JP2011052248A - Method for manufacturing round bar of large diameter - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a round bar of a large diameter having excellent toughness as rolled without performing any heat treatment. <P>SOLUTION: A steel material having a composition consisting of, by mass, 0.35-0.60% C, 0.1-0.4% Si, 0.5-1.0% Mn, ≤0.20% Cr, ≤0.1% Al, and the balance Fe with inevitable impurities is heated at the heating temperature in a range of 900 to 1,200°C, and subjected to rough rolling, and naturally cooled to the predetermined temperature, and then, subjected to the finish rolling. The finish rolling is performed in the temperature range of 850-650°C with the reduction rate of area being ≥30% which is defined as reduction rate of area (%)=ä(sectional area before starting finish rolling)-(sectional area after completing finish rolling)}/(sectional area before finish rolling)×100. The steel material is naturally cooled after the finish rolling. In addition to the composition, one or two kinds selected among ≤0.25% Cu and ≤0.25% Ni may be contained. Thus, a round bar of a large diameter with both the strength and the toughness can be obtained. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for manufacturing a large-diameter round bar, and particularly to a method for improving the toughness of a large-diameter round bar without performing heat treatment. The “large diameter round bar” here refers to a round bar having a diameter of about 90 to 250 mmφ.

  In recent years, with the increase in size of machines and equipment, steel materials to be used are required to be extremely thick. For example, a round bar is required to have a large diameter of 90 mmφ or more, and even with such a large diameter, it is an essential requirement to have excellent toughness. For this reason, in order to obtain such a large-diameter round bar, the rolled material is further subjected to a heat treatment such as a normalizing treatment or a quenching and tempering treatment. However, the heat treatment has a problem that the manufacturing process becomes complicated and the manufacturing cost increases.

For such a problem, for example, Patent Document 1 describes a method for manufacturing a large-diameter non-heat treated steel bar. The technology described in Patent Document 1 includes C: 0.30 to 0.60%, Si: 1.8% or less, Mn: 0.60 to 2.0%, Cr: 0.50% or less, V: 0.03 to 0.20%, solAl: 0.020 to 0.060%, N: A steel ingot or continuous cast slab containing 0.008 to 0.020% is heated to 1100 to 1200 ° C, cooled in a furnace, extracted at 1000 to 1050 ° C, and 30% or more by split rolling in a temperature range of 750 to 1000 ° C. This is a method for producing a large-diameter non-tempered steel bar, in which a rolling amount is added and rolling is performed so that the final finish rolling temperature of the partial rolling is 750 to 900 ° C. According to the technique described in Patent Document 1, TS: 70 kg / mm ² (690 MPa) or more in a non-tempered state, test temperature using ² mm U notch test piece: impact value _U E ₂₀ at 20 ° C. It is said that a large-diameter round bar having high strength and high toughness of 4 kgm / cm ² (39 J) or more can be obtained.

  Further, in Patent Document 2, after ingot or continuous casting bloom is heated and divided and rolled, hot rolling including intermediate rolling and finish rolling is performed without reheating, before hot rolling or hot rolling. A method for manufacturing a steel bar is described, characterized by performing forging over the entire length in the middle of rolling. According to the technique described in Patent Document 2, a steel bar having good internal quality can be obtained even if the rolling ratio of roll rolling is small.

  Patent Document 3 describes a method for producing a round bar steel. The technique described in Patent Document 3 is a method of manufacturing a round bar steel in which a slab is subjected to ingot rolling and then formed into a product shape having a round cross section by forging press molding. According to the technique described in Patent Document 3, a product with good internal quality can be obtained even when a large slab by continuous casting is used.

JP 59-170222 JP 61-67501 A Japanese Patent Laid-Open No. 9-271884

  However, the technique described in Patent Document 1 requires a large amount of Si or Mn, and uses a steel material containing an alloy element such as Cr or V, so that the material cost increases and heating is performed. There is a problem that a subsequent furnace cooling is required, and there is a concern about a decrease in productivity. In addition, the techniques described in Patent Documents 2 and 3 both require forging during the process of hot rolling, which complicates the manufacturing process, and provides large equipment for forging. There was a problem such as need.

  The present invention solves the above-mentioned problems of the prior art, and can produce a large-diameter round bar with tensile strength TS: 600 MPa or more and excellent toughness at low cost without being subjected to heat treatment. It aims at providing the manufacturing method of a diameter round bar.

  Conventionally, when manufacturing a large-diameter round bar, it is rolled by one heating, and after rolling, in order to ensure uniformity, a normalizing treatment is performed to obtain a product. However, the normalization treatment is only for the purpose of homogenizing the structure, and a remarkable improvement in toughness cannot be expected. In order to achieve the above-described object, the present inventors diligently studied measures for improving the toughness of a large-diameter round bar in a rolled state. As a result, as shown in FIG. 1, the present inventors set the finish rolling as a rolling at a relatively low temperature in which the inlet side temperature is 850 ° C. or lower, so that a large-diameter round It has been found that the toughness of the rod is significantly improved.

FIG. 1 shows that various finishing roll temperatures (finishing entrance temperature) are used for finish rolling to produce round bars with various diameters, and the resulting round bars are subjected to Charpy impact in the as-rolled state. the impact value uE ₂₀ tests to investigate a graph showing the relationship between the rolling entry side temperature finishing and uE _20. 1 that the toughness of the round bar to be manufactured tends to be improved by setting the finish rolling entry side temperature to 850 ° C. or less. FIG. 1 includes the results of round bars having a diameter of 105 mmφ to 250 mmφ, and it is important that the finishing rolling entry temperature is 850 ° C. or lower regardless of the round bar size. I can say that.

  And as a result of further investigation, in order to make the finish rolling at a relatively low temperature as described above, the heating temperature of the steel material for hot rolling is made relatively low, and the finish after finishing the rough rolling is finished. It is preferable from the material uniformity (longitudinal direction and uniformity in the cross section) of the large-diameter round bar until the surface temperature of the material to be rolled reaches the finish rolling entry temperature in a predetermined range before rolling. I got the knowledge.

The present invention has been completed based on the above findings and further studies.
That is, the gist of the present invention is as follows.
(1) In mass%, C: 0.35 to 0.60%, Si: 0.1 to 0.4%, Mn: 0.5 to 1.0%, Cr: 0.20% or less, Al: 0.1% or less, and the balance consisting of Fe and inevitable impurities When heating a steel material having a composition and hot rolling to obtain a large-diameter round bar, the heating is heating to a heating temperature in the range of 900 to 1200 ° C., and the hot rolling is rough. It is rolling which consists of rolling and finish rolling, air cooling is carried out after completion | finish of the said rough rolling to the start of the said finish rolling, and this finish rolling is a temperature range of 850-650 degreeC, following (1) Formula
Area reduction ratio (%) = {(cross-sectional area before finishing rolling) − (cross-sectional area after finishing rolling)} / (cross-sectional area before finishing rolling) × 100
A method for producing a large-diameter round bar excellent in toughness, characterized in that the reduction in area is defined as 30% or more and is air-cooled after the finish rolling.

  (2) In (1), in addition to the above composition, the composition further contains one or two kinds selected from Cu: 0.25% or less and Ni: 0.25% or less by mass%. A manufacturing method of a large-diameter round bar.

  According to the present invention, a large-diameter round bar excellent in toughness can be easily and inexpensively manufactured in a rolled state without being subjected to heat treatment.

Test temperature: It is a graph which shows the relationship between the impact value uE20 in ₂₀ degreeC, and finish rolling entrance temperature. It is explanatory drawing which shows the manufacturing process of this invention typically.

The steel material used in the present invention includes C: 0.35-0.60%, Si: 0.1-0.4%, Mn: 0.5-1.0%, Cr: 0.20% or less, Al: 0.1% or less, or, further, Cu: 0.25 % Or less, Ni: One or two selected from 0.25% or less, and a steel material having a composition comprising the balance Fe and inevitable impurities.
First, the reasons for limiting the composition of the steel material used in the present invention will be described. Hereinafter, unless otherwise specified, mass% is simply referred to as%.

C: 0.35-0.60%
C is an element having an effect of increasing the strength of the round bar, and needs to be contained in an amount of 0.35% or more in order to ensure a predetermined strength of the large-diameter round bar. On the other hand, when the content exceeds 0.60%, the toughness is significantly lowered. For this reason, C was limited to the range of 0.35 to 0.60%. In addition, Preferably it is 0.40 to 0.50%.

Si: 0.1-0.4%
Si is an element that acts as a deoxidizer and has a function of increasing the strength by solid solution. Such an effect becomes remarkable when the content is 0.1% or more. However, when the content exceeds 0.4%, the elongation is significantly reduced. For this reason, Si was limited to the range of 0.1 to 0.4%.
Mn: 0.5-1.0%
Mn is an element that dissolves to increase the strength of the round bar and needs to be contained in an amount of 0.5% or more in order to ensure the predetermined strength of the large-diameter round bar. On the other hand, if the content exceeds 1.0%, the strength becomes too high, the toughness is significantly lowered, and segregation is promoted. For this reason, Mn was limited to the range of 0.5 to 1.0%. In addition, Preferably it is 0.6 to 0.9%.

Cr: 0.20% or less
Cr is an element that contributes to increasing the strength of the round bar through an increase in hardenability. In order to acquire such an effect, it is desirable to contain 0.01% or more, but inclusion exceeding 0.20% reduces toughness. For this reason, Cr was limited to 0.20% or less.
Al: 0.1% or less
Al is an element that acts as a deoxidizing agent and has an action of combining with N to form AlN to prevent coarsening of austenite grains and improve toughness. In order to acquire such an effect, it is desirable to contain 0.005% or more, but the content exceeding 0.1% causes a rise in manufacturing cost. For this reason, Al was limited to 0.1% or less. In addition, Preferably it is 0.005-0.06%.

Although the above-described components are basic components, one or two of Cu and Ni may be further contained as optional elements as required.
One or two selected from Cu: 0.25% or less, Ni: 0.25% or less
Cu and Ni are both elements that contribute to improving the strength by solid solution, and can be selected and contained as necessary.

Cu is an element that improves corrosion resistance in addition to increasing strength. In order to acquire such an effect, it is desirable to contain 0.1% or more, but inclusion exceeding 0.25% reduces hot workability. For this reason, when it contains, it is preferable to limit Cu to 0.25% or less.
Ni is an element that contributes to an increase in strength and toughness as well as Cu. Such effects are desirably contained in an amount of 0.1% or more. However, if the content exceeds 0.25%, the material cost increases. For this reason, when it contains, it is preferable to limit Ni to 0.25% or less.

The balance is Fe and inevitable impurities.
In the present invention, a steel material having the above composition is heated and hot-rolled to obtain a large-diameter round bar.
In the present invention, the manufacturing method of the steel material is not particularly limited, but the molten steel having the above-described composition is melted by a conventional melting method such as a converter, and blooms by a conventional casting method such as a continuous casting method. It is preferable to use a steel material. From the viewpoint of the uniformity of the steel material, it is preferable to apply a continuous casting method as the casting method.

  The steel material having the above composition is first heated for hot rolling. The heating temperature is in the range of 900 to 1200 ° C. When the heating temperature is less than 900 ° C., the deformation resistance of the steel material is high, the load on the rolling mill during rolling is excessive, and rolling may be difficult. On the other hand, when the heating temperature exceeds 1200 ° C., coarsening of austenite grains becomes remarkable. For this reason, the heating temperature was limited to a temperature in the range of 900 to 1200 ° C. In addition, Preferably it is 900-1100 degreeC.

The heated steel material is then extracted from the heating furnace and hot rolled. The extraction temperature is preferably as low as possible if hot rolling is possible from the viewpoint of improving toughness. In the present invention, the extraction temperature is preferably 900 to 1100 ° C.
Hot rolling consists of rough rolling and finish rolling. The heated steel material is roughly rolled to obtain a roughly rolled material (finished rolled material) having a predetermined shape. In the present invention, the conditions for rough rolling need not be limited, and any conditions can be applied as long as a desired dimension and shape can be secured.

In the present invention, the rough rolled material (finish rolled material) is retained and air-cooled after the rough rolling is completed until the surface temperature reaches a predetermined finish rolling temperature range. After the surface temperature of the rough rolled material has fallen to a predetermined finish rolling temperature range, that is, finish rolling is performed so that the temperature (surface temperature) of the rough rolled material (finish rolled material) is within a predetermined finish rolling temperature range. It starts after becoming.
In the present invention, the predetermined finish rolling temperature range is set to a range of 850 to 650 ° C. In the finish rolling, a rough rolled material (finished rolled material) is repeatedly rolled (reduced) a plurality of times (pass) to obtain a round bar having a desired dimension (diameter). If rolling is performed in the above-described finish rolling temperature range, austenite (γ) grains are elongated by reduction in each pass, and the elongated γ is recrystallized to become fine γ grains. Such refinement of crystal grains by reduction is repeated for each pass, and the refinement of crystal grains proceeds. This refinement of crystal grains improves toughness. The predetermined finish rolling temperature range means that both the entry side temperature and the exit side temperature of finish rolling fall within the above-described temperature range.

If the inlet side temperature and the outlet side temperature of finish rolling are less than 650 ° C., the rolling temperature becomes too low and the load on the rolling mill becomes too high. On the other hand, if it exceeds 850 ° C., the degree of grain refinement is small, and the effect of improving toughness is not significant. In addition, from the viewpoint of preventing coarsening of crystal grains, the temperature is preferably 830 ° C. or lower.
Further, in order to promote the above-described refinement of crystal grains, the area reduction ratio (reduction) in finish rolling is set to 30% or more. Here, the area reduction rate is expressed by the following equation (1)
Area reduction ratio (%) = {(cross-sectional area before finishing rolling) − (cross-sectional area after finishing rolling)} / (cross-sectional area before finishing rolling) × 100
Defined by

If the area reduction ratio in finish rolling is less than 30%, the desired crystal grain refinement cannot be promoted. The upper limit of the area reduction rate is determined depending on the capability of the rolling equipment, but is preferably about 80% or less from the viewpoint of the load on the rolling mill. Preferably, it is 30 to 80%.
After finishing rolling, the obtained round bar is air-cooled. Thereby, it becomes a large-diameter round bar having homogeneous characteristics as a whole.

Although the present invention has been described by taking a round bar as an example, it goes without saying that the same effect can be obtained with a square bar having a square cross section.
Next, the present invention will be described in more detail based on examples.

Molten steel having the composition shown in Table 1 was melted to obtain a steel material (bloom) having a size shown in Table 2 by a continuous casting method. After the steel material is charged into the heating furnace 2 and heated, the steel material is extracted at the temperature shown in Table 2, roughly rolled by the roughing mill 4, and then finish-rolled by the finishing mill group 6, and the size shown in Table 2 is obtained. A round bar. In addition, as shown in FIG. 2, the extraction temperature, finish rolling entry side temperature, and exit side temperature are thermometers 3, 5 installed on the exit side of the heating furnace, the entrance side, and the exit side of the finish rolling mill group, respectively. 7, the surface temperature was measured. Moreover, the amount of reduction by finish rolling is the following (1) formula
Area reduction ratio (%) = {(cross-sectional area before finishing rolling) − (cross-sectional area after finishing rolling)} / (cross-sectional area before finishing rolling) × 100 (1)
Expressed by the area reduction rate defined in.

From 1 / 4D of the obtained round bar, a JIS No. 4 tensile test piece was collected in accordance with JIS Z 2201, and a tensile test was conducted in accordance with JIS Z 2241. Tensile properties (yield point YP , Tensile strength TS, elongation EL).
In addition, from 1 / 4D of the obtained round bar, a 2 mm U notch test piece was sampled in accordance with JIS Z 2242, a Charpy impact test was conducted at a test temperature of 20 ° C., and an impact value _U E ₂₀ was obtained. Asked. In addition, 6 impact test pieces were collected with each round bar, and an impact test was performed. Then, the maximum value, the minimum value, and the average value were obtained.

 The obtained results are shown in Table 2.

Each of the examples of the present invention is a large-diameter round bar (product) having high strength of TS: 600 N / mm ² or more and excellent toughness. On the other hand, in the comparative example that is out of the range of the present invention, the minimum value of the impact value _U E ₂₀ is lower than that of the present invention example. Moreover, compared with the same steel and the same product size, the example of the present invention has less variation in impact value (maximum value-minimum value) than the comparative example.

1 Steel material 2 Heating furnace 3 Thermometer 4 Coarse rolling mill 5 Thermometer 6 Finish rolling mill group 7 Thermometer

Claims (2)

% By mass
C: 0.35-0.60%, Si: 0.1-0.4%,
Mn: 0.5 to 1.0%, Cr: 0.20% or less,
Al: In the case of heating a steel material having a composition including 0.1% or less and the balance Fe and inevitable impurities, and hot rolling to obtain a large-diameter round bar,
The heating is heating with a heating temperature in the range of 900-1200 ° C;
The hot rolling is rolling consisting of rough rolling and finish rolling. After the rough rolling, the hot rolling is air-cooled until the start of the finish rolling, and the finish rolling is performed at a temperature range of 850 to 650 ° C. A method for producing a large-diameter round bar excellent in toughness, characterized in that the reduction in area defined by the formula is 30% or more, and air cooling is performed after the finish rolling.
Record
Area reduction ratio (%) = {(cross-sectional area before finish rolling) − (cross-sectional area after finish rolling)} / (cross-sectional area before finish rolling) × 100   2. The composition according to claim 1, wherein the composition further contains one or two kinds selected from Cu: 0.25% or less and Ni: 0.25% or less by mass% in addition to the composition. Manufacturing method of large diameter round bar.

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