JP4699269B2 - Cr-containing steel sheet having excellent workability and method for producing the same - Google Patents

Description

  The present invention relates to a Cr-containing steel sheet excellent in workability used for automobile exhaust system materials and the like and a method for producing the same.

  Conventionally, it is known that a Cr-containing steel sheet having C + N of 300 ppm or less and containing Ti and Nb as stabilizing elements exhibits excellent workability. Among them, Cr-containing steel plates with 10 to 14% Cr content, such as SUH409L (JIS G 4312), with addition of Ti are soft and stretchable, and have excellent deep drawability. It is widely used as a member that is used in moderately moisturizing environments and at temperatures of 800 ° C. or lower.

  On the other hand, in recent years, improvement in fuel efficiency of automobiles has been demanded in order to reduce the burden on the environment. For this reason, it is important to reduce the weight of the vehicle body, and in many cases, to reduce the size and weight of the vehicle body in the same living space. In that case, the required property of the exhaust system material is an improvement in workability, and this requirement is increasingly high.

  The reason why SUH409L is relatively excellent in workability is that so-called IF (Interstitial Free) is performed in which C and N are reduced as much as possible and carbonitride-forming elements such as Ti and Nb are added. C + N is about 100 to 300 ppm, and Ti is added about 0.2%.

Several methods for further improving the workability have been disclosed. For example, in Patent Document 1, in hot rolling, a steel plate is wound at 600 ° C. or higher so that the hot work sheet annealing is omitted even if the hot-rolled sheet annealing is omitted. A method for producing Cr-containing steel that is excellent in the above is disclosed.
Japanese Patent Laid-Open No. 05-320772

  However, demands for improving the workability of Cr-containing steel in recent years have become increasingly severe, and it has been difficult to meet such demands with the above-described conventional technology.

  Then, this invention aims at providing the Cr containing steel plate excellent in workability, and its manufacturing method.

The gist of the present invention is as follows.
(1) By mass%, Cr: 10% or more and 14% or less, and further Si and Al, Si: 0.1% or less, Al: 0.01% or less, Si + Al: 0.001% or more, 0 And C, N, Mn, P, S, Ti, Nb, Zr, and V, C: 0.003% or less, N: 0.003% or less, C + N: 0.0. 005% or less, Mn: 0.1% or less, P: 0.04% or less, S: 0.01% or less, Ti + Nb + Zr + V: limited to 0.01% or less, remaining Fe and unavoidable impurities A Cr-containing steel sheet having excellent workability.
(2) The sheet thickness is 0.5 mm or more and 2.5 mm or less, the elongation in the rolling direction is 40% or more, and the average r value represented by the following (Formula 1) is 1.8 or more. The Cr containing steel plate excellent in workability as described in said (1).

Average r value = (rL + 2 × rD + rC) / 4 (Formula 1)
However, rL, rD, and rC are r values in the rolling (L) direction, 45 ° (D) direction, and sheet width (C) direction, respectively.
(3) By mass%, Cr: 10% or more and 14% or less, and further Si and Al, Si: 0.1% or less, Al: 0.01% or less, Si + Al: 0.001% or more, 0 And C, N, Mn, P, S, Ti, Nb, Zr, and V, C: 0.003% or less, N: 0.003% or less, C + N: 0.0. 005% or less, Mn: 0.1% or less, P: 0.04% or less, S: 0.01% or less, Ti + Nb + Zr + V: 0.01% or less, and a steel slab comprising the balance Fe and inevitable impurities , Hot rolling under conditions of a heating temperature of 1000 to 1300 ° C., a finishing temperature of 800 to 1000 ° C., a winding temperature of 500 to 800 ° C., and the recrystallization rate of the hot rolled sheet after hot rolling to 90% or more, , Cold rolling, final annealing, excellent workability Cr content A method of manufacturing a steel sheet.

  By this invention, Cr containing steel excellent in workability can be provided, a great profit can be obtained not only for the manufacturer but also for those who use this steel, and the industrial value is extremely high.

  The best mode and limiting conditions for carrying out the present invention will be described in detail.

  The present inventors have conducted a detailed study on Cr-containing steel having excellent workability. Among them, so-called IF (Interstitial Free), in which C and N are reduced as much as possible and carbonitride forming elements such as Ti and Nb are added, is contrary to the common sense that has been said to be a means of improving workability. It has been clarified that if the IF is advanced to the limit, the strengthening allowance for the solid solution of Ti and Nb added to stabilize C and N is large, and the processability cannot be improved sufficiently.

  Therefore, the inventors changed the way of thinking and based on the idea that workability can be improved by reducing C and N as much as possible without adding carbonitride-forming elements such as Ti and Nb. We proceeded with examination. As a result, the steel sheet produced by reducing C and N as much as possible and reducing other solid solution elements as much as possible has a very high recrystallization speed in the hot rolling process, and the production conditions are appropriate. It has been found that a recrystallized structure can be obtained in a hot-rolled sheet after hot rolling. This is difficult with conventional Cr-containing steel sheets. Furthermore, the workability (elongation, r value) of the steel sheet from which the recrystallized structure was obtained by hot-rolled sheet greatly exceeds the conventional example even though no carbonitride-forming elements such as Ti and Nb are added. I found out.

  Based on the above knowledge, the present inventors proceeded with detailed studies and completed the present invention.

  First, the limiting conditions for each component will be described.

  Cr is a basic element necessary for ensuring corrosion resistance and oxidation resistance. If its content is less than 10%, sufficient corrosion resistance cannot be obtained. Moreover, since Cr is also a solid solution strengthening element, when it exceeds 14%, workability will fall. Therefore, the Cr addition amount is 10% or more and 14% or less.

  Since Si is a solid solution strengthening element, it is reduced as much as possible. The upper limit is 0.1%.

  Al is reduced as much as possible in order to reduce workability. In order to prevent deterioration of workability, the upper limit is made 0.01%.

  However, Si and Al are useful as deoxidizing elements, and a trace amount needs to be added. Therefore, Si and Al are added together in an amount of 0.001% to 0.1%. This is because if the total amount is less than 0.001%, it is not effective as a deoxidizer, and if it exceeds 0.1%, the workability is lowered.

  C is an unavoidable impurity element, but is reduced as much as possible in the present invention. Therefore, the upper limit is set to 0.003%. It is because workability will fall when it contains more than this.

  N is an unavoidable element like C and is reduced as much as possible in the present invention. Therefore, the upper limit is set to 0.003%. It is because workability will fall when it contains more than this.

  It is desirable to reduce C + N as much as possible. In order to prevent deterioration of workability, the upper limit is made 0.005%.

  Mn is a component inevitably contained in the steel, but is reduced as much as possible. In order to prevent deterioration of workability, the upper limit is made 0.1%.

  P is a component inevitably contained in the steel, but if it exceeds 0.04%, the toughness decreases, so 0.04% was made the upper limit.

  S is a component inevitably contained in the steel, but if it exceeds 0.01%, the corrosion resistance decreases, so the upper limit is made 0.01%.

  Ti, Nb, Zr, and V are stabilizing elements that fix C and N as carbonitrides, but are also solid solution strengthening elements. In the state where C and N are sufficiently reduced to the scope of the present invention, the present inventors have added Ti, Nb, Zr, and V elements to a total amount exceeding 0.01%. It has been found that the effect of solid solution strengthening is greater than the stabilization effect, and contrary to the conventional wisdom, the workability is reduced. Therefore, the upper limit of the sum of these elements, Ti + Nb + Zr + V, was set to 0.01%. However, it is very difficult to reduce these elements to the limit, and the cost is too high, so 0.001% is preferably set as the lower limit.

  The steel plate of the present invention having a suitable component in the above range preferably has a thickness of 0.5 mm to 2.5 mm. If the thickness is less than 0.5 mm, the necessary workability can be obtained even if the present invention is not present, and if it exceeds 2.5 mm, the necessary workability is difficult to obtain even with the present invention.

  Further, the steel sheet of the present invention preferably has an elongation value in the rolling direction of 40% or more and an r value of 1.8 or more in average r value. This is because if the elongation value is less than 40% and the average r value is less than 1.8, it cannot be said that the steel sheet is more excellent in workability than in the past.

  Here, the evaluation of elongation uses the breaking elongation value in the rolling (L) direction as an index, and the r value measures r values in the rolling (L) direction, 45 ° (D) direction, and 90 ° (C) direction, respectively. RL, rD, rC, and the average r value obtained from the following (Equation 1) is used as an index. These measurement methods were performed using a JIS 13B test piece conforming to JIS Z 2241, the elongation conforming to JIS Z 2241, and the r value conforming to JIS Z 2254.

Average r value = (rL + 2 × rD + rC) / 4 (Formula 1)
Next, the manufacturing method will be described in detail.

  Since the steel of the present invention restricts the additive elements as much as possible, carefully select raw materials with few impurities, use a converter-secondary refining method, or a vacuum melting furnace, and slabs, ingots, etc. of desired components It is desirable to melt a steel piece (hereinafter also simply referred to as a slab). The obtained steel slab becomes a product by a manufacturing method comprising simple steps of hot rolling, cold rolling, and final annealing.

  In a hot rolling process, it is set as a hot-rolled sheet about 2-6 mm from a steel piece. If it is less than 2 mm, it is too thin and unfavorable because the rolling reduction required in the next step of cold rolling cannot be obtained, and if it exceeds 6 mm, it is too thick and the load of the cold rolling process becomes too large.

  What is important in this hot rolling process is that the recrystallization rate of the hot rolled sheet after hot rolling is 90% or more. Here, by recrystallizing 90% or more, the elongation and the r value after the final annealing can be greatly improved. If the recrystallization rate is less than 90%, coarse and elongated crystal grains in the rolling direction remain, which is not preferable because elongation and r value after final annealing are reduced. In order to increase the elongation and the r value, the higher the recrystallization rate, the better. The most preferable recrystallization rate is 100%.

  In conventional Cr-containing steel sheets, recrystallization in hot-rolled sheets was insufficient, and it was difficult to improve workability. However, in the Cr-containing steel sheets of the present invention, a recrystallized structure is easily obtained with hot-rolled sheets. It is presumed that this is because recrystallization in hot rolling is promoted because additive elements are reduced as much as possible.

  Here, the recrystallization rate refers to a cross section in the rolling direction (L cross section) of the hot-rolled sheet as an observation surface, and is observed by mirror polishing, etching, and re-measured within a range of total plate thickness x constant plate width (for example, 1 mm). A value obtained by dividing the area of crystallized crystal grains by the total area is expressed as a percentage.

  In the steel sheet of the present invention, the hot rolling conditions for realizing a recrystallization rate of 90% or more in the hot rolled sheet are as follows: the heating temperature is 1000 ° C to 1300 ° C, the finishing temperature is 800 ° C to 1000 ° C, and the winding temperature is 500 ° C. The temperature is 800 ° C. or lower. If the heating temperature is less than 1000 ° C., wrinkles are likely to occur during hot rolling, so it is necessary to set the heating temperature to 1000 ° C. or higher. On the other hand, if the heating temperature is higher than 1300 ° C., the oxide scale becomes strong and the pickling property is impaired. If the finishing temperature is less than 800 ° C., recrystallization of the hot-rolled sheet becomes insufficient, so that the finishing temperature is set to 800 ° C. or more. On the other hand, if the finishing temperature is higher than 1000 ° C., a facility for suppressing the temperature drop of the steel sheet during hot rolling is required and the manufacturing cost is increased. When the coiling temperature is less than 500 ° C., recrystallization of the hot-rolled sheet becomes insufficient, so that the coiling temperature is set to 500 ° C. or more. On the other hand, when the coiling temperature is higher than 800 ° C., an equipment that suppresses the temperature drop of the steel sheet during hot rolling is required and the manufacturing cost is increased.

  The obtained hot-rolled sheet is pickled by a general pickling method such as nitric hydrofluoric acid pickling, and then cold-rolled to obtain a cold-rolled sheet having a thickness of 0.5 to 2.5 mm. The cold rolling conditions are not particularly defined, but the roll diameter is preferably 50 mm or more. If it is 50 mm or less, the r value is difficult to improve, which is not preferable. Further, the cold rolling rate is preferably 40% or more. If it is less than 40%, it is difficult to improve the r value. Here, the cold rolling rate is a percentage obtained by dividing the difference between the thickness of the hot-rolled sheet and the thickness of the cold-rolled sheet after the cold rolling by the thickness of the hot-rolled sheet.

  Although final annealing is performed to obtain a product, the annealing temperature is preferably 750 to 900 ° C. If it is less than 750 ° C., recrystallization is insufficient, which is not preferable, and if it exceeds 900 ° C., crystal grains become excessively large and rough skin is caused during processing, which is not preferable.

  The atmosphere of the final annealing is not particularly defined, but may be an atmosphere used in general steel production. In the subsequent pickling, a product is obtained by using a general pickling method such as nitric hydrofluoric acid pickling. Of course, bright annealing may be performed as it is.

  In the manufacturing method of the present invention, since a highly workable Cr-containing steel sheet can be obtained by a simple method, hot-rolled sheet annealing is not performed, but even if hot-rolled sheet annealing is performed, a workable Cr-containing steel sheet can be obtained. Needless to say. Moreover, it goes without saying that a Cr-containing steel sheet having good workability can be obtained even by repeated annealing and cold rolling.

  Hereinafter, the present invention will be described in more detail with reference to examples.

  A 50 kg steel piece having a thickness of 90 mm having the chemical components shown in Table 1 was melted using a vacuum melting furnace, and a hot rolled sheet having a thickness of 4.5 mm was obtained by hot rolling. The hot rolling conditions at this time were a heating temperature of 1100 ° C. to 1200 ° C., a finishing temperature of 800 ° C. to 950 ° C., and a winding temperature of 550 ° C. to 650 ° C. After this hot-rolled sheet was pickled with nitric hydrofluoric acid, it was rolled to 1.5 mm by cold rolling, finish-annealed at 800 ° C. to 900 ° C., and pickled with nitric hydrofluoric acid to obtain a test material.

  The recrystallization rate of the hot-rolled sheet is measured by mirror polishing, etching and observing the cross-section in the rolling direction (L-section) of the hot-rolled sheet as an observation surface, and recrystallized crystal grains in the range of total plate thickness x 1 mm. The area was measured, and the value obtained by dividing the area by the total area was expressed as a percentage.

  The tensile test used the JIS13 test piece based on JISZ2201. A test piece having the longitudinal direction in the L direction for measuring the elongation and the L, D, and C directions for measuring the r value was prepared.

  The elongation measurement was performed in accordance with JIS Z 2241, and the elongation at break in the L direction was used as an index.

  The r value is measured according to JIS Z 2254, and the average r calculated from the r value, rL, rD, and rC measured in the L, D, and C directions, with the average r value = (rL + 2 × rD + rC). The value was used as an index.

  Table 1 shows the results. Steel A to steel I are examples of the present invention, and all recrystallization of hot-rolled sheets has an elongation of 40% or more and an average r value of 1.8 or more, indicating excellent workability.

  In contrast to these inventive examples, J steel with high C and K steel with high N have high C + N, and the recrystallization rate of hot-rolled sheet is reduced, so that the elongation and r value are lowered, especially the r value is greatly lowered. It has become. L steel with high Cr is elongated and the r value is lowered. Although M steel with low Cr shows excellent workability, it is out of the scope of the present invention because Cr is low and is inferior in corrosion resistance. N steel with high Si and Al is also stretched, and both r values are decreased. From O steel to R steel is a steel in which Ti, Nb, Zr, and V forming carbonitrides are added beyond the scope of the present invention. Since these contribute to solid solution strengthening, the recrystallization of the hot-rolled sheet is suppressed, and the elongation, r value, particularly r value is lowered. The S steel is SUH409L, which is a typical Cr-containing steel. However, since there are many solute elements, the recrystallization rate of the hot-rolled sheet is zero, and both the elongation and the r value are lower than those of the examples of the present invention.

  From the above, it is clear that the steel of the present invention has excellent workability.

  A large number of 90 mm-thick steel pieces having a chemical component corresponding to steel A in Table 1 were melted using a vacuum melting furnace, and then the production conditions were changed to obtain steel plates. The measurement of the hot-rolled sheet recrystallization rate, the tensile test, and the evaluation of elongation and r value were carried out in the same manner as in Example 1. Production conditions and results are shown in Table 2.

  The present invention example from A1 steel to A4 steel manufactured within the scope of the manufacturing method of the present invention has a high hot-rolled sheet recrystallization rate of 90% or higher, an elongation of 40% or higher, and an average r value of 1.8 or higher. Yes, showing excellent workability.

  Even when the A5 steel having a thickness of 3 mm and the manufacturing method of the present invention is used, the average r value is slightly inferior. A6 steel having a high final annealing temperature was rough in the tensile test. In A7 steel having a low final annealing temperature, unrecrystallized grains remain on the product plate, so both the elongation and the r value are low. Since A8 steel with a low heating temperature has a low hot-rolled sheet recrystallization rate, the r value is low. The A9 steel with a low finishing temperature also has a low r value because the hot-rolled sheet recrystallization rate is low.

  From the above, it is clear that the steel of the present invention is excellent in workability.

Claims (3)

% By mass
Cr: 10% or more and 14% or less,
Furthermore, Si, Al,
Si: 0.1% or less,
Al: 0.01% or less,
Si + Al: contained to satisfy 0.001% or more and 0.1% or less,
C, N, Mn, P, S, Ti, Nb, Zr, V,
C: 0.003% or less,
N: 0.003% or less,
C + N: 0.005% or less,
Mn: 0.1% or less,
P: 0.04% or less,
S: 0.01% or less,
Ti + Nb + Zr + V: A Cr-containing steel sheet excellent in workability, characterized by being limited to 0.01% or less and comprising balance Fe and inevitable impurities. The plate thickness is 0.5 mm or more and 2.5 mm or less, the elongation in the rolling direction is 40% or more, and
The average r value shown by the following (Formula 1) is 1.8 or more, The Cr containing steel plate excellent in workability of Claim 1 characterized by the above-mentioned.
Average r value = (rL + 2 × rD + rC) / 4 (Formula 1)
However, rL, rD, and rC are r values in the rolling (L) direction, 45 ° (D) direction, and sheet width (C) direction, respectively. % By mass
Cr: 10% or more and 14% or less,
Furthermore, Si, Al,
Si: 0.1% or less,
Al: 0.01% or less,
Si + Al: contained to satisfy 0.001% or more and 0.1% or less,
C, N, Mn, P, S, Ti, Nb, Zr, V,
C: 0.003% or less,
N: 0.003% or less,
C + N: 0.005% or less,
Mn: 0.1% or less,
P: 0.04% or less,
S: 0.01% or less,
Ti + Nb + Zr + V: Restricted to 0.01% or less, a steel piece composed of the remainder Fe and inevitable impurities is hot-rolled under the conditions of a heating temperature of 1000 to 1300 ° C., a finishing temperature of 800 to 1000 ° C., and a winding temperature of 500 to 800 ° C. And the recrystallization rate of the hot-rolled sheet after hot rolling shall be 90% or more, Furthermore, cold rolling and final annealing are performed, The manufacturing method of the Cr containing steel plate excellent in workability characterized by the above-mentioned.