JP2016191150A - Stainless steel plate with excellent toughness and method for producing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a high toughness stainless steel sheet for an exhaust pipe fastening component, having a Charpy impact value at a room temperature of 300 J/cmor more and a Charpy impact value at -40°C of 50 J/cmor more.SOLUTION: There is provided the stainless steel sheet for an exhaust pipe fastening component, excellent in toughness is provided which contains, by mass%, C:0.02% or less, N:0.02% or less, Si:0.005 to 1.0%, Ni:0.1 to 1.0%, Mn:0.1 to 3.0%, P:0.04% or less, S:0.0100% or less, Cr:10% or more and less than 18%, further one or two kind of Ti:0.05 to 0.30% and Nb:0.01 to 0.50% with the sum of Ti and Nb of 8(C+N) to 0.75% and the balance Fe with inevitable impurities, and in which γdefined by the Castro formula of (1) formula is 70% or more, ferrite particle diameter is 20 μm or less and martensite formation amount is 70% or less.SELECTED DRAWING: Figure 2

Description

  The present invention relates to a stainless steel plate having excellent toughness and a method for producing the same, and is suitably used as a stainless steel plate for exhaust pipe fastening parts that require particularly excellent toughness.

  Ferritic stainless steel sheets are used in a wide range of fields such as home appliances, kitchen equipment, and electronic equipment. For example, in recent years, from the viewpoint of rust prevention, conversion from plain steel to stainless steel plate is being studied as a material used for flanges and brackets (support parts) of automobiles and motorcycles. These parts are required to have high toughness in addition to corrosion resistance and strength in an exhaust environment and a fuel environment. However, although it is excellent in strength and corrosion resistance as compared with ordinary steel sheets, it is inferior in terms of toughness and cost, so its use has been limited. In particular, with regard to low toughness, cracks and plate breakage occur when the coil is rewound at room temperature or when cold rolling is performed, and in some cases, cold rolling may not be possible. In addition, there is a problem that cracks occur during punching and press molding during product processing.

  Ferrite steel also has a problem of low temperature toughness. Automobiles are used in a variety of environments, sometimes even at very low temperatures of -40 ° C. In such a low temperature environment, the toughness of ferritic steel having a body-centered cubic structure is significantly deteriorated.

  Several ideas have been made to solve the above-mentioned problems related to the toughness of ferritic stainless steel sheets. For example, a ferritic stainless steel that has been devised to control inclusions and improve toughness by adding Al as in Patent Document 1 is disclosed. However, these are based on the premise that they are high alloy steels containing 18% or more of Cr, and are not inventions related to low alloy steels to which the present invention is directed. In addition, an alloy having a large amount of addition of Cr, which is a ferrite stabilizing element, is less likely to undergo austenite phase transformation and is not suitable for refining the structure using phase transformation.

As in Patent Document 2, a high toughness ferritic stainless steel corresponding to a wide range of Cr: 10 to 25% is disclosed. However, these materials have a room temperature toughness of hot-rolled sheet of about 25 to 35 J / cm ² and are unsuitable for use as a flange material that requires high toughness. In addition, there is no knowledge of using phase transformation to improve toughness.

As in Patent Document 3, a ferritic stainless steel corresponding to a wide component of Cr: 10 to 20% and toughened by adding Cu is disclosed. However, these have no knowledge of utilizing phase transformation by hot-rolled sheet annealing to improve toughness, and the toughness is less than 300 J / cm ² .

  As in Patent Document 4, attempts have been made to increase toughness using the ferrite-austenite transformation in ordinary steel. However, it is not knowledge about stainless steel.

  There is also an attempt to increase toughness using austenite transformation in stainless steel as in Patent Document 5. However, since these need to contain much C in order to improve castability, if a martensite phase produces | generates, toughness will fall large.

JP 2011-179114 A JP 2012-140687 A JP 2012-188748 A JP-A-3-75309 International Publication No. WO2008-126944

Upon using the stainless steel as the exhaust pipe fastening parts, a Charpy impact value at room temperature and 300 J / cm ² or more, it is required to a Charpy impact value at -40 ℃ 50J / cm ² or more. Longitudinal cracks occur during secondary processing such as punching and pressing when manufacturing exhaust pipe fastening parts. In particular, toughness at low temperatures is important in order to prevent cracking in winter. In addition, when a car travels, there is a risk of cracking when the bounced pebbles hit the fastening parts, and particularly in cold regions, the risk increases, so room temperature and low temperature toughness are important.

  An object of the present invention is to provide a stainless steel plate that solves the problems of known techniques and is excellent in toughness at room temperature and low temperature, and particularly suitable for exhaust pipe fastening parts, and a method for producing the same. is there.

The gist of the present invention for solving the above problems is as follows.
(1) In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.005 to 1.0%, Ni: 0.1 to 1.0%, Mn: 0.00. 1 to 3.0%, P: 0.04% or less, S: 0.0100% or less, and Cr: 10% or more to less than 18%, Ti: 0.05 to 0.30%, Nb : 0.01 to 0.50% of 1 type or 2 types, the total of Ti and Nb is 8 (C + N) to 0.75%, the balance consists of Fe and inevitable impurities, γ _p A stainless steel plate excellent in toughness characterized by having a (gamma potential) of 70% or more and a ferrite grain size of 20 μm or less. Note that γ _p (%) is evaluated using the Castro equation (1).
γ _p = 420 (% C) +470 (% N) +23 (% Ni) +9 (% Cu) +7 (% Mn)
-11.5 (% Cr) -11.5 (% Si) -12 (% Mo) -23 (% V) -47 (% Nb)
-49 (% Ti) -52 (% Al) +189 (1)
In addition, (% X) shows the mass ratio of each component X.
(2) Further, in mass%, B: 0.0002 to 0.0030%, Al: 0.030 to 0.300%, Mo: 0.1 to 2.0%, Cu: 0.1 to 2. 0%, V: 0.05 to 1.00%, Sn: 0.005 to 0.500%, W: 0.005 to 3.00%, Co: 0.01 to 0.30%, Sb: 0 0.001 to 0.500%, REM: 0.001 to 0.200% of one type or two or more types, The stainless steel plate having excellent toughness according to (1).
(3) The stainless steel plate excellent in toughness according to (1) or (2), wherein the Charpy impact value at room temperature is 300 J / cm ² or more.
(4) The stainless steel plate excellent in toughness according to any one of (1) to (3), wherein the Charpy impact value at −40 ° C. is 50 J / cm ² or more.
(5) The stainless steel plate excellent in toughness according to any one of (1) to (4), wherein the Vickers hardness is 250 Hv.
(6) A method for producing a stainless steel plate having excellent toughness, characterized by hot rolling a stainless steel slab having the component composition described in (1) or (2), followed by hot rolling.
(7) In the hot rolling step, rough slab heating is performed at a slab heating temperature of 1100 to 1200 ° C, and finish rolling is performed so that the start temperature is 900 ° C or higher, the end temperature is 800 ° C or higher, and the difference is within 200 ° C. The method for producing a stainless steel plate having excellent toughness as described in (6), wherein winding is performed at 600 ° C. or higher.
(8) In the hot-rolled sheet annealing process, annealing is performed at a temperature condition within the upper limit temperature A ₁ point of the ferrite single phase and within ± 100 ° C. of the lower limit temperature A ₃ point of the austenite single phase, and then water-cooled. The manufacturing method of the stainless steel plate excellent in toughness as described in (6) or (7) characterized by making a structure | tissue. Incidentally, A ₁ point (℃) and A ₃ point (℃) is evaluated using equation (2), and (3).
A ₁ = 35 (% Cr) +60 (% Mo) +73 (% Si) +170 (% Nb) +290 (% V)
+620 (% Ti) +750 (% Al) +1400 (% B) -250 (% C) -280 (% N)
−115 (% Ni) −66 (% Mn) −18 (% Cu) +310 (2)
A ₃ = 35 (% Cr) +60 (% Mo) +73 (% Si) +170 (% Nb) +290 (% V)
+620 (% Ti) +750 (% Al) +1400 (% B) -250 (% C) -280 (% N)
-80 (% Ni) -31 (% Mn) -18 (% Cu) +360 (3)
In addition, (% X) shows the mass ratio of each component X.
(9) The stainless steel plate having excellent toughness according to any one of (1) to (5), which is used for exhaust pipe fastening parts.
(10) The method for producing a stainless steel plate having excellent toughness according to any one of (6) to (8), wherein the stainless steel is used for exhaust pipe fastening parts.

  ADVANTAGE OF THE INVENTION According to this invention, the stainless steel plate for exhaust pipe fastening components excellent in toughness can be provided efficiently, without introducing new equipment. Excellent toughness can be obtained even in a low temperature environment of −40 ° C.

It is a figure which shows the relationship between the crystal grain diameter of a product plate, and normal temperature (RT: 25 degreeC) Charpy impact value. It is a figure which shows the relationship between the crystal grain diameter of a product plate, and a low temperature (-40 degreeC) Charpy impact value. It is a figure which shows the relationship between (gamma) _p and normal temperature Charpy impact value of a product board. It is a figure which shows the relationship between (gamma) _p and a low temperature (-40 degreeC) Charpy impact value of a product board.

  There is a Charpy impact value as an index of toughness. One factor affecting toughness is the grain size of steel. The finer the structure, the better the steel Charpy impact value.

  In the present invention, a steel sheet with Ti added while reducing carbon is subjected to hot-rolled sheet annealing in a γ single-phase region or α + γ two-phase region, thereby miniaturizing the structure of the product plate and simultaneously reducing toughness. It has been found that toughness can be greatly improved by softening the causative martensite.

In the steel sheet of the present invention, the crystal grain size of the ferrite phase is adjusted to 20 μm or less. As shown in FIGS. 1 and 2, when the crystal grain size exceeds 20 μm, the toughness deteriorates significantly, so the upper limit was set to 20 μm. More desirably, it is 15 μm or less. In addition, as shown in FIG. 2, the low temperature impact value of the component removal comparative example in which the ferrite grain size is 20 μm or less is 50 J / cm ² or less. From this, it can be seen that in order to obtain high toughness, it is necessary to satisfy the present invention conditions not only in the particle diameter but also in the component range.

  Moreover, the steel plate of this invention adjusts the martensite production amount to 70% or less. When the amount of martensite produced exceeds 70%, deterioration of toughness becomes remarkable, so the upper limit was made 70%. More desirably, it is 50% or less.

  In addition, the influence of martensite on toughness is related not only to the amount of production but also to the hardness of the structure, and in order to obtain good toughness, the Vickers hardness of steel needs to be 250 Hv or less.

FIG. 3 shows an example of a hot-rolled annealed sheet deviated from the conditions of the present invention as a comparative example and the examples manufactured by the component conditions and process of the present invention, and shows the Charpy impact values at room temperature in terms of γ _p . the Charpy impact value in) to organize in gamma _p shown in FIG. steel sheet of the present invention that gamma _p is 70% or more, room temperature Charpy impact value 300 J / cm ² or more, and the Charpy impact value at -40 ℃ is 50 J / cm ² or more, with excellent toughness. Further, in FIG. 4, the impact value of the comparative component removal example in which γ _p is 70% or more is 50 J / cm ² or less, and not only γ _p but also the component range needs to satisfy the present invention conditions. I understand that.

  Next, the component range of steel will be described.

  C deteriorates moldability and corrosion resistance. When C is high, the amount of martensite produced exceeds the upper limit of the present invention, and when C dissolves in martensite, it hardens and deteriorates toughness. Therefore, the lower the content, the better. The upper limit is 0.02%. did. However, excessive reduction leads to an increase in refining costs, so the lower limit was made 0.001%. Furthermore, if considering the manufacturing cost and intergranular corrosion properties of the weld, 0.002 to 0.01% is desirable.

  N, like C, deteriorates formability and corrosion resistance, and also hardens when dissolved in martensite and deteriorates toughness. Therefore, the lower the content, the better. The upper limit was made 0.02%. However, excessive reduction leads to an increase in refining costs, so the lower limit was made 0.001%. Furthermore, if considering the manufacturing cost, workability and corrosion resistance, 0.005 to 0.015% is desirable.

  Si may be added as a deoxidizing element, and also improves oxidation resistance, but is a solid solution strengthening element, and excessive addition causes deterioration of toughness, so the upper limit was made 1.0%. . However, excessive reduction leads to an increase in refining costs, so the lower limit was made 0.005%. Furthermore, if considering the manufacturing cost and corrosion resistance, 0.2 to 0.5% is desirable.

  Ni is an austenite stabilizing element and is effective for refining crystal grains by phase transformation. It also promotes crevice corrosion suppression and repassivation. Since this effect appears at 0.1% or more, the lower limit was made 0.1%. However, excessive addition hardens and deteriorates moldability, and stress corrosion cracking tends to occur, so the upper limit was made 1.0%. In view of the raw material cost, 0.2% to 0.8% is desirable. More preferably, it is 0.2 to 0.5%.

  Mn, like Ni, is an austenite stabilizing element and is effective for refining crystal grains by phase transformation. It also contributes to improved scale adhesion and suppression of abnormal oxidation. Since this effect appears at 0.1% or more, the lower limit was made 0.1%. However, when added excessively, MnS is formed to lower the corrosion resistance, so the upper limit was made 3.0%. Furthermore, if considering the manufacturing cost and corrosion resistance, 0.1 to 2.0% is desirable.

  Since P is a solid solution strengthening element like Si, the lower the content, the better. Therefore, the upper limit was made 0.04%. Furthermore, if considering the manufacturing cost and corrosion resistance, 0.01 to 0.02% is desirable.

  Since S is an element that degrades corrosion resistance, the upper limit was made 0.01%. Furthermore, 0.0005 to 0.0050 is desirable in consideration of manufacturing cost and suppression of crevice corrosion when used as a part.

  Cr is an element that improves corrosion resistance and oxidation resistance. When considering the exhaust part environment, 10% or more is necessary from the viewpoint of suppressing abnormal oxidation. On the other hand, excessive addition of Cr causes hardening and deteriorates toughness. Further, since Cr is a ferrite stabilizing element, if added excessively, austenite phase transformation does not occur. Furthermore, from the viewpoint of cost increase, the upper limit was made less than 18%. In addition, when considering the sheet breakage and workability at the time of manufacturing the steel sheet due to the manufacturing cost and toughness deterioration, 10.5% or more and less than 15% are desirable.

  The present invention contains one or two of Ti: 0.05 to 0.30% and Nb: 0.01 to 0.50%, and the total of Ti and Nb is 8 (C + N) to 0.75%. The range.

  Ti is an element added to combine with C, N, and S to improve corrosion resistance, intergranular corrosion resistance, and deep drawability. In addition, when martensite is generated, there is an effect of softening by fixing C and N. Since the fixing action of C and N is expressed from 0.05%, the lower limit was made 0.05%. Moreover, since addition exceeding 0.3% hardens by solute Ti and toughness deteriorates, the upper limit was made 0.3%. Furthermore, if considering the manufacturing cost, 0.06 to 0.25% is desirable.

  Nb, like Ti, is an element added to improve the corrosion resistance, intergranular corrosion resistance, and deep drawability by combining with C, N, and S. In addition to improving workability and high-temperature strength, it is added as necessary to suppress crevice corrosion and promote repassivation. Since this effect appears at 0.01% or more, the lower limit was made 0.01%. However, excessive addition causes hardening and deteriorates moldability, so the upper limit was made 0.5%. Furthermore, if considering the manufacturing cost, 0.05 to 0.3% is desirable.

  Further, if the sum of Ti and Nb is less than 8 (C + N), excess C and N are dissolved in martensite and hardened, so the sum of Ti and Nb is 8 (C + N) or more. Furthermore, if the total of Ti and Nb exceeds 0.75%, solid solution Ti, solid solution Nb, and Nb and Ti carbonitrides and intermetallic compounds cause hardening and deteriorate toughness and formability. , Ti and Nb total is 0.75% or less. Here, Ti, Nb, C, and N all mean the content (mass%) of each element.

  This invention can contain the following elements further as needed.

  B is an element that improves the secondary workability of the product by segregating at the grain boundaries. In addition to suppressing vertical cracks during secondary processing of exhaust system parts, 0.0002% or more is preferably added in order to prevent cracks in winter. However, excessive addition causes a decrease in workability and corrosion resistance, so the upper limit was made 0.0030%. Furthermore, if considering the refining cost and ductility reduction, 0.0003 to 0.0015% is desirable.

  In addition to being added as a deoxidizing element, Al has an effect of suppressing oxide scale peeling. Since this effect is manifested at 0.03% or more, the lower limit was made 0.03%. On the other hand, addition of 0.3% or more causes a decrease in elongation, weld penetration and surface quality deterioration, so the upper limit was made 0.3%. Furthermore, if considering the refining cost and the pickling property at the time of manufacturing the steel sheet, 0.05 to 0.15% is desirable.

  Mo is an element that improves the corrosion resistance, and is an element that suppresses crevice corrosion, particularly when it has a crevice structure. Since this effect appears at 0.1% or more, the lower limit was made 0.1%. On the other hand, if it exceeds 2.0%, the moldability is remarkably deteriorated and the manufacturability is deteriorated. Considering the alloy cost and productivity, 0.1 to 0.5% is desirable.

  Cu, like Ni and Mn, is an austenite stabilizing element and is effective for refining crystal grains by phase transformation. Moreover, in order to promote suppression of crevice corrosion and repassivation, it adds as needed. Since this effect appears at 0.1% or more, the lower limit was made 0.1%. However, excessive addition hardens and deteriorates toughness and moldability, so the upper limit was made 2.0%. Considering alloy cost and productivity, 0.15 to 1.0% is preferable.

  V is added as necessary to suppress crevice corrosion. Since this effect appears from 0.05% or more, the lower limit was made 0.05%. However, since excessive addition hardens and deteriorates moldability, the upper limit was made 1.0%. In consideration of the raw material cost, 0.1 to 0.5% is desirable.

  Sn contributes to the improvement of corrosion resistance and high-temperature strength, so 0.005% or more is added as necessary. However, since addition of 0.50% or more may cause slab cracking during steel sheet production, the upper limit is made 0.50%. Furthermore, if considering refining costs and manufacturability, 0.003 to 0.30% is desirable.

  W contributes to improvement of corrosion resistance and high-temperature strength, so 0.005% or more is added as necessary. However, the addition of more than 3.0% hardens and leads to toughness deterioration and cost increase during the production of the steel sheet, so the upper limit is made 3.0%. Furthermore, if refining costs and manufacturing methods are taken into consideration, 0.01 to 0.10% is desirable.

  Co contributes to improving the high-temperature strength, so 0.01% or more is added as necessary. Since addition of more than 0.30% leads to toughness deterioration and cost increase at the time of manufacturing the steel sheet, the upper limit is made 0.30%. Furthermore, if refining costs and manufacturability are taken into consideration, 0.01 to 0.10% is desirable.

  Sb is an element that segregates at the grain boundary to increase the high temperature strength. Since this is expressed from 0.005% or more, the lower limit was made 0.005%. However, if it exceeds 0.50%, Sb segregation occurs and cracks occur during welding, so the upper limit is made 0.50%. Considering high temperature characteristics, production cost and toughness, 0.03 to 0.30% is desirable. More desirably, it is 0.05 to 0.20%.

  REM (rare earth element) is effective in improving oxidation resistance, and is added in an amount of 0.001% or more as necessary. Moreover, even if added over 0.20%, the effect is saturated and the corrosion resistance is lowered by sulfide of REM, so 0.001 to 0.20% is added. Considering the workability and manufacturing cost of the product, it is desirable that the lower limit is 0.002% and the upper limit is 0.10%. REM follows the general definition. It is a generic term for two elements of scandium (Sc) and yttrium (Y) and 15 elements (lanthanoid) from lanthanum (La) to lutetium (Lu). It may be added alone or as a mixture.

  Other components are not particularly defined in the present invention, but Ta and Hf may be added in an amount of 0.001% to 1.0% in order to improve the high temperature strength. Moreover, you may contain Bi 0.001 to 0.02% as needed. Note that it is desirable to reduce general harmful impurity elements such as As and Pb as much as possible.

The steel plate of the present invention is adjusted so that γ _p becomes 70% or more within the above-mentioned component range. When gamma _p is too low, ferrite - heating at high temperature austenite transformation is required, so that fine crystal grains becomes difficult, and the lower limit is 70%. γ _p (%) is evaluated using the Castro equation (1).
γ _p = 420 (% C) +470 (% N) +23 (% Ni) +9 (% Cu) +7 (% Mn)
-11.5 (% Cr) -11.5 (% Si) -12 (% Mo) -23 (% V) -47 (% Nb)
-49 (% Ti) -52 (% Al) +189 (1)
In addition, (% X) shows the mass ratio of each component X.

The stainless steel plate of the present invention contains the above-specified components, the ferrite grain size is 20 μm or less, and the martensite generation amount is 70% or less, so that the Charpy impact value at room temperature is 300 J / cm ² or more, By setting the Charpy impact value at −40 ° C. to 50 J / cm ² or more, it is possible to provide a stainless steel plate having excellent toughness at normal and low temperatures. Since it is excellent in toughness at both normal temperature and low temperature, it can be suitably used particularly for exhaust pipe fastening parts.

  Moreover, while containing the component prescribed | regulated by this invention and making a martensite production amount 70% or less, a Vickers hardness can be 250 Hv or less.

  Next, a manufacturing method will be described. The method for producing a steel sheet of the present invention comprises a step of annealing after steelmaking-hot rolling. In steelmaking, a method in which the steel containing the above essential components and components added as necessary is subjected to furnace melting followed by secondary refining. The molten steel is made into a slab according to a known casting method (continuous casting). The slab is heated to a predetermined temperature and is hot-rolled by continuous rolling to a predetermined plate thickness.

  The cast slab is heated at 1100 to 1200 ° C. From the viewpoint of furnace performance and economy, the upper limit temperature was set to 1200 ° C. When the heating temperature is too low, scale generation is reduced, and surface quality is deteriorated due to seizure of the rolling roll and steel material. Therefore, the slab heating temperature was set to 1100 to 1200 ° C. Furthermore, considering productivity and surface flaws, 1120 to 1160 ° C. is desirable.

After the slab heating, in the hot rolling process, multiple passes of rough rolling are performed, and finish rolling consisting of a plurality of stands is performed in one direction. After rough rolling, finish rolling is performed at a high speed and the coil is wound up. In the present invention, in order to obtain a fine structure at the time of winding, the finish rolling temperature and the winding temperature are defined. In order to prevent cracks and breaks during production, it is important to have a fine structure. For this reason, if the coiling temperature is too low, recrystallization and phase transformation do not occur at the time of coiling. Therefore, it is necessary to perform finish rolling at a high temperature and at a high speed. Therefore, finish rolling is performed so that the start temperature is 900 ° C. or higher, the end temperature is 800 ° C. or higher, and the difference is within 200 ° C. In addition, the coiling temperature is 600 ° C. or higher. The hot-rolled plate thickness may be appropriately selected according to the design of the fastening part, but is preferably about 2 to 15 mm in consideration of the winding shape, plate thickness accuracy, and surface properties. By using the above hot rolling conditions, the hot-rolled sheet normal-temperature impact value before annealing can be 50 J / cm ² or more, and cracks and breaks during production can be prevented.

In the present invention, at the time of hot-rolled sheet annealing, it is important to raise the temperature to the γ single-phase or α + γ two-phase region and to partially transform all or part of the structure made of ferrite to austenite. Therefore, annealing is performed at a temperature of A ₁ point or higher during annealing and a temperature condition within ± 100 ° C. of A ₃ point. At the time of annealing, it is necessary to anneal at least A ₁ point or more in order to cause phase transformation. In addition, if the temperature is too low at this time, the amount of γ transformation decreases, and it is necessary to perform annealing for a long time, so the lower limit of the hot-rolled sheet annealing temperature is the higher of A ₁ point or A ₃ point-100 ° C. did. Thereby, the ferrite phase can be sufficiently transformed into the austenite phase. Moreover, there is a possibility that the toughness deteriorates even when recrystallization does not occur sufficiently by annealing at a low temperature and the hot rolled structure remains as it is.

When hot-rolled sheet annealing is performed at an excessively high temperature, the crystal grains are coarsened, martensite is excessively generated, and the toughness is deteriorated. Therefore, the upper limit was set to A ₃ point + 100 ° C. That is, it contains components specified in the present invention, the hot-rolled sheet annealing temperature by an A ₃ point + 100 ° C. a temperature below the martensite amount may be 70% or less.

The ferrite grain size is 20 μm or less by containing the components specified in the present invention, particularly by setting γ _p in the formula (1) to 70% or more and the hot-rolled sheet annealing temperature to a temperature within the range of the present invention. can do.
The A ₁ point (° C.) and the A ₃ point (° C.) are evaluated using the Castro equation (2) and the equation (3).
A ₁ = 35 (% Cr) +60 (% Mo) +73 (% Si) +170 (% Nb) +290 (% V)
+620 (% Ti) +750 (% Al) +1400 (% B) -250 (% C) -280 (% N)
−115 (% Ni) −66 (% Mn) −18 (% Cu) +310 (2)
A ₃ = 35 (% Cr) +60 (% Mo) +73 (% Si) +170 (% Nb) +290 (% V)
+620 (% Ti) +750 (% Al) +1400 (% B) -250 (% C) -280 (% N)
-80 (% Ni) -31 (% Mn) -18 (% Cu) +360 (3)
In addition, (% X) shows the mass ratio of each component X.

  By cooling with water after annealing, the effect of preventing the growth of crystal grains during phase transformation from austenite to ferrite is exhibited, so that the microstructure can be obtained and high toughness can be obtained.

  Steel No. 1 in Table 1 Steels having the composition shown in 1-31 were melted and cast into slabs, hot-rolled to 10 mm, and then subjected to hot-rolled sheet annealing. Each steel was manufactured under the manufacturing conditions shown in Tables 2-1 and 2-2. After hot-rolled sheet annealing, water cooling was performed except for B23 in Table 2-2. B23 was air-cooled.

  The crystal grain size of the ferrite phase was measured by an EBSD (Electron Back Scattering Diffraction) method. The amount of martensite produced was measured by EBSD and image analysis. The measurement conditions of the particle size are the conditions of 0.3 to 0.6 μm step at a measurement magnification of 1000 times, and the obtained data is set to a grain boundary with an orientation difference of 15 ° or more by TIM OIM (Orientation Imaging Microscopy) analysis software. Two grain boundaries were set and the equivalent circle diameter was calculated. The value obtained by arithmetic average of the obtained equivalent circle diameter was defined as the crystal grain size. Similarly, for the amount of martensite produced, the fraction of ferrite phase and martensite was quantitatively measured using OIM.

  The toughness of the hot-rolled annealed sheet is measured at 25 ° C. and −40 ° C. in accordance with JIS Z 2242, evaluated by Charpy impact value, and the “product plate characteristics / impact value” in Table 2-1 and Table 2-2. Shown in the column. As a test piece, a V-notch Charpy test piece was taken in parallel with the rolling direction and used for the test.

A1 to A16 in Table 2-1 are examples of the present invention. It is clear that the steel of the example of the present invention has a high Charpy impact value of the hot-rolled annealed sheet at both room temperature and low temperature, and is excellent in toughness. Moreover, the normal temperature Charpy impact value of a hot-rolled sheet is also shown. It is confirmed that the steel of the present invention has high manufacturability such as hot rolled sheet having a normal temperature impact value of 50 J / cm ² or more, excellent toughness, and prevention of cracking during sheet passing.

  B1 to B23 in Table 2-2 are comparative examples. First, the results of the product plate (hot rolled annealed plate) will be described.

In B1 to B5 and B12, γ _p is out of the range of the present invention, and in B15, B21, and B22, the Nb, Co, and V contents are out of the upper limits, respectively, and the ferrite grain size is out of the upper limit and the toughness is lowered. As in B1 and B2 and B3 and B4, the toughness was insufficient even if the hot-rolled sheet annealing was simply reduced and the grain growth was reduced by suppressing the crystal grain growth.

  Both B6 and B7 are steel Nos. B6 having a ferrite single-phase structure annealed within the annealing condition range with the C content exceeding 19, the low temperature toughness was lowered, annealing was performed above the upper limit of the annealing condition, and a large amount of hard martensite was generated. As for B7, toughness fell in both normal temperature and low temperature. From this, it can be seen that it is important to set the C amount within an appropriate range regardless of the structure of ferrite, martensite, or the like.

  Since B8 is not subjected to hot-rolled sheet annealing and is a structure as it is hot-rolled, both the ferrite grain size martensite generation amount cannot be measured and the toughness is low.

  For both B9 and B10, the hot-rolled sheet annealing temperature deviated from the upper limit, and for both B9 and B10, the martensite generation amount deviated and the toughness decreased. In addition, since B9 is annealed at a higher temperature, a larger amount of martensite is generated and the crystal grain size is large.

  In B11, the hot-rolled sheet annealing temperature deviated from the lower limit, the phase transformation from ferrite to austenite did not occur, and the refinement effect did not occur. Moreover, since it was low-temperature annealing, recrystallization did not occur and almost all the processed structure of the hot-rolled sheet remained, so the ferrite grain size was not measurable, and the toughness decreased at both room temperature and low temperature.

  B13 and B20 have N and W contents outside the upper limits, respectively, and B14 has a total of Ti and Nb of 8 (C + N), both of which have a hardness exceeding 250 Hv, and have one or both toughness at normal temperature and constant temperature. Declined. In B16 to B19, the Cu, Ni, Si, and Mo contents deviated from the upper limit, respectively, and the toughness of one or both of room temperature and low temperature was lowered.

  The results of the hot rolled sheet before annealing in the comparative example will be described. In B15, the finish rolling was not performed at a high speed, and the difference between the finish rolling start temperature and the finish temperature exceeded 200 ° C., so the hot rolled sheet impact value was low.

  Since B23 was air-cooled after hot-rolled sheet annealing, the crystal grains grew excessively and exceeded 20 μm, so the toughness was reduced.

According to the present invention, it is possible to produce a ferritic stainless steel plate having excellent toughness, and the toughness of the hot-rolled material is also good. improves. In addition, when the material to which the present invention is applied is used for exhaust pipe fastening parts, the degree of freedom in molding is improved, and when it is subjected to cracks during press processing or impacts in a low temperature environment when mounted on an actual vehicle. It becomes possible to suppress cracking. That is, the present invention is extremely useful industrially.

Claims (10)

In mass%, C: 0.02% or less, N: 0.02% or less, Si: 0.005-1.0%, Ni: 0.1-1.0%, Mn: 0.1-3 0.0%, P: 0.04% or less, S: 0.0100% or less, Cr: 10% to less than 18%, Ti: 0.05-0.30%, Nb: 0.01 -0.50% 1 or 2 types, the total of Ti and Nb is 8 (C + N)-0.75%, the balance consists of Fe and inevitable impurities, γ _p is 70% or more And the stainless steel plate excellent in the toughness characterized by a ferrite particle size being 20 micrometers or less and a martensite production amount becoming 70% or less. Note that γ _p (%) is evaluated using the equation (1).
γ _p = 420 (% C) +470 (% N) +23 (% Ni) +9 (% Cu) +7 (% Mn)
-11.5 (% Cr) -11.5 (% Si) -12 (% Mo) -23 (% V) -47 (% Nb)
-49 (% Ti) -52 (% Al) +189 (1)
In addition, (% X) shows the mass ratio of each component X.   Furthermore, in mass%, B: 0.0002 to 0.0030% or less, Al: 0.030 to 0.300% or less, Mo: 0.1 to 2.0% or less, Cu: 0.1 to 2. 0%, V: 0.05 to 1.00%, Sn: 0.005 to 0.500%, W: 0.005 to 3.00%, Co: 0.01 to 0.30%, Sb: 0 The stainless steel plate excellent in toughness according to claim 1, comprising one or more of 0.005 to 0.500% and REM: 0.001 to 0.200%. The stainless steel plate excellent in toughness according to claim 1 or ² , wherein a Charpy impact value at room temperature is 300 J / cm ² or more. The stainless steel plate excellent in toughness according to any one of claims 1 to 3, wherein a Charpy impact value at -40 ° C is 50 J / cm ² or more.   The stainless steel plate excellent in toughness according to any one of claims 1 to 4, wherein the Vickers hardness is 250 Hv or less.   A method for producing a stainless steel plate having excellent toughness, comprising hot-rolling a slab of stainless steel having the component composition according to claim 1 or 2 and then annealing the slab.   In the hot rolling step, rough rolling is performed with a slab heating temperature of 1100 to 1200 ° C., and finish rolling is performed so that the start temperature is 900 ° C. or more, the end temperature is 800 ° C. or more, and the difference is within 200 ° C., 600 ° C. The method for producing a stainless steel plate having excellent toughness according to claim 6, wherein winding is performed as described above. In the hot-rolled sheet annealing process, the microstructure is obtained by annealing at a temperature not lower than the upper limit temperature A ₁ point of the ferrite single phase and within ± 100 ° C. of the lower limit temperature A ₃ point of the austenite single phase, and then water cooling. The manufacturing method of the stainless steel plate excellent in toughness of Claim 6 or Claim 7 characterized by the above-mentioned. Incidentally, A ₁ point (℃) and A ₃ point (℃) is evaluated using equation (2), and (3).
A ₁ = 35 (% Cr) +60 (% Mo) +73 (% Si) +170 (% Nb) +290 (% V)
+620 (% Ti) +750 (% Al) +1400 (% B) -250 (% C) -280 (% N)
−115 (% Ni) −66 (% Mn) −18 (% Cu) +310 (2)
A ₃ = 35 (% Cr) +60 (% Mo) +73 (% Si) +170 (% Nb) +290 (% V)
+620 (% Ti) +750 (% Al) +1400 (% B) -250 (% C) -280 (% N)
-80 (% Ni) -31 (% Mn) -18 (% Cu) +360 (3)
In addition, (% X) shows the mass ratio of each component X.   The stainless steel plate having excellent toughness according to any one of claims 1 to 5, wherein the stainless steel is used for exhaust pipe fastening parts. The said stainless steel is a manufacturing method of the stainless steel plate excellent in toughness of any one of Claims 6-8 used for exhaust pipe fastening components use.

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