JP3380032B2 - Method for producing high-strength steel wire with excellent drawability - Google Patents

Description

Detailed Description of the Invention

[0001]

The present invention relates to C: 0.3 to 0.92 quality.
The amount%, Si: 2.0 wt% or less, Mn: 1.0 wt% or less
Below, Cr: 0.1 to 2.0 mass % , balance iron and impurities
Steel composed of elements ( hypo-eutectoid steel, eutectoid steel or hyper-eutectoid steel ) , or Cu, Ni, Ti, Nb,
When casting a steel wire manufactured by rolling an alloy steel containing one or more of V, Mo and B (hereinafter “eutectoid steel etc.”) (hereinafter “rolled steel wire such as eutectoid steel”) The present invention relates to a manufacturing method in which, in a consistent process up to rolling of a wire rod, the steel material is rolled without completion of transformation from an austenite phase and immediately subjected to isothermal transformation heat treatment.

[0002]

2. Description of the Related Art Conventionally, rolled steel wire such as eutectoid steel is cast into a bloom, and is heated and held at a high temperature for a long time to reduce segregation in a slab and homogenize, and then rolled. The conventional rolling process is divided into two stages. The first stage is billet rolling and the second stage is wire rolling from the billet. In both rolling processes, bloom or billet is reheated before rolling.

Especially in the former rolling, the temperature around 1200 ° C.,
In some cases, it is heated at a high temperature near 1300 ° C,
This not only raises the manufacturing cost, but also prevents simplification of process control. The reason why such a heating process is adopted is as follows. That is, in the conventional manufacturing method, since a large cross-section bloom is adopted, segregation inside the slab is significant, so in order to reduce this, the growth of columnar crystals is suppressed by an external operation such as electromagnetic stirring,
A casting method is adopted in which free crystals are formed inside. For this reason, the solidification structure of the slab is coarse, and C, Mn, P,
Since a large number of local segregation (“semi-macro segregation”) portions such as S are scattered, it is inevitable to heat the bloom at high temperature and homogenize the internal structure by rolling. In particular, in the rolled steel wire such as eutectoid steel, which is the object of the present invention, if the internal structure is not sufficiently homogenized, the adverse effects of minute inclusions that inevitably remain in the steel are promoted and the product characteristics are likely to be impaired.

[0004]

DISCLOSURE OF THE INVENTION The present invention eliminates reheating of a bloom or billet before rolling, which is a conventional process, in producing a rolled steel wire such as eutectoid steel, and has a slab immediately after casting. Eutectoid steel that uses heat to perform rolling, uses the heat of the steel wire immediately after rolling to perform isothermal transformation without reheating after rolling, and has superior properties compared to materials produced by conventional processes The present invention relates to a method for manufacturing an equal rolled steel wire.

That is, by performing a series of steps without transforming from the austenite phase until the final isothermal transformation heat treatment after the completion of solidification, the heating energy cost required for the intermediate step can be reduced and the steps can be performed directly. In addition to the above, the purpose is to obtain an extremely fine transformation structure by utilizing the processed structure obtained by immediately applying the rolling process to the solidified structure that has been cast while securing the solidification rate above a predetermined speed. There is.

[0006]

Means for Solving the Problems The present inventor has set a temperature of 1200 ° C., which is performed prior to rolling of a slab, which is a preceding rolling step in a current consistent manufacturing process of rolled steel wire such as eutectoid steel. Heat treatment at a high temperature exceeding (hereinafter referred to as "bloom heating"),
The metallurgical function performed by the heat treatment (hereinafter referred to as "billet heating") preceding the wire rolling, which is the latter rolling, was studied in detail. As a result, although bloom heating reduces the semi-macro segregation at the center of the slab, the degree of segregation does not have an industrially significant segregation reduction effect as is generally considered in the past. The flattening of dendrite microsegregation (hereinafter referred to as "microsegregation") in the entire region is extremely remarkable.

This facilitates the growth and coarsening of austenite crystal grains during billet heating. Therefore, the cast piece subjected to such treatment cannot suppress coarsening of austenite crystal grains unless the heating temperature is set low. By the heat treatment in which the coarsening of the heated austenite is suppressed in this way, the carbides of Cr, Ti, Nb, V, Mo and B cannot be sufficiently dissolved in the steel matrix before the billet rolling, and the carbides thereof cannot be completely dissolved. Not only does it significantly reduce the effect on various material aspects in grain refinement or transformation from austenite during precipitation strengthening and rolling recrystallization, but also because of these undissolved carbides and nitrides, the final wire It was found that the cold workability was reduced.

On the other hand, the relationship between the casting structure and the rolling structure when eutectoid steel or the like is subjected to rolling immediately after casting, and
The metamorphosis after that was studied in detail. As a result, the working structure of the as-solidified austenite phase of eutectoid steel etc. when rolled is not introduced uniformly, but the concentrated part of microsegregation, that is, the dendrite inter-wood part (hereinafter referred to as "concentrated part" )) And the non-concentrated part, that is, the dendrite dendritic part (“non-concentrated part”) differ in the residual amount of the processed structure, and the concentrated part retains a processed structure with an extremely higher density than the non-concentrated part. . This non-uniform work structure has a function of a transformation nucleus upon transformation from austenite, and this transformation nucleus function is stronger than that of an austenite grain boundary. Therefore, it was found that if the unit size of the heterogeneous structure is made fine, an extremely fine transformation structure can be obtained even if the austenite crystal grains are extremely coarse.

As a result of research on the suitability of the casting conditions and the rolling conditions that can be industrially adopted based on such knowledge, the optimum casting conditions and the rolling processing conditions have been obtained. Furthermore, the present inventor has come to know that in order to obtain a fine transformation structure from a coarse cast austenite grain structure, it is important not to perform recovery / recrystallization from the rolling work structure until the start of transformation.

As manufacturing conditions for industrially realizing the above knowledge, not only the columnar crystal portion is developed as much as possible without causing molten steel flow during solidification of cast slab, but also the dendrite of the columnar crystal portion is refined. Therefore, it is necessary to set the solidified shell growth rate to a predetermined rate or more, and to set the range of solidification at the solidified shell growth rate to a predetermined ratio of the slab cross-sectional area in order to spread the fine work structure throughout the slab. It was found by repeating a number of full-scale experiments that it is necessary to secure a sufficient amount of austenite rolling and to perform rapid cooling so that the elapsed time until the isothermal holding process after rolling is as short as possible. The present invention has been completed.

That is, the main points of the present invention are: (1) C: 0.3 to 0.92 % by mass , Si: 2.0 % by mass
% Or less, Mn: 1.0 mass% or less, Cr: 0.1 to 2.
In a continuous casting method, steel containing 0 mass % of the balance iron and impurity elements ( hypo-eutectoid steel, eutectoid steel or hyper-eutectoid steel ) was
At least 7 of the cross section of the slab at the solidified shell growth rate of mm / min or more
A slab obtained by solidifying a portion of 0% or more is rolled at a cross-section reduction rate of 98% or more without lowering the transformation point from austenite in the cooling process after casting, and rapidly cooled within 30 seconds after the end of rolling to 600 A method for producing a high-strength steel wire having excellent wire drawability, which is characterized by holding at a temperature of not higher than 300 ° C and not lower than 300 ° C, and holding and transforming at that temperature.

(2) Rapid cooling after rolling and holding at a temperature for transformation are performed by immersing in a molten salt bath of potassium nitrate and sodium nitrate or by injecting the molten salt. A method for producing a high-strength steel wire having wire drawability. (3) Part or all of the quenching after rolling is performed by cooling with water or a refrigerant in which air and water are mixed, (1)
Alternatively, the method for producing a high-strength steel wire having excellent wire drawing workability according to (2).

(4) As steel components, Cu: 1.0% by mass or less Ni: 1.0% by mass or less Ti: 0.2% by mass or less Nb: 0.3% by mass or less V: 0.3% by mass % or less Mo: 0.3 mass% or less B: has excellent drawability according to any one of 0.005 wt% or less was added one or two or more (1) - (3) It is a method of manufacturing a high-strength steel wire.

Here, the reason for limiting the components will be described. Cr is an essential element for exhibiting the effects of the present invention. That is, C in the high temperature austenite state after casting
Since it is directly subjected to the rolling process from the solid solution state of r, it not only suppresses recovery and recrystallization of the processed austenite during and after rolling, but also reveals a uniform transformation structure in the transformation from the austenite after rolling. Can help.

Further, Si, Mn, Cu, Ni, Ti,
The reason why the upper limits of the amounts of Nb, V, and Mo added are set will be described. Ti, Nb, V, and Mo are all added for the purpose of forming a fine θ phase during precipitation strengthening by carbides or nitrides or transformation from austenite, but the effect is not enhanced even if added in excess. Absent. As such an upper limit value, Ti: 0.2 mass %, N
b: 0.3 mass %, V: 0.3 mass %, Mo: 0.3 quality
The amount was set to %.

Si is a desirable element for obtaining the fine transformation structure which is the object of the present invention, since Si increases the activity of C in austenite and facilitates the precipitation of the θ phase, but excessive addition causes casting. Since it is difficult, the upper limit is 2% by mass . M
All of n, Cu, and Ni strengthen the ferrite in the microstructure after transformation and at the same time expand the isothermal holding condition range, and are preferable additive elements. However, if added in excess, the surface quality of the slab is deteriorated during casting. : 1.0 mass % or less, Cu: 1.0 mass % or less, and Ni: 1.0 mass % or less.

Further, B suppresses the transformation from the grain boundary during transformation from austenite, and also accelerates the transformation shell function in the grain, so B is added with an upper limit of 0.005 mass %.

[0018]

According to the method of the present invention, the microstructure after transformation of the hypereutectoid steel rolled steel wire or the like is as fine as or finer than that of the conventional steel wire or the like. In addition, the concentrations of Cr, Ti, Nb, V and Mo, which are in a solid solution state in the steel matrix of the slab during casting, are higher than those in the billet after the billet is heated by the conventional method, and if the steel composition is the same. Higher strength is obtained. Alternatively, the amount of addition of these alloying elements required to obtain the same strength can be reduced. This means that the amount of elements that remain in the solid solution state in the steel matrix during solidification should be brought to that temperature even after the thermal history once below the Ar ₃ transformation point, even if the temperature is the same. Much more than.

On the other hand, according to the method of the present invention, it is obvious that the heating of the slab or the like can be omitted, but for practical production convenience,
Even if a heat retention treatment is performed to moderate the cooling of the slab, the metallurgical advantages of the method of the present invention described above are not reduced, and the additional heat energy cost can be significantly reduced. The present invention will be further described based on examples thereof.

[0020]

Example: Hypoeutectoid steel and eutectoid steel produced by the method of the present invention
And characteristic values of the same steel wire due to over-eutectoid steel rolled steel wire and conventional method shown in Table 1 and Table 2.

[0021]

[Table 1]

[0022]

[Table 2]

As in the examples, the examples of the present invention are R
> 5 mm / min, that is, at a solidified shell growth rate of 5 mm / min or more, 72 to 95% of the cross section of the slab was solidified, and the time from the end of rolling to the start of cooling was 10 to 30 seconds, which was rapidly cooled. The quenching rate is not particularly limited, but the cooling rate in the transformation region is preferably 50 ° C./sec or more.

Further, in the present invention, in order to quench and maintain the transformation temperature, it may be cooled by immersing it in a salt bath consisting of potassium nitrate and sodium nitrate, or jetting the molten salt in a jet or laminar form. The refrigerant used for part of the cooling may be mist or steam.

[0025]

According to the method of the present invention, in producing hyper-eutectoid steel rolled steel wire and the like, as compared with the conventional production method, the heating process of the slab is omitted and the slab is directly connected to the rolling, thereby reducing the heat energy cost. Can be significantly reduced, and the production process can be shortened by simplifying the production process. Moreover, in addition to providing higher strength than the conventional method, the obtained product
Notably, the secondary wire drawing workability is significantly enhanced. That is, by introducing a work structure into high temperature austenite and immediately performing isothermal transformation, the unit of the structure after transformation becomes fine and the orientation is random, and the workability in secondary working is enhanced.

─────────────────────────────────────────────────── ─── Continuation of front page (51) Int.Cl. ⁷ identification code FI C22C 38/00 301 C22C 38/00 301Y 38/18 38/18 (58) Fields investigated (Int.Cl. ⁷ , DB name) B21B 1/00-3/02 C21D 8/00-8/10 C21D 9/52 B22D 11/06

Claims (4)

(57) [Claims] 1. C: 0.3 to 0.92 % by mass , Si:
2.0 mass% or less, Mn: 1.0 mass% or less, Cr:
0.1 to 2.0% by mass , consisting of balance iron and impurity elements
In the continuous casting method of steel, a slab obtained by solidifying at least 70% or more of the cross section of the slab at a solidified shell growth rate of 5 mm / min or more must fall below the transformation point from austenite in the cooling process after casting. Without reducing the cross-section reduction rate of 98% or more, quenching within 30 seconds after the completion of rolling and 600 ° C or less 3
A method for producing a high-strength steel wire having excellent wire drawability, which is characterized in that the temperature is maintained at 00 ° C or higher and the material is held and transformed at that temperature. 2. The excellent elongation according to claim 1, wherein quenching after rolling and holding at a temperature for transformation are carried out by dipping in a molten salt bath of potassium nitrate and sodium nitrate or by injecting the molten salt. A method for producing a high-strength steel wire having wire workability. 3. A high-strength steel wire having excellent wire drawability according to claim 1 or 2, wherein part or all of the quenching after rolling is carried out by cooling with water or a refrigerant in which air and water are mixed. Production method. 4. As a steel component, Cu: 1.0% by mass or less Ni: 1.0% by mass or less Ti: 0.2% by mass or less Nb: 0.3% by mass or less V: 0.3% by mass hereinafter Mo: 0.3 mass% or less B: 0.005 mass% or less of one or high tensile steel having excellent wire drawability according to claim 1 with the addition of two or more Wire manufacturing method.