ES2787353T3 - Ferritic stainless steel sheet that has excellent heat resistance - Google Patents

Abstract

A method of producing a hot rolled annealed ferritic stainless steel, the ferritic stainless steel being characterized by consisting of, in mass%, Cr: 13.0 to 21.0%, Sn: 0.01 to 0.50 %, Nb: 0.05 to 0.60%, C: 0.001 to 0.015%, Si: 0.01 to 1.5%, Mn: 0.01 to 1.5%, N: 0.001 to 0.020%, P : 0.035% or less, S: 0.015% or less, and optionally contains one or more of, in mass%, Ti: 0.32% or less, Ni: 1.5% or less, Cu: 1.5% or less, Mo: 2.0% or less, V: 0.3% or less, Al: 0.3% or less, B: 0.0020% or less, W: 0.20% or less, Zr: 0.20% or less, Sb: 0.5% or less, Co: 0.5% or less, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.1% or less, and containing a Fe residue and unavoidable impurities, wherein the production method of hot rolled annealed ferritic stainless steel includes a step of performing a heat treatment at a temperature of 600 to 750 ° C so that a displayed L value by formula 3 become 1.91 × 104 or more, the ferritic stainless steel reco hot rolled acid satisfying Formula 1 'and Formula 2', and having a grain boundary Sn concentration of 2% or less where the ductile-brittle transition temperature of hot rolled annealed sheet having a thickness of 4.0 mm is 150 ° C or less; ** (See formula) ** where, T: temperature (° C), t: time (h).

Description

DESCRIPTION

Ferritic stainless steel sheet that has excellent heat resistance

Technical field

The present invention relates to a material for sheet metal structure which is used at a high temperature, in particular it relates to ferritic stainless steel which exhibits resistance to corrosion at normal temperature and which is resistant to brittleness due to use at a temperature high, such as a material for an automobile exhaust system.

Background of the technique

Ferritic stainless steel is inferior to austenitic stainless steel in workability, toughness, and high temperature resistance, but it does not contain a large amount of Ni and is therefore inexpensive. In addition, it has a small heat expansion, so in recent years it has been used for roofing and other building materials or materials for parts of automobile exhaust systems that have high temperatures and other applications where thermal stress becomes a problem. In particular, when used as a material for parts of automobile exhaust systems, high temperature resistance, normal temperature corrosion resistance, and high toughness associated with high temperature use are important. In general, SUH409L, SUS429, SUS430LX, SUS436J1L, SUS432, SUS444 and other steels are used as suitable ferritic stainless steel for these applications.

In these materials, PLT 1 describes a material that uses 0.05 to 2% Sn to increase resistance to high temperatures. Furthermore, PLT 2 describes the technique of adding 0.005 to 0.10% Sn to improve the surface quality of stainless steel sheet. Furthermore, in recent years, scrap containing surface treated steel plate has been used as a raw material, thus large amounts of Sn greater than 0.05% have been included in stainless steel as unavoidable impurities.

Documents EP 17434143 and JP 2006233278 describe a method of producing ferritic stainless steel for exhaust parts.

Appointment list

Patent bibliography

PLT 1: Japanese Patent Publication No. 2000-169943A

PLT 2: Japanese Patent Publication No. H11-92872A

Compendium of the invention

Technical problem

If the Sn-containing stainless steel described in the prior art is used at a high temperature, the previously unknown grain boundary brittle phenomenon is known to occur and the problem arises that the strength of the parts is affected. An object of the present invention is to provide ferritic stainless steel that does not deteriorate in toughness at normal temperature, even if it is exposed to high temperature for a long period of time, as in a material for an automobile exhaust system.

Solution to the problem

The inventors undertook several studies on the drop in toughness at normal temperature of ferritic stainless steel containing Sn after prolonged exposure to high temperatures. First, they investigated the temperature range in which a drop in toughness occurs when using SUS430LX containing 0.3% Sn, and found that the temperature range was 500 to 800 ° C. In addition, particularly, the temperature at which a drop in toughness occurred in a short time was 700 ° C, and it was known that a large drop in toughness occurred in just 1 hour. As shown in fig. 1, The fracture surface mode that occurs due to brittle fracture differs from a general split fracture surface and has the characteristic of a grain boundary fracture surface. The inventors cooled a sample to a low temperature in an AES (Auger Electron Spectroscopy) apparatus, then broke it and analyzed the fracture surface of the grain boundary, and a remarkable segregation of Sn was observed with a thickness of about 1 nm. That is, the drop in toughness due to long-term high-temperature use was believed to have occurred due to segregation of the Sn grain boundaries.

To prevent such brittleness of the grain boundary, lowering the Sn content is most effective. However, recycling of surface treated steel sheets is unavoidable for environmental protection, so scrap containing Sn should be used. Furthermore, removing Sn by refining is difficult for the existing technique. A material that is resistant to grain boundary brittleness has been desired, even if it contains Sn.

Therefore, the inventors investigated in detail the effects of various types of alloying elements to avoid brittleness due to segregation of Sn grain boundaries and found that to ensure corrosion resistance, stabilizing elements Ti and Nb that are added to immobilize C and N in stainless steel have a significant effect. That is, as shown in fig. 1 and 2, found that if Ti-stabilized steel contains Sn, the grain boundary brittleness associated with high temperature use becomes noticeable and that Nb-stabilized steel is brittle resistant even if it contains Sn.

Based on these findings, the inventors investigated the effects on toughness by adding the stabilizer elements Ti and Nb alone and adding them together and were able to develop steel resistant to drop in toughness due to high temperature use.

The present invention was achieved based on these findings. The solution to the problem of the present invention is as in the appended claims.

Advantageous effects of the invention

According to the Sn-containing ferritic stainless steel of the present invention, the stabilizing elements Nb and Ti are optimized, whereby a stainless steel sheet is obtained which has little deterioration of toughness even when used at high temperature and is also excellent. in corrosion resistance.

Brief description of the drawings

Fig. 1 shows photos of ferritic stainless steels of the present embodiment and comparative steels that are 4.0 mm thick hot rolled annealed sheets as such, and shows photos of fractured surfaces of test pieces showing brittle fracture in a Charpy impact test for ferritic stainless steels of the present and comparative embodiment after heat treatment at 700 ° C for 1 hour.

Fig. 2 is a graph showing ductile-brittle transition temperatures measured by conducting V-notch Charpy impact tests on 4.0 mm size test pieces for ferritic stainless steels of the present embodiment and comparative steels that are 4.0 mm thick hot rolled annealed sheets as such, and showing the ductile-brittle transition temperatures measured by conducting the Charpy V-notch impact tests on the test pieces for ferritic stainless steels of the present realization and comparative steels after heat treatment at 700 ° C for 1 hour.

Fig. 3 is a graph showing the relationship between a ductile-brittle transition temperature (DBTT) measured by conducting Charpy V-notch impact tests on 4.0 thickness size test pieces. mm and an indicator (GBSV) showing the tendency of segregation of the grain boundary of Sn when using ferritic stainless steels of the present embodiment and comparative steels which are hot rolled annealed sheets of thickness 4.0 mm and treatment additional heat of ferritic stainless steels at 700 ° C for 1 hour.

Fig. 4 is a graph showing the relationship between the concentration of Sn at the grain boundary and the ductile-brittle transition temperature (DBTT) by measuring the Sn concentration at the fracture surface of the grain boundary using AES and measuring the DBTT by a Charpy impact test and using ferritic stainless steels of the present embodiment and comparative steels which are 4.0 mm thick hot rolled annealed sheets and additional heat treatment of the ferritic stainless steels at 700 ° C for 1 hour.

Description of the realization

Next, an embodiment of the present invention will be explained. First, reasons for limiting the steel composition of the stainless steel sheet of the present embodiment will be explained. Note that the% indications for the composition mean% by mass unless otherwise indicated.

C: 0.001 to 0.015%

The C causes the formability, the corrosion resistance and the toughness of the hot rolled sheet to deteriorate, whereby the content is preferably as small as possible. Therefore, the upper limit becomes 0.015%. However, the excessive reduction causes an increase in the refining cost, so the lower limit is 0.001%. Also, if it is considered from the viewpoint of corrosion resistance, the lower limit is preferably made 0.002% and the upper limit is preferably 0.009%.

N: 0.001 to 0.020%

N, like C, causes the formability, corrosion resistance and toughness of hot rolled sheet to deteriorate, so the lower the content the better. Therefore, the content is made 0.02% or less. However, the excessive reduction leads to an increase in the refining cost, so the lower limit becomes 0.001%. To more reliably avoid a drop in corrosion resistance and deterioration of toughness, the upper limit is preferably set to 0.018%. More preferably, the upper limit can be made 0.015%.

Yes: 0.01 to 1.5%

The excessive addition of Si causes a drop in ductility at ordinary temperature, so the upper limit becomes 1.5%. However, Si is an element that is useful as a deoxidizing agent and is an element that improves resistance to high temperatures and resistance to oxidation. The deoxidizing effect is enhanced along with the increase in the amount of Si. The effect manifests itself at 0.01% or more and stabilizes at 0.05% or more, so the lower limit becomes 0.01%. Note that if oxidation resistance is considered when adding Si, the lower limit is more preferably 0.1% and the upper limit is more preferably 0.7%.

Mn: 0.01 to 1.5%

Excessive addition of Mn causes a drop in toughness of hot rolled sheet due to precipitation of the y-phase (austenite phase), and further forms MnS to cause a drop in corrosion resistance, so the upper limit becomes 1.5%. On the other hand, Mn is an element that is added as a deoxidizing agent and an element that contributes to increased resistance to high temperatures in the medium temperature region. In addition, it is an element by which during long-term use, Mn-based oxides are formed on the surface and contribute to the effect of suppressing the adhesion of scale (oxides) and abnormal oxidation. To make this effect manifest, Mn is added so that the Mn content in the stainless steel of the present invention becomes 0.01% or more. Note that if the high temperature ductility or adhesion property of scale and suppression of abnormal oxidation is considered, the lower limit is more preferably made 0.1 and the upper limit is more preferably made 1.0 %.

P: 0.035% or less

P is an element with a great capacity to strengthen the solution, but it is a stabilizing element of ferrite and it is also a detrimental element for corrosion resistance and toughness, so the content is preferably as small as possible. P is contained as an impurity in the ferrochrome material of stainless steel. Removal of P from the stainless steel melt is extremely difficult, so 0.010% or more is acceptable. Furthermore, the content of P is substantially determined by the purity and amount of the ferrochrome material used. The P content in the ferrochrome material is preferably low, but the low P-containing ferrochrome is expensive, so the content is set in a range that does not cause the quality or corrosion resistance to be greatly deteriorated , that is, 0.035% or less. Note that the content is preferably 0.030% or less.

S: 0.015% or less

S forms sulfur-based inclusions and deteriorates the general corrosion resistance of steel material (general corrosion or pitting corrosion). Therefore, the content of S is preferably as small as possible. Considering a range that does not affect corrosion resistance, the upper limit becomes 0.015%. Also, the lower the S content, the better the corrosion resistance, but to reduce the S, the desulfurization load increases and the manufacturing cost increases, so the lower limit can be 0.001%. Note that preferably the lower limit becomes 0.001% and the upper limit becomes 0.008%.

Cr: 13.0 to 21.0%

Cr is an essential element to ensure oxidation resistance and corrosion resistance in the present invention. If it is less than 13.0%, these effects are not manifested, while if it is greater than 21.0%, there is a drop in the ability to be worked or deterioration of toughness, so the lower limit is made 13.0 and the upper limit becomes 21.0%. Also, if the manufacturability and high temperature ductility are considered, the upper limit is preferably made 18.0%.

Sn: 0.01 to 0.50%

Sn is an effective element to improve resistance to corrosion or resistance to high temperatures. In addition, it also has the effect of not causing a great deterioration of mechanical properties at normal temperature. The effect on corrosion resistance manifests itself at 0.01% or more, so the lower limit becomes 0.01%. The contribution to the high temperature resistance is stably manifested with the addition of 0.05% or more, so that the preferable lower limit is made 0.05%. On the other hand, if it is added in excess, the manufacturability and weldability deteriorate markedly, so the upper limit becomes 0.50%. Take into account that, if oxidation resistance, etc. is considered, the lower limit is preferably 0.1%. Also, if weldability, etc. is considered, the upper limit is preferably made 0.3%. The manifestation of the brittle phenomenon in the use at high temperatures becomes noticeable by the inclusion of Sn: 0.05% or more, but by adding together Nb as explained below, the brittle phenomenon due to the inclusion of Sn can be suppressed. Furthermore, to make the DBTT (ductile-brittle transition temperature) lower than 50 ° C, the upper limit of the Sn content is more preferably made 0.21%.

Nb: 0.05 to 0.60%

Nb is a carbonitride-forming element and therefore has the effect of suppressing sensitization due to the precipitation of chromium carbonitrides in stainless steel and the drop in corrosion resistance. The effect manifests itself at 0.05% or more. Furthermore, the inventors found the fact that this also has the effect of suppressing the grain boundary brittleness in Sn-containing steel. The two effects of improving the corrosion resistance and suppressing the brittleness of the grain boundary are manifested at 0.05% or more, so the lower limit becomes 0.05%. To obtain the effects more reliably, the content is preferably made 0.09% or more. If 0.2% or more, the effects can be obtained substantially reliably. On the other hand, excessive addition causes the problem of a drop in manufacturability due to the formation of Laves phases. Taking these into account, the upper limit of Nb became 0.60%. Also, from the point of view of weldability and the ability to be worked as a sheet, the lower limit is sometimes made 0.3% and the upper limit is sometimes made 0.5%. Furthermore, the effect of suppressing grain boundary brittleness in Sn-containing steel can be obtained even by the joint addition of Ti and Nb. Also in this case, the effects are obtained with an addition amount of Nb of 0.05% or more. However, in both the single sum of Nb and the joint sum of Ti and Nb, the CI value explained below must be adjusted to fall within a predetermined range.

CI = (Ti 0.52Nb) / (CN) is set to not less than 8 to not more than 26. If it does not contain Ti, CI = 0.52Nb / (CN) is set to not more than 8 to not less than 26. Ti and Nb form carbonitrides and suppress the drop in corrosion resistance due to chromium carbonitride formation and sensitization. That is, addition amounts corresponding to the amounts of C and N in the steel are necessary. The CI value is an indicator to make C and N in steel precipitate as carbonitrides of Ti and Nb and suppress sensitization. The higher the CI value, the more sensitization is suppressed. To stably suppress the precipitation of chromium carbonitrides even in a cycle of welding heat, etc., the IC should be 8 or more. However, if Ti and Nb are added excessively, they form large inclusions and reduce the workability, so the IC is 26 or less. To stably ensure corrosion resistance and workability, CI is preferably set from 10 to 20.

Furthermore, in the present invention, GBSV = Sn Ti-2Nb-0.3Mo-0.2 is set to 0 or less. When it does not contain Ti and Mo, GBSV = Sn-2Nb-0.2 is set to 0 or less. The GBSV is an indicator that shows the tendency to segregation of the Sn grain boundaries. The higher the value, the more noticeable is the segregation of the grain boundary. The coefficients of the elements that make up the GBSV are to evaluate the effects on the segregation of grain boundaries. Sn is an element that is effective for high temperature resistance and corrosion resistance, but segregation of grain boundaries causes the material strength to drop to 400 ° C or less. On the other hand, Nb and Mo not only have actions to suppress the segregation of the grain boundary of Sn, but also effects of raising the strength of the grain boundary and have actions to suppress the brittleness due to the segregation of the grain boundary of Sn . As shown in fig. 3, it can be found that along with a drop in GBSV, the ductile-brittle transition temperature is lowered and that if the GBSV becomes 0 or less, the ductile-brittle transition temperature of a thick hot rolled annealed sheet 4.0mm becomes 150 ° C or less and that hardness is greatly improved. Therefore, the GBSV is set to 0 or less.

Next, the inventors used the concentration of Sn at the grain boundary fracture surface (in%) as an indicator of the segregation of the Sn grain boundary to investigate the relationship with the brittle ductile transition temperature. As shown in fig. 4, it was found that if the concentration of Sn in the grain boundaries exceeds 2.0%, the ductile-brittle transition temperature increases rapidly and the grain boundary brittleness easily occurs. Also in a high temperature service environment, it is important to make the Sn concentration at the grain boundaries 2.0% or less to suppress the brittleness of the grain boundary due to Sn.

Here, as an indicator that treats the temperature and time in a standardized way in the case of using at a high temperature for a long time, the L value was entered, which is generally used as an indicator for the evaluation of heat treatment and is displayed by formula 3. If you heat treat from 600 to 750 ° C so that the L value shown by formula 3 becomes 1.91 x 104 or more, you will see a noticeable segregation of Sn at the grain boundaries in the case of the addition of Ti. The inventors found that Sn segregation at grain boundaries has a detrimental effect on properties (transition temperature). In addition, the inventors confirmed that, in the case of the component composition in the present invention, the concentration of Sn at the grain boundary when performing a heat treatment giving an L value of 1.91 x 104 or more becomes by 2 in% or less. Take into account that, as a condition that further simplifies the provision under heat treatment conditions by the L value, the concentration of Sn at the grain boundary after performing the heat treatment at 700 ° C for 1 hour is preferably 2 , 0 in% or less.

The concentration of Sn at the grain boundaries is fractured and measured on an AES apparatus in an ultra-high vacuum. Auger electrons are emitted not only from atoms on the surface, but also at several nm inside the surface, so the value does not show only the concentration of Sn at the grain boundaries. Furthermore, the precision of the analysis differs with each device. However, in principle, the Sn concentration at the cleavage fracture surface is the same as the average Sn concentration of the base material. Therefore, the Sn concentration at the grain boundaries has been determined by calibrating the measurement values of the Sn concentration at the cleavage fracture surface so that the Sn concentration measured at the cleavage fracture surface is Convert to the average Sn concentration of the base material. To stably reduce the brittleness of the grain boundary, it is preferable to make the Sn concentration at the grain boundaries 1.7% or less. Also, making the concentration lower than the Sn concentration in the base material is difficult, so it is preferable to make 0.02% of the lower limit.

Furthermore, in the present invention, in addition to the above elements, it is preferable to add one or more of Ti: 0.32% or less, Ni: 1.5% or less, Cu: 1.5% or less, Mo: 2 , 0% or less, V: 0.3% or less, Al: 0.3% or less, and B: 0.0020% or less.

Ti: 0.32% or less

Ti, like Nb, is a carbonitride forming element and therefore has the effect of suppressing sensitization due to precipitation of chromium carbonitrides in stainless steel and decreased corrosion resistance. However, compared to Nb, this has a greater effect by exacerbating the brittleness of the grain boundary in steel containing Sn, so in steel containing Sn, this is an element that must be decreased. The effect on the segregation of the Sn grain boundary is manifested when the Ti content exceeds 0.05%. However, when Nb is included, it is possible to reduce the detrimental effect due to Ti. By adding it together with Nb, it was confirmed that if the upper limit was 0.32%, the grain limit concentration of Sn became 2.0% or less even in the above heat treatment. The preferable upper limit including Nb is 0.15%. Keep in mind that this enters from the starting materials as an unavoidable impurity, so excessive reduction is difficult, so the Ti content is preferably made 0.001% or more. From the viewpoint of improving workability by reducing inclusions, the lower limit is made more preferably 0.001 and the upper limit is more preferably 0.03%.

Ni: 1.5% or less

Ni enters the alloy materials of ferritic stainless steel as an unavoidable impurity and is generally contained in an amount in the range 0.03 to 0.10%. In addition, it is an element that is effective in suppressing the progression of bites. That effect is stably exhibited by adding 0.05% or more, whereby the lower limit is preferably made 0.05%. More preferably, the lower limit is 0.1%.

On the other hand, adding a large amount can cause the material to harden due to the strengthening of the solution, so the upper limit is made 1.5%. Note that if the cost of the alloy is considered, the upper limit is preferably 1.0%. More preferably the upper limit is 0.5%. Because of this, Ni is suitably 0.1 to 0.5%.

In the present invention, Ni is an element that improves corrosion resistance due to the synergistic effect with Sn. Joint addition with Sn is helpful. Furthermore, Ni has the action of reducing the drop in workability (elongation and r-value) that accompanies the addition of Sn. When added together with Sn, the lower limit of Ni is preferably made 0.2 and the upper limit is preferably 0.4%.

Cu: 1.5% or less

Cu is effective in improving corrosion resistance. In particular, it is effective in reducing the rate of crack corrosion progression after crack corrosion occurs. To improve the corrosion resistance, the inclusion of 0.1% or more is preferable. However, the excessive addition causes a deterioration of the ability to be worked. Therefore, Cu is preferably included with a lower limit of 0.1 and an upper limit of 1.5%. Cu is an element that improves corrosion resistance due to a synergistic effect with Sn. Joint addition with Sn is helpful. Furthermore, Cu has the action of reducing the drop in workability (elongation and r-value) that accompanies the addition of Sn. When this is added together with Sn, Cu is preferably included with a lower limit of 0.1 and an upper limit of 0.5%.

Due to the above, in the present invention, the joint addition of Sn and Ni and / or Cu is useful to improve corrosion resistance.

Also, Cu is an element that is required to raise the high temperature resistance that is used for use as a member for a high temperature environment such as a high temperature exhaust system of an automobile. Cu primarily exhibits a precipitation strengthening ability at 500 to 750 ° C and acts to suppress plastic deformation of the material and raise resistance to thermal fatigue by strengthening the solution at temperatures above that. Such action of hardening by precipitation of Cu or strengthening of the solution is manifested by the addition of 0.2% or more. On the other hand, excessive addition becomes a cause of abnormal oxidation and surface defects at the time of heating for hot rolling, so the upper limit becomes 1.5%. To make use of the high temperature reinforcing ability of Cu and stably suppress surface defects, the lower limit is preferably made 0.5 and the upper limit is preferably 1.0%.

Mo: 2.0% or less

Mo should be added as necessary to improve high temperature resistance and resistance to thermal fatigue. To exhibit these effects, the lower limit is preferably made 0.01%.

On the other hand, excessive addition can cause the formation of wash phases and a drop in the hardness of the hot rolled sheet. Considering this, the upper limit of Mo becomes 2.0%. Furthermore, from the viewpoint of productivity and manufacturability, the lower limit is preferably made 0.05% and the upper limit is preferably 1.5%.

V: 0.3% or less

V enters the alloying material of ferritic stainless steel as an unavoidable impurity and is difficult to remove in the refining process, so it is generally contained in a range of 0.01 to 0.1%. In addition, it forms fine carbonitrides and has the effect of giving rise to a precipitation strengthening action and helping to improve resistance to high temperatures, making it an element that is deliberately added as needed. This effect is stably manifested by adding 0.03% or more, whereby the lower limit is preferably made 0.03%.

On the other hand, if it is added in excess, it is likely to invite the thickening of the precipitates. As a result, the high temperature resistance drops and the thermal fatigue life ends up falling, so the upper limit is set at 0.3%. Note that if manufacturing cost and manufacturability are considered, the lower limit is preferably 0.03% and the upper limit is preferably 0.1%.

Al: 0.3% or less

Al is an element that is added as a deoxidizing element and also improves resistance to oxidation. In addition, it is useful as a solution strengthening element to improve resistance at 600 to 700 ° C. This action manifests itself in a stable way from 0.01%, so the lower limit is preferably carried out at 0.01%.

On the other hand, excessive addition causes hardening and causes uniform elongation to drop noticeably. In addition, it makes the hardness significantly decrease. Therefore, the upper limit becomes 0.3%. Furthermore, if the formation of surface defects and the weldability and manufacturability are considered, the lower limit is preferably made 0.01% and the upper limit is preferably made 0.07%.

B: 0.0020% or less

The B is effective to immobilize the N which is detrimental to the ability to be worked and to improve the ability to be secondary worked. It is added as needed at 0.0003% or more. Also, even if it is added by more than 0.0020%, the effect is saturated. B causes a deterioration in workability and a drop in corrosion resistance, so this adds 0.0003 to 0.002%. If workability and manufacturing cost are considered, the lower limit is preferably set at 0.0005% and the upper limit is preferably set at 0.0015%.

W: 0.20% or less

W is effective in improving high temperature resistance and is added as needed by 0.01% or more. Also, if it is added in more than 0.20%, the strength of the solution becomes too great and the mechanical properties fall, so 0.01 to 0.20% is added. If the manufacturing cost and toughness of the hot rolled sheet are considered, the lower limit is preferably made 0.02% and the upper limit is preferably 0.15%.

Zr: 0.20% or less

Zr, like Nb, Ti, etc., forms carbonitrides to suppress the formation of Cr carbonitrides and improve corrosion resistance, so it is added as needed at 0.01% or more. Also, even if it is added by more than 0.20%, the effect is saturated and the formation of large oxides causes surface defects, so it is added by 0.01 to 0.20%. Compared to Ti and Nb, this is an expensive item, so if the manufacturing cost is considered, the lower limit is preferably made 0.02% and the upper limit is preferably 0.05%.

Sb: 0.5% or less

Sb is effective in improving resistance to sulfuric acid and is added as needed at 0.001% or more. Also, even if it is added by more than 0.5%, the effect is saturated and brittleness occurs due to segregation of the grain boundary of Sb, so 0.001 to 0.20% is added. If workability and manufacturing cost are considered, the lower limit is preferably set at 0.002% and the upper limit is preferably set at 0.05%.

Co: 0.5% or less

Co is effective in improving wear resistance and improving high temperature resistance and is added as needed at 0.01% or more. Also, even if more than 0.5% is added, the effect is saturated and the mechanical properties are degraded due to the strengthening of the solution, so 0.01 to 0.5% is added. From the manufacturing cost and the stability of the high temperature resistance, the lower limit is preferably made 0.05% and the upper limit is preferably made 0.20%.

Ca: 0.01% or less

Ca is an important desulfurizing element in the steelmaking process and also has a deoxidizing effect, so it is added as needed at 0.0003% or more. In addition, even if more than 0.01% is added, the effect is saturated and the resistance to corrosion due to Ca granules or deterioration of workability due to oxides decreases, so this is added by 0.0003 to 0.01%. If waste treatment and other aspects of manufacturability are considered, the lower limit is preferably 0.0005% and the upper limit is preferably 0.0015%.

Mg: 0.01% or less

Mg is an element that is effective in refining the solidified structure in the steelmaking process and is added as needed at 0.0003% or more. Furthermore, even if it is added by more than 0.01%, the effect is saturated and the corrosion resistance due to Mg sulfides or oxides is easily lowered, so it is added by 0.0003 to 0.01%. The addition of Mg in the steelmaking process results in violent combustion by oxidation of Mg and a lower yield. If the large increase in cost is considered, the lower limit is preferably made 0.0005% and the upper limit is preferably 0.0015%.

REM: 0.1% or less

A REM is effective in improving oxidation resistance and is added as needed at 0.001% or more. Furthermore, even if it is added by more than 0.1%, the effect is saturated and the REM granules cause a drop in corrosion resistance, so 0.001 to 0.1% is added. If the workability of the products and the manufacturing cost are considered, the lower limit is preferably 0.002% and the upper limit is preferably 0.05%.

The number of grain size after cold rolling and annealing is made from 5.0 to 9.0.

If Sn is exposed by adding steel to a high temperature environment, even if the components are controlled by the GBSV value, the drop in toughness will not be completely eliminated. In this case, it is possible to increase the area of the grain boundaries into which Sn segregates to alleviate the brittleness of the grain boundary. For that reason, the grain size number should be made 5 or more. However, if the grain size number becomes too large, the grain refinement will cause the mechanical properties to change to low ductility and high strength, so the size is made from 5.0 to 9.0. If you consider the optimization of the Lankford value, which governs the improvement of the deep stretchability and the reduction of the roughness of the skin at the time of work, the size is preferably made from 6.0 to 8.5.

Furthermore, even if Sn is not used to add steel in a high temperature environment, in the manufacturing process, if Sn segregates at the grain boundaries, it becomes a cause of a drop in hardness of the sheet metal product, so after cold annealing, it becomes necessary to increase the cooling rate to suppress grain boundary segregation. The annealing temperature of the cold rolled strip becomes 850 ° C or higher where Sn segregation from the grain boundary will not occur easily and it becomes 1100 ° C or lower where the grain size number will not easily thicken. At the time of cooling, it is preferable to make the cooling rate 5 ° C / s or more in the temperature range of 800 to 600 ° C, where the segregation of Sn at the grain boundary is done in a short time.

(Example 1)

Examples will now be used to explain the effects of the present invention, but the present invention is not limited to the conditions used in the following examples.

In this example, first, the steel of each of the component compositions shown in Table 1-1 and Table 1-2 was melted and cast into a slab. This slab was heated to 1190 ° C, then given a final temperature in the range of 800 to 950 ° C and hot rolled to a thickness of 4 mm to obtain a hot rolled sheet. Note that in Table 1-1 and Table 1-2, numerical values that are outside the scope of the present invention are underlined. The hot rolled steel sheet was cooled by aerated water cooling below 500 ° C, then wound on a coil.

In Table 1-1 and Table 1-2, the inventive examples and comparative examples that do not contain Ti or Mo have Ti and Mo contents which are shown with the symbols "-". Also, in Table 1-1 and Table 1-2, the values CI and GBSV of the inventive examples and comparative examples not containing Ti or Mo were calculated based on formula 1 and formula 2. mentioned above. Furthermore, the CI and GBSV values of the inventive examples and comparative examples containing Ti and Mo were calculated based on the formula 1 'and the formula 2' mentioned above.

After this, the hot rolled coil was annealed at a temperature of 900-1100 ° C and cooled to normal temperature. At this time, the average cooling rate in the range of 800 to 550 ° C became 20 ° C / s or more. Next, the hot rolled annealed sheet was pickled and cold rolled to obtain a sheet thickness of 1.5mm, then the cold rolled sheet was annealed and pickled to obtain a sheet product. Numbers 1 to 34 in table 1-1 are examples of the invention, while numbers 35 to 56 in table 1-2 are comparative examples.

The hot rolled annealed sheet thus obtained was heat treated at 700 ° C for 1 hour (L value: 19460), then subjected to a Charpy impact test according to JIS Z 2242 and the ductile brittle transition temperature (DBTT) was measured. The measurement results are shown in Table 2-1 and Table 2-2. Furthermore, the test piece in this embodiment is a test piece smaller than the thickness of the hot rolled annealed sheet as is, whereby the absorption energy was divided by the cross sectional area (units: cm ² ) to compare and evaluate the hardness of the hot rolled annealed sheets in the examples. Note that the criterion for evaluating toughness was a ductile-brittle transition temperature (DBTT) of 150 ° C or less as "good".

Furthermore, from the hot rolled annealed sheet, 14 x 4 x 4 mm test pieces were prepared for Auger electron spectroscopy (AES). In the central parts of the test pieces in the longitudinal direction, notches with a depth of 1 mm and a width of 0.2 mm were formed. The test pieces were cooled with liquid nitrogen in the AES apparatus under super high vacuum and tapped to break apart, then the concentration of Sn at the grain boundary fracture surfaces was measured. Measurement results are shown as "grain boundary Sn concentration (in%)" in Tables 2-1 and 2-2. For the AES apparatus, a SAM-670 (manufactured by PHI, Model FE) was used. The beam size was made 0.05 pm. The concentration was calibrated so that the analysis value at the cleavage fracture surface is the same as the concentration of the base material. Auger electrons are emitted not only from the shallowest surface of the grain boundary fracture surface, but also from several nm deep. Therefore, with this method, although the precise concentration of Sn is not at the grain boundaries, as a general measurement value, using this technique, 2 in% or less was considered good.

Furthermore, the hot rolled annealed sheet was cold rolled to 1.5 mm, annealed at 840 to 980 ° C for 100 seconds, then stripped. Cold rolled annealed sheet metal was welded by plate bead MIG welding and subjected to a copper sulfide and stainless steel sulfuric acid corrosion test prescribed in JIS G 0575 to investigate the presence of any HAZ weld sensitization. However, the sulfuric acid concentration became 0.5% and the test time became 24 hours.

Plates exhibiting grain boundary corrosion were considered to be poor in corrosion resistance. The results of the evaluation are shown as "Improved Strauss Assay" in Tables 2-1 and 2-2.

In addition, the surfaces of the cold rolled, annealed and pickled veneers were polished with # 600 paper, then treated by the salt spray test method prescribed in JIS Z 2371 for 24 hours and the presence of rust was verified. Plates exhibiting rust were deemed failed. The evaluation results are shown as "salt spray test" in Table 2-1 and Table 2-2.

Furthermore, the heat treatment conditions of the hot rolled annealed sheets were changed and tests similar to those described in Table 2-1 and Table 2-2 were performed. The results are shown in Table 3. Some of the steels shown in Table 3 were evaluated by a repeated wet / dry test. The test solution was made one containing NO ^3- nitrate ions: 100 ppm, SO ^{42 -} sulfate ions: 10 ppm, and Cl- chloride ions: 10 ppm and with a pH = 2.5. A test tube with an outer diameter of 15 mm, a height of 100 mm and a thickness of 0.8 mm was filled with the test solution to 10 ml. In this, the different types of stainless steels obtained by cutting into 1 "t" x 15 x 100mm pieces and wet polishing the entire surface with # 600 sandpaper were half dipped. This test tube was inserted into a warm bath at 80 ° C. The sample, which was completely dried after 24 hours had elapsed, was washed lightly with distilled water, then a freshly washed test tube was filled with the test solution again to immerse the sample in half again and kept at 80 ° C for 24 hours. This was done for 14 cycles.

Furthermore, the annealing conditions of the cold rolled annealed sheet were changed as specified in Table 4 to obtain 1.5mm rolled products. These were subjected to an aging treatment at 600 ° C for 1 week, then subjected to a Charpy V-notch impact test at that thickness as is. The results are shown in Table 4. At this time, the ductile-brittle transition temperature which became -20 ° C or less became the pass-through condition.

Table 2-1

Table 2-2.

Table 4

As is clear from Table 1-1, Table 1-2, Table 2-1, Table 2-2 and Table 3, in steels with component compositions and grain boundary Sn concentrations according to the present invention , the ductile-brittle transition temperatures (DBTT) evaluated by the hot rolled annealed sheets were low, the corrosion resistances evaluated by the cold rolled annealed sheets were good, and the total elongations evaluated by a tensile test were also good 30% or more. Furthermore, no surface defects could be observed. On the other hand, comparative examples outside the present invention failed in at least one of the Charpy impact values (absorption energy), corrosion resistance, material quality and surface defects. Because of this, it is learned that the heat resistance and the corrosion resistance of the ferritic stainless steels in the comparative examples are inferior.

Specifically, Nos. 35, 39-41, 43, 44, 46, 49, and 50 had GBSV greater than 0 and had grain boundary segregation amounts of Sn after undergoing heat treatment at 700 ° C for 1 hour. greater than 2% as measured by AES. The toughness was low, as evidenced by the ductile-brittle transition temperatures, above 150 ° C. Nos. 43 to 45 and 47 to 49 had IC values less than 8, so the grain boundary corrosion resistance evaluated by the improved Strauss test and the oxidation resistance evaluated by the salt spray test were lower. Nos. 36, 37, 38, 52, 53 and 51 were respectively high in Si, Mn, P, Ni, Cu and Mo and were low in elongation due to the strengthening of the solution, so they were bad in mechanical properties . While # 39 was high in S and # 40 was low in Cr, # 42 was low in Sn, and # 55 was high in B, so they were poor in resistors corrosion evaluated by the salt spray test. Also, No. 42 was low in Sn, so it was good at toughness even if GBSV was greater than 0. No. 45 was high in Nb, while No. 47, 45, and 50 were high in Ti and # 54 was high in V so the defects occurred due to large inclusions and these were considered low quality. No. 41 was high in Cr, while No. 56 was high in Al and had surface defects, so it was considered poor quality.

In Table 3, all references a1 to a3 had grain boundary Sn concentrations of 2 in% or more after heat treatment with L values of 1.91 x 104 or more, and all a1 to a3 they had DBTT above 150 ° C and had poor toughness. Also, as with a4, if the L value is less than 1.91 x 104, Sn does not segregate at the grain boundaries, so the DBTT is at 80 ° C, but if the L value becomes large, Sn it segregates at the grain boundaries and the DBTT increases, which is why it was confirmed that it is necessary to evaluate the Sn segregation at the grain boundaries by an L value of 1.91 x 104 or more.

Furthermore, all the steels in the range of the present invention had maximum corrosion depths of 50 µm or less. Take into account that, in the case of steel containing Ni or Cu in the range of the present invention, the maximum corrosion depth was 20 pm or less or an extremely good result for corrosion resistance.

Furthermore, as will be clear from Table 4, sheets having component compositions, grain size numbers after cold rolling and annealing, cold rolled strip annealing temperatures and cooling rates according to the present invention they exhibited low ductile-brittle transition temperatures and good toughness.

On the other hand, b1 had an annealing temperature of cold rolled strip of 1100 ° C or more, and a grain size number which is defined by the Steel Crystalline Granularity Microscope Type Test Method established in JIS G 0551 was less than 5.0. Therefore, the cooling rate at 800 to 500 ° C was 20 ° C / s. However, the ductile-brittle transition temperature was high. Reference b2 had a cold rolled strip annealing temperature of less than 850 ° C and a grain size number greater than 9.0, so the mechanical properties were poor. Also, b3 and b6 had cooling rates less than 5 ° C / s at 800 to 500, so the annealing temperature was suitable and the grain size number was also suitable 8.0. However, the ductile-brittle transition temperature was high. In addition, b4 and b5 were compositions of the comparative examples, so the cold rolled strip annealing temperatures, cooling rates, and grain size numbers were in suitable ranges, but the ductile-brittle transition temperatures they were tall.

From these results, it was possible to confirm the above findings. Furthermore, it was possible to verify the reasons for limiting the compositions and constitutions of the steels explained above.

Industrial applicability

As is clear from the above explanation, according to the Sn-containing ferritic stainless steel of the present invention, the stabilizing elements Nb and Ti are optimized, whereby it is possible to produce stainless steel sheets having little deterioration in toughness, even if they are used at a high temperature and it is also excellent in resistance to sheet corrosion. Furthermore, by applying the material according to the present invention particularly to the parts of the exhaust system of automobiles and motorcycles, it becomes possible to extend the service life of the parts and thus raise the degree of contribution to the general society. That is, the present invention has sufficient applicability in industry.

Claims (11)

1. A method of producing a hot rolled annealed ferritic stainless steel, the ferritic stainless steel being characterized in that it consists of, in mass%, Cr: 13.0 to 21.0%, Sn: 0.01 to 0.50%, Nb: 0.05 to 0.60%, C: 0.001 to 0.015%, Yes: 0.01 to 1.5%, Mn: 0.01 to 1.5%, N: 0.001 to 0.020%, P: 0.035% or less, S: 0.015% or less, and optionally contains one or more of, in mass%, Ti: 0.32% or less, Ni: 1.5% or less, Cu: 1.5% or less, Mo: 2.0% or less, V: 0.3% or less, Al: 0.3% or less, B: 0.0020% or less, W: 0.20% or less, Zr: 0.20% or less, Sb: 0.5% or less, Co: 0.5% or less, Ca: 0.01% or less, Mg: 0.01% or less, REM: 0.1% or less, and containing a trace of Fe and unavoidable impurities, wherein the production method of the hot rolled annealed ferritic stainless steel includes a step of performing a heat treatment at a temperature of 600 to 750 ° C so that an L value shown by formula 3 becomes 1.91 x 104 or plus, hot rolled annealed ferritic stainless steel satisfying Formula 1 'and Formula 2' and having a grain boundary Sn concentration of 2% or less wherein the ductile-brittle transition temperature of the hot rolled annealed sheet having a thickness of 4.0 mm is 150 ° C or less; 8 <Cl = (Ti + 0.52Nb) / (C + N) <26 formula 1 GBSV = Sn + Ti-2Nb-0.3Mo-0.2 <0 formula 2 L = (273 T) (log (t) +20) formula 3 where, T: temperature (° C), t: time (h). 2. The production method of a hot rolled annealed ferritic stainless steel according to claim 1, wherein said heat treatment is carried out at a temperature of 700 ° C for 1 hour. The method of producing a hot rolled annealed ferritic stainless steel according to claim 1 or 2 which further contains, in mass%, one or more of Ti: 0.001 to 0.32%, Ni: 0.03 to 1.5%, Cu: 0.1 to 1.5%, Mo: 0.01 to 2.0%, V: 0.01 to 0.3%, Al: 0.01 to 0.3%, and B: 0.0003 to 0.0020%. The method of producing a hot rolled annealed ferritic stainless steel according to any one of claims 1 to 3 further containing, in mass%, one or more of W: 0.01 to 0.20%, Zr: 0.01 to 0.20%, Sb: 0.001 to 0.5%, Co: 0.01 to 0.5%, Ca: 0.0003 to 0.01%, Mg: 0.0003 to 0.01%, and REM: 0.001 to 0.1%. The method of producing a hot rolled annealed ferritic stainless steel according to any one of claims 1 to 4, further containing, in mass%, at least one of W: 0.01% to 0.20% and Sb: 0.001% to 0.5%. The method of producing a hot rolled annealed ferritic stainless steel according to claim 5 which further contains, in mass%, one or more of Zr: 0.01 to 0.20%, Co: 0.01 to 0.5%, Ca: 0.0003 to 0.01%, Mg: 0.0003 to 0.01%, and REM: 0.001 to 0.1%. 7. A hot rolled annealed ferritic stainless steel obtainable by the method of any one of claims 1 to 6. 8. A production method of the cold rolled annealed ferritic stainless steel sheet, the method being characterized by comprising preparing a hot rolled annealed ferritic stainless steel by the method according to any one of claims 1 to 6, cold rolling the hot rolled annealed ferritic stainless steel, anneal cold rolled ferritic stainless steel at an annealing temperature of cold rolled sheet of 850 ° C to 1100 ° C and then cool from the annealing temperature of cold rolled sheet by a cooling rate of 5 ° C / s or more in the temperature range of 800 to 500 ° C, wherein a grain size number after annealing the cold rolled sheet becomes 5.0 to 9.0. The production method of the cold rolled annealed ferritic stainless steel sheet according to claim 8, wherein a grain size number after annealing of the cold rolled sheet is made 6.0 to 8.5. 10. A cold rolled annealed ferritic stainless steel sheet characterized by being obtainable by the method of claim 8 or 9. A part of the exhaust system of an automobile or motorcycle comprising the hot rolled annealed ferritic stainless steel of claim 7 or the cold rolled annealed ferritic stainless steel sheet of claim 10.