JPH0142331B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to the production of cold rolled steel sheets for press forming,
Specifically, the production of cold-rolled steel sheets for press forming with improved chemical conversion properties without causing deterioration of press formability due to entrapment in a carburizing or nitriding atmosphere during cooling after annealing of the cold-rolled steel sheets. Regarding the method. In recent years, continuous annealing methods have been widely adopted as methods for annealing cold-rolled steel sheets for the purpose of saving energy or shortening delivery times. In order to obtain a steel plate with good press formability by continuous annealing, the content of C and N in the steel must be reduced as much as possible,
Furthermore, carbide-forming elements such as Ti, Nb, and Zr, as well as Al and B
It is well known that it is preferable to add nitride-forming elements such as nitride-forming elements to fix solid solution C and solid solution N in steel as carbides and nitrides. However, in cold-rolled steel sheets manufactured by this method, the phosphate film crystals become coarse or the phosphate crystals become coarse in the zinc phosphate chemical conversion treatment that is applied as a base treatment for painting after press forming. This has a fatal drawback as a steel sheet for automobiles: areas where precipitation does not occur, that is, sagging occurs, and as a result, corrosion resistance after painting is poor. As a result of studies to improve the above-mentioned drawbacks, the present invention has been completed by discovering that the chemical conversion treatability of cold-rolled steel sheets is dramatically improved when solid solution C or solid solution N is present in the steel. During cooling after annealing of a cold-rolled steel sheet, leaching, carburizing, or nitriding is performed to form a solid solution of carbon or nitrogen only in the surface layer of the steel sheet.
We propose a method for manufacturing a cold rolled steel sheet for press forming, which improves chemical conversion treatment properties without causing deterioration of press formability. The present invention will be explained in detail below. First, in order to obtain a steel plate with good press formability by continuous annealing, it is preferable that the amounts of C and N be as small as possible, such as C<0.01% and N<0.01%.
It is necessary to do so. Next, carbonitride-forming elements such as Ti, Nb, and Zr,
Nitride-forming elements such as Al and B are added. That is, when these elements are added, the extent to which press formability is inhibited by C and N in the steel is significantly reduced. However, when carbides and nitrides are present in large amounts, the ductility of the steel sheet deteriorates and press formability is inhibited. Furthermore, in order to obtain good press formability, it is desirable that C in the steel is not in a solid solution state before annealing. Therefore, in order to fix C as TiC, NbC or ZrC, it is necessary that the carbonitride-forming element be contained in the steel in an atomic ratio of 1 or more to C. Further, even when two or more carbonitride-forming elements are added in combination, there is no problem as long as the total atomic ratio of each element to C is 1 or more. However, if these elements are excessively contained, C and N that have been carburized or nitrided into the steel may become carbides or nitrides because they are retained in a carburizing or nitriding atmosphere as described below during cooling after annealing. It becomes a nitride, and on the contrary, the effect of improving chemical conversion treatment properties cannot be obtained. From this point of view, the total content of carbonitride-forming elements needs to be 5 or less in atomic ratio with respect to the amount of C contained in the steel sheet before annealing. Furthermore, although nitride-forming elements such as B and Al are difficult to form carbides, they easily form nitrides in steel. Therefore, in order to effectively generate carbides using carbonitride-forming elements, N in the steel is fixed as nitrides using B, Al, etc., thereby achieving good press formability and at the same time Since it is possible to reduce the amount of carbonitride-forming elements added, it is possible to fully exhibit the effect of improving chemical conversion treatment properties by carburizing treatment during cooling after annealing. In order to expect such an effect, the atomic ratio of B or Al to N in the steel must be 0.5 or more. On the other hand, as in the case of the carbonitride-forming element group, if the impregnation is excessive, the effect of improving chemical conversion treatment by nitriding during annealing and cooling will not be realized, so the total atomic ratio of N in steel will be 5. The following shall apply. As mentioned above, the annealing temperature is 750 for the above composition.
The material is heated to a temperature higher than .degree. C., because sufficient recrystallization is necessary to obtain good press formability. Next, after heating to 750°C or higher, when cooling, carburizing or nitriding is performed to improve chemical conversion properties.
10°C in a carburizing or nitriding atmosphere.
Keep it in place for more than a second. That is, in order to obtain good chemical conversion treatment properties, it is necessary that a suitable amount of solid solute C or solid solute N exist in the steel. Although the cause of this is not certain, it can be inferred as follows. When solid solution C and solid solution N are not present in steel, grain boundaries become preferential nucleation sites for phosphate crystals during chemical conversion treatment, and phosphate crystals nucleate and grow exclusively from grain boundaries. For this reason, one crystal grain of the steel sheet is covered with at most several phosphate crystals, and the phosphate crystal grains become coarse. On the other hand, if solute C or N exists in steel, it will segregate at grain boundaries,
By stabilizing the grain boundaries, the nucleation of phosphate crystals from the grain boundaries is suppressed, and the countless nucleation sites such as dislocations and precipitates that exist within the grains act as preferential nucleation sites, and an extremely large number of phosphorus Generates acid salt crystal nuclei. Therefore, the crystal grains of the phosphate coating become fine, forming a dense and good coating. Here, solid solution C and solid solution N that exist in steel significantly deteriorate press formability, so it is not a good idea to make C and N exist in solid solution in steel before annealing. By allowing solid solution C and solid solution N to exist only in the surface layer of the steel sheet after recrystallization annealing, chemical conversion treatment and press formability can be simultaneously improved. As mentioned above, when carburizing or nitriding,
When the temperature is 750°C or higher, carburized or nitrided C or N diffuses into the steel and becomes difficult to remain in the surface layer, and at the same time reacts with carbonitride-forming elements and nitride-forming elements in the steel, forming carbides, This is not preferable because it becomes nitride. Further, if the temperature is below 450°C, the speed of carburizing or nitriding becomes undesirable, and if the carburizing temperature is below 450°C, free C may adhere to the surface, which may even impair the chemical conversion properties. Therefore, carburizing or nitriding must be carried out at a temperature range of 750°C or lower and 450°C or higher. In this temperature range, if the steel plate is retained in a carburizing or nitriding atmosphere for 10 seconds or more, a sufficient effect of improving chemical conversion properties can be obtained, and press formability will not deteriorate in the slightest. The carburizing atmosphere includes, for example, CO,
(H ₂ + N ₂ ) gas that impregnates CH ₃ etc. or other mixed gas may be used. As the nitriding atmosphere, for example, (N ₂ + H ₂ ) gas containing NH ₃ or other mixed gas may be used. It is sufficient to use a mixed gas. Examples will be described below. Example 1 After melting the steel having the composition shown in Table 1, it was hot-rolled into a hot-rolled plate with a thickness of 2.8 mm, and after descaling, it was cold-rolled into a cold-rolled plate with a thickness of 0.7 mm. Figure a,
Three types of annealing were performed as shown in FIG. 1b and FIG. 1c. That is, FIG. 1a shows a normal annealing atmosphere cycle, where in a (7% H ₂ +N ₂ ) atmosphere,
After heating at 800°C for 20 seconds, it was cooled from 700°C to 450°C at a rate of 20°C/second, and then allowed to cool. Figure 1b shows the atmosphere cycle for carburizing treatment; after heating at 800°C for 20 seconds in a (7% H ₂ + N ₂ ) atmosphere, heating at 700°C in a (3% CH ₃ + 7% H ₂ + N ₂ ) atmosphere
450℃ is cooled at a rate of 10℃/sec, and then (7%
It was left to cool in a H ₂ +N ₂ ) atmosphere. Figure 1c shows the atmosphere cycle for nitriding treatment; after heating at 800°C for 20 seconds in a (7% H ₂ + N ₂ ) atmosphere, heating at 700°C in a (5% NH ₃ +7% H ₂ + N ₂ ) atmosphere. ~
450℃ is cooled at a rate of 10℃/sec, and then (7%
It was left to cool in a H ₂ +N ₂ ) atmosphere. In FIGS. 1a, 1b, and 1c, reference numeral 1 indicates the annealing start position, 2 the cold start position, 3 the treatment start position, and 4 the treatment end position. Also, is (7% H ₂ + N ₂ ) atmosphere, is (3
%CH ₃ + 7% H ₂ + N ₂ ) atmosphere, is (5% NH ₃
+7%H ₂ +N ₂ ) atmosphere. After annealing, the steel sheets were subjected to 0.8% temper rolling, and the material properties and chemical conversion treatability of the steel sheets were investigated. The results are shown in Table 2. In Table 2, all steel sheets other than No. 10, in which the amount of carbonitride-forming elements is 1 or less in atomic ratio to C, have materials with YS≦20Kgf/mm ² , El≧48%, and ≧1.8. It can be seen that it exhibits excellent press formability. Furthermore, the particle size of phosphate crystals that exhibit chemical conversion treatment properties and the oxygen reduction current value measured in a PH12 solution after chemical conversion treatment vary greatly depending on the annealing method.
In other words, when carburized, steels other than No. 2 and No. 6, which contain excessive amounts of carbonitride-forming elements, had fine phosphate crystals, and the oxygen reduction current measured after chemical conversion treatment was A low and good phosphate film is formed. Also, even when nitrided, No.
2. No. 6 and No. 9 containing excessive nitride-forming elements
It can be seen that a good phosphate film was formed in all other respects.

【table】

【table】

【table】

[Table] Example 2 The relationship between carburizing conditions, nitriding conditions, and chemical conversion treatability will be explained. Using No. 1 and No. 3 steel, after soaking and holding at 800℃, 800℃, 750℃, 700℃, 600℃, 500℃
C, 450 C, and 400 C for 15 seconds, and 600 C for 5 to 30 seconds, respectively. After cooling to room temperature, temper rolling was performed to examine chemical conversion treatment properties. The temperature dependence of the phosphate crystal size when held at 800°C to 400°C for 15 seconds was calculated as follows.
This is shown in Figure a and Figure 2b. In addition, FIG. 2a shows the case of carburizing treatment, FIG. 2b shows the case of nitriding treatment, black circles show the case of No. 1 steel, and white circles show the case of No. 3 steel. When carburizing or nitriding at 800°C or 400°C, the phosphate crystals become coarse and the effect of carburizing or carburizing is not recognized. It can be seen that when carburizing was carried out in the temperature range of 450°C to 75°C, the phosphate crystals became finer and the chemical conversion treatment properties were improved. On the other hand, in the case of nitriding treatment, the temperature is 450℃~700℃
A remarkable effect is observed when nitriding with 750
The effect is small at ℃, and no effect is observed at 800℃. The reason why the chemical conversion properties are not improved when carburized or nitrided at 800℃ is because carbides or nitrides are quickly formed in the steel after carburizing or nitriding, and the effect is not recognized at 450℃ or lower. This is because no carburizing or nitriding occurs. Next, FIGS. 3a and 3b show changes in the particle size of phosphate crystals and changes in oxygen reduction current depending on the treatment time when carburizing or nitriding at 600°C. In addition, Fig. 3a shows the case of carburizing treatment, Fig. 3b shows the case of carburizing treatment, and the black circles are No. 1 steel,
White circles indicate the case of No. 3 steel. In Figures 3a and 3b, if the carburizing or nitriding treatment time is less than 10 seconds, sufficient improvement in chemical conversion properties is not observed, and even if the treatment is performed for more than 20 seconds, phosphate crystals no longer crystallize. This shows that neither the particles nor the oxygen reduction current value change.

[Brief explanation of drawings]

Figures 1a, 1b, and 1c are explanatory diagrams showing the atmosphere cycles in normal treatment, carburizing treatment, and nitriding treatment, respectively, and Figure 2a
Figure 2b is a graph showing the relationship between treatment temperature and phosphate average crystal grain size in case of carburizing treatment, Figure 2b is a graph showing the relationship between treatment temperature and phosphate average crystal grain size in case of nitriding treatment, Figure 3a is a graph showing the relationship between treatment time, oxygen reduction current, and phosphate average crystal particle size in the case of carburizing treatment, and Figure 3b is a graph showing the relationship between treatment time, oxygen reduction current, and phosphate average crystal particle size in the case of nitriding treatment. It is a graph showing the relationship between average crystal particle diameters. Code 1...Annealing start position, 2...Cooling start position, 3...Processing start position, 4...Processing end position,
...(7%H ₂ +N ₂ ) atmosphere, ...(3%
CH ₃ +7% H ₂ + N ₂ ) atmosphere,...(5% NH ₃
+7%H ₂ +N ₂ ) Atmosphere.

Claims (1)

[Claims] 1 Contains C<0.01% and N<0.01%, Ti,
Contains one or more types of carbonitride-forming elements such as Nb and Zr, and the total atomic ratio of these carbonitride-forming elements to C is 1 to 5, and nitridation of B, Al, etc. The total number of nitride-forming elements including one or two of the nitride-forming element group is N.
When a cold-rolled steel sheet with an atomic ratio of 0.5 to 5 is heated to 750°C or higher by continuous annealing and then cooled, it is exposed to a carburizing or nitriding atmosphere in the temperature range of 750°C to 450°C. A method for producing a cold-rolled steel sheet for press forming with excellent chemical conversion treatment properties, characterized by retaining the steel for 10 seconds or more.