JP2004270026A - HIGH Al-CONTAINING FERRITIC STAINLESS STEEL HOT ROLLED STRIP HAVING EXCELLENT TOUGHNESS, AND PRODUCTION METHOD THEREFOR - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high Al-containing ferritic stainless steel hot rolled strip which has excellent toughness, and to provide a production method therefor. <P>SOLUTION: In the high Al-containing ferritic stainless steel hot rolled strip having a composition comprising, by mass, 12 to 30% Cr, 3 to 8% Al, and 0.05 to 0.5% Nb, if required, comprising one or more kinds of metals selected from V, Ti, and Zr, comprising ≤0.025% C and ≤0.025% N, and in which the content of C+N is ≤0.03%, and the balance Fe with inevitable impurities, the metallic structure at least in a region between the central part of the sheet thickness and the part in a thickness of 1/4 from the surface is a unrecrystallized structure. In the production method therefor, the steel is hot-rolled so as to be finished at 700°C to a recrystallization temperature Ts, the total draft in the temperature range of the recrystallization temperature Ts or below is controlled to ≥15%, and, after the hot rolling, it is coiled in the temperature range of >500 to <850°C, and is successively subjected to forced cooling. <P>COPYRIGHT: (C)2004,JPO&NCIPI

Description

  The present invention relates to a hot-rolled steel strip of a high Al-containing ferritic stainless steel excellent in toughness and a method for producing the same.

  In recent years, a ferritic stainless steel containing Al has attracted attention as a heat-resistant material. Such a high-Al-containing ferritic stainless steel has much better oxidation resistance than austenitic stainless steel in heat resistance, and also has a high electrical resistivity. This material is used for electromagnetic components, such as electric resistors, which require high electrical resistivity, exhaust gas components for automobiles, stove components, heating furnace walls, etc., which require excellent oxidation resistance. As the usage environment becomes severer, more heat resistance is required, and the Al content is increasing.

  However, the hot-rolled steel strip of a ferritic stainless steel containing Al has remarkably low toughness, so that when the coil is unwound or cold-rolled at room temperature, cracks or plate breakage occur, and in extreme cases, cold rolling occurs. Rolling may not be possible. In order to avoid this, the steel strip should be heated to a temperature higher than the transition temperature and passed through. However, in addition to the cost increase due to heating, workability and efficiency are poor when the heating temperature is high, and safety is not a factor. Is also not preferred. In the case of a steel type in which the transition temperature of the steel strip is extremely high, it may be practically difficult to avoid cracking and plate breakage even by the above heating. Further, when a hot-rolled steel strip is bent, cut, or punched into a product, there is a problem of cracking.

  Therefore, hot-rolled steel strip with high toughness, high heat resistance and high Al-containing ferritic stainless steel, which does not crack in the next manufacturing process or product processing such as unwinding or cold rolling of hot-rolled steel strip coil and There has been a strong demand for a manufacturing method thereof. As a conventional technique for preventing the embrittlement phenomenon of the high Al content ferritic stainless steel hot-rolled steel strip, as disclosed in Patent Document 1, for example, C and N are reduced to reduce 10 to 10 after hot rolling. There is a method of quenching at a cooling rate of not less than 450 ° C. and winding at a low temperature of not more than 450 ° C. Further, Patent Literature 2 discloses a technique in which the hot-rolling finishing temperature of a Cr-Al steel containing Ti is set to 900 ° C or lower, the winding temperature is set to 600 ° C or lower, and then water cooling is performed. The inventions described in the above documents relate to low-temperature winding after hot rolling. The reason for the low-temperature winding is to prevent precipitation of grain boundaries of carbonitride and precipitation of intermetallic compounds at the time of high-temperature winding.

  On the other hand, Patent Literature 3 discloses a technique in which a high Al-containing ferritic stainless steel containing Ti is hot-rolled at a temperature of 700 to 800 ° C and then rapidly cooled. This technique promotes AlN precipitation by winding in a temperature range of 700 to 800 ° C. and holding for 5 to 10 minutes, thereby improving the toughness by reducing the solute Al and lowering the transition temperature. The reason for keeping the holding time up to 10 minutes is to suppress grain boundary precipitation of carbonitride by holding for more than 10 minutes.

JP-A-60-228616 JP-A-5-148548 JP-A-5-331552

  The present invention further improves the toughness of a high heat-resistant ferritic stainless steel hot-rolled steel strip containing Al, thereby enabling cold rolling of a particularly thick hot-rolled steel strip and improving workability. It aims to improve and further improve the processability of the product.

  For this purpose, the present invention has been completed as a result of studying the components, metallographic structure, hot rolling conditions, cold rolling conditions, and winding conditions, and has been completed. It has been found that optimizing the addition of Nb and also V, Ti, and Zr and optimizing the winding temperature for improvement meet such a purpose. The summary is as follows.

  That is, the object of the present invention is achieved by a hot-rolled steel strip containing high Al-containing ferritic stainless steel as described in the following (1) to (8) and a method for producing the same.

The high Al-containing ferritic stainless steel hot-rolled steel strip excellent in toughness of the present invention,
(1) In mass%, Cr: 12 to 30%, Al: 3 to 8%, Nb: 0.05 to 0.5%, C: 0.025% or less, N: 0.025% or less , C + N: 0.030% or less, the balance being a hot-rolled steel strip of high Al-containing ferritic stainless steel comprising Fe and unavoidable impurities, at least 1/4 from the center and the surface of the steel strip. The metal structure in a region between the thickness portion and the region having the thickness is an unrecrystallized structure.
(2) V: 0.05 to 0.4% by mass.
(3) Further, one or more of Ti: 0.02 to 0.2% by mass and Zr: 0.02 to 0.2% by mass are contained.
Further, the method for producing a hot-rolled steel strip containing high Al-containing ferritic stainless steel having excellent toughness according to the present invention includes:
(4) In mass%, Cr: 12 to 30%, Al: 3 to 8%, Nb: 0.05 to 0.5%, C: 0.025% or less, N: 0.025% or less , C + N: 0.030% or less, with the remainder being hot-rolled in a temperature range of 700 ° C. or more and a recrystallization temperature Ts or less of a slab of high Al-containing ferritic stainless steel consisting of Fe and unavoidable impurities. The total rolling reduction R (%) in the temperature range of not more than the recrystallization temperature Ts is 15% or more, then wound around a coil, and then cooled.
Here, Ts is calculated by the following equation.
Ts (° C.) = 900 + 20Al-2R
Al: mass% of Al
(5) V: 0.05 to 0.4% by mass.
(6) The composition further comprises one or more of Ti: 0.02 to 0.2% by mass and Zr: 0.02 to 0.2% by mass.
(7) The hot-rolled slab of high-Al-containing ferritic stainless steel is hot-rolled, wound around a coil at a winding temperature of more than 500 ° C and less than 850 ° C, and subsequently cooled.
(8) The high-Al content ferritic stainless steel hot rolled steel strip is forcibly cooled after winding.

  The hot-rolled high-Al-content ferritic stainless steel strip of the present invention simultaneously realizes high heat resistance and high toughness regardless of the thickness of the sheet, and enables cold rolling of a hot-rolled steel strip. Things. According to the present invention, if a high-Al-containing ferritic stainless steel hot-rolled steel strip is manufactured, cracks and sheet breaks during unwinding and cold rolling of the hot-rolled steel strip are prevented, and these steel strips are used as products. When bending, cutting, punching or the like is used, cracks are eliminated and workability is greatly improved. The steel of the present invention is an extremely excellent ferritic stainless steel hot-rolled steel strip that can be used for high electrical resistivity applications and high heat resistance applications, and its industrial value is remarkably large.

  The reason for limiting the present invention will be described in detail below.

  Incidentally, the steel slab in the present invention is a continuous cast slab, a slab that has been broken down on it, an ingot slab, and a slab that has been subjected to slab rolling. Since the ferritic stainless steel in the present invention has a composition of a ferrite single phase in the hot rolling temperature range, martensitic transformation does not occur due to cooling after completion of hot rolling. Although there is no martensite structure that is a nucleus for microcracks, which is one of the causes of toughness deterioration, in a ferritic stainless steel containing 3% or more of Al, sufficient toughness cannot be obtained by this effect alone.

  Further, since the ferritic stainless steel of the present invention has a ferrite single-phase structure in a hot rolling temperature range, a metal structure control utilizing phase transformation is performed, for example, a steel type represented by a general-purpose ferritic stainless steel SUS430. I can't hope for that. Therefore, in the present invention, the recovery process of the hot-rolled structure (meaning recovery and recrystallization) is utilized to improve the toughness.

  The present inventors examined the effects of components, hot rolling conditions, cooling conditions, and winding conditions on the toughness of the hot-rolled steel strip, and found that Nb or Nb and Vb, Ti, and Zr were added in appropriate amounts. Hot rolling of ferrite stainless steel slab with low nitrogen and high Al content is completed in the recovery temperature range of 700 ° C or higher and the recrystallization temperature or lower, then wound up at more than 500 ° C and less than 850 ° C, and then forcibly cooled. It has been found that the toughness is significantly improved by performing the method.

  First, the reasons for limiting each component range (% by mass) of stainless steel targeted by the present invention will be described.

  C, N: If C and N are present in excess of 0.025%, the toughness of the hot-rolled steel strip is reduced, so that the content of each is set to 0.025% or less, and the total amount of C + N is set to 0.03% or less. Preferred values of C and N are 0.005% or less, and preferred values of C + N are 0.01% or less.

  Cr: Cr is the most basic element for ensuring the heat resistance or oxidation resistance of stainless steel. In the present invention, if the content is less than 12%, these properties are not sufficiently ensured, while if the content is more than 30%, the toughness and ductility of the hot-rolled steel strip particularly deteriorate remarkably. Therefore, the component range of Cr is set to 12 to 30%. The preferred range is 14.5-16%.

  Al: Al is an element that significantly improves the oxidation resistance and electric resistivity of ferritic stainless steel. In the present invention, if this element is less than 3%, it is not enough to improve oxidation resistance. If the content exceeds 8%, the toughness of the hot-rolled steel strip is significantly reduced. Therefore, the component range of Al is set to 3 to 8%. The preferred range is 4-6%.

  Nb: Nb forms nitrides or carbides to reduce solid solution C and N, and precipitates on dislocations introduced by working during hot rolling to refine the structure, thereby improving the toughness of the hot-rolled steel strip. Further improve. If this effect is less than 0.05%, the effect is not sufficient, and if it exceeds 0.5%, the workability in the cold state is remarkably deteriorated. Therefore, the component range is set to 0.05 to 0.5%. The preferred range is 0.1-0.3%.

  Ti: Ti can be selectively added in the present invention. Ti is effective in improving the oxidation resistance of ferritic stainless steel and improves the adhesion of the oxide film. At the same time, it forms nitrides or carbides like Nb to reduce the solute C and N. Improves the toughness of the hot-rolled steel strip. This effect can be exhibited with a Ti content of 0.02% or more. However, the addition of excessive Ti deteriorates the toughness of the hot-rolled steel strip due to the increase of solid-solution Ti, and at the same time, forms coarse micron-order Ti nitride TiN at the time of casting, and the starting point of microcracks in the hot-rolled steel strip. It is thought that it becomes. In particular, if it exceeds 0.2%, the toughness significantly deteriorates. Therefore, the component range was set to 0.02 to 0.2%. The preferred range is 0.04 to 0.10%. It is desirable that the content of TiN in the hot-rolled steel strip containing high Al-containing ferritic stainless steel targeted by the present invention is 0.015% by mass or less.

  V: V can be selectively added in the present invention. With the same effect as Nb, the toughness of the hot-rolled steel strip is further improved. If the effect is less than 0.05%, the effect is not sufficient, and if it exceeds 0.4%, the workability in the cold state is remarkably deteriorated. Therefore, the component range is set to 0.05 to 0.4%.

  Zr: Zr can be selectively added in the present invention. Zr improves the toughness of the hot-rolled steel strip at 0.02 or more due to the same effect as Ti. However, the excessive addition of Zr deteriorates the toughness of the hot-rolled steel strip due to the increase of solid solution Zr, and at the same time, forms coarse Zr on the order of microns and an iron intermetallic compound, and the starting point of microcracks in the hot-rolled steel strip. It is thought that it becomes. In particular, if it exceeds 0.2%, the toughness significantly deteriorates. Therefore, the component range was set to 0.02 to 0.2%.

  Next, the metal structure of the stainless steel targeted by the present invention will be described. In the hot-rolled steel strip with high Al-containing ferritic stainless steel of the present invention, the metal structure at least in the region between the central portion of the plate thickness and a portion having a thickness of 1/4 from the surface is an unrecrystallized structure. In order to recrystallize the metal structure, hot rolling at a temperature higher than the recrystallization temperature is required, and preferably, heat treatment after the hot rolling is required. In this case, the crystal grains become very coarse, and the low toughness characteristic inherent in the high Al-containing ferritic stainless steel becomes apparent, and the toughness deteriorates. Since the degree of work due to shear strain in hot rolling is high in the vicinity of the surface, a fine recrystallized structure is easily formed, but at least in the region between the central part of the sheet thickness and the part １ ／ thickness from the surface. It is difficult to form a fine recrystallized structure by hot rolling because it is difficult to accumulate processing strain. Therefore, the metal structure in at least the region between the central portion of the plate thickness and the region having a thickness of 1/4 from the surface is softened by making the metal structure unrecrystallized and a recovered structure, thereby reducing the toughness of the hot-rolled steel strip. Improve.

  In the method for producing a hot-rolled steel strip containing high Al-containing ferritic stainless steel, which is a target of the present invention, the slab is subjected to hot rolling in a temperature range of 700 ° C. or more and a recrystallization temperature Ts or less, and then a coil is formed. And subsequently cooled. In the hot rolling, the final stage of rolling is performed in a recovery temperature range below the recrystallization temperature Ts (° C.), so that the dislocations introduced during the rolling pass form sub-grain boundaries as an energetically stable rearranged structure. However, the hot rolled structure is considered to have sub-grains in the crystal grains. Therefore, in order to improve the toughness of the hot-rolled steel strip containing high Al-containing ferritic stainless steel, it is effective to relieve the stress concentration due to the accumulation of dislocations at the crystal grain boundaries with a fine subgrain structure.

  In a hot-rolled ferritic stainless steel strip containing 3% or more of Al, dislocations necessary for forming such a fine sub-grain structure are introduced by hot rolling at a recovery temperature of Ts or less. The total sum R of the rolling reduction in the region is 15% or more. If R exceeds 60%, a large amount of introduced dislocations remain in the sub-grains, and stress concentration at crystal grain boundaries and sub-grain boundaries is promoted. Therefore, R is preferably 60% or less. A preferred range of the rolling reduction is 30 to 60%.

Furthermore, as a result of a detailed study of the effects of the Al content (% by mass) and the total reduction R (%) in the recovery temperature range on the recrystallization temperature Ts (° C.), it was found that Ts is given by the following equation. I found it.
Ts (° C.) = 900 + 20Al-2R
Al: mass% of Al

  It has been found that the minimum sum of the reduction required in the above-mentioned recovery temperature range (referred to as the critical reduction) is 15%.

  In addition, in the hot rolling, it is preferable that the heavy rolling under the recrystallization temperature is performed not only in the final rolling pass but also in at least two passes including the final rolling pass.

  The reason why the hot-rolling end temperature of the high-Al-containing ferritic stainless steel slab was set to 700 ° C. or higher is that it is not practical because hot rolling at a temperature lower than 700 ° C. increases the deformation resistance of the stainless steel and causes insufficient mill power. In addition, when the hot rolling end temperature is extremely low, a large amount of processing strain introduced in the hot rolling remains, and the recovery of dislocations does not sufficiently proceed. A preferred value of the hot rolling end temperature is 850 ° C. or more.

  The winding temperature of the high Al content ferritic stainless steel hot rolled steel strip of the present invention is desirably more than 500 ° C and less than 850 ° C. If the winding temperature is 500 ° C. or lower, the recovery of dislocations introduced by hot rolling does not proceed sufficiently, so that improvement in toughness of the hot-rolled steel strip cannot be expected. On the other hand, at 850 ° C. or higher, although the recrystallization and grain growth of the processed structure formed by hot rolling proceed, the fine structure is not formed and the low toughness inherent to the high Al-containing ferritic stainless steel becomes apparent. Therefore, the toughness of the hot-rolled material is deteriorated. A practically more preferred winding temperature is from 600C to 750C. This is because, when the hot rolling is completed at a temperature lower than the recrystallization temperature, the temperature is a temperature which naturally reaches by air cooling and a winding temperature range in which the effect of improving toughness is high.

  In the high Al-containing ferritic stainless steel of the present invention containing Nb or Nb and one or more of V, Ti, and Zr in combination, unlike the Ti-added steel shown in the prior art, Nb-added or Nb-added. Since the addition of one or more of V, Ti, and Zr can sufficiently fix carbon and nitrogen in the grains, the influence of embrittlement due to precipitation of grain boundaries of carbonitrides and precipitation of intermetallic compound AlN is small.

  Further, it is preferable that the cooling rate after the winding is completed is forcibly cooled as quickly as possible. Specifically, water cooling is realistic. In principle, it is difficult to avoid the influence of so-called 475 ° C brittleness during cooling after winding, and the slower the cooling rate is, the more the formation of a fine Cr-rich phase, which is the structure causing 475 ° C brittleness, is promoted. As a result, the toughness of the hot-rolled material is deteriorated, and the effect of improving the toughness by promoting the formation of a recovery structure by winding at a temperature higher than 500 ° C. and less than 850 ° C. is offset, so that the winding temperature region effective for improving the toughness is narrowed. The case of air cooling corresponds to this.

  Hereinafter, the present invention will be described specifically with reference to Examples.

(Example 1)
A high Al-containing ferritic stainless steel shown in Table 1 was melted by a converter AOD method or a vacuum melting method. These steels were manufactured according to the conditions shown in Table 2 to obtain a hot-rolled steel strip having a thickness of 5 mm.

Evaluation of toughness was carried out parallel to collected impact test and the rolling direction of V-notch Charpy test pieces of the sub-size (thickness 5mm) in accordance with JIS standards, the impact value is 2 kgf / cm ² Temperature (VT2: ° C.) Was evaluated. When vT2 is 30 ° C. or less, cold rolling of a hot-rolled steel strip is possible without heating. In the case of 30 ° C. to 70 ° C., cold rolling of a hot-rolled steel strip is possible by performing hot water heating by a hot bath in advance. In the case where the temperature exceeds 70 ° C., the risk of plate breakage due to impact or the like becomes extremely high if cold rolling is performed even if heating with hot water is performed.

  The metal structure was observed with an optical microscope after corrosion with a reverse aqua regia-based etching solution.

The oxidation resistance was evaluated by using a sample whose surface had been polished with a count of # 400 and increasing the oxidation after 950 ° C. × 100 hours in the air. The case where the weight increase by oxidation was 0.5 mg / cm ² or less was indicated by ○, and the case where the oxidation amount was more than 0.5 mg / cm ² was indicated by ×.

  In Example 1, the hot rolling end temperature was 700 ° C. or more and the recrystallization temperature Ts or less at any level, and the total draft R (%) in the temperature range of the recrystallization temperature Ts or less was 15% or more. The metal structure in the region between the central portion of the steel strip and a portion having a thickness of 1/4 from the surface is an unrecrystallized structure.

  Inventive steel No. It can be seen that the hot-rolled steel strips 1 to 10 have the components of the present invention, have greatly improved toughness, and do not cause troubles such as plate breakage during cold rolling.

  No. of the comparative example. No. 13 has a component of sample symbol 13 and Cr is out of the lower limit of claims. Sample No. 15 has the component of Sample No. 15, and Al departs from the lower limit of claims, and all have poor oxidation resistance.

  Comparative Example No. In the case of No. 11, the value of C was too high. In the case of No. 12, the Cr content was higher, and In the case of No. 14, the Al content was higher than that of No. 14; In the case of No. 16, the N was deviated to a higher value. In No. 17, Nb came off at a low level. No. 18 had a higher Nb, and In the case of No. 19, the value of Ti was too high. In No. 20, Zr was deviated to a higher level. In No. 21, the V is deviated to a high value, and vT2 exceeds 70 ° C., so that cold rolling cannot be performed.

  The value of the electrical resistivity at room temperature is 100 μΩcm or more except for the sample code 13 and the sample code 15.

(Example 2)
For the sample having the component of sample symbol 7 (No. 7) in Table 1, the hot rolling end temperature, the hot rolling winding temperature and the cooling method were changed as shown in Table 3 to obtain a hot rolled steel strip. Manufactured. The hot-rolled steel strip (Nos. 7, 22, 23, 28, 29, 30) produced by the method of the present invention has greatly improved toughness, and does not cause troubles such as sheet breakage during cold rolling. Understand.

  Comparative Example No. In Nos. 24 and 25, the R is less than 15%, the rolling end temperature exceeds the recrystallization temperature Ts, and in each case, the metal structure in the region between the central portion of the steel strip and a portion 1/4 thick from the surface is not yet formed. Does not have a recrystallized structure. Comparative Example No. No. 26 has a hot rolling end temperature of less than 700 ° C. No. No. 27 has a winding temperature of 850 ° C. or higher, and No. 31 has a winding temperature of 500 ° C. or less. 32 is air cooling for cooling after winding. In any of these comparative examples, vT2 exceeds 70 ° C., so that cold rolling cannot be performed.

  FIG. 1 shows the effect of the winding temperature after hot rolling on the toughness of a hot-rolled sheet. The test was conducted on the sample having the component of sample code 7 with the sample No. 7 in Table 3. Hot rolling under the conditions of 31 and winding at a winding temperature of 400 ° C., and thereafter annealing at a temperature of 350 to 950 ° C. for 0 to 100 hours using a laboratory electric furnace, followed by water cooling, and a change in the winding temperature. It was a simulation. The impact test was performed at 50 ° C., and evaluated by an average value of n = 3. Above 500 ° C. and below 850 ° C., a remarkable improvement in impact value is observed. That is, it is understood that there is an appropriate temperature range of the winding temperature with respect to the toughness of the hot-rolled sheet.

The figure which shows the influence which the winding temperature after hot rolling gives on the toughness of a hot rolled sheet.

Claims (8)

In mass%,
Cr: 12-30%,
Al: 3 to 8%,
Nb: containing 0.05 to 0.5%;
C: 0.025% or less,
N: 0.025% or less,
C + N: 0.030% or less;
The remainder is a hot-rolled steel strip of a high Al-containing ferritic stainless steel comprising Fe and unavoidable impurities, the metal being present in at least a region between the central portion of the steel strip and a portion having a thickness of 1/4 from the surface. A hot-rolled steel strip containing a high Al content ferritic stainless steel having excellent toughness, characterized in that the structure is an unrecrystallized structure.   2. The hot-rolled steel strip according to claim 1, further comprising V: 0.05 to 0.4% by mass.   The high toughness-excellent high Al according to claim 1 or 2, further comprising one or more of Ti: 0.02 to 0.2% by mass and Zr: 0.02 to 0.2% by mass. Hot rolled steel strip containing ferritic stainless steel. In mass%,
Cr: 12-30%,
Al: 3 to 8%,
Nb: containing 0.05 to 0.5%;
C: 0.025% or less,
N: 0.025% or less,
C + N: 0.030% or less;
Hot rolling is completed in a temperature range of 700 ° C. or higher and a recrystallization temperature Ts or lower of a slab of a high Al-containing ferritic stainless steel having a balance of Fe and inevitable impurities in a temperature range of a recrystallization temperature Ts or lower. A method for producing a hot-rolled high Al-containing ferritic stainless steel strip excellent in toughness, characterized in that the total draft R (%) is 15% or more, subsequently wound up into a coil, and subsequently cooled.
Here, Ts is calculated by the following equation.
Ts (° C.) = 900 + 20Al-2R
Al: mass% of Al   The method for producing a hot-rolled steel strip with high toughness and high Al-containing ferritic stainless steel according to claim 4, further comprising V: 0.05 to 0.4 mass%.   The high Al having excellent toughness according to claim 4 or 5, further comprising one or more of 0.02 to 0.2% by mass of Ti and 0.02 to 0.2% by mass of Zr. Production method of hot-rolled steel strip containing ferritic stainless steel. The method for producing a hot-rolled high-Al-content ferritic stainless steel strip according to any one of claims 4 to 6, wherein the coil has a coiling temperature of more than 500 ° C and less than 850 ° C. The method according to any one of claims 4 to 7, wherein the cooling is forcible cooling.

Images