Production method of bainite steel rail for crane

Abstract

The invention discloses a production method of a bainite steel rail for a crane. The production method comprises the following steps: 1) carrying out KR molten iron pretreatment, converter top and bottom combined blowing smelting, LF external refining, VD vacuum treatment, square billet continuous casting and casting billet slow cooling on the steel rail to obtain a 280 * 380mm square billet; (2) a casting blank is subjected to stepping type heating, high-pressure water descaling, blooming through a BD1 roughing mill, rolling through a BD2 roughing mill, rolling through a CCS finishing mill, controlled cooling, cooling through a cooling bed, straightening and tempering heat treatment; the chemical composition of the steel rail is also defined in percentage by mass. The bainite steel rail for the crane has excellent obdurability, high hardness, high wear resistance and high contact fatigue performance, and the mechanical property indexes of the bainite steel rail are obviously superior to those of an existing crane steel rail.

Description

Technical Field

[0001] The invention belongs to the technical field of metallurgical materials, and in particular relates to a production method of a bainite steel rail for cranes. Background Art

[0002] Crane rails are mainly used for cranes or load-carrying vehicles to transport loads. my country's crane rails are generally made of pearlite materials such as U71 Mn or U75V, with a tensile strength of only 880MPa and 980MPa, a hardness of only about 270HB and 290HB, and an impact of about 10J. The sections include QU120, QU100, QU80, and QU70. Since the compression ratio of rail manufacturers is reduced when using continuous casting billets of the same specification to produce crane rails, the density of the steel is significantly reduced, and its performance deteriorates. During the use of the rails, due to excessive loads, the rails are crushed, fractured, and have poor wear resistance. With the development of modern industry and transportation, cranes are promoted to develop in the direction of large-scale and high-speed. Therefore, it is necessary to develop high-strength and toughness crane rails to solve the problems of crushing and fracture during the use of crane rails, and further improve the service life and safety of crane rails.

[0003] At present, domestic rails are mainly divided into pearlite rails and bainite rails. The process routes are divided into air cooling state and online heat treatment state. Pearlite rails can improve the strength and hardness of the rails through alloy strengthening and online quenching treatment. However, the domestic crane rail market share is relatively low, and the existing rail heat treatment production line is not suitable for crane rails.

[0004] Bainitic rails have a lower structural transformation cooling rate and higher hardenability. Even in the air-cooled state, they can obtain a bainite and martensite duplex structure with excellent toughness. Bainitic rails have contact fatigue resistance that pearlite rails cannot match. Therefore, it is of great significance to develop high-strength and toughness bainitic rails for heavy machinery.

[0005] The patent with application number 201911042676.6 discloses a high-strength and toughness crane rail and its manufacturing method. The patent uses C, Si, Mn, Cr, Mo, Ni, Nb, and Ti alloying elements to control the rolling interval time and temperature of each rolling process to obtain a crane rail with a lath bainite structure. The tensile strength of the rail is ≥1300MPa, and the impact strength at room temperature is ≥70J. The rail sections are QU100 and QU120. The patent only provides the composition and rolling process, and does not provide a detailed description of the smelting process that affects the quality and performance of heavy rail steel. In addition, the patent only rolls and produces two sections, QU100 and QU120.

[0006] The patent with application number 202110036244.5 discloses a method for producing high-strength and toughness American standard CR175 crane rails. The patent obtains pearlite rails through the ratio of C, Si, Mn, and Cr strengthening elements. The tensile strength of the rails is ≥1080MPa, the elongation after fracture is ≥9%, and the tread hardness is 321-388HB. The strength and hardness of this patent are improved compared with pearlite rail materials such as U71 Mn or U75V, but it still does not meet the performance indicators for heavy-load cranes, and the toughness of the rails is only about 9%, which does not meet the requirements for use in low temperatures or special environments.

[0007] Patent application number 201380065881.9 discloses a method for manufacturing high-strength crane rails. The alloy strengthening elements of the patented rails are C, Si, Mn, Cr, V, Ti, and N. Pearlite rails are obtained by water cooling the rail head, rail waist, and rail bottom. The rail elongation after fracture is between 9.5-11.3%, and the tread hardness is 302-378HB. The strengthening mechanism of Ti and N elements adopted in the patent is not applied to heavy rail steel. Heavy rail steel has strict restrictions on N elements (≤0.0080%). The addition of Ti elements is likely to cause corner cracks in continuous casting billets, which is not in line with the continuous casting production of heavy rail steel. In addition, domestic rail manufacturers do not have water cooling control equipment for the rail head, rail waist, and rail bottom of crane rails. At the same time, water cooling of rails is also likely to produce abnormal structures such as martensite on the surface, which is not in line with actual domestic production conditions. Summary of the invention

[0008] The purpose of the present invention is to provide a production method of a bainite steel rail for cranes. The bainite steel rail for cranes of the present invention has excellent toughness, high hardness, high wear resistance and high contact fatigue performance, and is significantly better than the mechanical performance indicators of existing crane rails.

[0009] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0010] The present invention provides a method for producing a bainite steel rail for a crane, comprising:

[0011] 1) The rails are subjected to KR hot metal pretreatment, converter top and bottom combined blowing smelting, LF refining outside the furnace, VD vacuum treatment, billet continuous casting, and billet slow cooling to obtain 280*380mm billets; wherein:

[0012] The [S] content of the molten iron after pretreatment is ≤0.010%; the aluminum-free deoxidation process is used for smelting, and the C content of the steel is controlled to be ≥0.10%, and the steel tapping temperature is ≥1600℃; the LF furnace refining controls the slag basicity R≥2.0, and all the alloys required in the steel are supplemented. The molten steel is heated to a suitable temperature, and the out-of-position temperature is ≥1585℃; the deep vacuum degassing time is ≥20min, the soft blowing time after vacuum degassing is ≥25min, and the deep vacuum degree is ≤0.10kPa; the billet drawing speed is controlled between 0.60-0.65m / min; the billet slow cooling heating temperature is ≥600℃, and the insulation time is ≥30h hours; the gas content of the billet is controlled, wherein [H]≤1.7ppm, [O]≤20ppm, and [N]≤80ppm;

[0013] 2) The ingot is then subjected to step-by-step heating - high-pressure water descaling - ingot opening by BD1 roughing mill - rolling by BD2 roughing mill - rolling by CCS finishing mill - controlled cooling - cooling on cooling bed - straightening - tempering heat treatment; wherein:

[0014] The temperature of heating furnace heating section 1 is ≤1000℃, the temperature of heating section 2 is ≤1200℃, the temperature of heating section 3 is 1150-1300℃, the temperature of soaking section is 1150-1280℃, and the total heating time is ≥3h; the surface of steel billet must be descaled by high-pressure water, the rolling temperature is controlled at 1080-1150℃, and the final rolling temperature is ≤900℃; the cooling speed of rail is controlled at ≤2.0℃ / s, and the horizontal step of cooling bed is ≥600mm; the straightening temperature is ≤60℃, the rail after straightening should be straight, without wave bend, and only one roller straightening is allowed; the tempering heat treatment temperature of rail is controlled at 250-300℃, and the holding time is ≥20h;

[0015] The chemical composition of the steel rail includes, by mass percentage, C: 0.18-0.35%, Si: 0.80-1.20%, Mn: 2.00-2.50%, Cr: 0.80-1.20%, Ni: 0.30-0.90%, Mo: 0.25-0.80%, Nb: 0.03-0.06%, V: 0-0.10%, RE: 0-0.0020%; P≤0.022%, S≤0.015%, Al≤0.010%, and the rest is Fe and unavoidable impurities.

[0016] Furthermore, the deoxidizer is silicon calcium barium.

[0017] Furthermore, the produced bainite rails meet the following requirements: tensile strength Rm≥1330MPa, yield strength Rp0.2≥1150MPa, elongation after fracture A≥14.0%, normal temperature impact KU2≥100J, tread hardness≥400HB, and retained austenite content≤16.0%.

[0018] Furthermore, the microstructure of the produced bainite rail is a bainite + martensite complex phase structure.

[0019] Furthermore, the chemical composition of the rail includes, by mass percentage, C: 0.22%, Si: 1.00%, Mn: 2.30%, Cr: 0.95%, Ni: 0.60%, Mo: 0.45%, Nb: 0.06%, V: 0.10%, RE: 0.0015%, and the rest is Fe and unavoidable impurities.

[0020] Furthermore, the sections of bainite rails for cranes include QU70, QU80, QU100 and QU120.

[0021] Compared with the prior art, the beneficial technical effects of the present invention are:

[0022] The bainite steel rail for cranes obtained by the present invention has the following characteristics: tensile strength Rm≥1330MPa, yield strength Rp0.2≥1150MPa, elongation after fracture A≥14.0%, normal temperature impact KU2≥100J, tread hardness≥400HB, residual austenite content≤16.0%, and microstructure of bainite+martensite complex phase structure. The bainite steel rail for cranes of the present invention has excellent toughness, high hardness, high wear resistance and high contact fatigue performance, and is significantly better than the mechanical performance indicators of existing crane steel rails. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The present invention will be further described below in conjunction with the accompanying drawings.

[0024] Figure 1 This is the rail head microstructure of Example 4. DETAILED DESCRIPTION

[0025] The present invention is further described in detail below in conjunction with specific implementation examples. The chemical compositions of rail materials in different implementation examples are shown in Table 1.

[0026] Table 1 Metallurgical composition of rails in different embodiments (mass percentage / %)

[0027]

[0028] The rail heating process, cooling process and tempering process of different embodiments are shown in Table 2.

[0029] Table 2 Rail rolling and heat treatment process of different embodiments

[0030]

[0031] The mechanical properties of rails in different embodiments are shown in Table 3.

[0032] Table 3 Mechanical properties of rails in different embodiments

[0033]

[0034] From Table 1, Table 2, Table 3 and Figure 1 It can be seen that the bainite steel rail for crane obtained in Example 4 of the present invention has a tensile strength Rm≥1330MPa, a yield strength Rp0.2≥1150MPa, an elongation after fracture A≥14.0%, a room temperature impact KU2≥100J, a tread hardness≥400HB, a residual austenite content≤16.0%, and a microstructure of bainite+martensite duplex structure by optimizing the proportion of C, Si, Mn, Cr, Ni, Mo, Nb, V and RE elements, and combining the design of smelting process, rolling process and heat treatment process. The bainite steel rail for crane of the present invention has excellent toughness, high hardness, high wear resistance and high contact fatigue performance, which is significantly better than the mechanical performance indicators of existing crane rails.

[0035] The embodiments described above are only descriptions of the preferred modes of the present invention, and are not intended to limit the scope of the present invention. Without departing from the design spirit of the present invention, various modifications and improvements made to the technical solutions of the present invention by ordinary technicians in this field should all fall within the protection scope determined by the claims of the present invention.

Claims

1. A method for producing a bainite rail for a crane, characterized in that: include: 1) The rails are subjected to KR hot metal pretreatment, converter top and bottom combined blowing smelting, LF refining outside the furnace, VD vacuum treatment, billet continuous casting, and billet slow cooling to obtain 280*380mm billets; wherein: The [S] content of the molten iron after pretreatment is ≤0.010%; the aluminum-free deoxidation process is used for smelting, and the C content of the steel is controlled to be ≥0.10%, and the steel tapping temperature is ≥1600℃; the LF furnace refining controls the slag basicity R≥2.0, and all the alloys required in the steel are supplemented. The molten steel is heated to a suitable temperature, and the out-of-position temperature is ≥1585℃; the deep vacuum degassing time is ≥20min, the soft blowing time after vacuum degassing is ≥25min, and the deep vacuum degree is ≤0.10kPa; the billet drawing speed is controlled between 0.60-0.65m / min; the billet slow cooling heating temperature is ≥600℃, and the insulation time is ≥30h hours; the gas content of the billet is controlled, wherein [H]≤1.7ppm, [O]≤20ppm, and [N]≤80ppm; 2) The ingot is then subjected to step-by-step heating - high-pressure water descaling - ingot opening by BD1 roughing mill - rolling by BD2 roughing mill - rolling by CCS finishing mill - controlled cooling - cooling on cooling bed - straightening - tempering heat treatment; wherein: The temperature of heating furnace heating section 1 is ≤1000℃, the temperature of heating section 2 is ≤1200℃, the temperature of heating section 3 is 1150-1300℃, the temperature of soaking section is 1150-1280℃, and the total heating time is ≥3h; the surface of steel billet must be descaled by high-pressure water, the rolling temperature is controlled at 1080-1150℃, and the final rolling temperature is ≤900℃; the cooling speed of rail is controlled at ≤2.0℃ / s, and the horizontal step of cooling bed is ≥600mm; the straightening temperature is ≤60℃, the rail after straightening should be straight, without wave bend, and only one roller straightening is allowed; the tempering heat treatment temperature of rail is controlled at 250-300℃, and the holding time is ≥20h; The chemical composition of the steel rail includes, by mass percentage, C: 0.18-0.35%, Si: 0.80-1.20%, Mn: 2.00-2.50%, Cr: 0.80-1.20%, Ni: 0.30-0.90%, Mo: 0.25-0.80%, Nb: 0.03-0.06%, V: 0-0.10%, RE: 0-0.0020%; P≤0.022%, S≤0.015%, Al≤0.010%, and the rest is Fe and unavoidable impurities.

2. The method for producing a bainite steel rail for crane according to claim 1, characterized in that: The deoxidizer is silicon calcium barium.

3. The method for producing a bainite steel rail for crane according to claim 1, characterized in that: The produced bainite rails meet the following requirements: tensile strength Rm ≥ 1330MPa, yield strength Rp0.2 ≥ 1150MPa, elongation after fracture A ≥ 14.0%, normal temperature impact KU2 ≥ 100J, tread hardness ≥ 400HB, and retained austenite content ≤ 16.0%.

4. The method for producing a bainite steel rail for crane according to claim 1, characterized in that: The microstructure of the produced bainite rail is a bainite + martensite complex phase structure.

5. The method for producing a bainite steel rail for crane according to claim 1, characterized in that: The chemical composition of the steel rail includes, by mass percentage, C: 0.22%, Si: 1.00%, Mn: 2.30%, Cr: 0.95%, Ni: 0.60%, Mo: 0.45%, Nb: 0.06%, V: 0.10%, RE: 0.0015%, and the rest is Fe and inevitable impurities.

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