JP2745839B2 - Manufacturing method of martensitic stainless steel slab - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a martensitic stainless steel generally called 13Cr stainless steel, more specifically, C: 0.40% or less, Si: 1.0% or less, Mn: 1 0.0% or less, Cr: 10 to 15%, Ni:
The present invention relates to a method for producing a billet of martensitic stainless steel containing 0.80% or less.

[0002]

2. Description of the Related Art In general, martensitic stainless steel is widely used as a corrosion-resistant high-alloy seamless oil country tubular good or the like, and its component is generally the one having the above composition. In the production, as shown in FIG. 3, the high temperature slab obtained through the melting step (I) and the continuous casting step (II) is charged into a soaking furnace at about 850 ° C. After (III), hot rolling (IV) is performed and about 7
A process of gradually cooling (V) from 50 ° C. to about 200 ° C., followed by softening annealing (VI), cutting the outer periphery (VII), and forming a pipe (VIII) is employed.

[0003] As described above, in the conventional method, the soft annealing step was essential. The reason for this is that if soft annealing is not performed,
Because of the good hardenability of 13Cr steel, martensitic transformation is likely to occur in the cooling process of the round billet production stage of the shaped material, and transformation cracks are likely to occur. This is because it becomes extremely difficult to cut the outer periphery.

[0004]

However, in the above-mentioned method, usually, hot rolling is performed at a temperature of 1000.degree.
It has to be heated for 15 hours at a temperature of 750 ° C. and a temperature of 750 ° C., so that it takes a long time for the treatment, and there is a problem that the energy consumption is large economically.

Accordingly, a main object of the present invention is to sufficiently develop a ferrite + pearlite structure in a cooling process after hot rolling,
An object of the present invention is to provide a method for economically producing martensitic stainless steel in a short time by suppressing the formation of martensitic transformation, thereby enabling the softening annealing step to be omitted.

[0006]

Means for Solving the Problems The above-mentioned problems are as follows: C: 0.4
0% or less, Si: 1.0% or less, Mn: 1.0% or less,
After casting a martensitic stainless steel containing Cr: 10 to 15% and Ni: 0.80% or less by the continuous casting method, from the austenite temperature range, from γ structure to α structure
Up to a temperature of 300 ° C or higher below the transformation temperature of 20 ° C / H
After cooling at a cooling rate of r or less, 1200-1250 ° C
This can be solved by performing hot rolling at a final rolling temperature of 800 ° C. or higher and then cooling at a slow cooling rate of 10 ° C./Hr or lower.

[0007]

According to the present invention, a martensitic stainless steel slab having the above composition is subjected to γ-structure formation from the austenite temperature range.
Temperature below 300 ° C below the transformation temperature to
And cool. That is, the present invention is intended to cause transformation by cooling to a temperature as low as possible below the γ → α transformation temperature, thereby making the crystal grains fine .

The lower limit of the cooling temperature is set to 300 ° C. in order to prevent martensite transformation cracking just above the martensite transformation start temperature (MS point). At this time, as the cooling rate increases, crystal grains become finer as the number of α nucleation sites increases. However, in order to prevent thermal stress cracking due to a temperature difference between the inside and outside of the slab , the slab cooling rate is set to 20 ° C./Hr. It was as follows.

Next, the slab is heated for hot rolling to cause α → γ retransformation. However, since α grains are fine, the number of γ nucleation sites increases and the growth of γ is suppressed. Is done. That is, when the slab is once cooled to a temperature below the γ → α transformation temperature and then reheated, the crystal grains become finer than when reheated from a temperature above the γ → α transformation point.

The temperature at which the slab is heated for hot rolling is set at 1200 ° C. to 1250 ° C.
If the ratio exceeds 1, grain boundary embrittlement occurs, causing cracking.
If the temperature is lower than 200 ° C., the deformation resistance of the slab is large and hot rolling becomes difficult, and the final rolling temperature is lower than 800 ° C.

When the temperature is in the above range, when the slab is hot-rolled and then gradually cooled, the crystal grains before the slow cooling are fine, so that the nucleus of the γ → α transformation, which is a transformation accompanied by diffusion, is performed. The grain boundaries, which are the generation sites, increase, and γ → α transformation is likely to occur, and martensitic transformation is suppressed.

In the present invention, the final rolling temperature is 800 ° C.
The reason for this is that if the final rolling temperature is lower than 800 ° C., the ferrite + pearlite transformation cannot be performed sufficiently even if the cooling rate is reduced. The reason for setting the slow cooling rate of the steel slab after hot rolling to 10 ° C./Hr or less is to sufficiently promote the ferrite + pearlite transformation.

As described above, according to the present invention, the martensitic stainless steel slab obtained by the continuous casting method is γ
→ Martensitic transformation onset temperature (Ms
Point) By cooling to just above, crystal grains can be refined, ferrite + pearlite transformation is promoted in the slow cooling process after hot rolling, and martensite transformation is suppressed to reduce hardness and improve toughness. However, this makes it possible to omit the soft annealing step.

Next, the steps according to the present invention will be described with reference to the drawings. In FIG. 1, after melting the martensitic stainless steel of the above component (1), for example, a width of 530 mm and a thickness of 41
Continuous casting (2) to a thickness of 0 mm is performed, and this is cooled to a slab temperature of 300 ° C. at a cooling rate of 20 ° C./Hr or less. Next, the slab is heated (3) at a temperature of 1200 ° C. to 1250 ° C., and then hot-rolled (4).
mm round billet shape. Thereafter, it is gradually cooled at a rate of 10 ° C./Hr or less (5), and the obtained round billet is cut (6) to make a pipe (7). By adopting such a step, the soft annealing step can be omitted for the reason described later.

Next, the reasons for limiting the numerical values of each component in the present invention will be described. C: 0.40% or less C is necessary to secure strength, but is 0.40%
This is because, if it exceeds, the strength increases and the toughness decreases. Si: 1.0% or less Si is effective as a deoxidizing agent, but if it exceeds 1.0%, the toughness is significantly reduced. Mn: 1.0% or less It is effective for improving the strength and toughness, but if the upper limit exceeds 1.0%, the mechanical properties deteriorate. Cr: upper limit 15%, lower limit: 10% In order to obtain the required corrosion resistance, 10% or more is necessary, and the corrosion resistance for the application of martensitic stainless steel is 15% or less. Ni: 0.80% or less Although it is an element for improving corrosion resistance and toughness, since it is an expensive element, the upper limit is 0.8 in consideration of cost.
0%.

Next, FIG. 2 shows a CCT diagram obtained by a heat pattern according to the method of the present invention and a conventional method for steels having the representative components shown in Table 1. In FIG.
The line indicates the example of the present invention, and the broken line indicates the conventional example. Table 2 is a supplementary description of FIG.

[0017]

[Table 1]

[0018]

[Table 2]

Therefore, as shown in FIG.
Thus, it was found that by cooling the slab obtained by the continuous casting method to below the γ → α transformation point, the crystal grains before slow cooling became finer, and the start of diffusion transformation shifted to a shorter time side. Also, as shown in Table 2, the slow cooling rate was 10 ° C./Hr
In the following, ferrite + pearlite transformation is promoted, martensitic transformation is prevented, and a sufficiently low Vickers hardness is obtained.

Therefore, by refining the crystal grains before slow cooling, the martensitic transformation is suppressed, the hardness is reduced, and even if the softening annealing step is omitted, the prevention of the martensitic transformation crack and the outer peripheral cutting can be performed.

[0021]

EXAMPLES Next, the effects of the present invention will be clarified by examples. As test steels, martensitic stainless steels shown in Table 3 ( Examples: Nos. 1 to 5, Comparative Examples: Nos. 1 to 6,
Conventional examples: Nos . 1 to 5 ) were used.

[0022]

[Table 3]

In Examples and Comparative Examples, martensitic stainless steels having the components shown in Table 3 were melted in a refining furnace, and then subjected to continuous casting to obtain 530 mm wide × 410 mm thick.
Bloom got. According to the present invention, the obtained bloom has a temperature of about 300 ° C. at a cooling rate of 20 ° C./Hr or less.
After cooling to time, after heating and held for 6 hours to 1200 ° C. to 1250 ° C., and a round billet of the terminated diameter 187mm hot-rolled at 800 ° C. or higher. Then, 10 ℃
It cooled to 200 degreeC at the slow cooling rate of / Hr or less, and obtained martensitic stainless steel.

In the conventional example, the bloom obtained by the continuous casting method is heated from 850 ° C. to 1200 ° C. to 125 ° C.
After heating to 0 ° C. and holding for 6 hours, hot rolling was completed at a temperature of 800 ° C. or more, and a slow cooling rate of 10 ° C./Hr or less was applied.
It cooled to 00 degreeC, and obtained the martensitic stainless steel.

The results are also shown in Table 3.

In Table 3, ○ indicates that no martensitic transformation cracks occurred and the hardness (Vickers hardness) was sufficiently low, and x indicates that martensite transformation cracks occurred and the hardness was high. Comparative example in which C content exceeds 1.0%
No. 1 , Comparative Example No. 2 containing more than 1.0% of Si
In Comparative Example No. 4 in which the n content exceeds 1.0%, martensitic transformation cracking occurs and the hardness is high. Also the slab temperature
Comparative Example No. 3 at 255 ° C., steel slab slow cooling rate of 15 ° C./Hr
Comparative Example No. 4 and comparison of billet slow cooling rate 20 ° C / Hr
In Example No. 5 , a martensitic transformation crack occurred in each case. Furthermore, the billet cooling rate is 25 ° C / Hr.
Comparative Example No. 3 and Comparative Example No. 6 at 28 ° C./Hr
In each case, a martensitic transformation crack occurred.

Further , for the same test steel as in each embodiment,
Production was performed under the same conditions by the conventional method.
o.1~5 all martensitic transformation cracking occurs, and shows the high hardness.

[0028]

As described above, according to the present invention, the martensitic stainless steel slab can be obtained in a short time and economically, since the softening annealing step which has been conventionally required can be omitted.

[Brief description of the drawings]

FIG. 1 is a process chart according to the method of the present invention.

FIG. 2 is a CCT diagram of the stainless steel according to the present invention.

FIG. 3 is a process chart according to a conventional method.

Claims (1)

(57) [Claims] 1. C: 0.40% (% by weight, the same applies hereinafter), Si: 1.0% or less, Mn: 1.0% or less, Cr:
After casting a martensitic stainless steel containing 10 to 15% and Ni: 0.80% or less by a continuous casting method,
Transformation from γ-structure to α-structure from austenite temperature range
After cooling at a cooling rate of 20 ° C./Hr or less to a temperature of 300 ° C. or more at a temperature of not more than the temperature, it is heated to 1200 to 1250 ° C., and hot-rolled at a final rolling temperature of 800 ° C. or more , and then 10 ° C./Hr A method for producing a martensitic stainless steel slab characterized by cooling at the following slow cooling rate .