JPS58921B2 - Slab rolling method - Google Patents

Description

[Detailed Description of the Invention] In recent years, the continuous casting method (hereinafter abbreviated as CC method) has become mainstream as a method for manufacturing slabs from the viewpoint of productivity and manufacturing cost. It is known that there are a large number of shrinkage defects (hereinafter referred to as "Zaku") in the center of the steel, and if they are not crimped even after being rolled, they will have a significant negative effect on the quality of the rolled steel material.

Therefore, the important role of rolling is to apply a considerable amount of processing to the slab and press the corks.

On the other hand, inclusions (mainly Mn5) contained in the slab elongate in the rolling direction due to rolling, deteriorating the mechanical properties of the steel material, particularly in the thickness direction.

The present invention relates to a rolling method for manufacturing a steel material having excellent material properties from a CC slab by taking into account the above-mentioned factors that deteriorate the material quality of the slab.

A detailed explanation of the present invention will be given below.

First, we will discuss the rolling method that is effective for crimping Zaku.

It is well known that the dents present in the center of slabs produced by the CC method have an adverse effect on the mechanical properties of the steel material.

In particular, the hydrogen contained in Zaku can cause delayed destruction, so great efforts are being made to remove it.

For example, slowing down the drawing speed of slabs in the CC method is considered to be one effective measure to suppress the occurrence of burrs.

However, with this method, the productivity of slabs is extremely low, and CC
It is not possible to take advantage of the high productivity and low manufacturing cost characteristics of the method.

Currently, when manufacturing steel products from slabs by rolling, no particular consideration is given to the progress of the rolling pass up to the product thickness d required from the viewpoint of crimp crimping. It is believed that if the ratio D/d (hereinafter abbreviated as rolling reduction ratio) between D and product thickness d is sufficiently large, the Zaku will be crimped.

However, even if the reduction ratio is set to a sufficiently large value, the steel material rolled from an actual CC slab may have ultrasonic flaw detection defects due to non-corrosion bonding, or its mechanical properties may deteriorate.

Moreover, as will be described later, increasing the rolling reduction ratio generally deteriorates the tensile properties and impact properties of the steel material in the thickness direction and in the direction perpendicular to rolling.

However, it has been thought that it is unavoidable to increase the rolling reduction ratio in order to press the Zaku.

Recently, attempts have been made to manufacture thick steel plates from 00 cast slabs, and Susumu Metal Vol. 1.24. No. 1, pp. 79-86 (
1972) 2 Japan Steel Pipe Technical Report No. 69, pp. 1-9 (1
(976) reported that when hot rolling was carried out at a rolling reduction ratio of 3 to 4, the center porosity in the CC slab was crimped and no defects were observed.

However, these reports do not clearly explain the reason why the reported steel sheets are crimped with a lower reduction ratio, although the reduction ratio of 6 or more is required for conventional crimping.

Therefore, it is predicted that if the size, distribution, etc. of the corrugations in the CC slab change, the rolling reduction ratio required for crimping the corrugations will change.

In the first place, it has been thought that the rolling condition that mainly controls uneven crimping is the reduction ratio, but this condition itself has only been obtained empirically, and the rolling conditions that mainly control uneven crimping are The situation was not fully understood.

Therefore, the inventors of the present invention first aimed at crimping the ridges in the slab by rolling, and studied in detail the relationship between each rolled bust and the crimping.

As a result, it has been found that even if the rolling reduction ratio is large, depending on how the amount of rolling is taken in each rolling pass, the zaku may or may not be crimped.

In other words, it was clarified that what is important in crimping the corks by rolling is not so much the size of the rolling reduction ratio, but rather the size of the shape ratio, which is determined by the steel thickness and roll diameter before and after each rolling pass.

However, the shape ratio is the steel thickness before and after each rolling pass, hl,
h2, and the roll radius is R. The most distinctive feature of the invention is that it has been found that the necessary condition for crimp contact is that the shape ratio is 0.8, preferably 1 or more.

It is known that in the center of the rolled steel material in the thickness direction, where there are no dents, the shape ratio is 1 or more and compressive stress acts in the rolling direction. It becomes a discontinuum, and therefore the stress distribution around it becomes complicated, and conventional research results cannot be applied.

((1) V, E, Potapkin and 1.A
, Bobukh: 5teel in the USSR
(1971), 641) The present inventors first conducted detailed basic experiments regarding crimping of cavities within slabs, and clarified the relationship between rolling conditions, deformation of the cavities, and strain distribution around the cavities.

FIG. 1 is an example of the results of measuring the strain distribution around the cavity after creating an artificial cavity inside the slab and performing hot rolling with various rolling shape ratios.

The strain in the rolling direction when the strain in a portion sufficiently far away from the cavity inside the slab is 0.31 is shown by the numbers and contour lines in FIG. 1.

FIG. 2 also shows the change in the diameter of the cavity in the rolling direction when the rolling shape ratio changes.

Note that the rolling characteristic values in the cases of FIGS. 1 and 2 are as follows.

Roll radius = 175mm Rolling schedule Rolling shape ratio 0.68: 30mm (D) → 27.5m
m → 25.5 mm → 24.0 mm → 22.5 mm → 21.5 mm (d) In the case of rolling shape ratio 1.05, 29.5 mm (D) → 25.
Omm → 21.5mm (d) 30.0mm (D) → 21. in case of rolling shape ratio 1.50.
5mm (d) From these, if the rolling shape ratio is 1 or less, the cavity will extend in the rolling direction to a greater extent than the average amount of deformation of the slab, and the amount of strain in the surrounding areas at both ends will be equal to the average strain of the healthy part of the slab. However, it can be seen that when the rolling shape ratio exceeds 1, the length of the cavity itself decreases, and the amount of strain around both ends becomes larger than the average amount of strain.

In other words, it has become clear that when the rolling shape ratio is greater than 1, crimping of the cavities tends to occur due to non-uniform deformation near the cavities during rolling.

FIG. 3 shows the relationship between the apparent density of the central part of the thickness of the slab and the rolling shape ratio for an actual continuously cast slab containing a hollow.

As the shape ratio increases, the apparent density increases,
When the shape ratio becomes 1 or more, it approaches a constant value and the Zaku can be considered to be crimped.

Figure 4 shows the reduction of area value in the plate thickness direction (the value of "reduction of area" defined in JIS Z2241, which is the cross-sectional shrinkage rate observed using a round bar tensile test piece, and is a guideline for ductility; the larger this value is, the more ductile it is. Are better.

) and shape ratio.

When the shape ratio is less than 1, the aperture value is low due to white spot defects due to uncrimped edges, but when it becomes 1, the edges are crimped and the aperture value increases.

Secondly, we will discuss how the elongation of inclusions due to rolling has a negative effect on the material quality.

The degree of elongation of A-type inclusions in steel materials can be determined not by the rolling pass schedule but by the rolling reduction ratio.As the rolling reduction ratio increases, the inclusions elongate more, which affects the mechanical properties of the steel material, especially the plate thickness. Degrades the directional characteristics.

FIG. 5 shows that the reduction of area in the thickness direction rapidly decreases with the rolling reduction ratio.

Therefore, from this point of view, it is desirable that the rolling reduction ratio be small.

In addition, one of the reasons why the reduction ratio is usually set to 6 or more in conventional manufacturing methods is to improve the toughness value in the rolling direction.
The required rolling reduction ratio can be reduced by increasing the austenite grain size during rolling heating to ASTM number 2, preferably number 4 or higher.

As mentioned above, when manufacturing steel products by rolling from slabs produced by the CC method, we have developed a new rolling method that can obtain excellent material quality by taking into account unbonded parts and elongation of inclusions as factors that cause material deterioration of the steel material. Invented it.

In other words, a rolling pass with a shape ratio smaller than 0.8 is not only ineffective for crimping, but also promotes elongation of the material.
It is actually harmful to the steel material.

Therefore, it was concluded that the most preferable rolling method is a rolling pass in which the shape ratio is 0.8, preferably 1.0 or more, and the reduction ratio is small.

The reasons for the limitations that form the gist of the present invention will be described below.

As is clear from Figure 3, the shape ratio is 0 for Zaku crimping.
．． 8, preferably 1.0 or more rolling passes are required.

Therefore, the shape ratio was limited to 0.8 or more.

Furthermore, if there is at least one rolling pass with a shape ratio of 0.8 or more, the zaku will be crimped, so the limiting condition for the number of rolling passes is one or more.

It is most effective to carry out a rolling pass with a shape ratio of 0.8 or more preferably 1.0 or more at the beginning of the rolling schedule, but it may also be carried out at the middle or end stage depending on the capacity and slab thickness of the rolling mill.

When loads are applied in the thickness direction, such as in offshore structures, lamellar tear resistance becomes a particular problem.

The aperture value in the plate thickness direction is used to evaluate the lamella tear resistance, and it is considered that if the aperture value is 15% or more, it is good, and if the aperture value is 25% or more, there is almost no problem with the lamella tear resistance.

Therefore, in consideration of the lamb latitude property of the steel material, the reduction ratio was set to 4 or less.

However, this value can be increased depending on the amount of sulfur and the form of sulfide, which is the cause of reduction of area deterioration, and in reality, product thicknesses vary, and complete lamellar tear resistance is not possible for all steel materials. Taking into account that this is not a requirement, the limiting conditions can be extended to a reduction ratio region where an aperture value of 15% or more is obtained, which is considered to have good lamellar tear resistance.

Therefore, the reduction ratio limit was set to 6 or less.

Although the lower limit of the total reduction ratio of the slab is not specified, there are porosity in the slab that occurs during solidification, and a certain amount of deformation is required for crimping, as shown in Figure 5. As such, a reduction ratio of 15 or more is usually required.

The present invention does not impose any restrictions on the thickness of the slab as long as it satisfies the limiting conditions.

Although CC slabs have been described as slabs, they are not necessarily limited.

Further, although the above description has mainly been given to steel plates, the present invention can also be applied to shaped steel, steel bars, and the like.

Next, examples of the present invention will be described.

The basic composition of the CC slab used is equivalent to JIS standard 5M50.

Example 1 Table 2 shows test results in which the rolling method according to the present invention shown in Table 1 and the conventional rolling method were performed and the mechanical properties were compared.

The tensile test piece was a JIS standard No. 4 round bar test piece.

As is clear from Table 2, when the rolling method according to the present invention is carried out, the mechanical properties, especially in the thickness direction, are significantly improved.

(1) The L direction means the rolling direction and the Z direction means the plate thickness direction. (2) * Midway break The value indicates the breaking stress.

Example 2 Continuously cast slabs cast to a thickness of 250 mm (basic components equivalent to SM50) were rolled by the conventional method and the method of the present invention, and mechanical property values in the thickness direction were investigated.

The rolling schedule is shown in Table 3.

Table 4 shows the mechanical property values investigated using JIS standard No. 4 round bar tensile test pieces, and it is clear that the rolling method of the present invention significantly improves the properties.

(1) Z direction means the plate thickness direction.

(2) Take five test pieces and show their mechanical properties.

(3) *Break in the middle, value indicates breaking stress.

[Brief explanation of drawings]

Figure 1 shows the strain distribution around the cavity and the rolled shape during rolling after an artificial cavity is created in the center of the slab and hot rolled so that the strain in the rolling direction at a sufficient distance from the cavity is 0.31. This shows the relationship with the ratio. FIG. 2 shows the relationship between the change in the diameter of the artificial cavity in the rolling direction due to rolling expressed in logarithmic strain and the rolled shape ratio. FIG. 3 shows the relationship between the density and shape ratio at the center in the thickness direction of a steel material rolled from a CC slab. FIG. 4 shows the relationship between the shape ratio and the aperture value in the plate thickness direction. FIG. 5 shows the relationship between the rolling reduction ratio and the reduction of area in the thickness direction.

Claims (1)

[Claims] 1. When manufacturing steel products from slabs by hot rolling, slab thickness D, product thickness d, and slab thickness before and after each rolling pass are h1. h2. (h1>h2), where R is the roll radius, D/d≦6, and at least one rolling pass is performed.