阅读说明：本技术 薄规格700MPa级低合金高强钢的生产方法 (Production method of thin 700 MPa-grade low-alloy high-strength steel ) 是由 张星 侯明山 王彬 宋志岗 郑英辉 高小尧 董跃星 栗建辉 于程福 于 2021-07-13 设计创作，主要内容包括：一种薄规格700MPa级低合金高强钢的生产方法,属于炼钢技术领域。该方法包括冶炼、连铸、加热、控轧控冷、卷取工序。冶炼工序,低合金高强钢化学成分避开凝固包晶区；连铸工序,使用与低合金高强钢成分近似的钢水开浇,开浇钢水浇注状态稳定后将低合金高强钢水逐步混入中间包和结晶器中；卷取工序,合理设定成卷张力T与产品厚度t、钢卷重量W之间的关系：T≥23×W/t。本发明可有效抑制连铸生产粘结、漏钢事故的发生,改善薄规格钢带卷形缺陷,且不会出现塌卷缺陷和卷取事故,在薄板坯连铸连轧线应用时,可有效实现薄规格高强钢批量稳定生产。(A production method of thin 700 MPa-grade low-alloy high-strength steel belongs to the technical field of steel making. The method comprises the working procedures of smelting, continuous casting, heating, controlled rolling and controlled cooling and coiling. Smelting, namely, chemical components of low-alloy high-strength steel are kept away from a solidification and peritectic region; a continuous casting process, namely, using molten steel with similar components to the low-alloy high-strength steel for casting, and gradually mixing the low-alloy high-strength molten steel into a tundish and a crystallizer after the casting state of the molten steel for casting is stable; and in the coiling process, reasonably setting the relationship between coiling tension T and product thickness T as well as steel coil weight W: t is more than or equal to 23 xW/T. The invention can effectively inhibit the occurrence of continuous casting production adhesion and breakout accidents, improve the coil shape defect of the thin-gauge steel strip, avoid the coil collapse defect and the coiling accident, and effectively realize the batch stable production of the thin-gauge high-strength steel when being applied to a thin slab continuous casting and rolling line.)

1. A production method of thin 700 MPa-grade low-alloy high-strength steel is characterized by comprising the following steps of: comprises the working procedures of smelting, continuous casting, heating, controlled rolling and controlled cooling and reeling;

in the smelting process, the chemical components of the low-alloy high-strength steel are kept away from a solidification and crystallization-coating area, and the weight percentage of the chemical components meet the following relation: [C] 0.0935-0.008[ Si ] -0.022[ Mn ] +0.047[ P ] -0.6[ S ] +0.035[ Als ], and Nb and Ti are added in a compounding way to strengthen;

in the continuous casting process, molten steel similar to the low-alloy high-strength steel is used for casting, the content of corresponding elements in the alloy high-strength steel with similar C, Mn and low S content in the molten steel is reduced by 0-30%, the content of components Si, P and Als is within +/-20% of the content of corresponding elements in the low-alloy high-strength steel, and the low-alloy high-strength molten steel is gradually mixed into a tundish and a crystallizer after the casting state of the molten steel is stable;

in the coiling procedure, the relationship between the coiling tension of the coiling machine and the thickness and weight of the steel coil is as follows: t is more than or equal to 23 xW/T, wherein T: coiling tension of a coiling machine in unit MPa; w: the steel coil weight, unit t; t: product thickness in mm.

2. The method for producing the thin gauge 700MPa grade low alloy high strength steel according to claim 1, wherein: in the continuous casting process, the thickness of the continuous casting billet is more than or equal to 65mm, and the casting speed is 3.5-5.5 m/min.

3. The method for producing the thin gauge 700MPa grade low alloy high strength steel according to claim 2, wherein: in the heating process, the heating temperature is more than or equal to 1200 ℃.

4. The production method of the thin 700MPa grade low-alloy high-strength steel according to claim 3, wherein in the controlled rolling and controlled cooling process, the rough rolling reduction rate is less than or equal to 75%, the finish rolling inlet temperature is 1040-1060 ℃, and the total rolling reduction of the fourth and fifth rolling mills in the finish rolling controlled pass is more than or equal to 30%.

5. The method for producing the thin gauge 700MPa grade low alloy high strength steel according to any one of claims 1 to 4, wherein the thickness of the low alloy high strength steel is 1.2-4.0 mm.

6. The method for producing the thin 700MPa grade low alloy high strength steel according to claim 5, wherein the tensile strength of the low alloy high strength steel is not less than 700MPa, the yield strength is not less than 600MPa, A_80mm≥12%。

Technical Field

The invention belongs to the technical field of steel making, and relates to a production method of thin 700 MPa-grade low-alloy high-strength steel.

Background

The technology for producing thin steel strips on the basis of a thin slab continuous casting and rolling line lasts for nearly two decades in China, and the production process of most low-carbon steel and medium-strength high-strength low-alloy steel is mature day by day at present. With the high-end development of steel products, market competition is intensified day by day, and numerous scholars in China try to develop high-strength steel which is 'cold instead of hot' based on a thin slab continuous casting and rolling process flow so as to reduce cost and improve product competitiveness.

Chinese patent CN101525717 discloses a 700MPa grade Ti microalloying ultra-fine grain hot rolled steel and a production method thereof, and the molten steel after alloying treatment in the refining process comprises the following chemical components in percentage by weight: [C] the method comprises the following steps 0.03 to 0.07%, [ Si ]: <0.3%, [ Mn ]: 1.0-2.10%, [ P ] is less than or equal to 0.02%, [ S ] is less than or equal to 0.008%, [ Ti ]: 0.10-0.14%, and the balance of Fe and inevitable impurity elements in the steel; the thickness of the continuous casting billet is 50-60 mm, and the continuous casting drawing speed is 4.1-5.0 m/min; the initial rolling temperature of the hot continuous rolling is 1050-1080 ℃, the final rolling temperature is 900-930 ℃, and the reduction rate of the 1 st pass of the rolling is controlled to be 50-55%; the cooling speed after rolling is 15-30 ℃/s, and the coiling temperature is 580-630 ℃. According to the scheme, the material strength is improved by using higher Ti content, a controlled rolling process is not designed, the cooling strength is 15-30 ℃/s, and the medium-strength cooling is realized, so that the strong controlled cooling effect of a thin slab continuous casting and rolling production line can not be achieved.

Chinese patent CN101254527 discloses a method for producing low-carbon bainite high-strength steel based on a thin slab continuous casting and rolling process, and the molten steel comprises the following components: [C] the method comprises the following steps 0.03 to 0.08%, [ Si ]: 0.10-0.40%, [ Mn ]: 1.0-2.0%, [ P ] less than or equal to 0.020%, [ S ] less than or equal to 0.008%, [ Nb ]: 0.03 to 0.06%, [ Ti ]: 0.01 to 0.10%, [ Mo ]: 0.0 to 0.4%, [ Cr ]: 0.20 to 0.60%, [ B ]: 0.0 to 0.002%, [ Alss ]: 0.02-0.06%; continuous casting of thin slab at a casting speed of 3.5-4.5m/min, a first stand reduction rate of the hot continuous rolling unit of not less than 40%, and a last stand reduction rate of the hot continuous rolling unit of not less than 12%; the final rolling temperature of the hot rolled plate is 840-860 ℃ and the thickness is 3.0-12.0mm, laminar cooling is adopted, and the rolled plate is coiled underground at the temperature of 550-600 ℃ to form a plate coil. According to the scheme, Nb and Ti composite reinforcement is adopted, and although the content of Ti is reduced, Cr, Si and other elements are added, and production practices show that the elements can deteriorate the surface quality of the steel strip; although the controlled rolling scheme is adopted, the rolling temperature is low, the coiling temperature is low, the thickness of the steel strip is more than 3.0mm, the coiling tension is high, and the equipment burden is increased.

Chinese patent CN102787273A discloses a thin slab produced hot-rolled pickled steel for automobiles with thickness less than or equal to 1.8mm and a production method thereof, and develops a thin hot-rolled pickled steel for automobiles with thickness less than or equal to 1.8mm and tensile strength more than or equal to 400MPa, which has the characteristics of low yield ratio, high surface quality, short production period and low cost. But the method is difficult to produce steel strip products with the strength of over 700 MPa.

Chinese patent CN102335681A discloses a coiling method for preventing a hot-rolled strip steel from being flatly coiled, wherein the coiling temperature is 500-600 ℃, and the coiled steel coil stays on a coiling drum of a coiler for 20-60 s. The method also adopts the modes of low-temperature coiling and delayed coil unloading to increase the phase transformation amount and reduce the tendency of flat coil, but the phase transformation of the steel strip is inhibited under the condition of high-strength layer cooling, so that the problem of the flat coil tendency of the steel strip after high-strength controlled cooling is difficult to effectively solve.

However, the inherent equipment function design of the thin slab continuous casting and rolling line and the traditional medium slab continuous casting and rolling line have inherent defects in the production of high-strength steel, especially low-alloy high-strength steel, the continuous casting stability is poor, the rolling compression ratio from a casting blank to a steel strip is low, the length of a heating furnace is short, the temperature is low, the cooling mode is single, the cooling strength is insufficient, and the like, so that various process defects such as steel leakage in continuous casting production, steel piling in the rolling process, large product performance fluctuation, severe flat coil of a steel coil, high repair rate caused by multiple head and tail rolling defects, and the like are all limited to a certain extent in the batch stable production of the high-strength steel and the ultrahigh-strength steel. Therefore, although many development attempts are made on the thin slab continuous casting and rolling process flow, the high-strength steel is still in the production test stage, and reports of products with controllable quality can be stably produced in batches are rare; the technical bottleneck of low-cost production of thin-specification high-strength steel on a thin slab continuous casting and rolling line under the condition of large-scale industrialized production exists, and needs to be solved urgently.

Disclosure of Invention

In order to solve the technical problems, the invention provides a production method of thin 700 MPa-grade low-alloy high-strength steel, which adopts the following technical scheme:

a production method of thin 700 MPa-grade low-alloy high-strength steel comprises the working procedures of smelting, continuous casting, heating, controlled rolling and controlled cooling and coiling;

in the smelting process, the chemical components of the low-alloy high-strength steel are kept away from a solidification and crystallization-coating area, and the weight percentage of the chemical components meet the following relation: [C] not more than 0.0935-0.008[ Si ] -0.022[ Mn ] +0.047[ P ] -0.6[ S ] +0.035[ Als ], and Nb and Ti are added compositely for reinforcement, wherein the content of Nb and Ti meets the requirement of the standard GB/T3273.

In the continuous casting process, molten steel similar to the low-alloy high-strength steel is used for casting, the content of corresponding elements in the alloy high-strength steel with similar C, Mn and low S content in the molten steel is reduced by 0-30%, the content of components Si, P and Als is within +/-20% of the content of corresponding elements in the low-alloy high-strength steel, and the low-alloy high-strength molten steel is gradually mixed into a tundish and a crystallizer after the casting state of the molten steel is stable; the thickness of the continuous casting billet is more than or equal to 65mm, and the casting speed is 3.5-5.5 m/min.

In the heating process, the heating temperature is more than or equal to 1200 ℃.

In the controlled rolling and controlled cooling process, the rough rolling reduction rate is less than or equal to 75%, the finish rolling inlet temperature is 1040-1060 ℃, and the total rolling reduction of the fourth and fifth rolling mills in the finish rolling controlled pass is more than or equal to 30%.

In the coiling procedure, the relationship between the coiling tension of the coiling machine and the thickness and weight of the steel coil is as follows: t is more than or equal to 23 xW/T, wherein T: coiling tension of a coiling machine in unit MPa; w: the steel coil weight, unit t; t: product thickness in mm.

The thickness of the low-alloy high-strength steel produced by the method is 1.2-4.0 mm; tensile strength is more than or equal to 700MPa, yield strength is more than or equal to 600MPa, A_80mm≥12%。

The 700MPa grade high-strength low-alloy steel product standard of the invention refers to 700L mark in GB/T3273.

Adopt the produced beneficial effect of above-mentioned technical scheme to lie in: the invention is based on the empirical formula of the influence of alloy elements on ladle crystal reaction, changes the high-temperature solidification characteristic of steel by controlling the content proportion of the alloy elements, improves the solidification behavior in the thin slab continuous casting crystallizer, and simultaneously uses molten steel with similar contents of C, Mn, S, P, Si and Als in the components of low-alloy high-strength steel for casting according to the main alloy elements influencing the ladle crystal reaction, thereby effectively inhibiting the occurrence of steel continuous casting production bonding and steel leakage accidents, having particularly remarkable application effect on a thin slab continuous casting and rolling line, and the proven single-casting yield can reach 2250 t. The invention utilizes the comprehensive control of microalloy composite reinforcement, structure refinement and rolling and cooling control to realize the improvement of material strength, effectively reduces the alloy addition by utilizing the deformation effect under thin specification and large pressure, and reduces the alloy cost. According to the invention, the steel strip coiling process and the coil weight are reasonably set to match, so that the coil shape defect of the thin-specification steel strip is effectively improved, the thickness of the steel strip produced in batches can reach 1.2mm, and the coil collapse defect and the coiling accident can not occur.

Drawings

FIG. 1 is an appearance and appearance diagram of a low-alloy high-strength steel coil in example 1;

FIG. 2 is an appearance and appearance diagram of a low-alloy high-strength steel coil in example 2;

FIG. 3 is an appearance and appearance diagram of a low-alloy high-strength steel coil in example 3;

FIG. 4 is an appearance and morphology diagram of a low-alloy high-strength steel coil in example 4.

Detailed Description

Example 1

The thin 700MPa grade low-alloy high-strength steel of the embodiment has a thickness specification of 4.0mm and comprises the following chemical components in percentage by mass: c: 0.043%, Mn: 1.22%, Si: 0.06%, S: 0.004%, P: 0.015%, Als: 0.027%, Nb: 0.043%, Ti: 0.075%, the balance being Fe and unavoidable impurity elements.

The production method of the thin 700 MPa-grade low-alloy high-strength steel comprises the working procedures of smelting, continuous casting, heating, controlled rolling and controlled cooling and coiling. The continuous casting uses molten steel which has similar components with low-alloy high-strength steel for casting, and the molten steel for casting comprises the following main chemical components in percentage by mass: c: 0.040%, Mn: 1.17%, Si: 0.05%, S: 0.003%, P: 0.016%, Als: 0.030%, Ti: 0.022 percent, and the balance of Fe and inevitable impurity elements, and gradually mixing the low-alloy high-strength molten steel into a tundish and a crystallizer after the casting state of the molten steel is stable. In particular, in the present example, the addition of Ti element which does not affect the solidification characteristics of molten steel to the near-composition molten steel is intended to reduce the number of continuous casting and mixing and the ratio of degraded products.

The thickness specification of the continuous casting billet is 72mm, the stable casting speed of a continuous casting machine is 4.0m/min, the continuous casting of the tundish is 900t, the heating temperature of a hot charging furnace is 1280 ℃, the rough rolling reduction rate is 69.85%, the mean temperature of a finish rolling inlet is 1040 ℃, and the total reduction of a finish rolling controlled pass secondary fourth mill and a finish rolling controlled pass fifth mill is 30.11%.

And (4) rolling and cooling the steel coil, and then feeding the steel coil into a coiling machine, wherein the coiling tension is 135MPa, and the weight of the steel coil is controlled to be 22 t.

The mechanical property results of the low-alloy high-strength steel obtained in the embodiment are shown in table 1, and the appearance of the steel coil is shown in fig. 1.

Example 2

The thin 700MPa grade low-alloy high-strength steel of the embodiment has a thickness specification of 1.2mm and comprises the following chemical components in percentage by mass: c: 0.049%, Mn: 1.37%, Si: 0.23%, S: 0.003%, P: 0.011%, Als: 0.026%, Nb: 0.038%, Ti: 0.082%, the rest is Fe and inevitable impurity elements.

The production method of the thin 700 MPa-grade low-alloy high-strength steel comprises the working procedures of smelting, continuous casting, heating, controlled rolling and controlled cooling and coiling. The continuous casting uses molten steel which has similar components with low-alloy high-strength steel for casting, and the molten steel for casting comprises the following main chemical components in percentage by mass: c: 0.042%, Mn: 1.01%, Si: 0.22%, S: 0.003%, P: 0.009%, Als: 0.031, Ti: 0.034%, the balance being Fe and inevitable impurity elements; and after the pouring state of the molten steel for casting is stable, the low-alloy high-strength molten steel is gradually mixed into a tundish and a crystallizer. In particular, in the present example, the addition of Ti element which does not affect the solidification characteristics of molten steel to the near-composition molten steel is intended to reduce the number of continuous casting and mixing and the ratio of degraded products.

The thickness specification of the continuous casting billet is 65mm, the stable casting speed of a continuous casting machine is 4.5m/min, the continuous casting of the tundish is 1800t, the heating temperature of a hot charging furnace is 1220 ℃, the rough rolling reduction rate is 74.60%, the mean temperature of a finish rolling inlet is 1058 ℃, and the total reduction of a fourth rolling mill and a fifth rolling mill of a finish rolling controlled way is 36.08%.

And (4) rolling and cooling the steel coil, and then feeding the steel coil into a coiling machine, wherein the coiling tension is 345MPa, and the weight of the steel coil is controlled to be 18 t.

The mechanical property results of the high-strength low-alloy steel strip obtained in the embodiment are shown in table 2, and the appearance of the steel coil is shown in fig. 2.

Example 3

The thin 700MPa grade low-alloy high-strength steel of the embodiment has a thickness specification of 3.5mm and comprises the following chemical components in percentage by mass: c: 0.056%, Mn: 1.23%, Si: 0.06%, S: 0.005%, P: 0.012%, Als: 0.027%, Nb: 0.035%, Ti: 0.076%, the balance being Fe and unavoidable impurity elements.

The production method of the thin 700 MPa-grade low-alloy high-strength steel comprises the working procedures of smelting, continuous casting, heating, controlled rolling and controlled cooling and coiling. The continuous casting uses molten steel which has similar components with low-alloy high-strength steel for casting, and the molten steel for casting comprises the following main chemical components in percentage by mass: c: 0.046%, Mn: 1.05%, Si: 0.05%, S: 0.004%, P: 0.010%, Als: 0.027%, Nb: 0.033%, Ti: 0.021%, and the balance of Fe and inevitable impurity elements; and after the pouring state of the molten steel for casting is stable, the low-alloy high-strength molten steel is gradually mixed into a tundish and a crystallizer. In particular, the reason why the Nb and Ti elements which do not affect the solidification characteristics of molten steel are added to the molten steel of similar composition in this example is to reduce the number of continuous casting and mixing and the ratio of degraded products.

The thickness specification of the continuous casting billet is 65mm, the stable casting speed of a continuous casting machine is 4.5m/min, 1200t of continuous casting of the tundish is carried out, the heating temperature of a hot charging furnace is 1280 ℃, the rough rolling reduction rate is 73.85%, the mean temperature of a finish rolling inlet is 1046 ℃, and the total reduction of a fourth rolling mill and a fifth rolling mill of a finish rolling controlled pass is 31.80%.

And (4) rolling and cooling are controlled, and then the steel enters a coiling machine, the coiling tension is 155MPa, and the weight of the steel coil is controlled to be 20 t.

The mechanical property results of the high-strength low-alloy steel strip obtained in the embodiment are shown in table 3, and the appearance of the steel coil is shown in fig. 3.

Example 4

The thin 700MPa grade low-alloy high-strength steel of the embodiment has a thickness specification of 2.0mm and comprises the following chemical components in percentage by mass: c: 0.063%, Mn: 1.22%, Si: 0.05%, S: 0.004%, P: 0.011%, Als: 0.029%, Nb: 0.040%, Ti: 0.084%, the balance being Fe and inevitable impurity elements.

The production method of the thin 700 MPa-grade low-alloy high-strength steel comprises the working procedures of smelting, continuous casting, heating, controlled rolling and controlled cooling and coiling. The continuous casting uses molten steel which has similar components with low-alloy high-strength steel for casting, and the molten steel for casting comprises the following main chemical components in percentage by mass: c: 0.055%, Mn: 1.19%, Si: 0.04%, S: 0.003%, P: 0.013%; and Als: 0.025%, Nb: 0.032%, Ti: 0.024%, and the balance of Fe and inevitable impurity elements; and after the pouring state of the molten steel for casting is stable, the low-alloy high-strength molten steel is gradually mixed into a tundish and a crystallizer. In particular, the reason why the Nb and Ti elements which do not affect the solidification characteristics of molten steel are added to the molten steel of similar composition in this example is to reduce the number of continuous casting and mixing and the ratio of degraded products.

The thickness specification of the continuous casting billet is 70mm, the stable casting speed of a continuous casting machine is 4.0m/min, the continuous casting of the tundish is 2250t, the heating temperature of a hot charging furnace is 1240 ℃, the rough rolling reduction rate is 74.28%, the mean temperature of a finish rolling inlet is 1045 ℃, and the total reduction of a fourth rolling mill and a fifth rolling mill of a finish rolling controlled pass is 33.88%.

And (4) rolling and cooling the steel coil, and then feeding the steel coil into a coiling machine, wherein the coiling tension is 229MPa, and the weight of the steel coil is controlled to be 19 t.

The mechanical property results of the high-strength low-alloy steel strip obtained in the embodiment are shown in table 4, and the appearance of the steel coil is shown in fig. 4.

Example 5

The thin 700MPa grade low-alloy high-strength steel of the embodiment has a thickness specification of 2.6mm and comprises the following chemical components in percentage by mass: c: 0.054%, Mn: 1.35%, Si: 0.10%, S: 0.003%, P: 0.008%, Als: 0.022%, Nb: 0.036%, Ti: 0.078%, the balance being Fe and unavoidable impurity elements.

The production method of the thin 700 MPa-grade low-alloy high-strength steel comprises the working procedures of smelting, continuous casting, heating, controlled rolling and controlled cooling and coiling. The continuous casting uses molten steel which has similar components with low-alloy high-strength steel for casting, and the molten steel for casting comprises the following main chemical components in percentage by mass: c: 0.045%, Mn: 1.28%, Si: 0.12%, S: 0.003%, P: 0.009%, Als: 0.026%, Ti: 0.031%, the balance being Fe and inevitable impurity elements; and after the pouring state of the molten steel for casting is stable, the low-alloy high-strength molten steel is gradually mixed into a tundish and a crystallizer. In particular, in the present example, the addition of Ti element which does not affect the solidification characteristics of molten steel to the near-composition molten steel is intended to reduce the number of continuous casting and mixing and the ratio of degraded products.

The thickness specification of the continuous casting billet is 68mm, the stable casting speed of a continuous casting machine is 5.5m/min, 1200t of tundish continuous casting is carried out, the heating temperature of a hot charging furnace is 1200 ℃, the rough rolling reduction rate is 71.65%, the mean value temperature of a finish rolling inlet is 1051 ℃, and the total reduction of a finish rolling controlled pass secondary fourth mill and a finish rolling controlled pass fifth mill is 31.82%.

And (4) rolling and cooling the steel coil, and then feeding the steel coil into a coiling machine, wherein the coiling tension is 200MPa, and the weight of the steel coil is controlled to be 21.3 t.

The mechanical property results of the low-alloy high-strength steel obtained in the embodiment are shown in table 1, and the appearance of the steel coil is similar to that of the steel coil shown in figures 1-4, so that the results are omitted.

Example 6

The thin 700MPa grade low-alloy high-strength steel of the embodiment has a thickness specification of 3.8mm and comprises the following chemical components in percentage by mass: c: 0.045%, Mn: 1.26%, Si: 0.17%, S: 0.003%, P: 0.010%, Als: 0.023%, Nb: 0.041%, Ti: 0.080%, and the balance of Fe and inevitable impurity elements.

The production method of the thin 700 MPa-grade low-alloy high-strength steel comprises the working procedures of smelting, continuous casting, heating, controlled rolling and controlled cooling and coiling. The continuous casting uses molten steel which has similar components with low-alloy high-strength steel for casting, and the molten steel for casting comprises the following main chemical components in percentage by mass: c: 0.033%, Mn: 1.21%, Si: 0.15%, S: 0.003%, P: 0.010%, Als: 0.024%, Nb: 0.031, Ti: 0.030%, the balance being Fe and unavoidable impurity elements; and after the pouring state of the molten steel for casting is stable, the low-alloy high-strength molten steel is gradually mixed into a tundish and a crystallizer. In particular, the reason why the Nb and Ti elements which do not affect the solidification characteristics of molten steel are added to the molten steel of similar composition in this example is to reduce the number of continuous casting and mixing and the ratio of degraded products.

The thickness specification of the continuous casting billet is 66mm, the stable casting speed of the continuous casting machine is 4.8m/min, and the continuous casting of the tundish is 1650

t, the heating temperature of a hot charging furnace is 1264 ℃, the rough rolling reduction rate is 69.85%, the mean value temperature of a finish rolling inlet is 1060 ℃, and the total reduction of a fourth rolling mill and a fifth rolling mill of a finish rolling controlled way is 31.02%.

And (4) rolling and cooling the steel coil, and then feeding the steel coil into a coiling machine, wherein the coiling tension is 125MPa, and the weight of the steel coil is controlled to be 18.7 t.

The mechanical property results of the low-alloy high-strength steel obtained in the embodiment are shown in table 1, and the appearance of the steel coil is similar to that of the steel coil shown in figures 1-4, so that the results are omitted.

Example 7

The thin 700MPa grade low-alloy high-strength steel of the embodiment has a thickness specification of 1.5mm and comprises the following chemical components in percentage by mass: c: 0.060%, Mn: 1.31%, Si: 0.19%, S: 0.002%, P: 0.008%, Als: 0.020%, Nb: 0.040%, Ti: 0.077%, the balance being Fe and unavoidable impurity elements.

The production method of the thin 700 MPa-grade low-alloy high-strength steel comprises the working procedures of smelting, continuous casting, heating, controlled rolling and controlled cooling and coiling. The continuous casting uses molten steel which has similar components with low-alloy high-strength steel for casting, and the molten steel for casting comprises the following main chemical components in percentage by mass: c: 0.055%, Mn: 1.22%, Si: 0.16%, S: 0.002%, P: 0.009%, Als: 0.022%, Ti: 0.029%, and the balance of Fe and inevitable impurity elements; and after the pouring state of the molten steel for casting is stable, the low-alloy high-strength molten steel is gradually mixed into a tundish and a crystallizer. In particular, in the present example, the addition of Ti element which does not affect the solidification characteristics of molten steel to the near-composition molten steel is intended to reduce the number of continuous casting and mixing and the ratio of degraded products.

The thickness specification of the continuous casting billet is 70mm, the stable casting speed of a continuous casting machine is 4.2m/min, the continuous casting of the tundish is 1500t, the heating temperature of a hot charging furnace is 1232 ℃, the rough rolling reduction rate is 74.60%, the mean temperature of a finish rolling inlet is 1053 ℃, and the total reduction of a finish rolling controlled pass secondary fourth mill and a finish rolling controlled pass fifth mill is 33.85%.

And (4) rolling and cooling are controlled, and then the steel enters a coiling machine, the coiling tension is 320MPa, and the weight of the steel coil is controlled to be 20.5 t.

The mechanical property results of the low-alloy high-strength steel obtained in the embodiment are shown in table 1, and the appearance of the steel coil is similar to that of the steel coil shown in figures 1-4, so that the results are omitted.

TABLE 1 mechanical Property results of the steel strips of examples 1-7

。