High-Strenth Low-Carbon Bainitic Fire-Resistant Steel And Preparation Method Thereof

Abstract

The present disclosure relates to a high-strength low-carbon bainitic fire-resistant steel and a preparation method thereof, and belongs to the technical field of low-carbon air-cooled bainitic fire-resistant steels. The present disclosure solves the problems of low yield strength, complicated production process and poor high-temperature mechanical properties of the fire-resistant steel in the prior art. The high-strength low-carbon bainitic fire-resistant steel, whose chemical components by mass percent are as follows: 0.07%-0.1% of C, 0.7%-0.9% of Si, 1.0%-1.5% of Mn, 0.7%-0.8% of Cr, 1.0%-1.3% of Ni, 0.3%-0.35% of Cu, 0.6%-0.8% of Mo, 0.025%-0.035% of Nb, 0.09%-0.15% of V, 0.01%-0.015% of Ti, <0.2% of Nb+V+Ti, <0.02% of Al, <0.003% of S, <0.008% of P, and the balance is Fe and inevitable impurities. The present disclosure improves the yield strength and high-temperature mechanical properties of the fire-resistant steel.

Claims

1. A high-strength low-carbon bainitic fire-resistant steel, comprising the following chemical components by mass percent: 0.07%-0.1% of C, 0.7%-0.9% of Si, 1.0%-1.5% of Mn, 0.7%-0.8% of Cr, 1.0%-1.3% of Ni, 0.3%-0.35% of Cu, 0.6%-0.8% of Mo, 0.025%-0.035% of Nb, 0.09%-0.15% of V, 0.01%-0.015% of Ti, <0.2% of Nb+V+Ti, <0.02% of Al, <0.003% of S, <0.008% of P, and the balance is Fe and inevitable impurities.

2. The high-strength low-carbon bainitic fire-resistant steel according to claim 1, wherein the fire-resistant steel comprises the following of chemical components by mass percent: 0.08%-0.10% of C, 0.75%-0.85% of Si, 1.1%-1.5% of Mn, 0.7%-0.78% of Cr, 1.0%-1.25% of Ni, 0.3%-0.34% of Cu, 0.6%-0.75% of Mo, 0.025%-0.032% of Nb, 0.09%-0.14% of V, 0.01%-0.013% of Ti, <0.18% of Nb+V+Ti, <0.02% of Al, <0.003% of S, <0.008% of P, and the balance is Fe and inevitable impurities.

3. A preparation method for a high-strength low-carbon bainitic fire-resistant steel comprising the following steps: step 1: rolling a slab to obtain a medium and heavy steel plate; and step 2: subjecting the medium and heavy steel plate to a heat treatment to obtain a fire-resistant steel; wherein the high-strength low-carbon bainitic fire-resistant steel comprises the following chemical components by mass percent: 0.07%-0.1% of C, 0.7%-0.9% of Si, 1.0%-1.5% of Mn, 0.7%-0.8% of Cr, 1.0%-1.3% of Ni, 0.3%-0.35% of Cu, 0.6%-0.8% of Mo, 0.025%-0.035% of Nb, 0.09%-0.15% of V, 0.01%-0.015% of Ti, <0.2% of Nb+V+Ti, <0.02% of Al, <0.003% of S, <0.008% of P, and the balance is Fe and inevitable impurities.

4. The preparation method for the high-strength low-carbon bainitic fire-resistant steel according to claim 3, wherein the step 1 comprises the following steps: step 11: loading the slab into a heating furnace for heating to obtain a heated slab; step 12: rolling the heated slab to obtain a rolled slab; and step 13: control-cooling the rolled slab to obtain a medium and heavy steel plate.

5. The preparation method for the high-strength low-carbon bainitic fire-resistant steel according to claim 4, wherein in step 11, the slab is heated to 1,180-1,240° C. in the heating furnace, and soaked for 1-4 h.

6. The preparation method for the high-strength low-carbon bainitic fire-resistant steel according to claim 4, wherein in step 12, an initial rolling temperature of the slab is 1,150-1,200° C.; the rolling comprises rough rolling and finish rolling; the rough rolling is performed in 3-6 passes, with a final rolling temperature of the rough rolling controlled at 950-1,100° C.; the finish rolling is performed in 5-10 passes, with a final rolling temperature of the finish rolling controlled at 880-920° C.

7. The preparation method for the high-strength low-carbon bainitic fire-resistant steel according to claim 4, wherein in step 13, the rolled slab is control-cooled to below 370° C.

8. The preparation method for the high-strength low-carbon bainitic fire-resistant steel according to claim 3, wherein the step 2 comprises the following steps: step 21: normalizing the medium and heavy steel plate; and step 22: air-cooling the normalized medium and heavy steel plate to room temperature, and then the tempering heat treatment is performed.

9. The preparation method for the high-strength low-carbon bainitic fire-resistant steel according to claim 8, wherein the medium and heavy steel plate is normalized at 880-920° C., soaked for 1-4 h after normalizing, and the medium and heavy steel plate is air-cooled to room temperature after the normalizing soaking.

10. The preparation method for the high-strength low-carbon bainitic fire-resistant steel according to claim 8, wherein the medium and heavy steel plate is tempered at 370-430° C., soaked for 1-3 h after tempering, and the medium and heavy steel plate is air-cooled to room temperature after tempering soaking, then obtaining a finished fire-resistant steel.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0035] The accompanying drawings are provided merely for illustrating the specific examples, rather than to limit the present disclosure. The same reference numerals represent the same components throughout the accompanying drawings.

[0036] FIG. 1 shows a diagram of a rolling process and heat treatment process of a fire-resistant steel plate.

[0037] FIG. 2 shows a microstructure observed from an optical microscope after a heat treatment (tempering time 1 h) in Example 1.

[0038] FIG. 3 shows a microstructure observed from an optical microscope after a heat treatment (tempering time 3 h) in Example 1.

[0039] FIG. 4 shows a microstructure observed from a scanning electron microscope (SEM) after a heat treatment (tempering time 1 h) in Example 1.

[0040] FIG. 5 shows a microstructure observed from an SEM after a heat treatment (tempering time 3 h) in Example 1.

[0041] FIG. 6 shows a microstructure observed from an optical microscope after a heat treatment (tempering time 1 h) in Comparative Example 1.

[0042] FIG. 7 shows a microstructure observed from an optical microscope after a heat treatment (tempering time 3 h) in Comparative Example 1.

DETAILED DESCRIPTION

[0043] The preferred examples of the present disclosure are described in detail below with reference to the accompanying drawings. As a part of the present disclosure, the accompanying drawings are used together with the examples of the present disclosure to explain the principles of the present disclosure, rather than to limit the scope of the present disclosure.

[0044] The present disclosure provides a high-strength low-carbon bainitic fire-resistant steel, whose chemical components by mass percent are as follows: 0.07-0.1% of C, 0.7-0.9% of Si, 1.0-1.5% of Mn, 0.7-0.8% of Cr, 1.0-1.3% of Ni, 0.3-0.35% of Cu, 0.6-0.8% of Mo, 0.025-0.035% of Nb, 0.09-0.15% of V, 0.01-0.015% of Ti, <0.2% of Nb+V+Ti, <0.02% of Al, <0.003% of S, <0.008% of P, and the balance is Fe and inevitable impurities.

[0045] In another specific example, the chemical components of the fire-resistant steel by mass percent are as follows: 0.08-0.10% of C, 0.75-0.85% of Si, 1.1-1.5% of Mn, 0.7-0.78% of Cr, 1.0-1.25% of Ni, 0.3-0.34% of Cu, 0.6-0.75% of Mo, 0.025-0.032% of Nb, 0.09-0.14% of V, 0.01-0.013% of Ti, <0.18% of Nb+V+Ti, <0.02% of Al, <0.003% of S, <0.008% of P, and the balance is Fe and inevitable impurities.

[0046] Based on a low-C—Si—Mn—Cr air-cooled bainitic alloy system, the present disclosure adopts a high-V and low-Nb—Ti micro-alloying technology route, and the microstructure of the steel is tempered bainite+residual austenite (or residual MA islands). The specific functions of the alloying elements in fire-resistant steel are as follows:

[0047] C: C is low in the low-carbon bainitic steel, and its function is to make the precipitation of carbide strengthened during the tempering process. The C content of the traditional low-carbon bainitic 690 MPa grade steel is controlled below 0.06%, and the purpose is to not form upper bainite and cementite in the transformation product to ensure the welding performance of the steel. The present disclosure increases the C content and aims to ensure that there is sufficient carbon content to promote the precipitation of high-temperature carbides at 600° C. In the present disclosure, the high Si content suppresses the diffusion of C to hinder the formation of the cementite, so as to improve the weldability of the steel, and at the same time, the ferrite/pearlite transformation zone and the bainite transformation zone are separated by adding Mn and Mo, and a certain amount of MA islands is retained in the steel, and this part of MA islands adjusts the stability of the size and performance of the steel through the subsequent low-temperature tempering process. In the present disclosure, the content of C element is 0.07-0.1%.

[0048] Si: The Si content in the traditional high-strength low-carbon bainitic steel is low. The present disclosure utilizes the mechanism that Si inhibits C diffusion to hinder the formation of cementite in the bainite lath. On the other hand, the present disclosure utilizes the mechanism that Si exists in ferrite or austenite in the form of solid solution, and has a strong solid solution function to improve the normal-temperature and high-temperature strength of the steel. Therefore, in the present disclosure, the Si content is preferably 0.7-0.9%.

[0049] Mn: As a main alloying element in the present disclosure, Mn forms a solid solution with Fe to improve the hardness and strength of ferrite and austenite in the steel. The ability of Mn to stabilize austenite structure is second only to Ni, and it also strongly increases the hardenability of the steel and promotes the phase transformation of bainite. However, if the Mn content is too high, that is, >1.6%, or the hardenability of the steel is greatly improved, it is easy to cause segregation of Mn or the presence of martensite structure in the steel formation, which will affect the performance stability of the steel. In the present disclosure, the Mn content is preferably 1.0-1.5%.

[0050] Mo: Mo is an essential component in the fire-resistant steel. The strengthening mechanism in steel includes solid solution strengthening and precipitation strengthening. Mo can significantly improve the high-temperature creep and durability of steel, so an increase of the Mo content can make the fire-resistant steel have better high-temperature fire resistance. Mo and interstitial elements (C, N) have a significant interaction to achieve solid solution strengthening, and the joint addition of 0.01% of C or N and 0.5% of Mo can significantly improve the high-temperature creep and durability of steel. The main purpose is to form more fine and stable carbides (Mo.sub.2C) at high temperatures to achieve the purpose of pinning grains to inhibit their recrystallization and growth and improve strength. Because of the large solid solubility of Mo in steel, therefore it is easy to use the mechanisms that aging precipitation of Mo at high temperature to make the steel have good fire resistance. Mo improves the hardenability, and is to promote the bainite structure to form element, which facilitates obtaining the air-cooled bainitic steel. Meanwhile, Mo can push up the temper brittleness temperature of the bainitic steel, and make the air-cooled bainitic steel have a larger tempering process adjustment window. However, the addition of Mo will increase the production cost, so in the present disclosure, the Mo content is controlled at 0.6-0.8%.

[0051] Cr: Cr is a main element in the air-cooled bainitic alloy system of the present disclosure, Cr can improve the hardenability of the steel and promote the formation of the air-cooled bainite. Cr works together with Mo, Cu, and Ni to improve the corrosion resistance of the steel. In the present disclosure, the content of Cr is 0.7-0.8%.

[0052] Ni: Ni can increase the strength of the steel without significantly reducing the toughness. It can reduce the brittle transition temperature of steel, that is to say, Ni can improve the low-temperature toughness of the steel, and improve the workability and weldability of the steel. Ni can improve the corrosion resistance of the steel, not only acid resistance, but also alkali and atmospheric corrosion. In the present disclosure, the Ni content is preferably 1.0-1.3%.

[0053] Cu: When Cu is added to steel, it can replace part of Ni to improve the hardenability of steel and its solid solution strengthening effect. Depending on the age hardening of Cu, high strength, especially Y/T ratio, can be obtained without causing obvious damage to the plasticity and toughness. It has no adverse effects on welding and toughness, and can improve the low-temperature toughness and weather resistance of the steel. Due to the low melting point, excessively high Cu in the steel is prone to cracking during hot working. Therefore, it is necessary to eliminate the hot cracking tendency of Cu-containing steel through high Ni. At the same time, Cu does not form carbide particles with C. The aging precipitation temperature of Cu is 500-600° C., which is precipitation strengthening in the form of Cu particles. In the present disclosure, the Cu content is 0.3-0.35%.

[0054] Nb: Nb can combine with C, Ni and O to form extremely stable compounds. It is usually precipitated in the high-temperature austenite to refine grains, reducing the overheating sensitization and temper brittleness of the steel. The joint addition of Nb and Mo can promote and the precipitation of Mo at high temperature and improve the fire resistance of the steel. Therefore, the design of fire-resistant steel usually uses a high Nb content (>0.06%) to replace part of Mo. However, the design idea that using Nb and Mo precipitate alloy by fire will increase the complexity of production process control. It is needed to adopt high-temperature solid solution and two-phase zone partition tempering process to ensure a considerable amount of Nb solid solution in the steel. The process window for the two-phase zone tempering is narrow, which is not conducive to the stability of mass production. Therefore, the present disclosure only uses the high-temperature precipitation of Nb to refine the austenite grains and suppress the size of the bainite laths after cooling phase transformation. In the present disclosure, the Nb content is relatively low, which is 0.025-0.035%.

[0055] Ti: A small amount of Ti element precipitates at high temperature to form dispersed fine second-phase particles in the steel, which are pinned in the austenite grain boundaries, inhibiting the growth of austenite in the heat-affected zone and improving the plasticity and toughness of the steel after welding. As a commonly used micro-alloying element in fire-resistant steel, Ti is added in combination with Nb for precipitation strengthening. Typically, Ti can be added up to 0.25%. However, an excessively high Ti content will cause mixed precipitation of different forms of Ti particles such as nitride, carbide and oxide, which will affect the effective Ti performance and microscopic grain size fluctuations. In the present disclosure, the Ti content is 0.01-0.015%.

[0056] V: A small amount of V in the steel has the characteristics of solid solution strengthening, fine grain strengthening and precipitation strengthening. Typically, V has an infinite solid solution mechanism with austenite or ferrite, and at the same time V (NC) can be precipitated in austenite and bainite ferrite laths. The carbide of V has good stability at high temperature and is not easy to dissolve and grow. At the same time, the carbide formed by V and C can keep coherent with the matrix, and can generate a strong stress field to prevent the movement of dislocations and improve the high-temperature performance of the steel. By using the full solid solution of V in the steel and the precipitation at 600° C. in fire to hinder the grain recrystallization and growth control of the performance mechanism of the steel plate, the design of high V content can be adopted in the design of the fire-resistant steel to ensure the fire resistance of the weathering steel. In the present disclosure, the V content is controlled to 0.09-0.15%.

[0057] P, S: P and S are often regarded as impurity elements in the steel. Clean steel will effectively reduce the contents of P and S, but it will increase the cost of steelmaking. Therefore, in the present disclosure, the contents of P and S are P≤0.008% and S≤0.003%, respectively.

[0058] The present disclosure specifies the total content range of the micro-alloying elements is Nb+V+Ti<0.2% so as to control the total precipitation amount of microalloy particles during heat treatment to ensure the steel has sufficient carbide precipitation and reduce the total amount of Nb+V when they encounter fire. And the total amount of Nb+V is too high to affect the welding performance.

[0059] The present disclosure adopts the design principle of a low Al content (Al<0.02%). Al is a deoxidizer, but Al has an adverse effect on the low-temperature toughness and high-temperature strength of the steel. Therefore, the present disclosure limits the content of Al.

[0060] Another aspect of the present disclosure provides a preparation method for the high-strength low-carbon bainitic fire-resistant steel. As shown in FIG. 1, the preparation method includes the following steps:

[0061] Step 1: Roll a continuously casting slab or casting slab to obtain a medium and heavy steel plate.

[0062] The continuously casting slab or casting slab is loaded into a heating furnace for heating, heated to 1,180-1,240° C., and soaking time is 1-4 h, and rolled the heated continuously casting slab or casting slab. An initial rolling temperature of the continuously casting slab or casting slab is 1,150-1,200° C. The rolling process is as follows: the rough rolling is performed in 3-6 passes, with a final rolling temperature of rough rolling controlled at 950-1,100° C. The finish rolling is performed in 5-10 passes, with a final rolling temperature of finish rolling controlled at 880-920° C. The rolled continuously casting slab or casting slab is air-cooled or laminar-cooled to below 370° C. to obtain a medium and heavy plate.

[0063] Step 2: Subject the medium and heavy steel plate after rolling to a heat treatment to obtain a fire-resistant steel.

[0064] After the hot rolling, the medium and heavy plate is performed normalizing and tempering heat treatment. The normalizing temperature is in the austenite temperature zone, and the temperature range of the normalizing treatment is 880-920° C., soaking time is 1-4 h, and then air-cooled to room temperature. The tempering temperature of the medium and heavy plate is in the bainitic temperature zone, and the tempering temperature is 370-430° C., soaking time is 1-3 h, and then air-cooled to room temperature to obtain a finished fire-resistant steel. The grain structure is refined through the precipitation mechanism of Nb and Ti in the steel at the high-temperature stage in the austenite temperature zone to improve the plasticity and toughness of the steel. The steel still retains a sufficient solid solution V content in the lath-like bainitic ferrite and the residual austenite at room temperature through the infinite solid solution mechanism of V and the bainitic ferrite. V and a small amount of solid-solution Mo and Nb can be coordinated to precipitate a second time at a high temperature of 600° C. to strengthen and pinning the grain, and achieve the purpose of stabilizing the strength of the steel.

[0065] In particular, in order to keep the residual stress of the steel at a low level and the overall performance of the steel to be uniform, a secondary tempering process can be used. Normalizing is performed to control the components of the microstructure and the uniformity of the grain size of the fire-resistant steel. Tempering is performed to further eliminate the residual stress in the steel so as to improve the plasticity and toughness of the steel, and to decompose the larger residual austenite so as to improve the stability of the microstructure and properties. As shown in FIGS. 2 to 5, the microstructure of the fire-resistant steel prepared in Example 1 is tempered bainite+residual austenite (a small amount of martensite-austenite structure) structure. As shown in FIGS. 6 to 7, the matrix structure of the fire-resistant steel in Comparative Example 1 is different from that in Example 1, and the grain structure of the fire-resistant steel in Example 1 is finer. The content of each element of the fire-resistant steel in the specific examples of the present disclosure is shown in Table 1, and the preparation method of the fire-resistant steel is shown in Table 2. Compared with Examples 1 to 3, Comparative Example 1 adopts a design scheme of increasing the content of Nb and Ti elements and reducing the content of V. The content of each element of the fire-resistant steel in the comparative example is shown in Table 1, and the preparation method of the fire-resistant steel is shown in Table 2.

TABLE-US-00001 TABLE 1 Element content (wt %) of examples of the present disclosure and comparative examples Implementations C Si Mn Cr Ni Mo Cu Nb Ti V Al S P Example 1 0.1 0.73 1.0 0.73 1.25 0.6 0.3 0.035 0.015 0.09 0.012 0.0026 0.0047 Example 2 0.07 0.75 1.5 0.78 1.25 0.7 0.35 0.025 0.010 0.14 0.012 0.0026 0.0047 Example 3 0.08 0.08 1.2 0.75 1.3 0.75 0.32 0.03 0.010 0.15 0.012 0.002 0.006 Example 4 0.09 0.09 1.3 0.8 1.0 0.8 0.34 0.028 0.012 0.12 0.012 0.002 0.006 Comparative 0.08 0.78 1.0 0.78 1.25 0.57 0.4 0.062 0.025 0.04 0.03 0.0028 0.0087 Example 1

TABLE-US-00002 TABLE 2 Rolling and heat treatment process of examples of the present disclosure and comparative examples Rolling and heat treatment process Heating Final Final temperature rolling rolling Temper- (° C.)/ temper- temper- ature soaking Initial ature of ature after time (h) rolling rough of finish laminar Implemen- of casting temperature rolling rolling cooling tations slab (° C.) (° C.) (° C.) (° C.) Example 1 1180/3 1150 950 880 340 Example 2 1240/3 1200 1100 920 370 Example 3 1200/3 1170 1070 900 350 Example 4 1210/3 1180 1080 890 360 Comparative 1240/3 1200 1100 920 370 Example 1

[0066] Table 3 shows mechanical properties, low-temperature impact performance and 600° C. fire resistance of Examples 1 to 4 of the present disclosure and Comparative Example 1.

[0067] Table 3 Heat treatment process and performance of examples of the present disclosure and

Comparative Example 1

[0068]

TABLE-US-00003 600° C. Tensile Yield Reduction yield −40° C. strength/ strength/ Y/T Elongation/ of strength/ impact Heat treatment process MPa MPa ratio % area/% MPa energy/J Example 1 910° C.*1 hAC + 400° C.*1 hAC 888 755 0.85 18.5 66 481 167/176/94 910° C.*1 hAC + 400° C.*3 hAC 908 745 0.82 17.5 67 482  69/124/78 Example 2 890° C.*1 hAC + 380° C.*1 hAC 918 775 0.84 15.5 56 491 157/146/94 890° C.*1 hAC + 380° C.*3 hAC 932 765 0.82 15.5 57 492  69/114/88 Example 3 900° C.*1 hAC + 430° C.*1 hAC 878 735 0.84 17.5 57 471  157/156/104 900° C.*1 hAC + 430° C.*3 hAC 882 745 0.84 16.5 51 472  82/114/98 Example 4 900° C.*1 hAC + 430° C.*1 hAC 898 745 0.83 17.5 62 485 168/175/99 900° C.*1 hAC + 430° C.*3 hAC 922 755 0.82 16.5 65 483  78/121/83 Comparative Example 1 910° C.*1 hAC + 380° C.*1 hAC 1013 765 0.76 17.5 61 459 74/24/16 910° C.*1 hAC + 380° C.*3 hAC 1059 677 0.64 15 62 432 20/42/57

[0069] Combining Table 1, Table 2 and Table 3, it can be concluded that the present disclosure adopts a high-V and low-Nb—Ti micro-alloying technology route. The grain structure is refined through the precipitation mechanism of Nb and Ti in the steel at the high-temperature stage in the austenite temperature zone to improve the plasticity and toughness of the steel. The steel still retains a sufficient solid solution V content in the lath-like bainitic ferrite and the residual austenite at room temperature through the infinite solid solution mechanism of V and the bainitic ferrite. V and a small amount of solid-solution Mo and Nb can be coordinated to precipitate a second time at a high temperature of 600° C. to strengthen and pinning the grain, to improve the strength of the steel.

[0070] The above are merely preferable particular embodiments of the present disclosure, and the protection scope of the present disclosure is not limited thereto. Any modification or replacement easily conceived by those skilled in the art within the technical scope of the present disclosure should fall within the protection scope of the present disclosure.