A high-strength and easy-to-weld bulb flat steel and its preparation method

Abstract

The invention discloses a high-strength and tough, easy-to-weld bulb flat steel and a preparation method thereof, which belongs to the technical field of steel materials and solves the problem that the strength and toughness matching and weldability of the bulb flat steel in the prior art cannot be effectively improved at the same time. The components of the high-strength and tough, easy-to-weld bulb flat steel include, by mass percentage: C: 0.03% to 0.07%, Si: 0.20% to 0.60%, Mn: 0.40% to 0.80%, Cu: 1.00% to 3.00%, S≤0.002%, P≤0.010%, Ni: 1.00% to 3.50%, Cr: 0.60% to 1.00%, Mo: 0.20% to 0.80%, Ti: 0.007% to 0.016%, Al: 0.02% to 0.09%, N: ≤0.013%, and the balance is Fe and other inevitable impurities. The high-strength and tough, easy-to-weld bulb flat steel of the invention has excellent strength and toughness and excellent welding performance.

Description

Technical Field

[0001] The invention relates to the technical field of steel materials, and in particular to a high-strength and easy-to-weld bulb flat steel and a preparation method thereof. Background Art

[0002] Bulb flat steel is a key material for building various hull structures and an indispensable special profile for my country's shipbuilding. At present, my country's shipbuilding mainly uses 355-440MPa bulb flat steel as a structural profile. With the requirements of large-scale ships, efficient construction, and resistance to low temperature environments, the use of bulb flat steel supporting hull structures has put forward the use requirements of high strength, toughness, and easy welding.

[0003] Bulb flat steel is a special-shaped steel with extremely asymmetric cross-section. It is usually rolled by pass-type rolling. Unlike steel plate rolling, steel rolling is often limited by equipment conditions and cannot achieve better strength and toughness matching through controlled rolling and controlled cooling like plates. Microalloying technology is an important means for bulb flat steel to obtain high strength. CN101229565A introduces a high-strength bulb flat steel and its production process. The strength of the steel is improved by adding 0.01-0.025% Nb to refine the final structure, but the maximum strength level of the produced bulb flat steel is A / D36. CN102400045A introduces the use of VN microalloying technology to achieve a longitudinal yield strength of not less than 390MPa in the web of large-size D40 bulb flat steel. The longitudinal impact energy of the web at -40℃ is greater than 41J.

[0004] Some of the current bulb flat steels have problems such as poor low-temperature toughness or the need for preheating before welding. The alloy composition design and manufacturing technology cannot meet the comprehensive control requirements of bulb flat steels with good strength-toughness matching and easy weldability. Summary of the invention

[0005] In view of the above situation, the present invention aims to provide a high-strength and toughness, easy-to-weld bulb flat steel and a preparation method thereof, so as to solve the problem that the strength and toughness matching and weldability of the existing bulb flat steel cannot be effectively improved at the same time.

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

[0007] The invention provides a high-strength and easy-to-weld bulb flat steel. The components of the high-strength and easy-to-weld bulb flat steel include, by mass percentage, C: 0.03%-0.07%, Si: 0.20%-0.60%, Mn: 0.40%-0.80%, Cu: 1.00%-3.00%, S≤0.002%, P≤0.010%, Ni: 1.00%-3.50%, Cr: 0.60%-1.00%, Mo: 0.20%-0.80%, Ti: 0.007%-0.016%, Al: 0.02%-0.09%, N: ≤0.013%, and the balance is Fe and other inevitable impurities.

[0008] Furthermore, in the composition of the high-strength, tough and easily weldable bulb flat steel, Ni / Cu>1.

[0009] Furthermore, in the composition of the high-strength and easy-to-weld bulb flat steel, Mn / Si is 1.5 to 3.0.

[0010] Furthermore, in the composition of high-strength and easy-to-weld bulb flat steel, when the N content is ≥ 0.0040%, it satisfies: 100Ti(7 / 24) / a 1 +100Al (14 / 27) / a 2 ≥100N-0.0006, where a 1 =1,a 2 =3, Ti, Al, N represent the mass percentage of Ti, Al, N elements.

[0011] Furthermore, the components of the high-strength, tough and easy-to-weld bulb flat steel include, by mass percentage: C: 0.03%~0.06%, Si: 0.25%~0.50%, Mn: 0.45%~0.75%, Cu: 1.10%~2.50%, S≤0.002%, P≤0.010%, Ni: 1.10%~2.50%, Cr: 0.60%~0.90%, Mo: 0.20%~0.70%, Ti: 0.007%~0.015%, Al: 0.02%~0.08%, N: 0.004%~0.010%, and the balance is Fe and other inevitable impurities.

[0012] The present invention also provides a method for preparing the above-mentioned high-strength and easy-to-weld bulb flat steel, comprising the following steps:

[0013] Step 1, using a secondary induction coil to perform secondary heating on the hot-rolled bulb flat steel;

[0014] Step 2, the bulb flat steel is quenched after secondary heating;

[0015] Step 3: The quenched bulb flat steel is subjected to high temperature tempering heat treatment and air-cooled to room temperature after tempering.

[0016] Furthermore, in step 1, the final surface heating temperature of the ball head portion is controlled to be higher than the surface heating temperature of the web portion.

[0017] Furthermore, in step 1, during the first-level heating, the surface heating temperature of the ball head part is 750-820°C, and the surface heating temperature of the web part is 780-860°C.

[0018] Furthermore, in step 1, during the secondary heating, the surface heating temperature of the ball head part is 980-1000°C, and the surface heating temperature of the web part is 960-990°C.

[0019] Furthermore, in step 3, the tempering temperature is 630-680°C.

[0020] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0021] The high-strength and easy-to-weld bulb flat steel of the present invention can achieve good matching of mechanical properties and welding properties by accurately controlling the combination of alloy elements such as Si, Mn, Cu, Cr, Mo, Ni and the like and matching the contents of different elements.

[0022] In the preparation method of the high-strength and easy-to-weld bulb flat steel of the present invention, by adopting the induction quenching + high-temperature tempering process, the medium-frequency induction heating quenching can save energy and improve the surface quality of the bulb flat steel; in addition, in view of the special-shaped structural characteristics of the bulb flat steel, the differentiated induction heating process control is adopted to enable the bulb flat steel to obtain good cross-sectional properties, and the high-temperature tempering ensures that the bulb flat steel has good strength and toughness matching. The microstructure of the high-strength and easy-to-weld bulb flat steel of the present invention is a tempered bainite structure. The structure is fine and uniform, and the organizational difference between the ball head core and the web is small.

[0023] The high-strength and easy-to-weld flat bulb steel of the present invention has excellent toughness and excellent welding performance, for example, the yield strength reaches 600MPa or more, for example, 650-700MPa, the tensile strength is ≥660MPa, for example, 700-730MPa, the elongation is ≥20%, for example, 20%-25%, and the ball head core is -84℃ KV 2 ≥200J, for example, ball head center -84℃KV 2 280~350J, belly plate -84℃KV 2 ≥200J, for example, abdominal plate -84℃KV 2 The welding temperature is 210~330J, and the bulb flat steel can be welded at 0℃ without preheating.

[0024] Other features and advantages of the present invention will be described in the following description, and part of them will become obvious from the description, or will be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the contents particularly pointed out in the written description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The accompanying drawings are only used for the purpose of illustrating specific embodiments and are not to be considered as limiting the present invention. In the entire drawings, the same reference symbols represent the same components;

[0026] Figure 1 It is a schematic cross-sectional view of a bulb flat steel according to the present invention;

[0027] Figure 2 The microstructure diagram of the ball head center of the flat bulb steel after tempering in Example 1;

[0028] Figure 3 The microstructure diagram of the web of the bulb flat steel after tempering in Example 1;

[0029] Figure 4 This is the microstructure diagram of the ball head center of the flat bulb steel after tempering in Example 2;

[0030] Figure 5 The microstructure diagram of the web of the bulb flat steel after tempering in Example 2;

[0031] Figure 6 The microstructure diagram of the ball head center of the flat bulb steel after tempering in Example 3;

[0032] Figure 7 The microstructure diagram of the web of the bulb flat steel after tempering in Example 3;

[0033] Figure 8 The microstructure diagram of the ball head center of the flat bulb steel after tempering in Example 4;

[0034] Fig. 9 The microstructure diagram of the web of the bulb flat steel after tempering in Example 4;

[0035] Fig.10 The microstructure diagram of the ball head center of the quenched flat bulb steel in Example 3;

[0036] Fig.11 The microstructure diagram of the web of the bulb flat steel after quenching in Example 3;

[0037] Fig.12 The original austenite grain morphology of the ball head center of the bulb flat steel in Example 3;

[0038] Fig.13 The original austenite grain morphology of the web of the bulb flat steel of Example 3;

[0039] Fig.14 The microstructure diagram of the ball head center of the quenched flat bulb steel of Comparative Example 3;

[0040] Fig.15 The microstructure diagram of the web of the bulb flat steel after quenching in Comparative Example 3;

[0041] Fig.16 The original austenite grain morphology of the ball head center of the bulb flat steel of comparative example 3;

[0042] Fig.17 This is the original austenite grain morphology of the web of the bulb flat steel of Comparative Example 3.

[0043] Reference numerals:

[0044] h—total height; b—ball head width; t—web thickness; r 1 —ball end fillet radius; r—web end fillet radius. DETAILED DESCRIPTION

[0045] 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 to explain the principles of the present invention together with the embodiments of the present invention.

[0046] The invention provides a high-strength and tough, easy-to-weld bulb flat steel. The components of the high-strength and tough, easy-to-weld bulb flat steel include, by mass percentage: C: 0.03%-0.07%, Si: 0.20%-0.60%, Mn: 0.40%-0.80%, Cu: 1.00%-3.00%, S≤0.002%, P≤0.010%, Ni: 1.00%-3.50%, Cr: 0.60%-1.00%, Mo: 0.20%-0.80%, Ti: 0.007%-0.016%, Al: 0.02%-0.09%, N: ≤0.013%, and the balance is Fe and other inevitable impurities.

[0047] Specifically, the composition of the high-strength and easy-to-weld bulb flat steel also satisfies the following conditions: Ni / Cu>1, and Mn / Si is 1.5-3.0.

[0048] Specifically, in the composition of the above-mentioned high-strength and easy-to-weld bulb flat steel, when the N content is ≥ 0.0040%, it satisfies: 100Ti(7 / 24) / a 1 +100Al (14 / 27) / a 2 ≥100N-0.0006, where a 1 =1,a 2 =3, Ti, Al, N represent the mass percentage of Ti, Al, N elements.

[0049] The functions and dosages of the components in the present invention are described in detail below, and only % is used to represent the mass percentage of the components.

[0050] C: has a significant solid solution strengthening effect and can improve the hardenability of steel, but the increase in C content in steel is very unfavorable to the low temperature toughness and weldability of steel. Therefore, the present invention adopts an ultra-low carbon design, and the C content is controlled at 0.03% to 0.07%.

[0051] Si: One of the deoxidizing elements in steel, it has a certain solid solution strengthening effect, but excessive silicon is not good for the low temperature toughness of steel, and the sensitivity of steel to welding cold cracks and hot cracks will increase. Taking comprehensive consideration, the Si content of the present invention is controlled to be 0.20% to 0.60%.

[0052] Mn: Manganese is an essential element to ensure the strength and toughness of steel. It can significantly improve the hardenability of steel and has a certain solid solution strengthening effect. However, when the Mn content is too high, it will cause center segregation, reduce toughness and weldability, and the Mn content should be controlled at 0.40% to 0.80%. In addition, the Mn and Si contents need to satisfy Mn / Si=1.5 to 3.0, which is an important element to be controlled in the present invention. This is mainly based on the inventor's in-depth research that when the Si content is low, the weldability is better but the uniformity effect during induction heating is poor. When the Si content is appropriately increased, the Mn content is kept within the lower limit of the present invention, which can effectively improve the continuous casting segregation and significantly improve the uniformity of the composition and organization.

[0053] Cu: The key precipitation strengthening element of the steel of the present invention. Cu can precipitate in the steel in the form of nanoclusters during the tempering process without relying on C and N elements, thereby producing a significant precipitation strengthening effect, which can make up for the strength loss caused by reducing C. At the same time, Cu can also improve the seawater corrosion resistance of the steel. When the Cu content is too little, the effect is not obvious; when the content is too much, it is easy to cause copper segregation, which deteriorates the processability and weldability. In the present invention, the Cu content is controlled at 1.0% to 3.0%.

[0054] Ni: Ni and Cu form a composite precipitation phase during the aging process. Ni can promote the precipitation kinetics of Cu and inhibit the coarsening of the Cu-rich phase during the aging process, thereby enhancing the precipitation strengthening effect of Cu. Ni can also improve the hardenability and low-temperature toughness of steel, and at the same time has a certain solid solution strengthening effect, and can also prevent the hot brittle effect of Cu. In order to improve the surface hot crack tendency of bulb flat steel, uniform cross-sectional properties, and high strength and toughness matching, the Ni content should be controlled at 1.0-3.50%, and the Ni and Cu contents should satisfy Ni / Cu>1.

[0055] Cr: Solid solution Cr can significantly improve the hardenability of steel (it has the effect of reducing A3 temperature and improving the stability of supercooled austenite). In the present invention, the Cr content is controlled in a relatively high content range of 0.60% to 1.0%, and the hardenability of the steel is significantly improved, which is conducive to obtaining a full-section hardened structure of the bulb flat steel.

[0056] Mo: Solid solution Mo can significantly improve the hardenability of steel and also has a certain solid solution strengthening effect. At the same time, Mo can improve tempering stability and significantly reduce tempering brittleness. In the present invention, the Mo content is controlled at 0.20% to 0.80%.

[0057] Ti: The present invention adds a trace amount of Ti element, which is combined with N element. On the one hand, it fixes the free N in the steel, reduces the aging sensitivity of the steel, and improves the toughness level of the steel; on the other hand, the formed TiN plays a role in improving weldability; the Ti content is controlled in the range of 0.007% to 0.016%.

[0058] Al: Adding Al element can promote the precipitation of AlN second phase, cooperate with TiN, and refine the austenite grains in the coarse grain area. The Al content is controlled in the range of 0.02%~0.09%.

[0059] N: A certain amount of N can form TiN with Ti, improve the toughness of the base material and the heat affected zone of welding, and form carbonitrides with Ti, Al, etc. to improve the strength. However, too high a N content will affect the quality of the ingot and the low-temperature toughness of the material. Therefore, the N content is controlled at ≤0.0130%; when the N content in the steel is ≥0.0040%, the content of Ti, Al, and N elements should meet 100Ti (7 / 24) / a. 1 +100Al(14 / 27) / a 2 ≥100N-0.0006, where a 1 =1,a 2 =3, Ti, Al, N represent the mass percentage of Ti, Al, N elements to optimize the organization and grain size of the coarse grained zone affected by welding heat.

[0060] P: Phosphorus is an impurity element in steel, which will damage the toughness of the base metal and the heat-affected zone of welding. Therefore, the P content is controlled below 0.010%.

[0061] S: Sulfur is an impurity element in steel, which will form sulfide inclusions and become a crack source. Therefore, the S content is controlled below 0.002%.

[0062] Specifically, in order to further improve the comprehensive performance of the above-mentioned high-strength, toughness and easy-to-weld bulb flat steel, the components of the above-mentioned high-strength, toughness and easy-to-weld bulb flat steel include, by mass percentage: C: 0.03%~0.065%, Si: 0.2~0.50%, Mn: 0.45%~0.75%, Cu: 1.10%~2.50%, S≤0.002%, P≤0.010%, Ni: 1.10%~2.50%, Cr: 0.60%~0.90%, Mo: 0.20%~0.70%, Ti: 0.007%~0.013%, Al: 0.03%~0.08%, N: 0.004%~0.010%, and the balance is Fe and other inevitable impurities.

[0063] Specifically, the composition of the high-strength and easy-to-weld bulb flat steel also satisfies the following requirements: Ni / Cu is 1-2, and Mn / Si is 1.5-2.7.

[0064] Specifically, the specifications of the high-strength and easy-to-weld bulb flat steel can be P8~P28 (see national standard GB / T9945). For example, the ball head width b of the high-strength and easy-to-weld bulb flat steel is 19~53mm, the web thickness t is 7~13mm, and the total height h of the bulb flat steel is 80~280mm.

[0065] Specifically, the microstructure of the high-toughness and easy-to-weld flat bulb steel is a tempered bainite structure. The structure is fine and uniform, and the difference between the structure of the ball head center and the web is small. For example, the microstructure of the high-toughness and easy-to-weld flat bulb steel has a grain size of 8.5-14 μm in the ball head center, a grain size of 9-14 μm in the web, and a grain size difference of less than 1 μm between the ball head center and the web.

[0066] Specifically, the yield strength of the high-strength and easy-to-weld bulb flat steel is 600-780MPa, the tensile strength is ≥660MPa, the elongation is ≥16%, and the ball head core is -84℃ KV 2 ≥200J. Bulb flat steel meets the requirements of 0℃ welding without preheating.

[0067] On the other hand, the present invention also provides a method for preparing the above-mentioned high-strength and easy-to-weld bulb flat steel, comprising the following steps:

[0068] Step 1, using a secondary induction coil to perform secondary heating on the hot-rolled bulb flat steel;

[0069] Step 2, the bulb flat steel is heated by a secondary induction coil and then quenched;

[0070] Step 3: The quenched bulb flat steel is subjected to high temperature tempering heat treatment, and then air-cooled to room temperature after tempering.

[0071] Specifically, in the above step 1, considering that the ball head of the flat bulb steel is thicker, in order to obtain an ideal hardened structure and good cross-sectional uniformity at the center of the flat bulb steel ball head, the final surface heating temperature of the ball head (i.e., the surface heating temperature after secondary heating) is controlled to be higher than the surface heating temperature of the web.

[0072] Specifically, in the above step 1, in order to ensure that the temperature of the cross section of the bulb flat steel is evenly controlled during the rapid heating process, a two-stage induction coil is used to perform two-stage heating on the hot-rolled bulb flat steel.

[0073] Specifically, in the above step 1, considering that the distance between the primary and secondary induction coils is too large, resulting in a faster temperature drop, and the distance is too small to affect the sufficient heat conduction of the cross-sectional temperature, thereby affecting the final quenching temperature control, therefore, the distance between the primary and secondary induction coils is set to 100-220 mm.

[0074] Specifically, in the above step 1, it is considered that if the heating power of the primary induction coil is too large, the heating will be fast and the hardening layer will be shallow; if the heating power is too small, the temperature will rise slowly. Therefore, the heating power of the primary induction coil is controlled to be 340-445KW, such as 350KW, 360KW, 370KW, 380KW, 390KW, 400KW, 410KW, 420KW, 430KW, 440KW; the surface heating temperature of the ball head is controlled to be 750-820℃, such as 760℃, 770℃, 780℃, 790℃, 800℃, 810℃; the surface heating temperature of the web is 780-860℃, such as 790℃, 800℃, 810℃, 820℃, 830℃, 840℃, 850℃.

[0075] Specifically, in the above step 1, if the heating power of the secondary induction coil is too large, the structure will be further coarsened and the mechanical properties will be further reduced; if it is too small, the final quenching temperature control requirements will not be met. Therefore, the heating power of the secondary induction coil is 220-380KW, such as 230KW, 260KW, 270KW, 280KW, 300KW, 320KW, 340KW, 370KW; the surface heating temperature of the ball head is controlled to be 980-1000℃, such as 985℃, 990℃, 995℃; the surface heating temperature of the web is 960-990℃, such as 965℃, 970℃, 975℃, 980℃, 985℃.

[0076] Specifically, in the above step 2, the bulb flat steel is heated by the secondary induction coil and then waits for temperature. A reasonable waiting distance can ensure uniform austenitization of the bulb flat steel cross section and obtain an ideal hardened structure. Therefore, the waiting distance can be controlled to be 80 to 250 mm, and water spray cooling is performed after waiting for temperature. After cooling, the temperature of the bulb flat steel is lower than 25°C.

[0077] Specifically, in the above step 3, considering that the tempering temperature is too high or the holding time is too long, the strength of the bulb flat steel will be lower than the lower limit assessment requirement of the index, and the tempering temperature is too low or the holding time is too short, which will cause the strength of the bulb flat steel to be higher than the upper limit assessment requirement of the index, and the toughness margin is small. Therefore, the tempering temperature is controlled to be 630-680℃ (for example, 640℃, 650℃, 660℃, 670℃), and the furnace is heated to a temperature, and the temperature is kept for 120-240min (for example, 130min, 140min, 150min, 160min, 170min, 180min, 190min, 200min, 210min, 220min, 230min).

[0078] The high-strength and easy-to-weld bulb flat steel of the present invention can achieve good matching of mechanical properties and welding properties by accurately controlling the combination of alloy elements such as Si, Mn, Cu, Cr, Mo, Ni and the like and matching the contents of different elements.

[0079] In the preparation method of the high-strength and easy-to-weld bulb flat steel of the present invention, by adopting the induction quenching + high-temperature tempering process, the medium-frequency induction heating quenching can save energy and improve the surface quality of the bulb flat steel; in addition, in view of the special-shaped structural characteristics of the bulb flat steel, the differentiated induction heating process control is adopted to enable the bulb flat steel to obtain good cross-sectional properties, and the high-temperature tempering ensures that the bulb flat steel has good strength and toughness matching. The microstructure of the high-strength and easy-to-weld bulb flat steel of the present invention is a tempered bainite structure. The structure is fine and uniform, and the organizational difference between the ball head core and the web is small.

[0080] The high-strength and easy-to-weld flat bulb steel of the present invention has excellent toughness and excellent welding performance, for example, the yield strength reaches 600MPa or more, for example, 650-700MPa, the tensile strength is ≥660MPa, for example, 700-730MPa, the elongation is ≥20%, for example, 20%-25%, and the ball head core is -84℃ KV 2 ≥200J, for example, ball head center -84℃KV 2 280~350J, belly plate -84℃KV 2 ≥200J, for example, abdominal plate -84℃KV 2 The welding temperature is 210~330J, and the bulb flat steel can be welded at 0℃ without preheating.

[0081] The advantages of precise control of the composition and process parameters of the steel of the present invention are demonstrated below with specific examples and comparative examples.

[0082] Embodiments 1-4 of the present invention provide a high-toughness, easy-to-weld bulb flat steel and a preparation method thereof. The embodiments and comparative examples respectively use the chemical composition of Table 1 to prepare bulb flat steels of different specifications and sizes. The outer dimensions of the bulb flat steels of the embodiments and comparative examples are as follows: Figure 1, as shown in Table 2. The outer dimensions of the bulb flat steel of Example 1 are the same as those of Comparative Example 1, and the outer dimensions of the bulb flat steel of Example 3 are the same as those of Comparative Examples 2 and 3. Comparative Example 1 is a conventional carbon equivalent, low Ni / Cu and Mn / Si composition design test steel, and Comparative Examples 2 and 3 use the same specifications, dimensions and chemical compositions as Example 3, but use different quenching processes.

[0083] Table 1 Chemical composition, wt%

[0084]

[0085] Table 2 Cross-sectional dimensions of the bulb flat steel of the embodiment and comparative example

[0086]

[0087] The preparation method of the embodiment and comparative example steel comprises:

[0088] Step 1, using a secondary induction coil to perform secondary heating on the hot-rolled bulb flat steel, during which the surface heating temperature of the bulb head is higher than the surface heating temperature of the web;

[0089] Step 2, the bulb flat steel is heated by a secondary induction coil and then quenched;

[0090] Step 3: The quenched bulb flat steel is subjected to high temperature tempering heat treatment, and then air-cooled to room temperature after tempering.

[0091] The specific process parameters of the embodiment and the comparative example are shown in Table 3; the microstructure of the bulb flat steel produced in the embodiment is tempered bainite structure, and the grain size is shown in Table 4; it can be seen that the structure of the bulb flat steel in the embodiment of the present invention is fine and uniform. The microstructure difference between the ball head core and the web is small.

[0092] Figure 2 This is the microstructure diagram of the ball head center of the flat bulb steel after tempering in Example 1. Figure 3 The microstructure diagram of the web of the bulb flat steel after tempering in Example 1 is shown in FIG. Figure 4 This is the microstructure diagram of the ball head center of the flat bulb steel after tempering in Example 2. Figure 5 This is the microstructure diagram of the web of the bulb flat steel after tempering in Example 2. Figure 6 This is the microstructure diagram of the ball head center of the flat bulb steel after tempering in Example 3. Figure 7 This is the microstructure diagram of the web of the bulb flat steel after tempering in Example 3. Figure 8 This is the microstructure diagram of the ball head center of the flat bulb steel after tempering in Example 4. Fig. 9 This is a microstructure diagram of the web of the tempered flat bulb steel of Example 4; it can be seen that the flat bulb steel of the embodiment of the present invention has a fine and uniform structure. The difference in structure between the ball head core and the web is small.

[0093] According to GB / T 228.1 standard, the room temperature tensile property test of the bulb flat steel was carried out, and according to GB / T 229 standard, the -84℃ Charpy impact energy of the ball head and the web was tested. The sampling locations for the tensile and impact tests were both at the center of the ball head and the longitudinal 1 / 3 position of the web. Two parallel specimens were taken for the tensile test, and three parallel specimens were taken for the impact test. The mechanical property test results of the bulb flat steel of the embodiment and the comparative example are shown in Table 5. The crack resistance test of the bulb flat steel was carried out, and the welding was carried out at 0℃ without preheating, and the relative humidity of the test environment was set to 80%. The test results are shown in Table 6.

[0094] From the results, it can be seen that the strength and toughness of the bulb flat steel of the embodiment of the present invention are well matched, with high strength and low temperature toughness, and the surface quality and bending performance of the bulb flat steel are good. The bulb flat steel of each embodiment meets the requirement of 0℃ welding without preheating.

[0095] Table 3 Specific process parameters

[0096]

[0097] Table 4 Microstructure of steel

[0098]

[0099] Table 5 General mechanical properties of bulb flat steel

[0100]

[0101] Table 6 T-type crack resistance test

[0102]

[0103] The inventor has conducted a large number of experimental studies during the research process, and now uses some solutions with poor performance as comparative examples.

[0104] Comparative Example 1

[0105] This comparative example provides a bulb flat steel, the components of which are shown in Table 1. The composition design adopts the traditional carbon equivalent design, and the Ni / Cu and Mn / Si ratios are not within the scope of the present invention. The preparation method is the same as that of Example 1, which will not be repeated here. From the results, it can be seen that due to the influence of the alloy composition, the mechanical properties of the test steel of Comparative Example 1 are lower than the index requirements, and welding cold cracks occur when welding at 0°C.

[0106] Comparative Example 2

[0107] This comparative example provides a bulb flat steel, the components of which are shown in Table 1. The components are the same as those in Example 3 and are not described in detail here. Compared with Example 3, the induction heating quenching temperature of Comparative Example 2 is lower, the induction heating is uneven, and the bulb flat steel is not fully quenched, resulting in the strength and impact performance of the ball head and web being lower than the index requirements.

[0108] Comparative Example 3

[0109] This comparative example provides a bulb flat steel, the components of which are shown in Table 1. The components are the same as those in Example 3 and will not be repeated here.

[0110] Fig.10 This is the microstructure diagram of the ball head center after quenching in Example 3; Fig.11 This is the microstructure diagram of the web after quenching in Example 3; Fig.12 This is the original austenite grain morphology of the ball head center after quenching in Example 3, and the grain size is 12.07 μm; Fig.13 This is the original austenite grain morphology of the web after quenching in Example 3, and the grain size is 12.98 μm; Fig.14 This is the microstructure diagram of the ball head center after quenching in comparative example 3; Fig.15 This is the microstructure diagram of the web after quenching in Comparative Example 3; Fig.16 This is the original austenite grain morphology of the ball head center after quenching in comparative example 3, and the grain size is 15.71 μm; Fig.17 This is the original austenite grain morphology of the web after quenching in Example 3, and the grain size is 23.72μm; it can be seen that compared with Example 3, the induction heating quenching temperature of the web of Example 3 is higher, the web structure of the comparison example is further coarsened, and the difference between the ball head and the web structure is further increased, resulting in a significant decrease in the mechanical properties of the web part and a further increase in the difference in the structure and performance of the ball head and the web section.

[0111] It should be noted that since the microstructure is observed using microscopes of different types, the fonts of the scales in the microstructure diagrams in the attached drawings are not exactly the same. There are two types, um and μm. Um and μm have the same meaning, both representing micrometers.

[0112] 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 high-strength and easy-to-weld bulb flat steel, characterized in that: The components of the high-strength and easy-to-weld bulb flat steel include, by mass percentage: C: 0.03% to 0.07%, Si: 0.20% to 0.40%, Mn: 0.45% to 0.80%, Cu: 1.00% to 3.00%, S≤0.002%, P≤0.010%, Ni: 1.00% to 3.50%, Cr: 0.60% to 1.00%, Mo: 0.20% to 0.80%, Ti: 0.007% to 0.016%, Al: 0.02% to 0.09%, N: ≤0.013%, and the balance is Fe and other inevitable impurities; The composition of the high-strength and easy-to-weld bulb flat steel is Ni / Cu>1; Mn / Si is 1.5-3.0; When the N content is ≥0.0040%, it satisfies: 100Ti (7 / 24) / a1+100Al (14 / 27) / a2≥100N-0.0006, where a1=1, a2=3, Ti, Al, N represent the mass percentage of Ti, Al, N elements; The ball head width b of the high-strength and easy-to-weld flat bulb steel is 19 to 53 mm, the web thickness t is 7 to 13 mm, and the total height h of the flat bulb steel is 80 to 280 mm; The microstructure of the high-strength and easy-to-weld flat bulb steel is a tempered bainite structure, which is fine and uniform, and the grain size difference between the ball head core and the web is less than 1 μm; The ball head center of the high-strength and easy-to-weld bulb flat steel is -84℃KV2≥200J, and the web is -84℃KV2≥200J.

2. The high-strength and easy-to-weld bulb flat steel according to claim 1 is characterized in that: In the components of the high-strength and easy-to-weld bulb flat steel, 2≥Ni / Cu>1.

3. The high-strength and easy-to-weld bulb flat steel according to claim 1 is characterized in that: In the composition of the high-strength and easy-to-weld bulb flat steel, Mn / Si is 1.5 to 2.

7.

4. The high-strength and easy-to-weld bulb flat steel according to any one of claims 1 to 3, characterized in that: The components of the high-strength and easy-to-weld bulb flat steel include, by mass percentage, C: 0.03% to 0.06%, Si: 0.25% to 0.40%, Mn: 0.45% to 0.75%, Cu: 1.10% to 2.50%, S≤0.002%, P≤0.010%, Ni: 1.10% to 2.50%, Cr: 0.60% to 0.90%, Mo: 0.20% to 0.70%, Ti: 0.007% to 0.015%, Al: 0.02% to 0.08%, N: 0.004% to 0.010%, and the balance is Fe and other inevitable impurities.

5. A method for preparing a high-strength and easy-to-weld bulb flat steel, characterized in that: The preparation method is used to prepare the high-strength and easy-to-weld bulb flat steel according to any one of claims 1 to 4, comprising the following steps: Step 1, using a secondary induction coil to perform secondary heating on the hot-rolled bulb flat steel; Step 2, the bulb flat steel is quenched after secondary heating; Step 3, the quenched bulb flat steel is subjected to high temperature tempering heat treatment, and air-cooled to room temperature after tempering; In the step 1, the final surface heating temperature of the ball head is controlled to be higher than the surface heating temperature of the web; In the step 1, during the first stage heating, the surface heating temperature of the ball head part is 750-820°C, and the surface heating temperature of the web part is 780-860°C; during the second stage heating, the surface heating temperature of the ball head part is 980-1000°C, and the surface heating temperature of the web part is 960-990°C.

6. The preparation method according to claim 5, characterized in that: In the step 1, during the first stage heating, the surface heating temperature of the ball head part is 760-820°C, and the surface heating temperature of the web part is 780-850°C.

7. The preparation method according to claim 5, characterized in that: In the step 1, during the secondary heating, the surface heating temperature of the ball head part is 980-995° C., and the surface heating temperature of the web part is 965-990° C.

8. The preparation method according to any one of claims 5 to 7, characterized in that: In the step 3, the tempering temperature is 630-680°C.

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