An economical hot-rolled coil with a strength of 490MPa and its production method

Abstract

This invention discloses a 490MPa grade economical hot-rolled coil and its production method. First, by designing the alloy composition of the coil, Ti alloy is used to replace Nb and Mn alloys. The process of continuous casting followed by direct hot-rolling is adopted to ensure that the coil has advantages such as high strength, low alloy cost, and low process cost. This ensures that Q390B high-strength coil can replace some low-strength thick plate products, meeting the technical requirements for replacing some thick plates with extra-thick specifications. Ultimately, it facilitates processing operations such as cutting, bending, and drilling, improves the structural stability and reliability of large machinery or engineering projects, and also reduces the procurement costs of downstream enterprises, thereby improving their profitability.

Description

Technical Field

[0001] This invention relates to the field of steel materials technology, specifically to a type of steel widely used in structural and supporting components of large factories, stadiums, high-rise buildings, and engineering machinery, namely hot-rolled high-strength steel with a tensile strength of 490MPa, especially for extra-thick hot-rolled coils required for building large spans and large spaces. Background Technology

[0002] Q235B-Q390B low-alloy medium-thick plates are widely used in important sectors of the national economy, such as bridge construction, building engineering, machinery manufacturing, boiler shells and plates, pressure vessels, shipbuilding, and the automotive industry. However, due to limitations such as the compression ratio and production lines of thick plates, medium-thick plates (4.5mm to 25mm) are easier to cut, bend, and drill due to their moderate thickness. This makes medium-thick plate steel more flexible in meeting various engineering needs. Whether in construction, bridges, shipbuilding, or other structural fields, medium-thick plate steel can adapt to complex structural design requirements through appropriate processing, ensuring the stability and reliability of the structure. Currently, many companies, considering production costs, are gradually choosing medium-thick plate products to replace relatively thinner thick plate products, such as using 24.0mm thick Q390B hot-rolled coils instead of 30.0mm thick Q235B thick plates. For example, invention patent CN117925972A discloses a method for producing 390MPa grade impact-resistant high-strength structural steel plates. This method uses Nb, V, and Ti composition design, resulting in high alloy costs and lack of repeatability. Invention patent CN118389941A discloses an economical, low-segregation, high-homogeneity 355MPa grade ship hull structural steel plate and its preparation method. However, because the C content of this method is designed in the peritectic region, the strength level is not ideal. Invention patent CN116254450A discloses a low-cost structural steel production method. This method uses a dual-path steelmaking process and lacks repeatability.

[0003] With the rapid development of my country's national economy, high-strength structural steel is highly sought after in the market due to its high strength, light weight, good seismic performance, ease of construction of large spans and large spaces, short construction period, more reliable quality, environmental friendliness, and reusability. Its high strength makes it suitable for constructing large-span, heavy-load steel structures; its good plasticity means it will not suddenly fracture under overload conditions. Therefore, there is an urgent need for an economical high-strength structural steel of 490MPa grade (Q390B) to achieve thinner steel plates and replace some thicker plates with thicker coil products. Summary of the Invention

[0004] In view of this, this patent provides an economical hot-rolled coil of 490MPa grade and its production method, with the aim of reducing production costs and realizing the replacement of thick plate products with thick coil products while ensuring the surface quality and core quality of extra-thick coils.

[0005] To achieve the aforementioned objectives, in a first aspect, the 490MPa grade economic hot-rolled coil disclosed herein has the following alloy composition by weight 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%, balance being Fe and unavoidable inclusions; upper yield strength ≥ 380 MPa, tensile strength 490–650 MPa, elongation after fracture A ≥ 20%.

[0007] Secondly, the production method of the 490MPa grade economic hot-rolled coil described in the first aspect includes a steelmaking process and a hot rolling process. The steelmaking process includes a raw material processing step, a converter smelting process, a refining process, and a continuous casting process. The hot rolling process includes a heating process, a roughing process, a finishing process, a cooling process, and a coiling process, wherein:

[0008] After the continuous casting process is completed, the hot rolling process is carried out directly in the hot overheating charging.

[0009] In the continuous casting process, the second cooling section adopts a weak cooling mode and a light reduction mode. In sections 5 to 7, the reduction is applied in sections of 1 to 5 mm. The constant casting speed is controlled at 1.0 to 1.5 m / min. The target for continuous casting superheat is ≤25℃.

[0010] When performing the hot rolling process, the furnace inlet temperature is ≥500℃, the furnace outlet temperature is ≥1150℃, the heating time in the furnace is greater than 130 minutes, and the target outlet temperature is 1180℃~1200℃.

[0011] In the present disclosure and possible embodiments, in the raw material process, lime powder and magnesium powder are used for desulfurization, with lime powder injection time ≤10min and magnesium powder injection time ≤8min; slag is removed, and refined scrap steel is added, with the scrap steel accounting for 5% to 15% of the molten iron, and the sulfur content in the refined scrap steel not exceeding 0.050%. After final pretreatment, S ≤0.009% is guaranteed. At the same time, high-carbon ferromanganese is added, with the high-carbon ferromanganese consumption ≤0.02 tons / furnace.

[0012] In the present disclosure and possible embodiments, in the converter smelting process, high carbon extraction operation, constant oxygen pressure variable lance position operation, and single slag smelting and blowing are adopted. The oxygen supply time is 14 min to 18 min. Argon gas is used to purge the ladle before tapping and slag is blocked in the early stage. Slag is blocked in the later stage of tapping and the slag thickness in the ladle is less than 100 mm. The tapping time is 4 min to 7 min. The shape of the tapping spout is controlled to avoid slag spillage. Deoxidation and alloying are carried out when 1 / 5 of the steel is tapped and alloying is completed when 4 / 5 of the steel is tapped.

[0013] In the present disclosure and possible embodiments, a single-path LF furnace is used in the refining process, and ferrotitanium is added during the refining process; LF refining uses active lime and fluorite to create a fluid reducing slag, and avoids the exposed molten steel as much as possible; soft argon blowing is performed for ≥10 minutes before the end of the process, and the LF departure temperature of a normal furnace 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 section, and between the zero section and the first sector section is ≥99%, and the arc connection qualification rate between the first and second sections, between the seventh and eighth sections, within the section and the main body is ≥98%, with a measurement standard of ±0.3mm; protective pouring is carried out throughout the process.

[0015] In this disclosure and possible embodiments, in the heating process, 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 target furnace exit temperature is 1180℃~1200℃.

[0016] In the roughing process, 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 is used for descaling at high temperature. The thickness of the intermediate billet is 40mm~60mm, and the cumulative reduction rate in the roughing stage is greater than 70%.

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

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

[0019] In this disclosure and possible embodiments, the winding temperature is set to 570°C to 630°C during the winding process.

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

[0021] The present invention relates to an economical hot-rolled coil of 490MPa grade and its production method. First, by designing the alloy composition of the coil, Ti alloy is used to replace Nb and Mn alloys. Then, a process of continuous casting followed by direct hot overheating and rolling is adopted to ensure that, while maintaining high strength, it also has advantages such as low alloy cost and low process cost. This ensures that Q390B high-strength coil can replace some low-strength thick plate products, meeting the technical requirements for replacing some thick plates with extra-thick specification coils. Ultimately, it facilitates processing operations such as cutting, bending, and drilling, improves the structural stability and reliability of large machinery or engineering projects, and also reduces the procurement costs of downstream enterprises, thereby enhancing their profitability. Attached Figure Description

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

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

[0024] Figure 3 This is the metallographic structure of Comparative Example 3. Detailed Implementation

[0025] The present disclosure is described below based on embodiments; however, it is worth noting that the present disclosure is not limited to these embodiments. In the detailed description of the present disclosure below, certain specific details are described in detail. However, those skilled in the art will fully understand the present disclosure for the parts not described in detail. Furthermore, unless the context explicitly requires it, the words "comprising," "including," and similar terms throughout the specification and claims should be interpreted as encompassing rather than exclusive or exhaustive; that is, meaning "including but not limited to."

[0026] The specific steps of the 490MPa grade economic hot-rolled coil production method described in the embodiments of this disclosure are as follows:

[0027] First, the alloy composition of the hot-rolled coil is designed as follows, by weight percentage:

[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%, with the balance being Fe and unavoidable inclusions. The roles of each major element are as follows:

[0029] C: 0.16-0.19 wt%. When the carbon mass fraction is <0.09%, low-carbon ferromanganese is often required as a raw material. Low-carbon ferromanganese is generally more expensive than medium-carbon ferromanganese, which is detrimental to cost control. When the carbon mass fraction is 0.09%–0.16%, the peritectic reaction L+δ→γ during steel solidification is more intense, with a maximum volume shrinkage rate reaching 0.38%, leading to large variations in the linear shrinkage of the solidified billet shell. When the linear shrinkage reaches a certain level, it causes the air gap between the continuously cast billet shell and the crystallizer to increase, resulting in depressions. Simultaneously, the heat flow decreases, the initial billet shell becomes thinner, and under thermal stress and other stresses, cracks form at the troughs of the depressions. Therefore, this invention designs the C content to be 0.16-0.19 wt%.

[0030] Si: ≤0.10wt%; Si has a strong affinity for O and is a strong deoxidizing element. It exists in steel in solid solution form. 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 scrap steel can meet the requirements, and at the same time, the surface quality of the steel plate can be guaranteed to a certain extent.

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

[0032] Als: not greater than 0.0030 wt%; Al often precipitates in the form of AlN during high-temperature processes. The lower the content of Al and N, the lower the precipitation temperature. Since the grain boundary has high interfacial energy, which is conducive to nucleation, AlN usually precipitates at the grain boundary of austenite, which reduces the grain boundary strength. It is subjected to tensile stress in the straightening section, which leads to corner cracks in the billet and is not conducive to heat treatment. Therefore, this invention controls the Al content at a low level.

[0033] Ti: 0.040~0.060wt%; Ti has the effects of grain refinement and precipitation strengthening. At high temperatures, it can dissolve into austenite, inhibiting the (γ→α) phase transformation. The 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. Moreover, Ti is abundant and inexpensive in my country. In terms of hot tempering of continuously cast billets, Ti has a better effect on the hot plasticity of steel than Nb and V. Therefore, this invention adopts a single Ti composition design.

[0034] N: not more than 0.0050 wt%; excessive N content will react with Al and Ti, easily producing AlN and sharp-cornered inclusions TiN, thus affecting the surface quality and performance of the product. Therefore, the present invention controls the N content at a low level.

[0035] P: Not more than 0.020 wt%; Generally speaking, phosphorus is a harmful element in steel, which increases the cold brittleness of steel, deteriorates the weldability, reduces plasticity, and worsens the cold bending performance. Therefore, the content of P element is controlled at a low level in this invention.

[0036] S: not more than 0.010 wt%; S is a harmful element that causes hot brittleness in steel, reduces the ductility and toughness of steel, and is prone to cracking during rolling. It is also detrimental to welding performance. Therefore, the content of S element is controlled at a low level in this invention.

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

[0038] I. Steelmaking process:

[0039] 1. Raw material processing

[0040] Desulfurization is achieved using lime powder and magnesium powder, with lime powder injection time ≤10 min and magnesium powder injection time ≤8 min. After removing slag, fine scrap steel is added, comprising 5%-15% of the molten iron, and the sulfur content in the scrap steel must not exceed 0.050%. The final pretreatment ensures S ≤0.009%. High-carbon ferromanganese is also added, with a consumption ≤0.02 tons / furnace.

[0041] 2. Converter process

[0042] In converter smelting, high-carbon drawing operation and constant oxygen pressure variable lance position operation are adopted, and single-slag smelting and blowing are used. The oxygen supply time is 14-18 minutes. Argon gas is used to purge the ladle before tapping, and slag is blocked in the early stage. Slag darts are used in the later stage of tapping to ensure that the slag thickness in the ladle is less than 100 mm. The tapping time is guaranteed to be 4-7 minutes, and the shape of the tapping spout is controlled to avoid slag spillage. 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 has been tapped, and alloying is required to be completed when 4 / 5 of the steel has been tapped.

[0043] 3. Refining process

[0044] The present invention uses a single-path LF furnace for refining. Titanium iron is added during the refining process, and the mass percentage of Ti element in the molten steel is 0.04-0.06%. The LF uses active lime and fluorite to create a fluid reducing slag, and avoids the exposed molten steel as much as possible. Soft argon blowing is performed for ≥10 minutes before the end of the treatment. The LF temperature at the station for normal furnace cycles is controlled at 1540-1560℃.

[0045] 4. Continuous casting process

[0046] The arc connection qualification rate between the casting machine crystallizer and the zero section, and between the zero section and the first sector section is ≥99%. The arc connection qualification rate between the first and second sections, between sections 7 and 8, within sections, and within the main body is ≥98%, with a measurement standard of ±0.3mm. Protective pouring is carried out throughout the process. Argon gas is used to purge the tundish before pouring. No exposed molten steel is allowed during the pouring process to avoid nitrogen and oxygen addition. The long nozzle must be cleaned after each pouring. The long nozzle must be kept vertical during the pouring process. The secondary cooling section adopts a weak cooling mode and a light reduction mode. In sections 5-7, the reduction is 1-5mm in stages. The constant casting speed is controlled at 1.0-1.5m / min. The target for continuous casting superheat control is ≤25℃.

[0047] Because the embodiments of this disclosure use a process of rolling directly after continuous casting and hot superheating, the energy consumption of cooling and then heating can be avoided, thereby achieving the goal of reducing process costs.

[0048] II. Hot Rolling Process

[0049] 1. Heating process

[0050] Because this steel grade is hot-overheated, 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 target exit temperature is 1180℃~1200℃.

[0051] 2. Rough rolling process

[0052] Control the final rolling temperature of R1 to 950-1030℃ and R2 to 980-1070℃. High-temperature water spraying for descaling can better remove iron oxide scale from the steel surface. The intermediate billet thickness is 40-60mm, and the cumulative reduction rate during the roughing stage is greater than 70%.

[0053] 3. Finishing rolling process

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

[0055] 4. Cooling and winding processes

[0056] The cooling mode is intermittent cooling, with the first 7 groups of strong cooling (ultra-dense cooling) at a cooling rate of 10–20 °C / s. The coiling temperature is set at 570 °C–630 °C, which is conducive to the full precipitation of "effective Ti" (TiC), resulting in good precipitation strengthening. If the coiling temperature is too low, it is not conducive to shape control and is also difficult to control during rolling; if the coiling temperature is too high, the TiC precipitation is insufficient, the cooling rate is insufficient, the precipitation strengthening and grain refinement strengthening effects are not obvious, and the strength is unsuitable.

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

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

[0059] Table 2 Chemical Composition / %

[0060]

[0061]

[0062] 2. The hot rolling processes of Examples 1-4 are 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 mechanical property testing is GB / T1591-2018.

[0066] Table 4 Mechanical Properties

[0067]

[0068] As can be seen from Table 4, compared with the low-alloy high-strength structural steel standard GB / T1591-2018, the hot-rolled coils prepared in Examples 1-4 of this 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 between 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 Hebei Steel, respectively.

[0073] Table 6 Comparison of Mechanical Properties and Costs

[0074]

[0075] As shown in Tables 5 and 6, despite the reduction in alloy cost and process cost, the mechanical properties of Example 2 are not significantly different from those of 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 from the two figures that both are ferrite + pearlite, the only difference being that Example 2 has slightly more pearlite, indicating that Example 2 did not have a substantial change in microstructure under the premise of cost reduction.

[0077] Figure 2-1 , 2-2 The solidification states of amorphous steel and peritectic steel in Example 2 and Comparative Example 3 are shown respectively. Obviously, the amorphous steel in Example 2 is of better quality.

[0078] The embodiments described above are merely illustrative of implementation methods of this disclosure, and while the descriptions are specific and detailed, they should not be construed as limiting the scope of this patent disclosure. It should be noted that those skilled in the art can make various modifications, equivalent substitutions, and improvements without departing from the concept of this disclosure, and these all fall within the protection scope of this disclosure. Therefore, the protection scope of this patent disclosure should be determined by the appended claims.

Claims

1. A 490MPa grade economical hot-rolled coil, characterized in that, Its 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%, balance being Fe and unavoidable inclusions; upper yield strength ≥380MPa, tensile strength 490~650MPa, elongation after fracture A≥20%; The production method of the 490MPa grade economic hot-rolled coil includes a steelmaking process and a hot rolling process. The steelmaking process includes a raw material process, a converter smelting process, a refining process and a continuous casting process. The hot rolling process includes a heating process, a roughing process, a finishing process, a cooling process and a coiling process. In the continuous casting process, the secondary cooling section adopts a weak cooling mode and a light reduction mode in sections 5 to 7. Segmented casting with a reduction of 1~5mm, constant casting speed controlled at 1.0~1.5m / min, and continuous casting superheat control target ≤25℃; In the heating process, the furnace entry temperature is ≥500℃, the furnace exit temperature is ≥1150℃, and the heating time in the furnace is greater than 130 minutes; In the roughing rolling process, 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 is used for descaling at high temperatures. The thickness of the intermediate slab is 40mm~60mm, and the cumulative reduction rate in the roughing rolling stage is greater than 70%. In the finishing rolling process, F1~F7 use hydraulic bending rolls and F2~F4 use CVC rolls; the entry temperature of finishing rolling F1 is 950℃~1030℃, the final rolling temperature of F7 is 840℃~880℃, and the finishing rolling time is 60s~100s. In the cooling process, intermittent cooling is adopted, with the first 7 groups of strong cooling at a rate of 10~20℃ / s.

2. The method for producing economical hot-rolled coils according to claim 1, characterized in that: In the raw material process, lime powder and magnesium powder are used for desulfurization. The lime powder injection time is ≤10 min, and the magnesium powder... Powder injection time ≤ 8min; remove slag, add refined scrap steel, the proportion of scrap steel to molten iron is 5%~15%, the sulfur content in the refined scrap steel shall not exceed 0.050%, and after final pretreatment, ensure S≤0.009%, and add high carbon ferromanganese, the consumption of high carbon ferromanganese is ≤0.02 tons / furnace.

3. The method for producing economical hot-rolled coils according to claim 2, characterized in that: In the converter smelting process, high carbon extraction operation, constant oxygen pressure variable lance position operation, and single slag smelting and blowing are adopted. The oxygen supply time is 14 min to 18 min. Argon gas is used to purge the ladle before tapping and slag is blocked in the early stage. Slag is blocked in the later stage of tapping and the slag thickness in the ladle is less than 100 mm. The tapping time is 4 min to 7 min. The shape of the tapping spout is controlled to avoid slag spillage. Deoxidation and alloying are carried out when 1 / 5 of the steel is tapped and the alloying is completed when 4 / 5 of the steel is tapped.

4. The method for producing economical hot-rolled coils according to claim 3, 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 create a fluid reducing slag, and the exposed molten steel is avoided as much as possible. Soft argon blowing is performed for ≥10 minutes before the end of the process, and the LF temperature at the station for normal furnace cycles is controlled at 1540℃~1560℃.

5. The method for producing economical hot-rolled coils according to claim 4, characterized in that: In the continuous casting process, the arc connection qualification rate between the casting machine crystallizer and the zero section, and between the zero section and the first sector section is ≥99%, and the arc connection qualification rate between the first and second sections, between the seventh and eighth sections, within the section, and the main body is ≥98%, with a measurement standard of ±0.3mm; protective pouring is carried out throughout the process.

6. The method for producing economical hot-rolled coils according to claim 1, characterized in that: In the winding process, the winding temperature is set at 570℃~630℃.

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