JPS6126757A - Cold rolled steel sheet for deep drawing having sintering hardness - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel sheet for deep drawing having sintering hardness by specifying the total amount of S and N in a dead soft steel contg. Ti. CONSTITUTION:The composition of a steel is composed of, by weight, 0.0005- 0.015% C, <1% Si, <1% Mn, 0.005%-0.1% Al, <0.15% P, <0.003% S, <0.004% N (S+N<=0.005%), available Ti represented by the equation by an amount 4-20 times the amount of C and the balance Fe with inevitable impurities. The steel is hot rolled and cold rolled to obtain a cold rolled steel sheet for deep drawing having burning hardenability (BH>=2kgf/mm.<2>). Said composition may further contain <3XC% Nb and/or <0.005% B.

Description

[Detailed Description of the Invention] (Field of Industrial Application) A cold-rolled steel sheet that has both ultra-deep drawability and bake hardenability is suitable for use that satisfies the following requirements.

In recent years, there has been an increasing demand for higher strength steel sheets for automobiles, with the aim of improving fuel efficiency by reducing the weight of automobiles.

On the other hand, from the viewpoint of press formability, properties such as low yield strength, low tensile strength, high elongation, and high r value are desired.

Against this background, there is a need for a steel sheet that is soft and exhibits good press formability during press forming, and has characteristics that increase yield strength and tensile strength during subsequent coating baking, that is, has bake hardenability.

This invention relates to a steel plate that has ultra-deep drawability among press formability as well as bake hardenability.

(Prior art) Regarding a cold rolled steel sheet having bake hardenability and its manufacturing method,
JP-A-53-114717 discloses T] additive steel,
Further, JP-A-57-70258 discloses Nb-added steel, and JP-A-59-31827 discloses Nb composite addition steel.

In all of these, the amount of solid solute C in the steel is controlled by controlling the amount of Ti and Nb added or the cooling rate during annealing, thereby imparting bake hardenability without causing material deterioration. It is something.

However, if an attempt is made to control the amounts of Ti and Nb added in this way to allow solid solution carbon to remain, the properties of the steel sheet will be significantly affected by subtle changes in the amounts added. That is, Ti
If the amount of Nb added is insufficient, it will lead to deterioration of the material that affects formability such as elongation and r value, while if the amount added is excessive, bake hardenability will be lost. Therefore, controlling the amount of added elements is considered to be a key issue in process production.

(Problems to be Solved by the Invention) It is necessary to avoid such disadvantages when limiting the amount of added carbonitride forming elements such as Ti and Nb (rather, to limit the content of S and N that should be combined with T1). An object of the present invention is to provide a cold-rolled steel sheet for deep drawing that has stable bake hardenability by limiting the following.

By the way, regarding limiting the respective contents of S and N, Japanese Patent Application Laid-open No. 110659/1983 discloses that S should be 0,001w or less (hereinafter simply expressed in %) to 0.020%, and N content should be 0.003%.
5%, and in Japanese Patent Application Laid-Open No. 58-42752, N is 0.
0.0025%, but the former is intended to prevent surface defects by reducing the amounts of Ti and B added, and the latter is likely to improve secondary processability and r-value. .

(Means for solving the problem) When the inventors were researching the relationship between the amount of S and N contained in the steel and the material quality, using a steel made by adding T1 to ultra-low carbon, they discovered that the amount of S and N contained in the steel We have discovered that high bake hardenability can be achieved by adjusting the total amount of

Based on this discovery, the present invention has achieved the following solutions to the above problems.

1, C: 0. (1005~0.0150″L%,
Si: 1. Ow or less Mn: 0150″L%
Hereinafter, l: 0.005 ~ 0.100 wt%P
: Contains 0.15wt% or less, S: 0.003
wL% or less, N: 0.0040w or less, and satisfies the conditions of (swt%] + [:Nwt%]≦0.0050, and the effective T1 (Ti”
) in a range of 4 to 20 times as much as [C″1%], with the balance being iron and unavoidable impurities, with bake hardenability (BH: 2 kg’/mm2 or more)
A cold-rolled steel sheet for deep drawing (first invention)).

2, C: 0.0005-0.0150wt%, S
i: 1, Ow%% or less Mn: 0150w or less, 8jg: 0.005 to 0.100w or less P
: 0.15" including 1% or less, S: 0.003 wt% or less, N: 0.00
40wt% or less [S w or less] + [Nwt%] ≦
0.0050, and further, the effective Ti (Ti”) given by the following formula is 4 to [Cwt%].
Contains 20 times as much Nb as [C″1%] and less than 3 times as much Nb as 0.0050wt%
Contains one or two of the following B, with the balance being iron and unavoidable impurities.
kg' 7mm2 or more) for deep drawing.

(Second invention), 3Z First, the contents of the experiments that led to the above-mentioned inventions will be explained. coo, 0015%, Mn: 0.1%, A
β: 0.04%.

Vacuum melted steel with varying amounts of N, S and Ti was melted in the laboratory and hot rolled to a thickness of 3.5 mm.
This cold-rolled sheet, which had been cold-rolled to 0.8+n+n, was heat-treated in a thermal cycle of soaking at 800° C. for 40 seconds, followed by temper rolling at a rolling reduction of about 0.8%, and then the material properties were investigated.

Note that a JIS No. S test piece was used for the tensile test.

Bake hardenability (hereinafter abbreviated as BH property) was determined by applying 2% prestrain as shown in FIG. 1, and then measuring the increase in yield point by aging treatment at 170° C. for 20 minutes equivalent to baking.

Figure 2 shows the relationship between the (S+N) quantity, Bβ, r value, and elf quantity. Bj here! , r is a value obtained by averaging the test results of test pieces taken parallel to the rolling direction, in the 45° direction, and in the 90° direction using the following formula.

FIG. 2 shows that when the [S] + [N amount is 50 ppm or less, the OH can be increased to 4 kg/mm'' or more.

At this time, r value and Eβ do not deteriorate ([S) + [N amount]
Improves as quantity decreases.

On the other hand, [S] is 40 ppm steel (・mark) and N is 45 ppm.
In pm (mu mark), ([S] + [N amount)] is 50pp.
Even with m, the BH was at most 1.5 lag'/m12 or less.

From the above results, [j) quantity, [N ) quantity, and ((3+
(N)) Effective Ti by controlling the amount: Ti
” = ((Ti amount) - - CN%) - 5C3%)) is 4 x [C%] or more, that is, even if the effective atomic ratio of T1 to C is 1 or more, appropriate bake hardenability cannot be obtained. It became clear that

(Function) Although it is not clear why bake hardenability appears by controlling the amounts of [S] and [N] in this way, TiS, Ti
It is thought that due to the decrease in N precipitates, TIC became unstable and solid solution C remained.

Furthermore, a reduction in S and N naturally leads to a reduction in precipitates such as TiS and TiN, and this is considered to be suitable for applications that are subject to severe processing, such as [+1 can].

The reason for limiting the ingredients in this invention will be explained.

The lower the CTC, the better the quality of the material, and if it exceeds 0.015%, good drawability cannot be obtained even if the amount of Ti added, which will be described later, is increased. On the other hand, 0. If it is less than 0.05%, the bake hardenability which is the object of this invention cannot be obtained. Therefore, the amount of C is set to 0.0005% to 0.0150%.

Si, Mn: Si and Mn are effective for increasing the strength of the steel sheet without deteriorating the deep drawability. However, Si
>0150% Mn >0150%, the addition of Mn significantly deteriorates the elongation and drawability of the steel sheet, so the amount added
i 0150% or less Mn' 1. 'Limit to 0% or less.

P: Like Si and Mn, P also increases the strength of the steel sheet without deteriorating its findability, but if it exceeds 0.15%, it will significantly deteriorate the elongation and drawability of the steel sheet, so it should be added up to 0.15%. do.

β: Hel is added in an amount of 0.005% or more for deoxidation and the like, but addition of more than 0.10% has a negative effect on the surface properties, so the upper limit is set at 0.10%.

S, N: The amounts of S and N in steel are the most important components in this invention, and as is clear from the previous experimental results, S≦0.00
30%, N amount 0.0040% and [S%] + [N%]
The reason for the limitation is that bake hardenability is advantageously improved when ≦0.0050.

Ti: Ti is added to fix S, N, and C, but unlike conventional bake-hardenable steel plates, effective Ti
(Ti” = [Ti amount)-7[N%]-5C8%
]) to c in an atomic ratio of 1 (that is, 0% by weight)
Good material quality and bake hardenability can be obtained by setting the atomic ratio to 1 or more, rather than less than 2 times).

However, adding too much T1 leads to deterioration of the surface properties of the steel sheet and is disadvantageous in terms of cost, so the upper limit is set at 2.
0X[:C%]. Therefore, the amount of Ti added is 4×[C
%〕≦([11%)--CN%)--[S%])T4
32 ≦20X [C%].

As mentioned above, even if one or both of Nb and B are added in combination with Ti, the bake hardenability, which is a feature of the present invention, is not lost, and the r value and Bl are improved. However, for Nb, 3×[C%] and for B, 0.005
Even if more than 0% is added, the curing will be saturated and it will be disadvantageous in terms of cost, so Nb<3x[C%]B≦0.005
0%.

Furthermore, 0.0150% or less Cr, Cu, V, 0.0
5% or less Pb.

Addition of Ca is possible because it does not deteriorate B11 properties and deep drawability.

The method for producing cold rolled steel sheets having the above composition is not particularly regulated. Steel tapped in a converter or electric furnace is made into a slab by the ingot-blowing method or continuous casting method, and is made into a cold-rolled plate by normal hot rolling or cold rolling. Recrystallization annealing is
The annealing may be performed by a box annealing method or by a continuous annealing method.

Further, the steel sheets manufactured in the following examples had excellent chemical conversion treatment properties, and had no problems in hot-dip galvanizing properties and secondary workability.

Example 1 Steel having the composition shown in Table 1 was melted in a converter, and after vacuum degassing treatment, it was made into a slab using a continuous casting machine.

This slab is subjected to normal hot rolling and cold rolling to achieve a plate thickness of 0.
．． After forming a cold-rolled sheet of 8 mm, continuous annealing (soaking condition = 80
0°C-305) was applied. Table 2 shows the mechanical properties of the product. All mechanical properties were measured using JIS No. 5 test pieces.

The YS, TS, Bβ, and r values shown in Table 2 are the average values of test results taken in the rolling direction (X, ), 45° from the rolling direction (X45), and 90° from the rolling direction (xso). This is the value of Also Y
B.A.

BH and aging index Aj (7,5% prestrain 100 ℃ 30
The increase in yield point after aging) is the result of a test using test pieces taken parallel to the rolling direction.

In this invention, BA of 50% or more, r of 1.9 or more
value, and an 8H amount of more than 3.2 kg' 7 mm2 was obtained.

However, comparative steel 6 whose [S] amount is outside the range, ([S%] +
[N%]) was outside the range for comparison steel 7, the BH amount was 1, respectively.
．． 2 kg'7mm2.0.8kg' 7mm2
It was low. Moreover, in comparative steel 8 with an excessive amount of C and comparative steel 9 with an insufficient amount of Tl, Bj! , deterioration of r value was observed.

(Effect of the invention) The above [:S:], (N) and [:S)+[:N)
Due to the limitation of the amount, it is found in the first and second inventions that even in steels in which Ti is added such that the atomic ratio of effective Ti (Ti") to C is 1 or more, appropriate bake hardenability is not achieved.
It can be combined according to deep drawability.

[Brief explanation of the drawing]

Fig. 1 is a diagram showing the procedure for measuring B) I property, and Fig. 2 is a graph showing the relationship between the amount of (S + N) in steel and the material. Figure 1 Predistortion (2.0%) Child Figure 2 (SfN (PPtn) Procedure Masa Sode Written November 8, 1980 Manabu Shiga, Commissioner of the Patent Office 1, Indication of the case 1988 Patent Application No. 146990 No. 2, Name of the invention Cold-rolled steel sheet for deep drawing with bake hardenability 3, Relationship with the case of the person making the amendment Patent applicant (125) Kawasaki Steel Corporation 4, Attorney 5, Subject of the amendment Description of the specification The title of the invention has been amended to ``I'' and ``cold rolled steel sheet for deep drawing with bake hardenability''.
-9/1, the claims of the specification are amended as follows. rl, C: 0.0005-0.0150wt%,
Si: 1. Ow'% or less Mn: 1,0" or less, Aβ: 0.005 to 0.100 w or less, P2 0.15 w or less, S: 0.003 w or less, N: 0.0040 w or less Dekatsu [:S
wt%] → [Nw or less] ≦0.0050, and the effective T1 (Ti”) given by the following formula
A cold-rolled steel sheet for deep drawing which is resistant to baking hard scratches and has a composition of 4 to 20 times as much as [CwL%], with the balance being iron and unavoidable impurities. 2. C: O-,0005~0.0150wt%,
Si: 1. Ow or less, Mn: 0150w or less, AA': 0.005 to 0.100 w or less, P: 0.15w or less, S: 0.003
w or less, N: 0.0040w or less, and satisfies the following conditions: [S″%) + (Nw or less] ≦0.0050, and furthermore, the effective T1 (Ti”) given by the following formula
Contains Nb in an amount of 4 to 20 times as much as [Cw or less], and Nb that is less than twice as much as [C°1%] and 0.0050°1%.
A cold-rolled steel sheet for deep drawing with bake hardenability, containing one or two of the following B, with the balance being iron and unavoidable impurities. (Note) T + * (w 1%) - ((Tiwt%:] --
□ (N wt%] - - □ [S w or less]) J 2. The title of the invention is corrected to "Cold rolled steel sheet for deep drawing with bake hardenability." Procedural amendment Written on June 5, 1985 Manabu Shiga, Commissioner of the Patent Office1, Indication of the case, Patent Application No. 146990 filed in 19812, Name of the invention Cold rolled steel plate for deep drawing with bake hardenability3, Relationship with the case of the person making the amendment Patent applicant (125) Kawasaki Steel Corporation 4, Agent 5, Subject of amendment Column 1 of “Detailed Description of the Invention” of the specification, page 10 of the specification, 6th from the bottom ~1 line “Ti:
For Ti, 111 can be obtained. "Ti: Ti is added to fix S, N and C, but the effective T1 (Ti"
= (Ti amount) -- [N%) -- (S%]) to C in an atomic ratio of 1 (that is, 4 times as much as 0% by weight) or more, with a high r value and 2kyf/- The above bake hardenability can be obtained. ” is corrected. 2. / in "Table 1" and "Table 2" on pages 18 and 15 of the same
169 data are deleted respectively. Procedural amendments June 29, 1985 Mr. Manabu Shiga, Commissioner of the Patent Office, Indication of the case, Patent Application No. 146 '990 2 of 1982, Name of the invention: Cold-rolled steel sheet for deep drawing with bake hardenability 3. Relationship with the case of the person making the amendment Patent applicant (125) Kawasaki Steel Corporation 4, Agent 1, r Nb J on page 8, line 20 of the specification, r Ti
, Corrected to Wb J+. 2. "ro,oogs%" on page 6, line 4 of the same page, jO,00
Corrected to 25% or less. 8. Correct "Comparative aS, comparative steel 9 with insufficient Ti amount" in line 7 of page 16 to "comparative steel 8". 4. Table 1 on page 18 and Table 2 on page 15 are corrected as follows.

Claims (1)

[Claims] 1. C: 0.0005 to 0.0150^w^t%, Si
: 1.0^w^t% or less Mn: 1.0^w^t% or less,
Al: 0.005-0.100^w^t%P: 0.15
In addition to including ^w^t%, S: 0.003^w^t% or less, N: 0.0040^w^t% or less, and [S^w^t
%]+[N^w^t%]≦0.0050, and furthermore, the effective Ti (Ti^*) given by the following formula is 4 to 20 times the [C^w^t%]. A cold-rolled steel sheet for deep drawing that has bake hardenability (BH: 2 kg^f/mm^2 or more) and has a composition of iron and unavoidable impurities. 2, C: 0.0005-0.0150^w^t%, Si
: 1.0^w^t% or less Mn: 1.0^w^t% or less,
Al: 0.005-0.100^w^t%P: 0.15
Including ^w^t% or less, S: 0.003^w^t%
Below, N: 0.0040^w^t% or less and [S^w
＾t%]+[N＾w＾t%]≦0.0050, and the effective Ti (Ti＾*
) in a range of 4 to 20 times as much as [C^w^t%], and less than 3 times as much Nb as [C^w^t%], and 0.0050^
Bake-hardening (
Cold-rolled steel sheet for deep drawing with BH: 2 kg^f/mm^2 or more. (Note) Ti^*(^w^t%) = ([Ti^w^t%]-48
/14 [N^w^t%] -48/32 [S^w^t%]
)