[case study] reason analysis of magnesium carbon brick erosion of 180t ladle slag line
The new technology and new process required to produce the high added value steel have put forward higher requirements for the service life of ladle refractory. Benxi steel plate steel pipeline steel, rectangular billet and other deep desulfurization of steel through the path of LF refining steel production increased year by year, and in 2011 with the ladle double air brick transformation completed, argon blowing intensity increase slag ladle slag line parts agitation, the slag line magnesia carbon brick has caused tremendous damage, leading to shorten the service life of the ladle slag line brick, leaking badly result in slag line steel. Therefore, it is an important direction for engineers to improve the service life of ladle slag line brick and reduce steel leakage in slag line.
magnesium carbon brick
The production status
There are 7 sets of continuous casting machines in the steel plate mill: 2 thick slab 1600mm (double flow), 2 thin slab 1700mm (single flow), 1 rectangular slab 350mmx470mm (four flow)1900mm wide slab | 1 (double flow), 1 wide slab 2200mm (single flow). 11 refining sites: LF refining furnace 5, RH vacuum refining furnace 5, AHF chemical heating furnace 1.
The process path is mainly BOF- lf-ccm, BOF- rh-ccm, BOF- ahf-ccm, BOF- lf-rh-ccm or BOF- rh-lf-ccm.
At present, there are 65 180t ladle turnover in benshan steel plate steelmaking plant, the slag line brick is mg-c brick, and the slag line thickness is 160mm. RH or AHF refining treatment site on the ladle slag line erosion less, so today mainly talk about the LF refining furnace slag line erosion impact.
Erosion mechanism of magnesium carbon brick
2.1 main physical and chemical indexes of mg-c brick of a certain manufacturer
The main physicochemical indexes of magnesia-carbon brick from a manufacturer are shown in table 1.
2.2 erosion mechanism
The LF furnace refining slag is mainly the oxides of Ca, Si, Al, Mg, Mn and Fe under certain alkalinity. The mg-c brick reacts with LF refining slag at high temperature to form calcium-magnesite olivine in which calcium oxide and silica and some magnesium oxide are derived from slag.
The equation is as follows:
Calcium and magnesium olivine has low melting point and low viscosity at high temperature, and it is easy to flow into slag in the agitation of slag. On the other hand, the graphite in the magnesium carbon material is oxidized by the oxide in the slag, or even dissolved directly into the steel in contact.
After the graphite is oxidized to CO(g), the pores in the original position are left as the channels for the slag to penetrate into the refractory. When the integrity of mg-c brick is destroyed, the magnesia particles become isolated and then flow into the slag, resulting in the erosion of refractory materials. Due to the presence of impurities such as silicon-calcium in magnesia raw materials, the low-melting phase of calcium-magnesite olivine will also be formed at high temperature, and they will also become channels for slag to enter into refractory materials.
In the actual smelting process, the graphite in the magnesium carbon brick located at the interface between molten slag and molten steel will melt into the molten steel, thus causing damage to the structure of the brick is not negligible.
Factors affecting the erosion of magnesium carbon brick
Slag line is the worst part of steel ladle. In actual production, the erosion of ladle slag line is affected by the quality of resistant material, the quality of masonry process and the smelting conditions. When these factors change, abnormal erosion and even steel leakage may occur in ladle slag line.
3.1 influence of resistant material quality
The corrosion resistance, high temperature strength and thermal shock resistance of mg-c bricks are affected by the quality of raw materials, the distribution of magnesia and graphite particles and the addition of additives. Due to the unreasonable grain size distribution of magnesia and graphite and improper additives, mg-c brick will expand greatly when used at high temperature, resulting in excessive thermal stress, which reduces the thermal shock resistance of mg-c brick, leading to the rapid erosion of weak spots such as slag line joint and brick joint, forming a “manton-like erosion”. Figure 1 shows the picture of steel leakage in slag line caused by “steamed bread erosion” of a tank service in no. 35 ladle of benxi iron and steel co., LTD.
3.2 influence of masonry process on abnormal erosion of piping
When the ladle repairs the slag line, the new slag line and the joint wrong platform is large, up to 50mm. During the mixing process of molten steel blowing argon in refining, a vortex is formed at the wrong part of the platform, which intensifies the erosion of this part, thus causing obvious pits. The depth between the bottom of the pit and the brick working face can reach up to 70mm. This kind of fault may also occur when the cement between the working floor and the permanent floor is not clean during masonry. Abnormal erosion of slag line resulted from the wrong platform of slag line brick, as shown in FIG. 2.
3.3 influence of smelting conditions on slag line erosion
In order to meet the requirements of steel temperature and composition, LF refining furnace needs electrode heating. The slag temperature of arc zone can reach 2000℃, and the high temperature time is long. Under the condition of high alkalinity and low viscosity, the slag line was corroded strongly by argon stirring. In general, the erosion rate of slag line is below 3.0mm/ time. Table 2 shows that the slag line of a certain tank in no.36 tank is treated by LF furnace for 22 times (no other refining path is taken), and the erosion rate reaches 4.0mm/ time.
Due to the large production of steel varieties, the requirements for ladle tank condition are strict, ladle tank along the “forehead”, “inner rise” is not allowed. The steel mill of this steel adopts slag-hook hook and special hook hook. In the process of hook “forehead” slag and “inner rising” slag, it is easy to destroy the slag-line brick and make the slag-line brick form pits or break. If the slag line brick body is damaged and not repaired in time or is covered by slag in the later process of use, it cannot be found. Figure 3 shows that the slag line itself of no.32 ladle was damaged in a tank service, but the slag line leakage accident was caused in the process of LF furnace treatment due to the workers’ error in judgment and continued turnover.
1. The quality of raw materials, the distribution of magnesia and graphite particles and the addition of additives and other factors affect the quality of magnesia carbon slag line brick, and then affect the erosion of magnesia carbon slag line brick.
2. Wrong platform phenomenon in ladle masonry accelerates the erosion of mg-c slag line brick.
3.LF refining slagging process used to get magnesium carbon brick erosion increased sharply, hook “forehead” slag, “rising” slag and so on slag line brick body damage.