JP4976985B2 - Manufacturing method of wire rod and steel bar with excellent low-temperature torsional characteristics - Google Patents

Description

  The present invention is a wire rod and steel bar (hereinafter referred to as “wire rod / bar”) that is excellent in deformability, particularly low-temperature torsional characteristics, and has high strength even in hot rolling without being subjected to soft annealing. , And its manufacturing method. The wire rod and bar of the present invention are used for manufacturing machine parts such as bolts, screws, nuts, special screw parts, and electrical parts by processing such as wire drawing, cold forging, cold forging, and cold rolling. Although it is useful, it is particularly useful as a material for a torsion bar for a seat belt that makes use of excellent low-temperature torsional characteristics.

  Cold working is more versatile as a method for efficiently producing machine parts and electrical parts such as bolts, screws, and nuts because it has higher productivity than hot working and cutting, and has a good yield of steel. Has been. Steel used for such cold working is required to exhibit excellent cold workability.

  As a specific index of cold workability, it is important that the deformation resistance during cold working is low and the deformability [ductility (elongation, drawing, twist value)] is high. If the deformation resistance of steel is high, the life of the tool used for cold working is reduced, while if the deformability is low, cracking is likely to occur during cold working.

  In recent years, with the improvement of cold working technology, especially cold forging technology, more complex and precise forged parts have been demanded, and higher functionality is also required for steel materials used in forged parts. It is supposed to be.

  By the way, the torsion bar for a seat belt exhibits a function of protecting a passenger's collision energy in an emergency such as a car collision by twisting the torsion bar (for example, Patent Document 1). These torsion bars have also been produced by cold forging in recent years, and achieve high deformability, in particular, high twist values from a low temperature of about −40 ° C. to room temperature (hereinafter, this characteristic is Called "low temperature torsional characteristics").

  In order to enhance the deformability of the steel material, softening heat treatment such as low temperature annealing, annealing, spheroidizing annealing, or the like may be performed before cold working, and the deformability of the steel material can be increased by such treatment. However, this softening heat treatment needs to be performed for a long time of several hours to several tens of hours, and the softening heat treatment is omitted from the viewpoint of improving productivity, energy measures, and cost reduction. However, the reality is that realization of a steel material exhibiting excellent deformability is required.

  For these reasons, various techniques for improving the deformability by adjusting the component surface and the structure surface of the material have been proposed. As such a technique, for example, Patent Document 2 proposes to use a steel material having a carbon content of 0.04% or less as a steel material useful as a material of the torsion bar. Patent Document 3 also proposes a technique for improving the deformability of a steel material by devising cold forging conditions to make the surface hardness lower than that of the central portion.

  On the other hand, in Patent Document 4, the C content is reduced (0.002 to 0.05%), and the solid solution N is reduced by addition of Al and Nb, and P is reduced to further increase the ductility. In addition, a technique for preventing grain boundary segregation of P by adding B is also proposed.

Patent Document 5 discloses that the structure is made of ferrite or ferrite pearlite, and the ratio of the grain boundary length of cementite to the total length of the ferrite crystal grain boundary is 30% or less, thereby improving the deformability. Techniques for making them disclosed are disclosed. In this technique, in order to control to such a structure, the elongation after breaking in a high-speed tensile test at a low temperature is improved by accelerating cooling after rolling or adding a carbide forming element.
Japanese Utility Model Publication No. 61-11085 Japanese Patent Laid-Open No. 2001-122077 JP 2001-163178 A JP 2003-313626 A JP 2006-22397 A

  As described above, the improvement of ductility including low temperature is made mainly in terms of the component and the structure, thereby improving the deformability by improving the ductility. However, it cannot be said that the conventional technology accurately captures the phenomenon of torsion, and the low-temperature torsional characteristics evaluated by the twist value are not necessarily good. It is.

  In other words, just improving the ductility of only the fractured part as in the tensile test does not necessarily improve the low-temperature torsional characteristics. To improve the low-temperature torsional characteristics, uniform ductility over the entire length of the torsional part. However, there is no technology improved from this point.

  The present invention has been made under such circumstances, and the object thereof is to exhibit excellent torsional characteristics from −40 ° C. to room temperature, and particularly useful as a material for a torsion bar for a seat belt, Another object of the present invention is to provide a useful method for producing such wire rods and bars.

The wire rods and bar steels of the present invention that can achieve the above object are: C: 0.002 to 0.02% (meaning mass%, the same shall apply hereinafter), Si: 0.3% or less (not including 0%) , Mn: 0.1 to 0.5% and P: 0.001 to 0.020%, S: 0.020% or less (excluding 0%), Al: 0.02% or less (Including 0%) and N: 0.01% or less (including 0%), respectively, the balance is composed of iron and inevitable impurities, and the microstructure is a structure of ferrite: 99 area% or more. The ferrite grain size number (A) on the surface is less than 3.0 to 7.0, and the ferrite grain size number (B) in D / 4 part (D: diameter of wire rod / bar) is less than 3.0 to 7.0, And these particle size numbers (A) and (B) have a gist in that there is a relationship of the following formula (1). The
| (A)-(B) | ≦ 0.5 (1)

  In manufacturing the wire rod / steel bar as described above, a steel piece satisfying the above chemical composition is heated to a temperature of 975 to 1200 ° C. and hot-rolled in a temperature range of 950 to 1150 ° C. to a predetermined shape. And then controlled cooling from a temperature range of 900 to 975 ° C. to a temperature range of 600 to 500 ° C. at a cooling rate of 0.1 to 10 ° C./second. In particular, it is useful to chemically or mechanically descal the surface of the rolled wire rod or steel bar obtained by this method, and after performing a film treatment, to perform a drawing process at a surface reduction ratio of 2 to 15%. is there.

  According to the present invention, by appropriately controlling the chemical component composition and the structure thereof, and by making the structure uniform, it is possible to realize a wire / bar with extremely good low-temperature torsion characteristics. As a material for screws, nuts and special screw parts, it is particularly useful as a material for torsion bars used in seat belts.

  The present inventor has ensured the minimum required mechanical strength (tensile strength TS of 280 to 350 MPa) and good cold workability, particularly excellent deformability [drawing: 85% or more, twist value: 150 times or more (100D conversion)] Aiming at a wire rod / steel bar exhibiting the same], it was examined from various angles.

  In order to exhibit the excellent deformability as described above, (a) a structure mainly composed of ferrite as a structure (ferrite area ratio is 99% or more), (b) a ferrite crystal grain size is too fine It has been found that it is effective to achieve homogenization without making it, (c) to contain as much of the alloy element as possible while reducing the C content. Based on these findings, further studies were made. As a result, the present inventors have found that the above-mentioned object can be achieved brilliantly with wire rods and steel bars that satisfy the above requirements, and have completed the present invention. Hereinafter, each requirement prescribed | regulated by this invention is demonstrated sequentially.

  In the wire rod / steel bar of the present invention, it is also important to make the structure mainly composed of ferrite (the ferrite area ratio is 99% or more). This is for exhibiting good deformability without performing softening heat treatment. In the wire rod / steel bar of the present invention, the ferrite area ratio may be 99% or more (including 100%), and may contain a small amount (1 area% or less) of pearlite. When the area ratio increases, the deformability of the wire rods and steel bars decreases due to the structure.

It is also an important requirement that the wire rod and steel bar of the present invention be structurally uniform. As an index indicating such uniformity, in the present invention, the ferrite particle size number (A) on the outermost surface is less than 3.0 to 7.0, and the ferrite particle size number (B: D is the diameter of the wire rod / bar) is 4/4. ) Is 3.0 to less than 7.0, and the particle size numbers (A) and (B) are defined to have the relationship of the following formula (1). The ferrite grain size number is a value measured based on the crystal grain size test method described in JIS G 0551. According to this test method, for example, ferrite grain size number 3.0 means average diameter: 125 μm, and ferrite grain size number 7.0 means average diameter: 31.2 μm.
| (A)-(B) | ≦ 0.5 (1)

  In the wire rod / bar of the present invention, both the ferrite particle size number (A) on the outermost surface and the ferrite particle size number (B) of D / 4 part (D: diameter of wire rod / bar) are both less than 3.0 to 7.0 ( 3.0 to less than 7.0), and the relationship between the particle size numbers (A) and (B) is made substantially the same [the relationship of the above formula (1)]. Satisfying these requirements achieves good deformability [elongation: 85% or more, twist value: 150 times or more (100D conversion)] in combination with the structure mainly composed of ferrite and the chemical composition. (The principle of improving the twist value will be described later).

  The relationship between the ferrite particle size number (A) and the ferrite particle size number (B) is usually because the surface is cooled by direct cooling (the surface has a higher cooling rate) and the crystal growth time is short. The surface ferrite grain size number (A) takes a larger value (the crystal grains are smaller), but the reverse may be possible depending on circumstances such as recuperation after cooling. Considering these points, the above formula (1) is defined in the form of an absolute value. In short, the difference between the ferrite grain size number on the surface and the inside (D / 4 part) may be 0.5 or less.

  In the wire rod / steel bar of the present invention, the chemical component composition (C, Si, Mn, P, S, Al, N, etc.) needs to be appropriately controlled. The reasons for limiting these ranges are as follows.

[C: 0.002 to 0.02%]
C is an element necessary for ensuring the necessary strength of steel (tensile strength: 280 to 350 MPa). In order to ensure the necessary strength even if the rolling conditions are changed, 0.002% or more (preferably Is 0.005% or more). However, the C content is preferably 0.02% or less (preferably 0.015% or less) in order to exhibit good deformability [drawing: 85% or more, twist value of 150 times or more (100D conversion)]. It is necessary to suppress.

[Si: 0.3% or less (excluding 0%)]
Si is an element that effectively acts as a deoxidizer and solid solution strengthening. However, in the present invention, when Si is contained, ductility is lowered, and in particular, drawing: 85% or more cannot be obtained. % Or less (preferably 0.2% or less, more preferably 0.05% or less).

[Mn: 0.1 to 0.5%]
Mn acts effectively as a deoxidizing agent and exhibits an effect of suppressing embrittlement due to S by forming MnS by combining with S in steel. In order to exert such an effect, it is necessary to contain Mn in an amount of 0.1% or more (preferably 0.2% or more). However, if the Mn content is excessive, bainitic ferrite is generated and it becomes difficult to obtain a uniform structure. Therefore, the Mn content is determined to be 0.5% or less (preferably 0.4% or less).

[P: 0.001 to 0.020%]
P is an element that contributes to work hardening (see the principle of improving twist value described later), and in order to exert its effect, it is useful to contain 0.001% or more. However, if the P content is excessive, the deformability is lowered, so it is necessary to make it 0.020% or less.

[S: 0.020% or less (excluding 0%)]
Since S mainly forms sulfide-based inclusions of MnS and lowers the deformability of the steel material, the smaller the amount, the better. Therefore, the S content is determined to be 0.020% or less (preferably 0.01% or less). However, S is an impurity inevitably mixed in the production of steel, and it is difficult to make the amount 0% industrially.

[Al: 0.02% or less (including 0%)]
In addition to being used for deoxidation, Al is an element that supplements solid solution N to become AlN and promotes refinement of crystal grains. Since such Al causes the size of the crystal grains to be affected by the presence or absence of AlN, to stabilize the crystal grains large (to stably produce ferrite having a desired average crystal grain size). Therefore, it is better that Al is as little as possible. From such a viewpoint, the Al content is determined to be 0.02% or less (preferably 0.01% or less).

[N: 0.01% or less (including 0%)]
As described above, N forms nitrides with Al and the like to refine crystal grains, and N that is not fixed by Al or the like remains in the steel as solute N, and is deformed by strain aging. This causes an increase in resistance. From this point of view, the N content should be kept as low as possible, but considering the actual operational aspects of steel production, the upper limit is 0.01%, which can be suppressed to the extent that the above-mentioned adverse effects can be substantially ignored. (Preferably 0.007% or less).

  The basic component composition of the steel sheet of the present invention is as described above, and the balance is substantially iron. However, as a matter of course, the inevitable impurities (for example, Cu, Ni, Cr, Mo, Sn) brought in depending on the situation of raw materials, materials, manufacturing equipment, etc. are allowed in the steel sheet. However, these impurities should be suppressed to 0.1% or less in order not to lower the ductility.

  In the present invention, in addition to appropriately adjusting the chemical composition of the steel, by controlling the production conditions (particularly hot rolling and subsequent cooling conditions), the wire / bar steel having the above structure is obtained. However, each condition in the production method of the present invention will be described in detail. In each of the following conditions, each temperature (heating temperature, rolling temperature) is a value measured by a radiation thermometer, and therefore the temperature management position is “steel surface”.

<Heating temperature of steel slab during hot rolling: 975 to 1200 ° C>
Since the steel slab targeted in the present invention has a low C content, it is necessary to set the temperature as high as possible in order to ensure the austenite state even in such a low C steel. When this temperature is less than 975 ° C., the austenite temperature may be partially interrupted by the influence of cooling water or the like during rolling. However, if the heating temperature exceeds 1200 ° C., the final crystal grain size may become coarse.

<Hot rolling temperature: 950 to 1150 ° C.>
The hot rolling temperature is a condition for ensuring that the steel material is in an austenite state, as with the heating temperature. If this temperature is less than 950 ° C., the austenite temperature may be partially interrupted by the influence of cooling water or the like during rolling. However, if the heating temperature exceeds 1150 ° C., the final crystal grain size may become coarse. In this case, “hot rolling” means rough rolling, intermediate rolling, and finish rolling. Therefore, in these series of steps, the rolling temperature is preferably in the above range. By hot rolling in such a temperature range, a hot rolled material (wire rod / bar) having a predetermined shape is obtained.

<Controlled cooling start temperature: 900 to 975 ° C.>
After the hot rolling is completed, rapid cooling is performed using water or the like as a medium, and then controlled cooling is performed. After the rapid cooling, the temperature of the surface of the wire rod / steel bar rises due to recuperation, but it is necessary to make the temperature range including the temperature rise amount. When this temperature is less than 900 ° C., the ferrite structure becomes fine even if the surface of the wire or bar is recrystallized by reheating after rapid cooling, and a uniform structure cannot be obtained. On the other hand, if the controlled cooling start temperature exceeds 975 ° C., the scale after cooling becomes too thick and partly peels off, making it impossible to obtain good surface properties in the subsequent wire drawing.

<Control cooling end temperature: 600 to 500 ° C., control cooling rate: 0.1 to 10 ° C./second>
The average cooling rate when cooling from the controlled cooling start temperature (900 to 975 ° C.) to a temperature of 600 to 500 ° C. needs to be in the range of 0.1 to 10 ° C./second. This is to ensure the strength of the hot rolled material (wire rod / bar). That is, when the cooling rate is less than 0.1 ° C./second, cooling control is difficult, and when it exceeds 10 ° C./second, the formation of bainitic ferrite and the crystal grains are likely to be unstable. The preferable lower limit of the cooling rate is 0.5 ° C./second, and the preferable upper limit is 3 ° C./second. The cooling end temperature needs to be 600 ° C. or lower in order to obtain a structure mainly composed of ferrite, but it needs to be 500 ° C. or higher from the viewpoint of obtaining a stable scale structure.

  By performing the hot rolling and cooling as described above, the wire rod / steel bar of the present invention can be obtained. If necessary, the obtained wire rod may be chemically (pickled) or mechanically (shot). It is preferable to perform a drawing process at a surface area reduction ratio of 2 to 15% after descaling and coating treatment (lime, chemical conversion coating, etc.).

By performing such a drawing process, the subsequent wire rods and bars (including steel wires) are not only processed to a predetermined size, but also torsional characteristics, especially when a torsion test is performed at low and normal temperatures. The effect of twisting is exhibited. That is, when the area reduction ratio at the time of drawing is less than 2%, not only a perfect circular wire rod / bar (that is, steel wire size) can be obtained, but also a good torsional characteristic is not exhibited and the area reduction ratio is 15 If it exceeds 50%, the torsional characteristics at room temperature will deteriorate significantly. A more preferable lower limit of the area reduction ratio is 3%, and a more preferable upper limit is 12%. The area reduction rate is defined by the following equation (2).
Area reduction ratio (%) = [1- (D ₁ / D ₀ ) ² ] × 100 (2)
However, D ₁ : Diameter of wire rod and bar after drawing (mm)
D ₀ : Diameter (mm) of wire rod and bar (ie steel wire) before drawing

When the drawing process (drawing) is performed with the above-mentioned area reduction rate, the torque generated when twisting has a specific torsion angle θ ₁ represented by the following formula (3) of 0.0105 to 0.063 (rad / mm). In this range, increasing with an increase in torsion angle θ ₀ is an important requirement for improving torsional characteristics. That is, such an increase in torque means that a work hardening phenomenon has occurred, and the torsion is reliably transmitted. On the other hand, if the torque falls within the range of the specific torsion angle θ ₁ or if the increase amount is very small (within 0.5%), it will be twisted without causing work hardening. In this case, the twisted portion is softened by heat generation, and the twist is not propagated but is concentrated and breaks early.
Specific torsion angle θ ₁ (rad / mm) = θ ₀ / L (3)
Where θ ₀ : twist angle [rad: a (deg) / 360 × 2π], L: gauge distance (mm)

Such a situation will be described with reference to the drawings. FIG. 1 is a graph showing a case where torques T1 and T2 generated when twisting increase with an increase in torsion angle θ ₀ in a range where the specific torsion angle θ ₁ is 0.0105 to 0.063. T1 indicates the torque when the torsion angle θ ₀ is 30 ° (1/12 rad) (that is, when the specific torsion angle θ ₁ is 0.0105 when the gauge distance is 50 mm), and T2 indicates the torsion. This shows the torque when the angle θ ₀ is 180 ° (1/2 rad) (that is, when the specific torsion angle θ ₁ is 0.063 when the gauge distance is 50 mm). The state shown in FIG. 1 shows a state where the wire is twisted while being work-hardened (that is, T1 <T2). In such a situation, the twist is surely propagated throughout the wire, and good twist characteristics are exhibited.

FIG. 2 is a graph showing a case where the torques T1 and T2 generated when twisting do not increase with an increase in the torsion angle θ ₀ when the specific torsion angle θ ₁ is in the range of 0.0105 to 0.063. The meanings of T1 and T2 are the same as described above. That is, the situation shown in FIG. 2 shows a state in which the torque stagnates at the beginning of twisting (that is, T1 ≧ T2). In such a situation, twisting is concentrated on a part, and it becomes easy to break early, and good twisting characteristics are not exhibited.

  EXAMPLES Hereinafter, the present invention will be described in more detail with reference to examples. However, the present invention is not limited by the following examples, and appropriate modifications are made within a range that can meet the above and the following purposes. Of course, it is possible to implement them, and they are all included in the technical scope of the present invention.

  Using test steels a to d having chemical composition shown in Table 1 below, various wire rods were manufactured under the conditions shown in Table 2 [hot rolling conditions, cooling conditions before hot rolling] (diameter: 11 .5 mm). About the obtained wire, while obtaining a ferrite particle size number (outermost surface, D / 4 part) based on JIS G 0551, a JIS Z2201 No. 9 test piece was sampled and subjected to a tensile test to obtain a tensile strength (TS). And the aperture was measured. The results are also shown in Table 2. Each wire was drawn at various surface reduction rates. The dimensions after drawing (diameter: drawing dimension) and the area reduction ratio are shown in Table 2. The test steels c and d are JIS SWRCH 10K equivalent steel and JIS SWRCH 35K equivalent steel, both of which have a high C content.

Subsequently, the wire drawing material was subjected to a torsion test at normal temperature (room temperature) and low temperature (−40 ° C.). At this time, the torsion test at room temperature was carried out using a gauge distance (wire length): 50 mm, a rotation speed: 1 rpm, the above-described torque T1 (torque when the specific torsion angle θ ₁ is 0.0105) and torque T2. (Torque when specific torsion angle θ ₁ is 0.063) was measured, and the difference [(T2−T1) / T1] × 100 (%) was measured (room temperature low-speed torsion characteristics).

In addition, the torsion test at low temperature (−40 ° C.) was performed by cooling the wire with dry ice to −40 ° C., the gauge distance (length of the wire): 50 mm, and the rotation speed: 100 rpm. The twist value (100D conversion) calculated | required by a type | formula was measured. In addition, the twist value calculated | required by the following (4) formula becomes a conversion value (100D conversion) when the diameter of a wire is 100 mm (low-temperature high-speed torsion characteristic).
Twist value (100D conversion) = [twist number / (reference point distance / D ₁ )] × 100 (4)
However, D ₁ : Diameter (mm) of wire rod (steel wire) after wire drawing

  The torsion test results are shown in Table 3 below, and it can be seen that good torsional characteristics are exhibited when the requirements specified in the present invention are satisfied (Test Nos. 1, 2, and 5). On the other hand, if the requirements (or preferable requirements) deviated from the present invention are not satisfied (Test Nos. 3, 4, 6-8), at least one of the room temperature low speed torsion characteristics and the low temperature high speed torsion characteristics has It turns out that it has deteriorated.

Based on these results, the relationship between the number of twists (same meaning as “rotational speed” shown in FIGS. 1 and 2) and the amount of increase in torque is shown. FIG. 1 shows the relationship between the number of twists and the torque in FIG. 1 [FIG. 3A shows the number of twists one time (that is, the twist angle θ ₀ is up to 360 °), and FIG. 3B shows the number of twists 11 times. Until]. FIG. 3 shows the relationship between the number of twists and the torque in FIG. 3 [FIG. 4A shows the number of twists one time (that is, twist angle θ ₀ is up to 360 °), and FIG. 4B shows the number of twists 11 times. Until]. FIG. 4 shows the relationship between the number of twists and the torque in FIG. 4 [FIG. 5A shows the number of twists one time (that is, the twist angle θ ₀ is up to 360 °), and FIG. 5B shows the number of twists ten times. Until].

It can be seen that the sample satisfying the requirements defined in the present invention (Test No. 1) increases as the torsion angle θ ₀ increases (see FIG. 1). On the other hand, in the case where the requirements defined in the present invention are not satisfied (test Nos. 3 and 4), it can be seen that the torque is stagnant in the initial torsion (see FIG. 2).

  For reference, test no. A transmission electron micrograph (TEM photograph) of the structure of wire 1 is shown in FIG. 6 (drawing substitute photograph). Both the surface and D / 4 part have a ferrite grain size number (FGC) of 6.0. You can see that it is obtained.

It is the graph which showed the case where the torque T1 and T2 which arise when twisting increase with the increase in torsion angle (theta) ₀ . It is the graph which showed the case where torque T1, T2 produced when twisting did not increase with the increase in torsion angle (theta) ₀ . Test No. 2 is a graph showing the relationship between the number of twists and torque in 1. FIG. Test No. 3 is a graph showing the relationship between the number of twists in 3 and the torque. Test No. 4 is a graph showing the relationship between the number of twists and torque in FIG. Test No. It is a drawing substitute TEM photograph which shows the structure | tissue in 1 wire.

Claims (1)

C: 0.002 to 0.02% (meaning mass%, the same shall apply hereinafter), Si: 0.3% or less (not including 0%), Mn: 0.1 to 0.5%, and P: 0.0. 001 to 0.020%, S: 0.020% or less (excluding 0%), Al: 0.02% or less (including 0%), and N: 0.01% or less (0 % and respectively inhibited the containing) the balance being a billet ing of iron and inevitable impurities, having a predetermined shape is subjected to hot rolling in the temperature range of heating to 950 to 1150 ° C. to a temperature of 975 to 1,200 ° C. heat Then, the surface of the obtained hot rolled wire rod / steel bar is controlled and cooled from a temperature range of 900 to 975 ° C. to a temperature range of 600 to 500 ° C. at a cooling rate of 0.1 to 10 ° C./second. After descaling chemically or mechanically and applying a film treatment, the area reduction rate is 2 to 15%. By punching, microstructure ferrite of the wire rod, bar steel: with a 99 area% or more of the tissue, the ferrite grain size number of the top surface (A) is less than 3.0 to 7.0, D / 4 parts (D : ferrite grain size number in diameter) of the wire-bars (B) is less than 3.0 to 7.0, and the following (1 these grain size number (a) and (B)) and related ones of formula A method for producing a wire rod and bar steel excellent in low-temperature torsion characteristics .
| (A)-(B) | ≦ 0.5 (1)

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