JP3175919B2 - Bulk rolling method for martensitic stainless steel slabs - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

TECHNICAL FIELD The present invention relates to SUS410,
The present invention relates to a method of bulk rolling a martensitic stainless steel slab represented by SUS420.

[0002]

2. Prior Art Prior to slab rolling into a billet, a Cr-based stainless steel slab is heated and soaked in a soaking furnace. During the heating and soaking, decarburization of the slab corner progresses, and δ ferrite is removed. It is known that precipitation is promoted and cracks occur at corners.

[0003] In addition, the slab-rolled Cr stainless steel billet is liable to undergo martensitic transformation during cooling.
It is known that in normal annealing, cracks accompanying martensitic transformation called transformation cracking occur at a position where the cooling rate is high, such as at the end of an annealing furnace or at both ends of a billet.

[0004] As a method of preventing the occurrence of cracks at the corners of the above-mentioned Cr-based stainless steel slab during heating and soaking,
A method of reheating to a temperature 20 to 200 ° C. lower than the boundary temperature between the austenite single phase region and the ferrite-austenite two phase region, maintaining the soaking temperature for 2 hours to 10 hours, and then forming a round billet by ordinary rolling (particularly, Fairness 6-7
No. 8567) is known.

Further, as a method for preventing the occurrence of transformation cracks due to martensitic transformation during cooling, a method of using an annealing furnace with a burner, reheating once to an austenite region, and then gradually cooling, is used. A method of inserting a torch into an annealing furnace to prevent a sudden drop, and a method of preventing transformation cracks due to a fall in the temperature of the billet end due to the combustion heat of the torches. A method of recharging a billet into a furnace equipped with equipment and performing a heat treatment is known.

[0006]

SUMMARY OF THE INVENTION The above Japanese Patent Publication No. Hei 6-785.
The heat equalizing method disclosed in Japanese Patent No. 67 cannot prevent the occurrence of cracks and cracks in a region where the Cr equivalent is high, requires treatment such as billet care and shaving, and has a problem that the lead time becomes long. Have.

[0007] The method using the annealing furnace with a burner requires a large capital investment to install the burner in the annealing furnace. In addition, the method of inserting the torches into the lehr
Not only is the running cost of purchasing a torch necessary,
The hot work of inserting a torch and the ash removal after the torch burns require a lot of man-hours. Furthermore, the method of performing the heat treatment again after slow cooling has the disadvantage that extra fuel for the heat treatment is required and the lead time is longer than that of the process of only slow cooling.

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned disadvantages of the prior art, to avoid cracks during slab rolling without capital investment and man-hours, and to have a billet free from transformation cracks associated with martensitic transformation. To provide a method for bulk rolling of a martensitic stainless steel slab which can obtain the following.

[0009]

In order to achieve the above-mentioned object, the present inventors have set the soaking conditions for bulk-rolling, that is, the relationship between the heating temperature and δ-ferrite, and the relationship between δ-ferrite and cracks. , Soaking temperature / time and Cr + 4Si- @ 22C
As a result of investigating and examining the relation of Cr equivalent obtained at + 0.5Mn + 1.5Ni + 30N} using an actual machine, it was found that when the heating temperature was increased, the generation of δ-ferrite was increased, the crack generation rate was increased, and the Cr equivalent was increased by 8%. By setting the product of soaking temperature and time to 7500 or less, the generation of cracks during soaking can be suppressed, and dummy materials should be inserted into the upper and lower stages during slow cooling and slowly cooled. Thus, the present inventors have found that martensitic transformation can be suppressed and transformation cracks can be avoided, and the present invention has been achieved.

[0010] The present invention provides a method for producing C: 0.1 to 0.3%,
r: In the bulk rolling of a continuously cast martensitic stainless steel containing 11 to 13%, Cr equivalent = Cr
+4Si-@22C+0.5Mn+1.5Ni+30
Cr of martensitic stainless steel required by N｝
Adjust so that the equivalent is 8.4 or less, soaking temperature (℃)
And time (hr) so that the product is within 7500,
After rolling into billets, dummy materials are charged into the upper and lower stages of the martensitic stainless steel billet at the time of slow cooling in the slow cooling furnace, and then gradually cooled. Thus, the Cr equivalent of the martensitic stainless steel was adjusted to be 8.4 or less, and the product of the soaking temperature (° C.) and the time (hr) was 75.
By soaking so that the temperature is not more than 00, the occurrence of cracks can be prevented. Further, by inserting the dummy material into the upper and lower stages of the martensitic stainless steel billet and gradually cooling, the martensitic transformation in the annealing furnace is suppressed, and transformation cracks can be avoided.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION A martensitic stainless steel slab having a Cr equivalent of 7.73 manufactured by a normal AOD furnace-continuous casting process was soaked at a soaking temperature of 1220 to 1270 ° C.
Specimens were sampled from the billet subjected to slab-rolling while changing the temperature and the relationship between the soaking temperature and the amount of δ-ferrite and the relationship between Cr equivalent and the amount of δ-ferrite were investigated. as a result,
As shown in FIG. 2, the relationship between the soaking temperature and the amount of generated δ ferrite was confirmed to increase as the soaking temperature increases. Also, when the soaking temperature was the same, it was confirmed that the higher the Cr equivalent, the larger the amount of δ ferrite generated. Further, the relationship between the crack occurrence rate during the ingot rolling of the billet from which the specimen was sampled and the δ ferrite generation amount estimated from the heating conditions in FIG. 2 shows that the δ ferrite generation amount increases as the soaking temperature increases. Increased, and it was confirmed that this led to the generation of cracks in the billet.

[0013] Furthermore, as a result of performing bulk slab rolling using a continuously cast slab of martensitic stainless steel while changing the soaking temperature condition, the soaking temperature (° C) and the soaking time (hr) were obtained.
It can be confirmed that by setting the product of 7500 or less and the Cr equivalent to 8.4 or less, it is possible to prevent the occurrence of cracks in the billet due to the generation of δ ferrite.

In the present invention, as shown in FIG.
First, a plurality of dummy materials 32 are charged in a plurality of stages, a martensitic stainless steel billet 33 is charged thereon, and a plurality of dummy materials 32 are further charged thereon.
4 and then slowly cooled.

In the present invention, Cr equivalent = Cr + 4Si
The Cr equivalent of the martensitic stainless steel determined by − {22C + 0.5Mn + 1.5Ni + 30N} is 8.
The reason for setting it to 4 or less is that when the Cr equivalent exceeds 8.4, even if the product of the soaking temperature (° C.) and the soaking time (hr) is 7500 or less, the generation amount of δ ferrite increases, and This is because cracks occur during rolling. In the present invention, the product of the soaking temperature (° C.) and the soaking time (hr) is set to 7500 or less because the product of the soaking temperature (° C.) and the soaking time (hr) exceeds 7500, This is because, even when the equivalent is 8.4 or less, the amount of δ ferrite increases, and cracks occur during slab rolling.

Further, in the present invention, the dummy material is charged into the upper and lower stages of the martensitic stainless steel billet at the time of slow cooling because only the martensitic stainless steel billet is charged into the annealing furnace and gradually cooled. Both ends of the billet and the lower billet are quenched, and the hardness of the billet surface increases due to martensitic transformation.However, quenching concentrates on the dummy material by loading the dummy material into the upper and lower stages, which are the fast cooling parts in the annealing furnace. However, rapid cooling of the martensitic stainless steel billet is avoided, martensitic transformation is suppressed, and a martensitic stainless steel billet with low hardness can be manufactured. Therefore, in the slow cooling method of the present invention, as long as there is a dummy material to be subjected to slab rolling at the same timing, capital investment such as installation of a burner, extra torch insertion, and extra work of heat such as processing of torch combustion ash are required. Since no treatment such as softening heat treatment is required, the lead time can be shortened.

[0016]

【Example】

Example 1 C: 0.1 to 0.3%, Cr: 11 to 13%,
Steel type A with Cr equivalent of 7.3 to 8.5, Cr equivalent of 7.7
~ 8.8 steel type B, Cr equivalent 5.7 ~ 6.5 steel type C
Of martensitic stainless steel in a normal AOD furnace and then 540 mm ×
A slab of 410 mm was formed, and the slab was subjected to bulk-rolling under various soaking conditions to finish a round billet having an outer diameter of 147 to 187 mm. The relationship was investigated. The result is shown in FIG. In FIG. 1, the circles indicate steel type A, the triangles indicate steel type B, and the squares indicate steel type C.
Semi-black is 10% to 50% of crack, black is 50% of crack
% Or more.

As shown in FIG. 1, in order to prevent the occurrence of cracks due to the generation of δ ferrite, it is necessary to set the Cr equivalent to 8.4 or less and the soaking temperature × time to 7500 or less.

Example 2 Steel No. 1 having the chemical composition shown in Table 1 was used. The martensitic stainless steels Nos. 1 to 9 were converted into slabs of 540 mm x 410 mm by a normal AOD furnace-continuous casting method, and the slabs were slab-rolled under the conditions shown in Table 2 and then the lower and upper stages of the lehr. , And a martensitic stainless steel billet was charged in the middle stage, followed by slow cooling. The crack generation rate, transformation crack rate, and hardness of each of the obtained martensitic stainless steel billets were investigated. Table 2 shows the results. In addition, the hardness measured Shore hardness according to the Shore hardness test method prescribed | regulated to JISZ2246.

[0019]

[Table 1]

[0020]

[Table 2]

As shown in Table 2, if the conditions of the method of the present invention are satisfied, there is almost no crack at the time of bulk rolling and transformation crack at the time of slow cooling, and an austenitic system having a low surface hardness that can be processed in a lower step. Stainless steel billets can be supplied.

Example 3 A martensitic stainless steel slab having a Cr equivalent of 7.3 to 7.5 obtained by a normal AOD furnace-continuous casting method was subjected to slab rolling to obtain an outer diameter of 187 mm and a length of 8 m. When slowly cooling the martensitic stainless steel billet of
In the case of conventional annealing, in which only the martensitic stainless steel billet is slowly cooled by charging it into the lehr, The relationship between the annealing time and the temperature of the center and the end of the billet at the lower end and the center of the upper end of the martensitic stainless steel billet was investigated for each case of the gradually cooled sandwich.
The results are shown in FIG. 4 for the case of conventional slow cooling, and FIG. 5 for the case of sandwich slow cooling. In addition, the Shore hardness distribution in the longitudinal direction of the upper, middle, and lower martensite stainless steel billets was measured. The results are shown in FIG. 6 for the case of conventional slow cooling, and FIG. 7 for the case of sandwich slow cooling.

As shown in FIG. 4, in the case of the conventional slow cooling, the end of the billet at the upper end is rapidly cooled to have a large temperature difference from the center, but the billet at the lower end has a temperature difference between the end and the center. Although it is small, it is quenched and the temperature is considerably lower than the center of the upper billet. On the other hand, as shown in FIG. 5, in the case of the sandwich slow cooling, the billet temperature at the lower end portion and the upper end center portion of the martensitic stainless steel billet hardly changed because dummy materials were charged in the upper and lower stages. Moreover, no temperature decrease due to rapid cooling was observed.

Further, as shown in FIG. 6, in the case of conventional slow cooling, a remarkable increase in hardness due to martensitic transformation due to rapid cooling is observed at the end of the upper billet, and the entire length of the billet at the lower end is rapidly cooled by the rapid cooling. A remarkable increase in hardness due to site transformation is observed. On the other hand, as shown in FIG. 7, in the case of the sandwich gradual cooling, a slight increase in hardness is observed at the end of the martensitic stainless steel billet, and the surface hardness that can be processed in the lower process is substantially extended over the entire length. Was holding.

[0025]

According to the bulk rolling method of the present invention, a 13% Cr-based stainless steel slab having a Cr equivalent of 8.4 or less is produced by heating the soaking temperature (° C.) and the soaking time (hr) to 7500 or less. After soaking, the bulk is rolled into a billet, and a dummy material is charged into the upper and lower stages of the martensitic stainless steel billet and cooled slowly, so that capital investment such as burner installation, torch insertion, and torch burning It is possible to avoid cracks at the time of slab rolling without the need for extra steps during heating such as ash treatment, and without any treatment such as softening heat treatment, and to prevent transformation cracks due to martensitic transformation. You can get no billet.

[Brief description of the drawings]

FIG. 1 is a graph showing a relationship between Cr equivalent, soaking temperature × time, and a good region without cracks in Example 1.

FIG. 2 is a graph showing the relationship between the soaking temperature and the amount of δ-ferrite generated.

FIG. 3 is a perspective view for explaining the slow cooling method of the present invention.

FIG. 4 is a graph showing the relationship between slow cooling time and temperature in the case of conventional slow cooling in Example 3.

FIG. 5 is a graph showing the relationship between annealing time and temperature in the case of sandwich annealing in Example 3.

FIG. 6 is a graph showing the Shore hardness distribution in the longitudinal direction of the upper, middle, and lower martensitic stainless steel billets in the case of conventional slow cooling in Example 3.

FIG. 7 is a graph showing a Shore hardness distribution in the longitudinal direction of the billet in the case where the sandwich is gradually cooled in Example 3.

[Description of Signs] 31 Annealing furnace 32 Dummy material 33 Martensitic stainless steel billet 34 Annealing furnace cover

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-3-193818 (JP, A) JP-A-6-100931 (JP, A) JP-B-6-78567 (JP, B2) Japan Institute of Metals "Revised 4th Edition Metal Handbook (2nd Printing)" (April 20, 1986), published by Maruzen Co., Ltd., pp. 825-827 (58) Fields investigated (Int. Cl. ⁷ , DB name) B21B 1/02 B21B 3/02 B21B 45/00-45/02 C21D 8/00

Claims (1)

(57) [Claims] 1. C: 0.1-0.3%, Cr: 11-1
In the bulk rolling of a martensitic stainless steel continuous cast slab containing 3%, Cr equivalent = Cr + 4Si−
After melting 13% Cr-based stainless steel having a Cr equivalent of {8.4C + 0.5Mn + 1.5Ni + 30N} of 8.4 or less, it is continuously cast into continuous cast slabs, soaking temperature (° C) and soaking. After the product of time (hr) is equalized to 7500 or less, the material is soaked, then slab-rolled into a billet, and at the time of slow cooling in an annealing furnace, the dummy material is charged into the upper and lower stages of the martensitic stainless steel billet and gradually cooled. A method for bulk rolling of martensitic stainless steel slabs.