An excellent low-temperature impact toughness non-quenched and tempered steel and a method for manufacturing the same

Abstract

The present application relates to a kind of excellent low temperature impact toughness non-quenched and tempered steel and its production method, the chemical composition of non-quenched and tempered steel and its mass percent are as follows: C:0.23~0.28%, Si:0.25~0.50%, Mn:1.20~1.60%, V:0.08~0.20%, Nb:0.016~0.020%, N:0.01~0.02%, P≤0.025%, S≤0.015%, the balance is Fe and inevitable impurities;Its production method includes obtaining casting blank according to the formula amount, casting blank heating and controlled rolling and controlled cooling.The present application adds V, Nb, N element in smelting process, so that it forms carbonitride precipitation, and the structure is regulated and controlled in combination with controlled rolling and controlled cooling process, forms high strength and toughening effect, and the impact energy of the obtained non-quenched and tempered steel can reach more than 27J at-28℃, can satisfy the use demand under low temperature service condition.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of steel material manufacturing, and particularly relates to a non-quenched and tempered steel with excellent low-temperature impact toughness and a preparation method thereof. BACKGROUND

[0002] Traditional mechanical structural steel has good strength, plasticity and toughness coordination, but the quenched and tempered treatment (quenching + high-temperature tempering) adopted by the traditional mechanical structural steel consumes a huge amount of energy and does not conform to the current development concept of low-carbon environmental protection and cost saving. At present, the ferrite-pearlite type non-quenched and tempered steel has been successfully applied in batch production of engine crankshafts and connecting rods and other components, but the further popularization and application of the non-quenched and tempered steel in safety components such as steering knuckles, half shafts, stabilizer bars and new energy automobile motor shafts are hindered, and the main problem is how to improve and stabilize the strength and toughness under the premise of controlling the alloy cost, especially under the extreme service environment of low temperature, to improve the impact toughness. The V-notch impact energy of the general non-quenched and tempered steel at-20 ℃ is less than 10 J, which is difficult to meet the demand. Therefore, under the premise of ensuring high strength, improving the low-temperature toughness will help the popularization and application of the non-quenched and tempered steel.

[0003] At present, the carbon content of the non-quenched and tempered steel is usually 0.3-0.5wt% C, and micro-alloying elements such as V, Nb and Ti are added to the steel, combined with controlled rolling (forging) and controlled cooling process to refine the grain, and at the same time, carbonitride is dispersedly precipitated in the microstructure of ferrite and pearlite as a strengthening phase, so that the mechanical properties similar to those of quenched and tempered steel are obtained. However, the addition of alloy elements to improve the strength and toughness of the non-quenched and tempered steel will increase the cost. In view of this, the present application designs and manufactures a non-quenched and tempered steel containing only a small amount of micro-alloying elements such as V, Nb and N, which controls the cost while ensuring the strength and toughness. SUMMARY

[0004] The purpose of the present application is to provide a non-quenched and tempered steel with excellent low-temperature impact toughness and a preparation method thereof. By adjusting the composition of the steel plate and the preparation process, the V-notch impact energy of the finally obtained non-quenched and tempered steel at-28 ℃ reaches 27 J or more, and the cost is controlled on the basis of low alloy content.

[0005] To achieve the above purpose, the technical solution adopted by the present application is as follows:

[0006] A non-quenched and tempered steel with excellent low-temperature impact toughness, the chemical composition and the mass percentage thereof are as follows: C: 0.23-0.28%, Si: 0.25-0.50%, Mn: 1.20-1.60%, V: 0.08-0.20wt%, Nb: 0.016-0.020%, N: 0.01-0.02%, P≤0.025%, S≤0.015%, and the balance is Fe and unavoidable impurities.

[0007] The application adjusts the element composition of the non-quenched and tempered steel, so that the final obtained medium-carbon pearlite-ferrite type non-quenched and tempered steel reaches the V-notch impact function of 27J or more at-28℃.

[0008] The effects of each element in the steel plate composition are as follows:

[0009] C has the greatest impact on the strength and hardness of the steel, but the content needs to be controlled, otherwise it will seriously affect the plasticity and toughness of the steel. The carbon content of the non-quenched and tempered steel is generally not allowed to exceed 0.5%, and too low carbon content will cause insufficient mechanical strength of the steel, therefore the C addition amount of the steel is designed to be 0.20-0.32%, preferably 0.23-0.28%.

[0010] Mn is one of the most important elements of the non-quenched and tempered steel, which has a strong solid solution strengthening effect, can reduce the phase transition temperature of the steel, slow down the transformation speed of austenite to pearlite, improve the hardenability and refine the grain, and the appropriate amount of Mn added to the steel is beneficial to the strength and toughness of the non-quenched and tempered steel. However, it should be noted that the Mn content should not be too high, otherwise it will produce bainite in the quenched structure and deteriorate the plasticity and toughness of the steel, and the Mn content of the medium-carbon non-quenched and tempered steel is generally not more than 1.5%, therefore the Mn addition amount of the steel is designed to be 1.00-1.80%, preferably 1.20-1.60%.

[0011] Si exists in the form of solid solution in the steel, has a strong solid solution strengthening effect, and is an important ferrite strengthening element. Higher content of Si element can increase the ferrite content and refine the grain to improve the toughness. However, excessive Si will increase the activity of C and promote the occurrence of decarburization during rolling, thereby seriously reducing the toughness of the steel, therefore the Si addition amount of the steel is designed to be 0.25-0.50%.

[0012] V has a low precipitation temperature in the steel, mainly precipitates between phases during the transformation of austenite to ferrite and precipitates in ferrite, and improves the strength of the steel through precipitation hardening. The high-efficiency strengthening effect of V provides a basic guarantee for the high strengthening of the non-quenched and tempered steel, but excessive V will deteriorate the toughness. Therefore, the V addition amount is designed to be 0.08-0.20%.

[0013] Nb exists in the steel mainly in the form of solid solution and precipitation. The solid solution Nb element can improve the hardenability of the steel, expand the temperature interval and cold speed range of the ferrite-pearlite phase transformation. The precipitation of Nb(C, N) also refines the austenite grain, and promotes the dispersion precipitation of V(C, N) on the grain boundary and dislocation, which is beneficial to the improvement of toughness. The Nb(C, N) precipitates can hinder the coarsening of austenite grains, refine the pearlite interlamellar spacing, and strengthen the non-quenched and tempered steel, and can also induce intracrystalline ferrite nucleation to refine the grain.

[0014] N can promote the precipitation of vanadium in vanadium-containing steel, enhance the precipitation strengthening effect of vanadium, and significantly improve the strength of non-quenched and tempered steel. By using cheap nitrogen element, the toughness of the steel is improved, which is a very effective method to reduce cost. The steel addition amount is 0.01-0.02%.

[0015] The preparation method of the excellent low-temperature impact toughness non-quenched and tempered steel provided by the application comprises obtaining a casting blank according to a formula amount, heating the casting blank, and controlling rolling and cooling.

[0016] Further, the application provides a method for obtaining a casting blank according to a formula amount, which comprises converter smelting, LF ladle refining, VD vacuum refining and continuous casting according to a formula amount; wherein V and Nb elements are added in the converter smelting process; and Nb elements are added in the VD vacuum refining process.

[0017] Further, the application provides that the heating of the casting blank comprises preheating, heating and soaking the casting blank in sequence.

[0018] Preferably, the preheating temperature of the casting blank is not more than 900 DEG C, and the preheating time is 40-50 min.

[0019] Preferably, the heating section of the casting blank comprises first heating and second heating; the first heating temperature is 1050-1120 DEG C; and the second heating temperature is 1150-1220 DEG C.

[0020] Preferably, the soaking temperature of the casting blank is 1180-1250 DEG C.

[0021] Further, the application provides that the controlled rolling and cooling comprises rough rolling, medium rolling, finish rolling, water cooling and cold bed cooling of the casting blank in sequence.

[0022] Preferably, the rough rolling opening temperature is 1080-1120 DEG C.

[0023] Preferably, the inlet temperature of the water cooling is 1000-1040 DEG C, and the outlet temperature is 900-940 DEG C.

[0024] Preferably, the upper cold bed temperature of the cold bed cooling is 820-850 DEG C, and the lower cold bed temperature is 530-560 DEG C.

[0025] The technical scheme of the application has the following beneficial technical effects:

[0026] 1. In the chemical composition design, by adding the basic elements and micro-alloying elements V, Nb and N, the strengthening effect of V and Nb on carbonitride and nitride is fully utilized, and the effects of Nb element on hindering austenite grain coarsening, refining pearlite interlamellar spacing and inducing intracrystalline ferrite nucleation are utilized, so that the strength and toughness level of the steel is further improved.

[0027] 2. On the basis of controlling the composition of the steel plate, the application makes the non-quenched and tempered steel have excellent mechanical properties by controlling the process conditions of the casting blank heating and controlled rolling and controlled cooling, the tensile strength is higher than 700 MPa, the yield strength is higher than 500 MPa, the elongation is higher than 15%, the reduction of area is higher than 35%, and the V-notch impact energy at-28 ℃ reaches more than 27 J. The production method has the advantages of simple control, low manufacturing cost, strong operability and the like.

[0028] 3. The application obtains the non-quenched and tempered steel with good low-temperature impact toughness by controlling the cooling mode and cooling speed of the micro-alloyed non-quenched and tempered steel after rolling and designing the composition of the steel grade, and has high popularization value in actual production and application. BRIEF DESCRIPTION OF DRAWINGS

[0029] Figure 1 It is the optical microstructure diagram of the non-quenched and tempered steel of the embodiment 1 of the application. IMPLEMENTATION

[0030] In order to make the purpose, technical scheme and advantages of the application more clear and explicit, the technical scheme of the application will be clearly and completely described below through embodiments. The described embodiments are only a part of examples of the application, and based on the embodiments of the application, those skilled in the art should understand that the described embodiments only help to understand the application and are not used to limit the application. EMBODIMENT 1

[0031] A kind of excellent low-temperature impact toughness non-quenched and tempered steel, its preparation method includes obtaining casting blank according to formula amount, casting blank heating and controlled rolling and controlled cooling;Specifically as follows:

[0032] (1) obtaining casting blank according to formula amount: according to formula amount, converter smelting, LF refining, VD vacuum and continuous casting are carried out, and casting blank is obtained;Wherein, V element is added in the converter smelting process;Nb element is added in the VD vacuum process;

[0033] (2) casting blank heating: preheating, heating and soaking of the casting blank are carried out;Wherein the preheating temperature is 865 ℃, and the preheating time is 50 min;The heating section includes first heating and second heating, the first heating temperature is 1080 ℃, and the heating time is 1 h;The second heating temperature is 1180 ℃, and the heating time is 1 h;The soaking temperature is 1180 ℃, and the heating time is 1 h, and the high-pressure water phosphorus removal water pressure is 22 Pa;

[0034] (3) controlled rolling and controlled cooling: the casting blank is sequentially subjected to rough rolling, medium rolling, finish rolling, water cooling and cooling bed cooling;Wherein the rough rolling opening temperature is 1090 ℃, and the casting blank is rolled through 5 sets of rough rolling mill + No. 1 flying shear + 6 sets of medium rolling mill + No. 2 flying shear + 6 sets of finish rolling mill + No. 3 flying shear + reducing diameter mill;

[0035] Water tank water cooling is adopted, the inlet temperature is 1020℃, the outlet temperature is 920℃; the upper cooling bed temperature is 835℃, the lower cooling bed temperature is 535℃; then stack cooling is adopted, the cooling speed is 0.2℃ / min, the end temperature is 80℃, then air cooling is adopted until the room temperature, and the non-quenched and tempered steel is obtained.

[0036] The non-quenched and tempered steel prepared in the embodiment is detected in terms of composition, and the detection results in terms of mass percentage are as follows: C is 0.25%, Si is 0.35%, Mn is 1.40%, V is 0.12%, Nb is 0.020%, N is 0.017%, S is 0.0063%, and the balance is Fe and inevitable impurities, and P in the impurities is ≤100ppm.

[0037] The performance detection results of the non-quenched and tempered steel in the embodiment are shown in Table 1; the metallographic structure is shown in Fig. 1, and is medium-carbon pearlite-ferrite. Figure 1 Example 2

[0038] An excellent low-temperature impact toughness non-quenched and tempered steel, and a preparation method thereof, the preparation method comprises the following steps:

[0039] (1) obtaining a casting blank according to the formula amount: converter smelting, LF refining, VD vacuum and continuous casting are carried out according to the formula amount to obtain a casting blank; wherein, V element is added in the converter smelting process; Nb element is added in the VD vacuum process;

[0040] (2) casting blank heating: preheating, heating and soaking are carried out on the casting blank; the preheating temperature is 880℃, the preheating time is 50min; the heating section comprises first heating and second heating, the first heating temperature is 1100℃, the heating time is 1h; the second heating temperature is 1200℃, the heating time is 1h; the soaking temperature is 1210℃, the heating time is 1h, and the high-pressure water phosphorus removal water pressure is 22Pa;

[0041] (3) controlled rolling and controlled cooling: the casting blank is sequentially subjected to rough rolling, medium rolling, finish rolling, water cooling and cooling bed cooling; the rough rolling opening temperature is 1095℃, and the casting blank is rolled through 5 sets of rough rolling mill + No.1 flying shear + 6 sets of medium rolling mill + No.2 flying shear + 6 sets of finish rolling mill + No.3 flying shear + reducing diameter mill;

[0042] Water tank water cooling is adopted, the inlet temperature is 1020℃, the outlet temperature is 920℃; the upper cooling bed temperature is 835℃, the lower cooling bed temperature is 535℃; then stack cooling is adopted, the cooling speed is 0.2℃ / min, the end temperature is 80℃, then air cooling is adopted until the room temperature, and the non-quenched and tempered steel is obtained.

[0043] ​The non-quenched and tempered steel prepared in the embodiment is subjected to component detection, and the detection results in mass percentage are as follows: C is 0.25%, Si is 0.32%, Mn is 1.32%, V is 0.11%, Nb is 0.017%, N is 0.015%, S is 0.0072%, and the balance is Fe and inevitable impurities, wherein P in the impurities is ≤100 ppm.

[0044] The performance detection results of the non-quenched and tempered steel in the embodiment are shown in Table 1, and the metallographic structure thereof is the same as that of Example 1. Example 3

[0045] An excellent low-temperature impact toughness non-quenched and tempered steel, and a preparation method thereof, the preparation method comprising obtaining a casting blank according to a formula amount, heating the casting blank, and controlling rolling and cooling, and specifically as follows:

[0046] (1) obtaining a casting blank according to a formula amount: converter smelting, LF refining, VD vacuum, and continuous casting are performed according to a formula amount to obtain a casting blank; wherein V element is added in the converter smelting process; and Nb element is added in the VD vacuum process;

[0047] (2) heating the casting blank: preheating, heating, and soaking are performed on the casting blank; wherein the preheating temperature is 865 ℃, the preheating time is 50 min; the heating section comprises first heating and second heating, the first heating temperature is 1080 ℃, the heating time is 1 h; the second heating temperature is 1190 ℃, the heating time is 1 h; the soaking temperature is 1200 ℃, the heating time is 1 h, and the high-pressure water phosphorus removal water pressure is 22 Pa;

[0048] (3) controlling rolling and cooling: the casting blank is sequentially subjected to rough rolling, medium rolling, finishing rolling, water cooling, and cooling bed cooling; wherein the rough rolling opening temperature is 1110 ℃, and the casting blank is rolled through 5 sets of rough rolling mill + No. 1 flying shear + 6 sets of medium rolling mill + No. 2 flying shear + 6 sets of finishing rolling mill + No. 3 flying shear + reducing diameter mill;

[0049] water cooling is adopted, the inlet temperature is 1040 ℃, and the outlet temperature is 940 ℃; the upper cooling bed temperature is 850 ℃, and the lower cooling bed temperature is 535 ℃; then slow cooling is performed through stack cooling, the cooling speed is 0.2 ℃ / min, the end point temperature is 80 ℃, and then air cooling is performed to room temperature.

[0050] The non-quenched and tempered steel prepared in the embodiment is subjected to component detection, and the detection results in mass percentage are as follows: C is 0.26%, Si is 0.36%, Mn is 1.39%, V is 0.12%, Nb is 0.021%, N is 0.017%, S is 0.0061%, and the balance is Fe and inevitable impurities, wherein P in the impurities is ≤100 ppm.

[0051] The performance detection results of the non-quenched and tempered steel in the embodiment are shown in Table 1, and the metallographic structure thereof is the same as that of Example 1. Example 4

[0052] An excellent low-temperature impact toughness non-quenched and tempered steel, a preparation method thereof comprises obtaining a casting blank according to a formula amount, heating the casting blank and controlling rolling and cooling, and specifically as follows:

[0053] (1) obtaining a casting blank according to a formula amount: carrying out converter smelting, LF refining, VD vacuum and continuous casting according to a formula amount to obtain a casting blank; wherein, V element is added in the converter smelting process; Nb element is added in the VD vacuum process;

[0054] (2) heating the casting blank: preheating, heating and soaking the casting blank; wherein, the preheating temperature is 850℃, the preheating time is 50min; the heating section comprises first heating and second heating, the first heating temperature is 1080℃, the heating time is 1h; the second heating temperature is 1160℃, the heating time is 1h; the soaking temperature is 1210℃, the heating time is 1h, and the high-pressure water phosphorus removal water pressure is 22Pa;

[0055] (3) controlling rolling and cooling: sequentially carrying out rough rolling, medium rolling, finishing rolling, water cooling and cooling bed cooling on the casting blank; wherein, the rough rolling opening temperature is 1080℃, and the casting blank is rolled through 5 rough rolling mill groups+1 flying shear+6 medium rolling mill groups+2 flying shear+6 finishing rolling mill groups+3 flying shear+reducing diameter mill group;

[0056] water cooling is adopted by using a water tank, the inlet temperature is 1010℃, and the outlet temperature is 925℃; the upper cooling bed temperature is 820℃, the lower cooling bed temperature is 560℃, then the casting blank is cooled and slowly cooled by piling, the cooling speed is 0.2℃ / min, the end temperature is 80℃, and then the casting blank is air cooled to room temperature.

[0057] The non-quenched and tempered steel prepared in the embodiment is detected in terms of composition, and the detection results in terms of mass percentage are as follows: C is 0.25%, Si is 0.35%, Mn is 1.41%, V is 0.13%, Nb is 0.016%, N is 0.018%, S is 0.0058%, and the balance is Fe and unavoidable impurities, and P in the impurities is ≤100ppm.

[0058] The performance detection results of the non-quenched and tempered steel in the embodiment are shown in Table 1, and the metallographic structure thereof is the same as that of Example 1. Example 5

[0059] An excellent low-temperature impact toughness non-quenched and tempered steel, a preparation method thereof comprises obtaining a casting blank according to a formula amount, heating the casting blank and controlling rolling and cooling, and specifically as follows:

[0060] (1) obtaining a casting blank according to a formula amount: carrying out converter smelting, LF refining, VD vacuum and continuous casting according to a formula amount to obtain a casting blank; wherein, V element is added in the converter smelting process; Nb element is added in the VD vacuum process;

[0061] (2) slab heating: preheating, heating and soaking of the slab; the preheating temperature is 870 °C, the preheating time is 50 min; the heating section includes first heating and second heating, the first heating temperature is 1100 °C, the heating time is 1 h; the second heating temperature is 1210 °C, the heating time is 1 h; the soaking temperature is 1220 °C, the heating time is 1 h, and the high-pressure water phosphorus removal water pressure is 22 Pa;

[0062] (3) controlled rolling and controlled cooling: the slab is sequentially subjected to rough rolling, medium rolling, finish rolling, water cooling and cooling bed cooling; the rough rolling opening temperature is 1080 °C, and the slab is rolled through 5 rough rolling mill groups + No. 1 flying shear + 6 medium rolling mill groups + No. 2 flying shear + 6 finish rolling mill groups + No. 3 flying shear + reducing diameter mill group;

[0063] Water tank water cooling is adopted, the inlet temperature is 1040 °C, and the outlet temperature is 920 °C; the upper cooling bed temperature is 835 °C, the lower cooling bed temperature is 530 °C, then the cooling speed is 0.2 °C / min, the end point temperature is 80 °C, and then the air cooling is carried out to room temperature, thereby obtaining the product.

[0064] The non-quenched and tempered steel prepared in the example is subjected to component detection, and the detection results in mass percentage are as follows: C is 0.27%, Si is 0.34%, Mn is 1.38%, V is 0.11%, Nb is 0.019%, N is 0.017%, S is 0.0069%, and the balance is Fe and unavoidable impurities, and P in the impurities is ≤100 ppm.

[0065] The performance detection results of the non-quenched and tempered steel of the example are shown in Table 1, and the metallographic structure is the same as that of Example 1. Example 6

[0066] An excellent low-temperature impact toughness non-quenched and tempered steel, and a preparation method thereof, the preparation method comprises obtaining a slab according to a formula amount, slab heating and controlled rolling and controlled cooling; specifically as follows:

[0067] (1) obtaining a slab according to a formula amount: converter smelting, LF refining, VD vacuum and continuous casting are carried out according to the formula amount to obtain a slab; in the converter smelting process, V element is added; in the VD vacuum process, Nb element is added;

[0068] (2) slab heating: preheating, heating and soaking of the slab; the preheating temperature is 870 °C, the preheating time is 50 min; the heating section includes first heating and second heating, the first heating temperature is 1080 °C, the heating time is 1 h; the second heating temperature is 1160 °C, the heating time is 1 h; the soaking temperature is 1190 °C, the heating time is 1 h, and the high-pressure water phosphorus removal water pressure is 22 Pa;

[0069] (3) Controlling rolling and cooling: the cast blank is sequentially subjected to rough rolling, medium rolling, finish rolling, water cooling and cooling bed cooling; the rough rolling opening temperature is 1090°C, and the cast blank is rolled through 5 rough rolling mill groups + No. 1 flying shear + 6 medium rolling mill groups + No. 2 flying shear + 6 finish rolling mill groups + No. 3 flying shear + reducing sizing mill group;

[0070] The water tank is used for water cooling, the inlet temperature is 1020°C, and the outlet temperature is 930°C; the upper cooling bed temperature is 840°C, the lower cooling bed temperature is 540°C, then the cast blank is slowly cooled by stacking and cooling, the cooling speed is 0.2°C / min, the end point temperature is 80°C, and then the cast blank is air cooled to room temperature, thereby obtaining the cast blank.

[0071] The non-quenched and tempered steel prepared in the embodiment is subjected to component detection, and the detection results in terms of mass percentage are as follows: C is 0.28%, Si is 0.37%, Mn is 1.32%, V is 0.14%, Nb is 0.022%, N is 0.021%, S is 0.0056%, and the balance is Fe and unavoidable impurities, and P in the impurities is ≤100 ppm.

[0072] The performance detection results of the non-quenched and tempered steel in the embodiment are shown in Table 1, and the metallographic structure thereof is the same as that of Example 1. Example 7

[0073] An excellent low-temperature impact toughness non-quenched and tempered steel, a preparation method thereof includes obtaining a cast blank according to a formula amount, heating the cast blank and controlling rolling and cooling; specifically as follows:

[0074] (1) Obtaining a cast blank according to a formula amount: a converter smelting, LF refining, VD vacuum and continuous casting are performed according to a formula amount to obtain a cast blank; in the converter smelting process, V element is added; in the VD vacuum process, Nb element is added;

[0075] (2) Heating the cast blank: preheating, heating and soaking are performed on the cast blank; the preheating temperature is 850°C, the preheating time is 50 min; the heating section includes first heating and second heating, the first heating section temperature is 1080°C, the heating time is 1 h; the second heating section temperature is 1160°C, the heating time is 1 h; the soaking section temperature is 1190°C, the heating time is 1 h, and the high-pressure water phosphorus removal water pressure is 22 Pa;

[0076] (3) Controlling rolling and cooling: the cast blank is sequentially subjected to rough rolling, medium rolling, finish rolling, water cooling and cooling bed cooling; the rough rolling opening temperature is 1120°C, and the cast blank is rolled through 5 rough rolling mill groups + No. 1 flying shear + 6 medium rolling mill groups + No. 2 flying shear + 6 finish rolling mill groups + No. 3 flying shear + reducing sizing mill group;

[0077] The water tank is used for water cooling, the inlet temperature is 1000°C, the outlet temperature is 925°C; the upper cooling bed temperature is 830°C, the lower cooling bed temperature is 550°C, then the stack is cooled slowly, the cooling speed is 0.2°C / min, the end temperature is 80°C, then air cooling to room temperature, and the non-quenched and tempered steel is obtained.

[0078] The non-quenched and tempered steel prepared in the example is detected for composition, and the detection results are as follows in mass percentage: C is 0.23%, Si is 0.35%, Mn is 1.44%, V is 0.12%, Nb is 0.018%, N is 0.018%, S is 0.0061%, the balance is Fe and inevitable impurities, and P in the impurities is ≤100ppm.

[0079] The performance detection results of the non-quenched and tempered steel in the example are shown in Table 1, and the metallographic structure is the same as that in Example 1. Comparative Example 1

[0080] A non-quenched and tempered steel, and a preparation method thereof, are disclosed.

[0081] (1) The cast blank is obtained according to the formula amount: the converter smelting, LF refining, VD vacuum and continuous casting are carried out according to the formula amount, and the cast blank is obtained; wherein V element is added in the converter smelting process; Nb element is added in the VD vacuum process;

[0082] (2) The cast blank is heated: the cast blank is preheated, heated and soaked; wherein the preheating temperature is 860°C, the preheating time is 50 min; the heating section includes first heating and second heating, the first heating temperature is 1080°C, the heating time is 1 h; the second heating temperature is 1180°C, the heating time is 1 h; the soaking temperature is 1200°C, the heating time is 1 h, and the high-pressure water phosphorus removal water pressure is 22 Pa;

[0083] (3) Controlled rolling and controlled cooling: the cast blank is sequentially subjected to rough rolling, medium rolling, finishing rolling and cooling bed cooling; wherein the rough rolling opening temperature is 1100°C, and the cast blank is rolled through 5 sets of rough rolling mill + No. 1 flying shear + 6 sets of medium rolling mill + No. 2 flying shear + 6 sets of finishing rolling mill + No. 3 flying shear + reducing diameter mill;

[0084] The upper cooling bed temperature is 830°C, the lower cooling bed temperature is 530°C, then the stack is cooled slowly, the cooling speed is 0.2°C / min, the end temperature is 80°C, then air cooling to room temperature, and the non-quenched and tempered steel is obtained.

[0085] The non-quenched and tempered steel prepared in the example is detected for composition, and the detection results are as follows in mass percentage: C is 0.23%, Si is 0.35%, Mn is 1.44%, V is 0.12%, Nb is 0.018%, N is 0.018%, S is 0.0061%, the balance is Fe and inevitable impurities, and P in the impurities is ≤100ppm.

[0086] The performance detection results of the non-quenched and tempered steel of the present comparative example are shown in Table 1. Comparative Example 2

[0087] A non-quenched and tempered steel, the preparation method thereof comprises obtaining a casting blank according to a formula amount, heating the casting blank, and controlling rolling and cooling; specifically as follows:

[0088] (1) obtaining a casting blank according to a formula amount: carrying out converter smelting, LF refining, VD vacuum and continuous casting according to a formula amount to obtain a casting blank; wherein V element is added in the process of converter smelting;

[0089] (2) heating the casting blank: preheating, heating and soaking the casting blank; wherein the preheating temperature is 880℃, the preheating time is 50min; the heating section comprises first heating and second heating, the first heating temperature is 1080℃, the heating time is 1h; the second heating temperature is 1170℃, the heating time is 1h; the soaking temperature is 1190℃, the heating time is 1h, and the high-pressure water phosphorus removal water pressure is 22Pa;

[0090] (3) controlling rolling and cooling: sequentially carrying out rough rolling, medium rolling, finishing rolling, water cooling and cooling bed cooling on the casting blank; wherein the rough rolling opening temperature is 1100℃, and the casting blank is rolled through 5 sets of rough rolling mill + No.1 flying shear + 6 sets of medium rolling mill + No.2 flying shear + 6 sets of finishing rolling mill + No.3 flying shear + reducing diameter mill;

[0091] water cooling is adopted, the inlet temperature is 1010℃, and the outlet temperature is 930℃; the upper cooling bed temperature is 845℃, the lower cooling bed temperature is 535℃, then the casting blank is cooled and slowly cooled by stacking, the cooling speed is 0.2℃ / min, the end point temperature is 80℃, and then the casting blank is air cooled to room temperature.

[0092] The non-quenched and tempered steel prepared in the present comparative example is detected for composition, and the detection results are as follows in mass percent: C is 0.26%, Si is 0.38%, Mn is 1.43%, V is 0.12%, N is 0.015%, S is 0.0058%, and the balance is Fe and unavoidable impurities, and P in the impurities is ≤100ppm. Comparative Example 3

[0093] A non-quenched and tempered steel, the preparation method thereof comprises obtaining a casting blank according to a formula amount, heating the casting blank, and controlling rolling and cooling; specifically as follows:

[0094] (1) obtaining a casting blank according to a formula amount: carrying out converter smelting, LF refining, VD vacuum and continuous casting according to a formula amount to obtain a casting blank; wherein V element is added in the process of converter smelting;

[0095] (2) slab heating: preheating, heating and soaking of the slab; wherein the preheating temperature is 860℃, the preheating time is 50min; the heating section includes first heating and second heating, the first heating temperature is 1100℃, the heating time is 1h; the second heating temperature is 1190℃, the heating time is 1h; the soaking temperature is 1190℃, the heating time is 1h, the high pressure water phosphorus removal water pressure is 22Pa;

[0096] (3) controlled rolling and controlled cooling: the slab is sequentially subjected to rough rolling, medium rolling, finishing rolling and cooling bed cooling; wherein the rough rolling opening temperature is 1090℃, and the slab is rolled through 5 rough rolling mill groups + No. 1 flying shear + 6 medium rolling mill groups + No. 2 flying shear + 6 finishing rolling mill groups + No. 3 flying shear + reducing diameter mill group;

[0097] the upper cooling bed temperature is 830℃, the lower cooling bed temperature is 550℃, then the stack cooling is slow cooled, the cooling speed is 0.2℃ / min, the end point temperature is 80℃, and then air cooling to room temperature, thereby obtaining the product.

[0098] The non-quenched and tempered steel prepared in the present comparative example is subjected to component detection, and the detection results in mass percentage are as follows: C is 0.26%, Si is 0.36%, Mn is 1.40%, V is 0.13%, N is 0.016%, S is 0.0059%, and the balance is Fe and inevitable impurities, and P in the impurities is ≤100ppm.

[0099] Table 1: performance detection results of non-quenched and tempered steels of various examples and comparative examples

[0100]

[0101] In summary, the steel components are designed in the present application, the V, Nb and N micro-alloying elements are added in the smelting process to form (V, Nb)(C, N) carbonitride, the grain refinement and solid solution strengthening effects are formed; and then the controlled rolling and controlled cooling process in the rolling process is combined to control the cooling mode and cooling speed of the micro-alloyed non-quenched and tempered steel after rolling, the purpose of controlling the microstructure of the non-quenched and tempered steel is effectively achieved, the micro-alloyed ferrite-pearlite type non-quenched and tempered steel obtained finally has an impact energy of more than 27J at-28℃, and the cost is effectively controlled.

[0102] The above examples and comparative examples are only used for illustration but not for limiting the technical solutions of the present application, although the present application is described in detail with reference to the above examples, the skilled in the art can clearly understand that any modification or partial replacement can be made to the present application without departing from the true spirit and scope of the present application defined in the claims, and all of them should be covered in the claim scope of the present application.

Claims

1. A non-quenched and tempered steel with excellent low-temperature impact toughness, characterized in that, The chemical composition and mass percentage of the non-quenched and tempered steel are as follows: C: 0.23-0.28%, Si: 0.25-0.50%, Mn: 1.20-1.60%, V: 0.08-0.20%, Nb: 0.016-0.020%, N: 0.01-0.02%, P≤0.025%, S≤0.015%, with the balance being Fe and unavoidable impurities; The non-quenched and tempered steel has a tensile strength higher than 700 MPa, a yield strength higher than 500 MPa, an elongation higher than 15%, a reduction of area higher than 35%, and a V-shaped impact energy of more than 27 J at -28℃. The preparation method of the non-quenched and tempered steel includes obtaining a billet according to the formula, heating the billet, and controlled rolling and cooling. The controlled rolling and controlled cooling process specifically involves sequentially performing rough rolling, intermediate rolling, finish rolling, water cooling, and cooling bed cooling on the billet; the initial rolling temperature for rough rolling is 1080–1120℃. The inlet temperature of the water cooling system is 1000–1040℃, and the outlet temperature is 900–940℃. The upper cooling bed temperature is 820–850°C, and the lower cooling bed temperature is 530–560°C.

2. A method for preparing the non-quenched and tempered steel with excellent low-temperature impact toughness as described in claim 1, characterized in that, This includes obtaining the billet according to the formula, billet heating, and controlled rolling and cooling; The controlled rolling and controlled cooling process specifically involves sequentially performing rough rolling, intermediate rolling, finish rolling, water cooling, and cooling bed cooling on the billet; the initial rolling temperature for rough rolling is 1080–1120℃. The inlet temperature of the water cooling system is 1000–1040℃, and the outlet temperature is 900–940℃. The upper cooling bed temperature is 820–850°C, and the lower cooling bed temperature is 530–560°C.

3. The method for preparing a non-quenched and tempered steel with excellent low-temperature impact toughness as described in claim 2, characterized in that, The process of obtaining the billet according to the formula involves: smelting in a converter, refining in an LF ladle, refining in a VD vacuum, and continuous casting according to the formula; wherein V and Nb elements are added during the converter smelting process; and Nb element is added during the VD vacuum refining process.

4. The method for preparing a non-quenched and tempered steel with excellent low-temperature impact toughness as described in claim 2, characterized in that, The billet heating includes preheating, heating and homogenizing the billet in sequence; wherein the temperature of the preheating section of the billet does not exceed 900℃ and the time is 40 to 50 minutes.

5. The method for preparing a non-quenched and tempered steel with excellent low-temperature impact toughness as described in claim 4, characterized in that, The billet heating section includes a first heating and a second heating; the first heating temperature is 1050-1120℃; the second heating temperature is 1150-1220℃.

6. The method for preparing a non-quenched and tempered steel with excellent low-temperature impact toughness as described in claim 4, characterized in that, The temperature of the soaking zone for the cast billet is 1180–1250℃.

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