JP2007270181A - METHOD FOR ADJUSTING BAKE HARDENABILITY OF EXTRA-LOW CARBON STEEL CONTAINING Nb - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for adjusting a bake hardenability of an ultra-low carbon steel containing Nb, which can increase a degree of BH compared to a conventional method, regardless of a Nb/C atom ratio. <P>SOLUTION: This adjusting method is an improved technology of hot-dip galvanizing an extra-low carbon steel strip containing Nb by passing the steel strip in a line for producing a continuously hot-dip galvanized steel sheet, which has a vertical annealing furnace including a heating zone and a cooling zone, and has a plating tank. The adjusting method specifically includes controlling the temperature of the steel strip in an outlet side of the cooling zone to 550°C or lower, and simultaneously controlling a cooling rate to over 40°C/second while the ultra-low carbon steel strip containing Nb is in a range of 700 to 550°C in the cooling zone. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a method for adjusting the bake hardenability of an ultra-low carbon steel containing Nb, and more specifically, when the ultra-low carbon steel containing Nb is subjected to alloying hot dip galvanizing, the bake hardenability of the raw material The present invention relates to a technology for making an alloyed hot-dip galvanized steel sheet that has been improved compared to conventional steel.

2. Description of the Related Art In recent years, alloyed hot-dip galvanized steel sheets having characteristics (hereinafter referred to as bake hardenability) that are low in strength and excellent in workability and have a large increase in strength after paint baking are frequently used in automobile bodies. This bake hardenability is evaluated by the amount of BH (abbreviation of Bake Hardability) as in JIS standard: G 3135. The BH amount is a value obtained by subtracting the stress at the time of applying the pre-strain from the yield stress obtained by subjecting the test piece to a tensile test after the paint baking equivalent processing described later. And conventionally, the amount of BH was adjusted by changing the carbon (symbol: C) concentration, which is a chemical component of the steel sheet, and the Nb concentration that combines with carbon to produce carbide. That is, in order to obtain a larger amount of BH, the ratio of the Nb concentration to the carbon concentration in the steel is kept small (for example, the atomic ratio of Nb / C is set to less than 1), and carbon that does not generate carbide in the steel (so-called “ This is a technique in which a large amount of “free carbon” is left in the steel (for example, see Non-Patent Document 1).
Iron and steel, vol. 89 (2002) No. 11, page 109-111

  The conventional method for adjusting the amount of BH can be easily carried out industrially. However, when extremely low carbon steel (C <100 ppm) is targeted, the variation in carbon concentration is large, so a certain amount of Nb is added. Even so, there is a problem that the variation in the Nb / C atomic ratio is large, and the amount of BH inevitably varies greatly. In order to obtain a larger amount of BH by this method, the ratio of the Nb concentration to the carbon concentration in the steel is kept small (for example, the atomic ratio of Nb / C: 0.7 to 0.8). There is a need to keep free carbon rich farms high. Therefore, in the refining stage of the molten steel, the carbon concentration and Nb concentration in the molten steel are adjusted so as to be as narrow as possible. However, in the casting stage of the molten steel, the carbon pickup by the carbon supply from the mold powder or the like, The carbon concentration in the steel sheet after rolling varies due to the influence of carburization and the like when the slab is reheated immediately before the hot rolling stage, and as a result, the Nb / C atomic ratio of the steel sheet is 0.6 to 1. .0 greatly varies, and the fluctuation range of the BH amount also increases.

  In addition, since the free carbon concentration which does not exist as NbC in a steel plate is so high that the Nb / C atomic ratio of a steel plate is small, the larger amount of BH is obtained.

  However, when the Nb / C atomic ratio of the steel sheet is less than 0.7, when the steel sheet is pressed, a portion where the amount of free carbon is remarkably increased locally extends, and is referred to as “stretcher strain”. Another problem arises that the frequency of occurrence of “wrinkled surface defects” increases. Therefore, as a countermeasure, in the above-described conventional technology, the carbon concentration pickup in the molten steel casting stage and the slab hot rolling stage is taken into consideration, and the lower limit of the Nb / C atomic ratio is set to 0 at the time of molten steel ladle refining. As a result, there is a problem that the amount of BH of the steel sheet cannot be increased so much.

  In view of such circumstances, an object of the present invention is to provide a bake hardenability adjusting method for ultra-low carbon steel containing Nb that can increase the amount of BH compared to the conventional Nb / C atomic ratio.

  The inventor has intensively studied to achieve the above object, and the results have been embodied in the present invention.

  That is, the present invention passes a very low carbon steel strip containing Nb through a vertical annealing furnace comprising a heating zone and a cooling zone, and a continuous hot dip galvanized steel plate provided with a plating tank, When performing the plating, the cooling rate when the temperature on the cooling zone exit side of the ultra-low carbon steel strip containing Nb is 550 ° C. or less and the steel strip is in the range of 700 to 550 ° C. in the cooling zone. Is a bake hardenability adjusting method for ultra-low carbon steel containing Nb, characterized in that it is over 40 ° C./second. In this case, the Nb / C atomic ratio of the steel strip is preferably 0.7 to 1.0. Moreover, it is good to make Nb / C atomic ratio of the said steel strip more than 1.0, and to hold | maintain the temperature by the side of the heating strip of the said steel strip at 855-880 degreeC.

  In addition, the present invention passes a very low carbon steel strip containing Nb through a vertical annealing furnace composed of a heating zone and a cooling zone, and a continuous hot dip galvanized steel plate equipped with a plating tank. When plating, the Nb / C atomic ratio of the steel strip is less than 0.7, and the temperature on the cooling zone exit side of the steel strip is 650 to 680 ° C. This is a method for adjusting the bake hardenability of carbon steel.

  Furthermore, in the present invention described above, it is preferable to keep the temperature of the steel sheet after passing through the cooling zone so that the temperature when the steel strip enters the plating tank is maintained at 450 to 480 ° C. It is better to further heat and alloy the plated steel sheet.

  ADVANTAGE OF THE INVENTION According to this invention, it becomes possible to increase the BH amount of a 340 MPa class alloyed hot-dip galvanized steel plate from a conventional average of 24 MPa to 35 MPa using a very low carbon steel plate as a raw material. In addition, it is possible to reduce the variation in the BH amount (evaluated by 1σ) from the conventional 6 MPa to 3 MPa. Furthermore, it was possible to suppress the occurrence of stretcher strain defects that were sporadic when the carbon concentration was higher than the Nb concentration in the steel sheet and the Nb / C atomic ratio was less than 0.7.

  Hereinafter, the best embodiment of the present invention will be described based on the background of the invention.

  First, the inventor considered that in addition to the Nb / C concentration and free carbon concentration, there are factors that affect the amount of NbC produced in the steel. And all the processes which manufacture a hot-dip galvanized steel plate continuously in the manufacturing line of the continuous hot-dip galvanized steel plate provided with the vertical annealing furnace which consists of the heating zone 5 and the cooling zone 6 as shown in FIG. The steel strip 1 (hereinafter simply referred to as the “steel plate 1”) as a raw material is exposed to the heating zone 5 at the annealing stage before entering the plating bath 8 of the molten zinc bath, and the heating zone 5 exits the cooling zone 6. We focused on the cooling rate of the steel strip 1 to the side. This is because the temperature range in which NbC precipitates is generally 550 to 700 ° C.

  And before making it infiltrate into the plating tank 8 of a hot dip galvanization from the heating zone 5 (for example, it heats to an average of 850 degreeC by the indirect heating system by a radiant tube) of the steel plate 1 before this hot dip galvanization. In the process of cooling to 450 to 480 ° C. in the cooling zone 6 (for example, cooling by air cooling), the steel plate 1 passes through the NbC precipitation temperature region (550 to 700 ° C., hereinafter referred to as the NbC precipitation temperature region). We studied how to shorten the time as much as possible.

  As a result, specifically, when the steel sheet extracted from the heating zone is cooled in the cooling zone, the temperature of the steel strip on the cooling zone exit side is reduced from the conventional 630 ° C. as shown in Step 2 of FIG. The cooling rate of the steel sheet 1 was increased to 550 ° C. (Note that the broken line in FIG. 1 is the result of the conventional method, and step 2 is an example performed for comparison). Thereby, the cooling rate when the steel sheet passes through the NbC precipitation temperature region is increased from the conventional 35-40 ° C./second to 40 ° C./second, preferably 50 ° C./second or more, and passes through the NbC precipitation temperature region. The time can be shortened from the conventional 11 seconds to 5 to 8 seconds, and the average BH amount of the steel sheet 1 can be increased from the conventional 24 MPa to 35 MPa as an average value.

  Therefore, the inventor continues to study in detail, to set the temperature of the steel plate 1 on the cooling zone exit side to 550 ° C. or lower, and to cool the steel plate 1 when the steel plate 1 is in the range of 700 to 550 ° C. If it was satisfied that the speed was 50 ° C./second or more, the objective could be achieved by age hardening, and the present invention was completed based on these requirements.

  The reason why the ambient temperature in the cooling zone is set to 550 ° C. or lower is that NbC does not precipitate at less than 550 ° C., so that a large amount of free C (so-called “free C”) remains in the steel sheet. In addition, the steel plate 1 has a cooling rate exceeding 40 ° C./sec when the steel plate 1 is in the range of 700 to 550 ° C., because the time required to pass through the NbC precipitation temperature range becomes longer at 40 ° C./sec or less. This is because a large amount of NbC precipitates to reduce free C and decrease the amount of BH.

  In this case, the temperature of the steel plate 1 on the cooling zone exit side is limited by the lower limit of the steel plate temperature before entering the plating tank 8, but in order to obtain a sufficient amount of BH, the steel plate temperature on the cooling zone exit side is A heater having a heat retaining function may be provided in order to lower the temperature to 500 ° C. or lower, keep the temperature of the steel plate constant in the subsequent passage region, and keep the steel plate temperature before hot dipping at 450 to 480 ° C. as a target.

  Next, as described above, the inventor increases the cooling rate of the steel plate 1 and shortens the time during which the steel plate 1 is in the range of 700 to 550 ° C. to 5 to 8 seconds. Since it can be adjusted, it was considered that the amount of BH of the steel sheet can be adjusted by increasing or decreasing the cooling rate according to the chemical composition of the steel sheet or by increasing or decreasing the temperature of the heating furnace as necessary. When the Nb / C atomic ratio of the steel sheet is less than 0.7, the “stretcher strain” can be achieved by increasing the concentration of free carbon if the ambient temperature in the cooling zone is kept at about 600 ° C. ± 30 ° C. It was found that the occurrence of defects could be prevented, and this was also added to the present invention. Moreover, when the Nb / C atomic ratio of the steel sheet was set to 0.7 to 1.0, the effect of the above-described invention was further increased. Furthermore, when the Nb / C atomic ratio of the steel sheet exceeds 1.0, in addition to lowering the steel plate temperature on the cooling zone side, the steel plate temperature on the heating zone side is changed from the current 850 ° C. to 855 to 880 ° C. It was also considered to increase it. The reason for setting the temperature to 855 to 880 ° C. is that when the temperature is raised to 855 ° C., the amount of NbC dissolved in the steel starts to increase. If the temperature exceeds 880 ° C., the effect is saturated, and further temperature rise is wasted. Because. In addition, in the present invention, it is preferable to further heat the plated steel sheet to alloy zinc of the plating layer with iron.

  3 is a continuous hot-dip galvanized steel plate production line (alloying furnace not shown) equipped with a vertical annealing furnace comprising a heating zone 5 and a cooling zone 6 and a plating tank 8 as shown in FIG. An alloyed hot-dip galvanized steel sheet having a thickness of 0.6 to 1.0 mmt and a sheet width of 600 to 2000 mm was produced. The material has a chemical composition after hot rolling C ≦ 0.0050 mass%, Si ≦ 0.10 mass%, Mn: 0.05 to 0.100 mass%, P: 0.020 to 0.100 mass% , S ≦ 0.015% by mass, Nb: 0.005 to 0.020% by mass. At that time, the operation was carried out in five ways in the case of applying the bake hardenability adjusting method (4 ways) of the ultra-low carbon steel containing Nb according to the present invention and the conventional adjusting method. The conditions for plating and alloying were the same as normal operating conditions. The operation results are shown in Table 1.

  In the application of the present invention, the ambient temperature of the cooling zone is indirectly cooled with a radiant tube, and the temperature on the outlet side of the cooling zone 6 of the steel sheet 1 is set to 540 to 550 ° C., and in the application of the conventional method, it is 640 to 650 ° C. did. Moreover, the temperature of the steel plate 1 on the exit side of the heating zone 5 is set to 860 to 870 ° C. in the present invention, and is set to 830 to 850 ° C. in the conventional method.

  As a result, in the application of the present invention, as shown in Example 1 of Table 1, the cooling rate of the steel sheet 1 increased from 37 to 38 ° C./second to 56 to 58 ° C./second. The results of these operations were evaluated by the BH amount of the obtained galvannealed steel sheet and its standard deviation. That is, as shown in FIG. 2, in the conventional method, the BH amount, which was 24 MPa on average, was improved to 35 MPa by applying the present invention. In addition, the standard deviation of the BH amount was also reduced from 6 MPa of the conventional method to 3 MPa, and the variation in the BH amount was also reduced.

  Table 1 shows the results (Example 2) of the results of the operation of Example 1 and materials having different Nb / C atomic ratios, and the heating zone temperature of 860 to 870 when the Nb / C atomic ratio is 1.3. The result (Example 3) when increasing to ° C. is shown. In contrast, when the Nb / C atomic ratio is less than 0.7, the results when the cooling zone temperature is higher than before (670 to 680 ° C.) are also shown (Example 4).

  From these results, the Nb / C atomic ratio is 0.9, and the BH amount is the highest when the cooling zone temperature is less than 550 ° C. Further, when the Nb / C atomic ratio exceeds 1.0, the heating zone temperature is higher than the conventional one. Conversely, when the Nb / C atomic ratio is less than 0.7, the cooling zone temperature is increased. It becomes possible to make the amount of BH higher than the process method (conventional).

  The amount of BH was determined by subtracting the stress at the time of applying prestrain from the yield stress measured in the tensile test (JIS G 3135) after the paint baking equivalent treatment. The paint baking equivalent treatment is to immerse a test piece obtained from a steel plate in an oil bath under conditions of 170 ° C. × 20 min.

It is the figure which compared the heat pattern of the steel plate in an annealing furnace by the case where the bake hardenability adjustment method of the ultra-low carbon steel containing Nb which concerns on this invention, and the conventional adjustment method are applied. It is a figure which shows the frequency distribution of the BH amount of the manufactured galvannealed steel plate, (a) is a case where it manufactures by a conventional method, (b) is a case where it manufactures by this invention. It is a figure explaining the production line of a continuous hot dip galvanized steel plate.

Explanation of symbols

1 Coiled steel strip (steel plate)
2 Welding machine 3 Carriage 4 Flux treatment device 5 Heating zone 6 Cooling zone 7 Snout 8 Plating tank

Claims (6)

When passing a very low carbon steel strip containing Nb through a vertical annealing furnace consisting of a heating zone and a cooling zone, and a continuous hot dip galvanized steel plate equipped with a plating tank, and applying hot dip galvanization,
The temperature at the cooling zone outlet side of the ultra low carbon steel strip containing Nb is set to 550 ° C. or less, and the cooling rate when the steel strip is in the range of 700 to 550 ° C. within the cooling zone is 40 ° C./second The bake hardenability adjustment method of the ultra-low carbon steel containing Nb characterized by making it exceed.   2. The bake hardenability adjusting method for ultra-low carbon steel containing Nb according to claim 1, wherein the Nb / C atomic ratio of the steel strip is 0.7 to 1.0.   The Nb / C atomic ratio of the steel strip is made to exceed 1.0, and the temperature on the heating strip exit side of the steel strip is kept at 855 to 880 ° C. Method for adjusting bake hardenability of ultra-low carbon steel contained. When passing a very low carbon steel strip containing Nb through a vertical annealing furnace consisting of a heating zone and a cooling zone, and a continuous hot dip galvanized steel plate equipped with a plating tank, and applying hot dip galvanization,
Bake hardening of ultra low carbon steel containing Nb, characterized in that the Nb / C atomic ratio of the steel strip is less than 0.7, and the temperature on the cooling zone exit side of the steel strip is 650-680 ° C. Sex adjustment method.   The Nb according to any one of claims 1 to 4, wherein the steel plate is kept warm after passing through the cooling zone so as to keep the temperature when the steel strip enters the plating tank at 450 to 480 ° C. For adjusting the bake hardenability of ultra-low carbon steel containing iron.   6. The bake hardenability adjusting method for ultra-low carbon steel containing Nb according to any one of claims 1 to 5, wherein the hot-dip galvanized steel sheet is further heated and alloyed.

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