JPS61210121A - Thermomechanical treatment of steel - Google Patents

Abstract

PURPOSE:To give superplasticity by cold-working and heating a material which is heated to the A3 point or above and cooled rapidly. CONSTITUTION:As steel, hypo-eutectoid steel containing alloying elements of 1 or >=2 kinds among 0.05-2wt% Nb, 0.1-2wt% V, and 0.05-1wt% Al and having an improved grain growth-controlling effect is used. The hypo-eutectoid steel is heated to a temp. of the A3 point or above and cooled rapidly, which is subjected to cold working and then to heating to form a fine two-phase structure.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a process heat treatment method for steel, and more specifically, by subjecting practical steel such as structural steel to a specific process heat treatment, it is possible to develop so-called superplasticity. This paper relates to a steel processing heat treatment method that can be put to practical use. (Prior art and problems) Superplasticity means that a metal or alloy with fine crystal grains has 0,
This is a fine-grained superplastic phenomenon that shows abnormally large elongation at low loads at high temperatures above 4 Tm (Tm: melting point), and the material conditions that exhibit such characteristics are not simply that the crystal grains are fine. It is said that one of the important conditions is that such a fine grain structure is stable during deformation at high temperatures and is difficult to coarsen. For this purpose, it is convenient to have a fine two-phase structure, and various eutectic or eutectoid nonferrous alloys such as Zn-A1 alloy have superfine grains. It has been considered as a typical plastic alloy from an early stage. On the other hand, the so-called superplastic processing method, which actively utilizes such fine grain superplasticity phenomenon for plastic processing, has been attracting attention in recent years. There has been a demand for the development of structural materials, and in order to meet this demand, research has recently been conducted on the fine grain superplasticity of various iron-based alloys. However,
Research on this technology has just begun, and we have yet to see any research and development results that have reached the level of practical application. (Objective of the Invention) In view of the above circumstances, the present invention has been developed to develop fine grain superplasticity in iron-based alloys, especially practical steels used for various processing as structural materials, and to apply superplastic processing methods to them. The purpose is to provide a new method that can be used more effectively. (Structure of the Invention) In order to achieve the above object, the present inventors conducted basic experiments in order to find a processing heat treatment method that can make the austenite crystal grains of steel ultra-fine to an average grain size of 10 μm or less. As a result, for ultra-low carbon steel, solution treatment (1250℃
It was found that it was possible to refine the austenite grains by applying strong cold working (section reduction rate of 80%) to the material that was quenched after xLhr), rapidly heating it to just above the A3 point, and holding it for a short time. In addition, after quenching this, (α+
When heated to the γ) two-phase region, a fine (α+γ) two-phase structure is obtained, which can be subjected to superplastic working. However, the above method requires a long treatment process and increases manufacturing cost, so it cannot be said to be a practically advantageous method. Therefore, based on the results of the above basic experiment, the present inventors
Further research was conducted to find a process heat treatment method that has a simpler process and comparable superplasticity effects. As a result, in the processing heat treatment found in the above basic experiment, by tempering as necessary before cold working and heating to the (α + γ) two-phase region directly above the A3 point, the austenite It has been found that crystal grains can be ultra-fine to an average grain size of 4 to 6 μm or less, and that various practical steels can be advantageously subjected to superplastic processing. The experiment was conducted using high-strength, low-alloy steel (HS) with the chemical composition shown in Table 1.
The processing heat treatment shown in FIG. 1 was performed on KNb material and KCl material, which are LA steel. In addition, the amount of C is approximately 0.1 in each case.
5 wt%, and a trace amount of Nb is added to the KNb material. Table 1 (wt%) That is, plates with a thickness of 10 mm were cut out from the hot rolled material, subjected to lhr solution treatment at 1250° C., and then quenched in aqueous brine. Next, the processing rate was 80% (thickness 10mm →
The test pieces for measuring the 6-tensile elongation at break and the test pieces for measuring the m value (strain rate sensitivity index) which were cold rolled (2mm+) were each made using the wire cut electrical discharge machining method from plate materials that had been cold rolled by 80%. It was cut out into the shape and dimensions (mm) shown in FIGS. 2(a) and 2(b). Negative U Structure Observation In order to measure the prior austenite grain size of the specimen that has undergone the processing heat treatment shown in Figure 1, (picric acid + sodium laurylbenzenesulfonate + hydrogen peroxide solution)
Prior austenite grain boundaries were revealed by immersion in a boiling mixture of In addition, a vacuum furnace was used to confirm the state of the structure in the test piece at the start of the tensile test (described later). After heating and holding for 30 seconds at various set temperatures in the (α+γ) two-phase region under the same conditions as at the start of the tensile test, the microstructures of the brine-quenched specimens were examined using an optical microscope. In this case, Le Pera's reagent was used to reveal the tissue. Since these specimens have a (α+α') two-phase structure, the α' volume fraction was determined by image processing based on optical micrographs. (α+γ) is the volume fraction of the γ phase during heating in the two-phase region. mTensile rupture test and strain rate conversion test A tensile rupture test and strain rate conversion test were conducted using an Instron type tensile testing machine (maximum load 500 kg) equipped with an infrared radiation image furnace. These tests were conducted using a mixed gas (90%N2-10%H2) at a flow rate of 2L' to prevent oxidation of the test piece during tension.
I went while running it on min. During the test, the temperature difference at each heating and holding temperature was 10 cm at the center of the furnace containing the test piece.
The temperature was controlled within ±2°C over the entire temperature range. The test temperature was various temperatures in the (α+γ) two-phase region (700~
820'C), and the test piece was heated to a predetermined temperature at approximately 3°C/
The test was started after heating at a heating rate of S, and after reaching the set temperature, it was held for 30 minutes. Tensile break test: To measure the tensile break elongation, see Figure 2 (
A test piece with the shape and dimensions of a) is set to an initial strain rate of 5X1.
Tensile rupture tests were conducted at three constant crosshead moving speeds corresponding to 0-'', lXl0-2, and 5X10-11/win. Strain rate conversion test: The specimen was moved to a steady state at a constant crosshead moving speed. After stretching the crosshead until it reached The relationship between the speed t) and the true flow stress (σt) was determined, and the m value was determined from the relationship between the two.Furthermore, as described later, in this experiment, each sample steel showed a relatively large m value at 780°C. In this case, after 50% and 100% tensile deformation at this temperature with i = 5 ) The change in m value accompanying (b) A test piece having an ultra-fine structure (α' structure) as shown in (xcn material) was heated to various temperatures in the (α + γ) two-phase region in the same way as in the tensile test, and at those temperatures Figure 4 shows a representative example of the microstructure obtained by holding for a minute and then quenching in brine to obtain (α + α'). In the structure, the black part is α and the white part is α'. Note that the heating temperature and martensite (α′) quantity is (a
)...740'C130%. (b) -760℃, 45%, (c)・=800'c, 7
It is 0%. Thereby, the tissue state in the (α+γ) region can be estimated. Although FIG. 4 is for the KNb material, no major difference was observed for the KCl material. These organizations! l! From the observation, it can be seen that the volume fraction of α' (that is, the volume fraction of γ in the α+γ region) increases as the temperature in the (α+γ) two-phase region increases. As can be inferred from Figure 4, the (α+γ) fine two-phase structure obtained by this processing heat treatment is not only a structure in which the crystal grains are fine, but also a structure in which two equiaxed phases are uniformly distributed. (α+γ), and it is judged that the condition for exhibiting superplasticity is provided in the (α+γ) two-phase coexistence region. Results of tensile rupture test Figure 5 shows the change in elongation at break in the tensile rupture test with the test temperature and the volume fraction of 7 in the test piece. For KNb material, the second one is 5×10″ which is the slowest in this experiment.
The elongation is large over a wide temperature range in the case of ``1/n+in, and is faster than I x 10-'' 1/mi
In case of n, the elongation decreases overall. On the other hand, Kcm
For wood, conversely, t = I x 10-21/lll1n
The elongation is large in the case of , and the elongation is slow in the case of ;
/min, the elongation is decreasing. There are no major differences between the two in other respects. In both cases, the ratio of α phase and γ phase is equal to 740 to 780.
The maximum elongation occurs near ℃. Furthermore, in both cases, the structure shows that the elongation changes significantly due to changes in the test temperature (which changes the volume fraction of the α phase and γ phase). In the case of KNb material, t=5X10-'1
/min of 700% or more, in the case of KcII material, 1=
A large elongation at break of 600% or more was obtained at IX10-"1/l1in. In a tensile test at =5X10-11/a+in, this is considered to be close to the industrially possible practical deformation rate.
``All sample steels showed an elongation of 200 to 300%. FIG. 6 shows a case of KNb material as a representative example of an actual fracture test piece that showed the maximum elongation in all three types of 2. In addition, in the case of the KCII material, almost no difference was observed in the shape of the fracture test piece. The large difference between the fracture elongation and strain rate between the KNb material and the KCU material as described above is due to the N in the KNb material.
It is thought that this is closely related to the precipitation of b carbides (or Nb carbonitrides). That is, in the case of the KNb material, it is presumed that the presence of fine precipitates further impedes grain growth in the (α+γ) two-phase structure state where the grain growth rate is originally slow. From this point of view, KNb materials with a trace amount of Nb added have extremely low strain rates, and even when held at high temperatures for an extremely long period of time until fracture, they can sufficiently suppress grain growth and maintain superplasticity. However, KCI that does not contain Nb
In I material, there is no such effect, and in the case of tension at a significantly slow strain rate, grains grow during the test and cannot maintain superplasticity, which is seen in the above results. Therefore, it is interpreted that the elongation at break was larger when the strain rate was relatively high. Therefore, in order to examine the validity of the above speculation,
For both the KCII material and the KNb material, the test pieces in the processing heat treatment state shown in FIG.
The two organizations were compared. The results are shown in FIG. 2. Normally, it would be preferable to interrupt the tensile test and observe the quenched structure, but due to equipment limitations. Since such an operation is difficult, a separate vacuum annealing experiment was performed instead. From the same figure, the two-phase structure (α + γ
) is considerably rougher than that of KNb material ((a) in the same figure). This difference between the two is due to the grain growth inhibiting effect of precipitated Nb carbide (or carbonitride) in the KNb material, and when heated and held under stress such as during a tensile test, the difference between the two is The difference will become even more noticeable. Next, FIG. 8 shows the change in maximum deformation stress with the test temperature during the tensile test in three types. The figure shows the results obtained for the KNb material, but the results for the KC■ material were similar in terms of tendency. That is, at any strain rate, the deformation stress is lowest at the two-phase region temperature (780 to 790°C) where the volume fractions of the α and γ phases are equal, and the lowest value is; = 5 × 10−” 1 Approximately 9 in case of /win
kgf/III+2.1=I x 10-” 1
/win is approximately 2 kgf/mm2,
It decreases as the strain rate becomes slower, and i=
Approximately 1.5k in case of 5 x 10-' 1 /win
It is noteworthy that it shows an extremely low value of "gf/mm. The slope of the straight line in the figure corresponds to the m value, but in this case, the relationship between the two is divided into two regions, each of which is represented by a linear relationship.Similar trends was observed at all test temperatures for both KNb and KCII materials.If we assume that the region on the low IT side is stage ■ and the region on the high IT side is stage ■, then the m value at stage ■ is the same as that at stage ■. It is slightly larger than the m value. Note that 1=5X10''''31/llll1n adopted during the tensile rupture test is included in stage ■. Also, 1=IX10-''1/win is included in stage ■. Thereafter, the m value was examined by classifying it into stage ■ and stage ■. FIG. 10 shows the relationship between test temperature and m value for each steel material. From the figure, the m value in Stage I is larger in the temperature range before Stage II for both steel materials. In addition, the m value strongly depends on the test temperature, and the highest value is 740 to 780, where the volume fraction of the α phase and the γ phase is equal to 740 to 780.
Obtained at ℃. Elongation at break and m value The relationship between elongation at break and m value for the two types of steel materials obtained in this experiment is shown in Figures 11 (a) and (b). (a) shows the relationship between the elongation at break when = I x 10-'1/win and the corresponding m value of stage ■. (b) shows the elongation at break when j = 5X10-'1/win This figure shows the relationship between the elongation at break and the m value of stage I. In the case of (a), the same linear relationship holds between the elongation at break and the m value for both KNb and KCII materials. , it is possible to predict the amount of elongation at break based on the m value regardless of the material. However, in the case of (b), there is a significant difference between KNb material and KCII material, and for the same m value, This shows that the elongation at break varies significantly depending on the material.For example, when the m value is 0.6 (dashed line in the figure)
The elongation at break is 300 to 400% for the KCII material, while it is 600 to 700% for the KNb material. In the tensile test conducted in this experiment at 1=5 If the behavior is different, it is assumed that even if the initial m value is the same, the m value will change depending on the material during tensile deformation, resulting in a difference in the final elongation at break.To confirm this point. In addition, a test piece in the initial state that had not been subjected to a tensile test at a two-phase region temperature of 780°C, and 2 = 5
50% for each of From the same figure, the m value is 50% and 10% for all materials.
After 0% elongation, it decreases depending on the initial state, but the degree of decrease varies depending on the material, and the decrease in the case of KNb material is considerably smaller than that of KCn material. In the case of
What happens when the strain rate of Nb material is slower than that of KCII material? It can be considered to be related to the phenomenon of good stretching. That is,
In tensile tests at low strain rates, the elongation at break does not correspond to the initial m value, and it is related to how the m value is maintained during superplastic deformation, and good elongation means that a high m value is maintained during tensile deformation. It can be said that this is related to how long the temperature is maintained. In the case of the processing heat treatment method adopted in this experiment (Fig. 1), γ
Considering the equilibrium concentration product of Nb carbide or carbonitride in the KNb material, Nb in the KNb material is completely dissolved in solid solution by solution treatment at 1250°C, and most of it precipitates out during heating to the (α + γ) two-phase region. ing. Therefore, it can be said that these precipitates are extremely fine and very uniformly distributed. Therefore, the effect of inhibiting grain boundary bulges and forming crack nuclei is small, and it can be judged that the effect of suppressing crystal grain growth has priority. As mentioned above, according to the processing heat treatment method shown in FIG. The flow stress at 1.5 kgf/mu2 is a remarkable low value, especially at an initial strain rate of 5 x 10''11/win, which is close to the industrially possible practical deformation rate of 200-300 kgf/mu2. Furthermore, the high-strength low-alloy steel containing Nb showed a large elongation of up to 738% (648% for the latter) compared to the low-carbon steel, indicating that it had excellent overall superplasticity. Based on the above knowledge, we conducted more detailed experiments and found that superplasticity can also be effectively expressed by tempering before cold working in the above-mentioned heat treatment method (Figure 1). In other words, the gist of the present invention is that hypo-eutectoid steel is heated to a temperature of A1 point or higher and then rapidly cooled. A process heat treatment method for steel, which is characterized by subjecting the steel to cold working after tempering accordingly, and then heating it to form a fine two-phase structure of α + γ to develop superplasticity. The invention will be explained in detail based on Examples. In the heat treatment method of the present invention, it is necessary to first heat to a temperature of A1 point or higher and then rapidly cool to obtain a martensitic structure. Since this is heating to obtain
If a heating process is adopted in the steel manufacturing process, it can be used. The obtained martensitic structure is tempered if necessary, but the conditions are not particularly limited as long as the tempering treatment is sufficient to transfer unstable martensite to a stable phase of ferrite ten-grain cementite. After tempering, cold working is performed. Various forms of cold working, such as cold rolling, are possible. The cold working temperature is from room temperature to 100°C. Also, processing rate (section reduction rate 1%)
) can also be varied over a wide range from light processing to heavy processing. However, during superplastic processing, the higher the processing rate at the same heating temperature, the better the superplastic properties (elongation). It is necessary to select the processing rate. Next, (α + γ) 2-finger temperature range, sand A work point ~ A1
When the crystal grains are fine by heating to a temperature range between the points, a structure in which two equiaxed phases are uniformly distributed is obtained. The volume fraction of γ in the (α+γ) temperature range changes depending on the heating temperature, and generally the temperature To at which α:γ=50:50
It shows maximum elongation (superplasticity) at . (However, even for the same steel type, TO may vary depending on the strain rate, so use To as a rough guide.) Therefore, as shown in Figure 13, the above temperature To can be selected depending on the C content of the applicable steel type. can. This relationship is uniquely determined by the C content and the A2 point, so if a certain alloying element is added to raise the A2 point, α:
When γ=50:50, To becomes high, which is advantageous for superplastic processing. Moreover, it becomes possible to increase the C content, and it becomes possible to apply it to higher strength members. According to experiments conducted by the present inventors, such alloying elements include Si: 0.2-3%, Cr: 0.2-3%, Mo:
Examples include elements such as 0.1 to 1%, W: 0.1 to 3%, and V: 0.1 to 2%. When Mo and W are added in large amounts, the carbides become coarse and sufficient material properties cannot be obtained.
Furthermore, if too much Si and Cr are present, the toughness will deteriorate, so it is preferable that both Si and Cr be at the above upper limit values. In addition, in order to enhance the effect of suppressing grain growth during superplastic deformation, one of Nb: 0.05 to 2%, V: 0.1 to 2%, and AΩ: 0.05 to 1%. Alternatively, two or more alloying elements can be added. Too much Nb will lead to deterioration of toughness, and carbide (NbC) will form in the form of stringers during agglomeration, which will not become a solution during normal blooming and will be difficult to dissolve during subsequent heat treatment, resulting in poor material properties. lower. The same applies to V, which is also difficult to handle in steel manufacturing and causes deterioration of toughness. Also, if AΩ is too large, it may cause ground defects. because of these reasons,
It is preferable to add each element to the above upper limit. The above shows that the method of the present invention is applicable to all hypoeutectoid steels including practical steels. Of course, 5CR420 is an alloy steel widely used for shafts, gears, bolts, nuts, and other mechanical parts that require strong toughness.
, 5CR430, S 0M420.80M430 are also applicable steels, and other alloy steels including low carbon, medium carbon steel, and low alloy steel are also effective. Next, one embodiment of the present invention will be shown and explained in more detail. It goes without saying that the present experimental example already shown is also an example. (Example 1) The chemical components of the applied materials are shown in Table 2. In addition, the material steel blood,
Nos. 1 to 7 and Isao, and Nos. 13 to 22 were subjected to solution treatment by the processing heat treatment shown in FIG.
Na5~7.13~22 was quenched in brine at 1200°C for 1 hr), then subjected to 80% cold working, then (α+γ)
The maximum elongation was investigated by heating to a two-phase region temperature (superplastic working temperature shown in the same table). In addition, the material steels Nα8 to 12 were subjected to the processing heat treatment shown in Fig. 14, after solution treatment (1200'CX 1 hr), brine quenching, tempering at the temperature shown in the same table, and then 80% cold working. After applying, (α+γ) two-phase region temperature (
(Same as above) and the maximum elongation was investigated. The results are also listed in the same table. Test steels Na1 to Na4 are used to compare the effect of suppressing grain growth during superplastic deformation by adding Nb, V, and Al. Obviously, those containing one or more of Nb, V, and AΩ exhibit this effect and exhibit excellent superplasticity overall. The sample steel Na5-8 is 1 of Si, Cr, Mo, W and V.
This is a comparison of the effects of increasing the A1 point by adding a species or two or more species to increase the superplastic working temperature and making superplastic working advantageous. clearly. In those containing one or two of these maintenance methods, the effect appears and the superplastic working temperature becomes high. In addition, test steels Ha 19 to 22 were examined to see the influence of C, and although the maximum elongation tends to decrease as the amount of C increases, it can be seen that sufficient superplasticity is obtained for all the hypo-eutectoid steels. .

[Left below]

The results for the sample steel shown above are the same for other types of hypo-eutectoid steel, and the method of the present invention can be applied to all types of steel. (Effects of the Invention) As detailed above, in the present invention, in the processing heat treatment of hypo-eutectoid steel, heating is performed to the (α + γ) two-phase region without heating directly above the A3 point after cold working. It is possible to develop superplasticity in a wide range of steel types under favorable conditions.
In particular, the effects are extremely large, such as making it possible to expand the scope of application of the superplastic working method to practical steel as a structural material.

[Brief explanation of drawings]

FIG. 1 is a schematic diagram of processing heat treatment according to an embodiment of the present invention. Figure 2 is a diagram showing the shape and dimensions (m+s) of various test pieces, Figures 3 (a), (b), Figures 4 (a) to (c), and Figure 7 (a), (b) are 1 is a micrograph of a microstructure shown in each step in an example of the present invention. Figures 5 (a) and (b) are diagrams showing the relationship between temperature and fracture elongation at various strain rates; Figure 6 is a diagram showing the shape of the test piece after the fracture test when the temperature and strain rate are varied; Figure 8 shows the relationship between temperature and true stress at various strain rates, Figure 9 shows the relationship between true strain rate and true stress, Figure 10 shows the relationship between temperature and m value, and Figure 11 shows elongation. Figure 12 is a diagram showing the change in m value in the strain rate conversion test after pre-stretching, Figure 13 is the heating temperature and C content in the (α + γ) two-phase region. FIG. 14 is a schematic diagram of processing heat treatment according to another embodiment of the present invention. Patent applicant Masaharu Tokizane, Daido Steel Co., Ltd.

Claims (1)

[Scope of Claims] 1 Hypoeutectoid steel is heated to a temperature of A_3 or higher, then rapidly cooled and subjected to cold working, and then heated to form a fine two-phase structure and develop superplasticity. A processing heat treatment method for steel characterized by: 2 The steel has a weight ratio of Nb: 0.05 to 2%, V:
The steel according to claim 1, which is a steel containing one or more alloying elements of 0.1 to 2% and Al: 0.05 to 1%, and has an enhanced grain growth suppressing effect. Method. 3 The steel has a weight ratio of Si: 0.2 to 3% and Cr:
0.2-3%, Mo: 0.1-1%, W: 0.1-3%
The method according to claim 1, wherein the steel contains one or more alloying elements of V: 0.1 to 2% and has an increased A_3 point. 4. The method according to claims 1 to 3, wherein the steel is a low alloy steel with low to medium carbon. 5 Hypoeutectoid steel is heated to a temperature of A_3 or higher, then rapidly cooled, tempered and cold worked, and then heated to form a fine two-phase structure and develop superplasticity. Processing and heat treatment method for steel. 6 The steel has a weight ratio of Nb: 0.05 to 2%, V:
The steel according to claim 5, which is a steel containing one or more alloying elements of 0.1 to 2% and Al: 0.05 to 1%, and has an enhanced effect of suppressing grain growth. Method. 7 The steel has a weight ratio of Si: 0.2 to 3% and Cr:
0.2-3%, Mo: 0.1-1%, W: 0.1-3%
The method according to claim 5, wherein the steel contains one or more alloying elements of V: 0.1 to 2% and has an increased A_3 point. 8. The method according to claims 5 to 7, wherein the steel is a low-alloy steel with low to medium carbon.