JP4428185B2 - High strength and high ductility wire having ultrafine grain structure and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength and highly ductile wire having an ultrafine grain structure and a method for producing the same. It is what we planned.

In recent years, bolts, screws, springs, and wires used as their materials have been required to have higher strength in order to save energy in transportation equipment such as automobiles, increase the height of buildings, and reduce the size of various industrial equipment. Has been. As is well known, ductility decreases when the strength of steel materials is increased. When ductility decreases, workability such as cold forgeability and cold rollability deteriorates, resulting in a decrease in yield and efficiency of component manufacturing. It is necessary to use a suppressed high strength and high ductility wire.
Further, from the viewpoint of resource recyclability, it is desirable to achieve high strength and high ductility by using ordinary low carbon steel without adding alloying elements.

Several projects and studies have been made so far to increase the strength of steel materials without using alloy elements as much as possible.
For example, in the super metal (or super steel) project, the current 400 MPa class steel is used, and the crystal grain size is reduced to 1 μm or less, so it has a strength of about 800 MPa and has ductility. Ferritic steels that are easy to weld are being studied.
In this study, when the strength of the ferrite is increased by reducing the grain size of the ferrite, the Hall-Petch relationship is established, that is, if the crystal grain size of the ferrite is reduced, the yield strength and the tensile strength are increased. It has been shown that it rises and at the same time the toughness improves.
However, this technique has a problem that the elongation in the tensile test, which is a main index of the ductility of the steel sheet, is significantly reduced.

Further, in Non-Patent Document 1, studies have been made to increase the strength to 800 MPa class by using 400 MPa class composition steel that is easy to weld and setting the crystal grain size of ferrite + carbide structure to 1 μm or less. As a result, a sample with a thickness of 8 mm was austenitized (heated at 1100 ° C., held for 60 seconds), then cooled with water to obtain a martensite structure, and subjected to hot rolling at 640 ° C. with a total reduction ratio of 90%. It was reported that the ferrite structure was refined equiaxially to a nominal grain size of 0.77 μm, and a Vickers hardness Hv: 245 corresponding to a tensile strength of 760 MPa was obtained.
However, there is no report that a specimen for a tensile test was prepared and the tensile strength and ductility were directly measured. Further, the steel used as a sample has a high Mn content of 2.03% in order to ensure hardenability, which is not preferable from the viewpoint of recyclability.

In addition, there are many reports on increasing the strength by refining crystal grains without adding alloying elements, but these are all due to large strain processing and require special processing equipment. It is not suitable for commercial production.
The inventors also performed annealing at room temperature ARB (Accumulative Roll-Bonding), which is large strain processing, and annealing on ferrite + pearlite structure steel, and examined changes in the resulting structure and mechanical test values. Even after large strain processing, it becomes a non-uniform structure in which regions containing cementite and non-existing regions coexist. Ductile steel could not be obtained.

Therefore, the inventors have studied to obtain a high strength and high ductility steel material by a method applicable to actual commercial production, using ordinary low carbon steel as a raw material, without changing much of the conventional manufacturing process.
As a result, we found that when martensite obtained by forming a coarse austenite structure and then water-cooling is annealed after low strain processing, the ferrite structure becomes ultrafine, and based on this, a technology for obtaining a desired high strength and high ductility steel material is obtained. Developed (see Patent Document 1).

Here, in order to make the ferrite structure of ordinary low-carbon steel ultrafine, the idea of using the martensite structure as a starting material is used in the STX-21 project and the super metal project that promote the development of super steel. These are all directed to large strain processing, and as described above, high strength is achieved, but high ductility cannot be obtained.
On the other hand, Patent Document 1 is characterized in that a steel material having high strength and high ductility is obtained by low strain processing of martensite.

  However, the technique disclosed in Patent Document 1 is intended for steel sheets, and since martensite is processed by cold rolling, it cannot be applied to a wire that performs wire drawing instead of cold rolling. Actually, when the experiment was conducted by changing the cold rolling to wire drawing, the tensile strength was about 780 MPa, but the drawing, which is the main index of ductility of the wire, was only about 50%. It was.

CAMP-ISIJ, Vol 11 (1998) P.1031-1034 JP 2002-285278 A

As described above, in the prior art, a wire having high strength and high ductility cannot be produced using ordinary low carbon steel.
The present invention is a further improvement of the technique disclosed in Patent Document 1, and is a method applicable to actual commercial production using ordinary low-carbon steel rich in recyclability, without changing much of the conventional process. Then, it aims at providing a high intensity | strength highly ductile wire.
Here, since the conventional ordinary low carbon steel wire has a tensile strength of about 400 MPa and a drawing of about 73%, the present invention aims to obtain a tensile strength of 700 MPa or more and a drawing of 73% or more. . As a result, the strength can be sufficiently increased as compared with the conventional material, and it is not necessary to consider the ductility reduction and the workability deterioration caused by the increase in strength.

As described above, a technique for obtaining a high strength and high ductility ordinary low carbon steel sheet by low strain processing of martensite is disclosed in Patent Document 1, but this technique cannot be applied to a wire as it is.
Therefore, as a result of intensive studies on optimization of manufacturing conditions when applying low strain processing of martensite to a wire rod, the inventors once coarsen austenite grains in order to obtain a conventional martensite structure. Although it was thought that it was necessary, even if the austenite grains were not coarsened and cooled as they were fine austenite grains, if some martensite could be secured, by subsequent low strain processing and annealing, It has been found that the ferrite structure can be made ultrafine, and in this case, not only high strength but also high ductility can be obtained.

That is, conventionally, since ordinary low carbon steel is inferior in hardenability, it is necessary to coarsen austenite as described in Patent Document 1 in order to secure hardenability and obtain a martensite structure. It was thought.
On the other hand, the present invention is an ordinary low carbon steel inferior in hardenability, and it is one of the technical outlines to obtain fine martensite + ferrite structure by quenching fine austenite. Is something that did not exist before.

When fine austenite is quenched, it is difficult to make martensite entirely. However, if a predetermined amount of martensite can be secured even if the martensite structure is not entirely made, this is low. High strength and high ductility can be achieved by applying strain processing and appropriate annealing.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
(1) A high-strength, high-ductility wire having an ultrafine-grained ferrite structure with an average crystal grain size of 1 μm or less and a tensile strength of 700 MPa or more and a drawing of 73% or more. A steel material containing 80% or more of fine martensite obtained by making ordinary low carbon steel or ordinary low carbon steel containing B of 0.01 mass% or less into a fine austenite structure and then water-cooling the total area reduction ratio: 20 A high-strength, high-ductility wire having an ultrafine grain structure obtained by performing cold drawing and annealing at a rate of not less than 60% and less than 60% .

( 2 ) An annealing temperature in the annealing is 400 ° C. or higher and 550 ° C. or lower. The high strength and high ductility wire having an ultrafine grain structure as described in ( 1 ) above.

( 3 ) Ordinary low carbon steel or ordinary low carbon steel containing 0.01 mass% or less B is processed and heat-treated to reduce the average austenite grain size to 50 μm or less, and then water-cooled to obtain the obtained fine martensite. A steel material containing 80% or more of the site is cold-drawn under the conditions of a total area reduction of 20% or more and less than 60%, and then annealed to obtain an ultrafine-grained ferrite structure with an average grain size of 1 μm or less. A method for producing a high-strength, high-ductility wire having an ultrafine grain structure.

( 4 ) The method for producing a high-strength and high-ductility wire having an ultrafine grain structure according to ( 3 ), wherein the annealing is performed in a temperature range of 400 ° C. or higher and 550 ° C. or lower.

  According to the present invention, using ordinary low carbon steel that is highly recyclable, it can be applied to actual commercial production without changing much of the conventional manufacturing process, and has a tensile strength of 700 MPa or more and a drawing of 73% or more. It is possible to stably obtain a high-strength and high-ductility wire having a diameter, which is extremely useful industrially.

The preferred component composition range, microstructure and production conditions of the present invention are described in detail below.
In the present invention, the component composition of steel is usually low carbon steel from the viewpoint of resource recyclability.
The ordinary low carbon steel in the present invention is a steel material having a mass% of C ≦ 0.2%, Si ≦ 0.5%, Mn ≦ 1.6%, P ≦ 0.05%, S ≦ 0.05%, Al ≦ 0.08% and N ≦ 0.01%. Point to. Moreover, B which has the effect | action which improves hardenability and accelerates | stimulates a martensitic transformation may be contained in 0.01% or less of range.

In the present invention, the steel having the above component composition is first heated to an austenite region and then water-cooled to obtain a martensite structure, but the austenite grains before cooling may be limited to an average particle size of 50 μm or less. is important.
This is because, if the average particle size of austenite before cooling exceeds 50 μm, high strength can be obtained but high ductility cannot be obtained.

  Here, in order to suppress the average particle size of austenite to 50 μm or less, it is important to set the heating temperature to 850 to 980 ° C. and the heating time to 1 to 30 minutes. This is because if the heating temperature is less than 850 ° C or the heating time is less than 1 minute, the structure becomes ferrite + austenite during heating, and 80% or more of martensite cannot be obtained after water cooling. If the temperature exceeds 980 ° C. or the heating time exceeds 30 minutes, the average particle size of austenite exceeds 50 μm, and a good drawing value, that is, high ductility cannot be obtained.

After the above-described austenitizing treatment, the structure is martensite by water cooling. However, in the present invention, if the volume fraction of martensite is 80% or more, an ultrafine-grained ferrite structure is formed by low strain processing described later. Since desired characteristics can be obtained, the amount of martensite may be 80% or more in terms of volume fraction.
The remaining structure may be a structure other than martensite, such as Widmanstatten-like pro-eutectoid ferrite.

Next, low strain processing is performed to obtain an ultrafine ferrite structure.
The low strain processing referred to in the present invention means cold drawing at an appropriate processing rate and subsequent low-temperature annealing. By this treatment, the microstructure is made into ultrafine ferrite having an average crystal grain size of 1 μm or less.
Here, for cold wire drawing, if the total area reduction ratio (= (cross-sectional area before drawing−cross-sectional area after drawing) / cross-sectional area before drawing × 100%) is 20% or more, the particle size : 1μm or less ultra-fine ferrite is obtained . On the other hand, if the total area reduction ratio is 60% or more , not only the ductility after annealing is lowered, but also there is a risk of disconnection during wire drawing. The area ratio should be 20% or more and less than 60%.

  Also, for annealing after cold drawing, if the annealing temperature is 400 ° C or higher, it becomes easy to obtain a drawing of 73% or more with an ultrafine ferrite structure by continuous recrystallization, and the annealing temperature is 550 ° C or lower. If there is, discontinuous recrystallization does not occur and a tensile strength of 700 MPa or more can be easily obtained, so the annealing temperature is preferably about 400 to 550 ° C.

According to the present invention, the reason why a high-strength and high-ductility wire having both a tensile strength of 700 MPa or more and a drawing of 73% or more can be obtained from ordinary low carbon steel is considered to be due to the effective refinement of ferrite. .
The reason why the ferrite becomes ultrafine is thought to be because fine martensite obtained by water-cooling fine austenite is processed with low strain. In other words, martensite transformed from fine austenite is fine, and when it is processed with low strain, non-uniform deformation occurs and ferrite nucleation represented by grain boundaries (former austenite grain boundaries), packet boundaries, block boundaries, etc. Since the number of sites increases, it is considered that the ferrite becomes ultrafine. In addition, the suppression of ferrite grain growth by the pinning effect of the carbide is presumed to contribute to the ultrafine ferrite.
Thus, when the ferrite is refined according to the present invention, it is considered that the strength is increased, and crack propagation is frequently suppressed at the grain boundary, so that the local ductility is increased and the drawing is also improved.

Next, effects and the like of the present invention will be described with reference to examples.
A wire rod having a diameter corresponding to JIS SWRCH12A shown in Table 1 and having a diameter of 5.5 mm was pickled, heated in the austenite region for 5 minutes, and then immediately water-cooled to obtain a martensite structure. At that time, the austenite grain size was changed by setting the austenitizing temperature to two levels of 950 ° C. and 1100 ° C. When the structure of the martensite obtained at this point was observed, the average particle size of the austenite before quenching was 40 μm for the 950 ° C. heating material and 120 μm for the 1100 ° C. heating material. The martensite volume fractions were 85% and 96%, respectively, and the balance was acicular ferrite.
Subsequently, the obtained wire was pickled, subjected to a phosphate film treatment and a metal soap lubrication treatment, and then cold-drawn with a total area reduction ratio of 50% in three passes. Thereafter, the wire was annealed at 300, 400, 450, 500, 550, 575, and 600 ° C. for 30 minutes, and after annealing, it was air-cooled to room temperature.
A JIS No. 2 tensile test piece was prepared from the wire thus obtained and subjected to a tensile test.
FIG. 1 shows the tensile test results as a balance of tensile strength (TS) -drawing (RA).

As shown in the figure, the austenite grain size was refined to 40μm, and after cold drawing, annealing was performed at a temperature of 400-550 ° C. The aperture is obtained at the same time.
On the other hand, when the austenite grain size is 120 μm, which is outside the proper range of the present invention, there is no material that satisfies the tensile strength of 700 MPa or more and the drawing of 73% or more at any annealing temperature.
When the structure of the 950 ° C heating material was observed in a thin film using a transmission electron microscope, when subjected to 400-550 ° C annealing that simultaneously satisfies a tensile strength of 700 MPa or more and a drawing of 73% or more, the structure is composed of carbides and polygones. The ferrite was composed of null ferrite, lath ferrite, or subgrain ferrite, and the ferrite had ultrafine grains with an average grain size of 1 μm or less.

It is a figure which shows the relationship between the tensile strength (TS) single drawing (RA) balance of a wire, the austenite grain size before quenching, and annealing temperature.

Claims (4)

A high-strength, high-ductility wire with an ultrafine-grained ferrite structure with an average grain size of 1 μm or less and a tensile strength of 700 MPa or more and a drawing of 73% or more. A steel material containing 80% or more of fine martensite obtained by making carbon steel or ordinary low carbon steel containing B of 0.01 mass% or less into a fine austenite structure and then water-cooling , total area reduction ratio: 20% or more, A high strength and high ductility wire having an ultrafine grain structure obtained by performing cold drawing and annealing of less than 60% . The annealing temperature in the annealing, 400 ° C. or higher, high strength and high ductility wire having an ultrafine grain structure of claim 1, wherein a is 550 ° C. or less.   Ordinary low carbon steel or ordinary low carbon steel containing 0.01 mass% or less B is processed and heat-treated to reduce the average austenite grain size to 50 μm or less, then water-cooled, and the resulting fine martensite is 80 % Of the steel material containing at least 20% is cold drawn under conditions of 20% or more and less than 60%, and then annealed to obtain an ultrafine-grained ferrite structure with an average crystal grain size of 1 μm or less. A method for producing a high-strength, high-ductility wire having an ultrafine grain structure. The method for producing a high strength and high ductility wire having an ultrafine grain structure according to claim 3 , wherein the annealing is performed in a temperature range of 400 ° C or higher and 550 ° C or lower.

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