JPH0135908B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for producing killed steel slabs for hot rolling using a continuous casting method.
This was done to prevent surface cracks that occur on the surface of slabs peculiar to Nb steel, V-containing steel, and Nb-V-containing steel. As a conventional method of fixing [N] by adding a small amount of [Ti], it is effective as a countermeasure against surface defects, but if the amount of [Ti] exceeds the equivalent balance between [N] and [Ti],
The drawback was that the TiC compound precipitated and adversely affected the toughness, making it impossible for [Ti] to fulfill its original role. The gist of the present invention is to replace the addition of [Ti] with [Zr] as a measure to prevent cracking of the slab surface of Nb-containing steel, V-containing steel, and Nb-V steel. The point is that it fixes and controls the morphology of inclusions due to excess [Zr], and does not reduce toughness. It is generally used in the manufacture of products that require high strength and toughness, such as steel plates for line pipes, thick steel plates, thick and medium plates for hot rolling, thin plates for cold rolling, long steel, etc., either as rolled or after heat treatment. Nb steel, V-containing steel, and Nb-V-containing steel are mainly used. The reason for this is that high strength can be obtained even as rolled, and by appropriately selecting the heating step, rolling step, and heat treatment conditions, both high strength and high toughness can be obtained. The mechanism of these strength improvements is NbCN,
Carbo nitrides such as VCN, V ₃ C ₄ , etc.
-Nitride) is known to be caused by the precipitates.
Although Nb-containing steel, V-containing steel, and Nb-V-containing steel have these advantages, they have a major drawback in that surface cracks are likely to occur in slabs. Currently, the steel industry is trying to change its operating format to a continuous casting method, which is cheaper in production cost than the conventional ingot-blowing method. However, Nb-containing steel, V-containing steel,
When manufacturing Nb-V steel using the continuous casting method, surface cracks (cracks, etc.) are more likely to occur on the slab than when using the ingot casting method, so a large amount of cost is required to remove the defects. In addition, if the degree of flaw is severe, it becomes extremely difficult to remove it by hand, making it impossible to produce slabs of practical use. This is a contributing factor. Causes of surface cracks on slabs caused by continuous casting methods include thermal strain in high temperature ranges, distortion or bending due to poor roll alignment in continuous casting machines, external stress due to bending and unbending of slabs in curved continuous casting machines, etc. is possible. The crack generation mechanism is that nitrides precipitate at austenite grain boundaries, and voids are generated with these grain boundary precipitates as nuclei.In this state, when the above conditions are added, the voids grow and connect with each other. However, it is thought that cracks, which should be called intergranular cracks, occur when these voids crack along the prior austenite grain boundaries. To help explain this, Nb-containing steel, V-containing steel,
Figure 2 shows the high temperature tensile data of Nb-V steel.
This figure shows that these steels exhibit the lowest reduction of area just below the Ar ₃ point, indicating that they become brittle in a specific temperature range. The reason for this is that when the temperature drops below the two-phase coexistence region of ferrite and austenite,
This is due to the rapid acceleration of the precipitation of NbCN and VCN, and their selective precipitation at austenite grain boundaries. By adding the above conditions to this, deformation occurs from grain boundaries with weak strength, and NbCN,
Due to the presence of VCN, the deformability of the austenite grain boundaries is low, which appears to cause the reduction of area to drop rapidly and develop into cracks. From the above, it has been found that there is a embrittlement temperature range in which the aperture value rapidly decreases in a specific temperature range.
The cause of cracks can be explained. By the way, in the case of continuous casting machines that are currently in practical use, bending straightening is performed in the embrittlement temperature range of Nb-containing steel, V-containing steel, and Nb-V-containing steel, and therefore there are conditions that cause cracks to occur. The method of operation is to apply external stress. Recently, methods have been proposed to avoid this temperature in the embrittlement range, such as changing the amount of secondary cooling water or adding a small amount of Ti to prevent cracking, but these have not been completely solved. An object of the present invention is to completely avoid the occurrence of surface flaws in Nb-containing steel, V-containing steel, and Nb-V-containing steel by continuous casting. As a measure to prevent the occurrence of surface defects on slabs of the above steel types,
A method of fixing [N] by adding a small amount of [Ti] is effective, but if the equivalence balance between [N] and [Ti] is broken and there is an excess of [Ti], TiC compounds will precipitate, which will have an adverse effect on the toughness [Ti] It has the disadvantage that it cannot fulfill its original role. In order to eliminate such drawbacks, the present invention uses Ti instead of Ti.
Zr content in steel is 0.005 to 0.06% and Total [Zr]−
6.5Total[N]>0.
We provide a method for producing killed steel slabs for hot rolling that not only avoids the occurrence of surface defects in slabs but also further improves the required quality characteristics by adding Nb steel, V-containing steel, and Nb-V-containing steel. That is. The basic idea of the present invention is to reduce the reduction in cross-sectional shrinkage in the embrittled region. Therefore, reduction in cross-sectional shrinkage is achieved by preventing Nb and V from turning into nitrides and precipitating at austenite grain boundaries. That is, when NbCN and VCN precipitate, the above-mentioned external stress is applied and the cross-sectional shrinkage rate decreases, so it is sufficient to prevent NbCN and VCN from precipitating. In order to prevent such things from precipitating, there are two methods: one is to completely eliminate the input amount of N so that nitrides do not precipitate, and the other is to use other types of elements that have a greater affinity for N than Nb and V. can give. Since this method is almost impossible in terms of steel manufacturing, the present invention aims to minimize the input amount of N, and also fixes N as ZrN by adding an appropriate amount of Zr. Free
By setting [N]=0, it is possible to completely eliminate the occurrence of surface defects on slabs due to the continuous casting method, and also to satisfy the required quality characteristics. Figure 1 shows the relationship between the temperature at the straightening point and the number of surface cracks (cracks) during continuous casting of steel slabs satisfying Total [Zr] - 6.5 Total [N] > 0. This figure shows the operating conditions of the continuous casting method (changes in the secondary cooling water pattern, etc.; in the case of 1 strand, the normal cooling pattern is adopted; in the case of 2 strands, slow cooling is used to raise the upper limit temperature of the embrittlement region). Despite this, no surface cracking occurred in both patterns. As is clear from Figure 1, when a normal cooling pattern is used for one strand, no surface cracks occur even though both the long and short sides of the slab are within the embrittlement zone. Therefore, Total〔Zr〕−
It can be explained that the decrease in cross-sectional shrinkage rate is reduced in steels satisfying 6.5Total [N]>0, and it can also be seen that the cause of the decrease in cross-sectional shrinkage rate is the formation of nitrides. This is also clear from FIG. 2, which shows the high temperature tensile test results of the Nb-V-ZrN steel according to the present invention. In other words, ○A steel containing Nb, ○B steel containing V, ○C containing Nb
In the case of -V steel, there is a deep valley where the cross-sectional shrinkage rate decreases, whereas
In the case of V-ZrN steel, the embrittlement region of ○ho is not seen at all.
The cross-sectional shrinkage rate is the same as that of Si-Mn steel. The reason why the present invention achieves the effect of preventing slab surface cracking in Nb-containing steel, V-containing steel, and Nb-V-containing steel is that
By adding Zr, ZrN can be used during continuous slab production.
This is because NbCN and VCN precipitate in a high temperature range and do not precipitate at all. Next, the reason for limiting the composition range of the target steel of the present invention will be explained in detail. The target steels of the present invention are line pipe steel plates (API standards, etc.), thick steel plates (JISG3211, JISG3212,
BS4360), hot and cold rolling steel plate (JISG3313,
Nb,
V, Nb-Since it is a steel type with V added, Nb,
It is necessary to take measures to prevent surface cracks in continuously cast slabs caused by V, Nb-V, etc. Mn is 2.0
If it exceeds 2.0%, the toughness of the weld will deteriorate.
was set as the upper limit. The amount of Zr added is 0.005 in the steel.
If it is less than 0.06%, ZrN in the slab will be insufficient and cracks on the slab surface cannot be prevented, and if it is more than 0.06%, ZrO ₂
Because the amount of oxides increases and the quality deteriorates significantly.
The content was limited to 0.005-0.06%.
The lower the N content, the better, considering surface cracking of the slab, but it was set to 0.008% or less in order to maintain an equivalence relationship with the upper limit of Zr, 0.06%, from the relationship with Zr. Also Total〔Zr〕−
The reason for setting 6.5Total [N] > 0 is that it is an essential condition to prevent nitride precipitation, which is the basic concept of the present invention. ]=0. When Free [N] > 0, cracks occur on the slab surface and the effect of Zr addition is no longer recognized. Table 1 shows whether or not cracks occurred on the slab surface of the steel according to the present invention and the conventional steel. Free [N] from this result
It is clear that cracks occur on the surface of slabs in conventional steels with >0.

[Table] In the invention of claim 2, Mo is further added to the steel composition of the invention of claim 1.
Applies to steel containing 0.50% or less. The upper limit of the amount of Mo added was set at 0.50% because if it exceeded 0.50%, it would have a negative effect on the base metal and the toughness of the weld. Although Mo cannot be expected to improve the effect of preventing slab surface cracking, it does not in any way interfere with the above-mentioned preventive effect of Zr, so adding it to improve strength will not impair the characteristics of the present invention. In the invention of claim 3, in addition to the steel components of the invention of claim 1, Ni5.0
% or less, Cr1.0% or less, Cu1.0% or less, W1.0% or less, or REM0.0005 to 0.03%,
The object is steel containing one or more of the elements of the group consisting of Ca0.0005 to 0.03% and Hf0.01% or less. Ni, Cr, Cu, and W are elements that improve strength and toughness, but too much of them deteriorates the toughness of the base metal and weld, so the upper limit for each is set to 5.0% Ni,
Cr1.0%, Cu1.0%, W1.0%. REM, Ca, and Hf are known to metamorphose sulfides, but they are ineffective if REM is less than 0.0005%, so the lower limit was set at 0.0005%. If the amount is too large, not only will the REM sulfide become large, but also a large amount of REM oxysulfide will be generated, forming large inclusions and significantly impairing cleanliness. For this reason, the upper limit of the amount of REM added was set at 0.03%. Similarly, the range of Ca is 0.0005-0.03%, and
Hf shall be 0.10% or less. As described above, according to the present invention, a slab with fewer surface crack defects can be obtained during continuous casting.

[Brief explanation of drawings]

FIG. 1 is a diagram showing the relationship between the slab surface temperature and the number of surface cracks during the continuous casting process, and FIG. 2 is a diagram showing the high temperature tensile test results of the steel according to the present invention and comparative steel.

Claims (1)

[Claims] 1 C 0.6% or less, Si 0.75% or less, Mn 2.0% or less,
P0.04% or less, S0.05% or less, Al0.09% or less,
N0.008% or less, Zr0.005~0.06% plus Nb0.01
~0.10%, V0.01~0.20%, and the remainder consists of Fe and unavoidable impurities,
A method for manufacturing killed steel slabs for hot rolling, characterized by continuously casting steel that satisfies the relationship: Total [Zr] - 6.5Total [N] > 0 to obtain a slab. 2 C0.6% or less, Si0.75% or less, Mn2.0% or less,
P0.04% or less, S0.05% or less, Al0.09% or less,
N0.008% or less, Mo0.50% or less, Zr0.005~0.06%
In addition to Nb0.01~0.10% and V0.01~0.20%, it contains one or two types, the balance consists of Fe and unavoidable impurities, and Total [Zr] - 6.5Total [N]
A method for producing killed steel slabs for hot rolling, characterized by continuously casting steel satisfying the relationship >0 to produce slabs. 3 C0.6% or less, Si0.75% or less, Mn2.0% or less,
P0.04% or less, S0.05% or less, Al0.09% or less,
N0.008% or less, Zr0.005~0.06% plus Ni5.0%
Below, elements of the group consisting of Cr1.0% or less, Cu1.0% or less, W1.0% or less or REM0.0005 to 0.03%,
Contains one or more elements selected from one or both of the elements in the group consisting of Ca0.0005~0.03% and Hf0.01% or less, and further Nb0.01~0.01%.
0.10%, V0.01~0.20%, the balance consists of Fe and unavoidable impurities, and Total [Zr]-
6.5 A method for producing killed steel slabs for hot rolling, characterized by continuously casting steel that satisfies the relationship of Total [N] > 0 to obtain slabs.