JPH0135907B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing killed steel slabs for hot rolling using a continuous casting method, and in particular, the present invention is directed to preventing surface cracks that occur on the surface of slabs peculiar to B-containing steel and effectively utilizing [B]. It was done for. In order to improve high strength and toughness through controlled rolling or heat treatment when manufacturing line pipe steel plates, thick steel plates, hot rolling thick medium plates, cold rolling thin plates, long steel plates, etc. B-added steel is generally used. The reason for this is that B has the highest quenching multiplier and when added in a small amount, it significantly improves hardenability and easily forms a quenched structure, and when subjected to heat treatment, etc., it forms a fine structure and improves strength and toughness. This is because they can be improved together. However, despite the above-mentioned advantages, slabs produced by continuous casting of B-added steel have the disadvantage that surface cracks are likely to occur. 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, when B-added steel is produced by a continuous casting method and as disclosed in JP-A No. 49-91012, B-added steel is added in the ratio of Zr% and N%.
When melting steel, as mentioned above, surface cracks (cracks, etc.) are more likely to occur on the slab than in the ingot-forming method, so a large amount of cost is required to remove the defects, and If the degree of corrosion is severe, it is extremely difficult to remove it by hand, making it impossible to produce slabs of practical use. There is. The causes of surface cracks on slabs produced by continuous casting methods include thermal strain in high-temperature ranges, distortion or bending due to poor roll alignment in the continuous casting machine,
Possible causes include external stress caused by bending and unbending of the slab by the curved continuous caster. The mechanism of this cracking is that BN precipitates at the 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 each void is It is thought that these voids connect with each other and crack along the prior austenite grain boundaries, resulting in cracks that can be called intergranular cracks. To help explain this, high-temperature tensile data for B-added steel is shown in Figure 2. This figure shows the lowest aperture value just below the Ar ₃ point, indicating that it becomes brittle in a specific temperature range. The cause of this is ferrite,
When the temperature drops below the two-phase coexistence region of austenite, the precipitation of BN is rapidly accelerated, and moreover, it is selectively precipitated at the austenite grain boundaries.
When the above conditions are added to this, the deformability of the austenite grain boundaries is low due to the presence of BN, and it appears that the reduction of area decreases rapidly and develops into cracks. From the above, it is clear that there is a embrittlement temperature range in which the aperture value rapidly decreases in a specific temperature range, which can explain the cause of cracking. 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 B-added steel. It's summery. Recently, as an operation method to avoid this embrittling temperature range, changing the amount of secondary cooling water or controlling the cooling rate has been carried out.
Although methods have been proposed, such as methods of adding The object of the present invention is to completely avoid the occurrence of surface flaws in B-added steel slabs caused by continuous casting. As a countermeasure against surface defects on B-added steel slabs, the present inventors previously proposed Total[Ti]-
3.4 We proposed a method to eliminate surface defects on slabs by satisfying the relational expression Total [N] ≧0.
Although this is effective as a countermeasure against surface flaws,
The drawback is that TiC compounds are precipitated by Ti, which impairs toughness. In order to improve this drawback, the present invention uses Zr instead of Ti at a content of 0.005 to 0.06% in the steel and a total [Zr]
−6.5 Total [N] > 0 is added to the killed steel slab for hot rolling, which not only avoids the occurrence of surface defects on the slab but also further improves the required quality characteristics. The purpose is to provide a manufacturing method. The basic concept of the present invention is to reduce the reduction in cross-sectional shrinkage in the embrittled region, and to prevent the phenomenon in which B combines with N to form BN and precipitates at austenite grain boundaries. In other words, B combines with N
When BN is generated, the above-mentioned external stress is applied and the cross-sectional shrinkage rate decreases, so it is sufficient to prevent BN from precipitating. In order to prevent this BN from precipitating, there are two methods: completely eliminating the input amount of N so as not to precipitate nitrides, and using other types of alloying elements that have a greater affinity for N than B. It will be done. Since this method is nearly impossible due to steel melting,
The present invention adopts the method described above, reduces the input amount of N as much as possible, and adds an appropriate amount of Zr to fix N as ZrN, thereby making it free.
By setting [N]=0, it is possible to completely eliminate the occurrence of surface flaws in slabs due to the continuous casting method, and also to satisfy the required quality characteristics of mechanical properties. Figure 1 shows the relationship between the temperature at the straightening point and the number of surface cracks during continuous casting of B-containing steel slabs satisfying Total [Zr] - 6.5Total [N] > 0. This figure shows the operating conditions for the continuous casting method (changing the secondary cooling pattern). In the case of one strand, a normal pattern is adopted, and in the case of two strands, a slow cooling pattern is adopted to avoid the upper limit temperature of the embrittlement region. It can be seen that, despite the fact that no surface cracking occurred in both patterns, there were no surface cracks. As is clear from Fig. 1, when the normal cooling pattern is used for one strand casting, the long side of the slab,
Although both short sides are within the embrittlement area,
No surface cracks occurred, Total〔Zr〕−
It can be seen that in the steel satisfying 6.5Total [N] > 0, the decrease in the cross-sectional shrinkage rate is reduced, and it is also understood that the cause of the decrease in the 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 B-ZrN-containing steel according to the present invention and the B-containing steel that is a comparative steel. In other words, curve A for B-containing steel shows a deep valley of reduced cross-sectional shrinkage, whereas curve B for B-ZrN steel shows curve C for Si-Mn steel with no embrittlement region. It shows the same cross-sectional shrinkage rate. The reason why the present invention can achieve the effect of preventing cracking on the slab surface is that by adding an appropriate amount of Zr,
This is because ZrN precipitates in the high temperature range during continuous cast slab production, and BN does not precipitate at all. Excess Zr is also effective in controlling the form of inclusions in steel sheets, improving the anisotropy of slabs, and improving toughness. 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 steel plates for line pipes, thick-walled steel plates, and steel plates for pressure vessels (JIS G3115, JIS
G3120) is a hot-rolled or cold-rolled B-containing steel plate, etc., in which B is added to ensure the strength and toughness of the steel plate, and requires measures to prevent surface cracking of continuous cast slabs due to B. If Mn exceeds 2.0%, the weld toughness deteriorates, so 2.0% was set as the upper limit. If the Zr content in the steel is less than 0.005%, the ZrN content in the slab is
If the amount is insufficient, cracking of the surface of the slab cannot be prevented, and if it exceeds 0.06%, the amount of ZrO ₂ oxide increases and the quality deteriorates significantly, so it was limited to a range of 0.005 to 0.06%. The lower the N value, the better when considering surface cracking of the slab, but this also depends on the upper limit of Zr due to its relationship with Zr.
In order to maintain the equivalence relationship with 0.06%, it was set to 0.007% or less.
Also, the reason for setting Total [Zr] - 6.5Total [N] > 0 is that it is an essential condition not to precipitate nitrides, which is the principle of the present invention. Free〔N〕 by setting
This is because the goal is to achieve =0.
When Free [N] > 0, cracks occur on the slab surface,
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.
Use 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
%, 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 is set at 0.03%.
And so. Similarly, the range of Ca is 0.0005 to 0.03%, and the range of Hf is 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 surface temperature of a slab 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 steel according to the present invention, ordinary steel, and B-containing steel. .

Claims (1)

[Claims] 1 C 0.50% or less, Si 0.75% or less, Mn 2.0% or less,
P0.04% or less, S0.05% or less, Al0.09% or less,
N0.007% or less, Zr0.005~0.06% plus B0.003
% or less, the balance consists of Fe and unavoidable impurities, and Total [Zr] − 6.5Total [N] > 0
A method for manufacturing killed steel slabs for hot rolling, characterized by continuously casting steel that satisfies the following relationship to produce slabs. 2 C0.50% or less, Si0.75% or less, Mn2.0% or less,
P0.04% or less, S0.05% or less, Al0.09% or less,
N0.007% or less, Mo0.50% or less, Zr0.005~0.06%
In addition to the following, contain 0.003% or less of B, and the balance is 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: Total [N] > 0 to obtain slabs. 3 C0.50% or less, Si0.75% or less, Mn2.0% or less,
P0.04% or less, S0.05% or less, Al0.09% or less,
N0.007% 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 to 0.03% and Hf0.01% or less, and further contains B0.003%.
Killed steel for hot rolling, characterized in that it is made into a slab by continuous casting of steel that contains the following and the balance is Fe and unavoidable impurities and satisfies the relationship Total [Zr] - 6.5Total [N] > 0 Method of manufacturing slabs.