JPS6364496B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for directly quenching martensitic stainless steel. Martensitic stainless steel contains a large amount of Cr, so it has extremely good hardenability, and has a wide range of dimensions that can be hardened by air cooling (natural cooling) from the austenitizing temperature. However, actual hardening treatment is often performed by oil cooling or water cooling, which has a faster cooling rate than air cooling. The reason for this is that when the cooling rate during quenching is insufficient, carbides may precipitate at austenite grain boundaries during cooling. These grain boundary carbides do not decompose and disappear at all during the subsequent tempering treatment, lowering the grain boundary strength, making intergranular fracture more likely to occur, and deteriorating toughness.
A decrease in the amount of Cr near the grain boundaries reduces the corrosion resistance of the grain boundaries and makes intergranular corrosion more likely to occur, which is harmful to the characteristics of martensitic stainless steels used as structural materials and tool materials. On the other hand, the most common test method for evaluating the toughness of materials is the Charpy impact test, and the temperature at which the brittle fracture ratio becomes 50% when the test temperature is changed is defined as the ductile-brittle transition temperature. There is a method of evaluation in which it is determined that the lower the temperature, the better the stability of toughness at the service temperature. According to such an evaluation, the ductile-brittle transition temperature of martensitic stainless steel that has been quenched and tempered using conventional methods is generally from around 0°C to around room temperature (20°C). Alternatively, the toughness of steel tends to vary significantly due to changes in operating temperature, etc. For this reason, a large safety factor is required when designing the components, and even when the conditions of use are strictly restricted, variations in service life are unavoidable. This invention has the potential to improve the toughness of steel, as the structure after hot working under appropriate conditions is much finer than the structure obtained through normal quenching. The purpose of this work was to obtain stainless steel that has a fine martensitic matrix structure without grain boundary precipitation after quenching through hot working, and has stable toughness with a low ductile-brittle transition temperature. The purpose of this invention is to eliminate the drawbacks specific to martensitic stainless steel mentioned above. This invention utilizes recrystallization induced by hot working to refine the structure, and then adjusts the cooling rate to prevent grain boundary precipitation of carbides, thereby improving ductility.
It is designed to obtain martensitic stainless steel with a low brittle transition temperature. Martensitic stainless steel is hot worked, and the processing is completed at a temperature higher than the recrystallization start temperature, and the heat is kept until the recrystallization is completed. (including heating), then slow cooling or a combination of heat retention and slow cooling, cooling the carbide precipitation temperature range at a rate that avoids the carbide precipitation, and cooling to room temperature to cause martensitic transformation. It is characterized by producing a stainless steel with a fine martensitic base structure without grain boundary precipitation and a low ductile-brittle transition temperature. The martensitic stainless steel referred to in this invention has a γ phase that is stable at high temperatures and has a martensite structure at room temperature, for example, has a Cr content of 10% by weight or more and a C content of 0.1 to 1.2% by weight. %
or, if necessary, other alloying elements (corrosion resistance improving elements such as Ni, Mo, Cu, W,
Strength-improving elements such as V and Si, free-cutting improving elements such as S, Se, Pb, and Ca, and other elements are added. The heating temperature when hot working such martensitic stainless steel is preferably 1100~1100℃.
The temperature should be around 1250℃. In this case, if the heating temperature is too low, sufficient solid solution of the carbide will not be achieved, making it difficult to ensure the finishing temperature described later, and hot workability will also decrease. On the other hand, if the heating temperature is too high, δ-ferrite increases rapidly, reducing hot workability and performance of the steel, and coarsening of crystal grains becomes significant, resulting in finer structure due to hot working. inhibited. When performing hot working such as rolling or forging on martensitic stainless steel heated to a predetermined hot working temperature, the processing rate (reduction rate, forging rate)
is preferably 50% or more. This is because if the processing rate is too small, the structure will not be sufficiently refined by processing, and subsequent recrystallization will be insufficient. Such processing is completed at a temperature equal to or higher than the recrystallization start temperature, and the material is kept heated or slowly cooled until the recrystallization is completed. The finishing temperature in this case is preferably in the range of about 950 to 1050°C. This is because if the working end temperature is too low, recrystallization after hot working will be insufficient, and if it is too high, it will be difficult to refine the material after recrystallization. Thereafter, it is kept heated until recrystallization is completed, and then slowly cooled. When a combination of heat preservation and slow cooling is used, for example, it is cooled to 900° C. by a combination of heat preservation (including heating) and slow cooling, or by only slow cooling. In this case, slow cooling is carried out to sufficiently cause recrystallization after hot working to create a fine grained structure. However, if the slow cooling temperature is too low, the grain boundary precipitation temperature range may be reached, or the austenite grain boundaries may form. Since there is a possibility that carbides will precipitate, the temperature is preferably 850° C. or higher and appropriately selected. Further, heat retention or slow cooling may be performed by allowing the material to cool when the dimensions of the quenched material are sufficient. Next, in the carbide precipitation temperature range, the cooling rate is adjusted so that precipitation of the carbide is avoided. In other words, the nose at which carbides precipitate at austenite grain boundaries (the temperature at which grain boundary precipitation begins in the shortest time during cooling) is approximately 750°C, and cooling slower than the cooling rate that reaches this nose causes grain boundary precipitation to occur. become noticeable. Therefore, for example, from 750°C + 150°C = 900°C to 750°C - 150°C = 600°C, it is preferable to perform rapid cooling, blast cooling, or air cooling at a cooling rate that does not reach the nose. Next, below the precipitation temperature range of the carbide, for example,
In the temperature range below 600°C, a complete martensitic structure can be obtained unless the bainite transformation region is reached, so in practice it is sufficient to let it cool naturally. There is no problem even with wind chill. The drawing shows an example of a preferable cooling curve in the direct quenching method of the present invention, where K is the grain boundary carbide precipitation initiation curve, N is the nose, that is, the shortest time position of the curve K, P is the pearlite transformation region, and B is the The bainitic transformation region, M is the martensitic transformation region, M _S is the martensitic transformation start line, and the line ← is the constant cooling range for quenching without grain boundary precipitation, and the line ← is the grain that can be quenched. The constant velocity cooling range in which boundary precipitation occurs, and the line indicates a cooling curve according to an example of implementing the present invention. The cooling curve shown by the line is
Hot working end temperature is 1000℃, then 900℃
The temperature range of 900 to 600℃ is rapidly cooled to prevent carbide precipitation at austenite grain boundaries, and the line a is quenched from 600℃ to room temperature to cause martensitic transformation. , line b
This shows the case where martensitic transformation is caused by cooling from 600°C or higher to room temperature. By doing so, it is possible to obtain a stainless steel having a fine martensitic base structure without grain boundary precipitation and a low ductile-brittle transition temperature. Example 1 Steel having the chemical composition shown in Table 1 was melted in a 50 kg vacuum induction furnace, and then a billet was prepared.The billet was heated to the heating temperature shown in Table 2, and hot rolling was started from this temperature. After rolling was carried out under the conditions of a rolling dimension of 30 mmφ and a rolling reduction of 65%, and the rolling was completed at the hot rolling end temperature shown in Table 2, it was cooled by the cooling method shown in Table 2. After tempering, the austenite grain size number and ductile-brittle transition temperature were measured. The results are also shown in Table 2. In addition, when comparative method 1 is slowly cooled between 900 and 600℃, which includes the carbide precipitation temperature range, comparative methods 2 and 3
The case of rapid cooling after hot rolling was also investigated. The results are also shown in Table 2. Furthermore, as conventional methods 1 to 4, cases were also investigated in which the steel was cooled once after completion of hot rolling, and then reheated to perform quenching and tempering. The results are also shown in Table 2.

【table】

【table】

[Table] As shown in Table 2, in the case of methods 1 to 23 of the present invention, the austenite crystal grain size is very fine with No. 8.5 to 9.1, while the crystal grain size of conventional methods 1 to 4 is No. 8.5 to 9.1. It is significantly better than 5.2 to 6.3, and it is clear that the ductile-brittle transition temperature is lowered by about 30°C or more, and at the same time, the variation is also reduced. On the other hand, in Comparative Method 1, slight grain boundary precipitation occurs due to slow cooling in the carbide precipitation temperature range, and the ductile-brittle transition temperature is considerably high because the refinement effect is not exhibited. In addition, in Comparative Methods 2 and 3, recrystallization after processing is incomplete due to rapid cooling after completion of hot rolling, and although there is a grain refining effect, it is small.
The ductile-brittle transition temperature is not reduced by the process of the present invention. In addition, with the C content and rolling method of the martensitic stainless steel used in this example, even when the carbide precipitation temperature range was allowed to cool without rapid cooling, there was no grain boundary precipitation of carbides, and good results were obtained. Obtainable. Similar good results were also obtained when cooling up to 900°C using a combination of heat retention and slow cooling. Example 2 Steel having the chemical composition shown in Table 3 was melted in a 50 kg vacuum induction furnace, and then a billet was prepared.The billet was heated to the heating temperature shown in Table 4, and hot rolling was started from this temperature. After rolling was carried out under the conditions of a rolling dimension of 30 mmφ and a rolling reduction of 70%, the rolling was completed at the hot rolling end temperature shown in Table 4, and then cooled by the cooling method shown in Table 4. After tempering, the austenite grain size number and ductile-brittle transition temperature were measured. The results are also shown in Table 4. In addition, comparative method 31 in which slow cooling was performed between 900 and 600° C., which includes the precipitation temperature range of carbides, and comparative method 32 in which rapid cooling was performed after completion of hot rolling were also investigated. The results are also shown in Table 4. Furthermore, as conventional methods 31 to 34, cases were also investigated in which the steel was cooled once after completion of hot rolling, and then reheated to perform quenching and tempering. The results are also shown in Table 4.

【table】

[Table] As shown in Table 4, in the case of methods 31 to 47 of the present invention, the austenite crystal grain size is extremely fine with No. 8.6 to 9.1, and the crystal grain size of conventional methods 31 to 34 is very fine. It is significantly better than 6.2 to 6.5, and the ductile-brittle transition temperature is also considerably lower. On the other hand, in Comparative Method 31, slight grain boundary precipitation occurs due to slow cooling in the carbide precipitation temperature range, and Comparative Method 32
In these cases, the recrystallization after working is incomplete because the hot rolling is rapidly cooled, and the ductile-brittle transition temperature is not as low as that obtained by the method of the present invention. Note that the C content of the martensitic stainless steel used in this example and the rolling method are such that the carbide precipitation temperature range is set to a cooling rate higher than blast cooling, thereby achieving a stably low ductile-brittle transition temperature. can be obtained. Similarly good results were also obtained when cooling up to 900°C using a combination of heat retention and slow cooling. In each of the above examples, as the temperature division of the cooling method, the slow cooling period until the end of recrystallization is 900°C.
Based on the above, we have shown an example where the cooling temperature range to prevent grain boundary precipitation of carbides is 900 to 600°C, and then the temperature range for cooling to room temperature is 600°C or less. It goes without saying that it can be changed as appropriate depending on the chemical composition of the martensitic stainless steel used, and that it may be desirable to change it. As explained above, according to this invention,
By making use of recrystallization induced by hot working to refine the structure, and then adjusting the cooling rate to prevent grain boundary precipitation of carbides, in the state after direct quenching, fine and grain boundary It is possible to obtain a stainless steel that has a martensitic matrix structure without precipitation and stable toughness with a low ductile-brittle transition temperature, eliminating the drawback of unstable toughness peculiar to conventional martensitic stainless steels. It has a very good effect of being able to do this.

[Brief explanation of drawings]

The drawing is an explanatory diagram showing an example of a preferable cooling curve in the direct quenching method of the present invention.

Claims (1)

[Claims] 1. Hot working martensitic stainless steel, finishing the working at a temperature higher than the recrystallization start temperature, retaining heat until recrystallization is completed, and cooling slowly or by combining heat retention and slow cooling to obtain a carbide precipitation temperature range. By cooling the steel at a rate that avoids the precipitation of carbides and then transforming it into martensitic material, it is possible to obtain a stainless steel with a fine martensitic base structure without grain boundary precipitation and a low ductile-brittle transition temperature. Direct quenching method for martensitic stainless steel.