JPH0236648B2 - KOKYODOKOENSEIKONOSEIHO - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing heat-treated steel with high strength and excellent ductility and toughness under short thermal cycles, and more specifically, the present invention relates to a method for producing heat-treated steel having high strength and excellent ductility and toughness, and more particularly, martensite, martensite,
This relates to a method for producing high-strength, high-ductility steel in which a three-phase coexistence steel of bainite and retained austenite is produced while accelerating bainite transformation while stabilizing retained austenite, and is related to a method for producing high-strength, high-ductility steel, which is based on a previous patent application filed by the same applicant. This paper concerns further improvement of the "pulling austempering method" proposed in No. 57-21654. Conventionally, austempered steel and quenched and tempered steel are well known as heat-treated steel. Comparing the two, the former generally has higher toughness, but when producing bainite through austempering, it becomes significantly softer than the latter if the processing temperature is high, and if the processing temperature is lowered, the holding time becomes shorter. It is subject to the restriction that the amount increases significantly and manufacturability deteriorates. On the other hand, the latter makes it easier to obtain high-strength materials, but its toughness is inferior to the former. For this reason, in the previous patent application No. 1987-21654, the present inventors proposed a new manufacturing method called the pulling austempering method as a method to obtain high strength and excellent toughness while shortening the processing time. did. The purpose of the present invention is to further advance this pulling austempering method, and in particular, to actively stabilize retained austenite, which was not directly intended in the previous pulling austempering method. According to subsequent follow-up research by the present inventors, by stabilizing retained austenite and creating a three-phase coexistence steel of martensite, bainite, and retained austenite, a reduction in processing time, which is a characteristic of the pulling austempering method, was achieved. However, it has been found that steel can be obtained that satisfies the high strength and high ductility (T.S150Kg/mm ² , TS x El1500) required for cutlery materials and spring materials, and in particular by applying this pulling austempering method. It has been found that when obtaining a three-phase steel with retained austenite by using the same method, an appropriate amount of Si acts extremely effectively to stabilize this retained austenite. That is, in the present invention, among the elements in steel, C is 0.40
A medium-high carbon steel containing ~1.10 wt% and 0.8~2.7 wt% Si is austenitized by holding it in the temperature range of the Ac ₃ transformation point of the steel to Ac ₃ transformation point + 150℃, and from this austenitic region, the steel After being quenched to a temperature range of Ms point to M ₈₀ % point, and then retaining at least 20% by volume of untransformed austenite.
It is reheated to a temperature range of 300℃ to 450℃, and during this reheating step, martensite is tempered and untransformed austenite undergoes bainite transformation. The present invention provides a method for producing high-strength, high-ductility steel, which comprises cooling to room temperature within a time period during which the amount of bainite becomes at least 5% by volume or more, and interrupting bainite transformation by regulating the holding time. According to the present invention, the three-phase structure of tempered martensite, bainite and retained austenite can be formed while the heat treatment time (more specifically, the holding time at the reheating temperature) is significantly shortened (for example, within 4 minutes). High strength and high ductility steel can be obtained. A preferred embodiment of this three-phase structure is a structure consisting of 10 to 80% by volume of tempered martensite, 5% or more by volume of retained austenite, and the remainder substantially of bainite, and has mechanical properties when the plate thickness is 1 mm. satisfies T.S150Kg/mm ² and TS×El1500. 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. As a medium (refrigerant) for this hardening, a salt bath, an oil bath, a non-ferrous metal or alloy bath, and other known baths are used. The T ₂ temperature is a temperature range from Ms to M ₈₀ %, that is, a temperature range in which less than 80% (volume ratio) of martensite is generated at that temperature, and is different from the normal quenching temperature below the Mf 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. but,
Depending on the type of refrigerant and material dimensions, there will be some differences in the time it takes for the material to be completely cooled to this T ₂ temperature. Therefore, this t ₂ hours is the goal.
The time may be as long as necessary for the material temperature to reach _T2 temperature, but it should not be too long. Because by quenching to this temperature (Ms~M ₈₀ %), 20% by volume
The above supercooled austenite is made to exist, but if this supercooled austenite is kept at a temperature of T ₂ for a long time, it will transform into lower bainite, making it impossible to secure residual austenite. In other words, the retention time _t2 in the refrigerant bath needs to be a time during which 20% by volume or more of untransformed austenite exists. The stage is a process of reheating the material to a temperature T ₃ of 300-450 °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 transforms into bainite. but,
In the present invention, the holding time t ₃ at this temperature T ₃ must be set before the time when bainite transformation ends. One feature of the present invention is that this t ₃ is terminated at a time such that 5% or more of untransformed austenite exists in terms of volume ratio. When cooling to room temperature after the completion of the step treatment, as long as an appropriate amount of bainite transformation has been completed, either rapid cooling in water or slow cooling may be used, and there is no substantial difference in material quality between the two. It doesn't appear above. 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. and,
By containing an appropriate amount of Si, retained austenite can be stabilized, and steel with improved ductility can be obtained even in a short treatment time. Table 1 shows the chemical composition and transformation characteristic points of the test steel. The test steel was hot-rolled using a conventional method, and the plate thickness was
It is a cold-rolled steel strip finished to 1.0mm. The material properties were evaluated using JIS No. 13 B tensile test pieces.

[Table] Figure 2 shows sample steel D, T ₁ = 880℃, t ₁ =
After austenitizing for 20min, T ₂ = 204℃, t ₂
= 1 min, after obtaining about 20% primary martensite, T ₃ was set to 350°C, and the tensile test characteristics of the pulled austempered material when the t ₃ holding time was changed, and the constant temperature holding temperature was set to 350°C. The characteristics are compared with those of ordinary austempered materials. In the pulled austempered material, the elongation is high in the range of 30 seconds to 2 minutes, with a maximum value at 1 minute. On the other hand, in normal austempering, the elongation is extremely low at 2 minutes or less, high in the range of 4 to 20 minutes, and reaches a maximum value at 8 minutes. Figure 3 shows the C% in the retained austenite at this time, but in normal austempering treatment,
Because bainite transformation is slow, C concentration in austenite does not progress, and martensite (α″), which is a secondary product, is more likely to occur than untransformed austenite during the cooling process.This causes unstable fracture and extremely low elongation. On the other hand, in pulling austempering, bainite transformation is accelerated and C enrichment in austenite progresses in a short period of time. The C% of
(1.2% for normal austempering and 1.0% for pulling austempering) the retained austenite is stabilized. This is because in pulling austempering, the structure is fine and C concentration in austenite tends to occur on average as austenite is formed, whereas in normal austempering, the structure (bainite laths) is coarse and C concentration is difficult to occur. I think this is because it becomes heterogeneous. In this way, in pulling austempering, by generating primary martensite, bainite transformation is shortened and retained austenite is easily stabilized. Such an effect becomes greater as the amount of primary martensite increases. However, when the amount of primary martensite is increased to 80% or more, there is a problem that the absolute value of the amount of retained austenite obtained decreases. Therefore, in the present invention, the quenching bath temperature T ₁ is set to the M ₈₀ % point or higher so that the amount of primary martensite is within 80%. Figure 4 shows the four types of steel A to D in Table 1.
The mechanical properties and amount of retained austenite (γ _R ), arranged by Si content. As is clear from Fig. 4, the TS is significantly improved by the pulling austempering treatment according to the present invention, and as the Si content increases, the amount of retained austenite increases and the ductility increases. Recognize. In this way, Si provides an effective effect in obtaining stable retained austenite, but Si
If it is less than 0.8%, the stabilization of retained austenite will be insufficient and the target properties will not be obtained. Furthermore, even if Si exceeds 2.7%, non-metallic inclusions tend to increase, which tends to cause surface roughness and deterioration of ductility and toughness, so the Si content is preferably 2.7% or less. On the other hand, C, along with Si, is an important element in stabilizing retained austenite, but if C is less than 0.40%, the degree of stabilization of austenite decreases, making it difficult to obtain stable retained austenite. . On the other hand, when C exceeds 1.10%,
The ductility-improving effect of retained austenite becomes difficult to recognize. In addition, other ordinary elements such as
The amount of Mn, Ni, etc. added is permissible within the range of the component system where the Ms point is at room temperature or higher. Example: Chemical components C, D and E shown in Table 1 of Maeage
The steel was heat treated under the conditions shown in Table 2, and the microstructure and tensile test properties of the obtained steel are also listed in Table 2. In the same table, the comparative method refers to normal austempering treatment, and the invention method refers to pulling austempering treatment. (1) Nos. 1 to 7 are the cases where the constant temperature treatment temperature (T ₃ ) of Steel C was 300°C. In the comparative methods (No. 1 to 3), unstable fracture occurs when the retention time (t ₃ ) is less than 4 minutes, and TS×El is
Less than 1500. In contrast, the Invention Law (No. 4
~7), even if the retention time (t ₃ ) is less than 4 minutes,
Indicates TS>150Kg/mm ² and TS×El>1500. (2) Nos. 8 to 11 are for Steel D at a constant temperature treatment temperature (T ₃ ) of 350°C. Comparative method No. 8 has an insufficient bainite transformation amount of 44%, resulting in 33% secondary martensite formation and low elongation (however, the retention time (t ₃ ) is as long as 4 minutes). (shows good ductility). In contrast, the Invention Act (No. 10-11)
Even when the holding time (t ₃ ) is 2 minutes or less, it shows good strength and elongation. (3) Nos. 12 to 15 are cases where the constant temperature treatment temperature was 400°C. Comparative methods (No. 12 and 13) show good elongation, but
TS<150Kg/mm ² , whereas the invented method (No. 13, 14) has TS>150Kg/mm ² , TS×El>
Showing 1500. (4) No. 16 shows the results of treating Steel C using the invention method at a constant temperature treatment temperature of 450°C. TS＞150Kg／
Indicates TS×El>1500 in mm ² . (5) Nos. 17 to 20 are the results when the amount of primary (primary) martensite was changed for Steel E. The amount of primary martensite is 75% or less (No.17~
19) showed good strength and ductility, and TS×El
>1500, but the amount of primary martensite
When it is 85% (No. 20), the amount of retained austenite is 5% or less, and TS×El<1500. Regarding the TS x El value, for quenched and tempered steel strip with a thickness of 1.0 mm, TS = 150 ~
A material having a strength of 200 Kg/mm ² usually has a strength of 800 to 1100 in the JIS No. 13 B test.

【table】

【table】 [Brief explanation of drawings]

FIG. 1 is a pattern diagram showing the heat treatment steps according to the present invention, FIG. 2 is a diagram comparing the relationship between constant temperature treatment time and mechanical properties between a normal austempering method and a pulling austempering method according to the present invention. FIG. 3 is a diagram comparing the relationship between constant temperature treatment time and the structural components of retained austenite between the conventional austempering method and the pulling austempering method according to the present invention.
FIG. 4 is a diagram comparing the relationship between Si content in steel, mechanical properties, and retained austenite amount (γ _R ) between a conventional austempering method and a pulling austempering method according to the present invention.

Claims (1)

[Claims] 1 Among the elements in steel, C is 0.40 to 1.10% by weight, Si
A medium-high carbon steel with a carbon content of 0.8 to 2.7% by weight is austenitized by holding it in the temperature range of the Ac ₃ transformation point of the steel to Ac ₃ transformation point + 150°C, and from this austenitic region, the Ms point of the steel ~ M ₈₀ % Quenched to a point temperature range,
Then, it is reheated to a temperature range of 300°C to 450°C while retaining at least 20% by volume of untransformed austenite, and in this reheating step, martensite is tempered and untransformed austenite is transformed into bainite. , the holding time in this reheating temperature range is regulated within the time during which the amount of retained austenite stable at room temperature is at least 5% by volume or more, and the bainite transformation is interrupted by regulating the holding time. A manufacturing method for high-strength, high-ductility steel. 2 Steel cooled to room temperature has a structure in which tempered martensite is 10 to 80% by volume, retained austenite is 5% or more by volume, and the majority of the remainder is bainite, and the plate thickness is 1 mm. The characteristics of
The manufacturing method according to claim 1, wherein T.S150Kg/mm ² and TS×El1500. 3 Heat retention time in reheating temperature range 300℃~450℃ 4
The manufacturing method according to claim 1 or 2, wherein the manufacturing method is within minutes.