JPH07110362B2 - Internal defect pressure bonding rolling method of continuous cast material - Google Patents

Description

TECHNICAL FIELD The present invention relates to a rolling method in which a continuous casting bloom or a billet is used as a raw material and a steel bar / wire is manufactured by hot rolling using a continuous rolling mill.

(Prior art) Generally, steel bars made from continuous casting blooms and billets.
In rolling the wire rod, a continuous rolling mill in which vertical and horizontal stands are alternately arranged is used. In this case, the horizontal stand reduces the thickness of the material, and the vertical stand reduces the width of the material. In this rolling method, it is best to make no tension between the stands from the viewpoint of dimensional accuracy of the product. However, in rolling in the intermediate / finishing row, a tensile force of about several percent of the material deformation resistance may be applied between the stands in order to improve the material permeability. On the other hand, in the rough row, since the rolling pattern changes depending on each size, depending on the size, a compressive force may be generated between the stands, but it also remains within several percent of the material deformation resistance. In such rolling of steel bars and wire rods, the material cross-sectional shape is close to a square or a circle, and the material width spread in each pass is significantly larger than that of sheet rolling. Therefore, in particular, internal defects (porosity) near the surface portion in the thickness direction central portion and the width direction end portion tend to remain.

By the way, porosity is likely to occur in the central portion of low carbon steel continuously cast bloom billets, and microporosity is likely to occur in the surface layer portion of medium carbon steel.
The porosity generated in the slab disappears by sufficient pressure bonding in the rough rolling / intermediate rolling process when the width spread is small in one direction rolling such as plate rolling, but the width spread in each pass is remarkable like the bar steel. In this case, the reduction may not penetrate to the central portion, and the porosity in the central portion may remain in the product without being pressure bonded. Regarding microporosity in the vicinity of the surface layer as well, in the vicinity of the center of the end face where the width greatly expands, the porosity is difficult to be pressed in the pressing direction and remains in the product. Therefore, it may be necessary to cut from this porosity as a starting point when the product is further processed.

Conventionally, in order to crimp such porosity, it has been attempted to crimp this porosity by increasing the amount of reduction in the thickness direction and the width direction. As a result, in recent years, it has become common to use bloom as a material, and the cross-sectional size of the bloom is becoming larger than 300 mm × 400 mm. For wire rods and steel bars that are cold forged,
Since strict internal quality is required, ultrasonic inspection is performed at the stage of billet that has been slab-rolled from bloom, and if there are defects due to porosity in the surface layer / center part, care or reject is performed and only billets made of good steel are used. Are sent to the product factory.

(Problems to be Solved by the Invention) As described above, in the conventional tensionless rolling method, it is impossible to completely press-bond and eliminate porosity in the slab with a normal reduction amount. For this reason, conventionally, a large cross-section bloom was used as the material to increase the amount of reduction in the width and thickness directions.
I try to crimp the porosity as much as possible. For this reason, the billet is first rolled from the bloom in a slab, where it is used as a cold material for inspection and maintenance. Therefore, it requires cold maintenance in addition to adopting the two-heat rolling method, resulting in a large cost and time loss, which is contrary to the current direct steelmaking-rolling connection.

The most cost-effective method is a method in which a continuously cast billet having a small cross section is used as a raw material, is carried into a product factory (bar steel mill, wire rod mill, etc.) by hot charging, and is rolled into a product in one heat. However, if the cross-sectional area of the continuous cast material becomes smaller, the reduction rate also decreases and the possibility of porosity remaining in the product increases, so in the conventional rolling method, if continuous casting billets with a small cross section are used as the material, there is a risk of residual porosity. Is inevitable. Therefore, for high-grade steel products, two-heat rolling using a continuous casting bloom having a large cross section has been generally used in the past, which has been a factor of increasing the manufacturing cost and lengthening the manufacturing time.

Accordingly, an object of the present invention is to provide a rolling method for completely pressing and extinguishing porosity in a slab by one heat rolling of a high-grade steel product using a continuously cast billet having a small cross section.

(Means for Solving the Problem) The present inventor has conducted experiments and research to achieve this object,
I got the following idea.

By applying a strong inter-stand compression force between the stands of the continuous rolling mill, in the rolling of the upstream side stand, the compression force is evenly applied to the roll bite exit side, allowing the reduction to penetrate to the center of the thickness direction and the width direction of the material. Even in the case of, it is possible to uniformly permeate. Further, when a compressive force is applied between the stands in this way, the material is compressed (rolled down) in the rolling longitudinal direction, and the width and thickness are increased. Therefore, the effect of pressure bonding in the rolling direction of porosity can be expected especially in the rolled bite. This method of applying compressive force to the material can be performed between each stand, but if the material buckles between the stands due to the inter-stand compressive force, the porosity compression effect is lost and the rolling continues. Even impossible. Buckling occurs when the distance between the roll axes of the stands is too large with respect to the thickness and width of the material between the stands. Therefore, it is most effective to apply the compressive force for porosity compression in the bloom slab rolling process and billet rough rolling process.

Thus, the gist of the present invention is that in hot continuous rolling of a continuous cast material, the value of the hot deformation resistance of the material being rolled between at least one set of stands at the stage of slabbing or rough rolling. It is an internal defect crimping rolling method for continuous casting materials for bar steel and wire rod, which is characterized by applying a compressive stress between stands of 25% or more.

(Operation) Adjust the magnitude of the compressive stress between the stands by adjusting the roll drive motor rotation speed etc. between the adjacent stands, and apply the compressive stress necessary to crimp internal defects such as porosity between the stands to both rolls. Generate between. The specific value of this compressive stress was experimentally determined. This experiment will be described next.

Experimental apparatus Fig. 1 shows the arrangement of rolling mills used in the experiment. Driven horizontal roll H1 and non-driven vertical roll V2, and driven horizontal roll H3
Are continuously arranged, and the rolled material 1 is rolled in the direction of the arrow while applying a compressive force between the rolls H1 and V2. The specifications of this device are as follows.

H1 roll diameter: 300mm V2 roll diameter: 200mm H3 roll diameter: 300mm H1-V2 axial center distance: 700mm V2-H3 axial center distance: 700mm Rolling material As shown in Fig. 2, the rolling material has a square cross section. In order to confirm the effect of internal defect (porosity) pressure bonding by rolling using a billet (steel type SS41), artificial porosity was created by forming holes inside as shown in the figure. These holes (artificial porosity) consist of a hole 1a at the center of thickness that simulates the porosity of the center and holes 1b and 1c 10 mm below the surface that simulate microporosity near the surface. Among the holes 1b and 1c near the surface layer, the hole 1b is present in the central portion of the thickness in horizontal rolling. As shown in Fig. 2, these holes 1a, 1b and 1c are made by forming a hole with a diameter of 2 mm in the billet in the width direction, inserting a wire of the same diameter to leave a space of 2 mm in length, and welding the others. Closed.

The dimensions of this rolled material are summarized below.

Material thickness t: 100mm Material width w: 100mm Hole length l: 2mm Hole diameter d: 2mm Distance x ₁ , x ₂ : 10mm Distance y ₁ , y ₂ : 40mm Rolling conditions and results Producing artificial porosity as described above A large number of different materials (billet) 1 were prepared for the experiment. Rolling material 1 was heated to 1250 ℃ in either case, and the rolling temperature at H1 was 11
It was kept constant at 00 ° C. Further, with the reduction ratio at H1 being constant, the material thickness was reduced by 20% from t ₀ = 100 mm to t ₁ = 80 mm. On the other hand, the amount of reduction at V2 was variously changed from 0 to 20 mm to change the compressive stress between both rolls H1 and V2. In addition,
The final drive horizontal roll H3 reduced the thickness to 70 mm.

In this experiment, the compression state of artificial porosity in H1 mil and 1 pass rolling was investigated for each compressive stress. Fig. 3 schematically shows the material cross section in the rolling longitudinal direction before and after H1 rolling, and the circular cross section of artificial porosity 1a as the material thickness was reduced from t ₀ = 100 mm to t ₁ = 80 mm. Shows a state in which the cross section changes from S ₀ to S ₁ due to crushing into an ellipse. Since the cross-sectional area of porosity decreases from S ₀ to S ₁ with rolling, the porosity reduction rate γ =
(S ₀ −S ₁ ) / S ₀ × 100 (%) is used. γ = 100% means that the porosity is completely crimped. On the other hand, the plastic deformation of a material during rolling is determined by the relationship between the stress inside the material and the hot deformation resistance k _f of the material. Therefore, the magnitude of the compressive stress σ effective for internal defect (porosity) crimping is It is effective to use σ / k _f as an index.

Next, the conditions and results of this experiment are summarized in Table 1. In this table, γ _c and γ _s (%) are the hole reduction ratios of the hole 1a located in the center in both the thickness direction and the width direction, and the hole 1b near the surface layer in the thickness direction center part and the width direction edge part, respectively. Show. The hot deformation resistance k _f of SS41 material is constant at 6.0 kg / mm ² .

FIG. 4 is a graph showing the results of Table 1.

From the results shown in Table 1 and FIG. 4, it is concluded as follows. When σ / k _f is from 0.2 to 0.3, the porosity reduction effect becomes remarkable and the porosity compression effect becomes remarkable, and when σ / k _f = 0.4, the central hole 1a is completely pressure-bonded (γ _c = 100 %). Further, when σ / k _f = 0.6, γ _c = γ _s = 100%, and all the porosities including the surface layer porosity 1b in the vicinity of the surface layer in the central portion in the thickness direction and the end portion in the width direction where the pressure bonding effect is the smallest are pressure bonded.

Therefore, by repeating rolling of σ / k _f = 0.6 or more, the porosity in the central portion and near the surface layer can be completely eliminated by pressure bonding. In this experiment, the effect of the compression force between the stands appears that σ / k _f is 0.25 or more, and if this value is 25% or more, even if the porosity of a fairly large diameter of about 2 mm is completely crimped on all sections. Can be expected to do. In addition, when the porosity is 60% or more, the pressure bonding of porosity is completed.

Therefore, the compressive force between the stands when carrying out the present invention is σ / k _f sufficient for porosity crimping in the range where buckling of the material does not occur, considering the above-mentioned experimental results and the magnitude of porosity in the material, etc. It should be selected to have a value that has a certain effect.

(Example) Example 1 In this example, in a continuous mill having a VH arrangement of 6 stands for coarse row, 10 stands for intermediate row, 6 stands for finishing row and a total of 22 stands, a steel bar having a side length of 20 mm and a steel bar having a side length of 20 mm was produced. This is an example in which the method of the present invention is applied to a steel bar mill to be manufactured, and FIG. 5 shows a mill layout of this example.

The mill of this embodiment is driven individually on all stands,
50% of hot deformation resistance of rolled material between stands between V3
The rotation speed and output of each motor are set so that a compression force of ² kg / mm ² between the stands will be applied. The coarse row pass schedule for this example is shown in Table 2.

The compressive force between the stands tends to increase the dimensional variation over the entire length, but in this embodiment, the compressive force is applied only upstream of the coarse row, and the dimensional variation occurring between H2 and V3 is: It is well absorbed in the rolling of 19 stands downstream, and does not make much difference in size from the case of no tension.

Example 2 In this example, a horizontal tandem mill with a twist of 7 rows of coarse rows, 10 horizontal rows of intermediate rows, and 4 horizontal stands of finishing rows was used, and one side was formed from a 125 mm continuous casting billet.
This is an example in which the method of the present invention is applied to the step of producing a wire rod of 0 mm, and FIG. 6 shows a mill layout of this mill.

In this example, between the first stand H1 and the second stand H2, a compressive stress between the stands of 2.2 kg / mm ² , which corresponds to 60% of the hot deformation resistance of the rolled material, is applied, and the material is heated to 90 ° at the outlet side of the H2 stand. Twist and install on the H3 stand. The coarse row pass schedule for this example is shown in Table 3.

Example 3 In this example, an HV 8-stand steel billet continuous mill was used, and a side of a continuous casting bloom having a thickness of 300 mm and a width of 300 mm was used.
This is an example in which the method of the present invention is applied to a slab for rolling a 0 mm billet, and the arrangement of stands is shown in FIG. 7.

In this mill, V2 is not driven, H1-V2, V4-H5
Two times during this period, a compressive stress between the stands, which is 40% of the material deformation resistance, is applied. Furthermore, the dimensional fluctuation caused by these two compression rolling operations is reduced by lightly reducing with V8. The pass schedule of this embodiment is shown in Table 4. In the case of this embodiment, the dimensional accuracy of the billet is deteriorated by about several percent as compared with the conventional method, but due to the action of the compressive force, most of the microporosity under the surface layer is crimped / disappeared, and the billet for the next step is treated. Can be directly hot-charged without.

(Effect of the invention) As described above, the present invention provides a compressive stress of 25% or more of the deformation resistance of a rolled material between arbitrary stands in the continuous rolling of a continuously cast bloom billet, whereby the inside of the rolled material is provided. Press the microporosity of. Therefore, even if a large-section continuously cast bloom is used, a steel bar / wire can be rolled by two-heat hot charging, and a small-section continuously cast billet can be rolled in one heat. As a result, the manufacturing cost and the time required for manufacturing can be significantly reduced.

[Brief description of drawings]

FIG. 1 is a diagram showing the arrangement of rolling mills used in an experiment for investigating the relationship between the magnitude of the compressive force between stands and the porosity pressure bonding efficiency in the present invention; FIG. 2 is an experiment using the apparatus of FIG. FIG. 3 is a perspective view showing a cross section of the rolled material used; FIG. 3 is a sectional view schematically showing a longitudinal cross section of the rolled material before and after the first horizontal rolling of the rolled material of FIG. 2; Graphs showing the relationship of porosity pressure bonding efficiency; and FIGS. 5 to 7 are plan views showing the arrangement of rolling mills in the first to third embodiments of the present invention, respectively. 1: Rolled material 1a, 1b, 1c: Hole (artificial porosity)

Claims (1)

[Claims] 1. In the hot continuous rolling of a continuously cast material, at least one set of at least one pair of stands at the stage of slabbing or rough rolling has a value of 25% or more of the value of hot deformation resistance of the material being rolled. Internal defect pressure-bonding rolling method for continuous steel bar and wire rod rolling, which is characterized by applying compressive stress between stands.