JP2005226129A - Method for manufacturing ferritic stainless steel cast slab - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for manufacturing a ferritic stainless steel cast slab in which no internal cracks are present, and defects attributable to cavity oxidation are reduced while minimizing the impairment of the equipment cost, the manufacture cost and the manufacture step on the degree of versatility in a ferritic stainless steel continuous cast slab in which the toughness is very low because all the phases are ferritic at every temperature, and internal cracks caused by the residual stress are caused during the cooling after the casting. <P>SOLUTION: The ferritic stainless steel is continuously cast, the average cooling rate in the temperature range of 1,200-900°C is controlled to be ≥ 0.25°C/second, and the steel slab is charged in a re-heating furnace while ensuring the temperature of at least 200°C, and reheated for 1-10 hours in the temperature range of 650-1,000°C. <P>COPYRIGHT: (C)2005,JPO&NCIPI

Description

  The present invention relates to a production method that eliminates internal cracks, which is a problem in ferritic stainless steel continuous cast slabs, reduces scale loss, and minimizes cost and efficiency.

  Since ferritic stainless steel represented by SUS430 steel does not contain a large amount of expensive Ni and is inexpensive, it is suitable for applications that require a beautiful surface in a relatively mild corrosive environment such as automobile exterior parts and building interior parts. Widely used. Although its corrosion resistance is generally inferior to that of austenitic stainless steel represented by SUS304 steel, its application has been limited. However, in recent years, as shown in Patent Document 1, for example, high Cr and Nb, Ti, Mo, etc. With the addition of SUS304 and some steel types, steel types with the same level of weather resistance as SUS316 have appeared and are widely used in applications that take advantage of the same as austenitic stainless steels or no stress corrosion cracking susceptibility. .

  In the case of the above high corrosion resistance steel types, it is necessary to reduce C and N, which deteriorate the corrosion resistance, as much as possible. Since these are austenite-generating elements, all ferrite steel that does not transform into the austenite phase at all temperatures is obtained. Ferritic stainless steel is originally inferior in toughness compared to austenitic, but all ferritic steel or steel that produces only a small amount of austenitic phase has a very small amount that does not cause solid phase transformation during casting. It becomes a very large unit, and particularly has low toughness. In addition, Nb or the like added for improving corrosion resistance further reduces toughness.

  In the case of this low toughness ferritic stainless steel, a problem that occurs at the time of manufacture is an internal crack in which a slab formed by continuous casting cracks during cooling. This is a large crack that occurs parallel to the cross section or the surface of the slab due to the tensile residual stress caused by the temperature difference between the outer and inner surfaces during slab cooling, and the slab breaks during reheating. It may cause a serious accident.

  Conventionally, as a countermeasure, a method of gradually cooling from about 800 ° C. to about 100 ° C. by using a slow cooling furnace or the like, the ductile-brittle transition temperature of the slab is near 300 ° C., and reheating without cooling to 300 ° C. or lower. The method (Patent Document 1), the method of reheating and lowering the pressure before reaching 300 ° C (Patent Document 2), and further heating to 800 ° C and heating at 800 to 1300 ° C for 1 to 10 hours A method of slow cooling to 300 ° C. at an average of 40 ° C./hr or less (Patent Document 3) is disclosed.

  However, these methods are inappropriate for solving the above-mentioned problems and cause other problems. First, the method of slow cooling is ineffective even if the rate is reduced to about 5 ° C./hr, and it is disadvantageous in terms of cost even if it is effective at lower than that. The measure of not cooling to below 300 ° C is a reliable and appropriate method in the sense that the ductile-brittle transition temperature is about 200 ° C in the steel, and therefore it is not necessary to pass through the brittle region. Unless you take a very large manufacturing constraint method that always performs rolling at the same timing, it is necessary to perform a holding heat treatment that keeps the temperature above 300 ° C for several days to several weeks. In addition to requiring large-scale equipment, it can be said that the method greatly hinders the freedom and cost of the manufacturing process. Although the method of light reduction can solve the above-mentioned obstacles, it is necessary to install equipment under light pressure.

  On the other hand, the method disclosed in Patent Document 4 has a high degree of freedom in the manufacturing process because it can be made into a cold piece after the treatment, and only a heat treatment furnace is required as equipment, and the heat treatment time is only a few hours. This is the most effective method because the inhibition of efficiency is minimal. However, as a remaining problem, since a high-temperature heat treatment of 800 to 1300 ° C. is performed, scale loss cannot be ignored.

JP-A-57-140860 JP 58-39732 A JP 59-107022 A Japanese Patent Application Laid-Open No. 60-2628

  The present invention has very low toughness because almost all of it becomes a ferrite phase at all temperatures. In a ferritic stainless steel continuous cast slab that causes internal cracks due to residual stress during cooling after casting, the equipment cost, manufacturing cost, manufacturing process It is an object of the present invention to provide a manufacturing method in which there are no internal cracks and defects due to cavity oxidation are reduced while minimizing the hindrance to the degree of freedom.

  The present invention pays attention to defining the conditions at the time of casting slab casting in addition to the temperature pattern control after casting, which has been studied in the prior art, as a measure for achieving the above-mentioned problems. It is a result of finding and realizing a condition that can suppress this.

The gist of the present invention is as follows.
(1) In mass%,
C: 0.001 to 0.07%
Si: 0.01 to 1.0%
Mn: 0.01 to 2.0%
P: 0.01-0.05%
S: 0.0001 to 0.01%
Cr: 11-25%
Ni: 0.01 to 0.7%
Al: 0.0005 to 0.1%
N: 0.001 to 0.05%
Nb: 0.001 to 0.8%
Containing
The balance consists of Fe and inevitable impurities, and γpot. In the method for producing a ferritic stainless steel slab satisfying -50 or more and 20 or less,
Continuous casting of the ferritic stainless steel,
Subsequently, the average cooling rate in the temperature range of 1200 to 900 ° C is controlled to 0.25 ° C / second or more,
Subsequently, ensure a temperature of at least 200 ° C. and insert it into a reheating furnace,
Subsequently, reheating is performed in a temperature range of 650 to 1000 ° C. for 1 to 10 hours. Manufacturing method for ferritic stainless steel slabs.
γpot. = 420C + 470N + 23Ni + 9Cu + 7Mn-11.5Cr-
11.5Si-12Mo-47Nb-52Al + 189 Formula 1 (2) Further, in mass%,
Mo: 0.3-5.0%
Cu: 0.3 to 1.0%
The ferritic stainless steel slab manufacturing method according to claim 1, comprising at least one of the following.
(3) Ferritic stainless steel characterized in that the average cooling rate from the reheating temperature after reheating as described in (1) or (2) to 500 ° C or less is 5 ° C / hr or more and 40 ° C / hr or less Steel slab manufacturing method.

  According to the present invention, the austenite phase does not occur at all temperatures, or the remainder becomes a ferrite phase in a very small amount, so the toughness is very low, and a continuous ferritic stainless steel that causes internal cracks due to residual stress during cooling after casting. For cast slabs, it is possible to achieve both the prevention of internal cracking and the reduction of defects caused by cavity oxidation while minimizing the hindrance to equipment costs, manufacturing costs, and freedom of manufacturing processes. It becomes possible to manufacture high-quality steel materials.

  Hereinafter, the invention will be described in detail.

  As described above, the internal crack of the steel is a brittle crack due to the tensile residual stress caused by the temperature difference between the outer and inner surfaces when the slab is cooled, but the starting point of the crack is coarse carbonitride that precipitates at grain boundaries, etc. It is. Therefore, it can be said that it is effective to reduce the residual stress or change the form of the coarse carbonitride to reduce the internal crack.

  Of these, regarding the control of the form of carbonitride, in the above-mentioned Patent Document 4, the form is controlled by post-heat treatment, but it is most effective to make it finer when precipitated by cooling after casting. Nevertheless, it has never been studied.

  Therefore, the inventors conducted an experiment considering that carbonitride is finely precipitated by quenching the temperature range in which carbonitride precipitates after casting, thereby reducing the susceptibility to cracking. The present inventors have found that the above-described slab cavity oxidation defects due to high-temperature oxidation can be suppressed, and have led to the present invention.

  On the other hand, the residual stress is caused by uneven heat shrinkage when there is a temperature difference between the surface layer and the center during cooling. In the case of continuous casting, a sufficiently large tensile stress is generated by cooling during solidification, so cracking cannot be prevented even if the carbonitrides as described above are refined to increase cracking susceptibility. There is a need to add. For this purpose, the slab may be heat-treated under certain conditions. This is because the heat treatment at a certain temperature or more causes a creep phenomenon in the material and the residual stress disappears. The temperature required for this is equal to or higher than the temperature at which the material creeps, and the process is completed in a very short time as long as the entire slab is at a constant temperature. Therefore, a high temperature and a long time required for controlling the form of the precipitate are not required.

  On the other hand, as for the oxidation state by heat treatment, an experiment was conducted, and it was found that in the case of normal combustion atmosphere heat treatment, the oxidation amount rapidly increased above 1000 ° C., and the oxidation amount at 1000 ° C. or less was relatively small. . This is because Cr-rich scale plays the role of a protective film when the temperature is 1000 ° C. or lower, and oxidation does not proceed much more than a certain level, whereas this protective effect disappears when the temperature exceeds 1000 ° C.

  Hereinafter, the manufacturing method of the present invention will be described in the order of steps.

  First, when the steel slab is cast, solid solution C and N are precipitated as carbonitrides of Nb or Cr in a temperature range of 1200 to 900 ° C. By cooling this precipitation temperature range at a cooling rate of 0.25 ° C./second or more on average, carbonitrides are finely precipitated. Here, the slab temperature means the slab surface temperature.

  Next, it inserts into a heating furnace, without making it 200 degreeC or less which becomes a brittle region and a crack generate | occur | produces. There are no particular restrictions on the cooling rate and the heating rate in the heating furnace. This is because carbonitride has already precipitated, and the residual stress can be eliminated by subsequent heat treatment, and cracking does not occur even at high stress as long as the temperature is 200 ° C. or higher.

  Further, heat treatment is performed at an appropriate temperature and time according to the components. This is because the residual stress generated before the heat treatment is eliminated by the creep effect. In order to suppress scale loss due to oxidation and defects due to central cavity oxidation, it is desirable not to perform heat treatment at a higher temperature than necessary.

  After the reheating process is completed, the cooling to the cold piece is not essential because the residual stress generated thereby is not so large, but in order to reduce as much as possible, the average from the reheating temperature to 500 ° C What is necessary is just to anneal at a cooling rate of 40 ° C./hr or less to form a cold piece.

  As a result, no cracking occurs even when the slab is cooled to room temperature.

  Next, the reasons for limiting the constituent requirements of the present invention will be described.

  Since C deteriorates workability and corrosion resistance, the smaller the content, the better. However, excessive reduction leads to an increase in refining cost, so 0.001 to 0.07% was set. Furthermore, if considering economy and characteristics, 0.002 to 0.05% is desirable.

  Since Si is a deoxidizing element, it is added during refining. However, if it exceeds 1.0%, workability deteriorates, and if it is less than 0.01%, refining costs increase. Therefore, the range of Si is set to 0.01 to 1.0%. Furthermore, if considering material properties, 0.1 to 0.6% is desirable.

  Since Mn deteriorates workability and corrosion resistance, its content is preferably as small as possible. However, excessive reduction leads to an increase in refining costs and has a deoxidation effect similar to Si. %. Furthermore, if considering the economy and characteristics, 0.1 to 0.5% is desirable.

  P decreases the workability and corrosion resistance, so its content is preferably as small as possible. However, excessive reduction leads to an increase in refining costs, so 0.01 to 0.05% was made. Furthermore, if considering economy and characteristics, 0.02 to 0.04% is desirable.

  S is the same as P, and the smaller the content, the better. However, excessive reduction leads to an increase in refining costs. Therefore, the range of S is set to 0.0001 to 0.01%. Furthermore, if considering economy and characteristics, 0.0005 to 0.005% is desirable.

  Cr needs to be added in an amount of 11% or more in order to ensure corrosion resistance. However, the addition of more than 25% causes toughness deterioration, and internal cracking cannot be suppressed even with the method of the present invention. Therefore, the Cr range is 11-25%. Furthermore, 12 to 20% is desirable from the viewpoint of ensuring corrosion resistance and workability.

  Since Ni is expensive and deteriorates the stress corrosion cracking resistance, its content is preferably as small as possible. However, excessive reduction leads to an increase in refining costs, so 0.01 to 0.7% is set. . Furthermore, if considering the economy and characteristics, 0.01 to 0.5% is desirable.

  Since Al is a deoxidizing element, it is added during refining. However, if it exceeds 0.1%, workability deteriorates, and if it is less than 0.0005%, refining costs increase. Therefore, the Al range is set to 0.0005 to 0.1%. Furthermore, if considering economy and characteristics, 0.001 to 0.07% is desirable.

  N, like C, deteriorates workability and corrosion resistance, so its content is preferably as small as possible. However, excessive reduction leads to an increase in refining costs, so 0.001 to 0.05% was set. Furthermore, if considering economy and characteristics, 0.005 to 0.02% is desirable.

  Nb has the effect of forming precipitates with C and N, reducing the solid solution C and N, and improving the corrosion resistance. However, excessive addition causes severe toughness reduction, so 0.001 to 0.8% It was. Furthermore, if considering economy and characteristics, 0.001 to 0.5% is desirable.

  Mo and Cu are added as necessary.

  Mo has an effect of improving the corrosion resistance, but the effect is from 0.3% or more, and if it exceeds 5.0%, the workability is lowered. Therefore, the desirable range of Mo is set to 0.3 to 5.0%. Furthermore, if considering economy and corrosion resistance, 0.5 to 2.5% is more desirable.

  Cu also has an effect of improving the corrosion resistance, but the effect is from 0.3% or more, and if it exceeds 1.0%, the workability and the stress corrosion cracking resistance are reduced. Therefore, the desirable range of Cu is set to 0.3 to 1.0%. Furthermore, if considering the characteristics, 0.3 to 0.7% is more desirable.

  Furthermore, the residual stress reduction step shown in the present invention requires ferrite, which is particularly low toughness among ferritic stainless steels, or becomes a ferrite at all temperatures from casting to room temperature, or only a very small amount of austenite phase is generated. There is no steel grade. For other steel types, internal cracking can be prevented by slow cooling in the furnace after casting.

  Which component steel type requires this step can be calculated by γpot. Therefore, the processing of the present invention is required for the component steel types in which the value of this equation is 20 or less.

On the other hand, γpot. Is less than −60, the toughness decreases as internal cracks occur even with the treatment of the present invention. Therefore, γpot. The range is from -60 to 20 inclusive.
γpot. = 420C + 470N + 23Ni + 9Cu + 7Mn-11.5Cr
-11.5Si-12Mo-47Nb-52Al + 189 ... Formula 1
In Formula 1, each element symbol represents the content (mass%) of the element.

  Next, regarding the cooling rate at the time of casting, between 1200 and 900 degreeC is made into an average 0.25 degreeC / second or more. This temperature range is a temperature at which Nb or Cr carbonitride precipitates, and if this interval is cooled at 0.25 ° C./second or more, the precipitated carbonitride becomes finer and crack susceptibility can be reduced.

  The reason why the minimum temperature until insertion into the heating furnace is 200 ° C. is that if the temperature is lower than 200 ° C., the transition from the ductile region to the brittle region occurs, and cracks occur at this point.

  When the heating temperature is less than 650 ° C., the creep phenomenon does not occur at all and the residual stress cannot be reduced. Therefore, when the temperature exceeds 1000 ° C., oxidation proceeds rapidly and defects due to cavity oxidation occur, resulting in 650 ° C. to 1000 ° C. It was.

  If the heating time is less than 1 hour, the temperature at the center of the slab does not rise and the residual stress is not reduced. If the heating time exceeds 10 hours, the effect is saturated and the cost is deteriorated and the scale amount is increased. More than 10 hours.

  Regarding the cooling rate after the heat treatment, if the average exceeds 40 ° C./hr, residual stress is generated again during cooling, which may cause internal cracking, so the average is 40 ° C./hr or less. On the other hand, if the average is less than 5 ° C./hr, there is almost no effect of lowering the residual stress, but the annealing time becomes very long and the efficiency is lowered, so the average is set to 5 ° C./hr or more. If the temperature is less than 500 ° C., the heat shrinkage is not plastic deformation but elastic deformation and the residual stress does not increase. Therefore, the cooling rate is limited to 500 ° C. However, the increase in residual stress due to the removal of this rule is not essential since it is relatively small.

  Examples will now be described. After casting a ferritic stainless steel continuous cast slab, heat treatment was performed under various conditions to form a cold slab, and heated again to perform hot rolling to produce a wire. The presence or absence of internal cracks was evaluated by cutting out a portion when the slab was made into a cold piece and observing the cross section, and the occurrence rate was calculated from the number of slabs with internal cracks. Internal cavity oxidation-induced defects were evaluated by cutting out part of the wire and observing the cross section, and the occurrence rate was calculated from the number of wires. Table 1 shows the components of the test steel, and Table 2 shows the heat treatment conditions and evaluation results until the specimens are made into cold pieces. The cooling rate after casting was determined by actually measuring the slab surface temperature.

  No. in Table 1 1 and 2 are embodiments of claim 1 of the present invention, and 3 to 5 are embodiments of claim 2 of the present invention.

  No. 6 to 27 are comparative examples of claim 1. No. In No. 6, C is too low, so the refining cost increases. Since 7 is too high, workability and corrosion resistance deteriorate. No. In No. 8, since Si is too low, the refining cost increases. Since 9 is too high, workability deteriorates. No. No. 10 has a too low Mn, so the refining cost increases. Since 11 is too high, workability and corrosion resistance deteriorate. No. No. 12 has too low P, so the refining cost increases. Since 13 is too high, workability and corrosion resistance deteriorate. No. No. 14 has a refining cost increased because S is too low. Since 15 is too high, workability and corrosion resistance deteriorate. No. No. 16 has poor corrosion resistance because Cr is too low. Since No. 17 is too high, toughness deteriorates. No. No. 18 has a refining cost increased because Ni is too low. Since 19 is too high, the raw material cost increases, and the stress corrosion cracking resistance deteriorates. No. In No. 20, since Al is too low, the refining cost increases. Since 21 is too high, workability deteriorates. No. No. 22 has a refining cost increased because N is too low. Since 23 is too high, workability and corrosion resistance deteriorate. No. No. 24 has a low refining cost because Nb is too low. Since 25 is too high, toughness deteriorates.

  No. 26 is γ. Since the pot value is too low, the toughness is poor. Since No. 27 is high, it is excluded because it does not cause internal cracking only by slow cooling without reheating as in the present invention.

  Next, with respect to Table 2, no. A to E are examples of the present invention. By setting the heating pattern to a predetermined one, it is possible to manufacture one having neither internal cracks nor internal cavity oxidation defects.

  No. F to N are comparative examples. No. F is a case where the average cooling rate of 1200 to 900 ° C. after casting is less than 0.25 ° C./second, but internal cracking occurred even when heat treatment was performed at 800 ° C. thereafter. No. Internal cracking can be prevented by heating at a high temperature as in I, but defects due to internal cavity oxidation occur. No. G has a temperature lower than 200 ° C. before charging the heating furnace. H has a heating temperature of less than 650 ° C. Since J had a heating time of less than 1 hour, all of them had internal cracks. No. I caused defects due to internal cavity oxidation because the heating time exceeded 10 hours.

  No. L has an average cooling rate after heating of less than 5 ° C./hr, and there is no problem in characteristics, but it takes several days to take out the slow cooling furnace, and the efficiency is very low. No. M has an average cooling rate after heating exceeding 40 ° C./hr. N has a slow cooling end temperature of over 500 ° C. and then air cooling, so that although it is a very small amount, an internal crack is generated. Therefore, N should be avoided to prevent any internal crack.

Claims (3)

In mass%
C: 0.001 to 0.07%
Si: 0.01 to 1.0%
Mn: 0.01 to 2.0%
P: 0.01-0.05%
S: 0.0001 to 0.01%
Cr: 11-25%
Ni: 0.01 to 0.7%
Al: 0.0005 to 0.1%
N: 0.001 to 0.05%
Nb: 0.001 to 0.8%
Containing
The balance consists of Fe and inevitable impurities, and γpot. In the method for producing a ferritic stainless steel slab satisfying -60 or more and 20 or less,
Continuous casting of the ferritic stainless steel,
Subsequently, the average cooling rate in the temperature range of 1200 to 900 ° C is controlled to 0.25 ° C / second or more,
Subsequently, ensure a temperature of at least 200 ° C. and insert it into a reheating furnace,
Subsequently, it is reheated in a temperature range of 650 to 1000 ° C. for 1 to 10 hours,
Manufacturing method for ferritic stainless steel slabs.
γpot. = 420C + 470N + 23Ni + 9Cu + 7Mn-11.5Cr-
11.5Si-12Mo-47Nb-52Al + 189 ≦ 20 1 set Furthermore, in mass%,
Mo: 0.3-5.0%
Cu: 0.3 to 1.0%
The ferritic stainless steel slab manufacturing method according to claim 1, comprising at least one of the following.   3. The production of a ferritic stainless steel slab characterized in that the average cooling rate from the reheating temperature after reheating according to claim 1 or 2 to 500 ° C. or less is 5 ° C./hr or more and 40 ° C./hr or less. Method.