JPS648051B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing high-strength, high-toughness steel in which tempered martensite and bainite coexist while accelerating bainite transformation. I'll decide. Austempered steel and quenched and tempered steel are well known as heat-treated steel.
Comparing the two, the former has higher toughness, but when producing bainite through austempering,
If the treatment temperature is high, the material will be significantly softer than the latter, and if the treatment temperature is lowered with the intention of hardening, the holding time will be significantly increased, which will impair manufacturability. On the other hand, the latter makes it easy to obtain a high-strength material, but its toughness is inferior to the former. The present invention has been made with the aim of eliminating the drawbacks of both, and provides a method for producing high-strength, high-toughness steel with good manufacturability, which has the advantages of both while compensating for their drawbacks. . That is, the present invention uses medium-high carbon steel containing carbon in the range of 0.40 to 1.10% as a material, and austenitizes it by holding it in a temperature range of A ₃ transformation point of this steel to A ₃ transformation point + 150 ° C. After quenching _, at least
Reheating is performed from this temperature range to a temperature range of 300 to 550°C within a time period in which 10% by volume or more of untransformed austenite exists, and in this reheating temperature range, the supercooled austenite transforms to bainite and primary marten occurs. By tempering the site, base
The present invention provides a method for obtaining high-strength, high-toughness steel having a mixed composition of tempered martensite and bainite in a shortened time while accelerating the night transformation. The pulling austempering method of the present invention accelerates bainite transformation, and is therefore particularly advantageous when applied to a continuous line of steel strips or wire rods. However, it can of course also be applied to the production of single items such as semi-finished products and molded products. The details of the method of the present invention will be explained below. FIG. 1 is a basic diagram for explaining each heat treatment step of the pulling austempering method of the present invention. As shown in the figure, this method consists of the following steps: Step... temperature T ₁ , holding time t ₁ step... temperature T ₂ , ₂ stages of holding time t... Consisting of 3 stages of processing: temperature T ₃ and holding time t ₃ . First, the stage is a treatment for austenitization of the material, with T ₁ in the temperature range from A ₃ to _{A 3} +150°C. The reason why we set an upper limit on T ₁ (A ₃ +150℃) is because
This is because if the temperature exceeds this, the austenite grains will become coarser and this will cause a decrease in the toughness of the product. _t1 is determined to be an appropriate time depending on the heating method and material dimensions, and substantially uniform austenitization can be achieved if the time required for the undissolved carbide to become 10% or less, for example, 0.5 to 15 minutes, is sufficient. The step is a process in which the material is immersed and quenched in a medium held at T ₂ temperature from the step. The medium (refrigerant) for this quenching is a salt bath,
Use oil baths, non-ferrous metal or alloy baths, and other known baths. _T2 temperature ranges from Ms to M ₉₀ %,
In other words, it is the temperature range in which less than 90% (volume ratio) of martensite is generated at that temperature, and normal Mf
It is different from the quenching temperature below the point. Since martensitic transformation is a non-diffusion transformation, the amount of martensitic transformation is controlled by the quenching temperature, but is hardly affected by the holding time _t2 at that temperature. However, there are some differences in the time it takes for the material to be completely cooled to this T ₂ temperature depending on the type of refrigerant and material dimensions. Therefore, this t ₂ time may be the time necessary for the material temperature to reach the target T ₂ temperature, but it must not be too long. In other words, by quenching to this temperature (Ms ~ M ₉₀ %), over 10% by volume of supercooled austenite should be present, but this supercooled austenite must not complete its transformation into lower bainite. . In other words, the holding time _t2 in the refrigerant bath must be set to a time during which untransformed austenite exists in an amount of 10% by volume or more, preferably 40% by volume or more. It is more preferable that the amount of primary martensite produced by the treatment at this stage is 60% or less. The stage is a process of reheating the material to a temperature T ₃ of 300-550 °C above Ms from the stage. This heating also uses a heating furnace or bath held at this _T3 temperature. At this stage, the primary martensite produced at this stage is tempered, and the untransformed austenite is transformed into bainite. Therefore, the holding time t ₃ at this temperature T ₃ needs to be longer than the time when bainite transformation is completed. If this t ₃ is terminated at a time when 5% or more of untransformed austenite exists in terms of volume ratio, most of it will become secondary martensite, which will greatly impair ductility and toughness. The situation should be avoided. When cooling to room temperature after completion of the step treatment, as long as bainite transformation has been completed, either rapid cooling in water or slow cooling may be used, and there is virtually no difference in material quality between the two. However, if T ₃ > 350℃, it is best to perform slow cooling at a cooling rate of 10℃/sec or less until 350℃.
It is necessary to avoid further rapid cooling. When the method of the present invention, which consists of such three-stage treatment, is carried out, it is possible to accelerate the bainitic transformation, which can be called quenching stress, compared to the conventional austempering treatment. This will be explained below using test examples. A cold-rolled annealed plate with the chemical composition in Table 1 was used as a test material,
T ₁ = 850℃, t ₁ = 12 minutes: T ₂ = 120℃, t ₂ = 1 minute:
When T ₃ = variable and t ₃ = variable, the bainite transformation start point (Bs) and bainite transformation end point (Bf) were determined using a thermal dilatometer (Formaster F), and the results are shown in the TTT diagram in Figure 2. Indicated by a broken line.
In addition, austempering treatment (T ₁ =
The isothermal transformation (Bs) (Bf) at 850°C, t ₁ = 12 minutes, rapidly cooled to a temperature above the Ms point, and kept constant is shown by the solid line in Figure 2.

[Table] As is clear from the results in Figure 2, when the pulling austempering treatment according to the method of the present invention is performed,
Bs and Bf move much further to the left (for a short time) than austempering (Bs) and (Bf). The reason for this is
This is thought to be due to the effect of stressing of supercooled austenite due to the formation of martensite due to the stepwise treatment. This acceleration of bainite transformation enables high-speed heat treatment that could not be achieved with conventional austempering treatment.
All of the steps may be suitably carried out in a continuous processing line in which the steel plate (strip) or wire is passed through continuously. Moreover, in the case of the method of the present invention, compared to austempered steel, a mixed structure with bainite in which tempered martensite is present can be obtained, so that high strength properties as well as toughness can be exhibited. This will be illustrated below using test examples. Cold-rolled annealed steel plate with the chemical composition values shown in Table 1 (plate thickness
2.0mm) was used as the test material, and the stage was T ₁ in a salt bath.
= 850℃, t = 12 minutes constant, stage and stage temperature (T ₂ and T ₃ are variables (where t ₂ and t ₃ are constant conditions of 3.5 minutes and 2 minutes, respectively), and the stage is an oil bath , the Vickers hardness and Sharpie impact value (U notch 2 mm) were measured when treated in a salt bath, and the results are summarized in Figure 3. In Figure 3, curve a is _T3
= 400℃, curve b is T ₃ = 350℃, curve c is T ₃ = 300
℃, and the symbol for each curve a, b, c is ◎
The mark is T ₂ = T ₃ , the □ mark is T ₂ = 180℃, the ◇ mark is T ₂ = 150
℃, 〇 mark is T ₂ = 120℃, △ mark is T ₂ = 40℃, ▽ mark is
It shows T ₂ = 10°C. Curve d is the value when normal quenching and tempering treatment is performed; after the same austenitizing treatment, quenching is performed in an 80℃ oil bath and held for 10 minutes, and then ☆1 is 400℃×30
minutes, ☆2 is 350℃ x 30 minutes, ☆3 is 300℃ x 30 minutes, ☆
No. 4 was tempered at 250°C for 30 minutes. The following is clear from the results shown in Figure 3. First of all, terminating the bainitic transformation is a fundamental condition for obtaining good toughness, and T ₃ =
At 300° C. and T ₂ >150° C. (marks ◎ and □ on curve c), there is a large amount of untransformed austenite and bainite transformation has not been completed, so secondary martensite is generated and the toughness is reduced. Further, as seen in this curve c, the impact value shows a maximum near T ₂ =150°C. This indicates that as the T ₂ quenching temperature decreases, the amount of supercooled austenite decreases, and the amount of bainite transformation decreases. Then T ₃
= 350℃ and T ₃ = 400℃, the bainite transformation is completed and a mixed structure of primary martensite and upper bainite is formed. In this case, if the amount of bainite formed is 10% or more, the curve d changes. Compared to quenched and tempered materials, the hardness (and therefore strength) and toughness properties are extremely good. Furthermore, from this hardness-toughness characteristic diagram, it can be seen that it is desirable that the amount of primary martensite be 60% or less, that is, T ₂ 150° C. (see point M in FIG. 2). Figure 4 shows changes in characteristics when T ₃ and t ₃ are changed. In the case of T ₂ = 150°C (M ₅₀ %), A is T ₃ = 350°C, B is T ₃ = 300℃, C is T ₃ =
It shows the relationship between t ₃ and hardness-impact value at 280°C. As is clear from Figure 4,
The lower the _T3 temperature, the longer the appropriate processing time (dashed line) to obtain good toughness. In order not to lose the advantage of short-time processing by accelerating bainite transformation, which is one of the features of the present invention, it is desirable that t ₃ be approximately 10 minutes or less.
For this purpose, it can be seen from FIG. 4 that T ₃ needs to be 300° C. or higher. But T ₃
At >550°C, tempering progresses more than necessary and softens, so _T3 of the stage is preferably 300 to 550°C. The steel type to which the method of the present invention is applied is preferably medium-high carbon steel with a carbon content of 0.40 to 1.10%. The method of the present invention is advantageous when applied to obtain high-strength steel (TS>100Kg/mm ² , Hv>320), and therefore it is suitable for steel types that are applied to ordinary austempering and quenching and tempering treatments. Although they are not essentially different, since the toughness is further increased by utilizing the bainitic transformation of supercooled austenite, the effect of applying the present invention will be limited unless the degree of stabilization of austenite is high to some extent. cannot be fully demonstrated. That is, the more the nose and bay of the S curve of the material in the isothermal transformation diagram are located on the long time side (to the right), the more suitable the material is for the method of the present invention. If the position of the bay is short-term, isothermal transformation will proceed in the quenching bath and the amount of supercooled austenite will decrease. Because of this, C0.40
It is desirable to stabilize austenite in a component system where % is satisfied. On the other hand, C, Si,
The amount of added elements such as Mn, Ni, W, and Mo can be applied when the Ms point is above room temperature, but
If C exceeds 1.10%, even if the method of the present invention is applied, no improvement in toughness will be observed. Example Steel having the chemical composition values shown in Table 2 was treated according to the treatment pattern shown in FIG. 1, and its microstructure and mechanical properties were investigated, and the results shown in Tables 3 and 4 were obtained. However, Table 3 shows the results for steel (1) in Table 2, and Table 4 shows the results for steel (2) in Table 2.

【table】

【table】

【table】

[Table] As is clear from the results in Tables 3 and 4, by carrying out the three-step treatment according to the present invention,
A high-strength, high-toughness steel with excellent strength and toughness can be obtained. In addition, when the method of the present invention is followed, when the primary martensite is 90% or less, a mixed structure of tempered martensite and bainite is formed in which secondary martensite is not generated. If strength is to be emphasized, the amount of martensite in this mixed structure may be increased, and this can be done by adjusting the T ₂ temperature, T ₃ temperature, and t ₃ time.

[Brief explanation of drawings]

Fig. 1 is a pattern diagram showing the processing steps according to the present invention, Fig. 2 is a isothermal transformation diagram comparing the pulling austempering method of the present invention and the conventional austempering method;
FIG. 3 is a diagram showing the relationship between Vickers hardness and impact value when the method of the present invention is carried out, and FIG. 4 is a diagram showing the relationship between t ₃ time, impact value, and hardness according to the method of the present invention.

Claims (1)

[Claims] 1. A medium-high carbon steel containing carbon in the range of 0.40 to 1.10% is austenitized by holding it in a temperature range from the _A3 transformation point of the steel to _{the A3} transformation point + 150°C, and the austenite is After quenching so that supercooled austenite exists in the temperature range from Ms point to M ₉₀ % point of the steel, the steel is quenched from this temperature range to 300% while retaining at least 10% by volume of untransformed austenite.
A method for producing high-strength, high-toughness steel, which comprises reheating to a temperature range of ~550°C, and transforming supercooled austenite to bainite and tempering primary martensite in this reheating temperature range. 2. The manufacturing method according to claim 1, wherein the steel has a band-like or linear shape and the entire process is carried out in a continuous line. 3. The manufacturing method according to claim 1 or 2, wherein the time for holding in the reheating temperature range is the time required for 95% by volume or more of the terminally transformed austenite to transform into bainite.