JPS61279620A - Working heat treating method for steel - Google Patents

Abstract

PURPOSE:To produce superplasticity and to obtain a practicable steel by subjecting a hypereutectic steel to heating to a specified temp., rapid cooling, cold working and heating to a specified temp. to form a structure consisting of fine ferrite grains and spheroidal cementite. CONSTITUTION:A hypereutectic steel is heated to a temp. above the A3 point and rapidly cooled to form a structure contg. a large amount of carbide in the matrix as solid soln. such as a martensite or bainite structure. The heated steel may be slowly cooled to form a pearlite structure. The steel is then cold or warm worked at about 100 deg.C-A1 point and heated to a temp. just below the A1 point to form a structure consisting of fine ferrite grains and spheroidal cementite. Thus, superplasticity is provided to a practical hypereutectic steel such as a bearing steel.

Description

[Detailed description of the invention]

(Industrial Application Field) The present invention relates to a method for processing and heat treating steel, and more particularly, by subjecting practical steel such as bearing steel to a specific heat treatment, so-called superplasticity is developed. This paper relates to a steel processing heat treatment method that enables its practical application. (Prior art and problems) Superplasticity refers to metals or alloys with fine crystal grains! ′″
0.4T"(T""1 point is J"oa;m"c"・Low height 1
iri. It is a fine grain superplastic phenomenon that always shows large elongation.
The material conditions for exhibiting such characteristics are not only that the crystal grains are fine, but also that such a fine grain structure is stable during deformation at high temperatures and does not become coarse. It is said to be one of the conditions. There is,
・1 For this purpose, it is advantageous to have a fine two-phase structure. type non-ferrous alloys have long been considered as typical alloys exhibiting fine-grained superplasticity.On the other hand, in recent years, so-called superplastic working methods that actively utilize such fine-grained superplastic phenomena for plastic working have been developed. This has led to the development of inexpensive structural materials that exhibit superplasticity and are strong at room temperature.
. 1' has begun to be requested, and in order to meet this demand, a variety of
・Research on fine grain superplasticity has also been conducted for various iron-based alloys. However, this research has only just begun, and we have yet to see any research and development results that have reached the level of practical application. In particular, there have been only a few reports on the possibility of practical application of hypereutectoid steels such as bearing steels, as described below. That is, ultra-high carbon steel (UHC) containing 1 to 2.1% carbon
5IIE is a series of processing heat treatment methods that transform steel (steel) into a structure consisting of extremely fine ferrite grains and spheroidal cementite.
Developed by RBY and his collaborators. Materials processed by these methods exhibit superplasticity at warm temperatures and have excellent strength and toughness at room temperature, so they are attracting attention as new structural materials. Furthermore, this processing heat treatment method. It has already been shown that it can be applied to some practical high carbon steels such as 52100 bearing steel (1% C-1,5% Cr steel) to impart superplasticity. However, as shown in Figure 2, in the conventional processing heat treatment method described above, it is necessary to first heat the block to be treated at 1150°C for 1 hour, then air-cool it to 650°C, and then roll it isothermally at this temperature. be. In other words, the rolling reduction rate per 1 pass is 15%.
Before and after, heating is performed in the furnace between each pass at a constant temperature (6
Repeated rolling (16 passes, true/false ε=-2,
62). Next, sufficient superplasticity cannot be obtained unless the rolled plate material is further subjected to a heat treatment cycle as shown in the figure. As described above, the conventional processing heat treatment method requires many rolling passes under strict temperature control and a complicated heat treatment cycle, resulting in high costs and problems in practical application. (Object of the invention) In view of the above situation, the present invention has been made to provide iron-based alloys,
) Among practical steels such as bearing steel, 1-fine grain superplasticity can be expressed in a simple process, and it is useful for superplastic processing -IM-"°°15"6~"@kart:b
= “′). It is the object. (Structure of the invention)
i In order to achieve the above object, the inventors:
As a result of reexamining the usage of warm working and its effects in the conventional process heat treatment method, and conducting various basic experiments for this purpose, it was found that rolling, which requires a high isothermal working rate as in the conventional technique, was not used. It has been found that fine crystal grains can be easily obtained by relatively simple warm working or cold working. That is, in view of the purpose of the present invention, basic experiments were conducted. Currently, the practical bearing steel 5 is most widely used in Japan.
UJ-2 (high carbon chromium bearing #I) was used as a test material. Its chemical composition is shown in Table 1. As seen in the micrograph of FIG. 3 (5% nital corrosion), the structure of this sample material consists of a ferrite base with a grain size of 10 to 20 μm and spherical cementite grains of 0.5 to 2.0 μm. A round bar of 80 m φ in the spheroidized annealed state is
[55X50X100 at a temperature around 1100℃
It was forged into a 0 mm billet, and 10 blocks approximately 50 mm in length were cut from it. And these blocks: 1o□1@I(a)XL
t(b)4. -Oh, 48□, □□ 1 serving 5f
-.1 That is, first, the block was heated to 1150'C for 1 hour and then rapidly cooled to completely dissolve the carbide in the matrix.
Next, this was subjected to warm working (rolling) at 650°C. During this warm working at 650°C, the martensitic structure changes to a ferrite + fine semeitite structure, but the semeitite precipitated by the warm working becomes more spheroidal. After warm processing, two different treatments were performed. Figure 1 (a
) is a case in which superplasticity is achieved by heating directly to a temperature range of 650 to 730°C just below the A1 point (735°C) after warm working, while the process in Fig. 1(b) After temporary working, the material is heated once to a cementite+austenite two-phase region and quenched in oil, and then heated to a temperature region of 650 to 730° C. just below the A□ point to perform superplastic working. The differences between these two processes are as follows. As a result of the warm working at 650°C, finely distributed spherical cementite (particle size of approximately 0.05 mm) is produced, as shown in Figure 4(a).
It has a structure consisting of microcrystalline ferrite (grain size of about 1 μm) and fine crystal grain ferrite (grain size of about 1 μm). Therefore, even if the material is heated to a superplastic temperature range and processed as shown in FIG. 1(a), sufficient superplasticity can be obtained. However, in the rolling (warm working) state, the distribution of precipitated semeitite is quite uneven, so it is difficult to control the temperature range where austenite and cementite coexist, as shown in Figure 1 (b). After heating and oil quenching, the fourth
As shown in Figure (b), although the semeitite grains have grown into N, the non-uniform distribution of the semeitite has now become uniform. In other words, the average diameter is 0.8
Fine ferrite crystal grains on the order of μm and average grain size of 0.3μ
A uniform structure consisting of spherical cementite of about I was obtained. The heat treatment cycle shown in Figure 1(b) homogenizes the structure in this way, but it also converts the low-angle grain boundaries included in the warm rolling state into high-angle grain boundaries, thereby It is considered that superplasticity can be further improved. In fact, superplasticity was also confirmed by the following tensile rupture test and strain rate conversion test. The tensile test was conducted using the test piece that had been processed into the shape and dimensions shown in Figure 5 after the above-mentioned processing heat treatment shown in Figure 1 (a) or (b), using an Instron model equipped with an infrared radiation image furnace. The test was carried out using a universal testing machine while flowing a mixed gas (90:% N, +10% H2) to avoid oxidation of the test piece during the test. :■ Tensile rupture test To measure tensile ductility, IX10-'S-' to 8x10-
A tensile rupture test was conducted at a constant crosshead movement speed over an initial strain rate in the range of 'S''-'. Among the specimens that were tensilely deformed to failure, the test in the case of the processing heat treatment method shown in Figure 1(b) The specimen is shown in Figure 6. We also plot the elongation at break obtained from these results as a function of strain rate.
As shown in the figure. Note that the circles in FIG. 7 are for the heat treatment method shown in FIG. 1(a), and the triangle marks in FIG. 7 are for the heat treatment method shown in FIG. 1(b). From Figure 7, the overall trend is that the smaller the strain rate and the higher the deformation temperature, the higher the elongation at break, but at a deformation temperature of 730°C and a strain rate of 1.4 x 10-
In the case of 'S-', the elongation at break is decreased. This is considered to be due to the progress of crystal grain growth during the long-term test at such high temperatures. ■ Strain rate conversion test After the specimen was pulled at a low crosshead movement speed until it reached a steady deformation stress, the crosshead movement speed was changed one after another, and a small amount of strain was added at each crosshead movement speed. . In this way, each deformation temperature (650, 6
The relationship between strain rate and flow stress at temperatures (65, 680, 695, 710 and 730°C) was determined. All strain rate conversion tests were conducted within the range of elongation of 30% or less.
In this range, local necking of the specimen occurs;
Uniform elongation was obtained. FIG. 8 shows flow stress strain rate curves at various deformation temperatures obtained from the results of the strain rate conversion test. From these results, the strain rate sensitivity index m value shows a value close to 0.33 (n23, where n is the stress index) in most of the range of this experiment. This (7) mHo, 33. Th value 4,6 0UHCII (7) '□M□. Yuiaaaa, wow. = o, 5 engineering, □, i□I This is a relatively low value. However, the above processing heat
(Processing method 1 obtained. The diameter was 5".)
g40%), it is considered that superplastic flow plays an important role in the deformation process of this sample material in this region (m2 0.33). In some regions spanning the low temperature-high strain rate region, [□7th
As seen in the figure < m = 0, 2 (n = 5
) is shown. The existence of a region with a small m value □) in such a high strain rate region is the reason for the existence of a region with a small m value in conventional superplastic alloys. However, it is also commonly recognized in UHC-fi.
:1, and it can be said that in such a region, plastic flow progresses due to dislocation creep. Instead of the solution treatment shown in FIGS. 1(a) and 1(b), it is also possible to perform an annealing treatment to obtain a pearlite structure, and then perform warm working on this. In this case, the warm working at 650°C is a process of finely crushing the cementite in the pearlite to make it spheroidal and at the same time refining the ferrite grains, and it is thought that working and recovery or recrystallization proceed alternately during this process. After warm processing, the process is divided into two parts. Figure 1 (C) shows the case where superplastic working is performed by heating directly to a temperature range of 650 to 730 °C just below point A (735 °C) after warm working, and Figure 1 (d) shows the case where superplastic working is performed directly after warm working. After processing, the material is once heated to a cementite + austenite two-phase region and quenched in oil, and then heated to a temperature region of 650 to 730° C. directly below the A1 point to perform superplastic working. The difference between FIGS. 1(c) and 1(d) is as follows. By warm working at 650℃, most of the pearlite structure is crushed and finely distributed spherical cementite (grain size 0
．． It has a structure consisting of microcrystalline ferrite (grain size of about 1μ) and fine crystal grain ferrite (grain size of about 1μ). Therefore, it can be expected that sufficient superplasticity will be obtained even if the material is heated to a superplastic temperature range and processed as shown in FIG. 1(c). However, in the as-rolled state, small areas containing pearlite remain here and there and the microstructure is somewhat non-uniform, so this is changed to a temperature range where austenite and cementite coexist as shown in Figure 1(d). It is better to perform oil quenching after heating. Through this treatment, remaining pearlite disappears and spherical cementite is evenly distributed. Note that cementite grows rapidly and has an average particle size of about 0.3μ.
) remains sufficiently fine. Needless to say, the heat treatment cycle shown in Figure 1(d) only homogenizes the cementite structure in this way.
Instead of □, the low-angle grain boundaries of ferrite contained in the interpressure state are converted into high-angle grain boundaries, thereby further improving superplasticity. FIG. 9 shows the results of a tensile test performed on the processed and heat-treated materials shown in FIG. 1 (0) or (d). Figure 1(a),
Although the values of Σ and N elongation are lower than the heat-treated material in (b), it shows sufficient ductility necessary for superplastic working within the experimental range. The overall trend is that the strain rate decreases. Furthermore, the higher the deformation temperature, the more the elongation at break increases.
In case i, the elongation at break has decreased. This is considered to be due to the fact that the growth of crystal grains progressed during the long test at such high temperatures. Instead of the warm working shown in Figures 1(a) and (b),
As shown in e) and (f), a mechanical heat treatment process in which cold working is performed after tempering at around 650° C. is also possible. Furthermore, as shown in FIGS. 1(g) and 1(h), it is also possible to perform annealing treatment instead of solution treatment and directly perform cold working. Because the energy stored during cold working is larger than that of warm working, ferrite forms in more places when directly brought to the superplastic temperature range after cold working than during warm working. recrystallization and precipitation of cementite occur. Therefore, it is thought that the resulting crystal grains will become more refined, and greater superplasticity can be expected. Furthermore, even when heated to the γ+Fe5C two-phase region after cold working, austenite precipitation occurs in more locations than in warm working, resulting in a finer γ+Fe5G structure. Therefore, it is estimated that greater superplasticity can be obtained if this is quenched and processed in the superplastic temperature range. In addition, the annealing treatments shown in FIGS. 1(g) and (h) are shown in FIG. 1(e).
), (f), solution treatment + tempering is replaced with annealing, and the basic effects of cold working are as shown in Figure 1 (e), (f).
) is no different from that. Figure 10 shows the tensile test results for the processed and heat-treated materials shown in Figures 1 (e) and (f), and Figure 11 shows the tensile test results for the processed and heat-treated materials shown in Figures 1 (g) and (h). When cold working is performed, there is a difference in fracture ductility between when the material is heated to a two-phase region after cold working (triangle mark) and when it is directly subjected to superplastic working (circle mark). Furthermore, the annealed material shown in FIG. 11 has a smaller fracture ductility. However, within the experimental range, it shows sufficient ductility necessary for superplastic processing. The overall trend is that the smaller the strain rate and the higher the deformation temperature, the higher the elongation at break, but when the deformation temperature is 730°C and the strain rate is 1.4X10-'S-1, the elongation at break increases. It is declining. This is considered to be due to the progress of crystal grain growth during the test at such high temperatures and over a long period of time. Based on the knowledge obtained from the above basic experiments, and as a result of repeated similar experiments on various practical hypereutectoid steels, the present invention was developed in 2N. That is, the processing heat treatment method according to the present invention involves heating hypereutectoid steel to a temperature of A7 point or higher, then rapidly cooling or slowly cooling the steel, subjecting it to warm or cold working, and then directly treating the steel after warm or cold working. Fine ferrite can be produced by heating to a temperature just below the A1 point, or directly after warm or cold working, heating and holding in the two-phase region of cementite and austenite, then quenching, and then heating this to a temperature just below the A4 point. The main idea is to create a structure consisting of crystal grains and spherical cementite, and to develop superplasticity. The present invention will be explained in detail below. In the present invention, when carrying out processing heat treatment, the material is first heated to a temperature above point A. After that, the material is transformed by cooling, but this cooling method is by rapid cooling. It is also possible to create a structure in which many carbides are dissolved in the matrix, such as martensitic structure or bainite structure, or by slow cooling (
(F, C, etc.) may be used to form a pearlite structure. In either case, the transformation is caused by cooling. Next, warm working or cold working is performed. In the case of warm working, it is preferable to carry out at a temperature of 100° C. to point A□. The processing mode of this warm working is not particularly limited, and may be performed by rolling or the like. However, strict conditions such as conventional 16-pass rolling at a constant temperature (650° C.) are not required. Further, in the case of cold working, it is preferable to use a tempered material or an annealed material. Two methods are possible for treatment after warm or cold working: the first method is to directly heat the material to a temperature just below the 〇 point;
There is a second method in which after warm working, the material is heated and held in a two-phase region of cementite and austenite, then quenched, and then heated to a temperature just below the A0 point. In either method, it is important to perform superplastic working by heating to a temperature just below the A1 point, and thereby superplasticity can be developed. Note that directly below point A□ differs slightly depending on the applied steel type, but
Generally, it is preferable that the temperature is from the work point A to the work point A -100°C. The processing heat treatment method of the present invention can be applied not only to the bearing steel mentioned above, but also to
Needless to say, it can be applied to various hypereutectoid steels such as tool steel. (Example) Examples of the present invention are shown below, and test examples related to the aforementioned basic experiments are also included in the Examples of the present invention. The test materials having the chemical components shown in Tables 2 to 5 were processed under the conditions of the processing and heat treatment methods shown in the same table (however, the signals of each method correspond to the methods of each signal in Figure 1). , superplastic processing was carried out. The results are also listed in the same table. As can be seen from the same table, Figure 1 (a), (b), (c)
, (d), (e), (f), (g), and (h) show ductility that allows superplastic working.

[Left below]

:. 1) ゛[,,' □ -g (Effects of the Invention) As detailed above, according to the present invention, warm working is particularly introduced into the working heat treatment of hypereutectoid steel, so that the working heat treatment process is improved. The effect is extremely large because it is simple and allows practical hypereutectoid steel of various types such as bearing steel to exhibit superplasticity under practically advantageous conditions.

[Brief explanation of the drawing]

FIGS. 1(a) to (h) are schematic diagrams of processing heat treatment according to the present invention, and FIG. 2 is a schematic diagram of conventional processing heat treatment. FIG. 3 is a micrograph showing the microstructure of the sample material used in an example of the present invention before processing heat treatment, and FIGS. 4(a) and (b)
) is a micrograph showing the microstructure of the sample material in Fig. 3 subjected to the processing heat treatment of the present invention, Fig. 5 is a plan view showing the shape and dimensions (++uw) of the tensile test piece; (a) (b) shows the test piece for elongation measurement; Figure 6 shows the shape of the test piece after the breaking test; Figures 7, 9, 10, and FIG. 11 is a diagram showing the relationship between crosshead speed and elongation, and FIG. 8 is a diagram showing a flow path Kerr strain rate curve.

Claims (1)

[Claims] 1. Hypereutectoid steel is heated to a temperature of A_3 point or higher, then rapidly cooled or slowly cooled, subjected to cold or warm working, and then heated to a temperature just below A_1 point to form fine particles. A process heat treatment method for steel, characterized by creating a structure consisting of ferrite crystal grains and spheroidal cementite, and developing superplasticity. 2. The method for processing and heat treating steel according to claim 1, which comprises performing cold or warm working, then heating and holding to a two-phase region of cementite and austenite, and then quenching.