JP4254583B2 - Cr-containing alloy with excellent strain aging resistance of welds - Google Patents

Description

  The present invention relates to a Cr-containing alloy that is excellent in strain aging resistance characteristics of a welded part and that is particularly suitable as an automobile part such as an exhaust system or a suspension.

  For automobile parts, pipes of Cr-containing alloys such as SUH409L are frequently used. These pipes are formed by welding the ends of a steel plate by methods such as TIG welding, MIG welding, laser welding, and plasma welding. After pipe making, processing such as bending, drawing, and pipe expansion is performed according to the design of the car. In recent years, the pipe shape has become complicated, and in addition to rapid pipe processing, etc. There is a problem that various deformations may be applied and processing cracks occur. In this processing, the temperature of the material sometimes reaches 300 ° C. or more, and there is a problem that cracking occurs.

  Various methods have been proposed for improving the workability of welds of Cr-containing alloys or ferritic stainless steels from the viewpoint of improving ductility and toughness of welds.

  For example, Patent Document 1 discloses a technology that regulates the amounts of C, Si, Mn, and Al, further refines the structure of the welded portion by setting the amounts of Ti, O, N, and Cr to appropriate values and improves ductility. Presented. However, this method can reduce the size of crystal grains, but on the other hand, there is a problem that the welded portion is easily hardened by processing and cracks are easily generated.

Patent Document 2 proposes a method for preventing a decrease in the toughness of the weld heat affected zone by defining the distribution state of the Mg-based oxide. However, this method has a problem that the effect greatly depends on the flow rate of the shielding gas, and it is difficult to realize a predetermined oxide distribution state, and as a result, sufficient workability cannot be obtained.
JP-A-2002-275590, claims, [0007] to [0010], etc. JP 2003-3242 A, claims, [0005], etc.

  As a point common to said technique, the point which aims at improving workability by refine | miniaturizing the structure | tissue of a welding part is mentioned. However, strain aging associated with rapid deformation at a high temperature becomes a problem in the actual sequential processing. When strain aging occurs, the weld becomes hard, ductility and toughness decrease, and cracks occur during and after processing.

  The present invention has been proposed to solve such problems, and an object of the present invention is to provide a Cr-containing alloy whose workability has been remarkably improved by suppressing strain aging of welds. Yes.

  The inventors have conducted a thorough investigation and inspection on the influence of elements on the strain aging of the weld.

  As a result, by adding a small amount of Mo, it was found that the interaction between Mo, P, B, and Si can be effectively utilized, and the strain aging characteristics of the welded portion are greatly improved. In particular, when metalloid elements such as P, Si, and B are present alone in steel, the dislocations introduced by deformation are combined with these elements, strain aging is likely to occur, and the material is likely to deteriorate. Mo has a strong chemical bonding force with P, Si, B, and when the welded portion is melted by welding to form a liquid state, the atomic correlation of Mo-P, Mo-Si, Mo-B (short-range order) Can be considered. These atomic correlations are maintained even when the solution is brought into a solid solution state by rapid cooling, and cannot be solved even if the temperature of the weld rises due to processing or use at a high temperature. As a result, it is considered that P, Si, and B cannot be combined with dislocations, and strain aging is suppressed.

  The present invention has been completed through further studies based on the above-described findings. That is, the gist configuration of the present invention is as follows.

(1) By mass%, C: 0.0020% to 0.0120%, N: 0.0030% to 0.0120%, C + N: 0.0050% to 0.0200%, Si: 0.00. 05% to 1.0%, Mn: 0.10% to 1.0%, P: 0.04% or less, S: 0.005% or less, Cr: 9% to less than 12%, Ni: 0 0.5% or less, Mo: 0.02% to 0.2%, Al: 0.005% to 0.05%, B: 0.0001% to 0.0010%, Ti: 0.20% 0.40% or less, O: 0.0030% or less, remaining Fe and inevitable impurities (provided that ([% Ti] -5 × [% O]) × [% N] / [% Cr] ≦ 0.000120)),

A Cr-containing alloy excellent in strain aging resistance of a welded portion, characterized by being

  [% X] represents mass% of the component X in the steel.

(2) In addition to the composition of (1), V: 0.01% to 0.3%, W: 0.001% to 0.1%, and Co: 0.01% in mass% A Cr-containing alloy characterized by containing one or more selected from 0.2% or less.

(3) In addition to (1) or (2), a Cr-containing alloy containing Cu: 0.05% to 2.0% in mass%.

  The Cr-containing alloy of the present invention is excellent in the strain aging resistance characteristics of the welded portion, has little decrease in ductility and toughness due to strain aging, can prevent cracking during and after processing, and has a remarkable industrial effect. Have.

  The present invention will be specifically described below.

  First, the reason for the addition of steel components and the reason why the component composition is limited to the above range in the present invention will be described. In addition, "%" regarding a component shall mean the mass% unless there is particular notice.

C: 0.0020% or more and 0.0120% or less C not only lowers the toughness but also the workability, so the lower content is desirable. However, if it is too low, the crystal grains will be coarsened and the toughness will be lowered. On the other hand, excessive reduction will increase the steelmaking cost. Therefore, in the present invention, the C amount is C: 0.002% or more and 0.0120% or less. Preferably, the content is 0.002% or more and 0.008% or less.

N: 0.0030% or more and 0.0120% or less N, as well as C, not only lowers toughness, but also lowers workability, so a lower content is desirable. However, excessive reduction not only causes an increase in steelmaking cost, but also causes coarsening of crystal grains, which in turn reduces toughness. Therefore, in the present invention, the N amount is N: 0.0030% or more and 0.0120% or less. Preferably, the content is 0.0070% or more and 0.0100% or less.

C + N: 0.0050% or more and 0.0200% or less The total of C and N is made 0.0050% or more and 0.0200% or less due to the reason for the addition of C and N and the interaction between C and N. Preferably it is 0.0080% or more and 0.0180% or less.

Si: 0.05% or more and 1.0% or less Si lowers the workability and toughness of the welded part, so it is desirable to reduce the content from these aspects, but on the other hand, to improve oxidation resistance and corrosion resistance. It is an effective element. However, since addition exceeding 1.0% causes a problem of elongation reduction due to hardening or work cracking, Si: 0.05% to 1.0% is set. Preferably, it is 0.20% or more and 0.8% or less.

Mn: 0.10% or more and 1.0% or less Mn is an element effective for improving the strength, and is contained in an amount of 0.10% or more in order to exert the effect. However, if it is contained excessively, not only the toughness is lowered, but also the corrosion resistance is deteriorated. Therefore, Mn is specified to be 1.0% or less. Preferably it is 0.8% or less.

P: 0.04% or less P is an element effective for improving the strength. However, if excessively contained, the toughness is lowered, so the content is limited to 0.040% or less. In addition, Preferably it is 0.030% or less.

S: 0.005% or less S forms precipitates and inclusions with Ti and the like, and thus tends to be a starting point of breakage during processing. In particular, precipitates and inclusions segregated in the center of the plate thickness due to metal flow by welding are exposed on the surface, which not only serves as a starting point for fracture, but also causes deterioration in corrosion resistance. Therefore, S is set to 0.005% or less. Preferably it is 0.004% or less.

Cr: more than 9% and less than 12% Cr is an element indispensable for improving the corrosion resistance and the oxidation resistance, and if the content is 9% or less, a sufficient effect cannot be obtained. However, addition of 12% or more leads to a decrease in toughness and elongation, so Cr is over 9% and less than 12%.

Ni: 0.5% or less Ni contributes to improvement of toughness and corrosion resistance. However, addition of 0.5% or more causes not only deterioration of oxidation resistance but also an increase in manufacturing cost. .5% or less. Preferably it is 0.4% or less.

Mo: 0.02% or more and 0.2% or less Generally, Mo is an element effective for improving corrosion resistance. However, by adding a small amount of about 0.02%, Mo and P, B or It forms an atomic correlation with elements such as Si, suppresses strain aging of welds and base materials, and further improves toughness. However, when Mo is added excessively, the elongation is lowered and, on the contrary, the strain aging resistance is lowered, which causes deterioration of workability and cracking. Therefore, Mo is set to 0.02% or more and 0.2% or less. Preferably, the content is 0.05% or more and 0.18% or less.

Al: 0.005% or more and 0.05% or less Al is an element that is very effective as a deoxidizing material, and it is desirable to add about 0.005 in order to maintain the cleanliness of the steel. However, if added over 0.05%, surface defects due to clusters of Al ₂ O ₃ -based oxides are likely to occur, so Al was made 0.005% or more and 0.05% or less. Preferably, the content is 0.01% or more and 0.04% or less.

B: 0.0001% or more and 0.0010% or less B is known to strengthen the bond between crystal grains and improve secondary work brittleness. However, excessive addition exceeding 0.0010% reduces elongation. Therefore, the addition amount of B is 0.0001% or more and 0.0010% or less. Preferably, the content is 0.0002% or more and 0.0008% or less.

Ti: 0.20% or more and 0.40% or less Ti preferentially forms carbonitride to reduce solid solution C and N, and suppresses the formation of Cr carbonitride, so that ductility, toughness, It has the effect of improving weldability and corrosion resistance. However, if the content is less than 0.20%, the effect of addition is poor. On the other hand, addition over 0.40% not only lowers toughness but also precipitates TiN and causes surface defects. . Therefore, Ti is made 0.20% or less and 0.40% or more. Preferably, the content is 0.21% or more and 0.30% or less.

O: 0.0030% or less O intervenes in steel in the form of inclusions or solid solutions, and produces Al or Ti-based oxides during welding. If the amount of O exceeds 0.0030%, the amount of oxide increases, which tends to cause cracks during processing. Therefore, O is set to 0.0030% or less. Preferably it is 0.0025% or less.

  Further, regarding Ti, O, N, and Cr that are constituent elements of the present invention, the relational expression ([% Ti] −5 × [% O]) × [% N] / [% Exclude regions that satisfy Cr] ≦ 0.000120.

  The remainder other than the above components is Fe and inevitable impurities. Examples of inevitable impurities include elements such as Mg: 0.0015% or less, Ca: 0.0020% or less.

As described above, the appropriate range of the basic components has been described. However, in the present invention, it is not sufficient that each component simply satisfies the above composition range, and the relationship of the following formulas must also be satisfied.

  In addition, [% X] shows the mass% of the component X in steel.

  The above formula (1) is a condition necessary for preferentially fixing solute C and N with Ti. By satisfying the above formula, not only the workability is improved but also Cr carbonitride is precipitated. Accordingly, the formation of the Cr removal layer can be suppressed and the corrosion resistance can be improved.

Satisfying the above formula (2) is very important in improving the strain aging resistance. Mo is chemically bonded to P, B, and Si in the melting and solidification process in welding and processed. Has the effect of suppressing distortion aging of time. However, when the value of M is less than 1.0 × 10 ⁻² , the effect cannot be obtained.

  Although the basic components have been described above, the following elements can be appropriately contained in the present invention.

V: 0.01% or more and 0.3% or less, W: 0.001% or more and 0.1% or less, Co: 0.01% or more and 0.2% or less V, W and Co are all affected by heat due to welding. Although it is an effective element to improve the cracking susceptibility of the joints, the addition effect cannot be obtained unless the respective contents are lower than the lower limit. In the above range. Preferably, V: 0.02% to 0.25%, W: 0.001% to 0.08%, Co: 0.01% to 0.15%.

Cu: 0.05% or more and 2.0% or less Cu is an element that improves the corrosion resistance. If higher corrosion resistance is desired, 0.05% or more is preferably added as necessary. However, if added over 2.0%, the hot workability deteriorates and the weld toughness deteriorates. Therefore, the addition amount of Cu is 0.05% or more and 2.0% or less. Preferably they are 0.1% or more and 1.5% or less.

  Next, the suitable manufacturing method of this invention steel is demonstrated.

  The molten steel having the above component composition is melted by a known method such as a converter, an electric furnace, a vacuum melting furnace or the like, and is made into a slab by a continuous casting method or an ingot-bundling method. The slab is then heated, or directly hot-rolled without heating to form a hot-rolled steel strip. The hot-rolled steel strip is usually annealed by a continuous annealing furnace or a box furnace, but depending on the application, the steel strip annealing may be omitted. Next, after pickling, it is cold rolled into a cold-rolled steel strip, subjected to recrystallization annealing and pickling to obtain a product. It should be noted that adjusting the steel composition to the component composition of the present invention can be easily performed by those skilled in the art using conventional means, including preparation within the range of the formulas (1) and (2). it can.

<Example>
Hereinafter, the present invention will be described in more detail based on examples.

  Steels having the components shown in Table 1 were melted in a vacuum melting furnace to obtain a small steel ingot (50 kg). No. A to H are steels of the present invention, No. I to O are comparative steels. A specimen having a thickness of 175 mm was cut out from these small steel ingots, heated at 1100 ° C. and hot-rolled to obtain a hot-rolled sheet having a thickness of 3.5 mm. Subsequently, these hot-rolled sheets were annealed at 700-800 ° C. corresponding to box-type annealing for 8 hours, pickled, and then cold-rolled to obtain 1.5 mm-thick cold-rolled sheets. Furthermore, 40 s of final annealing was performed at 750-880 degreeC, and it was set as the cold rolled annealing board.

  For the welding, the surface of the cold-rolled annealed plate was subjected to TIG welding in parallel with the rolling direction after # 500 emery polishing. TIG welding is carried out at a current of 140 to 150 A, a voltage of 10 V, a welding speed of 0.01 m / s, a gas shield of 99.99% Ar, and a flow rate of 20 l / min on the front bead side and 10 l / min on the back bead side. It was.

  In order to measure the degree of strain aging in the weld zone, a JIS 13B test piece was taken in parallel with the weld bead and subjected to a heat treatment at 300 ° C for 30 minutes with a stress σ when 10% pre-strain was applied. After that, the difference (equation (3)) with 0.2% YS when pulled again was evaluated.

S value = 0.2% YS−σ (3)
The smaller the S value, the smaller the degree of strain aging resistance, and it can be said that the strain aging resistance is superior.

  In addition, in order to evaluate the toughness of the welded part, Charpy specimens were taken from the as-welded plate so that the V-notch was positioned at the center of the bead, and five steels were tested at room temperature to determine the impact absorption energy. The average value was obtained. Furthermore, after pre-straining 10%, heat treatment was performed at 300 ° C. for 30 minutes, and then a Charpy test piece was taken so that the V notch was positioned at the center of the bead and heated to 300 ° C. to perform the Charpy test. went. The test was performed five times for each steel, and the average value of the impact absorption energy was obtained. The difference ΔE between these average values is obtained and shown in Table 1.

It can be seen that the steels of the present invention (No. A to H) have excellent strain aging characteristics with less hardening due to strain aging and lower toughness than the comparative examples (No. I to O). .

Claims (3)

C: 0.0020% to 0.0120%, N: 0.0030% to 0.0120%, C + N: 0.0050% to 0.0200%, Si: 0.05% or more 1.0% or less, Mn: 0.10% or more and 1.0% or less, P: 0.04% or less, S: 0.005% or less, Cr: more than 9% and less than 12%, Ni: 0.5% Hereinafter, Mo: 0.02% to 0.2%, Al: 0.005% to 0.05%, B: 0.0001% to 0.0010%, Ti: 0.20% to 0.000. 40% or less, O: 0.0030% or less, balance Fe and inevitable impurities (however, ([% Ti] -5 × [% O]) × [% N] / [% Cr] ≦ 0.000120 )except for),

A Cr-containing alloy excellent in strain aging resistance of a welded portion, characterized by being
[% X] represents mass% of the component X in the steel.   Further, by mass%, V: 0.01% to 0.3%, W: 0.001% to 0.1%, and Co: 0.01% to 0.2% were selected. The Cr-containing alloy according to claim 1, comprising one or more kinds.   The Cr-containing alloy according to claim 1 or 2, further comprising Cu: 0.05% or more and 2.0% or less by mass%.