JPH07110361B2 - Internal defect pressure bonding rolling method of continuous cast material - Google Patents

Description

Description: TECHNICAL FIELD The present invention relates to a hot rolling method for a steel strip, a steel plate, or the like made from a continuously cast slab. More specifically, the present invention relates to a hot rolling method in which internal defects of a rolled material are pressed and removed during rolling.

(Prior Art) Generally, in a continuously cast slab, bloom, and billet, porosity due to segregation occurs in the central portion, and microporosity due to bubbles easily occurs in the surface portion. These may remain after rolling to cause deterioration of product quality.

For example, in the field of plates, in the case of thick plates such as rack materials with a plate thickness of 100 mm or more used for offshore rigs for oil drilling, the continuous casting slab thickness is 300 mm at the maximum, so due to insufficient reduction, segregation at the center part The resulting porosity, scratches, etc. do not bond even after rolling and remain in the product. For this reason, for an extremely thick plate that requires high quality, a steel ingot must be used as a material, resulting in an increase in product cost.

In the field of bar steel, horizontal thickness reduction and vertical width reduction are performed alternately.
Compared to the one-way rolling like a plate, the width spread in each pass is very large. For this reason, the porosity in the central portion may not be completely pressure-bonded even under high pressure, and the product may remain. Further, in the vicinity of the surface layer at the end portion in the width direction, the effect of reduction is remarkably reduced due to the width expansion. As a result, the microporosity existing about 10 mm below the surface layer at the center of the end face is likely to remain in the product without being almost pressure bonded. By the way, in a bar steel product, secondary processing such as wire drawing and cold casting is often performed, and when microporosity remains, troubles such as breakage easily occur in the secondary processing step.

Therefore, conventionally, the following measures have been taken in order to remove the porosity existing in the continuous cast material.

First, in the steel strip, it has been attempted to press the porosity by increasing the reduction amount in the thickness direction and the width direction as much as possible. For example, bloom continuous cast material with a large cross section of 300 mm in thickness and 400 mm in width is used, and the two heat rolling method of slab rolling and product rolling is adopted to increase the reduction amount. In this case, in slabbing, the surface is ablated with a hot scarf, and after slabbing, it is cold-maintained to check the quality. This is against the hot-charge / single-heat rolling from the continuous cast material, which is becoming the mainstream at present, and is a cause of increasing the manufacturing cost of high-grade steel bars / wires.

Also in the pressure plate, a steel ingot is used as a material for a high-grade extra-thick plate, and two heat rolling processes of slab rolling and plate rolling are performed to remove porosity.

(Problems to be Solved by the Invention) Thus, conventionally, the reduction amount is increased by adopting the two-heat rolling method of slab rolling-product rolling, whereby the porosity inside the slab is pressure-bonded. For this reason, one-heat rolling cannot be performed in the product factory, and the manufacturing cost has risen significantly. In addition, as a result of the long time required from order receipt to delivery, it may not be possible to deal with urgent products. Especially for high-end products, there are many small lots and quick delivery
The conventional manufacturing method has problems in cost and time.

Therefore, an object of the present invention is to perform a one-heat hot-charge rolling using a continuous casting material having a small cross section as a raw material by pressing and eliminating internal defects such as porosity generated during casting by rolling with a relatively small reduction. To provide a hot rolling method capable of manufacturing a steel strip product of higher quality than ever before at a low price in a short time.

(Means for Solving the Problems) The present inventor has come to pay attention to the following facts while conducting experiments and research to achieve the above-mentioned object. That is, conventionally, it has been considered best not to apply a compressive force / tension between the stands in order to prevent the dimensions of the material from changing. However, if a large compressive stress of 1/4 or more of the deformation resistance of the rolled material is applied between the stands of the rolling mill to the conventional tensionless rolling, the roll bite output side and the downstream side of the upstream side stand where the inter-stand compression force acts As a result of the uniform compressive force acting in the stand roll bite, the rolling force associated with the roll lowering easily penetrates evenly in the width direction to the center. However, when such a strong inter-stand compression force is applied, there is a risk that the material will buckle between the stands. In order to prevent the buckling of the material, it is necessary to reduce the distance between the rolls. For this purpose, the adjacent upstream rolls are driven, the downstream rolls are not driven, and one housing is used. If both the driven and non-driven rolls are housed inside, the distance between the roll axes becomes sufficiently small, and it becomes possible to generate the compressive force necessary for crimping the porosity. If you drive both the upstream and downstream rolls,
This is because the spindle and the pinion stand interfere with each other and the distance between the roll axes cannot be made sufficiently small, but the distance between the axes can be made sufficiently small by driving only the upstream roll.

It is effective to provide such a roll stand at the first at least one stand because of the pressure bonding of porosity found in the continuously cast material.

Thus, the gist of the present invention is that, in the hot rolling of the continuous casting material by the 1-heat rolling method, at least the first stand of the rolling mill accommodates a pair of driving rolls and non-driving rolls in a single stand. By rolling the rolled material with the non-driven rolls in addition to the driven rolls, the rolled material is rolled between these rolls, and the value of the hot deformation resistance of the rolled material is 25%.
It is an internal defect crimping rolling method for a continuous cast material, characterized in that the above-mentioned compressive stress is applied to crimp defects existing inside the rolled material.

(Operation) Driven by changing the amount of reduction in the non-driven roll
The magnitude of the compressive stress between the non-driven rolls can be adjusted. Therefore, the reduction amount of the non-driving roll is selected to be an appropriate value, and the compressive stress necessary to press the internal defect (porosity) between the driving and non-driving rolls is generated between the both rolls. The specific value of this compressive stress was experimentally determined. This will be described in detail later.

Further, since the driving roll and the non-driving roll are housed in the single stand, the axial distance between both rolls can be made extremely small. Therefore, even if a compressive stress necessary for crimping internal defects such as porosity is applied to the rolled material between both rolls, the rolled material does not buckle.

Next, an experiment conducted to determine the value of the compressive stress required to effectively crimp and eliminate internal defects (porosity) and the results thereof will be described.

Experimental apparatus Fig. 1 shows the combination stand used in the experiment. This combination stand accommodates a driving horizontal roll H1 and a non-driving vertical roll V2 in a single housing, and rolls the rolled material 1 in the direction of the arrow while applying a compressive force between both rolls H1 and V2. The specifications of this device are as follows.

H1 roll diameter: 300 mm V2 roll diameter: 200 mm Center distance D: 430 mm Rolling material As shown in Fig. 2, the rolling material is a square section billet (steel grade SS41), and internal defects (porosity) due to rolling. In order to confirm the effect of crimping, artificial porosity was formed 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. Holes 1b, 1c near the surface
Among them, the hole 1b exists in the central portion in the thickness direction.
As shown in Fig. 2, these holes 1a, 1b, 1c are made by forming a hole with a diameter of 2 mm in the billet in the rolling width direction, inserting a wire of the same diameter into this hole, and then leaving a space of 2 mm in length for welding. And then closed.

The respective dimensions of this rolled material 1 are collectively shown 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 rolled materials (billet) 1 were prepared and tested. Material 1 was heated to 1250 ℃ in all cases, and the rolling temperature at H1 was 11
It was kept constant at 00 ° C. Also, the rolling reduction at H1 is 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 changed from 0 to 20 mm to change the compressive stress between both rolls H1 and V2.

In this experiment, the compression state of artificial porosity after H1 mill and 1 bath rolling was investigated for each compressive stress value. Figure 3 shows
The longitudinal cross-section of the material before and after H1 rolling is shown schematically. As the material thickness was reduced from t ₀ = 100 mm to t ₁ = 80 mm, the circular cross-section of artificial porosity 1a collapsed into an ellipse and the cross-sectional area was reduced.
The change from S ₀ to S ₁ is illustrated. As a result of rolling in this way, the porosity cross-sectional area decreases from S ₀ to S ₁ , so the porosity reduction rate γ
It is suitable to use = (S ₀ −S ₁ ) / S ₀ × 100 (%). A porosity reduction rate γ = 100% means perfect pressure bonding of porosity. On the other hand, since the plastic deformation of a material during rolling is determined by the relationship between the stress acting inside the material and the hot deformation resistance k _f of the material, the magnitude of the compressive stress σ effective for internal defect (porosity) crimping It is effective to use σ / k _f as the index of.

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 hole 1a located in the central portion in both the thickness direction and width direction, and hole 1b near the surface layer in the thickness direction central portion and width direction edge portion, respectively. Indicates. The hot deformation resistance k _f of SS41 material is 6.0 kg / mm ²
It is constant at.

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 inter-stand compression force becomes apparent when σ / k _f is 0.25 or more, and this value is 25
%, It is expected that even a large porosity with a diameter of about 2 mm will be completely crimped over the entire cross section.

Therefore, the rolling reduction of the non-driven rolling mill when carrying out the present invention is
In consideration of the above experimental results and the size of porosity in the material, etc., select so that σ / k _f is sufficiently large within the range that does not cause excessive dimensional deviation of the rolled material and does not cause buckling of the material. Should.

(Example) Next, the Example of this invention is described in detail.

Example 1 FIG. 5 shows a mill layout of a bar steel mill coarse row in a first example in which the method of the present invention was carried out in a coarse row of a steel bar mill. In this embodiment, two combination stands (HV) 1 and (HV) 2 in which a driving horizontal roll and a non-driving vertical roll are assembled in one housing are used, and a continuous side of 200 mm per side is used. A cast billet is used, and the surface is reduced to 70 mm on each side with 8 rows of rough rows. Table 2 shows the pass schedule and the roll-to-roll compressive force of the rough row of this example. In the case of a continuously cast billet of low carbon steel, the porosity found in the center was completely pressed and extinguished by two compression rollings in (HV) 1 and (HV) 2.

Example 2 FIG. 6 shows a mill layout in a second example in which the method of the present invention was carried out in a plate mill. No in this example
1 mil H1 and No2 mil H3 are arranged in tandem.
A non-driven horizontal roll H2 is installed on the exit side of 1 mil H1. A continuous cast slab with a thickness of 300 mm and a width of 1200 mm was used as a material, and the slab was rolled into an extremely thick plate with a thickness of 100 mm by 3-pass revers rolling with No. 1 and No. 2 tandem mills. Table 3 shows the pass schedule and the compressive force between the rolls at this time. The porosity in the central part due to segregation, which occurred when continuous cast materials were used, was almost completely crimped and disappeared due to insufficient conventional reduction.

Example 3 FIG. 7 shows a mill layout for a third example in which the method of the present invention was carried out in a plate mill. No1
Install a non-driven vertical roll V on the exit side of the horizontal mill H1,
-V forms a single housing and is the No. 2 horizontal mill.
H2 is placed in tandem close to No.1 horizontal mill H1. The material is a continuous cast slab with a thickness of 30 mm × 1200 mm width,
It was rolled into a 100 mm thick x 1150 mm wide extremely thick plate by 3-pass revers rolling. Table 4 shows the pass schedule and the compression force between rolls.

In this embodiment, since the non-driving roll is the vertical roll V, when the non-driving roll is strongly pressed, the material buckles in the width direction. Therefore, the rolling reduction of the roll V cannot be made very high, but if the universal type in which a non-driving horizontal roll is also placed on the same surface as the non-driving vertical roll, it is possible to control the width and improve the inner quality.

(Effects of the Invention) Since the method of the present invention is configured as described above, it becomes possible to crimp internal defects (porosity) present in continuous cast pieces on the rolling line. Therefore, high quality extra thick plates and steel bars
There is no need to use steel ingots or large cross-section blooms in the production of bar steel as in the past, and instead of 2 heat rolling, 1
By adopting heat rolling, it is possible to significantly reduce rolling work. As a result, the manufacturing cost of these products can be reduced and the time required for manufacturing can be shortened.

[Brief description of drawings]

FIG. 1 is a side view of an HV combination stand used for investigating the relationship between the compressive force and the crimping effect in the present invention; FIG. 2 is a perspective view of a material showing an artificial porosity shape of a material cross section; FIG. 4 is a schematic cross-sectional view in the longitudinal direction of the material showing the artificial porosity shape before and after the horizontal rolling of the stand of FIG. 1; FIG. 4 is a graph showing the relationship between the roll compression force and the porosity pressure bonding effect; Drawing-Drawing 7 are top views showing the arrangement of the rolling mill of the 1st-3rd example of the present invention, respectively. 1: Rolled material 1a, 1b, 1c: Hole (artificial porosity)

Claims (1)

[Claims] 1. In hot rolling of a continuous cast material by a one-heat rolling method, a pair of driving rolls and non-driving rolls are housed in a single stand at the first at least one stand of the rolling mill, and In addition, by rolling the rolled material with non-driving rolls, a compressive stress of 25% or more of the value of the hot deformation resistance of the rolled material is exerted on the rolled material between these rolls, and it exists inside the rolled material. The internal defect crimping and rolling method for continuous cast material, which is characterized by crimping the following defects.