490MPa-grade economical hot-rolled coiled plate and production method thereof

Abstract

The invention discloses a 490MPa-grade economical hot-rolled coiled plate and a production method thereof.The production method comprises the steps that firstly, alloy components of the coiled plate are designed, then Nb and Mn alloys are replaced with Ti alloy, and the technology that rolling is conducted through direct overheat charging after continuous casting is adopted, so that it is guaranteed that the coiled plate has the advantages of being low in alloy cost, low in procedure cost and the like on the basis that the coiled plate has high strength; the Q390B high-strength rolled plate is used for replacing part of low-strength thick plate products, the technical requirement for replacing part of thick plates with extra-thick rolled plates is met, finally, convenience of machining operation such as cutting, bending and drilling is achieved, the structural stability and reliability of large machines or projects are improved, meanwhile, the purchase cost of downstream enterprises is reduced, and the profitability of the enterprises is improved.

Description

Technical Field

[0001] The present invention relates to the technical field of steel materials, and in particular to a steel widely used for structural parts and supporting parts of large factories, stadiums, high-rise buildings, engineering machinery, etc., namely, hot-rolled high-strength steel with a tensile strength of 490 MPa, especially hot-rolled coils with extra-thick specifications required for building large spans and large spaces. Background Art

[0002] Q235B-Q390B low alloy medium and thick plates are widely used in important areas of the national economy such as bridge construction, construction engineering, machinery manufacturing, boiler shells and furnace plates, pressure vessels, shipbuilding industry, and automobile industry. However, due to many restrictions on the compression ratio and production line of thick plate products, medium and thick plates (4.5mm to 25mm) can be cut, bent, drilled and other processing operations more easily due to their moderate thickness, which makes medium and thick plate steel more flexible and changeable in meeting various engineering needs. Whether it is construction, bridges, ships or other structural fields, medium and thick plate steel can adapt to complex structural design requirements through appropriate processing to ensure the stability and reliability of the structure. At present, many companies, considering the production cost, gradually use medium and thick plate products to replace relatively thin plate products, such as using 24.0mm thick Q390B hot rolled coils instead of 30.0mm thick Q235B thick plates. For example, the invention patent with publication number CN117925972A discloses a method for producing 390MPa-grade impact-resistant high-strength structural steel plates. This method adopts the composition design of Nb, V, and Ti, the alloy cost is relatively high, and it is not repeatable; the invention patent with publication number CN118389941A discloses an economical, low-segregation, high-homogeneity 355MPa-grade hull structure steel plate and a preparation method thereof. Because the C content of this method is designed in the peritectic zone, the strength level is not ideal; the invention patent with publication number CN116254450A discloses a method for producing low-cost structural steel. This method adopts dual paths for steelmaking and is not repeatable.

[0003] With the rapid development of my country's national economy, high-strength structural steel has been highly praised by the market for its high strength, light weight, good seismic performance, easy construction of large spans and large spaces, short cycle, more reliable quality, environmental protection and reusability. It has high strength and is suitable for building steel structures with large spans and heavy loads; it has good plasticity and will not break suddenly due to overload under normal conditions. Therefore, a 490MPa grade (Q390B) economical high-strength structural steel is urgently needed to achieve steel plate thinning and realize the replacement of thick plate products with some thick-gauge rolled plate products. Summary of the invention

[0004] In view of this, this patent provides a 490MPa grade economical hot-rolled coil and its production method, the purpose of which is to reduce production costs while ensuring the surface quality and core quality of extra-thick coils, so as to achieve the replacement of thick plate products with thick coil products.

[0005] To achieve the above-mentioned object of the invention, in a first aspect, the 490MPa grade economical hot-rolled coil disclosed in the present invention has the following alloy components, calculated by mass percentage:

[0006] C: 0.16%~0.19%, Si≤0.10%, Mn: 1.15%~1.35%, Als≤0.030%, Ti: 0.040%~0.060%, P≤0.020%, S≤0.010%, N≤0.0050%, the remainder is Fe and unavoidable inclusions; upper yield strength ≥380MPa, tensile strength 490~650MPa, elongation after fracture A≥20%.

[0007] In the second aspect, the production method of the 490MPa grade economical hot-rolled coil described in the first aspect includes a steelmaking process and a hot rolling process, wherein the steelmaking process includes a raw material process, a converter smelting process, a refining process and a continuous casting process, and the hot rolling process includes a heating process, a rough rolling process, a finishing rolling process, a cooling process and a coiling process, wherein:

[0008] After the continuous casting process is completed, directly hot-rolling the hot rolling process;

[0009] In the continuous casting process, the second cooling stage adopts weak cooling mode and light pressure reduction mode during continuous casting, and 1-5 mm pressure is applied in stages 5-7, the constant casting speed is controlled at 1.0-1.5 m / min, and the continuous casting superheat control target is ≤25°C;

[0010] When the hot rolling process is carried out, the furnace entry temperature is ≥500°C, the furnace exit temperature is ≥1150°C, the heating time in the furnace is greater than 130 minutes, and the furnace exit target temperature is 1180°C to 1200°C.

[0011] In the present disclosure and possible embodiments, in the raw material process, lime powder and magnesium powder are used for desulfurization, the lime powder blowing time is ≤10min, and the magnesium powder blowing time is ≤8min; the slag is scraped off, and refined scrap steel is added, the scrap steel accounts for 5% to 15% of the molten iron, and the sulfur content in the refined scrap steel shall not be greater than 0.050%. After final pretreatment, S is guaranteed to be ≤0.009%, and high carbon ferromanganese is added at the same time, and the unit consumption of high carbon ferromanganese is ≤0.02 tons / furnace.

[0012] In the present disclosure and possible embodiments, in the converter smelting process, high carbon drawing operation, constant oxygen pressure change gun position operation and single slag method smelting and blowing are adopted, the oxygen supply time is 14min~18min, the ladle is purged with argon before tapping, and slag is blocked in the early stage; slag darts are blocked in the later stage of tapping, and the slag thickness of the ladle is less than 100mm; the tapping time is 4min~7min, and the shape of the tapping port is controlled to avoid tapping dispersion; deoxidation and alloying are started when 1 / 5 of the steel is tapped, and the alloy is added when 4 / 5 is tapped.

[0013] In the present disclosure and possible embodiments, in the refining process, a single-path LF furnace is used, and ferrotitanium is added during the refining process; LF refining uses active lime and fluorite to make reducing slag with good fluidity to avoid exposure of molten steel as much as possible; soft argon blowing is performed for ≥10 minutes before the end of the treatment, and the normal furnace LF leaving the station temperature is controlled at 1540℃~1560℃.

[0014] In the present disclosure and possible embodiments, in the continuous casting process, the arc connection qualification rate between the casting machine crystallizer and the zero segment, the zero segment and the fan-shaped segment 1 is ≥99%, and the arc connection qualification rate between the 1st segment and the 2nd segment, between the 7th and 8th segments, within the segment and the main body is ≥98%, and the measurement standard is ±0.3mm; protective casting is carried out throughout the whole process.

[0015] In the present disclosure and possible embodiments, in the heating process, the furnace entry temperature is ≥500°C, the furnace exit temperature is ≥1150°C, the heating time in the furnace is greater than 130 minutes, and the furnace exit target temperature is 1180°C to 1200°C;

[0016] In the rough rolling process, the R1 final rolling temperature is controlled at 950°C to 1030°C, the R2 final rolling temperature is controlled at 980°C to 1070°C, and water spraying is performed for descaling at high temperature; the intermediate billet thickness is 40mm to 60mm, and the cumulative reduction rate in the rough rolling stage is greater than 70%.

[0017] In the present disclosure and possible embodiments, in the finishing rolling process, F1 to F7 use hydraulic bending rolls, and F2 to F4 use CVC rolls; the finishing rolling F1 inlet temperature is 950°C to 1030°C, the F7 final rolling temperature is 840°C to 880°C, and the finishing rolling time is 60s to 100s.

[0018] In the present disclosure and possible embodiments, in the cooling process, the cooling mode adopts intermittent cooling, the first 7 groups are strong cooling, and the cooling rate is 10-20°C / s.

[0019] In the present disclosure and possible embodiments, in the coiling process, the coiling temperature is set at 570°C to 630°C.

[0020] The present invention has the following beneficial effects:

[0021] The 490MPa-grade economical hot-rolled coil and its production method of the present invention firstly designs the alloy composition of the coil, then replaces Nb and Mn alloys with Ti alloy, and adopts a process of directly hot-rolling and rolling after continuous casting, thereby ensuring that on the basis of high strength, it also has the advantages of low alloy cost, low process cost and the like, ensuring that Q390B high-strength coil replaces part of low-strength thick plate products, meeting the technical requirements of replacing part of thick plates with extra-thick specification coils, and finally realizing the convenience of processing operations such as cutting, bending, and drilling, improving the structural stability and reliability of large-scale machinery or engineering, while also reducing the procurement cost of downstream enterprises and improving the profitability of enterprises. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 is the metallographic structure of Example 2;

[0023] Figure 2-1 , 2-2 They are respectively the solidification states of the non-peritectic steel and the peritectic steel of Example 2 and Comparative Example 3;

[0024] Figure 3 This is the metallographic structure of Example 3. DETAILED DESCRIPTION

[0025] The present disclosure is described below based on embodiments, but it is worth noting that the present disclosure is not limited to these embodiments. In the detailed description of the present disclosure below, some specific details are described in detail. However, for the parts that are not described in detail, those skilled in the art can also fully understand the present disclosure. At the same time, unless the context clearly requires, the words "including", "comprising" and the like in the entire specification and claims should be interpreted as including rather than exclusive or exhaustive meanings; that is, the meaning of "including but not limited to".

[0026] The 490MPa grade economical hot rolled coil production method described in each embodiment of the present disclosure has the following specific steps:

[0027] First, in terms of mass percentage, the alloy composition of the hot rolled coil is designed to be:

[0028] C: 0.16-0.19%, Si≤0.10%, Mn: 1.15-1.35%, Als≤0.030%, Ti: 0.040~0.060%, P≤0.020%, S≤0.010%, N≤0.0050%, the remainder is Fe and unavoidable inclusions. The functions of each main element are as follows:

[0029] C: 0.16-0.19wt%; when the carbon mass fraction is less than 0.09%, low-carbon ferromanganese is often required for raw material addition. The price of low-carbon ferromanganese is generally higher than that of medium-carbon ferromanganese, which is not conducive to low-cost control. When the carbon mass fraction is 0.09% to 0.16%, the peritectic reaction L+δ→γ is more intense during the solidification of the molten steel, and the maximum volume shrinkage rate can reach 0.38%, resulting in a large change in the linear shrinkage of the solidified shell; when the linear shrinkage reaches a certain level, it will cause the air gap between the continuous casting shell and the crystallizer to become larger, and cause the occurrence of depressions; at the same time, the heat flow is reduced, the primary shell becomes thinner, and under the action of thermal stress and other stresses, cracks are formed at the depression trough; therefore, the C content of the present invention is designed to be 0.16-0.19wt%

[0030] Si: ≤0.10wt%; Si has a strong affinity with O and is a strong deoxidizing element. It exists in steel in the form of solid solution. Si can improve the strength, fatigue limit, corrosion resistance and wear resistance of steel. However, if the Si content is too high, oxides are easily generated during hot rolling, which reduces the surface quality of the steel. When Si>0.10wt%, it is often necessary to add a certain amount of ferrosilicon to the molten steel, which increases the cost. When Si≤0.10wt%, the Si content in the scrap steel can meet the requirements and can also guarantee the surface quality of the steel plate to a certain extent.

[0031] Mn: 1.15-1.35wt%; Mn exists in the steel in a solid solution state and is a solid solution strengthening element, which can improve the strength of ferrite. Mn in low carbon steel has a significant effect on improving strength; but Mn and S are easy to form MnS plastic inclusions, which are elongated along the rolling direction during hot rolling, deteriorating the formability of the steel; as the Mn content increases, segregation defects will also occur, so the Mn content should not be too high, so the Mn content designed in the present invention is 1.15-1.35wt%.

[0032] Als: not more than 0.0030wt%; Al is often precipitated in the form of AlN during high temperature processes, and the lower the content of Al and N, the lower the precipitation temperature; since the grain boundary has high interface energy, which is conducive to nucleation, AlN is usually precipitated at the grain boundary of austenite, degrading the grain boundary strength, and being subjected to tensile stress in the straightening section, resulting in corner cracks of the ingot, which is not conducive to thermal overheating, so the present invention controls the low content of Als element.

[0033] Ti: 0.040-0.060wt%; Ti has the effects of grain refinement and precipitation strengthening. At high temperature, it can be dissolved into austenite to block the (γ→α) phase transformation. TiN and TiC precipitated in the steel can prevent grain growth in austenite and hinder the recrystallization of deformed austenite, thereby refining the grains. At the same time, the precipitated "effective Ti" TiC has a strong strengthening effect. In addition, my country has abundant Ti content and low price. In terms of continuous casting billet heat treatment, Ti also has a better influence on the thermoplasticity of steel than Nb and V elements. Therefore, the present invention adopts a single Ti component design.

[0034] N: not more than 0.0050wt%; too high N content will react with Al and Ti, easily produce AlN and sharp inclusions TiN, thus affecting the surface quality and performance of the product, so the present invention controls the N element to a low content.

[0035] P: not more than 0.020wt%; Generally speaking, phosphorus is a harmful element in steel, which increases the cold brittleness of steel, deteriorates welding performance, reduces plasticity, and deteriorates cold bending performance. Therefore, the present invention controls the low content of P element.

[0036] S: not more than 0.010wt%; S is a harmful element that makes steel hot brittle, reduces the ductility and toughness of steel, is prone to cracks during rolling, and is also detrimental to welding performance. Therefore, the present invention controls the S element content to be low.

[0037] Then, the hot rolled coil is produced and prepared according to the above alloy composition design requirements. The specific production process is as follows:

[0038] 1. Steelmaking process:

[0039] 1. Raw material process

[0040] Lime powder and magnesium powder are used for desulfurization, and the lime powder injection time is ≤10min and the magnesium powder injection time is ≤8min. Slag is removed and refined scrap steel is added, with scrap steel accounting for 5%-15% of the molten iron, and the sulfur content in the refined scrap steel shall not be greater than 0.050%. After final pretreatment, S is guaranteed to be ≤0.009%. At the same time, high carbon ferromanganese is added, with a unit consumption of ≤0.02 tons / furnace.

[0041] 2. Converter process

[0042] In converter smelting, high carbon pulling operation and constant oxygen pressure gun position operation are adopted, single slag method is adopted for smelting and blowing, oxygen supply time is 14-18min, argon gas is purged in the ladle before tapping, slag is blocked in the early stage; slag darts are blocked in the later stage of tapping to ensure that the slag thickness of the ladle is less than 100mm; tapping time is guaranteed to be 4-7min, the shape of the tapping port is controlled, and the tapping flow is avoided. The above measures can effectively reduce the oxygen content in the steel, thereby indirectly controlling the aluminum content. Deoxidation and alloying are started when 1 / 5 of the steel is tapped, and the alloy is required to be added when 4 / 5 is tapped.

[0043] 3. Refining process

[0044] The invention adopts a single-path LF furnace for refining, adds ferrotitanium during the refining process, and the mass percentage of Ti element in molten steel is 0.04-0.06%; LF adopts active lime and fluorite to make reducing slag with good fluidity, and avoids exposure of molten steel as much as possible; soft argon blowing is performed for ≥10 minutes before the end of the treatment, and the leaving-station temperature of LF for a normal furnace is controlled at 1540-1560°C.

[0045] 4. Continuous casting process

[0046] The qualified rate of arc connection between casting machine crystallizer and zero section, zero section and fan-shaped section 1 is ≥99%, and the qualified rate of arc connection between section 1 and section 2, between sections 7-8, within the section and the main body is ≥98%, and the measurement standard is ±0.3mm; protective pouring is carried out throughout the process; argon gas is used to purge the middle bag before pouring, and no molten steel is exposed during the pouring process to avoid nitrogen and oxygen increase. The long nozzle must be cleaned after each furnace pouring, and the long nozzle is required to remain vertical during the pouring process; during continuous casting, the second cooling section adopts weak cooling mode and light pressure reduction mode. In sections 5-7, 1-5mm pressure is applied in stages, and the constant pulling speed is controlled at 1.0-1.5m / min. The continuous casting overheat control target is ≤25℃.

[0047] Because the embodiment of the present disclosure adopts a process of directly heating the hot superheater for rolling after continuous casting, the energy consumption of heating up after cooling can be avoided, thereby achieving the purpose of reducing the process cost.

[0048] 2. Hot rolling process

[0049] 1. Heating process

[0050] Since this type of steel is hot-superheated, the furnace entry temperature is ≥500℃, the furnace exit temperature is ≥1150℃, the heating time in the furnace is greater than 130 minutes, and the furnace exit target temperature is 1180℃~1200℃.

[0051] 2. Rough rolling process

[0052] The final rolling temperature of R1 is controlled at 950-1030℃, and the final rolling temperature of R2 is controlled at 980-1070℃. Water spraying and descaling at high temperature can better remove the iron oxide scale on the surface of the steel. The thickness of the intermediate billet is 40-60mm, and the cumulative reduction rate in the rough rolling stage is greater than 70%.

[0053] 3. Finishing process

[0054] F1-F7 hydraulic bending rolls and F2-F4 CVC rolls are used to effectively control the strip shape. The finishing rolling F1 inlet temperature is 950-1030℃, the F7 final rolling temperature is 840-880℃, and the finishing rolling time is 60-100s.

[0055] 4. Cooling process and coiling process

[0056] The cooling mode is intermittent cooling, the first 7 groups of strong cooling (ultra-intensive cooling), and the cooling rate is 10-20℃ / s. The coiling temperature is set at 570℃-630℃, which is conducive to the full precipitation of "effective Ti" TiC and plays a good precipitation strengthening role. If the coiling temperature is too low, it is not conducive to plate shape control, and it is not easy to control during rolling; if the coiling temperature is too high, the TiC precipitation amount is insufficient, the cooling rate is insufficient, the precipitation strengthening and fine grain strengthening effects are not obvious, and the strength is not appropriate.

[0057] The following are preferred embodiments 1-4 and comparative examples 1-3 of the present disclosure.

[0058] 1. The chemical compositions and their mass percentages of Examples 1-4 are shown in Table 2 below:

[0059] Table 2 Chemical composition / %

[0060]

[0061]

[0062] 2. The hot rolling process of Examples 1-4 is shown in Table 3 below:

[0063] Table 3 Process parameters

[0064]

[0065] 3. The mechanical properties of Examples 1-4 are shown in Table 4 below. The standard used for the mechanical property test is GB / T1591-2018:

[0066] Table 4 Mechanical properties

[0067]

[0068] It can be seen from Table 4 that, compared with the low-alloy high-strength structural steel standard GB / T1591-2018, the hot-rolled coils prepared in Examples 1-4 of the present disclosure reach the 490MPa level, meet the standard requirements, and achieve the goal of replacing thick plate products with thick-gauge coil products.

[0069] 4. The composition and performance comparison of Example 2 and Comparative Examples 1-3 are shown in Tables 5 and 6 below:

[0070] Table 5 Comparison of chemical composition / %

[0071]

[0072] Note: Comparative Examples 1, 2 and 3 are hot-rolled coils of the same grade produced by Benxi Steel, Baotou Steel and Hesteel respectively.

[0073] Table 6 Comparison of mechanical properties and costs

[0074]

[0075] As shown in Table 5 and Table 6, on the basis of the reduction in alloy cost and process cost, the mechanical properties of Example 2 do not change significantly compared with Comparative Examples 1-3.

[0076] Figure 1 and Figure 3 By comparing the metallographic structures of Example 2 and Comparative Example 3, it can be seen that both are ferrite + pearlite, with the only difference being that Example 2 has slightly more pearlite, indicating that Example 2 has no substantial change in structure under the premise of cost reduction.

[0077] Figure 2-1 , 2-2 They are the solidification states of the non-peritectic steel and the peritectic steel of Example 2 and Comparative Example 3, and it is obvious that the non-peritectic steel of Example 2 has better quality.

[0078] The above-described embodiments are only embodiments of the present disclosure, and the descriptions thereof are relatively specific and detailed, but they cannot be construed as limiting the scope of the present disclosure. It should be noted that, for a person of ordinary skill in the art, without departing from the concept of the present disclosure, several variations, equivalent substitutions, improvements, etc. may be made, all of which fall within the scope of protection of the present disclosure. Therefore, the scope of protection of the present disclosure shall be subject to the attached claims.

Claims

1. A 490MPa grade economical hot rolled coil, characterized in that: The alloy composition by mass percentage is: C: 0.16%~0.19%, Si≤0.10%, Mn: 1.15%~1.35%, Als≤0.030%, Ti: 0.040%~0.060%, P≤0.020%, S≤0.010%, N≤0.0050%, the remainder is Fe and unavoidable inclusions; upper yield strength ≥380MPa, tensile strength 490~650MPa, elongation after fracture A≥20%.

2. The method for producing the 490MPa grade economical hot-rolled coil according to claim 1 comprises a steelmaking process and a hot rolling process, wherein the steelmaking process comprises a raw material process, a converter smelting process, a refining process and a continuous casting process, and the hot rolling process comprises a heating process, a rough rolling process, a finishing rolling process, a cooling process and a coiling process, and is characterized in that: After the continuous casting process is completed, directly hot-rolling the hot rolling process; In the continuous casting process, the second cooling stage adopts weak cooling mode and light pressure reduction mode during continuous casting, and 1-5 mm pressure is applied in stages 5-7, the constant casting speed is controlled at 1.0-1.5 m / min, and the continuous casting superheat control target is ≤25°C; When the hot rolling process is carried out, the furnace entry temperature is ≥500°C, the furnace exit temperature is ≥1150°C, the heating time in the furnace is greater than 130 minutes, and the furnace exit target temperature is 1180°C to 1200°C.

3. The method for producing an economical hot-rolled coil according to claim 2, characterized in that: In the raw material process, lime powder and magnesium powder are used for desulfurization, the lime powder blowing time is ≤10min, and the magnesium powder blowing time is ≤8min; the slag is scraped off, and refined scrap steel is added, the scrap steel accounts for 5% to 15% of the molten iron, and the sulfur content in the refined scrap steel shall not be greater than 0.050%. After final pretreatment, S is guaranteed to be ≤0.009%, and high carbon ferromanganese is added at the same time, and the unit consumption of the high carbon ferromanganese is ≤0.02 tons / furnace.

4. The method for producing an economical hot-rolled coil according to claim 3, characterized in that: In the converter smelting process, high carbon drawing operation, constant oxygen pressure change gun position operation and single slag method smelting and blowing are adopted, the oxygen supply time is 14min-18min, the ladle is purged with argon before tapping, and slag is blocked in the early stage; slag darts are blocked in the later stage of tapping, and the ladle slag thickness is less than 100mm; the tapping time is 4min-7min, and the shape of the tapping port is controlled to avoid tapping dispersion; deoxidation and alloying are started when 1 / 5 of the steel is tapped, and the alloy is added when 4 / 5 is tapped.

5. The method for producing an economical hot rolled coil according to claim 4, characterized in that: In the refining process, a single-path LF furnace is used, and ferrotitanium is added during the refining process; LF refining uses active lime and fluorite to make reducing slag with good fluidity to avoid exposure of molten steel as much as possible; soft argon blowing is performed for ≥10 minutes before the end of the treatment, and the LF leaving station temperature of a normal furnace is controlled at 1540°C to 1560°C.

6. The method for producing an economical hot-rolled coil according to claim 5, characterized in that: In the continuous casting process, the qualified arc connection rate between the casting machine crystallizer and the zero section, the zero section and the fan-shaped section 1 is ≥99%, and the qualified arc connection rate between the 1st section and the 2nd section, between the 7th and 8th sections, within the section and the main body is ≥98%, and the measurement standard is ±0.3mm; protective casting is carried out throughout the process.

7. The method for producing an economical hot-rolled coil according to claim 6, characterized in that: In the heating process, the furnace entry temperature is ≥500°C, the furnace exit temperature is ≥1150°C, the heating time in the furnace is greater than 130 minutes, and the furnace exit target temperature is 1180°C to 1200°C; In the rough rolling process, the R1 final rolling temperature is controlled at 950°C to 1030°C, the R2 final rolling temperature is controlled at 980°C to 1070°C, and water spraying is performed for descaling at high temperature; the intermediate billet thickness is 40mm to 60mm, and the cumulative reduction rate in the rough rolling stage is greater than 70%.

8. The method for producing an economical hot-rolled coil according to claim 7, characterized in that: In the finishing rolling process, F1-F7 use hydraulic bending rolls, and F2-F4 use CVC rolls; the finishing rolling F1 inlet temperature is 950°C-1030°C, the F7 final rolling temperature is 840°C-880°C, and the finishing rolling time is 60s-100s.

9. The method for producing an economical hot-rolled coil according to claim 8, characterized in that: In the cooling process, the cooling mode adopts intermittent cooling, the first 7 groups are strong cooling, and the cooling rate is 10-20°C / s.

10. The method for producing an economical hot-rolled coil according to claim 9, characterized in that: In the coiling step, the coiling temperature is set at 570°C to 630°C.

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