A semi-continuous rolling heating and rolling process for free-cutting stainless steel bars

Abstract

The present invention discloses a semi-continuous heating and rolling process for free-cutting stainless steel bars, which can avoid problems such as splitting, accumulating steel, and roll entanglement. The present invention employs the following technical solutions: ① Furnace charge control: strictly controlling the charge for each ingot type to ensure uniform through-burning of the ingots; ② Furnace charging method selection: loading the ingot tail-upward, with the ingot tail inserted into the rolling mill first, to ensure uniform temperature at the tail end of the ingot; ③ Ingot heating temperature control: setting a reasonable ingot temperature to avoid overheating and melting of sulfides, as well as splitting caused by insufficient temperature and poor rolling plasticity; and ④ Rolling system optimization: employing two-stage rolling to timely supplement the steel billet with secondary heating, thus avoiding plasticity loss caused by temperature drop and splitting during rolling. The present invention has the following advantages: strict control of the heating and rolling processes ensures 100% roll-free splitting during semi-continuous rolling of free-cutting stainless steel bars; and eliminates accumulating steel from high-strength, difficult-to-cut stainless steel.

Description

Technical Field

[0001] The invention relates to a rolling method for die-cast free-cutting stainless steel finished steel bars, in particular to a semi-continuous rolling heating and rolling process for free-cutting stainless steel bars. Background Art

[0002] Free-cutting steel has good cutting performance due to the addition of high content of sulfur, lead, tin, calcium and other free-cutting elements. In China, the control level of free-cutting stainless steel composition varies from unit to unit, and a separate standard has not yet been formed. The free-cutting stainless steels such as Y1Cr13, Y3Cr13 and Y10Cr17 in the GB / T1220-2007 stainless steel bar standard mainly control the sulfur content to be no less than 0.15%, and allow the addition of molybdenum up to 0.60%. The American ASTM A582 / A582M has formed the standard for free-cutting stainless steel bars, which mainly adjusts the cutting performance by controlling the sulfur, molybdenum and Se elements. Studies have shown that the splitting of sulfur-containing stainless steel is related to the morphology of sulfides in the steel. After processing, the sulfides are distributed in strips. At high temperatures, their plasticity decreases, resulting in anisotropic properties. Under the combined action of the rolling force and friction of the rollers, the center of the micro-area at the head of the rolled piece is subjected to tensile stress. Splitting occurs after exceeding the high-temperature strength limit. The splitting rate of sulfur-containing stainless steel rolling reaches more than 50% (such as Figure 1 How to control the heating process and rolling rhythm to prevent the free-cutting steel from splitting is an urgent problem to be solved. Summary of the Invention

[0003] The present invention discloses a semi-continuous rolling heating and rolling process for free-cutting stainless steel bars, which can avoid the occurrence of problems such as split steel piles and roller wrapping.

[0004] To achieve the above object, the present invention adopts the following technical solutions:

[0005] ⑴Ingot solidification

[0006] If the diameter of the bar (Φ) is not more than 150mm, a 1.7t ingot is used; if the diameter of the bar (Φ) is between 150mm and not more than 200mm, a 3t ingot is used; if the diameter of the bar (Φ) is between 200mm and not more than 250mm, a 5.2t ingot is used.

[0007] ⑵Steel ingot transmission

[0008] ① Give priority to high temperature red delivery;

[0009] ② If the temperature at a position 200mm away from the big end of the ingot body is not lower than 750℃, it is considered as hot delivery, otherwise it is heated as a cold ingot.

[0010] ⑶ Heating furnace capacity

[0011] ① For 1.7t ingots, the loading quantity shall not exceed 12 ingots per furnace. Regardless of whether the furnace is combined with other varieties, the total amount shall not exceed 21t.

[0012] ② For 3t ingots, the loading quantity shall not exceed 10 ingots per furnace. If combined with ingots less than 3t, the total loading quantity shall not exceed 21t; if combined with ingots not less than 3t, the total loading quantity shall not exceed 33t;

[0013] ③ For 5.2t ingots, the loading quantity shall not exceed 8 ingots per furnace. When combined with ingots less than 3t, the total loading quantity shall not exceed 21t; when combined with ingots not less than 3t, the total loading quantity shall not exceed 33t;

[0014] (4) Furnace loading method

[0015] ①The steel ingot cannot block the burner;

[0016] ②The steel ingot is loaded into the furnace for heating with the tail facing upwards;

[0017] ③ During the process of unloading, the air flow rate is controlled at no more than 2500m 3 / h.

[0018] ⑸ Red ingot heating process

[0019] ①After loading the furnace, heat it to a surface temperature of 1150℃±10℃ at a rate of no more than 200℃ / h and keep it warm for 0.5h;

[0020] ② Raise the temperature to 1210℃±10℃ at a rate not exceeding 200℃ / h;

[0021] ③ For 1.7t ingots, they are allowed to be taken out of the furnace after the ingot temperature is kept at least 3.5h; for 3t ingots, they are allowed to be taken out of the furnace after the ingot temperature is kept at least 4.5h; for 5.2t ingots, they are allowed to be taken out of the furnace after the ingot temperature is kept at least 5.5h.

[0022] ⑹Cold ingot heating process

[0023] ① Load the material into the heating furnace with the temperature of the ingot not exceeding 150℃;

[0024] ② Raise the temperature to 850℃±10℃ at a rate not exceeding 200℃ / h and keep warm for 0.5h;

[0025] ③ Raise the temperature to 1150℃±10℃ at a rate not exceeding 200℃ / h and keep warm for 1h;

[0026] ④ Raise the temperature to 1210℃±10℃ at a rate not exceeding 200℃ / h;

[0027] ⑤ 1.7t ingots are allowed to be taken out of the furnace after being kept at ingot temperature for not less than 4 hours; 3-ton ingots are allowed to be taken out of the furnace after being kept at ingot temperature for not less than 5 hours; 5.2t ingots are allowed to be taken out of the furnace after being kept at ingot temperature for not less than 6 hours.

[0028] ⑺Rolling process

[0029] The steel is rolled in two stages: the first stage is rolled in an 850 mill; the second stage is melted and re-fired; the third stage is rolled in a 750 mill.

[0030] The first step is 850 rolling mill:

[0031] ① Before rolling, use at least 3 other non-cracking varieties to turn off the roller cooling water for rolling to preheat the rollers;

[0032] ② During the formal rolling process, the cooling water is turned off and rolling is carried out. 50% of the water can be used for cooling when there is no steel gap.

[0033] The second step is to return to the furnace and burn again:

[0034] ①850 rolling mill rolls the billet into 380mm×400mm shape and then returns it to the furnace for re-firing. The re-firing is controlled at a surface temperature of 1240℃±10℃ and the holding time is 2.5h~3h before it is allowed to be taken out of the furnace;

[0035] ② Return the ingot to the furnace with the tail facing upwards.

[0036] The third step is 750 rolling mill:

[0037] ①The tail is not cut before rolling;

[0038] ② Before rolling, use at least 3 other non-cracking varieties to turn off the cooling water of the rolls for rolling to preheat the rolls;

[0039] ③ During the formal rolling process, the cooling water is turned off and 50% of the water can be used for cooling when there is no gap between the steel bars;

[0040] ④Measure the inlet temperature before 750 rolling, the temperature must be no less than 940℃. If the temperature is not satisfied, stop rolling and change the semi-finished product to slow cooling after it is off the line;

[0041] ⑤750 After rolling 6 steel bars, the furnace must be stopped and kept warm for more than 0.5 hours before circulating out of the furnace.

[0042] ⑻Steel cooling process

[0043] ① The temperature of the steel should not be lower than 450℃ and it should be put into slow cooling pit for not less than 30h, or it can be put into annealing furnace after hot-pressing.

[0044] Analysis of the invention points of the present invention:

[0045] ①Control of furnace loading

[0046] By strictly controlling the loading amount of each ingot type, the uniform burning of the ingots is guaranteed, and the phenomenon of ingots not burning through and uneven temperature caused by excessive loading is avoided.

[0047] ②Choice of furnace loading method

[0048] Because the tail of the ingot bites into the rolling mill first during rolling, the temperature uniformity of the tail of the ingot is ensured by loading the furnace with the tail of the ingot facing upward.

[0049] ③Control of ingot heating temperature

[0050] Set a reasonable ingot temperature to avoid overheating and melting of sulfides, and also avoid splitting caused by insufficient temperature and poor rolling plasticity.

[0051] ④ Optimization of rolling system

[0052] The use of two-fire rolling on the re-line can timely reheat the steel billet to avoid the decrease in plasticity caused by temperature drop and the interruption of the rolling process; it can better ensure the rolling smoothness of the 750 continuous rolling mill and prevent the occurrence of steel piling accidents that are difficult to handle.

[0053] Compared with the prior art, the present invention has the following advantages:

[0054] This method strictly controls the heating and rolling processes, ensuring that semi-continuous rolling of easy-to-cut stainless steel bars is 100% free of head-wrapping accidents. It can also eliminate the problem of steel piling up of strong and difficult-to-cut stainless steel, reduce the failure rate of the main machine, and improve the product yield rate to ensure contract delivery. BRIEF DESCRIPTION OF THE DRAWINGS

[0055] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0056] Figure 1 This is a photo of the surface condition of steel rolled using the existing rolling method, showing a serious problem of steel pile-up.

[0057] Figure 2 This is a surface photo of a Φ150mm steel bar rolled using the method of the present invention;

[0058] Figure 3 This is a surface photograph of a Φ120mm steel bar rolled using the method of the present invention. DETAILED DESCRIPTION

[0059] According to the above technical solution, two preferred embodiments are further described in detail.

[0060] Example 1

[0061] Rolled stainless steel 416, Φ150mm steel bar.

[0062] Adopt 1.7 ton ingot;

[0063] The steel ingots are sent to the rolling mill with the mold, and the ingot temperature is 852℃.

[0064] The steel ingots were heated by the following process: a furnace pit was prepared at 880°C in advance; 12 ingots were loaded into the furnace, and the temperature was raised at a rate of 150°C / h to a surface temperature of 1150°C, and kept at that temperature for 0.5h; the temperature was raised at a rate of 200°C / h to an ingot temperature of 1220°C, and kept at that temperature for 4h before being taken out of the furnace;

[0065] Rolling is divided into three steps.

[0066] In the first step, the 850 rolling mill is water-tight rolled to 380mm×400mm;

[0067] The second step is to return to the furnace and re-fire, with the surface temperature controlled at 1240℃±10℃ and the holding time being 3h;

[0068] The third step is rolling at 750℃ with closed water, inlet temperature 1040℃, and keeping warm for 0.5h after rolling 6 pieces.

[0069] After rolling, the steel is pit-cooled for 30 hours and then annealed. After straightening, magnetic flux leakage and ultrasonic flaw detection are carried out. The surface quality and internal quality meet the flaw detection requirements (such as Figure 2 shown).

[0070] Example 2

[0071] Rolled stainless steel Y1Cr13, Φ125mm steel bar.

[0072] Adopt 1.7 ton ingot;

[0073] The steel ingots are sent to the rolling mill with the mold, and the ingot temperature is 705℃;

[0074] The steel ingots are heated by the following process: prepare a 750℃ furnace pit in advance; load 12 furnaces, heat up to the surface temperature of 1150℃ at a rate of 150℃ / h, and keep warm for 0.5h; heat up to the ingot temperature of 1220℃ at a rate of 200℃ / h, keep warm for 4h and then take out of the furnace.

[0075] Rolling is divided into three steps.

[0076] In the first step, the 850 rolling mill is water-tight rolled to 380mm×400mm;

[0077] The second step is to return to the furnace and re-fire, with the surface temperature controlled at 1240℃±10℃ and the holding time being 3h;

[0078] The third step is closed water rolling on the 750 rolling mill with an inlet temperature of 1040℃, and keeping warm for 0.5h after rolling 6 pieces.

[0079] After rolling, the steel is pit-cooled for 30 hours and then annealed. After straightening, magnetic flux leakage and ultrasonic flaw detection are carried out. The surface quality and internal quality meet the flaw detection requirements (such as Figure 3 shown).

Claims

1. A semi-continuous heating and rolling process for free-cutting stainless steel bars, characterized in that: The heating: ⑴Steel ingot transmission: ①High temperature red feeding is preferred, and the temperature at a position 200mm away from the big end of the ingot body is not less than 750℃; ② The temperature at a position 200mm away from the big end of the ingot is lower than 750℃ and is heated as a cold ingot; ⑵ Heating furnace capacity; ① The diameter of the bar is not more than 150mm, and the ingot is 1.7t. The loading quantity is not more than 12 pieces per furnace. Regardless of whether it is combined with other varieties, the total amount is not more than 21t. ② For bars with a diameter of 150mm to no more than 200mm, 3t ingots shall be used, with a loading capacity of no more than 10 ingots per furnace. When combined with ingots less than 3t, the total loading capacity shall not exceed 21t; when combined with ingots no less than 3t, the total loading capacity shall not exceed 33t. ③ The diameter of the bar is between 200mm and not more than 250mm, and the ingot is 5.2t. The loading capacity is not more than 8 ingots per furnace. When combined with ingots less than 3t, the total amount is not more than 21t; when combined with ingots not less than 3t, the total amount is not more than 33t; ⑶ Furnace loading method: ①The steel ingot cannot block the burner; ②The steel ingot is loaded into the furnace for heating with the tail facing upwards; ③ During the process of unloading, the air flow rate is controlled at no more than 2500m 3 / h; ⑷ Red ingot heating process: ①After loading the furnace, heat it to a surface temperature of 1150℃±10℃ at a rate of no more than 200℃ / h and keep it warm for 0.5h; ② Raise the temperature to 1210℃±10℃ at a rate not exceeding 200℃ / h; ③ For 1.7t ingots, they can be taken out of the furnace after the ingot temperature is kept at a temperature of not less than 3.5h; for 3t ingots, they can be taken out of the furnace after the ingot temperature is kept at a temperature of not less than 4.5h; for 5.2t ingots, they can be taken out of the furnace after the ingot temperature is kept at a temperature of not less than 5.5h; ⑸Cold ingot heating process ① Load the material into the heating furnace with the temperature of the ingot not exceeding 150℃; ② Raise the temperature to 850℃±10℃ at a rate not exceeding 200℃ / h and keep warm for 0.5h; ③ Raise the temperature to 1150℃±10℃ at a rate not exceeding 200℃ / h and keep warm for 1h; ④ Raise the temperature to 1210℃±10℃ at a rate not exceeding 200℃ / h; ⑤ For 1.7t ingots, they can be taken out of the furnace after being kept at the ingot temperature for not less than 4 hours; for 3t ingots, they can be taken out of the furnace after being kept at the ingot temperature for not less than 5 hours; for 5.2t ingots, they can be taken out of the furnace after being kept at the ingot temperature for not less than 6 hours; The rolling process: Two-step rolling: the first step is 850 rolling mill, the second step is re-melting and re-firing, and the third step is 750 rolling mill; The first step is 850 rolling mill: ① Before rolling, use at least 3 other steel ingots that are not easy to crack, turn off the cooling water of the rolls and roll to preheat the rolls; ② During the formal rolling process, the cooling water is turned off and the gap without steel can be cooled with 50% water; The second step is to return to the furnace and burn again: ①850 rolling mill rolls the billet into 380mm×400mm shape and then returns it to the furnace for re-firing. The re-firing is controlled at a surface temperature of 1240℃±10℃ and the holding time is 2.5h~3h before it is allowed to be taken out of the furnace; ② Return the ingot to the furnace and load it with the tail of the ingot facing upwards; The third step is 750 rolling mill: ①The tail is not cut before rolling; ② Before rolling, use at least 3 other non-crackable steel billets and turn off the cooling water of the rolls to preheat the rolls; ③ During the formal rolling process, the cooling water is turned off and 50% of the water can be used for cooling when there is no gap between the steel bars; ④ Before the formal rolling of the 750 rolling mill, measure the inlet temperature, which must be no less than 940°C. If the temperature is not high enough, stop rolling and change the semi-finished product to slow cooling after it is taken off the line; ⑤After 750 rolling mill rolls 6 billets, it must stop to take out the furnace, keep warm for more than 0.5h, and then cycle out of the furnace; ⑥ The temperature of the steel should not be lower than 450℃ and it should be put into slow cooling pit for not less than 30h, or it can be put into annealing furnace after hot-pressing.

2. A semi-continuous heating and rolling process for free-cutting stainless steel bars according to claim 1, characterized in that: Rolled stainless steel 416, Φ150mm steel bar, using 1.7 ton ingot, The heating process is as follows: the steel ingot is sent to the rolling mill with the mold, and the ingot temperature is 705℃; the steel ingot is heated as follows: a furnace pit at 880℃ is prepared in advance; 12 furnaces are loaded, and the temperature is increased at a rate of 150℃ / h to a surface temperature of 1150℃, and kept at this temperature for 0.5h; the temperature is increased at a rate of 200℃ / h to an ingot temperature of 1220℃, and kept at this temperature for 4h before being taken out of the furnace; The rolling: The first step is to use 850 mill to roll the steel sheet to 380mm×400mm in closed water; The second step is to return to the furnace and re-fire, with the surface temperature controlled at 1240℃±10℃ and the holding time being 3h; The third step is 750 closed water rolling, the inlet temperature is 1040℃, and the temperature is kept for 0.5h after rolling 6 pieces; After rolling, the steel is pit cooled for 30 hours and then annealed.

3. The semi-continuous heating and rolling process for free-cutting stainless steel bars according to claim 1, characterized in that: Rolled stainless steel Y1 Cr13, Φ125mm steel bar, using 1.7 ton ingot, The heating process is as follows: the steel ingot is sent to the rolling mill with the mold, and the ingot temperature is 852°C; the steel ingot is heated as follows: a 750°C furnace pit is prepared in advance; 12 furnaces are loaded, and the temperature is increased at a rate of 150°C / h to a surface temperature of 1150°C, and kept at this temperature for 0.5h; the temperature is increased at a rate of 200°C / h to an ingot temperature of 1220°C, and kept at this temperature for 4h before being taken out of the furnace; The rolling: The first step is to use 850 mill to roll the steel sheet to 380mm×400mm in closed water; The second step is to return to the furnace and re-fire, with the surface temperature controlled at 1240℃±10℃ and the holding time being 3h; The third step is closed water rolling in 750 rolling mill, with inlet temperature of 1040℃, and heat preservation for 0.5h after rolling 6 pieces; After rolling, the steel is pit cooled for 30 hours and then annealed.

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