JPH10212514A - Method for producing highly clean ultra-low sulfur steel with excellent resistance to hydrogen-induced cracking - Google Patents

Abstract

(57) [Abstract] [Problem] To provide highly clean ultra-low sulfur steel excellent in resistance to hydrogen-induced cracking.
It is produced by adding Ca and Ca alloy. SOLUTION: A pretreatment step of preliminary deoxidation and slag adjustment during converter tapping, a slag reforming / desulfurization step by a stirring treatment using an inert gas, and then an inclusion reforming step by a Ca or Ca alloy treatment, Then, a casting process may be performed, and if desired, the RH treatment process may be performed before or after the stirring process using the inert gas. Stirring with inert gas
It is performed so as to satisfy the following equations (1) to (4). In the slag (% CaO) / (% Al 2 O 3) = 1.0~3.0 ·
・ ・ ・ ・ (1) In slag (% FeO) + (% MnO) ≤ 1.0%
・ ・ ・ ・ (2) In slag (% CaF ₂ )> 5% ・
・ ・ ・ ・ (3) In molten steel [S] ≤ 10 ppm
····(Four)

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for producing a highly purified ultra-low sulfur steel having excellent resistance to hydrogen-induced cracking. The present invention relates to a method for casting a steel having extremely high HIC resistance by casting with a continuous casting machine at a temperature of at least 20 ° C. higher than the linear temperature.

[0002]

2. Description of the Related Art HIC (hydrogen-induced cracking, also referred to as HIC cracking, hereinafter the same) is generated from MnS-based inclusions or linear crushed inclusions elongated during rolling. Since MnS-based inclusions are formed by the reaction between Mn and S in the steel segregated during solidification, this MnS formation can be suppressed by sufficiently desulfurizing in the molten steel stage to reduce the S content in the steel and adding Ca. how to fix the S of the molten steel inclusions as CaO-Al ₂ O ₃ -CaS it is effective.

[0003] There are many conventional techniques for this Ca addition method. For example, Japanese Patent Application Laid-Open Nos.
In each of JP-A-58-3913 and JP-A-4-259352, the processing order and processing conditions of the three processing steps of degassing, desulfurization and Ca addition are optimized. However, there are very few patents that specify slag components, and there is no prior art that mentions the casting temperature in a continuous casting machine.

[0004] The prior art which referred to the slag component includes:
For example CaO 55 to 60% for only JP 56-98415 is of the above publication is slag composition, SiO ₂ 5~10%,
It is disclosed that it is preferable to adjust Al ₂ O _{3 to} 30 to 40% and [FeO + MnO] to 0.1% or less.
In 63-7318 discloses, after stirring the slag (% CaO) / (% SiO 2)
> 2.5, (Total Fe) + (MnO) <2.0%, and discloses a method of adding Ca. Japanese Patent Application Laid-Open No. 64-75621 discloses a method in which the slag component is controlled to (% FeO) + (% MnO) <5% after tapping from a converter, and Ca is added.

[0005]

However, in such conventional techniques, for example, the methods disclosed in the above-mentioned JP-A-63-7318 and JP-A-64-75621, all of HIC steel (SIC exhibiting HIC property, the same applies hereinafter)), it is difficult to desulfurize with this slag component, and as described later, there is a large variation in the effect of modifying Ca inclusions, so that MnS, CaS It cannot prevent HIC cracking due to such factors.

[0006] It is an object of the present invention to provide a method for melting ultra-low sulfur steel which further improves HIC resistance and further enhances cleanliness. A specific object of the present invention is
HIC with excellent cleanliness and excellent hydrogen-induced cracking resistance of [S] ≦ 10 ppm in steel and (% FeO) + (% MnO) ≦ 1.0% in slab
It is to provide a method for melting steel.

[0007]

In the conventional HIC resistant steel smelting technology, Ca is added to sufficiently desulfurized molten steel to convert MnS and Al ₂ O ₃ clusters, which are the starting points of HIC, into CaO-. It has been considered that if the Al ₂ O ₃ -CaS-based spherical inclusions are rendered harmless and the generation of CaS observed when Ca is added is suppressed, practically sufficient HIC resistance can be obtained.

However, according to the studies by the present inventors so far, it has been found that the above method cannot sufficiently suppress the generation of HIC. The steel material treated with Ca after desulfurization was evaluated by the HIC resistance test, and after observing the morphology of inclusions in the sheet material after rolling of the steel material in which HIC was generated and analyzing the composition, it was found that the starting point of the HIC crack was MnS and Al ₂ O ₃
It was a combination of clusters, large spherical inclusions, or CaS.

[0009] All the steel materials in which the combination of MnS and Al ₂ O ₃ clusters were found were (% FeO) + in the slag before the Ca treatment.
The total of (% MnO) (hereinafter, lower oxide concentration) was more than 1.0%, and the Ca yield during Ca treatment was very poor. Ca is originally added for the purpose of modifying Al ₂ O ₃ clusters in molten steel into CaO-Al ₂ O ₃ system spherical inclusions.However, since the lower oxide concentration in slag is high, strong deoxidizer It is considered that the lower oxide in the slag consumed Ca, which resulted in a very poor Ca yield as shown in FIG. As a result, it is considered that the addition of Ca was insufficient and Al ₂ O ₃ clusters and MnS remained in the steel material. Also, since the amount of Al ₂ O ₃ clusters in the molten steel increases with the passage of time from the Ca treatment to the casting, reoxidation of the molten steel by lower oxides in the slag is also conceivable.

[0010] On the other hand, in the case of steel materials in which large spherical inclusions are found, the temperature in the tundish at the time of casting is lower than the liquidus temperature of molten steel by 20 ° C (hereinafter referred to as temperature ΔT).
= Temperature in tundish-liquidus temperature of molten steel).

As shown in FIG. 2, ΔT in the tundish
Is low, the viscosity of the molten steel itself increases and the friction coefficient between the inclusions and the molten steel increases, so large inclusions that normally float easily cannot float during casting, resulting in large spherical inclusions in the steel material. It is considered that the thing remained.

[0012] In the steel material in which CaS was found, S in the molten steel when Ca was added was more than 10 ppm. This is S 10ppm
This is because, in the case of exceeding, CaS always occurs under any Ca addition conditions.

Regarding desulfurization, as shown in FIG. 3, the desulfurization rate and (% CaO) / (% Al ₂ O ₃ ) and (% CaF ₂ )
Has a very strong correlation with (% CaO) / (% Al ₂ O ₃ ) = 1.0
3.03.0, (% CaF ₂ )> 5% was found to have a good desulfurization rate. In particular, (% CaO) / (Al 2 O 3) = 1.2 ~2.5, (% CaF 2)
When ≧ 10%, desulfurization was best performed and stable.

Therefore, in order to improve the HIC resistance of steel materials, stable ultra-low sulfurization of molten steel, suppression of CaS generation when Ca is added,
Prevents reoxidation of molten steel by reducing lower oxides in slag, suppresses the formation of Al ₂ O ₃ clusters and MnS by stabilizing the effect of modifying Ca inclusions, and aims to float inclusions by controlling ΔT high temperature in the tundish As a result, it has been found that the extremely low sulfurization and the high purification can be stably and reliably realized by their synergistic effects.

That is, the present inventors evaluated the Ca-treated steel by an HIC resistance test, observed the form of inclusions in the rolled steel after the HIC was generated, and analyzed the composition. The knowledge was obtained and the present invention was completed.

The concentration of the lower oxide in the slag before the Ca treatment is 1
%, The addition of Ca is insufficient, and Al ₂ O ₃
Clusters and MnS remain. If the lower oxide in the slag is more than 1%, the reoxidation of the molten steel increases the amount of Al ₂ O ₃ clusters in the molten steel. If ΔT in the tundish is lower than 20 ° C., large spherical inclusions remain in the steel material. The higher ΔT, the higher the number of spherical inclusions in the slab and the higher the Al
_The number of ₂ O ₃ clusters is reduced. When S in molten steel at the time of adding Ca exceeds 10 ppm, CaS always occurs under any Ca adding conditions. Desulfurization rate and slab composition (% CaO) / (% Al 2 O 3) ratio (%
There is a very strong correlation with CaF ₂ ).

Here, the gist of the present invention is that molten steel and slag in a ladle, which have been subjected to preliminary deoxidation and slag composition adjustment during converter tapping, are subjected to slag by a stirring treatment using an inert gas. The reforming and desulfurization treatment is performed, and then the Ca or Ca alloy treatment is performed under the conditions satisfying the following equations (1) to (4) .After that, the temperature of the molten steel in the tundish of the continuous casting machine is changed to the liquidus line of the molten steel. This is a method for producing a high-purity ultra-low sulfur steel having excellent resistance to hydrogen-induced cracking, characterized by casting while maintaining the temperature at 20 to 50 ° C. higher than the temperature.

In slag (% CaO) / (% Al ₂ O ₃ ) = 1.0-3.0 (1) In slag (% FeO) + (% MnO) ≦ 1.0% (2) ) In slag (% CaF ₂ )> 5% ・ ・ ・ ・ ・ (3) In molten steel [S] ≤ 10 ppm ・ ・ ・ ・ ・ ・ ・ (4)

[0019]

Here, the method of the present invention will be described in detail as follows. The method according to the present invention comprises: a pretreatment step of preliminary deoxidation and slag adjustment during converter tapping;
A slag reforming / desulfurization step by an inert gas stirring treatment, an inclusion reforming step by a Ca or Ca alloy treatment, and a casting step. If desired, the RH treatment step can be performed before or after the inert gas stirring treatment step. You may go to Crab.

Pretreatment step (preliminary deoxidation + slag adjustment): First, the molten steel at the time of tapping from the converter is preliminarily deoxidized with a desulfurizing agent such as Al, Si or Al + Si. In addition, slag modifier containing Al as a main component in converter slag flowing away from the converter at the end of tapping,
And a medium solvent for adjusting the slag composition.

In a preferred embodiment of the present invention, the composition of the converter slag at this time is (% CaO) / (% Al ₂ O ₃ ) = 1.0 to 3.0,
(% CaF ₂ )> 5%. Also in order to obtain a high desulfurization rate stably, slag composition, most desulfurization rate is excellent as shown in FIG. 3 (% CaO) / (% Al 2 O 3) = 1.2 ~2.5, (% CaF 2)
It is preferable to adjust to ≧ 10%.

Slag reforming / desulfurization step: Subsequently, slag Al or a slag modifier containing Al as a main component is added to the molten steel, and inert gas stirring of ₂ to 5 Nm ³ / min or CaO, CaF ₂ The desulfurizing agent is blown together with the above inert gas and stirred for 2 minutes or more.

In this slag reforming treatment, the lower oxide in the slag is reduced by Al, and the lower oxide concentration after this step,
That is, (% FeO) + (% MnO) is reduced to 1.0% or less. This is to prevent the reoxidation of the molten steel after this step and to prevent the generation of Al ₂ O ₃ clusters and MnS by stabilizing the effect of modifying Ca inclusions.

Subsequently, stirring is performed for ₂ minutes to 5 Nm ³ / minute with an inert gas or with blowing of a desulfurizing agent composed of CaO and CaF ₂ together with the inert gas while stirring for 3 minutes or more. In this desulfurization treatment, desulfurization in the molten steel is performed, and S in the molten steel after this step is reduced to ≦ 10 ppm. This is to prevent generation of CaS during Ca addition. The slag reforming and desulfurization by the agitation treatment with the inert gas described above may be performed at the same time in order to simplify the process, but are preferably performed continuously.

In addition, since the above treatment lowers the temperature of the molten steel, the ΔT of the molten steel in the tundish is ensured to be 20 ° C. or more.
A heating step may be added later. However, if the heat-up step is performed using the combustion heat of O ₂ gas + Al, (% CaO) / (% Al ₂ O ₃ ) in the slag decreases due to the amount of Al ₂ O ₃ generated. However, if such a heating step is performed before the desulfurization step, CaO should be added according to the heating rate, since the desulfurization efficiency will decrease.
(% CaO) / (% Al 2 O 3) = 1.0 ~3.0, it is necessary to adjust the _{(% CaF 2)> 5%} .

RH treatment step: The molten steel is circulated by an RH degassing apparatus, which degasses, and may simultaneously carry out the above-mentioned heating step. Further, the RH degassing apparatus may perform the reflux treatment before and after the slag reforming / desulfurization step, but it is more preferable to perform the reflux treatment after the desulfurization step because the slag entrained during the desulfurization step causes inclusions caused by the slag involved. , And the cleanliness of the molten steel is preferably improved.

Inclusion reforming step (Ca or Ca alloy treatment):
By the above treatment, metal Ca or Ca alloy is not applied to the molten steel, for example, in the form of a coated wire, for the molten steel reduced to S ≦ 10 ppm in the molten steel and the lower oxide concentration in the slag (% FeO +% MnO) ≦ 1.0%. It is added into molten steel while stirring with active gas.

As described above, the inclusion reforming treatment is performed in the following (1) to
Perform under conditions that satisfy equation (4). In the slag (% CaO) / (% Al 2 O 3) = 1.0~3.0 ····· (1) in the slag (% FeO) + (% MnO ) ≦ 1.0% ····· (2) in the slag (% CaF ₂ )> 5% ・ ・ ・ ・ ・ (3) In molten steel [S] ≤ 10 ppm ・ ・ ・ ・ ・ ・ ・ (4) The inert gas blowing method at this time is a lance method that is charged from above. Any of the porous brick method of stirring from the bottom may be used, but the flow rate is 50 to 500 Nl / min (0.05 to 0.5 Nm ³
/ Min) is desirable. If it is less than 50 Nl / min (0.05 Nm ³ / min), Ca is not sufficiently mixed with the molten steel, so that the modification of the Al ₂ O ₃ cluster in the steel varies. 500Nl / min (0.5 Nm
^If it is larger than ³ / min), the stirring becomes too strong and the molten steel surface breaks the slag surface and is exposed to the atmosphere, so that the molten steel is reoxidized by the atmosphere. Preferably it is 0.06-0.30 Nm < ³ > / min.

Further, as shown in FIG. 1, if the lower oxide concentration in the slag is ≦ 1%, the Ca yield is stable.
The amount of addition may be sufficient to match the amount of inclusions in the molten steel. Therefore, 0.1 to 0.4 kg / ton of molten steel in pure Ca is desirable.

Further, the Ca or Ca alloy to be added may be an injection method in which the Ca or Ca alloy is blown together with a carrier gas. However, a coating wire method is most preferable because slag may be involved.

Casting step: The molten steel produced through the above steps is cast by a continuous casting machine. As shown in FIG. 2, if ΔT in the tundish is controlled at 20 to 50 ° C. at this time, a floating effect of inclusions is exhibited, and the inclusions can be reduced. 20 ℃
If it is smaller, the viscosity of the molten steel increases and the floating of inclusions is suppressed. If the temperature exceeds 50 ° C., the effect of floating inclusions becomes the upper limit, and the cost of the refractory / heat-raising Al increases, so 50 ° C. is set as the upper limit. After casting, HIC steel may be formed through necessary processing and processing such as conventional hot rolling and cold rolling.

[0032]

[Example] After tapping the converter, 250 tons of molten steel in the ladle were subjected to Al deoxidation, and fine slag Al was used as a modifier in slag and as a solvent.
Add 1.0 to 1.5 tons of CaO and 200 to 500 kg of CaF ₂ and add
Was adjusted to the components shown in Table 1. In addition, these slag modifier and medium solvent (desulfurizing agent) were used simultaneously with Ar gas injection.

Next, a stirring treatment and a Ca treatment were performed according to the following three treatment patterns. The molten steel was stirred with Ar gas at 4 Nm ³ / min for 5 to 10 minutes using a lance immersed in a ladle to perform slag modification and desulfurization, and then Ca was added to the molten steel using a CaSi wire.

Using a lance immersed in a ladle, 4 Nm ³ /
The molten steel was stirred with Ar gas for 5 to 10 minutes to perform slag reforming and desulfurization, and then a reflux treatment was performed for 10 minutes with an RH degassing device, and then Ca was added to the molten steel using a CaSi wire. .

O ₂ + Al heat-up and reflux treatment are performed in the RH degassing apparatus, and then CaO corresponding to the generated Al ₂ O ₃ is charged, and 4 Nm ³ / min. Ar
After slag modification and desulfurization were performed by stirring the molten steel with a gas for 5 to 10 minutes, Ca was added to the molten steel using a CaSi wire.

Next, the three types of molten steel thus obtained were cast by a continuous casting machine, and after hot and cold rolling to obtain a test material, an HIC test under NACE conditions was performed. HIC resistance was determined.

Table 2 shows the above processing conditions and the results of the evaluation of the HIC resistance. In the table, items marked with * are conditions that are outside the conditions defined in the present invention. In the "HIC resistant" item, the symbol "o" indicates "no HIC occurred" and the symbol "x" indicates "occurred".

As shown in Table 2, the above-mentioned equation defined in the present invention
When processing is performed with all of the conditions specified in (1) to (4), HIC does not occur at all. However, HIC occurred when some of the processing conditions did not satisfy the conditions defined in the present invention.

[0039]

[Table 1]

[0040]

[Table 2]

[0041]

According to the present invention, the removal of large inclusions in molten steel, the prevention of reoxidation by low-grade oxides in slag, and the stabilization of the Ca reforming effect can be achieved. Spherical inclusions, Al ₂ O ₃ clusters, MnS, and CaS are no longer present, and ultra-low sulfur and high purification of steel can be achieved.
Thereby, a steel material having extremely excellent HIC resistance can be manufactured.

[Brief description of the drawings]

FIG. 1 is a graph showing the relationship between the lower oxide concentration in slag and the Ca yield.

FIG. 2 is a graph showing the relationship between ΔT and the effect of floating inclusions.

FIG. 3 is a graph showing a relationship between a desulfurization rate and a slag composition.

──────────────────────────────────────────────────の Continued on the front page (51) Int.Cl. ⁶ Identification code FI C21C 7/064 C21C 7/064 Z

Claims (3)

[Claims] [1] The molten steel and slag in the ladle that have been subjected to preliminary deoxidation and slag composition adjustment during the tapping of the converter are subjected to slag reforming / desulfurization treatment by a stirring treatment using an inert gas. Perform Ca or Ca alloy treatment under the conditions satisfying the following formulas (1) to (4), and then maintain the molten steel temperature in the tundish of the continuous casting machine at a temperature 20 to 50 ° C higher than the liquidus temperature of the molten steel A method for producing a highly clean ultra-low sulfur steel having excellent resistance to hydrogen-induced cracking, characterized by being cast and cast. In the slag (% CaO) / (% Al 2 O 3) = 1.0~3.0 ····· (1) in the slag (% FeO) + (% MnO ) ≦ 1.0% ····· (2) in the slag (% CaF ₂ )> 5% ・ ・ ・ ・ (3) In molten steel [S] ≤ 10 ppm ・ ・ ・ ・ ・ (4) 2. The method according to claim 1, wherein after the stirring process using the inert gas, the molten steel is refluxed by an RH degassing device prior to the Ca or Ca alloy treatment. Melting method for highly clean ultra low sulfur steel with excellent hydrogen induced cracking. 3. The method according to claim 1, wherein after performing the slag composition adjustment, the molten steel is subjected to a reflux treatment in an RH degassing apparatus before the stirring treatment using the inert gas. For producing highly clean ultra-low sulfur steel having excellent hydrogen-induced cracking resistance.