A short-process low yield ratio 690MPa grade bulb flat steel and its preparation method

Abstract

The invention discloses a short-process low yield ratio 690MPa grade bulb flat steel and its production process, belonging to the technical field of ship structural steel; solves the problem that the existing bulb flat steel cannot simultaneously meet the requirements of high strength, high toughness, low yield ratio and low cost. The chemical composition of the short-process low yield ratio 690MPa grade bulb flat steel of the invention includes, by mass percentage: C: 0.12% to 0.16%, Si: 0.05% to 0.20%, Mn: 1.50% to 1.85%, P≤0.015%, S≤0.010%, V: 0.10% to 0.15%, Ti: 0.005% to 0.010%, Nb: 0.014% to 0.020%, N: 0.020% to 0.030%, and the balance is Fe and unavoidable impurities. The invention realizes the demand for high-performance bulb flat steel in the construction of large ships, and improves the structural safety and bearing capacity of ships.

Description

Technical Field

[0001] The invention relates to the technical field of ship structural steel, and in particular to a short-process low-yield ratio 690MPa grade bulb flat steel and a preparation method thereof. Background Art

[0002] Bulb flat steel is a special structural steel for ships. It plays a vital role in the hull structure just like human ribs. It is an indispensable special profile for building 10,000-ton ships and various ocean-going, coastal and inland ships. In recent years, as the world's shipbuilding industry develops towards large-scale, high-speed and automated directions, marine bulb flat steel is facing the problem of product upgrading. High strength, high toughness and easy welding are the mainstream development direction of marine bulb flat steel.

[0003] At present, most of the domestic high-strength bulb flat steels with a yield strength of more than 440MPa adopt the Ni-Cr-Mo component system. However, with the addition of expensive alloy elements such as Ni, Cr, and Mo, the hardenability of the steel is improved, and a large amount of bainite is easily formed during continuous cooling from γ to α. On the one hand, the yield strength ratio is increased (generally above 0.90), which seriously affects the machinability and service safety of the bulb flat steel. On the other hand, due to the phase change characteristics of the rapid cooling process of this steel, the structural stress caused by the phase change leads to defects such as poor plate shape, and normalizing + tempering / tempering heat treatment after rolling is required to make the structure uniform, the production cycle is long, and the cost is too high.

[0004] In summary, the existing 440MPa grade bulb flat steel has a high alloy cost due to the addition of precious alloy elements Ni, Cr, and Mo; in addition, the hot rolling + tempering production process it adopts has a long manufacturing process and high production cost, and the highest strength level of the bulb flat steel it produces is 440MPa grade bulb flat steel, which cannot meet the demand for high-performance bulb flat steel in large ship construction. Summary of the invention

[0005] In view of the above analysis, the present invention aims to provide a short-process low yield ratio 690MPa grade bulb flat steel and its production process, so as to solve the problem that the bulb flat steel prepared by the existing technology cannot simultaneously meet the requirements of high strength, high toughness, low yield ratio and low cost.

[0006] The purpose of the present invention is mainly achieved through the following technical solutions:

[0007] The invention provides a short-process low yield ratio 690MPa grade bulb flat steel. The chemical composition of the bulb flat steel comprises, by mass percentage, C: 0.12%-0.16%, Si: 0.05%-0.20%, Mn: 1.50%-1.85%, P≤0.015%, S≤0.010%, V: 0.10%-0.15%, Ti: 0.005%-0.010%, Nb: 0.014%-0.020%, N: 0.020%-0.030%, and the balance is Fe and inevitable impurities.

[0008] In one possible design, the chemical composition of the bulb flat steel includes, by mass percentage: C: 0.12% ~ 0.158%, Si: 0.06% ~ 0.19%, Mn: 1.53% ~ 1.83%, P ≤ 0.015%, S ≤ 0.010%, V: 0.1% ~ 0.144%, Ti: 0.005% ~ 0.010%, Nb: 0.014% ~ 0.0184%, N: 0.020% ~ 0.030%, and the balance is Fe and unavoidable impurities.

[0009] In a possible design, in the short process low yield ratio 690MPa grade bulb flat steel, the V and Nb additions satisfy V / Nb≥6.0.

[0010] In a possible design, in the short process low yield ratio 690MPa grade bulb flat steel, the N and C additions satisfy C / N≤6.0.

[0011] In a possible design, in the short process low yield strength ratio 690MPa grade bulb flat steel, the V and N additions satisfy V / N ≥ 3.64.

[0012] On the other hand, the present invention also provides a method for preparing a short-process low yield ratio 690MPa grade bulb flat steel, which is used for the above-mentioned short-process low yield ratio 690MPa grade bulb flat steel, and the preparation method comprises the following steps:

[0013] Step 1, smelting is performed in an electric furnace, and steel is tapped after smelting. Argon blowing is ensured throughout the steel tapping process, and the steel is transferred to the LF furnace after the argon blowing time is greater than 3 minutes;

[0014] Step 2, using the LF furnace to perform refining outside the furnace to obtain molten steel;

[0015] Step 3, continuously casting the molten steel smelted in step 2 to obtain a cast billet;

[0016] Step 4, opening the ingot;

[0017] Step 5, rolling the steel billet;

[0018] In step 5, the steel billet is heated before rolling, the heating temperature is 1150-1250°C, the heating time in the furnace is less than or equal to 3.5h, the starting rolling temperature is 1100-1180°C, the final rolling temperature is 900-1000°C, and the stack is cooled to room temperature after rolling to obtain 690MPa grade bulb flat steel.

[0019] Furthermore, in step 1, the electric furnace smelting temperature is 1550°C-1650°C, the smelting time is 1-4h, and the steel tapping temperature T≥1630°C.

[0020] Further, in step 2, the refining temperature of the LF furnace is 1580-1600° C., and the refining time is 30-60 min.

[0021] Furthermore, in step 2, VN16 alloy powder is added in the middle stage of refining, and argon is blown throughout the refining process.

[0022] Furthermore, in step 2, the amount of VN16 alloy powder added is 1.1 to 2.0 kg per ton of molten steel.

[0023] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0024] (1) Existing bulb flat steels contain precious alloying elements such as Cr, Ni, and Mo, resulting in a high alloy cost. Compared with the prior art, the short-process low yield ratio high-strength bulb flat steel of the present invention does not contain the expensive alloying elements Cr, Ni, and Mo that are traditionally used to increase strength. The present invention uses V, Nb, and N microalloying elements to effectively reduce the amount of alloy used, thereby saving resources and reducing production costs.

[0025] (2) The hot rolling + tempering production process used in the existing flat ball steel has a long manufacturing process and high production cost, and the highest strength level of the flat ball steel produced is 440MPa. Compared with the prior art, the short-process low yield ratio high-strength flat ball steel of the present invention improves the toughness while ensuring the strength of the flat ball steel through a reasonable component system and good process, omits the post-rolling heat treatment process, simplifies the production process, shortens the production process, reduces carbon emissions, and finally obtains a short-process, high-strength, low yield ratio 690MPa flat ball steel, which can meet the urgent demand of the rapidly developing shipbuilding industry for high-strength flat ball steel with special performance requirements.

[0026] In the present invention, the above-mentioned technical solutions can also be combined with each other to achieve more preferred combination solutions. Other features and advantages of the present invention will be described in the subsequent description, and some advantages can become obvious from the description, or can be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained through the contents particularly pointed out in the embodiments of the description and the drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0027] The drawings are only for the purpose of illustrating particular embodiments and are not to be considered limiting of the present invention. Like reference symbols denote like components throughout the drawings.

[0028] Figure 1 This is the microstructure diagram of the short process low yield ratio 690MPa grade bulb flat steel of the present invention. DETAILED DESCRIPTION

[0029] The preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used together with the embodiments of the present invention to illustrate the principles of the present invention, but are not used to limit the scope of the present invention.

[0030] The invention provides a short-process low yield ratio 690MPa grade bulb flat steel. The chemical composition of the bulb flat steel comprises, by mass percentage, C: 0.12%-0.16%, Si: 0.05%-0.20%, Mn: 1.50%-1.85%, P≤0.015%, S≤0.010%, V: 0.10%-0.15%, Ti: 0.005%-0.010%, Nb: 0.014%-0.020%, N: 0.020%-0.030%, and the balance is Fe and unavoidable impurities.

[0031] It should be noted that the functions of the various components of the above 690MPa grade bulb flat steel are as follows:

[0032] V: Vanadium is a key microalloying element in the flat bulb steel of the present invention. V precipitates in austenite, which can refine austenite grains. At the same time, the interface energy between these grains and ferrite is small, which can promote the nucleation of ferrite, play a role in inducing intracrystalline ferrite nucleation and refining ferrite grains. In addition, V dispersedly precipitates in ferrite and can play a precipitation strengthening role, further improving the strength of the flat bulb steel. When the vanadium content is less than 0.10%, the minimum strength requirement of the present invention cannot be met. At the same time, considering the comprehensive cost, the upper limit of the vanadium content should be controlled at 0.15%.

[0033] Nb: Niobium is a key microalloying element in the bulb flat steel of the present invention. The addition of Nb element can inhibit the recrystallization of austenite during the rolling process of the steel plate, flatten the austenite of the steel plate during the rolling process, increase the area of ​​deformed austenite, and precipitate fine Nb (CN) particles during the rolling process, inhibit the growth of austenite grains during the rolling process and the cooling process after rolling, and refine the grains. However, when the Nb content is high, it will expand the window for the formation of bainite / Widmanstatten structure, promote the formation of bainite / Widmanstatten structure, and reduce the toughness of the steel. Therefore, taking comprehensive considerations, the Nb content in the present invention is controlled at a level of 0.014% to 0.020%.

[0034] Ti: Titanium is an important microalloying element in the bulb flat steel of the present invention, and is also a precipitation strengthening element in the microalloyed steel. It can form very stable TiN particles in the high temperature zone, which can effectively prevent the growth of austenite grains. To achieve this goal, the minimum Ti content should be controlled at 0.005%. However, too high a Ti content will make the TiN particles in the steel coarse and reduce the low-temperature toughness of the steel. Therefore, the present invention stipulates that the range of Ti is 0.005% to 0.010%.

[0035] N: Nitrogen is a key microalloying element in the bulb flat steel of the present invention. N can combine with V, Nb, and Ti in the steel to form second phase particles for precipitation, and the nitrogen in the steel can control the precipitation of V in austenite, changing the distribution of V that plays a role in fine grain strengthening and precipitation strengthening, and can play the process route of V fine grain strengthening, using fine grain strengthening to replace part of the precipitation strengthening effect, improving the plasticity and toughness of the matrix without changing the strength, and the increase of nitrogen promotes the precipitation of V, thereby reducing the amount of V, which plays a role in reducing costs. Combined with the contents of V, Nb and Ti, the N content is controlled at 0.020% to 0.030%.

[0036] C: Carbon is an essential element to improve the strength of steel. In order to obtain the required strength, the C content needs to be above 0.12%. However, when its content exceeds 0.16%, the low-temperature toughness and weldability of the steel will be significantly reduced, and the tough-to-brittle transition temperature will be increased. Therefore, its range is specified to be 0.12% to 0.16%.

[0037] Si: Silicon is a deoxidizing element and also a solid solution strengthening element. In order to ensure the deoxidation effect and the required strength, the Si content needs to be above 0.03%. However, when its content exceeds 0.40%, the toughness and weldability of the steel will deteriorate. Therefore, in order to ensure the strength, toughness and weldability of the bulb flat steel, the present invention stipulates that the range of Si is 0.05% to 0.20%.

[0038] Mn: Manganese is an essential element to ensure the strength and toughness of steel. It can not only delay the time of high-temperature phase transformation, reduce the phase transformation temperature, and improve the hardenability of steel, but also, in the present invention, an important role of manganese is to enrich in reverse transformed austenite, improve the stability of reverse transformed austenite, and thus improve the toughness level of steel. The Mn content can be controlled at different content levels to achieve different ultra-low temperature toughness requirements. Taking all factors into consideration, the Mn content is controlled at 1.50% to 1.85% in the present invention.

[0039] P is an impurity element in steel. The less the P content is, the better the performance of the bulb flat steel in the present invention is. Therefore, P is limited to 0.015%.

[0040] S is an unavoidable harmful element in steel. It will form MnS inclusions, reducing the toughness and weldability of steel. Therefore, its content should be reduced as much as possible and controlled below 0.01%.

[0041] It should be pointed out that in the above-mentioned short-process low yield ratio 690MPa grade bulb flat steel, the addition amounts of N and C satisfy C / N≤6.0. This is mainly because when the N content is low, the remaining V will combine with C to precipitate, which in turn reduces the strength of the steel.

[0042] In the above-mentioned short process low yield ratio 690MPa grade bulb flat steel, the addition amounts of V and N satisfy V / N ≥ 3.64. This is mainly because when the V content is low, the fixed nitrogen element is reduced, so that the remaining N in the steel will become free N, increasing the aging brittleness of the steel and reducing the low temperature toughness of the steel.

[0043] It should be noted that my country is the world's largest vanadium producer, with the largest vanadium resource reserves in the world. The development of VN microalloyed steel in my country has its natural resource advantages. China's vanadium reserves rank first in the world, about 9.5 million tons, accounting for 37% of the world's total reserves; vanadium can convert the cheap N element in steel into a useful alloying element. On the one hand, it promotes the precipitation of V in austenite, promotes the nucleation of intragranular ferrite, plays a role in fine grain strengthening, and improves the toughness of steel. On the other hand, it promotes the precipitation of V in ferrite during the slow cooling process, plays a role in precipitation strengthening, improves the strength of steel, and greatly improves the precipitation driving force of V through the addition of N, reduces the use of vanadium, and effectively reduces the development cost of high-strength bulb flat steel. In addition, V microalloyed steel has low rolling force requirements for rolling mills, can be rolled in a higher austenite recrystallization zone, has a wide process window, and can better adapt to the requirements of production processes.

[0044] In the above-mentioned short-process low yield ratio 690MPa grade bulb flat steel, the addition amounts of V and Nb satisfy V / Nb≥6.0. This is mainly to ensure that a sufficient ferrite content can be obtained and to improve the low-temperature toughness of the bulb flat steel. Adding Nb elements to the VN microalloy system is beneficial to refining the austenite grain size and further improving the strength and low-temperature toughness of the bulb flat steel.

[0045] In the above-mentioned short-process low yield ratio 690MPa grade bulb flat steel, 0.10%≤V+Nb+Ti≤0.16%, which ensures strength and toughness while reducing costs.

[0046] It should be pointed out that the width of the high-strength bulb flat steel of the present invention is 140-430 mm, the height of the bulb head is 26-83 mm, and the thickness of the web is 7-20 mm.

[0047] The microstructure of the short process low yield ratio 690MPa grade bulb flat steel includes ferrite F+pearlite P, such as Figure 1As shown, the volume fraction ratio of ferrite F and pearlite P is 1.5-2.5:1.

[0048] The above short-process low yield ratio 690MPa grade bulb flat steel also contains a small amount of granular bainite, whose volume percentage (RA%) is 3% to 10%, and the equivalent diameter (DRA) of the granular bainite is 8 to 17nm. This is because a small amount of fine granular bainite can improve the yield ratio and reduce the problem of processability of bulb flat steel. If the granular bainite is controlled at 3 to 10%, the heat treatment of traditional steel can be omitted, and a short process and low cost can be achieved, which can improve the low-temperature impact toughness of steel.

[0049] In one embodiment of the present invention, the chemical composition of the short-process low yield ratio 690MPa grade bulb flat steel includes, by mass percentage: C: 0.12% ~ 0.158%, Si: 0.06% ~ 0.19%, Mn: 1.53% ~ 1.83%, P ≤ 0.015%, S ≤ 0.010%, V: 0.1% ~ 0.144%, Ti: 0.005% ~ 0.010%, Nb: 0.014% ~ 0.0184%, N: 0.020% ~ 0.030%, and the balance is Fe and unavoidable impurities.

[0050] In one embodiment of the present invention, the chemical composition of the short process low yield ratio 690MPa grade bulb flat steel includes, by mass percentage: C: 0.125% ~ 0.142%, Si: 0.06% ~ 0.16%, Mn: 1.55% ~ 1.83%, P ≤ 0.015%, S ≤ 0.010%, V: 0.106% ~ 0.136%, Ti: 0.005% ~ 0.010%, Nb: 0.015% ~ 0.0162%, N: 0.0210% ~ 0.028%, and the balance is Fe and unavoidable impurities.

[0051] In one embodiment of the present invention, the chemical composition of the short-process low yield ratio 690MPa grade bulb flat steel includes, by mass percentage: C: 0.130% ~ 0.140%, Si: 0.09% ~ 0.14%, Mn: 1.58% ~ 1.68%, P ≤ 0.015%, S ≤ 0.010%, V: 0.12% ~ 0.130%, Ti: 0.005% ~ 0.010%, Nb: 0.015% ~ 0.0160%, N: 0.0220% ~ 0.0247%, and the balance is Fe and unavoidable impurities.

[0052] In one embodiment of the present invention, the chemical composition of the short-process low yield ratio 690MPa grade bulb flat steel includes, by mass percentage: C: 0.12%, Si: 0.18%, Mn: 1.72%, P: 0.006%, S: 0.003%, V: 0.121%, Ti: 0.008%, Nb: 0.015%, N: 0.022%, and the balance is Fe and unavoidable impurities; wherein, C / N is 5.45; V / N is 5.50, V / Nb is 8.07, and V+Nb+Ti is 0.144.

[0053] In one embodiment of the present invention, the chemical composition of the short-process low yield ratio 690MPa grade bulb flat steel includes, by mass percentage: C: 0.13%, Si: 0.16%, Mn: 1.59%, P: 0.007%, S: 0.006%, V: 0.136%, Ti: 0.008%, Nb: 0.015%, N: 0.024%, and the balance is Fe and unavoidable impurities; wherein, C / N is 5.42; V / N is 5.67, V / Nb is 9.07, and V+Nb+Ti is 0.159.

[0054] In one embodiment of the present invention, the chemical composition of the short-process low yield ratio 690MPa grade bulb flat steel includes, by mass percentage: C: 0.14%, Si: 0.14%, Mn: 1.68%, P: 0.007%, S: 0.005%, V: 0.115%, Ti: 0.006%, Nb: 0.017%, N: 0.028%, and the balance is Fe and unavoidable impurities; wherein, C / N is 5.00; V / N is 4.11, V / Nb is 6.76, and V+Nb+Ti is 0.138.

[0055] In one embodiment of the present invention, the chemical composition of the short-process low yield ratio 690MPa grade bulb flat steel includes, by mass percentage: C: 0.15%, Si: 0.09%, Mn: 1.83%, P: 0.009%, S: 0.005%, V: 0.128%, Ti: 0.005%, Nb: 0.016%, N: 0.026%, and the balance is Fe and unavoidable impurities; wherein, C / N is 5.77; V / N is 4.92, V / Nb is 8.00, and V+Nb+Ti is 0.149.

[0056] Compared with the prior art, the short-process low yield ratio high-strength bulb flat steel of the present invention has a yield strength greater than or equal to 690MPa, a tensile strength greater than or equal to 870MPa, a -40°C low-temperature impact energy greater than or equal to 170J, and a yield ratio less than or equal to 0.80.

[0057] Existing bulb flat steels add precious alloy elements Cr, Ni, and Mo, and the alloy cost is relatively high. At the same time, the hot rolling + tempering production process adopted by the invention has a long manufacturing process and high production cost, and the highest strength level of the bulb flat steel produced by the invention is 440MPa bulb flat steel. Compared with the prior art, the short-process low yield ratio high-strength bulb flat steel of the present invention does not contain the traditional expensive alloy elements Cr, Ni, and Mo for improving strength. The invention adopts V, Nb, and N microalloying elements to effectively reduce the amount of alloy, save resources and reduce production costs.

[0058] The present invention also provides a short-process production method of a 690 MPa-grade bulb flat steel with a low yield ratio, comprising the following steps:

[0059] Step 1: Use an electric furnace to smelt. The tapping temperature after smelting is T≥1630°C. During the tapping process, argon blowing is ensured throughout the process. After the argon blowing time is greater than 3 minutes, the steel is transferred to the LF furnace;

[0060] In the above step 1, the electric furnace smelting temperature is 1550°C-1650°C, the smelting time is 1-4h, and the tapping temperature T≥1630°C; the tapping temperature is controlled to be higher than 1630°C. On the one hand, the inclusions in the molten steel can be reduced, and on the other hand, a refined structure can be obtained in the subsequent rapid solidification process. In addition, the occurrence of defects such as cracks and segregation caused by too low temperature can be reduced.

[0061] In the above step 1, the present invention utilizes the high N content of electric furnace steel to reduce the addition of N content.

[0062] Step 2, using LF furnace for refining outside the furnace;

[0063] The refining temperature of the LF furnace is 1580-1600℃, and the refining time is 30-60min. VN16 alloy powder is added in the middle of refining, and argon is blown throughout the refining process. After adjusting the composition, stirring with large amounts of argon is strictly prohibited.

[0064] In the above step 2, the refining temperature is controlled within the range of 1580-1600°C. On the one hand, it can improve the removal effect of inclusions and improve the fluidity of molten steel to ensure uniform composition. On the other hand, it can avoid the accelerated oxidation of molten steel caused by excessively high temperature and reduce high-temperature erosion of refractory materials. The LF refining time is controlled within the range of 30-60min. On the one hand, sufficient refining time can ensure the clean removal of inclusions and uniform composition. On the other hand, it can avoid the oxidation of molten steel and the increase of production costs and the decrease of efficiency caused by too long a time.

[0065] In the above step 2, the purpose of adding VN16 alloy powder in the middle stage of LF refining is to prevent local supersaturation of nitrogen. Argon is blown throughout the refining process to increase the recovery rate of vanadium and nitrogen in the steel.

[0066] In the above step 2, the amount of VN16 alloy powder added is 1.1-2.0 kg / ton. The use of VN16 alloy powder can ensure that the yield of V and N is above 90%, while the addition of other materials, such as ferrovanadium, ferrovanadium nitride, etc., cannot ensure the yield of V and N.

[0067] Step 3, continuously casting the molten steel smelted in step 2;

[0068] During continuous casting, the molten steel is poured into the open pouring furnace and the continuous pouring furnace in turn. The superheat of the molten steel is controlled as follows: △T≤35℃ for the open pouring furnace, △T≤25℃ for the continuous pouring furnace, and the continuous casting straightening temperature ≥950℃; wherein, △T refers to the temperature difference caused by various factors when the molten steel is poured from the ladle into the tundish.

[0069] The specific process of continuous casting includes:

[0070] Step 31: Rapidly cool the molten steel using a crystallizer to solidify the molten steel into a billet;

[0071] Step 32, drawing and straightening;

[0072] In the crystallizer, the molten steel gradually solidifies into a shell. The pulling and straightening machine and the crystallization vibration device work together to pull the shell out of the crystallizer and straighten it.

[0073] Step 33, secondary cooling;

[0074] The pulled out billet shell enters the secondary cooling zone by spraying water or other cooling methods to ensure that the billet shell is completely solidified.

[0075] Step 33, cutting and conveying;

[0076] After the billet shell is completely solidified, it is cut into billets of a certain length using cutting equipment and transported to the next process via a roller conveyor.

[0077] Step 4, blanking;

[0078] The ingot is continuously rolled, the starting rolling temperature is ≤1200℃, the final rolling temperature is ≤1000℃, and it is air-cooled after rolling. The specification of the hot-rolled steel billet is 150×150mm.

[0079] In the above step 4, the rolling temperature is 1100-1180°C to ensure that the bulb flat steel has good plastic deformation ability; the final rolling is carried out in the temperature range of 900-1000°C to give full play to the beneficial effect of V, Nb, Ti, N, and C elements synergistically precipitating carbonitrides. On the one hand, the precipitation of carbonitrides in austenite can refine the original austenite grains; on the other hand, the precipitated carbonitrides can provide more nucleation cores for the transformation of austenite to ferrite, thereby further refining the ferrite structure and obtaining a better strength and toughness match. Traditional ferrite nucleates on the austenite grain boundary and is relatively coarse. The present invention can simultaneously realize the simultaneous nucleation of ferrite on the austenite grain boundary and in the grain, so as to obtain a fine final structure.

[0080] In the above step 4, the process of continuous rolling includes:

[0081] Step 41, performing medium frequency induction heating on the ingot;

[0082] The ingot (continuous casting billet) is fed into a medium frequency induction heating furnace for heating; the purpose of heating is to increase the plasticity of the billet and reduce the difficulty of rolling, while ensuring that the billet maintains a uniform temperature distribution during the rolling process.

[0083] It should be noted that the temperature needs to be strictly controlled during the heating process (rolling temperature ≤ 1100℃) to avoid overheating or overcooling which may lead to a decrease in the performance of the billet.

[0084] Step 42: Rough rolling the blank using a three-roller rolling mill:

[0085] The heated billet is fed into a three-roll mill for rough rolling; the purpose of rough rolling is to roll the billet into an intermediate billet close to the final shape, while removing the oxide scale and defects on the billet surface.

[0086] Step 43: Use a finishing mill to finish-roll the intermediate billet:

[0087] The intermediate billet after rough rolling is sent to the finishing mill for finishing rolling. The purpose of finishing rolling is to roll the intermediate billet into the required shape and size of the bulb flat steel.

[0088] Temperature and deformation control: During the finishing rolling process, the temperature and deformation need to be strictly controlled to ensure that the structure and performance of the bulb flat steel meet the design requirements. Excessive temperature or excessive deformation may cause the performance of the bulb flat steel to deteriorate.

[0089] Step 44, cooling the ball flat steel after finish rolling;

[0090] The bulb flat steel is cooled by natural cooling or forced cooling to remove the heat generated during the rolling process and prevent it from thermal deformation.

[0091] Step 5, rolling: the heating temperature of the hot-rolled steel billet is 1150-1250°C, the heating time in the furnace is less than or equal to 3.5h, the starting rolling temperature is 1100-1180°C, the final rolling temperature is 900-1000°C, and the stack is cooled to room temperature after rolling to obtain a 690MPa grade bulb flat steel.

[0092] In the above step 5, during the rolling of the ball flat steel, the heating temperature of the steel billet is higher than the rolling temperature. On the one hand, this is to ensure that the steel billet is fully heated to improve the plastic deformation capacity of the steel billet. On the other hand, the heating temperature of the steel billet is set higher than the rolling temperature so that there is enough temperature reserve during the rolling process to cope with the temperature drop of the steel material during the subsequent rolling process.

[0093] In the above step 5, the billet heating temperature is 1150-1250°C, the start rolling temperature is 1100-1180°C, the final rolling temperature is 900-1000°C, and air cooling is performed after final rolling; controlled rolling in the austenite recrystallization zone has lower requirements on the rolling force of the rolling mill and a small load, which is beneficial to protecting the rolling mill.

[0094] The present invention controls the start rolling temperature at a relatively high temperature of 1100-1180°C, which can ensure that the bulb flat steel has good plastic deformation ability. The present invention controls the final rolling temperature at a temperature range of 900-1000°C for final rolling, which can give full play to the beneficial effect of V, Nb, Ti, N, and C elements in co-precipitating carbonitrides. On the one hand, the precipitation of carbonitrides in austenite can refine the original austenite grains; on the other hand, the precipitated carbonitrides can provide more nucleation cores for the transformation of austenite to ferrite, thereby further refining the ferrite structure and obtaining a better strength and toughness match. Traditional ferrite nucleates on the austenite grain boundary and is relatively coarse. The present invention can simultaneously realize the simultaneous nucleation of ferrite on the austenite grain boundary and in the grain to obtain a fine final structure.

[0095] Compared with the prior art, the short-process production of low yield ratio high-strength bulb flat steel of the present invention fully utilizes the high N content of electric furnace steel, combined with the addition of VN16 alloy powder, can effectively ensure that the recovery rate of vanadium and nitrogen is above 90%, the process is simple, and the final rolling is carried out at 900-1000℃, which has a small load on the rolling mill and can effectively ensure the service life of the rolling mill.

[0096] It should be pointed out that compared with adding VN16 alloy powder, adding by other methods, such as ferrovanadium and ferrovanadium nitride, cannot guarantee the yield of V and N; while the present invention adopts the method of adding VN16 alloy powder, which can ensure the yield of V and N is above 90%.

[0097] Furthermore, the short-process, low-yield ratio, high-strength bulb flat steel of the present invention improves its toughness while ensuring its strength through a reasonable component system and good process, omits the post-rolling heat treatment process, simplifies the production process, shortens the production process, and reduces carbon emissions. Ultimately, a short-process, high-strength, low-yield ratio 690MPa grade bulb flat steel is obtained, which can meet the urgent demand of the rapidly developing shipbuilding industry for high-strength bulb flat steel with special performance requirements.

[0098] Example 1

[0099] In this embodiment, a 70-ton electric furnace is used to smelt the bulb flat steel. The chemical composition of the bulb flat steel is shown in mass percentage in Table 1 below.

[0100] The bulb flat steel of this embodiment adopts the process flow of smelting, refining outside the furnace, continuous casting, blanking and rolling to prepare No. 10 bulb flat steel. Its production process and technical parameters are controlled as follows:

[0101] Step 1, electric furnace smelting: 70 tons electric furnace smelting, tapping temperature is 1630 ℃, argon blowing is ensured throughout the tapping process, and the LF furnace is transferred after argon blowing time of 3.2 minutes;

[0102] Step 2, refining outside the furnace: the refining temperature is 1582℃, VN16 alloy powder is added in the middle of refining, the amount of alloy powder added is 1.6 kg / ton, argon is blown throughout the refining process, after adjusting the composition, it is strictly forbidden to stir with a large amount of argon to expose the molten steel;

[0103] Step 3, continuous casting: the superheat of molten steel is controlled to be △T≤35℃ for the opening furnace and △T≤25℃ for the continuous casting furnace, and the continuous casting straightening temperature is 975℃;

[0104] Step 4, billeting: the ingot is continuously rolled, the starting rolling temperature is 1110°C, the final rolling temperature is 920°C, air cooling is performed after rolling, and the specification is 150×150mm.

[0105] Step 5, rolling: the billet heating temperature is 1220°C, the heating time in the furnace is 3h, the starting rolling temperature is 1120°C, the final rolling temperature is 920°C, and the stack is cooled to room temperature after rolling to obtain a 690MPa grade bulb flat steel.

[0106] In the microstructure of the 690 MPa grade bulb flat steel obtained in this embodiment, the ratio of ferrite to pearlite is 1.5:1, and the content of granular bainite is 3.2%.

[0107] Example 2

[0108] The embodiment adopts a 70-ton electric furnace to smelt the bulb flat steel. The chemical composition of the bulb flat steel of this embodiment is shown in mass percentage in Table 1 below.

[0109] The bulb flat steel of this embodiment adopts the process flow of smelting, refining outside the furnace, continuous casting, blanking and rolling to prepare No. 22 bulb flat steel. Its production process and technical parameters are controlled as follows:

[0110] Step 1, electric furnace smelting: 70 tons electric furnace smelting, tapping temperature is 1635 ℃, argon blowing is ensured throughout the tapping process, and the LF furnace is transferred after argon blowing time of 3.5 minutes;

[0111] Step 2, refining outside the furnace: the refining temperature is 1585℃, VN16 alloy powder is added in the middle of refining, the amount of alloy powder added is 1.8 kg / ton, argon is blown throughout the refining process, after adjusting the composition, it is strictly forbidden to stir with a large amount of argon to expose the molten steel;

[0112] Step 3, continuous casting: the superheat of molten steel is controlled to be △T≤35℃ for the opening furnace and △T≤25℃ for the continuous casting furnace, and the continuous casting straightening temperature is 975℃;

[0113] Step 4, billeting: the ingot is continuously rolled, the starting rolling temperature is 1100°C, the final rolling temperature is 980°C, air cooling is performed after rolling, and the specification is 150×150mm.

[0114] Step 5, rolling: the billet heating temperature is 1180°C, the heating time in the furnace is 3.2h, the starting rolling temperature is 1140°C, the final rolling temperature is 940°C, and the stack is cooled to room temperature after rolling to obtain a 690MPa grade bulb flat steel.

[0115] In the microstructure of the 690 MPa grade bulb flat steel obtained in this embodiment, the ratio of ferrite to pearlite is 1.7:1, and the content of granular bainite is 4.8%.

[0116] Example 3

[0117] The embodiment adopts a 70-ton electric furnace to smelt the bulb flat steel. The chemical composition of the bulb flat steel in this embodiment is shown in mass percentage in Table 1 below.

[0118] The bulb flat steel of this embodiment adopts the process flow of smelting, refining outside the furnace, continuous casting, blanking and rolling to prepare No. 36 bulb flat steel. Its production process and technical parameters are controlled as follows:

[0119] Step 1, electric furnace smelting: 70 tons electric furnace smelting, tapping temperature is 1638 ℃, argon blowing is ensured throughout the tapping process, and the LF furnace is transferred after argon blowing time of 3.8 minutes;

[0120] Step 2, refining outside the furnace: the refining temperature is 1595℃, VN16 alloy powder is added in the middle of refining, the amount of alloy powder added is 1.8 kg / ton, argon is blown throughout the refining process, after adjusting the composition, it is strictly forbidden to stir with a large amount of argon to expose the molten steel;

[0121] Step 3, continuous casting: the superheat of molten steel is controlled to be △T≤35℃ for the opening furnace and △T≤25℃ for the continuous casting furnace, and the continuous casting straightening temperature is 975℃;

[0122] Step 4, billeting: the ingot is continuously rolled, the starting rolling temperature is 1050°C, the final rolling temperature is 950°C, air cooling is performed after rolling, and the specification is 150×150mm.

[0123] Step 5, rolling: the billet heating temperature is 1200°C, the heating time in the furnace is 3h, the starting rolling temperature is 1160°C, the final rolling temperature is 980°C, and the stack is cooled to room temperature after rolling to obtain a 690MPa grade bulb flat steel.

[0124] In the microstructure of the 690 MPa grade bulb flat steel obtained in this embodiment, the ratio of ferrite to pearlite is 2.0:1, and the content of granular bainite is 7.3%.

[0125] Example 4

[0126] The embodiment adopts 70-ton electric furnace for smelting. The chemical composition of the bulb flat steel in this embodiment is shown in Table 1 in terms of mass percentage.

[0127] The bulb flat steel of this embodiment adopts the process flow of smelting, refining outside the furnace, continuous casting, blanking and rolling to prepare the bulb flat steel No. 43. The production process and technical parameters thereof are controlled as follows:

[0128] Step 1, electric furnace smelting: 70 tons electric furnace smelting, tapping temperature is 1640 ℃, argon blowing is ensured throughout the tapping process, and the LF furnace is transferred after argon blowing time of 4.0 minutes;

[0129] Step 2, refining outside the furnace: the refining temperature is 1600℃, VN16 alloy powder is added in the middle of refining, the amount of alloy powder added is 1.3 kg / ton, argon is blown throughout the refining process, after adjusting the composition, it is strictly forbidden to stir with a large amount of argon to expose the molten steel;

[0130] Step 3, continuous casting: the superheat of molten steel is controlled to be △T≤35℃ for the opening furnace and △T≤25℃ for the continuous casting furnace, and the continuous casting straightening temperature is 975℃;

[0131] Step 4, billeting: the ingot is continuously rolled, the starting rolling temperature is 1080°C, the final rolling temperature is 1000°C, air cooling is performed after rolling, and the specification is 150×150mm.

[0132] Step 5, rolling: the billet heating temperature is 1240°C, the heating time in the furnace is 3.1h, the starting rolling temperature is 1175°C, the final rolling temperature is 990°C, and the stack is cooled to room temperature after rolling to obtain a 690MPa grade bulb flat steel.

[0133] In the microstructure of the 690 MPa grade bulb flat steel obtained in this embodiment, the ratio of ferrite to pearlite is 2.5:1, and the content of granular bainite is 9.7%.

[0134] Table 1 Chemical composition of bulb flat steel of embodiment and comparative example (wt%)

[0135]

[0136]

[0137] As can be seen from Table 1, Examples 1 to 4 of the present invention are four kinds of test steels produced in accordance with the component content requirements of the bulb flat steel defined in the present invention, and their chemical compositions all meet the requirements of the present invention, wherein C / N, Ti / V, and Mn / Ni also meet the requirements of the present invention.

[0138] As a control, the present invention also provides comparative examples 1 to 4; wherein, the preparation conditions of comparative example 1 are the same as those of embodiment 1 of the present invention, and the C and V contents in comparative example 1 do not meet the requirements of the present invention; the preparation conditions of comparative example 2 are the same as those of embodiment 2 of the present invention, and the C / N ratio and N content in comparative example 2 do not meet the requirements of the present invention; the preparation conditions of comparative example 3 are the same as those of embodiment 3 of the present invention, and the Nb content and V / Nb ratio in comparative example 3 do not meet the requirements of the present invention; the preparation conditions of comparative example 4 are the same as those of embodiment 4 of the present invention, and the V+Nb+Ti content and V / N ratio in comparative example 4 do not meet the requirements of the present invention.

[0139] Samples were taken from Example 1 to Example 1 and Comparative Examples 1 to Comparative Examples 4, and the sampling locations were all at the ball head center. A room temperature tensile test was conducted on a WE-300 hydraulic tensile testing machine according to GB / T228-2002 "Room Temperature Tensile Test Method for Metallic Materials", and a low temperature impact test was conducted on a JBN-300C impact testing machine according to GB / T229-1994 "Metallic Charpy Notch Impact Test Method", and the fracture impact absorption energy of the -40°C test steel was measured. The results are shown in Table 2.

[0140] Table 2 Mechanical properties of bulb flat steel in Example 1-Example 4 and Comparative Example 1-Comparative Example 4

[0141] type Specification Rm / MPa Rel / MPa A / ％ Z / ％ Rel / Rm Akv-40℃ / J Example 1 10 898 715 29.5 75 0.796 224 Example 2 22 917 726 31.5 76.5 0.792 218 Example 3 36 909 721 32 75.5 0.793 196 Example 4 43 940 745 34.5 73.5 0.793 181 Comparative Example 1 10 678 586 31.5 68.5 0.864 171 Comparative Example 2 22 796 691 28.5 65.5 0.868 66 Comparative Example 3 36 923 786 25.3 61.5 0.852 58 Comparative Example 4 43 671 536 31.6 71.5 0.799 182

[0142] As can be seen from Table 2, the yield strength of the No. 10, No. 22, No. 36 and No. 43 bulb flat steels rolled by Examples 1 to 4 of the present invention is 715MPa-745MPa, all greater than 690MPa; the low-temperature impact energy at -40℃ is 181-224J, all greater than 170J; and the yield strength ratio is 0.792-0.796, all less than 0.80; the tensile strength (Rm) is 898-940MPa; and the cross-sectional shrinkage is 73.5-76.5%. However, the yield strength and yield strength ratio of the bulb flat steel rolled by the steel of Comparative Example 1 do not meet the requirements, the yield strength ratio and low-temperature toughness of the bulb flat steel rolled by the steel of Comparative Example 2 and Comparative Example 3 do not meet the requirements, and the yield strength of the bulb flat steel rolled by the steel of Comparative Example 4 does not meet the requirements.

[0143] In summary, the short-process low yield ratio 690 MPa grade bulb flat steel and its production process of the present invention optimize and adjust the composition and process, reasonably control the microstructure, effectively reduce the yield ratio and improve the low-temperature toughness.

[0144] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A short-process low yield ratio 690MPa grade bulb flat steel, characterized in that: The chemical composition of the bulb flat steel includes, by mass percentage, C: 0.12% to 0.16%, Si: 0.05% to 0.20%, Mn: 1.50% to 1.85%, P≤0.015%, S≤0.010%, V: 0.10% to 0.15%, Ti: 0.005% to 0.010%, Nb: 0.014% to 0.020%, N: 0.020% to 0.030%, and the remainder is Fe and unavoidable impurities.

2. The short process low yield ratio 690MPa grade bulb flat steel according to claim 1 is characterized in that: The chemical composition of the bulb flat steel includes, by mass percentage, C: 0.12% to 0.158%, Si: 0.06% to 0.19%, Mn: 1.53% to 1.83%, P≤0.015%, S≤0.010%, V: 0.1% to 0.144%, Ti: 0.005% to 0.010%, Nb: 0.014% to 0.0184%, N: 0.020% to 0.030%, and the remainder is Fe and unavoidable impurities.

3. The short process low yield ratio 690MPa grade bulb flat steel according to claim 1 is characterized in that: In the short process low yield ratio 690MPa grade bulb flat steel, the addition amounts of V and Nb satisfy V / Nb≥6.

0.

4. The short process low yield ratio 690MPa grade bulb flat steel according to claim 1, characterized in that: In the short process low yield ratio 690MPa grade bulb flat steel, the addition amounts of N and C satisfy C / N≤6.

0.

5. The short process low yield ratio 690MPa grade bulb flat steel according to claim 1, characterized in that: In the short process low yield ratio 690MPa grade bulb flat steel, the added amounts of V and N satisfy V / N≥3.

64.

6. A method for preparing a short-process low yield ratio 690MPa grade bulb flat steel, characterized in that: For preparing the short process low yield ratio 690MPa grade bulb flat steel according to any one of claims 1 to 5, the preparation method comprises the following steps: Step 1, smelting is performed in an electric furnace, and steel is tapped after smelting. Argon blowing is ensured throughout the steel tapping process, and the steel is transferred to the LF furnace after the argon blowing time is greater than 3 minutes; Step 2, using the LF furnace to perform refining outside the furnace to obtain molten steel; Step 3, continuously casting the molten steel smelted in step 2 to obtain a cast billet; Step 4, opening the ingot; Step 5, rolling the steel billet; In the step 5, the steel billet is heated before rolling, the heating temperature is 1150-1250°C, the heating time in the furnace is less than or equal to 3.5h, the starting rolling temperature is 1100-1180°C, the final rolling temperature is 900-1000°C, and air cooling is performed to room temperature to obtain 690MPa grade bulb flat steel.

7. The method for preparing a short-process low yield ratio 690MPa grade bulb flat steel according to claim 6, characterized in that: In step 1, the electric furnace smelting temperature is 1550°C-1650°C, the smelting time is 1-4h, and the steel tapping temperature T≥1630°C.

8. The method for preparing a short-process low yield ratio 690MPa grade bulb flat steel according to claim 6, characterized in that: In step 2, the refining temperature of the LF furnace is 1580-1600° C., and the refining time is 30-60 min.

9. The method for preparing a short-process low yield ratio 690MPa grade bulb flat steel according to claim 6, characterized in that: In step 2, VN16 alloy powder is added in the middle stage of refining, and argon is blown throughout the refining process.

10. The method for preparing a short-process low yield ratio 690MPa grade bulb flat steel according to claim 9, characterized in that: In step 2, the amount of VN16 alloy powder added is 1.1-2.0 kg / ton of molten steel.

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