8.8-grade corrosion-resistant bolt steel rolling and heat treatment method

Abstract

The invention discloses a rolling and heat treatment method for 8.8-grade corrosion-resistant bolt steel, which adopts a rolling method of'high-temperature heating in a heating furnace at 1050-1200 DEG C + low-temperature finish rolling in a KOCKS rolling mill at 750-850 DEG C 'and a quenching and tempering heat treatment method of'heat preservation at 870-920 DEG C for 1 hour and quenching + tempering at 500-550 DEG C', and meets the mechanical properties of the 8.8-grade corrosion-resistant bolt steel. The obtained corrosion-resistant bolt steel meets the requirements that the tensile strength ranges from 830 MPa to 1030 MPa, the yield strength is larger than or equal to 600 MPa, the percentage elongation after fracture is larger than or equal to 12%, the percentage shrinkage after fracture is larger than or equal to 45%, the normal-temperature impact KU2 is larger than or equal to 63 J, and the hardness ranges from 24 HRC to 31 HRC. The finished product in the quenched and tempered state is detected according to the MSC289 standard, and the annual average corrosion rate CR is smaller than or equal to 1.0 mm / a when the finished product is soaked in a 10% NaCl solution with the PH of 0.85 for 72 h.

Description

Technical Field

[0001] The invention relates to the field of bolt steel, and in particular to a rolling and heat treatment method of 8.8 grade corrosion-resistant bolt steel. Background Art

[0002] At present, the construction of transportation tunnels is developing in a blowout manner, and the amount of tunnel projects is increasing year by year. The corrosion environment of these submarine, river-crossing, mountain, subway and other transportation tunnels is complex, including sand and gravel bottom conditions, riverside water-rich environment, ground fissure environment, complex urban environment, high temperature and high humidity environment, etc.

[0003] Commonly used bolt steel grades include 5.8, 6.8, and 8.8. The existing bolt materials combined with anti-corrosion measures still have serious corrosion problems and cannot meet the long life requirements of complex environments such as tunnels.

[0004] Therefore, it is necessary to develop new 8.8 grade corrosion-resistant bolt steel rolling and heat treatment methods to meet the long-term use requirements of bolts in complex environments such as tunnels. Summary of the invention

[0005] Purpose of the invention: In view of the shortcomings and defects of the prior art, the present invention provides a rolling and heat treatment method for 8.8 grade corrosion-resistant bolt steel, which adopts the rolling method of "high-temperature heating at 1050℃-1200℃ in a heating furnace + low-temperature final rolling at 750℃-850℃ on a KOCKS rolling mill" and the tempering heat treatment method of "870℃-920℃ insulation for 1h quenching + 500℃-550℃ tempering" to meet the mechanical properties of 8.8 grade bolt steel.

[0006] Technical solution: A rolling and heat treatment method for 8.8 grade corrosion-resistant bolt steel of the present invention is characterized by: adopting a rolling method of "high temperature heating at 1050℃-1200℃ in a heating furnace + low temperature final rolling at 750℃-850℃ in a KOCKS rolling mill", and a tempering heat treatment method of "870℃-920℃ insulation for 1h quenching + 500℃-550℃ tempering".

[0007] The 8.8 grade corrosion resistant bolt steel rolling and heat treatment method of the present invention is characterized in that it specifically comprises the following steps:

[0008] 1) The heating furnace adopts high temperature heating of 1050℃-1200℃, low air-fuel ratio of 0.7-1.0, maintains the reducing atmosphere of the heating furnace, and the heating time is 150min-200min to avoid the surface enrichment of microelements of corrosion-resistant bolt steel;

[0009] 2) High-pressure water is used to remove phosphorus when the furnace is discharged, and the phosphorus removal pressure is ≥19MPa;

[0010] 3) The final rolling is carried out by KOCKS rolling mill, the final rolling temperature is 750℃-850℃, and the low final rolling temperature refines the grains;

[0011] 4) Quenching and tempering heat treatment, quenching temperature 870℃-920℃, keep warm for 1h, tempering temperature 500℃-550℃, tempering time 40min-60min, air cooling after tempering.

[0012] Wherein, in the step 1), surface enrichment of microelements of the corrosion-resistant bolt steel is avoided, and the microelements include Cu.

[0013] Wherein, in the step 2), the dephosphorization pressure is ≥19MPa to ensure that the iron oxide scale on the surface of the steel billet is completely removed.

[0014] Among them, the corrosion-resistant bolt steel obtained meets the requirements of tensile strength of 830MPa-1030MPa, yield strength ≥600MPa, elongation after fracture ≥12%, shrinkage after fracture ≥45%, normal temperature impact KU2≥63J, and hardness of 24HRC-31HRC, and is judged to be qualified.

[0015] The obtained corrosion-resistant bolt steel quenched and tempered finished product was tested according to the MSC289 standard, immersed in a 10% NaCl solution with a pH of 0.85 for 72 hours, and the annual average corrosion rate CR was ≤1.0mm / a, which was judged to be qualified.

[0016] Beneficial effects: Compared with the prior art, the present invention has the following significant advantages: the present invention adopts the rolling method of "heating furnace 1050℃-1200℃ high temperature heating + KOCKS rolling mill 750℃-850℃ low temperature final rolling", and the quenching and tempering heat treatment method of "870℃-920℃ insulation 1h quenching + 500℃-550℃ tempering", which meets the mechanical properties of 8.8 grade bolt steel. The mechanical properties of the quenched and tempered state meet the requirements of tensile strength 830MPa-1030MPa, yield strength ≥600MPa, elongation after fracture ≥12%, shrinkage after fracture ≥45%, normal temperature impact KU2 ≥63J, and hardness 24HRC-31HRC; the quenched and tempered finished product is tested according to the MSC289 standard, and is immersed in a 10% NaCl solution with a pH of 0.85 for 72h, and the annual average corrosion rate (CR) is ≤1.0mm / a. The mechanical and corrosion-resistant properties of bolt steel are guaranteed to meet the needs of engineering applications, reducing the difficulty of bolt design, bolt inspection and maintenance in design, construction and maintenance units. DETAILED DESCRIPTION

[0017] The technical solution of the present invention is further described below in conjunction with specific implementation methods.

[0018] The 8.8-grade corrosion-resistant bolt steel rolling and heat treatment method of the present invention adopts a rolling method of "high-temperature heating at 1050°C-1200°C in a heating furnace + low-temperature final rolling at 750°C-850°C in a KOCKS rolling mill", and a quenching and tempering heat treatment method of "870°C-920°C insulation for 1h quenching + 500°C-550°C tempering".

[0019] The specific steps include:

[0020] 1) The heating furnace adopts high temperature heating of 1050℃-1200℃, low air-fuel ratio of 0.7-1.0, maintains reducing atmosphere of the heating furnace, and heating time of 150min-200min to avoid surface enrichment of microelements such as Cu in corrosion-resistant bolt steel;

[0021] 2) High-pressure water is used to remove phosphorus when the steel billet leaves the furnace, and the dephosphorization pressure is ≥19MPa to ensure that the iron oxide scale on the surface of the steel billet is completely removed;

[0022] 3) The final rolling is carried out by KOCKS rolling mill, the final rolling temperature is 750℃-850℃, and the low final rolling temperature refines the grains;

[0023] 4) Quenching and tempering heat treatment, quenching temperature 870℃-920℃, keep warm for 1h, tempering temperature 500℃-550℃, tempering time 40min-60min, air cooling after tempering.

[0024] Example:

[0025] The method of the present invention is used to prepare Φ25 mm corrosion-resistant bolt steel, and the steel is tested. The results of the quenched and tempered mechanical properties and corrosion resistance are as follows:

[0026] Table 1. Mechanical properties test results

[0027]

[0028] Table 2. Corrosion resistance test results

[0029]

[0030] The obtained Φ25mm corrosion-resistant bolt steel has a tensile strength of 926MPa-935MPa, a yield strength of 847MPa-853 MPa, an elongation after fracture ≥15%, a shrinkage after fracture of 54%-56%, a normal temperature impact KU2 125J-128J, and a hardness of 29HRC-30HRC; the tempered finished product is tested according to the MSC289 standard, immersed in a 10% NaCl solution with a PH value of 0.85 for 72 hours, and the annual average corrosion rate CR is 0.283mm / a-0.360mm / a. The finished product tests after production all meet the technical requirements.

Claims

1. A method for rolling and heat treating 8.8 grade corrosion resistant bolt steel, characterized in that: The rolling method is "high temperature heating at 1050℃-1200℃ in heating furnace + low temperature final rolling at 750℃-850℃ in KOCKS rolling mill", and the tempering heat treatment method is "quenching at 870℃-920℃ for 1h + tempering at 500℃-550℃".

2. The method for rolling and heat treating 8.8 grade corrosion resistant bolt steel according to claim 1, characterized in that: The specific steps include: 1) The heating furnace adopts high temperature heating of 1050℃-1200℃, low air-fuel ratio of 0.7-1.0, maintains reducing atmosphere of the heating furnace, and heating time of 150min-200min to avoid surface enrichment of microelements of corrosion-resistant bolt steel; 2) High-pressure water is used to remove phosphorus when the furnace is discharged, and the phosphorus removal pressure is ≥19MPa; 3) The final rolling is carried out by KOCKS rolling mill, the final rolling temperature is 750℃-850℃, and the low final rolling temperature refines the grains; 4) Quenching and tempering heat treatment, quenching temperature 870℃-920℃, keep warm for 1h, tempering temperature 500℃-550℃, tempering time 40min-60min, air cooling after tempering.

3. The method for rolling and heat treating 8.8 grade corrosion resistant bolt steel according to claim 2, characterized in that: In the step 1), surface enrichment of microelements in the corrosion-resistant bolt steel is avoided, and the microelements include Cu.

4. The method for rolling and heat treating 8.8 grade corrosion resistant bolt steel according to claim 2, characterized in that: In the step 2), the dephosphorization pressure is ≥19 MPa to ensure that the iron oxide scale on the surface of the steel billet is completely removed.

5. The method for rolling and heat treating 8.8 grade corrosion resistant bolt steel according to claim 2, characterized in that: The corrosion-resistant bolt steel obtained meets the requirements of tensile strength of 830MPa-1030MPa, yield strength ≥600MPa, elongation after fracture ≥12%, shrinkage after fracture ≥45%, normal temperature impact KU2 ≥63J, and hardness 24HRC-31HRC, and is judged to be qualified.

6. The method for rolling and heat treating 8.8 grade corrosion resistant bolt steel according to claim 5, characterized in that: The obtained corrosion-resistant bolt steel quenched and tempered finished product was tested according to MSC289 standard, immersed in 10% NaCl solution with pH=0.85 for 72 hours, and the annual average corrosion rate CR≤1.0mm / a was judged to be qualified.

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