JPH0210855B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial field of application) A cooling material that is used for ultra-deep drawing of automobile exterior panels, etc., and has the property of increasing the yield point stress (bake hardening property; BH property) after baking to increase the rigidity of the car body. In connection with the production of rolled steel sheets, this specification describes a method for ultra-deep drawing that is highly ductile, has little anisotropy of the material, is free from strain aging deterioration, and has high BH properties, using the advantageous application of the continuous annealing method. This paper describes the results of research and development on an appropriate manufacturing method for cold-rolled steel sheets. Here, the BH property is 170℃, 20℃ with 2% prestrain.
It is expressed as the amount of increase in yield point stress before and after treatment when held for 30 minutes, and for BH steel sheets, BH≧
3Kg/mm ² is required, and strain aging is evaluated by the strain aging index AI value, and when AI > 3Kg/mm ² it is evaluated as deteriorated. (Conventional technology) Steel sheets for press working have conventionally been made with low carbon (C: 0.02
~0.07wt%; hereinafter simply expressed as %) Al-killed steel was generally manufactured using the box annealing method, but recently, in order to further improve pressability and high productivity, aluminum with C < 0.01% has been used. It is now manufactured using a continuous annealing method using low carbon steel. In ultra-low carbon steel, in order to prevent strain aging deterioration,
Carbonitride forming elements such as Nb and Ti are added.
Conventionally, these elements are often added alone because they are expensive, and the properties of Ti and Nb, which are most commonly used, are compared as follows. Ti-added steel has the advantage of good material quality even when low-temperature coiling is performed, which is advantageous in terms of descaling properties such as pickling, but the disadvantage is that the in-plane anisotropy of the steel sheet is large. On the other hand, Nb-added steel has the advantage of less in-plane anisotropy of the steel sheet, but on the other hand, it does not have sufficient mechanical properties when coiled at low temperatures during hot rolling. A compromise measure for simultaneously exhibiting the advantages of both Ti and Nb is disclosed in JP-A-58-107414. In this case, the upper limit of the Ti content is defined as (48/12C (%) + 48/14N (%)), the gist of which is that most of the Ti is preferentially consumed as TiN, and that solute C is The remaining effective Ti (totalTi−Ti as TiN) and
By fixing with Nb, deep drawability and anti-aging properties are ensured, but in actual experiments within the range of effective Ti according to the above disclosure, C in the steel could not be effectively bonded with Ti, resulting in This may cause significant deterioration in drawability and strain aging deterioration due to residual solid solution C. Regarding imparting BH properties to TiNb composite additive steel, the addition of a small amount of B is disclosed in JP-A-59-38337, and the reduction of Ti is disclosed in JP-A-59-31,827. ), and in the latter case, relative material deterioration and cost increase.
Since Ti preferentially combines with S in steel, solid solute N in steel is replaced by Ti.
It cannot be fixed by AlN, but rather by AlN, and fine AlN has its disadvantages in deterioration of the material quality, especially deep drawability. (Problems to be solved by the invention) A cold-rolled steel sheet for ultra-deep drawing that more fully demonstrates the effect of the composite addition of Ti and Nb and ensures BH properties without deteriorating the material quality. It is an object of this invention to establish a manufacturing method. (Means for solving the problem) In view of this situation, the inventors have developed the ultra-low carbon
BH while taking advantage of the advantages of Ti and Nb composite additive steel, such as press workability, especially good deep drawability, high ductility, and low material anisotropy.
We considered ways to ensure sex. As a result of a more detailed investigation of the combined addition effect of Ti and Nb, the inventors found that TiS and TiN preferentially precipitate during the slab heating stage or during rough rolling, which is a stage prior to hot finish rolling. It was found that the solid solution C was fixed by the remaining available Ti and Nb. In other words, the effective Ti is (total Ti−Ti as
It was found that TiN−Ti as TiS) should be used. Thus C, N, S, Ti and Nb of ultra-low C steel
By limiting the amount and also strictly limiting the coiling conditions during hot rolling and the soaking cooling conditions during continuous annealing after cold rolling, it is possible to make a cold rolled steel sheet with excellent BH properties for ultra-deep drawing. I found something that I was satisfied with. This invention has C: 0.0050wt% or less, Si: 1.0wt%
Below, Mn: 1.5wt% or less, Ti: (48/14N (%) + 48/32S (%)) ~ (2・48/12C (%) + 48/14N (%) + 48/32S (
%)) wt % Nb: (0.2・93/12C (%)) ~ (93/12C (%)) w
t% Al: 0.005~0.10wt%, P: 0.20wt% or less, N: 0.0050wt% or less, S: 0.015wt% or less, after hot rolling a steel consisting of Fe and inevitable impurities with the balance being 710~ After winding at a temperature of 530℃ and then cold rolling with a reduction rate of 50% or more,
It is characterized by soaking for 1 second or more in a temperature range exceeding 850℃ and up to Ac ₃ points, and then cooling at a rate of 5 to 300℃/s to 500℃ or less, with an anisotropy Δr of r value of 0.41 or less. This is a method for producing a cold-rolled steel sheet for ultra-drawing that has bake hardenability. Under strict control of the CNS, Ti, and Nb contents, and by dissolving the C fixed in Ti and Nb during continuous annealing at high temperatures, the solid solution C deteriorates due to strain aging. By leaving an appropriate amount remaining so as not to cause a problem, the BH properties are effectively ensured, and furthermore, the chemical conversion treatment properties are also improved due to the residual solid solution C. As already clear, in this invention, Ti, Nb
The elucidation of the effectiveness of this is an important matter for limiting the components of the starting materials, and the effects of the present invention will be explained in order from the background to this elucidation. (Function) First, the results of laboratory experiments conducted by the inventors will be explained. Chemical components: Si: tr ~ 0.02%, Mn: 0.10 ~
0.12%, P: 0.007-0.010%, Al: 0.02-0.04% are at the same level, and further, N: 0.0027%, C:
At 0.0020%, S: 0.006%, 0.013% and
3 levels of 0.018%, 3 levels of Ti: 0.015%, 0.025% and 0.034% and Nb: 0.008%, 0.020%
18 types of steel with two standards were melted in the laboratory, bloomed into a sheet bar with a thickness of 30 mm, heated to 1200°C, then hot rolled to a thickness of 2.8 mm in 7 passes, and then rolled at 900 ± 5°C. Finished with. 1.5 seconds after finishing rolling, this steel plate was cooled to 550°C at 35°C/s using water spray. Then, it was immediately charged into a furnace at 550°C, maintained for 5 hours, and then subjected to furnace cooling treatment. Through this treatment, a simulation was performed for a case where the winding temperature was 550°C within the range of 710 to 530°C. Then, after pickling, cold rolling was performed at a reduction rate of 75%.
Next, continuous annealing treatment is performed using a resistance heating device.
It was heated to 700°C at a rate of 4°C/s, then heated to 860°C at a heating rate of 3°C/s, held at 860°C for 25 seconds, and then cooled to room temperature at a rate of 30°C/s. Next, the steel plate was subjected to 0.5% temper rolling and then subjected to a tensile test. As a test item, a value (Rankford value) was used as a measure of deep drawability. As shown in the results in Figure 1, the materials of each experimental steel varied greatly with respect to the amounts of Ti, S, and Nb. If r≧1.8 is the standard material required for a steel plate for press working, the material that satisfies this is:
Ti≧48/14N (%) + 48/32S (%) (N=0.0027
%), and it can be seen that this is the case when Nb=0.008%. In other words, even with the same amount of C and the same amount of Nb, the drawability deteriorates due to an increase in S, and the Ti
It can be seen that it is necessary to increase the amount of C: Total elongation (El), which is the most important for steel sheets for processing
In order to improve the rank order value (r), the lower the C content, the better, and the C≦0.0050%, more preferably the C≦0.0035%. As C increases, it is advantageous for enhancing BH properties due to residual solid solution C, but on the other hand, it also tends to cause strain aging deterioration, so it should not exceed 0.0050%. Si: It may be added to increase the strength of high-strength steel sheets for deep drawing, but excessive addition of more than 1.0% is undesirable as it causes deterioration of weldability.
It shall be 1.0%. Mn: Mn also has an upper limit of 1.5 for exactly the same reason as Si.
%. N: Like S, which will be discussed next, N is fixed with Ti before hot rolling, so N alone is not harmful. However, TiN formed by adding a large amount reduces the total elongation and r value, so the upper limit is 0.0050%.
However, a more preferable range is 0.0035% or less. Also, if Ti is so small that N cannot be fixed,
N is fixed as AlN, and in this case, if the hot rolling coiling temperature is 710° C. or lower, the aggregation of AlN will not proceed, resulting in a hard material after continuous annealing, resulting in poor press workability. S: In this invention, S is the most important element in relation to the amount of Ti. S is rendered harmless as TiS during heating of a slab before hot rolling, but excess S is used to fix it.
The upper limit is set at 0.015% because the amount of Ti increases and causes material deterioration. Ti: Ti is the most important element among the chemical components of this invention. Ti fixes S and N before hot rolling before Al and Nb. The lower limit of Ti is determined by the amount that fixes S and N, i.e. Ti: (48/14N (%) + 48/32S (%))%. Solid solution C is fixed and redissolved in an appropriate amount during continuous annealing to impart BH properties.
In order to improve the material quality while dissolving the appropriate amount of
Considering these, the upper limit of Ti is Ti = (2・48/12C (%) + 48/14N (%) + 48/32S
(%)) % is the upper limit. Excess Ti exceeding this limit is
Not only does the ability to impart BH properties be lost, but the material deteriorates due to the increase in recrystallization temperature. Nb: In order for Nb to fix C during the hot working stage and help improve drawability and in-plane anisotropy of the steel plate,
Nb=(0.2・93/12C(%))% is required. However, exceeding the upper limit of Nb: (93/12C (%))% not only deteriorates ductility and material quality due to an increase in recrystallization temperature, but also prevents solid solution C from redissolving during continuous annealing, making it difficult to ensure BH properties. make it difficult Al: Al is required to be at least 0.005% in order to fix O in molten steel and improve the yield of Ti and Nb. On the other hand, since most of the N in molten steel is fixed by Ti as mentioned above, adding a large amount of Al increases the cost, so the upper limit is set at 0.10%. P: P is the most effective element for increasing strength without decreasing the value, but excessive addition impairs weldability, so the upper limit is set at 0.20%. Next, regarding the hot rolling conditions, the slab heating temperature before hot rolling is not particularly limited, but S and N are
The temperature is preferably 1280°C or lower, preferably 1230°C or lower, and more preferably 1150°C or lower. Note that similar effects can be expected by so-called direct slab rolling or by casting as a sheet bar with a thickness of about 30 mm and hot rolling as it is. The finishing temperature for hot rolling is preferably the usual Ar ₃ point or higher, but even if it is lowered to about 700°C, which is the alpha range, the material deterioration is small. It is difficult to obtain good material when the winding temperature is lower than 530°C, but the quality of the material is further improved if winding is performed at a high temperature of 600°C or higher, especially above 530°C. However, when the winding temperature exceeds 710°C, not only the material quality improvement effect is saturated, but also the descaling property deteriorates significantly, so the upper limit is set at 710°C. Next, regarding the cold rolling conditions, in order to improve drawability, the cold rolling rate after descaling is required to be 50% or more, and more preferably 70% to 90%. Continuous annealing conditions require soaking at a higher temperature than conventional methods, exceeding 850℃, in order to improve material properties such as drawability and ductility, as well as redissolve some of the C in the steel and impart BH properties. However, if the Ac is higher than ₃ points, the quality of the material, especially the drawability, will deteriorate significantly.
Over 850℃ and up to ₃ Ac points, and soaking time is 1
If the time is longer than 2 seconds, the re-solid solution of the solid solution C is completed. In this invention, the cooling rate during the above-mentioned continuous annealing after soaking and holding until the temperature reaches 500°C is 5°C/s.
If it is slower, solid solute C precipitates again and impairs BH properties, while if it exceeds 300°C/s, the remaining solid solute C becomes excessive and causes deterioration due to strain aging. Limited to 300℃/s or more. (Example) Steels (A) to (Q) whose compositions are shown in Table 1 were tapped from a converter furnace.
After RH degassing, it was made into a slab by continuous casting. The slab was then reheated to 1150℃, then 900℃
The material was finished to a thickness of 3.2 mm and then wound at various temperatures shown in Table 1. After pickling, cold rolling was performed at a reduction rate of 75% to obtain a cold rolled sheet with a thickness of 0.8 mm. Next, continuous annealing was performed by increasing the temperature to the homogeneous temperature T shown in Table 1 at a heating rate of 4°C/s and holding it for 20 seconds, and then heating to 450°C at various cooling rates also shown in Table 1 to room temperature at 10°C/s. It was cooled at a cooling rate of s.

【table】

[Table] * Comparative example ＿ is a component outside the range
48 48 48 48 48
93 93

Claims (1)

[Claims] 1 C: 0.0050wt% or less, Si: 1.5wt% or less,
Mn: 1.5wt% or less, Ti: (48/14N (%) + 48/32S (%)) ~ (2・48/12C (%) + 48/14N (%) + 48/32S (
%)) wt% Nb: (0.2・93/12C (%) ~ 93/12C (%)) wt% Al: 0.005 ~ 0.10wt%, P: 0.20wt% or less, N: 0.0050wt% or less, S: After hot rolling a steel containing 0.015wt% or less and the balance Fe and unavoidable impurities, it is coiled at a temperature of 710 to 530℃, then cold rolled at a reduction rate of 50% or more, and then rolled to 850℃. It is characterized by soaking for 1 second or more in a temperature range of up to ₃ points and then cooling at a rate of 5 to 300°C/s to below 500°C, with an anisotropy Δr of r value of 0.41
The following describes a method for producing a cold-rolled steel sheet for super drawing that has bake hardenability.