JP2833004B2 - Fine grain pearlite steel - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION <Industrial application field> The present invention relates to a high degree of work cold drawing, cold rolling, bending,
The present invention relates to a versatile fine-grained pearlite steel material having good workability such as twisting and punching.

<Prior art and its problems> Conventionally, “the characteristics of steel materials, such as workability (plastic workability, cutting characteristics, bending characteristics, weldability, etc.), low-temperature toughness, corrosion resistance, superplasticity, etc.” have increased as the structure becomes finer. The fact that "improves" is widely known, and based on that recognition, various techniques are used to refine the crystal grains of steel and to suppress grain growth. And, for example, Fe-13 to 18
Austenitic stainless steel of 8wt% Cr-8 ~ 12wt% Ni is cold-worked at room temperature to transform austenite into martensite, then heated to a stable austenite region and annealed to reverse martensite to austenite. It is reported that an austenitic steel material having an austenitic crystal structure with a grain size of 0.5 μm can be obtained by transformation [Iron and Steel, 74th year (1988) No. 6, No. 1052-1057].
Page] or low-carbon steel is strongly worked in the austenite region above the transformation point to induce fine ferrite, and immediately quenched to obtain an average grain size of 5 to 3 μm at a ratio of less than 1 to 50%. It has also been proposed to obtain a hot-rolled steel material containing ferrite crystal grains and the remainder having a quenched structure of martensite or bainite (Japanese Patent Publication No. 62-42021).

However, with these known techniques, a steel having a structure mainly composed of pearlite cannot be obtained, and a steel having a pearlite structure having a fine colony diameter of 5 μm or less has not been known at all.

In other words, the aforementioned “Iron and Steel”, 74th year (1988) No. 6
Nos. 1052 to 1057 "and JP-B-62-42021 also describe fine-grained steel, but the former relates to austenitic steel, and the latter relates to 50 to 99% martensite or The present invention relates to steel having a quenched structure of bainite, and is different from steel having a structure mainly composed of pearlite having a fine colony diameter.

By the way, high carbon steel having a structure mainly composed of pearlite has a large amount of hard and brittle cementite in the structure, and therefore has poor workability. However, the lamellar spacing in the pearlite structure (the difference between the cementite layer and the ferrite layer) It is well known that strength and ductility can be improved by reducing the layer spacing. However, regarding the pearlite colony diameter, another pearlite tissue factor, there are "theory that larger is better", "theory that smaller is better", and "theory that it has nothing to do". There is no such thing at all. Moreover, there was no pearlite structure having a colony diameter of 5 μm or less, and such fine pearlite remained completely unknown.

Therefore, the present inventors have investigated the “steel material having a structure mainly composed of pearlite grains” in which the average colony diameter of pearlite colonies is 5 μm or less in order to investigate the characteristic trend by further reducing the pearlite colony diameter in pearlite steel materials.
The research from various viewpoints was repeated aiming at realization of.

<Means for Solving the Problems> Here, a book aimed at realizing a “steel material having a structure mainly composed of pearlite grains” having an average colony diameter of pearlite colonies of 5 μm or less, which had never existed before. The inventors noted that, `` Even if existing austenite grains are worked by hot working, as in the case of steel structure refining means generally used in the past, new austenite grains can be obtained. There is a limit to the refinement of austenite, which is a high-temperature phase, as long as it is generated by recrystallization in hot working. It is difficult to manage the limitations naturally. " That is, the pearlite colony diameter of high carbon steel ultimately depends on the size of the austenite grains before hot working, and by any means, austenite grains before being worked are generated in a completely fine state. The research was advanced from the standpoint that only such measures could overcome the "limit of microstructural refinement of steel materials included in the prior art".

As a result, the present inventors have obtained completely novel findings as described below. (A) In the case of hot working steel, a heat history or a working history as in known hot working is passed through before the working,
Thereafter, after performing temperature control or the like so that at least a part of the steel structure exhibits a low-temperature phase structure, the temperature is increased while applying plastic working as a final stage of the processing to exceed the transformation point, and the low-temperature phase structure is obtained. Is transformed into an austenitic structure, an ultrafine austenitic structure that cannot be obtained by conventional controlled rolling or the like can be realized.

(B) In the ultrafine austenite structure generated by the reverse transformation, as described above, a prestructure (low-temperature phase structure) for the reverse transformation is temporarily obtained during the working before the hot working reaches the final stage. This can be achieved by placing the steel material under such temperature conditions and performing a process of raising the temperature to exceed the transformation point while applying plastic working to this low-temperature phase structure at the final stage of the subsequent processing, A pre-structure (low-temperature phase structure) for preparing an austenitic structure by reverse transformation from the stage is prepared, and firstly, processing in a cold temperature range or a warm temperature range is performed. The temperature is raised while the plastic working is performed so that the temperature exceeds the transformation point. "

(C) As described above, in the case where the temperature is increased while performing plastic working on the low-temperature phase structure and the transformation point is exceeded to reversely transform to the austenite structure, in order to sufficiently complete the reverse transformation, plastic working must be performed. After the completion of the temperature increase process, the A ₁ transformation point in a complete equilibrium state, that is, Ae ₁
It is also advantageous to employ means for maintaining the temperature at or above the point for a certain period of time.

(D) The hot-worked steel material having the ultrafine-grained austenite structure obtained in this manner is then subjected to various cooling means (for example, cooling, gradual cooling) conventionally applied to impart the desired properties to the product. , Heat retention, accelerated cooling, multi-stage cooling, cooling with additional processing, quenching, or a combination thereof) results in a uniform and ultrafine transformed structure that cannot be obtained by the prior art.

(E) Moreover, in the “steel material mainly composed of pearlite grains” obtained by treating high-carbon steel in this way, when the average colony diameter of pearlite colonies becomes 5 μm or less,
Various properties (especially workability) of the steel material show a remarkable improvement that was not expected from conventional knowledge.

The present invention has been made based on the above findings and the like,
"It has extremely excellent processability, which has not been heretofore existed."
It has realized a very fine grain pearlite steel material having the following structure mainly composed of pearlite grains ".

Here, the term “pearlite colony” is defined as a region in which a ferrite plate (layer) and a cementite plate (layer) are arranged in parallel in the same direction in a pearlite structure, and “pearlite colony diameter”. Means a grain diameter when the above-mentioned region is regarded as a grain. In addition, “a structure mainly composed of pearlite grains” means that the structure contains 50% pearlite.
%, And when the percentage of pearlite in the steel structure reaches 50%, the characteristics of the steel material are almost dominated by the characteristics of pearlite.

Further, the component composition of the steel material according to the present invention is not particularly limited as long as a structure mainly composed of pearlite can be obtained, and the components in the carbon content range in which pearlite is formed by eutectoid transformation, and further, Up to 3 wt% Si and Al,
B, Ca, V, Nb, Ti, Zr, W containing up to 18 wt% of Mn, up to 30 wt% of Cr, or Ni or Mo in necessary amounts, or even these alloy steel compositions Needless to say, the composition may be a composition in which one or more elements such as Ta and the like are added. Of course, depending on the purpose, rare earth elements such as La and Ce and S, Pb, Bi
In addition, the composition of components to which free-cutting elements including Se and Se are added is also an object.

Next, the reason why the average colony diameter of the pearlite colony in the steel material of the present invention is set to 5 μm or less, and the means for producing the steel material of the present invention will be described.

<Effects> There have been reports that the workability of steel materials improves when the pearlite colony diameter is reduced, but “Materials and Processes” Vol.2 (1989), No. 3, p. 896｝, When the colony diameter was around 10 μm, the processing characteristics tended to be saturated, and the change in characteristics in a fine colony of 5 μm or less was completely unknown. However, according to the study of fine pearlite colony steel material realized by the new means, the workability of the pearlite steel material having a colony diameter of 5 μm or less is unexpectedly improved as compared with the pearlite steel material having a colony diameter of more than 5 μm, and is particularly improved by 2 μm It has been found that the improvement effect becomes extremely remarkable below. For this reason, in the pearlite steel material of the present invention in which pearlite accounts for 50% or more of the steel structure and controls the properties of the steel material, the average colony diameter of the pearlite colonies is limited to 5 μm or less, but preferably 2 μm.
The following is preferred.

Incidentally, the steel material according to the present invention is realized by the following manufacturing means. That is, the material steel is at least partially made into a microstructure state composed of a low-temperature phase (such as pearlite), and the temperature is raised to a temperature above the transformation point (Ac ₁ point) while being subjected to plastic working, or Subsequently, Ae is a means for maintaining a temperature region of at least _one point for a certain period of time and temporarily transforming part or all of the structure composed of the low-temperature phase back to austenite to produce ultrafine austenite grains, and then cooling.

As the plastic working method applied at the time of the above reverse transformation, known plate rolling mills, various rolling mills for seamless steel pipes, punching machines,
Required processing at required temperature range by using well-known hammers, swagers, stretch reducers, stretchers, twisting machines, extruders, drawing machines, etc., in addition to hole rolling mills for strip steel and wire rods. Any method can be adopted as long as the method can perform the processing of the degree, and there is no particular limitation.

The amount of strain in the plastic working is important in that the following three actions occur. One is the effect that extremely fine austenite crystal grains are induced by processing from the low-temperature phase hardened by processing the low-temperature phase,
The second is the action of generating the heat generated during processing to raise the temperature of the workpiece to the transformation point where the low-temperature phase transforms to austenite.The third is work hardening of the fine austenite crystals that have been generated. This is the effect of forming a finer pearlite colony during the subsequent formation of a pearlite structure. From such a viewpoint, the amount of strain in the plastic working is preferably 20% or more, and more preferably 50% or more.

It is essential that the temperature of the steel material to be processed rises to a temperature at which the low-temperature phase reversely transforms to austenite, that is, to the Ac ₁ point or more. Of course, Ac although even _one point or more temperature range that temperature becomes a two-phase mixed structure of low-temperature phase and austenite to be Ac less than ₃ points, Ac ₃ according to the method of applying the plastic working while the temperature rise Even in the temperature range below the point, the crystal grains are sufficiently refined by processing and recrystallization. However, the characteristic action of "working the low-temperature phase produces extremely fine austenite crystal grains from the work-hardened low-temperature phase structure induced by the processing."
In order to exert the effect sufficiently, it is desirable to raise the temperature to the Ac ₃ point or more if possible. However, some products require a two-phase structure consisting of a low-temperature phase and austenite.
Needless to say, it is necessary to keep the temperature within the temperature range of less than Ac ₃ points.

As described above, the reason for raising the temperature while performing plastic working during the reverse transformation from the low-temperature phase to the austenite phase is as follows. This is for the purpose of "work-induced generation of fine austenite grains from the structure" and "refinement by processing of austenite grains" and "strain-induced transformation of fine low-temperature phase structure from work-hardened austenite grains". Next, the present invention will be described more specifically based on examples.

<Examples> Steels having the component compositions shown in Table 1 were melted in an induction heating melting furnace, then hot forged and allowed to cool, then heated to 950 ° C and subjected to normalizing treatment. Then, the normalized material was machined to a required outer diameter and subjected to subsequent rolling to produce a fine-grained pearlite steel material.

In addition, a part of the molten material was made into a 120 mm square piece by soaking cracking and subjected to normal wire rolling to produce a steel material as a trial.

 The test examples are shown below.

Test Example 1 In Test No. 1, first, a steel piece having a cross section of 120 mm × 120 mm made of steel B was subjected to normal hot rolling, and immediately after the rolling, a 5.5 mmφ direct patenting wire, which was blast-cooled on a Stellmore conveyor, was used. Was performed. That is, the direct patenting wire is cold drawn to 3.0 mmφ (drawing degree: 70%) and rapidly heated to 950 ° C. by high frequency heating (heating rate:
(90 ° C./sec), and immersed in a lead bath to apply a patenting treatment.

In Test Nos. 2 to 4, first, steel B externally machined to 30 mmφ
After heating steel bar to 900 ℃, 17.0m with 8 stand tandem mill
Rolled to mφ, wait for the temperature of the rolled material to drop to 600 ° C by allowing it to cool, then raise it to 700 ° C again by high-frequency heating, and apply a 90% reduction to 5.5mmφ with a 10-stand tandem mill. Rolled. At this time, the rolling end temperature increased to 920 ° C. in Test Nos. 2 and 3, and to 915 ° C. in Test No. 4, exceeding the transformation point.

After rolling, test No. 2 was spray-water cooled, test No. 2 was allowed to cool to room temperature, and test No. 4 was forced air cooled to room temperature.

Further, in Test No. 3, a 5.5 mmφ wire that had been allowed to cool to room temperature was rapidly heated to 700 ° C. by high frequency heating, and then rolled to 2.0 mmφ in a 10-stand tandem mill (rolling end temperature: 910).
° C), and immediately after the rolling, forced air cooling. Test number 4
Then, after rolling to 2.0 mmφ (rolling end temperature: 910 ° C.) in the same manner as in Test No. 3, spray cooling was performed.

Subsequently, the structure of the thus obtained wire rods of Test Nos. 1 to 4 was observed and the mechanical properties were examined.

Furthermore, these wires were drawn by a wire drawing machine, and the limit of the drawing process was investigated. In addition, the drawing limit was evaluated using the drawing path immediately before the drawing path broken during the drawing. That is, the degree of wire drawing limit (drawing limit) The evaluation was made by displaying the logarithmic distortion.

In addition, a wear resistance test was performed on the 5.5 mmφ wire obtained by the treatment of Test No. 1 and the 2.0 mmφ obtained by the treatments of Test Nos. 2 to 4. In the wear resistance test, for a 2.0 mmφ wire rod, this was wrapped into a 1.9 mmφ cylinder, rubbed with a bearing steel ball, and the value obtained by dividing the weight loss of the test piece by the distance traveled was used. Amount (g /
cm).

 The test results are shown in Table 2.

Test Example 2 Using the three types of steels A to C shown in Table 1 above, the same history as Test No. 3 in Test Example 1 was passed, and 5.5 mmφ
After the wire was rapidly heated to 700 ° C. by high-frequency heating, it was rolled with a 10-stand tandem mill (reduction ratio: 86.8%) to obtain a 2.0 mmφ wire.

The rolling end temperature at this time was 915 in Test Nos. 5 and 6.
° C and 910 ° C in Test No. 7, both rising to above the transformation point.

The cooling after the 2.0 mmφ rolling was spray cooling in Test Nos. 5 and 6, and forced air cooling in Test No. 7.

With respect to the wires obtained in Test Nos. 5 to 7 obtained in this way, the structure and mechanical properties were examined, and the drawing limit and wear resistance were also examined. The results are shown in Table 3 together with the trial production conditions. Was.

The drawing limit and wear resistance were examined in the same manner as in Test Example 1.

The following can also be confirmed from the results shown in Tables 2 and 3.

That is, a steel having a 100% pearlite structure like steel B and having a pearlite colony diameter of 5 μm or less as obtained in Test Nos. 2 to 4 has strength and ductility as compared with the conventional steel in Test No. 1. Is remarkably excellent, and it is apparent that the improvement of the wire drawing limit is particularly remarkable. The characteristics are as follows: the pearlite colony diameter is 2 μm or less, 1 μm
m, the workability of ultra-fine pearlite steel far exceeds the conventional knowledge. You can see that there is.

Furthermore, as can be seen from the results of Test Nos. 5 to 7, when the pearlite colony diameter was reduced to 5 μm or less, carbon steel, alloys from steel A having a pearlite amount of about 60% to hypereutectoid steel C were obtained. It is also clear that both steels can have excellent properties-improving effects.

By the way, Tables 2 and 3 also show the results of the wear resistance test of the obtained steel material, but the conventional eutectoid steel was 3.7 × 10 ⁻⁶ g.
/ cm, whereas the fine pearlite steel according to the present invention has a value of 1 × 10 ⁻⁶ g / cm or less, and especially when the pearlite colony diameter is 1 μm or less, it is reduced to about 1/10 of the conventional steel. It is clear that the effect of improving the properties by the refinement of the pearlite structure is not limited to the workability.

<Summary of Effects> As described above, according to the present invention, it is possible to provide an ultra-fine grained pearlite steel material that could not be actually realized by the conventional technology, and it has never before been possible including workability. Industrially extremely useful effects such as a stable supply of steel having excellent various properties can be obtained.

Claims (1)

(57) [Claims] 1. The pearlite colony having an average rolony diameter of 5
A fine-grained pearlite steel material having excellent workability, characterized by having a structure mainly composed of pearlite grains of μm or less.