JP4844087B2 - Method for adjusting nitrogen concentration in molten steel and method for producing steel for large heat input - Google Patents

Description

  The present invention relates to a method for adjusting a nitrogen concentration in molten steel by vacuum degassing treatment, and a method for producing steel for high heat input suitable for a thick plate for shipbuilding, for example.

As a method for adjusting the nitrogen concentration in molten steel, for example, there is a technique described in Patent Document 1. In Patent Document 1, as a first conventional technique in which nitrogen gas and argon gas are individually blown, (1) vacuum degassing is performed by blowing nitrogen for a nitrogen blowing time determined before vacuum degassing. After that, a technique is disclosed in which the inert gas to be blown is switched to argon gas and the argon gas is blown until the molten steel reaches a predetermined temperature.
As a second prior art, vacuum degassing is performed while blowing nitrogen gas, and the nitrogen concentration of the molten steel is measured during the degassing, and the pressure in the vacuum degassing apparatus is adjusted based on the measurement result. A method for adjusting the nitrogen concentration in molten steel is disclosed.
JP 2000-34513 A

However, in the first prior art, the target nitrogen concentration has a poor target ratio, and the argon gas blowing time fluctuates. As a result, the time required for the degassing treatment fluctuates, which adversely affects the subsequent process operation. For example, processing such as continuous casting in a subsequent process cannot be performed at a fixed time.
Further, in the second technique, it is necessary to measure the nitrogen concentration of the molten steel being processed during the vacuum degassing process, and it is necessary to control to change the pressure based on the measurement result, which may be complicated. is there. In particular, there is a possibility that fluctuation of the pressure during the vacuum degassing process may cause disturbance, and as a result, the accuracy of the nitrogen concentration may be reduced.
The present invention has been made paying attention to the above points, and can improve the accuracy of the nitrogen concentration by simple adjustment, and is intended even for a steel type having a small range of the required nitrogen concentration. The task is to make it easier to manufacture steel with a nitrogen concentration.

In order to solve the above-mentioned problem, the invention described in claim 1 of the present invention uses nitrogen gas or argon gas as an inert gas for a preset predetermined blowing time with respect to the molten steel accommodated in the vacuum degassing apparatus. When performing degassing by blowing, first nitrogen gas is blown for the nitrogen blowing time and then nitriding, and then the remaining blowing time, in the nitrogen concentration adjustment method in the molten steel that blows argon gas to perform denitrification,
Taken molten steel sample at the start of degassing, by analyzing the samples during degassing required in the nitrogen concentration of the molten steel at the start of degassing, on the basis of the nitrogen concentration obtained were determined in advance under The nitrogen blowing time Tn at which the nitrogen concentration at the end of the degassing process becomes the target nitrogen concentration Ntarget is calculated using the equation, and when the calculated nitrogen blowing time Tn is reached, the inert gas blown into the argon gas from the nitrogen gas is calculated. It is characterized by switching.
Tn = (Ntarget−K × N0 + Vr × T) / (Vn−Vr)
here,
K: Influence coefficient of nitrogen concentration before degassing treatment on nitrogen concentration after degassing treatment
N0: Nitrogen concentration before degassing
Vn: Nitrogen blowing speed by nitrogen blowing
Vr: Denitrification rate by blowing argon
T: Degassing processing time
It is.
Next, the invention described in claim 2 is for high heat input, which is a high strength steel having excellent weld heat affected zone toughness by supplying molten steel that has been vacuum degassed to a tundish and performing continuous casting. In the manufacturing method of steel for large heat input that manufactures steel slabs,
The above claim so that the nitrogen concentration in the molten steel supplied to the tundish is 40 to 60 ppm , the Ti concentration is 0.010 to 0.014 mass%, and the Ti / N mass ratio is 2.00 to 3.17. The vacuum degassing treatment is performed by the nitrogen concentration adjusting method described in Item 1.

According to the present invention, the accuracy of the nitrogen concentration can be improved by simple adjustment, and it becomes easy to produce a steel having a target nitrogen concentration even if the required steel concentration range is small.
This makes it easy to produce a high heat input steel having a desired nitrogen concentration with a high yield. At this time, by adjusting the nitrogen concentration to 40 to 60 ppm, preferably 40 to 55 ppm, the weld heat affected zone toughness can be improved, and a steel for high heat input that can hardly cause slab grain boundary cracking during continuous casting can be produced. become.

Next, embodiments of the present invention will be described with reference to the drawings.
This embodiment manufactures a slab of steel for high heat input, and the molten steel is degassed with an RH vacuum degassing apparatus as shown in FIG. The addition step (B) and the continuous casting step (C) are performed. The CaSi addition step (B) may be omitted.

  The vacuum degassing apparatus, as is well known, performs vacuum refining by sucking the molten steel 2 in the pan 1 into the vacuum chamber 3 and bringing it into contact with the vacuum. One of the two dip tubes 4 and 5 is used. While the inert gas is blown into the molten steel passing through the ascending dip tube 4, the molten steel is sucked into the vacuum chamber 3 by the principle of a lift pump, and the molten steel 2 is introduced into the pan 4 from the vacuum vessel 3 through the other dip tube 5. By degassing, degassing is performed while circulating the molten steel 2. Note that the atmospheric gas in the vacuum chamber 3 is exhausted. In the present embodiment, nitrogen gas and argon gas can be exclusively blown as the inert gas blown into the ascending dip tube 4.

  And the degassing time T of the degassing process by the vacuum degassing apparatus is set to a predetermined casting time by the continuous casting machine as a subsequent process, and as shown in FIG. During the degassing process, the first half is degassed while nitrogen gas is being blown in, the gas is switched halfway, and the second half is degassed while being blown in argon gas. Here, assuming that the nitrogen blowing time for blowing nitrogen gas is Tn and the argon blowing time for blowing argon gas is Tr, there is a relationship of detreatment time T = Tn + Tr, and a predetermined nitrogen blowing time Tn0 is set as an initial value. ing. The unit of time is minutes.

Nitrogen blowing rate Vn by nitrogen blowing, denitrification rate Vr by argon blowing, and influence coefficient of nitrogen concentration before degassing treatment on nitrogen concentration after degassing treatment under predetermined vacuum degassing treatment conditions K is obtained in advance by experiments or the like. The flow rates of the suffocation gas and argon gas are set constant.
In addition, since the nitriding speed Vn and the denitrification speed Vr vary depending on the nitrogen concentration before treatment, the accuracy of hitting can be further improved by obtaining in advance the influence of the steel type and the nitrogen concentration before treatment.

Then, the target nitrogen concentration of Ntarget, when the nitrogen concentration before degassing N0 (ppm), or it can be seen that if the control so as to satisfy the following equation.
Ntarget = K * N0 + Vn * Tn-Vr * Tr (1)
Here, since T = Tn + Tr, since it can be described as follows, the following equation is obtained.
Ntarget = K * N0 + Vn * Tn-Vr * (T-Tn)
If this equation is transformed,
Tn = (Ntarget−K × N0 + Vr × T) / (Vn−Vr) (2)

And in this embodiment, after taking a sample from the molten steel in a pan just before degassing, degassing processing is started, blowing nitrogen gas. During this degassing process, the collected sample is analyzed to calculate the nitrogen concentration N0 before the process. The analysis of the sample requires, for example, about 10 minutes. After calculating the nitrogen concentration N0 before the treatment, the nitrogen blowing time Tn is obtained based on the above equation (2). When the time Tn has elapsed from the start of the degassing treatment, the blowing gas is changed from nitrogen to argon, and the time of Tr Degassing is performed while blowing.
As a result, the target nitrogen concentration N target is set to the target nitrogen concentration after degassing.
When the degassing process is completed, as described above, a CaSi addition process and a continuous casting process are performed to form a slab.

Here, when producing the steel for large heat input having the components described in Table 1, the coefficients of the above equation (1) were determined using multiple regression,
Nitrogen blowing rate for nitriding Vn = 0.616
Denitrification rate Vr = 0.203 by blowing argon
Influence coefficient K = 0.025
It became.
That is, the equation (1) is expressed as follows: Ntarget = 0.025 × N0 + 0.616 × Tn−0.203 × Tr
... (3)
It was expressed. In addition, the said coefficient changes with conditions of degassing processing, for example, a vacuum degree, the temperature and quantity of molten steel. It is determined in advance by experiments or the like.

Then, the nitrogen blowing time Tn is calculated during the degassing process using the above formula (3), and when the nitrogen blowing time Tn is reached, the nitrogen blowing time Tn is blown into the argon gas so that the target nitrogen concentration Ntarget is obtained. When the degassing process was performed by switching the inert gas, the nitrogen concentration after the degassing process could be set to the target nitrogen concentration Ntarget with high accuracy.
In the experiment, the target nitrogen concentration Ntarget = 50 ppm was determined based on whether it was within 50 ± 10 ppm.

  Here, in the nitrogen concentration adjusting method, since the entire processing time T is not changed, the subsequent processing is not affected. Further, since the process is completed at a predetermined time T and the degree of vacuum of the degassing process is not changed, the molten steel temperature after the process can be sent to a subsequent process at a predetermined desired temperature. Further, since only the adjustment of the inert gas is adjusted, disturbance to the degassing process other than the nitrogen concentration adjustment can be reduced, and the degassing process can be performed with high accuracy.

By adjusting the nitrogen gas concentration in the vacuum degassing treatment as described above, the molten steel adjusted to a nitrogen concentration of 40 to 60 ppm, preferably 40 to 55 ppm, is supplied to the tundish of a continuous casting machine to perform continuous casting and perform large casting. Manufactures steel slabs for heat input.
Here, as shown in Table 1 above, by setting Ti to 0.010 to 0.014% by mass,
(Ti / N) is adjusted to a range of 2.00 to 3.17. Accordingly, that is, the component amounts of Ti and N are adjusted within the target range shown in FIG.

When continuous casting is performed with molten steel for high heat input with a nitrogen concentration as described above, slab intergranular cracking that may occur using Nb (C, N) or Al as a medium in the bent portion of the casting. I was able to prevent it.
Moreover, the high heat input steel made of the slab of the high heat input steel produced above becomes a steel material in which TiN is finely precipitated in the steel and the coarsening of the γ crystal grains is suppressed, and the heat affected zone (HAZ) It was confirmed that the steel was a high strength steel with excellent toughness.

Here, when nitrogen is less than 40 ppm, TiN hardly precipitates, and when nitrogen exceeds 60 ppm, it has been found that slab grain boundary cracking is likely to occur during continuous casting, so the nitrogen component is in the range of 40 to 60 ppm. It prescribes.
Here, when performing the addition process of CaSi, as shown in FIG. 4, considering that the nitrogen concentration increases due to the addition of CaSi, the target nitrogen concentration Ntarget after the vacuum degassing process is reduced accordingly, for example, Set to 40 ppm.

It is a figure which shows the process process which concerns on embodiment based on this invention. It is a figure explaining nitrogen concentration adjustment at the time of degassing processing concerning an embodiment based on the present invention. It is a figure explaining the component range which concerns on embodiment based on this invention. It is a figure which shows the behavior of the nitrogen concentration which concerns on embodiment based on this invention.

Explanation of symbols

1 Pan 2 Molten Steel 3 Vacuum Tank T Total Blow Time Tn Nitrogen Blow Time Tr Argon Blow Time N0 Nitrogen Concentration Before Treatment Ntarget Target Nitrogen Concentration Vn Nitrogenation Rate Vr Denitrification Rate

Claims (2)

When nitrogen gas or argon gas is blown into the molten steel contained in the vacuum degassing apparatus as an inert gas for a preset predetermined blowing time to perform the degassing treatment, nitrogen gas is first blown in for the nitrogen blowing time. In the method of adjusting the nitrogen concentration in the molten steel, after performing nitrogen, the remaining blowing time, argon gas is blown to denitrify,
Taken molten steel sample at the start of degassing, by analyzing the samples during degassing required in the nitrogen concentration of the molten steel at the start of degassing, on the basis of the nitrogen concentration obtained were determined in advance under The nitrogen blowing time Tn at which the nitrogen concentration at the end of the degassing process becomes the target nitrogen concentration Ntarget is calculated using the equation, and when the calculated nitrogen blowing time Tn is reached, the inert gas blown into the argon gas from the nitrogen gas is calculated. A method for adjusting the nitrogen concentration in molten steel, characterized by switching.
Tn = (Ntarget−K × N0 + Vr × T) / (Vn−Vr)
here,
K: Influence coefficient of nitrogen concentration before degassing treatment on nitrogen concentration after degassing treatment
N0: Nitrogen concentration before degassing
Vn: Nitrogen blowing speed by nitrogen blowing
Vr: Denitrification rate by blowing argon
T: Degassing processing time
It is. Supplying molten steel that has been vacuum degassed to a tundish and performing continuous casting to produce a high heat input steel slab that is a high strength steel with excellent weld heat affected zone toughness. In the manufacturing method,
The above claim so that the nitrogen concentration in the molten steel supplied to the tundish is 40 to 60 ppm , the Ti concentration is 0.010 to 0.014 mass%, and the Ti / N mass ratio is 2.00 to 3.17. A method for producing steel for high heat input, wherein the vacuum degassing treatment is performed by the method for adjusting a nitrogen concentration according to Item 1.

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