanizing low-alloy pickling steel prepared by the technical scheme of the present application are statistically analyzed, and the metallographic structure is sampled and analyzed. The yield strength is 423-471 MPa, the tensile strength is 512-570 MPa, and the elongation is 18-26%; the microstructure is ferrite and pearlite, wherein the volume ratio of ferrite is 96-98%, and the ferrite grain size is 10.5-12 grade.

[0099] The above examples are merely examples for clearly illustrating, but not limitation to the embodiments. Other different forms of changes or variations can be made by those skilled in the art on the basis of the above description, and here, all the embodiments cannot be exhausted, and thus the obvious changes or variations still fall within the protection scope of the present application.

Claims

1. A low-alloy pickled steel for hot-dip galvanizing at 420MPa, characterized in that, The chemical composition, by mass percentage, is: C: 0.04~0.06%, Si: 0.01~0.04%, Mn: 0.55~0.75%, P≤0.010%, S≤0.006%, Cr: 0.25~0.35%, Ti: 0.025~0.035%, Nb: 0.020~0.030%, Als: 0.02~0.05%, N≤0.006%, and satisfies the Si equivalent value Si eq =Si+Cr / 3-P≥0.1%, with the balance being iron and unavoidable impurities.

2. The 420MPa grade low-alloy pickled steel for hot-dip galvanizing according to claim 1, characterized in that, The yield strength of the low-alloy pickled steel used for hot-dip galvanizing is 423~471MPa, the tensile strength is 512~570MPa, and the elongation is 18~26%.

3. The 420MPa grade low-alloy pickled steel for hot-dip galvanizing according to claim 1, characterized in that, The microstructure of the low-alloy pickled steel for hot-dip galvanizing consists of ferrite and pearlite, with ferrite accounting for 96-98% of the volume and having a ferrite grain size of 10.5-12.

4. The preparation method of the 420MPa grade low-alloy pickled steel for hot-dip galvanizing according to any one of claims 1 to 3, the process comprising: Steel smelting → thin slab continuous casting → slab descaling before furnace → slab soaking furnace heating → pre-finish rolling descaling → finish rolling → laminar flow cooling → coiling, characterized in that the strip surface is cooled using a composite surface cooling method of roll gap cooling + inter-stand descaling + inter-stand strip cooling during the finish rolling process; wherein the roll gap cooling water flow rate of each stand F1~F4 is 150~165m³ / h. 3 / h; the descaling water pressure between F1~F2 racks and between F2~F3 racks is 140~160 bar, and the flow rate is 80~90 m³ / h. 3 / h; Cooling water flow rate between F1~F2 racks and between F2~F3 racks is 170~195m³ / h. 3 / h, the cooling water flow rate between F3 and F4 racks is 115~130m³ / h. 3 / h.

5. The method for preparing 420MPa grade low-alloy pickled steel for hot-dip galvanizing according to claim 4, characterized in that, During the finishing rolling process, the roll surface is cooled by work roll cooling and lubricated by lubricating oil. The cooling water parameters for the work rolls of stands F2 to F4 are 0.48~0.56m. 3 / (h·mm), the flow rates of lubricating rolling oil for F1~F4 are 70~75ml respectively. 3 / h, 90~95ml 3 / h, 90~95ml 3 / h, 50~55ml 3 / h; Furthermore, the unit area rolling force of the F2 stand is ≤300MPa, the unit area rolling force of the F3 stand is ≤500MPa, and the unit area rolling force of the F4 stand is ≤700MPa.

6. The method for preparing 420MPa grade low-alloy pickled steel for hot-dip galvanizing according to claim 4, characterized in that, During the laminar cooling and coiling stages, the effective Ti value (Ti) is calculated based on the Ti, N, and S content of the low-alloy pickled steel used for hot-dip galvanizing. eff Ti eff =Ti-3.4N-3S, according to Ti eff Different laminar flow cooling modes and winding temperatures are adopted; when Ti eff When the concentration is <0.01%, a rapid cooling mode is adopted in the front section, with a single-unit water volume of 1450~1500m³ in the coarse adjustment section. 3 / h, start 2~2.25 groups, winding temperature is 600~620℃; when Ti eff When the concentration is ≥0.01%, the initial rapid cooling mode is adopted, and the water volume of a single group in the coarse adjustment stage is 700~750m³. 3 / h, start 3.5~3.75 groups, and the winding temperature is 620~640℃.

7. The method for preparing 420MPa grade low-alloy pickled steel for hot-dip galvanizing according to claim 4, characterized in that, The thin slab continuous casting adopts a two-strand slab continuous casting machine. When both slabs are the 420MPa grade hot-dip galvanized low-alloy pickled steel, it is a same steel grade mode, and the thickness of both slabs is 58~62mm. When only one of the two slabs is the 420MPa grade hot-dip galvanized low-alloy pickled steel, it is a different steel grade mode. The thickness of the hot-dip galvanized low-alloy pickled steel slab is 58~62mm, and the thickness of the other auxiliary steel grade slab is 62~76mm. The finishing rolling process employs a seven-stand finishing mill with two-strand slabs cross-rolling, and the final rolling temperature for the low-alloy pickled steel used for hot-dip galvanizing is 850~870℃.

8. The method for preparing 420MPa grade low-alloy pickled steel for hot-dip galvanizing according to claim 7, characterized in that, When the two slabs are of the same steel grade, either slab is used for hot rolling within the rolling unit, and the thickness of the rolled product is 1.5~3.0mm; when the two slabs are of different steel grades, an auxiliary steel grade slab is used for hot rolling within the rolling unit, and the thickness of the rolled product of the low alloy pickled steel for hot-dip galvanizing is 1.5~3.0mm, while the thickness of the rolled product of the auxiliary steel grade is 4.0~8.0mm.

9. The method for preparing 420MPa grade low-alloy pickled steel for hot-dip galvanizing according to claim 4, characterized in that, The descaling pressure at the furnace entrance is 140~160 bar, and the descaling water flow rate is 80~90 m³ / h. 3 / h; The slab heating time in the homogenizing furnace is 25-35 minutes, and the furnace exit temperature is 1180-1200℃. The descaling pressure before finishing rolling is 300~380 bar.

10. The method for preparing 420MPa grade low-alloy pickled steel for hot-dip galvanizing according to claim 4, characterized in that, In the finishing rolling process, the thickness of low alloy pickled steel for hot-dip galvanizing is transitioned from 3.0mm to 2.0mm to 1.8mm to 1.5mm. When the rolling sequence number within a rolling unit is ≤6, the rolling gap time is 60~65s; when the rolling sequence number is >6, the rolling gap time is 45~50s.

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