JPH0360572B2 - - Google Patents

Description

[Detailed Description of the Invention] Industrial Application Field The present invention relates to a method for controlling solidification segregation of steel, and more specifically, to reducing solidification segregation that causes material defects in carbon steel products obtained by continuous casting. The present invention relates to a method for effectively controlling the double segregation between trees during solidification of steel.

Conventional technology Traditionally, continuous casting and ingot making have not only caused surface flaws and cracks in slabs due to segregation of solutes during solidification, but also caused problems with low-temperature toughness when processed into product plates such as thick plates. Improvements have been desired since the quality of the product deteriorates, such as deterioration of the parts and increase of cracks in the welded parts.

These improvement methods include adding Ca to molten steel, reducing solutes that cause harmful segregation through refining, shortening the gap between the rolls of a continuous casting machine to suppress bulging, and Methods such as electromagnetic stirring to reduce center segregation are being used.

In addition, from the point of view of energy saving and labor saving, it is possible to prevent surface cracking of the continuously cast slab during rolling in direct rolling where the continuously cast slab is hot rolled without cooling it to room temperature or in hot charge rolling where the slab is charged into a heating furnace and then rolled. Therefore, a method for suppressing surface cracks in cast slabs has been proposed in which ultra-slow cooling is performed during the cooling process following melt solidification and before the start of hot rolling (Japanese Patent Laid-Open No. 1983-1999).
84203).

In the above method, P, S, O, which is harmful to hot workability,
Simulation experiments were conducted in a specific temperature range where elements such as N precipitate as nonmetallic inclusions due to segregation, and surface cracks occur frequently when the minimum cross-sectional shrinkage rate is less than 60% in the 1300-900℃ temperature range. Focusing on this, hot cracking of slabs is suppressed by controlling the precipitation form of these elements.

Further, Japanese Patent Laid-Open Nos. 55-109503 and 55-110724 also disclose a method in which continuous cast slabs are slowly cooled before hot rolling and then directly rolled.

On the other hand, Japanese Patent Publication No. 49-6074 discloses a method of preventing cracks by cooling and heating the continuously cast strand in order to prevent the temperature difference between the surface and the center liquid from becoming too large.

Furthermore, a method for suppressing inverted V segregation in large steel ingots by adding Mo is also disclosed (Japan Steel Works Newsletter, 40
(1980) p.1). In this method, in the solid-liquid coexistence layer,
Because low-density solutes are enriched in the liquid phase, the density of the liquid phase in this part is lower than that of the bulk liquid phase, resulting in upward convection, and this rising line remains as a streak even after solidification. , there is inverted V segregation, so this method attempts to suppress inverted V segregation by adding Mo to increase the density of the liquid phase and blocking convection in the upward direction.

Problems to be Solved by the Invention The present inventors have discovered that the deterioration of slab quality is not simply due to the amount of solidification segregation, but also that α stabilizing elements (P,
When γ-stabilizing elements (Si, S, Cr, Nb, V, Mo, etc.) and γ-stabilizing elements (C, Mn, Ni, etc.) are concentrated in the same area, the synergistic negative effects due to overlapping segregation become even more significant. This paper focuses on the difference in solubility between these α-stabilizing elements and γ-stabilizing elements in the δ and γ phases, and attempts to provide an effective method for separating these solutes. be.

In particular, the continuous casting materials targeted by the present invention have the following problems: the slab size is small and the solidification time is short; the cooling rate is high, so the solidification time is short; and the molten steel flow discharged from the immersion nozzle slows down the solidification process. Since stirring of molten steel is promoted and the concentrated liquid phase is easily homogenized, inverted V segregation hardly occurs, but rather center segregation caused by inter-tree segregation and its accumulation is the main problem. Therefore, the present invention aims to provide a particularly effective method for suppressing interdendritic segregation and center segregation in solidification segregation of steel during continuous casting.

Means for Solving the Problems The present invention provides: (1) In continuous casting of carbon steel containing a C concentration of 0.53% by weight or less, Be, Cr, Nb, Sn,
At least one or two or more of Ti or V,
1.4% by weight or less for Be, 0.5% for Sn
After adding 2% by weight or less of each other than Be and Sn, the temperature is from just below the melting point (when δ phase primary crystals occur) to the end temperature of Ar ₄ transformation or peritectic reaction (until it becomes γ phase). range to 40
A method for controlling the solidification segregation of steel in continuous casting characterized by cooling at a cooling rate of ℃/min or less, and (2) in continuous casting of carbon steel containing a C concentration of 0.53% by weight or less, Be, Cr, Nb, Sn,
At least one or two or more of Ti or V,
1.4% by weight or less for Be, 0.5% for Sn
After adding 2% by weight or less for each other than Be and Sn, and further adding 2% by weight or less of Mo2, just below the melting point (when δ phase primary crystals occur)
A method for controlling solidification segregation of steel in continuous casting, characterized by cooling at a cooling rate of ₄₀ °C/min or less over the temperature range from be.

Effect Steel in a molten state is cooled and as the temperature decreases, a solid phase crystallizes out. Figure 1 shows the relationship between the change in state and the carbon temperature. Carbon concentration is 0.17
Steel with a concentration between ~0.53% (weight%, the same applies hereinafter) changes from the liquid phase (area above line 1) to (liquid phase + δ phase) by cooling to (liquid phase) below 1495℃ (line 3 in the figure). phase + γ phase), and further cooling progresses to form straight line 6.
All become γ phase at the following temperatures. At the interface between the liquid phase and the δ phase (liquid phase + δ
Using the so-called peritectic reaction, which changes from phase) to (γ phase), α-stabilizing elements P, Si, S,
By incorporating P and S, which are particularly problematic, such as Cr into the δ phase, which has high solubility, C, which is a γ stabilizing element,
Incorporate Mn, Ni, especially Mn into the highly soluble γ phase. When the cooling progresses further and the total amount reaches the γ phase, the above-mentioned α stabilizing element is unevenly distributed in the portion that is transformed into the γ phase the latest. As a result, for example, a portion where a peak concentration of P exists is separated from a portion where a peak concentration of Mn exists, and overlapping segregation of P and Mn is avoided.

In steel with a carbon content of 0 to 0.08%, cooling changes the phase from liquid phase to (liquid phase + δ phase) to δ phase to γ phase.
In this case, the transformation from the δ phase to the γ phase is called Ar ₄ transformation, which begins at the temperature of line 4 in FIG. 1 and continues up to the temperature of line 5. During this time, in the Ar ₄ transformation region, the α-stabilizing element and the γ-stabilizing element are separated by utilizing the difference in solubility by utilizing the coexistence of the δ phase and the γ phase. For example, P is transferred to the δ phase and Mn is transferred to the γ phase. Even when the cooling progresses further and the entire amount changes to the γ phase, the α stabilizing element is unevenly distributed in the portion that transformed to the γ phase most late. As a result, for example, a portion where a P concentration peak exists is separated from a portion where a Mn concentration peak exists, and overlapping segregation of P and Mn can be avoided.

For steels with a carbon concentration of 0.08% to 0.17%, both the aforementioned peritectic reaction and the separation in _Ar4 transformation can be utilized.

Here, the inventors believe that Be, which is an α-stabilizing element,
It has been found that when one or more of Cr, Mo, Nb, Sn, Ti, or V is added to molten steel, the δ phase (=α phase) region in the phase diagram of FIG. 1 expands.

Generally, the diffusion rate of impurities or additive elements in solid iron is 10 to 100 times higher in the δ phase than in the γ phase (Hiroshi Oikawa; Iron and Steel
Vol. 68 (1982), p. 1489). Therefore, as the amount and presence time of the δ phase increases during the solidification process of steel, the rate of diffusion from the high concentration segregation area to the surrounding low concentration area increases accordingly, making it possible to reduce segregation.

Furthermore, as shown in Figure 2, when an α-stabilizing element is added, the δ-phase region in the dendrite portion increases compared to the case without addition, and as a result, the region where the δ-phase and γ-phase coexist expands. Peritectal reaction rate or Ar ₄
The transformation rate (together referred to as δ → γ transformation rate) increases, and α occurs due to the difference in solubility between the δ and γ phases.
Stabilizing elements (e.g., P) and γ-stabilizing elements (e.g.,
Since the separation of Mn) is promoted, overlapping segregation in the tree 21 can be reduced.

In other words, Figures 1 and 2 are schematic diagrams showing the regions in which δ and γ phases exist in dendrites during solidification of molten steel.
2 is the case where no α-stabilizing element is added, and 2 is the case where the α-stabilizing element is added. In the figure, the δ→γ transformation rate is expressed by the formula f = (B/A) ² , and it is clear that the δ→γ transformation rate increases with the addition of the α stabilizing element.

In the present invention, Be, Cr, Nb, Sn, Ti, V,
Alternatively, one or more types of Mo are added to the molten steel, with Be at most 1.4% by weight, Sn at most 0.5% by weight, and elements other than Be and Sn at most 2% by weight. The reason for the limitation in the case of Be and Sn will be explained in Examples. For other than Be and Sn, even if the content exceeds 2% by weight, it is effective in reducing segregation, but it increases the cost. Also, the lower limit is not particularly limited, but
The inventor has confirmed that it is already effective at 0.005%.

Further, the present invention is effective even when the C concentration is 0 or extremely close to 0, for example, about 0.001%.

The method of addition is not particularly limited, and conventional methods for adding alloy elements, such as the ferroalloy dropping method, injection method, bullet firing method, wire addition method, etc., can be used.

In the method of the present invention, from just below the melting point (when δ phase primary crystals occur) to the end temperature of Ar ₄ transformation or peritectic reaction (γ
cooling rate in the temperature range of 40℃/
By making it less than 1 minute, even better effects can be obtained in terms of reducing segregation and promoting separation of double segregation. Immediately after entering the γ phase, heat to approximately 1000°C at a rate of 30°C/min or more.
By cooling to
No. 21940).

Embodiment Next, an embodiment of the present invention will be described with reference to FIGS. 3 and 4. Figure 3 shows alloying element concentration (weight%) and 1300℃
Figure 4 is a graph of the degree of Mn segregation between trees when cooled to 1300°C.
It is a graph of the degree of P segregation between trees when cooled to .

The composition of steel is C0.15%, Si0.2%, Mn1.0%,
P0.012%, in order to simulate solidification segregation in continuous casting, a unidirectional solidification experiment was conducted at a cooling rate of 27°C/min, and two-dimensional (area) analysis was performed using EPMA. .

Mn ₀ and P ₀ indicate the average concentrations of Mn and P, respectively;
The degree of inter-tree segregation is defined as the concentration of Mn and P between trees _.
and P divided by ₀ .

Here, Be exhibits a tendency different from other alloying elements. In other words, the segregation of Mn and P in trees decreases with the addition of Mn and P, but at a concentration of 0.2% by weight,
In this case, both Mn and P have the lowest degree of segregation, and in the case of P, the degree of segregation is negative. If further added, both Mn and P have a strong tendency to segregate between trees. If the average concentration when initially added to molten steel is defined as a degree of segregation of 1, it is not preferable for the degree of segregation to exceed 1, especially in the case of P. Although the adverse effects of Mn due to inter-tree segregation can be tolerated compared to P, an inter-dendrome segregation degree of 1.05 or less is still within the acceptable range. From Figure 3, for Mn segregation degree 1.05
Since the Be concentration is 1.7% by weight, and as shown in FIG. 4, the Be concentration for a P segregation degree of 1.0 is 1.4% by weight, the amount of Be added is preferably 1.4% or less.

On the other hand, as can be seen from FIGS. 3 and 4, the addition of Sn reduces the solidification segregation of carbon steel, and a sufficient effect can be expected in this respect. However, when its concentration exceeds 0.5%, the generally well-known negative effects of Sn's own grain boundary segregation become dominant, and the material quality is adversely affected, making cracks more likely to occur. Therefore, the Sn concentration is 0.5
% or less.

Effects of the Invention From the examples, it was found that Be of about 1.4% by weight or less was added to carbon steel.
After adding approximately 0.5% by weight or less of Sn and approximately 2% by weight or less of each element of Cr, Nb, Ti, V, and Mo, the molten steel undergoes Ar ₄ transformation or peritectic from just below the melting point (when δ phase primary crystals occur). Until the end temperature of the reaction (until it becomes γ phase)
It is clear that the degree of segregation of P, which is a harmful element, is significantly reduced by cooling at a cooling rate of 40°C/min or less in the temperature range of . On the other hand, although the degree of segregation of Mn decreases less than that of P, its effectiveness is clear. In addition, if the degree of P segregation between trees is 1 or less and the degree of Mn segregation between trees is 1 or more, P and Mn
This shows that the segregation peaks are completely separated. In this way, by adding the α stabilizing element and regulating the cooling rate, it is possible to promote the diffusion and separation of P and Mn, reduce double segregation, and reduce the segregation peak value.

As mentioned above, the reduction in interdensity segregation due to the addition of alloys has been described, but the addition of alloys also reduces center segregation at the same time.

[Brief explanation of drawings]

Figure 1 is a phase diagram of carbon steel, Figures 2 1 and 2 are principle diagrams of stabilizing the δ phase in dendrites by the addition of α stabilizing elements, and Figures 3 and 4 are examples of the present invention. It is a graph showing a change in degree of segregation due to addition of alloying elements. 20...Tree core, 21...Tree space.

Claims (1)

[Claims] 1. In continuous casting of carbon steel containing a C concentration of 0.53% by weight or less, Be, Cr, Nb, Sn, Ti,
or at least one or more types of V, 1.4% by weight or less for Be, and 0.5% by weight for Sn
Below, other than Be and Sn are each 2% by weight.
After adding the following, just below the melting point (when δ phase primary crystals occur)
A method for controlling solidification segregation of steel in continuous casting, which is characterized by cooling at a cooling rate of 40°C/min or less over a temperature range from 4 to the end temperature of Ar ₄ transformation or peritectic reaction (until it becomes γ phase). 2 In continuous casting of carbon steel containing a C concentration of 0.53% by weight or less, Be, Cr, Nb, Sn, Ti,
or at least one or more types of V, 1.4% by weight or less for Be, and 0.5% by weight for Sn
Below, other than Be and Sn are each 2% by weight.
After adding the following and further adding Mo2 wt% or less, the temperature range from just below the melting point (when δ phase primary crystals occur) to the end temperature of Ar ₄ transformation or peritectic reaction (until it becomes γ phase) is increased by 40℃/ A method for controlling solidification segregation of steel in continuous casting, characterized by cooling at a cooling rate of less than 1 minute.