JPH04354849A - Cold rolled steel sheet for working excellent in strength property in weld zone - Google Patents

Abstract

PURPOSE:To offer a cold rolled steel sheet for working excellent in the strength properties in the weld zone. CONSTITUTION:In an extra-low carbon cold rolled steel sheet obtd. by subjecting a hot rolled steel sheet to cooling and annealing, the hot rolled steel sheet before the cooling and annealing contains <=0.005% C, <=0.005% N and <=0.05% Nb, furthermore contains Ti, B and Al and in which, as nitrides, TiN, BN and AlN are precipitated, the ratio of N present as TiN and BN to the total N satisfies {(NasTiN + NasBN)/TotalN}X100>=70% and the quantity of B present in the state of solid soln. in the thin steel sheet is regulated to <=0.0004%. This steel sheet is excellent in the strength properties in the weld zone, particularly in the spot weld zone and withstanding exceedingly severe press forming for automotive parts, parts for electric apparatus or the like.

Description

[Detailed description of the invention]

0001

[Industrial Application Field] The present invention relates to cold-rolled steel sheets for processing.
More specifically, the present invention relates to a cold-rolled steel sheet that has excellent strength characteristics in welded parts, particularly spot welded parts, and has high workability that can withstand extremely severe press forming such as parts for automobiles or parts for electrical equipment.

0002

[Prior Art] Ultra-low carbon cold-rolled steel sheets to which carbonitride-forming elements are added can be manufactured by continuous annealing in a short period of time;
In addition, it has great advantages in terms of manufacturing and processing, such as excellent deep drawability and ductility, and press formability that can withstand increasingly complex parts shapes.
Its production volume is increasing year by year.

[0003] Conventionally, as such ultra-low carbon steel sheets,
Ti-added steel and Nb-added steel are known, and
Ti--Nb composite additive steel is known, as shown in No. 8-107414.

0004

[Problems to be Solved by the Invention] However, although these ultra-low carbon steel sheets have superior workability compared to low carbon steel sheets, the structure of the welded part easily breaks down during welding performed when assembling parts. There is a problem of coarsening, and there is also a problem that there is less hardening in the weld zone and its heat affected zone compared to low carbon steel sheets, so there is a problem that the strength of the weld zone is low.
At present, there is a demand for improvement in terms of rigidity.

The present invention was made under such circumstances, and it is an object of the present invention to provide a cold-rolled steel sheet for working that has excellent strength properties at the welded portion.

[0006]

[Means for Solving the Problems] In order to solve the above problems, the present inventors have conducted extensive research on the effects of additive elements on the spot weld strength of ultra-low carbon cold-rolled steel sheets. It is extremely effective, and when deep drawability is taken into account, both workability and weld strength are excellent when the presence state of B and the composition ratio of nitrides in the hot rolled steel sheet are appropriate. The present invention has been completed based on this knowledge.

That is, the present invention provides a cold rolled steel sheet obtained by cold rolling and annealing a hot rolled steel sheet, in which the hot rolled steel sheet before cold rolling and annealing contains C: 0.005% or less and N: 0.005%. % or less, Nb: 0.05% or less, and further contains Ti, B and A
TiN, BN, and AlN are precipitated as nitrides, and the ratio of N present as TiN and BN to the total N amount is {(NasTiN+NasBN)/T
otalN}×100≧70%, and the amount of B present in a solid solution state in the thin steel sheet is 0.0004% or less, and has excellent strength properties of welded parts. The gist is steel plates.

The present invention will be explained in more detail below.

[0009]

[Effect]

First, the reason for limiting the chemical composition of the steel sheet in the present invention will be described.

C: In order to improve ductility and deep drawability, it is preferable to reduce C as much as possible, and the upper limit is 0.00.
5%.

N: In the present invention, N is TiN, BN
From the viewpoint of material quality, especially deep drawability, it is necessary to completely precipitate AlN in the form of precipitates in hot rolled sheets.
If the amount increases, the cost increases due to the increase in the amount of Ti, B, and Al required for fixing the precipitation, and the amount of precipitates themselves increases, which deteriorates ductility. Therefore, the upper limit of the amount of N is set at 0.005.
%.

Nb: Nb is added for the purpose of precipitating and fixing C in the form of NbC and improving workability.
Addition in excess of this amount not only saturates the effect but also increases cost, and addition in excess increases the amount of NbC or dissolved Nb, leading to deterioration of the material. Therefore, the amount of Nb is set to 0.05% or less.

Ti, B, Al: Ti, B, and Al are all nitride-forming elements, and they are used to improve the material quality by fixing N by precipitation, and B to improve the strength of spot welds. Add complex. In particular, in order to achieve both workability and strength of spot welds, in the present invention, the composition ratio of nitrides formed by these three elements in the hot rolled steel sheet is changed, and B is not only used as a nitride but also as a solid. The amount of B present in the solution state is calculated as follows: {(NasTiN+NasBN)/TotalN}×1
00≧70%, Solid solution B≦0.0004%.

First, the composition ratio of nitride will be explained. The ease of bonding with N is in the order of Ti>B>Al, and Ti
In the case of composite addition of three elements: , B, and Al, TiN>BN>
It is easy to precipitate in the order of AlN.

[0016] In the present invention, TiN and BN are precipitated, and the ratio is specified so that the amount of N precipitated as TiN and BN is 70% or more of the total N amount in the steel, and the remaining N is
is precipitated in the form of AlN. That is, AlN is TiN
, BN, and AlN, and it is not preferable to precipitate a large amount from the viewpoint of material quality. It is necessary to keep the amount of N precipitated as AlN to 30% or less of the total amount of N.

Next, regarding solid solution B, this is B
It is defined from the influence of the form of existence on the material. In other words, B is essential for improving the strength of the weld in the present invention, but if B is present in solid solution in the steel, the deep drawability after subsequent cold rolling and annealing will be significantly affected. to degrade. Therefore, the upper limit of the amount of solid solution B in steel is 0.0004%.
By precipitating B in the form of BN as much as possible, the effects of solid solution B and solid solution N are rendered harmless at the same time.

[0018] However, it is extremely difficult to significantly reduce N with the current steelmaking capacity, and unless special treatment is performed, produced steel contains 0.002 to 0.004% N. As mentioned above, in order to neutralize the effects of both solid solution N and solid solution B, it is ideal to add B equivalent to N (= (14/11)N). However, considering the N content of normal steel, a large amount of B needs to be added. If a large amount of B is added, cracks will occur at the slab stage, which poses a problem. Under these circumstances, in order to reduce the amount of B added to an appropriate amount, that is, to reduce the amount of N that can combine with B, Ti, which forms nitrides more easily than B, is added.

[0019] Furthermore, the amount of B required to improve the strength of the weld zone must be at least 0.0006%, and considering the upper limit of the amount of B in solid solution of 0.0004%, it is impossible to fix N by precipitation with the entire amount of Ti. , it is necessary to precipitate N in the form of TiN+BN.

The composite addition of Ti, B, and Al has been described above.The addition amount is not particularly stipulated as long as the specified conditions of the composition ratio of nitrides and the amount of solid solution B are satisfied, but chemical conversion treatment Ti≦0.06% from the viewpoint of performance, 0.0006%≦B as an amount sufficient to improve the weld strength, and B≦0.0050% from the perspective of preventing slab cracking, for deoxidation and to reduce the amount of inclusions. From reduction 0.020%≦Al≦0.10
It is preferable to set it to 0%.

Note that other impurity elements such as Si and M
It is preferable to reduce n and P as much as possible from the viewpoint of workability, but on the other hand, in the case of high-strength cold-rolled steel sheets, Si≦0.70%, Mn≦1.0%, and P≦0, respectively.
．． It is possible to add up to 10%, and as long as it is within this range, it does not particularly impair the purpose of the present invention.

[0022] Regarding the manufacturing method of cold-rolled steel sheets, there is no need to particularly regulate them as long as the hot-rolled sheets satisfy the above-mentioned predetermined chemical composition, precipitate composition, and amount of solid solution B. Neither the casting method nor the hot rolling method requires any special regulations, and HDR and HCR are also possible. By obtaining a hot rolled sheet that satisfies the above-mentioned regulatory conditions, cold rolling and annealing it, a cold rolled steel sheet having desired properties can be obtained.

[0023] Preferable conditions are as follows. Slab heating temperature: 1000-1200℃ Hot rolling finishing temperature:
Ar 3 points or more Coiling temperature: 650 to 750°C Cold rolling reduction: 65 to 90% Annealing temperature: Recrystallization temperature or more and Ac less than 3 points (Both box annealing and continuous annealing are possible)

The cold-rolled steel sheet of the present invention is superior to conventional ultra-low carbon steel when used as a cold-rolled steel sheet or as a surface-treated steel sheet such as a hot-dip galvanized steel sheet or an electrogalvanized steel sheet. It has press formability equal to or higher than the weld strength. Although various welding methods are possible, spot welding is most effective.

Next, examples of the present invention will be shown.

[0026]

[Example] As a test steel, C: 0.002%, N: 0.
0035%, Si: 0.01%, Mn: 0.12%, P
: 0.012%, S: 0.006%, Al: 0.042
%, Ti: 0.008%, Nb: 0.027% as base components, and various steels with B added thereto in the range of 0 to 0.0040% were heated to 1100°C and finished at a finishing temperature of 900 to
After rolling to a plate thickness of 3.6 mm at 915°C, it was wound up at 680°C. Next, after pickling, the material was cold rolled to a thickness of 0.8 mm, continuously annealed at 830° C. for 40 seconds, and then subjected to 0.7% temper rolling. A summary of the material test results of the obtained cold-rolled annealed sheets and the analysis results of the amount of nitrides and solid solution B in the hot-rolled steel sheets.

[Table 1] Shown below.

[0027] The tensile property value is the value in the L direction (rolling direction) using a JIS No. 5 test piece, and the r value is the value in the directions of 0°, 45°, and 90° with respect to the rolling direction using a strip test piece. The average value is (r0°+2r45°+r90°/4). In addition, for testing the shear strength (TSS) and cross joint tensile strength (CTS) of the welded portion, each test piece was produced by spot welding under the following spot welding conditions. Electrode: Dome type electrode Welding time: 12 cycles Pressure force: 1960N Welding current: 8.0KA

As is clear from Table 1, the invention steels C and D
Not only has good elongation and r value and excellent press formability, but also the strength of the spot welded part shows sufficiently high values. On the other hand, comparative steels A and B with a low B content have low spot weld strength, and steel A with a high AlN content has a high yield point, a low r value, and is inferior to the inventive steel in terms of formability. ing. Steels E and F containing a large amount of B have sufficiently high strength at spot welds, but have a large amount of solid solution B in the hot-rolled sheet stage, and therefore have significantly poor deep drawability after cold rolling and annealing.

[0029]

[Effects of the Invention] As detailed above, according to the present invention,
It is possible to obtain an ultra-low carbon cold-rolled steel sheet that has both high weld strength and excellent press formability.

Claims (1)

[Claims] Claim 1: In a cold-rolled steel plate obtained by cold-rolling and annealing a hot-rolled steel plate, the hot-rolled steel plate before cold-rolling and annealing contains C: 0.005% or less by weight% (the same applies hereinafter); N:0.00
5% or less, Nb: 0.05% or less, and further contains Ti,
Contains B and Al, TiN, BN and A as nitrides
lN is precipitated, and the ratio of N existing as TiN and BN to the total N amount is {(NasTiN+NasB
N)/TotalN}×100≧70%, and the amount of B present in solid solution in the thin steel sheet is 0.0004
A cold-rolled steel sheet for processing that has excellent strength properties of welded parts, characterized by a strength of less than %.