JP2002241895A - Thin steel sheet having excellent ductility and strength stability after heat treatment - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a thin steel sheet in which high strength can securely be obtained by quenching, and excellent ductility can simultaneously be attained, and which further has excellent corrosion resistance, plating properties and spot weldability. SOLUTION: The thin steel sheet has a composition containing, by mass, 0.11 to 0.22% C 0.1 to <0.5% Mn, Cr and/or Mo of 0.1 to 0.5% in total and 0.0005 to 0.005% B, and also satisfying 0.19<= [C]+([Cr]+[Mo])/5} in the inequality, [C], [Cr] and [Mo] respectively denote the contents (mass%) of C, Cr and Mn}.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a thin steel sheet used for manufacturing members by molding in the industrial fields of automobiles, electric machines, machinery and the like, and has excellent ductility as a characteristic. In addition, high strength can be reliably obtained irrespective of fluctuations in heat treatment conditions (hereinafter, such characteristics may be simply referred to as "strength stability after heat treatment" or "strength stability after quenching"), Further, the present invention relates to a thin steel sheet excellent in corrosion resistance, plating properties and spot weldability. The thin steel sheet of the present invention is used in the various fields described above. Hereinafter, the description will be focused on a case where the thin steel sheet is used for a steel sheet for automobiles as a typical application example.

[0002]

2. Description of the Related Art From the viewpoint of safety, a required property of an automobile member obtained by forming a thin steel sheet is that the member is deformed without completely destroying it in the event of an automobile collision to absorb a shock. In order to secure such characteristics, the strength of the member is increased by partially increasing the plate thickness of the member or by overlapping a reinforcing member. By the way, in recent years, weight reduction has been promoted from the viewpoint of improving fuel efficiency of automobiles, and the strength of steel plates has been further enhanced so that safety can be ensured without the above-mentioned reinforcement or the like. However, since high-strength steel sheets generally have poor workability, it is required to ensure workability at the time of forming members.
As means for solving such a problem, Japanese Patent Laid-Open No. 11-15 / 1999
No. 2541 proposes a high-strength steel sheet member in which a relatively high ductility steel sheet is formed and then hardened at a necessary portion to partially increase the strength. Also, JP-A-2000-144
No. 319 discloses a technique for securing strength and workability by adding Mn.

In such a steel material, C, Mn, etc. are added in a relatively large amount for the purpose of increasing the strength after quenching. By increasing the amount of C added, the strength after quenching is increased. Therefore, the Mn content is increased in place of C since the weldability and the like are liable to deteriorate in inverse proportion. However, when the amount of Mn is increased, the temperature in the two-phase region of the steel decreases, so that a hard phase such as martensite or bainite is easily generated during recrystallization annealing after cold rolling, and the ductility of the material is reduced. Therefore, when it is used as a steel sheet for automobiles or the like which is subjected to complicated processing, it is important to suppress the Mn content to secure more excellent ductility.

By the way, as described above, quenching is performed to increase the strength of the member. However, in any case of quenching such as induction quenching or press quenching, the heating temperature or the cooling start temperature fluctuates by about 50 ° C. Since the strength after quenching tends to fluctuate along with such a change in quenching temperature, there is a problem that a constant high strength cannot be secured as a member.

FIG. 1 is a graph showing the relationship between the quenching temperature and the tensile strength after quenching for each Mn concentration. The experimental conditions are as follows. That is, C: 0.13%, Mn:
High Mn steel containing 1.5%, and C: 0.16%, M
n: Low Mn steel containing 0.38% is hot-rolled under the conditions of a finish rolling temperature (FDT) of 890 ° C. and a winding temperature (CT) of 650 ° C. to a thickness of 2 mm, and then cold-rolled. To a sheet thickness of 1 mm, then annealed at 720 ° C. for 60 seconds, and finally rolled 1% by skin pass. 1.0 mm x 30 mm x 300
mm flat plate, 700 ° C, 800 ° C, 850
After quenching at each temperature of ℃, 900 ℃, 950 ℃ or 1050 ℃, JIS No. 5 test piece was sampled and subjected to a tensile test to measure the tensile strength.

As shown in FIG. 1, fluctuations (variations) in strength after quenching due to changes in quenching temperature are caused by increasing the quenching temperature, adding a large amount of Mn, or improving equipment. However, if the quenching temperature is increased, the plating adhesion of the quenched part in the plated steel sheet is deteriorated or the plated layer is lost, and the paintability of the hot-rolled steel sheet or the cold-rolled steel sheet is deteriorated. As a result, the corrosion resistance deteriorates, which is not preferable.

FIG. 2 is a graph showing the relationship between the quenching temperature and the iron content in the plating layer, and FIG. 3 is a graph showing the relationship between the iron content in the plating layer and the maximum hole depth in the corrosion resistance test. It is. FIG. 2 shows that, as experimental conditions, a continuous cast slab was hot-rolled to 4.0 mm, pickled, cold-rolled to 2.0 mm, and then subjected to a plating treatment (a basis weight of plating: 45g / both sides
m ² ), annealing and alloying were performed, and quenching was performed in the same manner as in FIG. FIG. 3 shows the results of a corrosion resistance test using a steel sheet quenched as described above.
(Automotive material corrosion test method). The test uses a 2.0 mm × 70 mm × 150 mm test piece, and is sprayed with salt water (35 ° C., 5% salt water) for 8 hours.
4 hours dry (60 ° C, 30% relative humidity), wet (50
℃, relative humidity 90%) 2 hours as one cycle, 170
After the cycle, the maximum drilling depth was measured.

From FIGS. 2 and 3, when the quenching temperature of the plated steel sheet is too high, the alloying of the plating tends to proceed too much and the Fe content in the plated layer tends to increase. It can be seen that when the amount is increased, rust easily occurs, and the maximum drilling depth in the corrosion resistance test increases, that is, the corrosion resistance deteriorates.

As described above, when the material is a plated steel sheet, the corrosion resistance of the quenched portion depends on the degree of alloying by quenching or the remaining degree of the plated layer. When the quenching temperature is increased, the alloying of the plated alloy proceeds excessively or the plated layer disappears. As a result, the anticorrosion effect of the plating layer is lost.

FIG. 4 is a graph showing the relationship between the quenching temperature and the residual ratio of the coating film. As an experimental condition, a steel plate was manufactured under the same conditions as in FIGS. The residual rate was measured by subjecting the quenched steel sheet to phosphate treatment and electrodeposition coating, and then performing a grid test.

FIG. 4 shows that the higher the quenching temperature of the cold-rolled steel sheet and the hot-rolled steel sheet, the lower the residual ratio of the coating film. Is thicker, and even if the coating is applied on the scale layer, the coating film is easily peeled off together with the scale layer. You.

Further, when the quenching temperature is increased, there arises a problem that the thermal deformation of the molded product is increased. Further, when Mn is added in a large amount to suppress the fluctuation in strength after quenching, it becomes difficult to secure ductility as described above.

Therefore, in order to ensure a low Mn concentration in order to secure ductility, to further reduce the thickness of the oxide scale layer at the quenched portion or to suppress the alloying of the plating, and to make the corrosion resistance the same as that of the unquenched portion, 850 is required. It is necessary to perform quenching in a relatively low temperature range of up to 950 ° C., but in such a case, variation in strength after quenching becomes a problem.

However, to reduce the variation in strength after quenching, no remarkable technique has been developed so far, and the present inventors have disclosed a technique disclosed in
Japanese Patent Application No. 0-248338 has already proposed a steel sheet for induction hardening in which a wide range of Mn concentration is defined, but this technique examines the variation in strength after quenching in a low Mn concentration range as in the present invention. It is not what you are doing.

[0015]

SUMMARY OF THE INVENTION The present invention has been made in view of such circumstances, and has as its object to provide excellent ductility capable of performing complicated molding and fluctuations in heat treatment temperature conditions. Irrespective of the above, it is an object of the present invention to provide a useful thin steel sheet which can simultaneously attain high strength after quenching without fail, and which is excellent in corrosion resistance, plating properties and spot weldability.

[0016]

The thin steel sheet according to the present invention means, by mass%, C: 0.11 to 0.22%, Mn: 0.
1 to less than 0.5%, Cr and / or Mo: 0.1 in total
０．５0.5%, B: 0.0005-0.005%, and 0.19 ≦ {[C] + ([Cr] + [Mo]) / 5} (1) C], [Cr], and [Mo] are C, Cr,
It is characterized by satisfying｝ indicating the content (% by mass) of Mn, and the right side in the formula (1) is 0.2
It is preferably 8 or less.

[0017]

BEST MODE FOR CARRYING OUT THE INVENTION Under the above-mentioned circumstances, the present inventors can surely obtain excellent ductility and high strength after quenching, and furthermore, the corrosion resistance and weldability of a quenched part are improved. As a result of intensive research aimed at realizing excellent thin steel sheets,
After reducing the amount of Mn, it was found that it is effective to define the amount of C in combination with the amount of Cr and / or the amount of Mo, and further pursued quantitative effects of these chemical components. As a result, the present invention has been made.

Hereinafter, the reasons for defining the chemical components in the present invention will be described in detail.

C: 0.11 to 0.22% C is an element necessary for enhancing the hardenability of the steel and ensuring high strength. If the content is too small, the desired quenching will be obtained even if sufficient quenching is performed. Since it is difficult to obtain strength, 0.11% or more, preferably 0.12% or more is added. However, if the C content is too large, the spot weldability deteriorates, and the welded portion becomes brittle when welding is performed. Therefore, the content is suppressed to 0.22% or less, preferably 0.20% or less.

Cr and / or Mo: 0.1 to 0.1 in total.
5% Cr, Mo is an element essential for improving hardenability. In addition, although the strength stability after quenching is improved by increasing the amount of C, as described above, in the present invention, C is suppressed to a low concentration of 0.22% or less to secure spot weldability, In order to secure strength stability, Cr and Mo should be 0.1
% Or more, preferably 0.2% or more.
However, if the content of any of Cr and Mo is too large, it may cause non-plating, deterioration of chemical conversion properties such as phosphate treatment, and poor plating adhesion during production (non-plating). Therefore, it is necessary to suppress the total sum to 0.5% or less, preferably 0.45% or less.

0.19 ≦ {[C] + ([Cr] + [Mo]) / 5} (1) FIG. 5 shows the C content and (Cr + Mo) content using the results of the examples described later. 5 is a graph showing the effect of the relationship between the amounts on the strength variation after quenching, and the suffix of the plot indicates the strength variation defined by the present invention (quenching temperature 85%).
The difference between the tensile strength after quenching at 0 ° C. and the tensile strength after quenching at a quenching temperature of 950 ° C.). FIG.
Some of them have a small variation in strength despite being out of the range of the scope of the present invention, but these are due to the above-mentioned component defining reasons or other necessary characteristics as shown in the examples described later. It has poor plating properties or spot weldability after a certain plating treatment.

As shown in FIG. 5, the variation in strength after quenching (the difference between the tensile strength after quenching at 850 ° C. and the tensile strength after quenching at 950 ° C.) is within a desirable range (100%). Below), the C defined in the present invention
It is very difficult to include C, Cr and Mo such that the right side of the above formula (1) is 0.19 or more, preferably 0.20 or more within a range satisfying the total amount of Cr and / or Mo. It is valid. On the other hand, if the value on the right side of the above formula (1) is too large, the hardness of the weld after welding increases more than necessary, and there is a concern that the joining strength of the weld may decrease. , More preferably 0.27
It is better to do the following.

Mn: 0.1 to less than 0.5% FIG. 6 is a graph showing the elongation of a steel sheet with respect to the Mn content. As an experimental condition, steel containing C and Mn shown in Table 1 below was used. Finish rolling temperature (FDT) 890 ° C,
After performing hot rolling at a winding temperature (CT) of 650 ° C. to form a thin steel sheet having a thickness of 2 mm, a JIS No. 5 test piece is sampled from the thin steel sheet, and a tensile test is performed to measure elongation. is there. From FIG. 6, it can be seen that the elongation, that is, the ductility, is drastically improved by suppressing the Mn content. In the present invention, in order to secure excellent ductility, the Mn content is set to 0.1.
It is determined to be less than 5%, preferably less than 0.45%, more preferably 0.4% or less.

[0024]

[Table 1]

On the other hand, Mn, like C, is also an effective element for increasing the hardenability of steel and ensuring high strength, and as shown in FIG. 1, stabilizes the strength after quenching. Therefore, the lower limit of the amount of Mn is set to 0.1%, preferably 0.2%.

B: 0.0005% to 0.005% B is an element necessary for enhancing hardenability and obtaining a sufficient hardened structure even at a low temperature. To effectively exert such an effect, 0% is required. 0.0005% or more, preferably 0.001%
% Must be added. However, if the B content is too large, a large amount of iron nitride precipitates and causes the ductility to deteriorate, so that 0.005% or less, preferably 0.005% or less.
Keep it below 4%.

According to the present invention, as described above, the amount of C, B, Cr, and / or Mo specified in the present invention is added in combination after suppressing the Mn content and securing sufficiently excellent ductility. By doing so, the hardenability is enhanced, and the variation after quenching is suppressed, and the high strength of the thin steel sheet can be reliably obtained. Further, by defining the components as described above, spot weldability and corrosion resistance after quenching could be secured.

Although the typical chemical composition in the present invention is as described above, it is also effective to obtain the following improvement effects by adding appropriate amounts of Ti and Al as necessary.
That is, Ti is effective for enhancing the quenching effect of B by keeping B in a solid solution state without precipitating B as nitride.
It is preferable to add 0.01% or more, but if the addition amount is too large, ductility is deteriorated. Therefore, it is preferably suppressed to 0.04% or less. Al is effective as a deoxidizer,
If the content is too large, surface defects such as "sludge and sliver" increase, so the content is preferably 0.06% or less, more preferably 0.05% or less.

The elements contained in the steel sheet of the present invention are as described above, and the balance is substantially Fe. However, it is needless to say that a trace amount of unavoidable impurities is allowed in the steel sheet. Other elements can be positively contained as long as the effects of the present invention are not adversely affected. Examples of other elements that can be positively added include Si, Cu, Ni, and the like, which have a hardenability improving effect.

It should be noted that the present invention does not specify a method for producing a thin steel sheet, and the thin steel sheet of the present invention may be obtained by hot rolling or subsequent cold rolling, or may be further obtained after rolling. It may be plated to make a plated steel sheet,
Reheating temperature, finish rolling temperature,
Identify conditions such as cooling and winding, cold rolling rate in cold rolling, recrystallization annealing, etc., or conditions such as plating bath type, plating bath temperature, plating weight, and plating alloying treatment in plating. Nothing to do.

The present invention does not limit the method of quenching, but includes high-frequency heating and quenching (induction quenching), heating and quenching in a heating furnace, or quenching (press quenching) in a mold simultaneously with molding after heating. ) Or any other heat treatment method.

Hereinafter, the present invention will be described more specifically with reference to examples. However, the present invention is not limited to the following examples, and may be appropriately modified within a range that can be adapted to the spirits described before and after. It is also possible to implement by adding
All of them are included in the technical scope of the present invention. That is, in the following examples, a cold-rolled steel sheet or a plated steel sheet is used as the final product, and the heat treatment is performed by the induction hardening method. However, as described above, the present invention does not specify the manufacturing conditions of the thin steel sheet, and various Applying the present invention to one manufactured under the manufacturing conditions is also included in the scope of the present invention.

[0033]

EXAMPLE A steel having a chemical composition shown in Table 2 was melted to produce a slab having a thickness of 230 mm, and the slab was subjected to hot rolling under the conditions shown in Table 3 below. 0
mm, cold rolling was performed to obtain a thin steel sheet having a thickness of 1.0 mm. Nos. Shown in Tables 2 and 3 10-14
Is a steel sheet obtained by cold rolling at a temperature shown in Table 3 for 4 hours.
After annealing for 0 second, finally skin pass rolling (elongation rate 1
%). No. Nos. 1 to 9 were plated as shown below. That is, after the thin steel sheet obtained by cold rolling is annealed at a temperature shown in Table 3 for 40 seconds, it is subjected to a hot dip galvanizing treatment, and further, alloying of the plating is performed at a temperature shown in Table 3, and finally a skin pass is applied. Rolling (1% elongation) was performed.

From the thin steel sheet thus obtained, 1.0
A flat plate of mm × 30 mm × 300 mm is heated at 850 ° C., 90
Three pieces were cut out at each quenching temperature of 0 ° C. and 950 ° C., and induction hardening was performed. In the quenching, the flat plate was fed from a steel plate guide between high-frequency coils opposed to each other, and the entire flat plate was quenched at a quenching temperature of 850 ° C., 900 ° C., or 950 ° C. After reaching the quenching temperature, shower cooling was immediately performed. Thereafter, a JIS No. 5 test piece was prepared from each flat plate and subjected to a tensile test to measure a tensile strength (TS). The strength deviation (ΔTS) shown in Table 3 shows the difference between the lowest TS value of the samples quenched at 850 ° C. and the highest TS value of the samples quenched at 950 ° C.

The mechanical properties of the steel sheet before quenching shown in Table 3 were obtained by cutting a sheet having the same size as the above-mentioned flat plate from the steel sheet before quenching, producing a JIS No. 5 test piece, performing a tensile test, and performing a yield test ( YP), tensile strength (TS) and elongation (El)
Is measured.

The plating properties were evaluated by judging that the obtained plated steel sheet having good surface properties was rated as “○” and that having no plating was rated as “x”. Table 3 also shows these results.

[0037]

[Table 2]

[0038]

[Table 3]

From the experimental results shown in Tables 2 and 3, N
o. Nos. 2, 4, 6, 11, 12, and 13 satisfy the requirements of the present invention, and provide a thin steel sheet having good ductility, small variation in strength after quenching, and good plating properties without occurrence of non-plating. Was done. On the other hand, no. 1,5,7
Nos. 10 to 14 do not satisfy the requirements of the present invention. Nos. 1, 5, 9, and 14 resulted in inferior strength stability after quenching, and Nos. 7, 8, and 10 resulted in inferior plating properties, ductility, or spot weldability.

That is, No. In No. 1, since the amount of C was insufficient and the above-mentioned formula (1) was not satisfied, the variation in strength after quenching was large. No. In No. 9, both the total amount of Cr and / or Mo and the right side of the formula (1) were below the range of the present invention. 14
Satisfies the range of the present invention with respect to the amount of C and the total amount of Cr and / or Mo, but the right side of the formula (1) is 0.1.
Since it was less than 19, the variation in strength after quenching was large. No. In No. 7, since the total amount of Cr and / or Mo exceeded the specified range, an oxide was formed on the surface of the base steel sheet before plating, resulting in non-plating. No. 8, since Mn exceeds the specified amount,
Although the variation in strength after quenching was small, the result was inferior ductility.

In addition, No. Reference numeral 10 is a reference example. When C is added in a large amount beyond the specified range of the present invention, the variation in strength after quenching can be reduced, but it is difficult to secure spot weldability. Is not preferred.

[0042]

The present invention is configured as described above.
By appropriately controlling the chemical composition as described above, high strength can be reliably obtained by quenching and excellent ductility can be ensured at the same time.Furthermore, good corrosion resistance, plating properties and spot weldability are also obtained. We were able to secure it. By realizing such thin steel sheets with excellent ductility and strength stability after quenching, complicated forming is performed and high strength is required for steel sheets for automobiles, steel sheets for construction, and mechanical structural members. It is now possible to supply steel plates and the like.

[Brief description of the drawings]

FIG. 1 shows the relationship between quenching temperature and tensile strength after quenching
It is the graph shown according to n density | concentration.

FIG. 2 is a graph showing a relationship between a quenching temperature and an iron content in a plating layer.

FIG. 3 is a graph showing a relationship between an iron content in a plating layer and a maximum drilling depth in a corrosion resistance test.

FIG. 4 is a graph showing a relationship between a quenching temperature and a residual ratio of a coating film.

FIG. 5 is a graph showing the relationship between the C content and the (Cr + Mo) content, which affects the variation in strength after quenching.

FIG. 6 is a graph showing the relationship between the Mn content in a steel sheet and the elongation of the steel sheet.

Claims (2)

[Claims] C .: 0.1% by mass (hereinafter the same)
1 to 0.22%, Mn: less than 0.1 to 0.5%, Cr and / or Mo: 0.1 to 0.5% in total, B: 0.0
005 to 0.005%, and 0.19 ≦ {[C] + ([Cr] + [Mo]) / 5} where [C], [Cr], and [Mo] are C and Cr, respectively. ,
A thin steel sheet excellent in ductility and strength stability after heat treatment characterized by satisfying｝ indicating Mn content (% by mass). 2. The method according to claim 1, wherein Δ [C] + ([Cr] + [Mo]) /
The thin steel sheet according to claim 1, wherein 5% is 0.28 or less.