JP3475864B2 - Solution treatment of stainless steel slab - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for heat-treating a stainless steel slab with high strength. 2. Description of the Related Art In a typical hot extrusion tube forming method, a hot hollow billet is charged into a container provided with a die at a tip, and a mandrel is inserted into a hollow portion of the billet. After inserting the billet, the billet is pushed through the annular gap between the die and the mandrel by pushing the back of the billet through the dummy block with the stem, and the seamless pipe is manufactured. [0003] At the time of extrusion, one or more of powdered glass, fiber glass, and disk glass obtained by solidifying the same are used as a lubricant, and the glass softened by the heat of the heated billet causes a gap between the billet and the tool to be formed. Lubrication is performed. Since lubrication with glass has a characteristic of extremely high lubricity, the Eugene method is used for manufacturing a seamless steel pipe made of a material having poor workability such as stainless steel. A billet used for manufacturing a seamless steel pipe by the Eugene method is, for example, using a long steel slab produced by hot rolling or hot forging from a steel ingot as a raw material, heat treating the steel slab, It is manufactured by performing machining such as cutting and cutting. The heat treatment applied during the production of billets
For imparting a predetermined structure to the steel slab produced by hot rolling or hot forging, in the case of stainless steel slab,
A solution heat treatment of rapidly cooling after heating and holding at a temperature of 950 ° C. or higher is employed. [0005] By the way, when the above-mentioned billet is manufactured,
Depending on the material and outer diameter of the steel slab, machining cannot be performed, or even though machining can be performed, the life of a machining tool may be extremely short. Especially when the billet is a large diameter stainless steel bill, even simple cutting may be difficult. [0006] It is presumed that the reason why machining of stainless steel pieces becomes difficult is hardening due to residual stress generated in the stainless steel pieces. That is, at the time of cooling in the solution heat treatment of the stainless steel piece, the internal cooling rate becomes slower than the cooling rate of the surface layer part, and a temperature difference occurs between the surface layer part and the inside of the stainless steel piece. For this reason, stress is generated inside, and residual stress is generated in the stainless steel piece after the solution treatment. In particular, when the cross-sectional area of the stainless steel piece is large, the temperature difference becomes large, and a large residual stress is generated. [0007] As a method of removing the residual stress of a stainless steel piece, a stress relief annealing is known. This stress relief annealing is carried out by selecting the temperature and time conditions which do not adversely affect the basic properties of the stainless steel piece and can remove the residual stress. For example, as a stress relieving annealing of a duplex stainless steel structure, a method of heating to 500 to 650 ° C., maintaining the temperature for a certain period of time, and then rapidly cooling is disclosed in JP-A-62-222020. Is disclosed. However, stress relief annealing is intended for a material that has been once subjected to a solution heat treatment, and usually requires several hours for the treatment, which increases the number of heat treatment steps. In order to solve this drawback, a method of performing solution heat treatment while preventing generation of residual stress is disclosed in Japanese Patent Laid-Open No. 2-23.
No. 6220 is disclosed. This method cools the stainless steel casting after heating and holding it at the solution temperature,
In this method, the temperature is gradually cooled to 800 to 900 ° C., and thereafter, the temperature is rapidly cooled. According to this method, Japanese Patent Application Laid-Open No. 62-2220
The disadvantages in JP 20 are eliminated. However, when the target stainless steel material is high Cr-high Mo, it is gradually cooled to 800 to 900 ° C. after heating and holding at the solution temperature,
Carbides and intermetallic compounds are precipitated, and the solution heat treatment is not completely performed. SUMMARY OF THE INVENTION An object of the present invention is to provide a stainless steel slab which is sufficiently solid-solution treated and capable of suppressing the generation of residual stress to such an extent that it can be easily machined. It is an object of the present invention to provide a solution heat treatment for a piece. [0012] The gist of the present invention resides in the following solution heat treatment method for stainless steel slab. [0013] When the stainless steel piece is subjected to solution heat treatment,
The temperature at which cooling by water cooling is stopped is as follows: the surface temperature of the stainless steel piece is equal to or higher than the temperature specified by the following formula, and is 600 ° C or less for austenitic stainless steel and 400 ° C or less for duplex stainless steel. A solution treatment method for stainless steel slabs, wherein the temperature range is followed by cooling at room temperature. [0014] T = 0.5 · S ^1/2 ··· formula where, T is the surface temperature of the stainless steel strip (℃) However, the cross-sectional area of S stainless steel strip lower limit to 50 ° C. (mm ² The present inventors have paid attention to the cooling process in the solution treatment of the stainless steel slab, and obtained various findings as a result of various studies. The present invention has been completed based on these findings. (1) When the billet is rapidly cooled to room temperature,
First, the surface layer is cooled to room temperature, and then the inside is cooled with a delay. Since the inside of the steel slab that is cooled with a delay shrinks when cooled, compressive residual stress is generated in the surface layer of the steel slab that has already reached room temperature and has a predetermined strength. (2) Therefore, if the surface layer of the steel slab is plastically deformed by the stress caused by shrinkage when the inside of the slab is cooled, the residual stress is reduced or eliminated. (3) In order to plastically deform the surface layer of the slab, the surface temperature at which rapid cooling is stopped may be increased to lower the strength. (4) The lower limit of the surface temperature at which rapid cooling is stopped in order to reduce the residual stress on the surface layer of the stainless steel piece to such an extent that machining can be performed is determined by the cross-sectional area of the target stainless steel piece. BEST MODE FOR CARRYING OUT THE INVENTION The stainless steel piece to be used in the present invention has a cross section of 100 to 1300 cm in size.
² is a steel slab made of austenitic stainless steel or duplex stainless steel having high strength. As high strength austenitic stainless steel, for example, ASTM
UNSNo. An austenitic stainless steel based on the chemical composition specified in S31254, and a duplex stainless steel, for example, JIS SUS329J
1, JIS SUS329J3L, JIS SUS32
9J4L, ASTM UNS No. S32740 (S3
9274), ASTM UNS No. Duplex stainless steel based on the chemical composition defined in S32760 can be used. For example, an ingot having the above chemical composition,
A solution heat treatment is performed on a stainless steel piece manufactured by subjecting a stainless steel material such as a slab to forging, rolling and the like. The heating temperature and the holding time during the solution heat treatment may be the same as those conventionally used.
In the case of IS SUS329J1, the temperature is maintained at a temperature range of 950 ° C. or more for 30 minutes or more. After heating to the above-mentioned temperature, the retained stainless steel piece is cooled. In the present invention, this cooling is first performed by water cooling to a predetermined temperature, and then allowed to cool at room temperature. The lower limit of the surface temperature of the stainless steel piece when the water cooling is stopped is a temperature that satisfies the condition defined by the following equation. [0023] T = 0.5 · S ^1/2 ··· formula where stainless steel when T is the surface temperature of the stainless steel strip (° C.) S is to stop the cross-sectional area of the stainless steel strip (mm ²⁾ water-cooled The reason why the lower limit of the surface temperature of the steel slab is defined as in the above equation is that if water cooling is performed to a temperature range lower than the equation, the residual stress cannot be sufficiently reduced and the stainless steel slab cannot be easily machined. In addition, the above formula was obtained as a result of investigating the machinability when the solution treatment was performed by variously changing the dimensions (cross-sectional area) of the stainless steel piece and the surface temperature of the stainless steel piece when water cooling was stopped. It is a thing. When the cross-sectional area of the target stainless steel piece is small, the lower limit temperature calculated by the above equation may be lower than 50 ° C. in this case,
Since there is almost no difference from the conventional method of water cooling to room temperature, the residual stress is not reduced. Therefore, the lower limit temperature calculated by the equation is 50 ° C. In order to prolong the tool life during machining, the lower limit of the surface temperature is preferably set to a temperature that satisfies the condition shown in the following equation. [0025] T = 0.7 · S ^1/2 ··· formula where stainless steel when T is the surface temperature of the stainless steel strip (° C.) S is to stop the cross-sectional area of the stainless steel strip (mm ²⁾ water-cooled The upper limit of the surface temperature of the steel material varies depending on the steel type. In the case of austenitic stainless steel, the upper limit is set to 600 ° C. in order to prevent carbide and intermetallic compounds from being precipitated. The temperature is set to 400 ° C. to prevent the embrittlement. The surface temperature at the time of stopping the water cooling is preferably controlled by a water cooling time previously obtained by an experiment for each dimension (cross-sectional area) of the billet. The water-cooling method only needs to ensure a cooling rate of at least 200 ° C./min. For example, any of an immersion method and a spray method may be used. Further, mist cooling may be used. Following water cooling to the above temperature range, the solution is left to cool at room temperature. By this cooling, the inside of the stainless steel piece is continuously cooled, and the surface layer is cooled after the temperature once rises due to the heat inside. [0029] The residual stress is reduced because the surface temperature when the water cooling is stopped is considerably higher than room temperature, so that the strength of the surface layer portion is lower than that at room temperature, and the surface layer portion is plastically deformed by internal stress. That's why. EXAMPLES In an electric furnace, steel types A (austenitic stainless steel) and steel type B (duplex stainless steel) having the chemical compositions shown in Table 1 were melted to produce 450 × 450 mm ingots. [Table 1] After heating this ingot to 1250 ° C., it was subjected to hot forging to have a cross-sectional length of 2,000
Stainless steel pieces of 0 mm or more were manufactured. The stainless steel piece was heated to 1100 ° C. and maintained for 2 hours, then immersed in water to cool the surface temperature to the water cooling stop temperature shown in Table 2, and then allowed to cool in the air. A cutting test and a cutting test were performed on the cooled stainless steel pieces. The results are shown in Table 2. [Table 2] The cutting test was carried out by using a segment saw having a saw blade diameter of 960 mm at a peripheral speed of 5.1 m / min and a feed speed of 15 mm / min.
0 mm, when one section could be completely cut without replacing the saw blade, ○: when one section could not be completely cut, ×, the life of the saw blade When the value was more than twice as large as the time, it was evaluated as ◎. In the cutting test, a carbide tip was used as a cutting tool, a peripheral speed was 55 m / min, and a cutting depth was 0.9 m.
m / rev. 500mm length with 2mm cutting allowance
When the entire surface of a single stainless steel piece could be cut without replacing the cemented carbide chip with a lathe, the stainless steel chip was worn and the carbide chip was worn out and needed to be replaced midway Was evaluated as x when occurred, and as ◎ when the life of the cemented carbide tip was twice or more that of ○. As can be seen from Table 2, The inventive examples 1 to 7 have good results in both the cutting test and the cutting test. This is because the residual stress was suppressed to the extent that cutting and cutting were possible. On the other hand, no. In Comparative Examples 8 to 13, since the water cooling stop temperature was low, the residual stress was not suppressed to the extent that machining was possible, and good results could not be obtained in both the cutting test and the cutting test. According to the present invention, when a solution of a stainless steel piece is subjected to solution heat treatment, the surface cooling water stop temperature is specified, and then the stainless steel piece is allowed to cool at room temperature. Generation of residual stress can be suppressed to such an extent that machining can be performed.

──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Honda Politics, Amagasaki City, Hyogo Prefecture No. 1, Nishinocho, Higashimujima Island Inside the Amagasaki Metal Industry Cooperative Association (56) References JP-A-3-10017 (JP, A) (58) Surveyed field (Int. Cl. ⁷ , DB name) C21D 6/00

Claims (1)

(57) [Claims] [Claim 1] When performing solution heat treatment of a stainless steel piece,
The temperature at which cooling by water cooling is stopped is such that the surface temperature of the stainless steel piece is equal to or higher than the temperature defined by the following formula, and is 600 ° C or less for austenitic stainless steel and 400 ° C or less for duplex stainless steel. A solution heat treatment method for a stainless steel piece, wherein the temperature range is followed by cooling at room temperature. T = 0.5 · S ^1/2 formula where T is the surface temperature of the stainless steel piece (° C.) However, the lower limit is 50 ° C. S is the cross-sectional area of the stainless steel piece (mm ² )