JPS5884958A - Manufacture of killed steel slab for hot rolling - Google Patents

Abstract

PURPOSE:To prevent the surface of Nb steel or Nb-V steel provided with high strength and toughness from being cracked during hot rolling by adding a specified amount of Ti to the steel. CONSTITUTION:The Nb, V, Ti and V contents of Nb steel or Nb-V steel contg. <=0.6% C, <=0.75% Si, <=2.0% Mn, <=0.04% P, <0.05% S, <=0.09% Al, <=0.01% N, <0.04% Ti and 0.01-0.10% Nb and/or 0.02-0.15% V or further contg. 1 or >=2 kinds of elements selected from Mo, Ni, Cu, Ca, Cr, W, Zn, REM and Hf as the steel of a steel plate for a pipeline, a thick steel plate, a steel bar, etc. requiring high strength and toughness as rolled or after treatment under heating are adjusted so as to satisfy 0.05<=2(Nb%)+V%<=0.22 and Ti%-3.4N% >=0.006. Thus, the surface of a slab of the steel can be prevented from being cracked during hot rolling.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method of manufacturing high-strength steel by hot rolling a slab produced by continuous casting, and in particular, the present invention is applicable to the method of manufacturing a high-strength steel material by hot rolling a slab produced by continuous casting. This was done to prevent cracks.

In general, Nb-containing and Nb-V-containing steels are the mainstream for line pipe steel plates, thick-walled steel plates, 7-strip steel, and other steels that require high strength and toughness either as rolled or by heat treatment. This is because high strength can be obtained even as is, and both strength and toughness can be improved by appropriately selecting working conditions in the rolling process and heat treatment process.

Here, the strength improvement mechanism of Nb-containing steel and Nb-V steel is
It is believed that this is due to bon-Nitride precipitates.

Although Nb-containing and Nb-V steels have such advantages, they have a major drawback in that surface cracks are likely to occur in slabs.

Currently, all steel mills are trying to change their operating format from the conventional ingot making and blooming method to the continuous casting method, which has lower production costs. However, when Nb-containing steel and Nb-V steel are produced using the continuous casting method, surface cracks are more likely to occur in the slab than when using the ingot-forming method, and large costs are required to remove the defects. If the degree of flaws is severe, it becomes difficult to remove them manually, making it impossible to produce slabs of practical use, and becoming a major factor preventing the continuous casting method from expanding the range of steel types and improving the continuous casting ratio. There is.

Possible causes of these problems in continuous casting machines include: - thermal distortion in high-temperature ranges, - distortion or bending due to poor roll alignment, and - external stress due to bending and unbending caused by curved continuous casting machines. The generation mechanism is that NbNC, VNC, etc. precipitate at the austenite grain boundaries, and voids are generated with the grain boundary precipitates as nuclei.At this time, by adding the above conditions, the voids are promoted and connected, and these voids become old. Cracks, which are intergranular cracks, are thought to occur by cracking along austenite grain boundaries.

As part of these explanations, the Grieple test results for Nb-containing, Nb-V-containing, etc. are shown in FIG. This is Aζ.

It shows the lowest shrinkage value just below the temperature, indicating that it becomes brittle in a specific temperature range. The reason for this is thought to be that when the temperature drops below the two-phase coexistence region of ferrite and austenite, the precipitation of nitrides is rapidly accelerated, and moreover, they are selectively precipitated at the austenite grain boundaries. When the above-mentioned conditions are added to this, the presence of nitrides causes the deformation of the austenite grain boundaries to be low and the drawing strength value to drop rapidly, leading to the development of cracks.

In accordance with the above explanation, it has been found that there is a embrittlement temperature range in which the shrinkage value rapidly decreases in a specific temperature range, which can explain the cause of cracking. Continuous casting machines that are in practical use are bent and straightened in this embrittlement temperature range.
Therefore, the operating method is to apply external stress under conditions that cause cracks to occur.

Recently, as an operating method to avoid this embrittlement temperature range,
As a method for changing the amount of secondary cooling water and preventing cracks, JP-A No. 5
There is the addition of a trace amount of Ti, etc., which is described in the report No. 3-1-23325, but it has not been completely solved.

That is, in JP-A-53-123325, N%-0.29 (
The amount of TiL) is specified as -0,002 to 0.006.The reason is that if the content is excessive compared to the N content, TiC will precipitate and impede the toughness of the base metal and weld zone. That is what it is. However, even within the scope of this technical concept, surface cracking cannot be completely eliminated even if it is reduced, and (
N) If excessive, surface cracking will worsen and good slabs will not be obtained.

Therefore, as a result of analyzing many experimental examples, the present invention has developed a specific range within the region of excessive Ti, which has traditionally been avoided, i.e., when Ti is 4% and Ti is 3%. It was discovered that by satisfying the relationship: .4NtI6)0.006, surface cracks can be completely eliminated.

Since the main cause of surface cracks is nitrides, methods to prevent the precipitation of nitrides include: ■ No N in steel, ■ Ni
Lower the remaining N to Ti, which has a strong affinity with N. #) Ti
There is a method to prevent the precipitation of nitrides by completely fixing it as N and making it Free (N) = 0. Since method (■) is difficult due to current manufacturing technology, method (■) was adopted. .Figures 2 and 3 (See 1)
By setting e(N)=Q, we not only reduce the embrittlement temperature range but also improve the aperture value.
This is in good agreement with the technical concept of N) = 0.Also, as a result of conducting field experiments by changing the strands inside and outside the embrittlement temperature range, we found that there is a difference between the embrittlement temperature range and outside the embrittlement temperature range, as shown in Figure 4. No surface defects were observed on both strands.

Ti%-3,4N%) 0.0 which is the key point of the present invention
The reason why the relational expression 06 is obtained is as shown in Figs. As a result, surface flaws occur due to surface flaws (errors in measurement), etc., and the drawing depth value decreases in the Greeple test results, and it is difficult to obtain stable slab surface quality. Therefore, from the viewpoint of stable production, Ti%-3
, 4N%) 0.006.

Next, the reason for limiting the component range will be explained in detail.

The steel of the present invention is a line pipe steel plate (API, ASTM
.

BS (B53606)t DIN (DIN17
172) etc.], thick steel plate (JISG3211.J
Standards such as ISG3141) 9-bar steel (JISG3503.
Standards such as JI8G4051), etc.) Need to ensure the strength and toughness of the steel plate Steel types to which Nb, V, Nb-V are added This is a measure to prevent defects.

For C, Si, P, S, kl, etc., upper limits satisfying the above standards were used. If Mn exceeds 2.0 mm, the toughness of the weld zone deteriorates, so the upper limit value was set at 2.01 Th. When N exceeds 0.01S, the welding wire must be replaced to improve the toughness of the weld zone during welding.As a toughness measure, the upper limit was set at 0.01%. The most important amount of Ti added is 0.05s
f: If it exceeds it, it becomes a TiC-based component, which greatly impedes low-temperature toughness, so the upper limit was set at 0.04%. Also, as mentioned above, Free(N) = O, that is, Ti%-3,4N
Although it is effective in preventing surface scratches by satisfying the relationship Ti㉉0, there is large variation due to uniform mixing in the steel, errors in analytical equipment, etc. 0.0
It was set to 0'6.

To explain these, let us consider the case where the aperture value is reduced despite the fact that Free(N)=0 in a multi-component manner as shown in FIGS. 2, 3 and 4. In addition, it is an essential condition to set Free (N) = O, which can be said to be the principle of the method of the present invention, and to prevent the precipitation of nitrides. Add 0.0% Mo to the ingredients.
It can also be contained in an amount of 50% or less. The amount of Mo added is 0
．． If it exceeds 50, it will adversely affect the toughness of the base metal and weld zone, so the upper limit was set at 0.50%. MO cannot be expected to improve the effect of preventing slab surface cracking, but Ti does have the aforementioned preventive effect.

Since it does not impede the performance of the invention in any way, it does not impair the characteristics of the present invention even if it is added to improve strength.

Furthermore, in the present invention, in addition to the basic four inventions, Te, Ni
<5. Os, Cu<: 1.0%, CaO, 000
5 to o, o a s are included. Ni and Cu improve strength and toughness, and if they are present too much, the toughness of the base metal and weld zone deteriorates, so each upper limit fNr 5
．． Q.

Cu 1. It was set as O. Although Ca is known to denature sulfide, it has no effect at less than 0.0005%, so the lower limit was set at o, o o o s%. In addition, if the amount is too large, a large amount of Ca oxide will be generated, forming large inclusions and significantly impairing cleanliness.
``Take the upper limit, ta.

Furthermore, in addition to the basic invention, in the present invention, "<
5.0 fb t”<1.0%, Cr<1, O
Contains one or more elements of group A consisting of S, W, 1.0, or RBMO, OOO5-0
．． 03%, Ca0.0005~0.03 To, Z
A of one or more elements of group B consisting of r<0.10 s, Hf and 0.01 t! It contains FF or group B. Group A is an element that improves strength, elongation, and toughness, but if it is present in too much, it deteriorates the toughness of the base metal and weld, so the upper limit for each of these elements is tNi5.0% and Cr.
1. O%.

Cu　1.　OTo　*　W”, 0%.

REM, Ca, Zr, Hf are monkey 77 (change C)
However, if REMo is less than 0.0005%, there is no effect, so the lower limit was set at 0.0005%.

If too much REM sulfide is used, REM sulfide will become large.
The upper limit of the amount added was set at 0.03%. Similarly, the range of Ca is 0.005 to 0.03%, and the range of Zr and Hf is 0.
．． It shall be 10 inches or less.

As described above, according to the present invention, slabs without surface cracks can be obtained during continuous casting.

Figure 5: KTi%-3,4N%': 20.006 TE (D shows the occurrence of surface flaws, and it can be seen that within this range, almost no surface flaws occur.

[Brief explanation of drawings]

Figure 1 is a chart showing the Greeple test results for various steels, Figure 2
Figures 3 and 3 are charts showing the results of the Greeple test for steels obtained by the present invention and the conventional method, Figure 4 is a chart showing the relationship between slab temperature and the number of cracks, and Figure 5 is a chart showing the relationship between the steel slab temperature and the number of cracks. It is a chart showing the occurrence of surface flaws due to changes in the range of 4N. Patent applicant's representative Patent attorney Tomoyuki Yabuki (and 1 other person) Figure 1 Urjun Atsushi (0c) Figure 2 Keiyu A &(''C) Figure 3 Kei 1kion/i (°c)

Claims (1)

[Claims] C: 0.6%, 18i: 0.75%, 9Mn: 20%, P: 0.04%, S: 0.05%, At<0.09%, N
<0.01%, Ti and 0.04% NbO, 01~0
．． 10%. V 0.02 to 0.15% of one or two kinds, and if necessary, Mo, Ni, Cu, Ca, Cr,
W. Contains one or more of Zn, REM, and Hf, with the balance being free of Fe and unavoidable impurities, and 0.0
5ku2(Nb%)+(7%)<0.22. and 11%
A method for producing killed steel slabs for hot rolling, characterized in that steel satisfying the relationship of -3,4N%) 0.006 is made into slabs using a continuous casting machine.