JP4551428B2 - Method to improve surface hardness of martensitic stainless steel - Google Patents

Description

The present invention relates to a method for improving the surface hardness of martensitic stainless steel, and in particular, an oxide film of stainless steel is reduced by hydrogen in a mixed gas, and carbon atoms provided by propane are easily contained in a stainless steel substrate. By controlling the flow rate of propane to 0.05 to 0.08 liter / min, sensitization occurs due to excessive carbon atoms forming chromium and carbides, and the corrosion resistance of stainless steel is affected. The present invention relates to a method for improving the surface hardness of martensitic stainless steel by carburizing, which prevents receiving.

  In the prior art, when screws are screwed onto a steel plate, drilling is first performed using a drill, and then a tapping device is used to cut a thread and screwing the screws there is very inconvenient. . Thereafter, a self-drilling screw 5 as shown in FIG. 1 was devised. The self-drilling screw 5 is first drilled in the steel plate by a drill part 51 provided at the end, and then the self-drilling screw 5 is screwed directly into the hole drilled by the drill part 51 by the thread 52 part. It is convenient because it is worn, and it is widely used in construction and woodworking.

  The self-drilling screw 5 is generally manufactured from aluminum alloy, alloy steel, stainless steel or the like, and materials often used in the self-drilling screw 5 made of stainless steel include an austenite type and a martensite type. Since the austenite system cannot change the metal structure by the heat treatment technique, the hardness cannot be improved by the heat treatment. In the martensitic system, the metal structure can be changed by a heat treatment technique, so that the hardness of the martensitic stainless steel can be improved. Therefore, when used in a steel structure, the hardness of the steel is high, so it is necessary to select the self-drilling screw 5 made of martensitic stainless steel, and AISI 410 is usually selected from the viewpoint of hardness, toughness, corrosion resistance, and the like. The elemental components are shown below.

However, when austenitizing and quenching in the heat treatment technology for the martensite system, it reacts with oxygen in the air and the surface turns black and loses brightness, so it is necessary to add a protective atmosphere by adding hydrogen, Since hydrogen burns with oxygen at the furnace port, oxygen can be prevented from entering the furnace. 2 and 3 show a heat treatment method according to the prior art, which includes the following steps.

  Material input process 11: Stainless steel is charged into the continuous furnace 2.

  Heating step 12: The stainless steel charged in the material charging step 11 is heated by the heating furnace 21 of the continuous furnace 2. The heating furnace 21 is divided into a preheating region 211 and an austenitizing region 212, and the temperatures of the preheating region 211 and the austenitizing region 212 are controlled and adjusted independently.

  Decomposition gas injection process 13: When the temperature in the heating furnace 21 in the heating process 12 reaches 500 ° C. or more, the decomposition gas is input into the continuous furnace 2. The cracked gas is generated by the following process. Ammonia charging step 16: Ammonia 22 is charged into the shift furnace 23. Decomposition step 17: The ammonia 22 charged in the ammonia charging step 16 is decomposed into nitrogen and hydrogen by the shift furnace 23. Drying step 18: The gaseous moisture and undecomposed ammonia 22 decomposed in the decomposition step 17 are removed by a molecular sieve in the dryer 24.

  Cooling step 14: The stainless steel heated to austenite in the heating step 13 is rapidly cooled in the cooling furnace 25 of the continuous furnace 2. A downwardly inclined region 251 is provided at the rear portion of the cooling furnace 25.

  Nitrogen addition step 19: After the liquid nitrogen 26 is evaporated into a gas by the evaporator 27, it is sent to the cooling furnace 25 and the downward inclined region 251 in the cooling step 14.

  Extraction step 15: The stainless steel after the cooling step 14 is sent out or subjected to other heat treatment such as tempering.

  In the above-described bright heat treatment method of the prior art, ammonia 22 is decomposed by the shift furnace 23 to generate hydrogen and nitrogen decomposition gas, and the decomposition gas moisture and undecomposed ammonia are removed by the molecular sieve provided in the dryer 24. Remove and prevent moisture and undecomposed ammonia 22 from contacting the stainless steel under high temperature and the stainless steel from losing shine. Since the cracked gas is introduced after the temperature of the heating furnace 21 reaches 500 ° C. or higher, hydrogen in the cracked gas is self-ignited, and as the cracked gas fills the heating furnace 23, the flame is charged with the material of the heating furnace 23. It moves in the direction of the portion, and finally a frame curtain is generated at the material inlet portion of the continuous furnace 2 to prevent external oxygen from entering the continuous furnace 2, thereby achieving a glitter effect. Further, nitrogen is injected into the cooling furnace 25 and the downward inclined region 28, and the outlet portion is inclined downward, so that oxygen entry can be further prevented.

  However, since the surface hardness of the martensitic stainless steel after heat treatment is usually 490 to 500 Hv and the hardness of the center is 410 to 430, the self-drilling screw 5 of the martensitic stainless steel is 3 mm relative to the AISI 304 steel plate. When drilling, good surface drilling cannot be performed due to insufficient surface hardness.

  Patent Document 1 describes a method for producing a self-drilling tapping screw composed of austenitic stainless steel and low carbon steel or low carbon alloy steel. The base material is low carbon steel or low carbon alloy steel, and is previously forged or cut by cutting. A cylinder containing the shoulder and threadless handle or drill is formed, and after carburizing and appropriate heat treatment, it is brazed to a threadless austenitic stainless steel cylinder with a pre-formed head and then brazed. After the burr of the part and the shoulder of the carburized steel cylinder are cut, a non-carburized hardened area of appropriate width is formed, and finally the rolling screw machining and the complete quenching process are performed, and the handle with the head and the thread The self-drilling tack is made of austenitic stainless steel and the self-tapping thread and drill are low-carbon steel or low-carbon alloy. Ring screw is manufactured.

  The above-mentioned “manufacturing method of self-drilling tapping screw made of austenitic stainless steel and low carbon steel or low carbon alloy steel” is a self-tapping screw thread and drill part made of low carbon steel or low carbon alloy steel by brazing. And an austenitic head and a handle having a thread are joined to each other and can be used for a thick steel plate.

However, self-drilling tapping screws with a head and threaded handle fastening part made of austenitic stainless steel and self-tapping screw thread and drilling parts made of low-carbon steel or low-carbon alloy material are very complicated to manufacture. Therefore, the manufacturing cost increases and the production rate decreases. Further, since the manufacturing process is complicated, manufacturing errors are likely to occur accordingly, and the defective product rate is increased. Moreover, since the portion of the low-carbon steel or low-carbon alloy steel is not stainless steel, the corrosion resistance of the low-carbon steel or low-carbon alloy steel portion is inferior and the service life of the screw is reduced.
Taiwan Patent Publication No. 205072 JP 2005-533185 A

  An object of the present invention is to control the amount of carburization without affecting the toughness and corrosion resistance of the martensitic stainless steel and to improve the surface hardness of the martensitic stainless steel to the maximum extent.

In order to solve the above-described problems, the present invention provides a method for improving the hardness of the martensitic stainless steel surface, and adds a gas mixing step and a propane charging step after the drying step.

Propane charging step: Propane is charged into the gas mixer, and the flow rate of propane is controlled to 0.05 to 0.08 liter / min by a micro flow meter.

Gas mixing step: The gas after the drying step and propane are mixed by a gas mixer and then sent together into a heating furnace.

The hydrogen in the gas mixture reduces the oxide film of the stainless steel so that the carbon atoms provided by the propane can easily enter the stainless steel substrate, and the flow rate of propane is 0.05 to 0.08 liter / min. By controlling the amount of carbon steel, it is possible to prevent excessive carbon atoms from forming chrome and carbides to cause sensitization to affect the corrosion resistance of the stainless steel, and to improve the surface hardness of the martensitic stainless steel by carburizing.

The surface hardness of the stainless steel can be greatly improved by the treatment method of the present invention, and the toughness can be maintained because there is little change in the hardness at the center. Moreover, there is no rusting even if it is immersed in pure water for 24 hours. Therefore, the stainless steel after the treatment according to the present invention can be provided with excellent hardness, toughness and corrosion resistance, and the self-drilling screw of the present invention can perform 3-4 drilling in a 3 mm thick AISI 304 steel plate. It is possible to improve the practicality of the self-drilling screw of martensitic stainless steel.

Examples illustrating the objects, features, and effects of the present invention will be described in detail with reference to the drawings.

The present invention adds a gas mixing step and a propane charging step after the drying step, and includes the following steps as shown in FIGS.

Material input process 31: Stainless steel is charged into the continuous furnace 4.

  Heating step 32: The stainless steel charged in the material charging step 31 is heated by the heating furnace 41 of the continuous furnace 4. The heating furnace 41 is divided into a preheating region 411 and an austenitizing region 412, and the temperatures of the preheating region 411 and the austenitizing region 412 are controlled and adjusted independently. Moreover, the temperature of the preheating area | region 411 is set to 920 degreeC-1000 degreeC, the temperature of the austenitization area | region 412 is set to 1020 degreeC-1080 degreeC, and the austenitization time of stainless steel is 30 minutes.

Mixed gas charging step 33: When the temperature in the heating furnace 41 in the heating step 32 reaches 500 ° C. or higher, the mixed gas is charged into the continuous furnace 4. The mixed gas is generated by the following steps. Ammonia charging step 36: Ammonia 42 is charged into the shift furnace 43. Decomposition step 37: The ammonia 42 introduced in the ammonia introduction step 36 is decomposed into nitrogen and hydrogen by the shift furnace 43. The temperature of the shift furnace 43 is set to 920 ° C to 980 ° C. Drying step 38: The gas moisture and undecomposed ammonia 42 decomposed in the decomposition step 37 are removed with a molecular sieve in the dryer 44. The dew point of the decomposition gas after removing moisture by the dryer 44 is −50 ° C. or less, and a flow meter 441 is provided at the rear of the dryer 44, and the flow rate of the decomposition gas is 5-6 m ³ / Control to min. Propane charging step 310: propane 46 is charged into the gas mixer 47, and a micro flow meter 461 is provided in front of the gas mixer, and the micro flow meter 461 adjusts the flow rate of the propane 46 to 0.05 to 0.08 liter. / Min. Gas mixing step 39: The cracked gas after the drying step 38 and the propane 46 charged in the propane charging step 310 are mixed by the gas mixer 47 and then sent together into the continuous furnace 4.

  Cooling step 34: The stainless steel heated to austenite in the heating step 32 is rapidly cooled in the cooling furnace 45 of the continuous furnace 4. A downwardly inclined region 451 is provided at the rear portion of the cooling furnace 45. The cooling furnace 45 is immersed in a water tank, and the water temperature of the water tank is maintained at room temperature by an outdoor cold water tower.

  Nitrogen addition step 311: After the liquid nitrogen 48 is vaporized by the evaporator 49, it is sent to the cooling furnace 45 and the downward inclined region 451 of the cooling step 34.

  Extraction step 35: The stainless steel after the cooling step 34 is sent out or other heat treatment such as tempering is performed.

  In the present invention, the temperature of the shift furnace 43 is set to 920 ° C. to 980 ° C. to decompose the ammonia 42 to generate hydrogen and nitrogen decomposition gas, and the molecular sieve provided in the dryer 44 generates the decomposition gas. Moisture and undecomposed ammonia are removed, the dew point of the decomposition gas is set to −50 ° C. or less, and moisture and undecomposed ammonia 42 are prevented from coming into contact with the stainless steel at high temperature to prevent the stainless steel from shining. In addition, the gas mixture step 39 mixes the cracked gas and the propane 46 charged in the propane charging step 310 in the gas mixer 47, and the mixed gas after the temperature of the heating furnace 41 reaches 500 ° C. or higher in the heating step 32. Is introduced into the furnace, the hydrogen in the mixed gas is self-ignited, the flame moves in the direction of the material input portion of the heating furnace 41 as the cracked gas fills the heating furnace 41, and finally the continuous furnace 4 A flame curtain is generated at the material inlet, preventing external oxygen from entering the continuous furnace 4, and a glitter effect is achieved. Further, the oxide film of the stainless steel is reduced by hydrogen in the mixed gas, so that the carbon atoms provided by the propane 46 introduced in the present invention can easily enter the stainless steel base material and the carburizing time is shortened. By controlling the flow rate of propane 46 to 0.05 to 0.08 liter / min, excessive carbon atoms form chromium and carbides to cause sensitization and prevent the corrosion resistance of stainless steel from being affected. To do. Therefore, the present invention not only has the glitter holding function in the prior art, but also achieves the carburizing effect by the above-described method, and improves the surface hardness of the martensitic stainless steel.

  The table below shows the results of drilling and corrosion resistance tests on 3 mm AISI 304 steel sheets of self-drilled screws processed by the method of the present invention and self-drilled screws processed by the prior art.

  As can be seen from the above figure, the surface hardness of the stainless steel can be greatly improved by the treatment method of the present invention, and the toughness can be maintained because there is little change in the hardness at the center. Moreover, there is no rusting even if it is immersed in pure water for 24 hours. Therefore, the stainless steel after the treatment process according to the present invention has excellent hardness, toughness and corrosion resistance. The self-drilling screw according to the present invention can perform 3-4 drilling in a 3 mm thick AISI 304 steel plate, and can improve the practicality of the self-drilling screw of martensitic stainless steel.

  As described above, the present invention can solve the problems that cannot be solved by the prior art. The present invention is an unpublished technology, has an inventive step and practicality, and satisfies the requirements for patent application.

It is a side view which shows a self-drilling screw. It is a block diagram which shows the heat processing process by a prior art. It is a schematic diagram which shows the heat processing process by a prior art. It is a block diagram which shows the heat processing process by this invention. It is a schematic diagram which shows the heat processing process by this invention.

Explanation of symbols

4 Continuous furnace 41 Heating furnace 411 Preheating area 412 Austenitizing area 42 Ammonia 43 Transformation furnace 44 Dryer 441 Flow meter 45 Cooling furnace 451 Downward inclined area 46 Propane 461 Micro flow meter 47 Gas mixer 48 Liquid nitrogen 49 Evaporator

Claims (5)

A material charging process for charging stainless steel into a continuous furnace;
A heating step of heating the stainless steel charged in the material charging step by a heating furnace of a continuous furnace divided into a preheating region and an austenitizing region;
An ammonia charging process for charging ammonia into the shift furnace;
A decomposition step of decomposing ammonia input in the ammonia input step into nitrogen and hydrogen by a shift furnace;
A drying step of removing gaseous moisture and undecomposed ammonia decomposed in the decomposition step with a molecular sieve in a dryer;
A propane charging step of charging propane into a gas mixer for mixing with the cracked gas after the drying step;
A gas mixing step of mixing the cracked gas after the drying step and the propane charged in the propane charging step with a gas mixer;
When the temperature in the heating furnace in the heating step reaches 500 ° C. or higher, a mixed gas charging step of charging the mixed gas into a continuous furnace;
In the heating step, the stainless steel material heated to austenite is followed by the austenite region, and a cooling step for rapid cooling in a continuous furnace cooling furnace provided with a downwardly inclined region on the outlet side,
A nitrogen addition step of evaporating liquid nitrogen into a gas by an evaporator and then sending it to the cooling furnace and a downwardly inclined region;
Taking out the stainless steel material after the cooling step or performing a tempering heat treatment,
A method for improving the surface hardness of martensitic stainless steel, comprising: The temperature of the preheating region of the heating furnace in the heating step is set to 920 ° C. to 1000 ° C., the temperature of the austenitizing region of the heating furnace in the heating step is set to 1020 ° C. to 1080 ° C., and the austenitizing time of the stainless steel The method for improving the surface hardness of martensitic stainless steel according to claim 1, wherein is 30 minutes. The temperature of the shift furnace in the decomposition step of decomposing ammonia into nitrogen and hydrogen is set to 920 ° C. to 980 ° C., and the dew point of the decomposition gas after the moisture is removed by the dryer in the drying step of the gas generated in the decomposition is 2. The temperature is −50 ° C. or lower, and a flow meter is provided behind the dryer in the drying step, and the flow meter controls the flow rate of the decomposition gas to 5 to 6 m ³ / min. To improve the surface hardness of martensitic stainless steel.   The marten according to claim 1, wherein a micro flow meter is provided in front of the gas mixer in the propane charging step, and the micro flow meter controls the flow rate of propane to 0.05 to 0.08 liter / min. A method to improve the surface hardness of site-based stainless steel.   The surface hardness of the martensitic stainless steel according to claim 1, wherein the cooling furnace in the cooling step is immersed in a water tank and cooled, and the water temperature of the water tank is maintained at room temperature by an outdoor cold water tower. How to improve.

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