JP4905024B2 - Ferritic stainless steel sheet with high strength of spot welded joint and method for producing the same - Google Patents

Description

  The present invention relates to a ferritic stainless steel sheet that is excellent in corrosion resistance and has high spot welded joint strength during press working, and a method for producing the same, used in kitchens, household electric appliances, furniture, containers, and the like.

  Ferritic stainless steel plates are excellent in design and corrosion resistance, and are therefore used in various applications such as buildings, transportation equipment, home appliances, and kitchens. At this time, simple resistance spot welding is often used for joining the pressed parts. However, in a ferritic stainless steel sheet, the C and N contents are reduced and the Cr content is increased in order to achieve high corrosion resistance, and the ferrite grains in the vicinity of the weld are easily coarsened due to high purity. For this reason, there is no problem with welded steel toughness, which is a problem in the heat affected zone, but in the case of a thin steel plate, the joint strength is extremely lower than that of normal carbon steel due to the coarsening of the heat affected zone. As a result, there is a drawback that the joint strength is difficult to maintain.

On the other hand, Patent Document 1 discloses a high workability ferritic stainless steel sheet excellent in weldability that increases the oxide density of the weld metal and heat-affected zone by adding Mg and suppresses the coarsening of crystal grains. Has been. However, in this method, Mg receives buoyancy during solidification and concentrates in the vicinity of the surface layer, so that it can prevent coarsening of the crystal grains near the surface layer of the steel sheet, but it cannot prevent coarsening of the central part of the plate thickness. In spot welding where the structure is important, it is impossible to prevent the coarsening of the heat-affected zone and the joint strength is low.
Patent Document 2 discloses a high workability ferritic stainless steel sheet excellent in weldability that suppresses coarsening of crystal grains by defining the grain size and density of a composite oxide having a Ti or Al oxide as a core. Is disclosed. However, even with this method, it is impossible to prevent the central portion of the plate thickness from becoming coarse, and with the spot welding heat in which the structure of the central portion of the plate thickness is important, it is not possible to prevent the coarsening of the affected part and the joint strength is low. Patent Document 3 defines a ferritic stainless steel sheet in which the amount of O and N in the weld metal is defined in association with the amount of C and Cr, and the toughness of the laser weld is improved by reducing the precipitate density in the weld metal. Is disclosed. However, this method can increase the toughness of extremely thin welds such as laser welding, but it cannot increase the strength of spot welded joints, and it can only refine the structure of the solidified part. It is impossible to suppress the grain coarsening of important heat affected zone. As a result, the joint strength is not improved.
Patent Document 4 discloses a ferritic stainless steel sheet excellent in weldability for controlling the addition amounts of Al and Mg to refine the solidification structure of the molten part. However, this technique is also intended to refine the melt-solidified structure and does not affect the weld heat affected zone at all.
Japanese Patent Laid-Open No. 11-256285 Japanese Patent Laid-Open No. 2001-254153 Japanese Patent Laid-Open No. 7-286239 JP-A-9-217151

  In view of such circumstances, the present invention provides a ferritic stainless steel sheet having high spot-welded joint strength that could not be achieved by the conventional invention and a method for producing the same.

In order to solve the above-mentioned problems, the present inventors investigated the strength of the spot welded joint in detail with the welded portion structure. As a result, the following knowledge was obtained.
When the structure of the heat-affected zone around the nugget (melt-solidified portion) of the spot welded portion is fine, fracture occurs outside the heat-affected zone.
When the crystal grains of the heat-affected zone around the nugget (melt-solidified zone) of the spot weld are coarse, the fracture starts from the boundary between the nugget and the heat-affected zone.
Since some of the heat-affected zone was fine and some were coarse, the difference was investigated in detail. TiN was distributed.
Furthermore, as a result of investigating the appropriate existence state of such fine TiN, TiN having an average particle diameter of 50 nm to 100 nm is obtained at the central portion of the plate thickness (from the surface layer to 1/4 to 3/4 of the plate thickness). When it is distributed, crystal grain coarsening in the heat-affected zone is suppressed and the spot welded joint strength is high.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] In mass%, C ≦ 0.010%, Si ≦ 0.15%, Mn ≦ 0.2%, P ≦ 0.04%, S ≦ 0.005%, Al ≦ 0.05%, N: 0.006 to 0.010%, Cr: 20 to 23% Cu: 0.3 to 0.6%, Ni ≦ 0.5%, Ti: 0.2 to 0.4%, Ti is Ti / N: 20 to 50, the balance is made of Fe and inevitable impurities, and average in the center of the plate thickness A ferritic stainless steel sheet having high spot-welded joint strength, wherein TiN having a particle size of 50 nm to 100 nm is precipitated at a distribution density of 10 or more per 100 μm ^{2 of} the cross-sectional structure .
[ 2 ] After heating the steel to 1200 ° C or lower, hot rolling is performed at a finishing temperature: 950 ° C or lower, a coiling temperature: 400 ° C to 600 ° C, and then an annealing temperature: 1000 ° C or lower, annealing time: 100 seconds. The ferritic stainless steel sheet with high spot-welded joint strength according to claim 1 , wherein hot-rolled sheet annealing is performed, and cold-rolled sheet annealing is performed at a temperature of 800 ° C. or higher after pickling and cold rolling. How to manufacture .
In addition, in this specification, all% which shows the component of steel is mass%. Moreover, a plate | board thickness center part refers to the part of 1 / 4-3 / 4 of plate | board thickness.

  According to the present invention, a ferritic stainless steel sheet having high spot welded joint strength can be obtained.

The present invention is described in detail below. First, the component composition of the present invention will be described.
(1) C ≦ 0.010%
C combines with Cr to reduce the amount of solid solution Cr, thus deteriorating corrosion resistance. In addition, the precipitation of Cr carbide reduces the Cr content near the ferrite grain boundary and reduces the effect of ferrite crystal grain growth due to the Solutegr Cr. From the above, the C content is set to 0.010% or less.

(2) Si ≦ 0.15%
Si is a solid solution strengthening element that makes steel hard and low ductile. Therefore, the Si content is 0.15% or less.

(3) Mn ≦ 0.2%
Mn is an element that degrades corrosion resistance and is also a constituent element of MnS, which is a starting point of fracture of welded joints. Therefore, the Mn content is 0.2% or less.

(4) P ≦ 0.04%
P remarkably solidifies and strengthens steel and segregates at the grain boundaries to promote brittle fracture of the grain boundaries. In the present invention, it is necessary to lower P from the viewpoint of workability. Therefore, the P content is 0.04% or less.

(5) S ≦ 0.005%
S forms MnS or TiS at the ferrite grain boundary and becomes the starting point of weld joint fracture. Therefore, the S content is 0.005% or less. Preferably, it is 0.003% or less.

(6) Al ≦ 0.05%
Al is a deoxidizer, and positive addition is desired to improve the cleanliness of steel. Therefore, addition of 0.02% or more is desirable. However, if added in a large amount, the coarsening of crystal grains is facilitated and brittle fracture of the heat affected zone is caused. From the above, the Al content is 0.05% or less. Desirably, it is 0.03% or less.

(7) N: 0.006-0.010%
N combines with Cr to reduce the amount of solid solution Cr, thus deteriorating corrosion resistance. Also, the precipitation of Cr nitride reduces the Cr content in the vicinity of the ferrite grain boundary and reduces the effect of ferrite crystal grain growth due to Cr solute trugg. This also suppresses the precipitation of TiN. From the above, the N content is set to 0.010% or less. However, if it is less than 0.006%, fine TiN having an effect of suppressing crystal grain growth does not precipitate, so the lower limit is made 0.006%.

(8) Cr: 20-23%
Cr is an element that improves the corrosion resistance by forming a passive film on the surface of stainless steel. Ordinary ferritic stainless steel sheet contains 18% Cr as represented by SUS430, but the steel of the present invention is premised on use where corrosion resistance surpassing SUS430 is required. The amount should be 20% or more. However, if the Cr content exceeds 23%, the delay in recrystallization due to Cr becomes remarkable, and the quality of the steel becomes high. From the above, the Cr content is 20% or more and 23% or less.

(9) Cu: 0.3-0.6%
Cu has a function of improving corrosion resistance when the C content is 0.01% or less and the Cr content is 20% or more. Therefore, in the steel according to the present invention having a low C content and containing 20% or more of Cr, 0.3% or more of Cu is added to further improve the corrosion resistance. On the other hand, if added over 0.6%, CuS tends to precipitate, and if this precipitates at the grain boundary, it tends to become the starting point of cracks. For this reason, the spot welded joint strength decreases. From the above, the Cu content is 0.3% or more and 0.6% or less. Preferably, it is 0.5% or less.

(10) Ni ≦ 0.5%
Ni is an element that improves corrosion resistance, but if mixed in a large amount, it hardens the steel and causes ductility deterioration. Therefore, the Ni content is 0.5% or less.

(11) Ti: 0.2-0.4%, Ti / N: 20-50
Ti combines with N, C, and S to form nitrides, carbides, and sulfides. If the Ti content is less than 0.2%, these elements cannot be fixed as precipitates, and as a result, Cr carbide is formed and the corrosion resistance deteriorates. For this reason, Ti is added by 0.2% or more. On the other hand, if added over 0.4%, the nucleation of TiN and TiS is excessively promoted and fine precipitation occurs, and the steel becomes hard and low ductility. From the above, the Ti content is 0.2% or more and 0.4% or less.
Further, if Ti / N, which is the ratio of Ti content to N content, is less than 20, the amount of precipitated TiN is so small that ferrite grain coarsening in the heat affected zone cannot be prevented. On the other hand, when Ti / N exceeds 50, TiN becomes coarse, so that coarsening of the heat-affected zone cannot be suppressed. Therefore, Ti / N is 20 or more and 50 or less.

  The balance other than the above is Fe and inevitable impurities. Even if Nb ≤ 0.015%, B ≤ 0.001%, Mo ≤ 0.1%, V ≤ 0.05%, Mg ≤ 0.01%, Ca ≤ 0.01%, etc. are mixed as impurity elements, the effect can be obtained. The smaller the number, the better.

(12) TiN having an average particle diameter of 50 nm to 100 nm at the center of the plate thickness (portion from 1/4 to 3/4 of the plate thickness from the surface layer) is the most important requirement in the present invention. The heat-affected zone of a spot welded joint is directly stressed by a joint tensile test, and fracture starts from there. In particular, the stress is thought to be supported at the center of the plate thickness from its shape. Therefore, it is important to prevent coarsening during the welding of crystal grains in the center of the plate thickness. When the average grain size of TiN is less than 50 nm, the ferrite grain boundaries become brittle and the joint strength does not increase. On the other hand, when it exceeds 100 nm, it becomes impossible to prevent crystal grain coarsening. From the above, in the present invention, it is assumed that TiN having an average particle diameter of 50 nm to 100 nm is deposited at the center of the plate thickness. TiN suppresses ferrite grain growth because TiN can be stably present in steel even at high temperatures.
In addition, the distribution density of TiN in the central portion of the plate thickness is 10 or more, preferably 10 TiN having an average particle size of 50 nm to 100 nm per 100 μm ^{2 of the} cross-sectional structure from the viewpoint of preventing grain coarsening in the heat affected zone. The number is preferably 10,000 or less. If TiN having an average particle diameter of 50 nm to 100 nm is less than 10 per 100 μm ^{2 of the} cross-sectional structure, the ferrite grain growth during welding cannot be suppressed, and HAZ is mixed and joint strength decreases. On the other hand, if it exceeds 10,000, each one becomes fine, and the elongation of the steel sheet itself may decrease.
The distribution state of TiN having an average particle diameter of 50 nm to 100 nm in the central portion of the plate thickness can be confirmed by observing with a transmission electron microscope. The average particle size of TiN was the average of the lengths of 100 TiN cube side, number per 100 [mu] m ² of cross-sectional structure will be observed with SEM to 100 [mu] m ² of the sheet thickness direction cross-section parallel to the rolling direction It can ask for.

Next, the manufacturing method of the ferritic stainless steel sheet with high strength of the spot welded joint according to the present invention will be described.
The stainless steel plate of the present invention is a steel having the above composition heated to 1200 ° C. or lower, then hot-rolled at a finishing temperature of 950 ° C. or lower and a winding temperature of 400 ° C. to 600 ° C., and then 1000 ° C. or lower. It is obtained by performing hot-rolled sheet annealing at an annealing temperature of 100 seconds or less at an annealing temperature of the above, pickling and cold-rolling, and then annealing at a temperature of 800 ° C. or higher.

Hot rolling: When the steel ingot (slab) heating temperature during heating hot rolling below 1200 ° C exceeds 1200 ° C, TiN becomes coarse during slab heating, and the grain coarsening of the heat affected zone of the spot weld can be suppressed. Disappear. Therefore, the heating temperature during hot rolling is set to 1200 ° C. or lower.

Hot rolling and finishing temperature: 950 ° C. or less When the finishing temperature of the hot rolling of the steel sheet of the present invention exceeds 950 ° C., the ferrite grains expand and the dispersion of TiN appearing at the ferrite grain boundaries is biased. At this time, crystal grain growth occurs in the portion where the dispersion of TiN becomes thin, and as a result, the strength of the spot welded joint is lowered. Therefore, the finishing temperature is 950 ° C. or lower. Preferably, it is 850 ° C. or higher.

Hot rolling, coiling temperature: 400 ℃ ~ 600 ℃
The coiling temperature is important for the control of precipitates in the hot rolled sheet. When the temperature is lower than 400 ° C, TiC does not precipitate, and during subsequent annealing, TiN precipitates at the coarse hot-rolled grain boundaries and the distribution density of TiN becomes thin. As a result, the joint strength decreases. On the other hand, when the temperature exceeds 600 ° C., Ti ₄ C ₂ S ₂ precipitates around TiN and effectively coarsens TiN. This deteriorates the strength of the spot welded joint. From the above, the coiling temperature is set to 400 ° C. or more and 600 ° C. or less.

Hot-rolled sheet annealing: 1000 ° C. or less, 100 seconds or less When the hot-rolled sheet annealing temperature exceeds 1000 ° C., fine TiN is likely to be coarsened, and it becomes impossible to prevent the coarsening of the heat affected zone of the spot welded portion. Therefore, the hot-rolled sheet annealing temperature is set to 1000 ° C. or less. In addition, even if the annealing temperature is lower than 1000 ° C., TiN coarsens if the annealing time is maintained for more than 100 seconds. Therefore, the hot-rolled sheet annealing time is set to 100 seconds or less.

Pickling and cold rolling pickling can be performed by a method usually used for stainless steel plates. Moreover, the cold rolling conditions are not particularly limited. For example, the rolling rate during cold rolling is preferably 50% or more in order to ensure mechanical characteristics. Further, the cold rolling may be one or two or more cold rolling including intermediate annealing.

Cold-rolled sheet annealing, annealing temperature: 800 ° C. or more If the cold-rolled sheet annealing temperature is low, a cold-rolled structure stretched in the rolling direction tends to remain and joint strength decreases. Further, if recrystallization is insufficient, the elongation is extremely low. From the above, in the present invention, the cold-rolled sheet annealing temperature is set to 800 ° C. or higher. From the viewpoint of preventing TiN coarsening, the temperature is preferably 1000 ° C. or lower.

  The obtained annealed plate may be subjected to temper rolling. The temper rolling ratio at this time is preferably 1.5% or less and 0.5% or more.

Steel having the chemical components shown in Table 1 was melted, and hot rolling and hot rolled sheet annealing were performed under the conditions shown in Table 2. The thickness of the hot rolled steel sheet was 3 mm. Next, after pickling, it was cold-rolled to a thickness of 0.8 mm. Subsequently, cold-rolled sheet annealing was performed under the conditions shown in Table 2 to obtain a cold-rolled annealed sheet.

With respect to the cold-rolled annealed sheet obtained as described above, a JIS No. 13 B tensile test piece was collected so that the tensile direction was parallel to the rolling direction, and a tensile test was performed.
In addition, a strip with a width of 30 mm and a length of 100 mm was cut out, each strip was parallel in the longitudinal direction, and two plates were stacked so as to wrap 30 mm, and resistance spot welding was performed at the center of the overlapped 30 mm square area. went. A single-phase AC machine was used for spot welding, and a Cr copper DR type electrode was used. The applied pressure was 400 kgf, the squeeze was 50 cy / 50 Hz, the energization time was 20 cy / 50 Hz, and the hold after energization was 50 cy / 50 Hz. The welding current was 8 kA. The tensile shear of the welded joint was measured by pulling both ends of the joint in the length direction of 100 mm with a tensile tester and measuring the maximum load.
In addition, a thin film was prepared from the central part of the steel plate by a twin jet method, and TiN was observed with a transmission electron microscope.
The results obtained as described above are shown in Table 2 together with the conditions.

From Table 2, in the examples of the present invention, both the tensile shear strength and other characteristics are excellent, and in particular, a ferritic stainless steel sheet having a high spot-welded joint strength with EL of 30% or more is obtained.
In the examples of the present invention, TiN having an average particle diameter of 50 nm to 100 nm was precipitated at a distribution density of 10 or more per 100 μm ^{2 of the} cross-sectional structure in the central portion of the plate thickness.

  On the other hand, in the comparative example, the tensile shear strength is inferior. Further, a comparative example of No4 with high C outside the scope of the present invention, a comparative example of No5 with a low amount of N and the average particle size of TiN outside the scope of the present invention, and an average grain size of TiN with a low coiling temperature outside the scope of the present invention The comparative example of No. 14 having a small diameter outside the scope of the present invention and the comparative example of No. 24 having an annealing temperature outside the scope of the present invention and a low average TiN particle diameter outside the scope of the present invention have poor elongation.

  It is suitable as a member that requires the strength of spot welded joints during press processing, mainly in kitchens, household electric appliances, equipment, containers, building interior and exterior materials, and the like.

Claims (2)

mass%, C ≦ 0.010%, Si ≦ 0.15%, Mn ≦ 0.2%, P ≦ 0.04%, S ≦ 0.005%, Al ≦ 0.05%, N: 0.006 to 0.010%, Cr: 20 to 23%, Cu: Including 0.3-0.6%, Ni ≦ 0.5%, Ti: 0.2-0.4%, Ti is Ti / N: 20-50, the balance consists of Fe and inevitable impurities,
A ferritic stainless steel sheet having high spot-welded joint strength, wherein TiN having an average particle diameter of 50 nm to 100 nm is deposited at a distribution density of 10 or more per 100 μm ^{2 of} the cross-sectional structure in the center of the plate thickness. After heating the steel to 1200 ° C or lower, it is hot-rolled at a finishing temperature of 950 ° C or lower, a coiling temperature of 400 ° C to 600 ° C, and then annealed at a temperature of 1000 ° C or lower and an annealing time of 100 seconds or shorter. The method for producing a ferritic stainless steel sheet having high spot-welded joint strength according to claim 1, wherein the steel sheet is annealed and cold-rolled at a temperature of 800 ° C. or higher after pickling and cold rolling. .