JPH02173247A - Cold rolled steel sheet excellent in resistance to molten metal brittleness and having superior workability and its production and brazing method for same - Google Patents

Abstract

PURPOSE:To manufacture a cold rolled steel sheet excellent in molten metal brittleness resistance and having superior workability by hot-rolling a steel slab with a specific composition and successively subjecting the resulting steel plate to cooling, winding, cold rolling, and annealing under respectively specified conditions. CONSTITUTION:A steel slab having a composition consisting of, by weight, <=0.0050% C, <=1.0% Si, <=1.2% Mn, 0.010-0.100% Ti, 0.005-0.100% Al, <=0.100% P, <=0.015% S, <=0.0050% N, 0.0005-0.0040% B, and the balance Fe with inevitable impurities is hot-rolled. Subsequently, cooling is started within 1sec after the conclusion of finish hot rolling, and the hot rolled plate is cooled at >=10 deg.C/sec average cooling rate until winding is started, and then wound up at <=700 deg.C. Further, the above plate is cold-rolled at >=60% draft and the resulting sheet is annealed at a temp. between 700 deg.C and the Ar3 point. By this method, the dead-soft C steel excellent in resistance to molten metal brittleness as well as in workability can be obtained.

Description

Detailed Description of the Invention (Industrial Application Field) This invention is applicable to processes such as brazing after processing such as deep drawing, or hot-dip Zn plating after processing, so that the steel plate after processing becomes liquid metal. The present invention relates to a cold-rolled steel sheet with excellent workability and excellent resistance to molten metal embrittlement suitable for use in applications where it comes into contact with metals, a method for manufacturing the same, and a method for brazing the same.

(Prior art) Steel sheets for press working have conventionally been manufactured using low carbon [C: 0.01 to 0°05w tχ; hereinafter simply expressed as %] Al-killed steel using a box annealing method. Recently, from the viewpoint of surface texture, shape and productivity, C<0.01
% ultra-low carbon steel as a material by a continuous annealing method, which surpasses the box annealing method in terms of material quality.

In ultra-low C steel, Ti is used to fix interstitial solid solution elements C and N in the steel and improve aging resistance and ductility.

Carbonitride-forming elements such as Nb, V, Zr, and Ta are added. Currently, the most widely used substances are Ti and Nb, and in particular, combined addition of both has many advantages over single addition (for example,
8407414).

By the way, recently, applications have emerged in which steel sheets for processing as described above are subjected to hot-dip Zn plating after press forming from the viewpoint of rust prevention and corrosion resistance. In addition, in the assembly process after drawing and forming, such steel plates are
So-called brazing is often performed.

When the above-mentioned ultra-low carbon steel is used in such applications, embrittlement occurs, which was not observed when conventional low carbon steels were used, and the degree of embrittlement depends on the degree of working. Not only does it become necessary to suppress the degree of processing to a low level, but it also requires the use of expensive silver solder that is less likely to cause embrittlement, and the need to perform strain relief annealing, which poses a major problem. The above-mentioned embrittlement phenomenon belongs to so-called liquid metal embrittlement, and one of the causes is large microscopic residual stress due to intense processing.

(Problems to be Solved by the Invention) This invention takes advantage of the excellent properties of ultra-low carbon steel and solves the problem of molten metal embrittlement (wax embrittlement, molten Zn embrittlement) after processing, which is a phenomenon specific to ultra-low carbon steel. The purpose of the present invention is to propose a cold-rolled steel sheet for processing that can be used in a wider range of applications, along with an advantageous manufacturing method and an advantageous brazing method.

The molten metal embrittlement that is a problem in this invention is a problem that was first addressed in ultra-low carbon steel in the field of steel plates for processing.

In other words, conventional low carbon A1 killed steel has a drawing ratio of 2.3.
It has never been used for ultra-deep drawing applications with a thickness greater than 0, and this issue only became a problem after ultra-low carbon steel for such ultra-deep drawing applications was developed and used for such applications. It is.

The reason why the above-mentioned embrittlement phenomenon occurs especially in ultra-low carbon steel has not yet been clearly elucidated, but judging from the form of cracking and the form of penetration of solder, it seems that both of them occur along the ferrite grain boundaries. This is thought to be due to the fact that the properties of the ferrite grain boundaries are different from those of conventional low carbon steel, and more specifically, the solid solution C segregated at the grain boundaries has been extremely reduced.

(Means for Solving the Problem) In view of the above-mentioned circumstances, the inventors first investigated the above-mentioned melt brittleness phenomenon, and further conducted a simulation test to reproduce it, and studied measures to reduce the molten metal brittleness. .

At that time, it goes without saying that consideration was given to preventing deterioration of other properties (tensile properties, workability), and in particular, the drawing ratio was 2.3.
A thorough study was conducted on the embrittlement phenomenon that occurs when processing is performed to exceed 0.

As a result, it was found that the embrittlement phenomenon can be determined by the presence or absence of cracking when the material is subjected to hot creep and the test piece is brought into contact with liquid metal, or by the critical stress at break. Therefore, hot creep tests were conducted in liquid metal under various conditions to investigate the effects of steel composition and steel structure on the degree of embrittlement. As a result, they found that this embrittlement phenomenon depends on the alloy composition, and therefore, they found that by optimizing the alloy composition, it is possible to avoid the embrittlement phenomenon even in actual drawn parts.

This invention is based on the above knowledge.

That is, this invention has the following properties: C: 0.0050% or less, Si: 1.0% or less, Mn: 1.2% or less, Ti: 0.010 to 0.100%, Al: 0.
005 to 0.100%, P: 0.100% or less, S: 0.015% or less, N: 0.0050% or less, and B: 0.0005 to 0.0040%, and sometimes further includes Nb: 0.005% or less. It is a cold-rolled steel sheet with excellent workability and excellent resistance to molten metal embrittlement, with the remainder being Fe and unavoidable impurities.

Further, this invention provides the following properties: C: 0.0050% or less, Si: 1.0% or less, Mn: 1.2% or less, Ti: 0.010-0.100%, Al
: 0.005-0.100%, P: 0.10
0% or less, S: 0.015% or less, N: 0.0050% or less, and B: 0.0005 to 0.0040%, sometimes further containing Nb: 0.005 to 0.025%, and the balance A steel billet with a composition of Fe and unavoidable impurities was hot rolled, cooling started within 1 second from the end of finish hot rolling, and cooling was performed at an average cooling rate of 10°C/S or more until coiling. A melt-resistant metal characterized by being coiled at a temperature of 700'C or less, then cold-rolled at a rolling reduction of 60% or more, and then annealed in a temperature range of 700'C% AC. This is a method for producing a cold-rolled steel sheet with excellent brittleness and good workability.

Furthermore, when brazing the above-mentioned steel plates, this invention provides
The brazing temperature T is determined according to the composition of the steel sheet (
1) This is a method for brazing cold rolled steel sheets with excellent workability and excellent resistance to molten metal embrittlement, which is carried out under conditions that satisfy the following formula.

Note T('C) ≦2200[C%]+2167 (P%
) +6117 [Al%) +17220 (T
i%) +21833 [Nb%) +1225
00 [B%] (1) (Function) First, the reason why the composition of this invention steel was limited to the above range will be described.

c: less than o, ooso% In order to improve elongation (El) and Lankford value ('), which are important for processing steel sheets, it is more advantageous to have less C. Therefore, in this invention, the C content is less than 0.0%. 00
It is set at 50% or less, preferably 0.0035% or less. Note that when Cl increases, it is fixed as carbide,
In order to reduce the amount of solid solute C to a level that is harmless to aging properties, more Ti and Nb are required, which has disadvantages such as increased melting cost, complicated work, and increased continuous annealing temperature. .

Si: 1.0% or less Si is a preferable additive element for producing high-strength steel sheets for deep drawing because it can increase tensile strength without significantly reducing ductility.

However, excessive addition not only deteriorates the surface properties but also makes manufacturing processes such as rolling difficult, so the upper limit was set at 1.0%.

Mn: 1.2% or less Mn is also a useful element similar to Si, but excessive addition also causes deterioration of the surface properties, so the second limit was set at 1.2%.

Ti: 0.010-0.100% Ti fixes and renders C harmless in the form of TiC, and El! ,
It is a useful element that improves the r value, and at least 0.010% is required to coarsen precipitates to an ideal size in the steel sheet manufacturing process.

However, when added in a large amount exceeding 0.100%, the amount of solid solution Ti increases significantly, resulting in an increase in the recrystallization temperature and the disadvantage that the continuous annealing temperature must be increased. There are also disadvantages such as the tendency for nozzle clogging to occur during the slab casting stage. Therefore, in this invention, Ti
The content was limited to a range of 0.0'IO-0.100%.

Eight I: 0.005-0.100% AI is
It is a useful element that fixes 0 in molten steel and improves the cleanliness of steel, but if the amount added is less than 0.005%, the effect of addition is poor, while addition in large amounts exceeding 0.100% will only increase costs. Rather, it will cause deterioration of the material (so 0.0
It was supposed to be added in a range of 0.05 to 0.100%.

S: 0.015% or less It is desirable to reduce S from the viewpoint of workability.

Furthermore, since S combines with Ti and reduces Ti, which is effective for fixing C, the upper limit was set at 0.015%.

N: 0.0050% or lessN, like S, is fixed by Ti, Nb, and AI, but the presence of precipitates is not only harmful to ductility in itself, but it is also necessary to add Ti commensurate with the amount of N. This leads to an increase in costs. Therefore, it is preferable to suppress the incorporation of N as much as possible, but it is permissible at 0.0050% or less.

P: 0.100% or less P is a useful element that can strengthen steel without reducing the T value (and without significantly reducing ductility), and is a desirable element when manufacturing high-strength steel sheets. o, too
If it is added in excess of 0.100%, the resistance to secondary work brittleness will be significantly deteriorated, so it is limited to 0.100% or less.

B: 0.0005 to 0.0040% B not only greatly contributes to improving secondary processing embrittlement resistance when added in small amounts, but also effectively improves molten metal embrittlement such as wax embrittlement and hot-dip Zn plating embrittlement. Contribute. The above effect becomes noticeable when the amount is added at 5 ppm or more, and at 40 ppm or more.
Even if it is added in excess of pm, the effect reaches saturation, so 0.
The content was set to be in the range of 0.0005% to 0.0040%.

Nb: 0.005-0.025% By adding 0.005% or more of Nb in addition to Ti, the structure of the hot-rolled base sheet is mainly grain-sized and refined, and at the stage of cold-rolled steel sheet, in-plane anisotropy is achieved. It can be made of a steel plate with low strength.

Addition of Nb is also effective against molten metal embrittlement. However, addition of more than 0.0025% causes a decrease in elongation value and an increase in recrystallization temperature, so 0.005 to 0.02%
It was supposed to be added in a range of 5%.

Next, explaining the manufacturing method, it is necessary to control the following items.

■) Time from the end of finish rolling to the start of water cooling is within 1 second The ultra-low carbon mesh used in this invention method has extremely good grain growth properties at high temperatures, so the start of water cooling after the end of hot rolling is within 1 second after the end of rolling. If the delay exceeds seconds, extremely non-uniform grain growth progresses, resulting in non-uniform ferrite grain size in the hot-rolled base plate, a decrease in the r value of the cold-rolled steel sheet, and an increase in the in-plane anisotropy of the material. Not only that, but the secondary processing brittleness is also deteriorated due to the formation of a mixed grain structure.

2) Average cooling rate from the start of water cooling to winding 10
℃/s or more Same as 1), this is also specified to prevent uneven grain growth in the hot-rolled mother plate.

3) Winding temperature: -700°C or lower When winding is carried out at a temperature higher than 700°C, the effect of improving Ej2゜ lower value is saturated, as well as increasing troubles in the winding process and deterioration of desgering properties. There is a disadvantage.

4) Cold rolling reduction -60% or less The cold rolling reduction is particularly important in improving the r value, and a reduction of at least 60% is required to obtain an excellent r value. A more preferable range is 70 to 95%.

5) Annealing temperature -700℃-Ac 3-point annealing temperature is important to ensure preferable material quality.
If this temperature is less than 700"C, the texture that is advantageous for workability will not develop sufficiently, making it impossible to obtain a high r value. The heating temperature should not exceed 700℃ as the drawability and ductility will decrease rapidly.
It was limited to the range of Ac.

In addition, in the above-mentioned annealing treatment, since soaking is not particularly required, it is preferable to utilize a continuous annealing furnace for such treatment.

Furthermore, the inventors also investigated the influence of each element on molten metal embrittlement using steels of various chemical compositions.

That is, steels with various compositions shown in Table 1 were melted, hot-rolled, cold-rolled, and annealed, and then formed into a cylinder at a drawing ratio of 2.60.
After heating to 50° C., high-frequency heating brazing was performed, and the presence or absence of cracking at that time was examined.

□□□” The survey results are summarized and shown in Figure 1.

The vertical axis T c a L c in Fig. 1 is expressed by the following formula, T"" = 2200 [C%) +2167 (P
%) +6117 [Al%) +17220 (Ti
%) +21833 [Nb%) +122500 [B
%].

Further, the straight line in the figure is a line indicating '1' = T c + at c.

As is clear from the figure, if T c a L c ≧T, that is, if the actual brazing temperature T is equal to or less than the calculated value '1' c a t c determined according to the chemical composition of the steel, Good brazing was achieved without cracking.

(Example) Example 1 After melting steels having various compositions shown in Table 2, hot rolling, cold rolling, pickling, and annealing were performed under the conditions shown in Table 3.

The final plate thickness is 0.7 mm in all cases.

Table 4 shows the results of investigating the mechanical properties and embrittlement characteristics of the cold-rolled sheets thus obtained.

In Table 3, L is the time (S) until the start of water cooling after hot rolling, ■
is the average cooling rate (°C/s) from the end of hot rolling to winding, C
T is the coiling temperature (°C), RED is the cold rolling reduction ratio, T is the soaking temperature of continuous annealing, 1. is the soaking time.

In addition, in Table 4, Tcr is an index of resistance to secondary work brittleness. After blanking a steel plate, it was cup-formed using a conical die, and 5 kg of heavy weight was heated to 80 cm at various test temperatures.
This refers to the critical temperature at which brittle cracking occurs when a specimen is crushed by dropping it from a height of m. The aperture ratio at that time was kept constant at 2.0.

As is clear from Table 4, any product that satisfies the component composition and manufacturing conditions of the present invention provides low YP, high Ef, and high T values depending on the strength level, and has good workability. Rather, it exhibits excellent secondary processing brittleness with a brittle transition temperature of about 1100'C, which is at a level that poses no practical problems.

Example 2 Steels with Nα1 shown in Table 2 were treated under various conditions shown in Table 5.

Table 5 also shows the results of investigating the properties of the cold-rolled sheets obtained in this manner.

As is clear from the same table, when manufactured according to the method of the present invention, a cold rolled steel sheet with high E2, high T value, low Tcr, good workability, and excellent resistance to secondary work brittleness is obtained.

Example 3 Each sample material in Tables 4 and 5 was drawn to a drawing ratio of 2.30, then punched out in the vicinity of the cup rim, then expanded, and then brazed at temperatures of 650 to 900°C. A test was conducted, and after cooling, the presence or absence of cracking was examined. Note that this brazing involves locally applying high-frequency heating to only the brazed portion.

As a result, when the relationship between steel composition and brazing temperature satisfies the appropriate range of this invention, no cracking occurs due to brazing, and good brazing properties, in other words, good resistance to molten metal embrittlement are exhibited. Ta.

(Effects of the Invention) Thus, according to the present invention, it is possible to obtain an ultra-low C steel that not only has excellent workability but also has excellent resistance to molten metal embrittlement, and is also free from cracking during actual brazing or hot-dip Zn plating. There is no fear of

[Brief explanation of the drawing]

FIG. 1 is a graph showing the influence of brazing temperature and steel composition on brazing cracking. Figure 1 Patent Applicant Kawasaki Steel Co., Ltd. L lower (0C)

Claims (1)

[Claims] 1. C: 0.0050 wt% or less, Si: 1.0 wt% or less, Mn: 1.2 wt% or less, Ti: 0.010 to 0.100 wt%, Al: 0.005 to 0. .100 wt%, P: 0.100 wt% or less, S: 0.015 wt% or less, N: 0.0050 wt% or less, and B: 0.0005 to 0.0040 wt%, with the remainder being Fe and unavoidable impurities. A cold-rolled steel sheet with excellent workability and excellent resistance to molten metal embrittlement. 2, C: 0.0050 wt% or less, Si: 1.0 wt% or less, Mn: 1.2 wt% or less, Ti: 0.010 to 0.100 wt%, Al: 0.005 to 0.100 wt%, P: 0.100 wt% or less, S: 0.015 wt% or less, N: 0.0050 wt% or less, and B: 0.0005 to 0.0040 wt%, and Nb: 0.005 to 0.025 wt%, The remainder consists of Fe and unavoidable impurities, making it a cold-rolled steel sheet with good workability and excellent resistance to molten metal embrittlement. 3. C: 0.0050 wt% or less, Si: 1.0 wt% or less, Mn: 1.2 wt% or less, Ti: 0.010 to 0.100 wt%, Al: 0.005 to 0.100 wt%, P: A steel piece containing 0.100 wt% or less, S: 0.015 wt% or less, N: 0.0050 wt% or less, and B: 0.0005 to 0.0040 wt%, with the balance being Fe and unavoidable impurities. After hot rolling, start cooling within 1 second from the end of finish hot rolling, cool at an average cooling rate of 10°C/s or more until coiling, then winding at a temperature of 700°C or less, and then rolling. After cold rolling at a rate of 60% or more,
A method for manufacturing a cold-rolled steel sheet with excellent workability and excellent resistance to molten metal embrittlement, characterized by performing annealing in a temperature range of 700°C to Ac_3 points. 4, C: 0.0050 wt% or less, Si: 1.0 wt% or less, Mn: 1.2 wt% or less, Ti: 0.010 to 0.100 wt%, Al: 0.005 to 0.100 wt%, P: 0.100 wt% or less, S: 0.015 wt% or less, N: 0.0050 wt% or less, and B: 0.0005 to 0.0040 wt%, and Nb: 0.005 to 0.025 wt%, A steel billet with the remainder consisting of Fe and unavoidable impurities is hot-rolled, cooling begins within 1 second from the end of finish hot rolling, and is cooled at an average cooling rate of 10°C/s or more until coiling. After that, it is rolled up at a temperature of 700°C or less, and then cold-rolled at a rolling reduction of 60% or more.
A method for manufacturing a cold-rolled steel sheet with excellent workability and excellent resistance to molten metal embrittlement, characterized by performing annealing in a temperature range of 700°C to Ac_3 points. 5. A melt-resistant metal characterized in that when brazing the steel plate according to claim 1 or 2, the brazing temperature T is performed under conditions that satisfy the following formula (1), which is determined depending on the composition of the steel plate. A method for brazing cold-rolled steel sheets with excellent brittleness and good workability. T (℃)≦2200 [C%] +2167 [P%] +61
17 [Al%] + 17220 [Ti%] + 21833 [
Nb%]+122500[B%]...(1)