JPH06256900A - Cold rolled steel sheet excellent in secondary working brittleness and its production - Google Patents

Abstract

PURPOSE:To obtain a cold rolled steel sheet excellent in secondary working brittleness by using IF steel as a base, specifying the E.N.T value, i.e., the index related to solid solution C, adding a suitable amt. of B and reducing the content of P. CONSTITUTION:A cold rolled steel sheet contg., by weight, 0.0015 to 0.01% C, <=0.5% Si, 0.05 to 2.2% Mn, <=0.017% P, 0.003 to 0.02% S, <=0.1% sol.Al, <=0.004% N, 0.0001 to 0.0008% B and 0.005 to 0.1% Ti and contg., at need, 0.005 to 0.05% Nb, and the balance iron is prepd. This steel sheet is annealed at 700 deg.C sheet temp. Then, the E.N.T value by the formula E.N.T=Ti*/(4XC) (wherein Ti*=Ti-{(48/14)XN+(48/32)XS} is regulated to 1 to 4, by which the objective steel is obtd.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a cold-rolled steel sheet excellent in secondary work embrittlement resistance, which is provided for use as a steel sheet for automobiles, and a method for producing the same, and is suitable for use as inner and outer sheets of automobiles. Cold rolled steel sheet and its manufacturing method. Here, cold-rolled steel sheet refers to a cold-rolled steel sheet that is used as it is for automobile steel sheets, etc., and the cold-rolled steel sheet is subjected to various surface treatments such as electrogalvanizing, hot dip galvanizing, or tin plating before use. It also includes a cold-rolled steel plate as an original plate for surface treatment.

[0002]

2. Description of the Related Art In recent years, environmental problems have been highlighted, and from the viewpoint of regulating exhaust gas of automobiles, there is a movement to improve fuel efficiency of automobiles by reducing the thickness of automobile body materials. Further, due to the diversification of automobile designs, cold-rolled steel sheets used for automobile bodies are subject to press formability in various ways, and therefore extremely severe formability is required.

As described above, the characteristics required for steel sheets for automobiles are extremely advanced and diversified. Conventionally, as cold-rolled steel sheets having excellent formability, IF steels made by adding carbonitride forming elements such as Ti and Nb to ultra-low carbon steels, and solid solution strengthening elements such as Si, Mn and P based on these IF steels. A high-strength cold-rolled steel sheet to which is added is manufactured. As the press formability of these steel sheets is improved, the forming process becomes more severe, and the drawback that brittle cracks are likely to occur in the subsequent secondary process is becoming more and more remarkable.

As a method of improving the above-mentioned secondary working brittleness, many methods of adding a trace amount of an element for increasing the grain boundary strength have been disclosed. For example, as disclosed in JP-A-1-309942, Zr, Techniques for adding Hf, JP-A-1-246325 and JP-A-62-24703
Although there is a technique of adding Sb, As, Ge, and Se as disclosed in Japanese Patent Publication No. 0, the manufacturing cost is high because an expensive element such as Zr, Hf, Sb, As, Ge, and Se is added to the steel sheet. There is a problem of becoming. Further, it also has a drawback of impairing various properties such as chemical conversion processability required for automobile steel sheets.

Further, Japanese Patent Publication No. 1-282817 discloses that
A technique is also disclosed in which B is added to improve the resistance to secondary working brittleness, the amount added is minimized, and the amounts of Ti and Nb are controlled so that the solid solution B can be effectively used. Since the amount of Nb added is regulated, improvement in press formability of steel sheet cannot be expected.

[0006]

In the recent automobile industry, on the premise that the steel sheet has high formability, weight reduction of the vehicle body has become a top priority in order to further improve fuel consumption. Is required to have higher strength. Therefore, although a cold rolled steel sheet having high strength and excellent secondary work embrittlement resistance is desired, a cold rolled steel sheet having improved secondary work embrittlement resistance and a manufacturing method thereof are not disclosed. An object of the present invention is to provide a cold-rolled steel sheet excellent in secondary work embrittlement resistance by using IF steel as a base material composition and further adding B as a component composition of the steel sheet in order to solve the problems of the above-mentioned conventional techniques. To do.

[0007]

Since the secondary work embrittlement resistance of cold-rolled steel sheets generally depends on the grain boundary strength of ferrite, the ultra-low carbon steel containing almost no solute C deteriorates the secondary work embrittlement resistance. It has been pointed out that it is easy to do. We have
As a result of various investigations on the factors governing the secondary work embrittlement resistance, repeated investigations have been made to solve the above-mentioned problems in the prior art.

As a result, solid solution C, which is a main controlling factor of the secondary work embrittlement of steel sheet, is utilized, and an index related to the solid solution C,
By introducing the ENT value, making the composition of the composition the ENT value is in the range of 1 or more and 4 or less, further adding an appropriate amount of B, and reducing the amount of P which is an element that deteriorates the secondary work embrittlement resistance,
It has succeeded in imparting extremely excellent secondary processing brittleness resistance.

That is, the first invention of the present application is a cold-rolled steel sheet having the following component composition (the component composition is wt%) excellent in secondary work embrittlement resistance. (A) C: 0.0015 to 0.01%, Si: 0.5% or less, Mn: 0.05
~ 2.2%, P: 0.017% or less, S: 0.003 ~ 0.02%, Sol.
Al: 0.1% or less, N: 0.004% or less, B: 0.0001 to 0.0008%
, Ti: 0.005 to 0.1%, the balance consisting of iron and inevitable impurities. (B) When ENT is defined by the following formula (1), ENT
Is 1 to 4. ENT = Ti * / (4 × C) …… (1) However, Ti * = Ti − {(48/14) × N + (48/32) × S}

The second invention of the present application is a cold-rolled steel sheet having the following component composition (the component composition is wt%) excellent in secondary work embrittlement resistance. (A) C: 0.0015 to 0.01%, Si: 0.5% or less, Mn: 0.05
~ 2.2%, P: 0.017% or less, S: 0.003 ~ 0.02%, Sol.
Al: 0.1% or less, N: 0.004% or less, B: 0.0001 to 0.0008%
, Ti: 0.005-0.1%, Nb: 0.005-0.05%, the balance consisting of iron and inevitable impurities. (B) When ENT is defined by the following formula (2), ENT
Is 1 to 4. ENT = Ti * / (4 * C) + (12 * Nb) / (93 * C) (2) However, Ti * = Ti-{(48/14) * N + (48/32) * S }

Further, the third invention of the present application is a method for producing a cold-rolled steel sheet excellent in secondary work embrittlement resistance, which comprises the following steps. (A) a step of preparing any one of the cold-rolled steel sheets described above, and (b) a step of annealing the cold-rolled steel sheet at a temperature of 700 ° C. or higher.

[0012]

The details of the present invention and the reasons for limiting the components will be described below. In the following, the component composition is wt% unless otherwise specified. The ENT value is defined by the following equation (2). ENT = Ti * / (4 * C) + (12 * Nb) / (93 * C) (2) However, Ti * = Ti-{(48/14) * N + (48/32) * S }
Is.

The ENT value is closely related to the amount of solid solution C in the steel, and as the value increases, the amount of solid solution C becomes almost zero. By adding Ti, or Ti and Nb,
When the ENT value is set to a steel composition of 1 or more, stoichiometrically, since C is precipitated and fixed by Ti and Nb, the solid solution C content should be 0, but When the relationship of the amount of solute C was investigated in detail, it was found that the amount of solute C was present in a small amount even when the ENT value was in the range of 1 or more and 4 or less.

Therefore, C: 0.0015 to 0.01%, Si: 0.5% or less, Mn: 0.05 to 2.2%, P: 0.017% or less, S: 0.00
3 to 0.02%, Sol.Al: 0.1% or less, N: 0.004% or less,
Ti: A slab in which the addition amount of each element was variously changed within the composition range of 0.005 to 0.1%, and Nb was added in the range of 0.005 to 0.05% in addition to the composition of these ranges. After hot rolling the slab, a test piece for measuring internal friction was taken from the hot rolled sheet.

Then, the internal friction of this test piece was measured, and the relationship between the solid solution C amount calculated from this result and the ENT value was investigated. The results are shown in FIG. 3, and it was also found that even if the ENT value is 1 or more, the amount of solute C on the order of several ppm is present, although it is a trace amount. It is considered that the reason why the amount of solid solution C remains is that the state of complete chemical equilibrium is not reached even after the heat cycle of slow cooling after hot rolling and winding, and therefore a trace amount of C remains. Because.

Since the residual amount of solid solution C suggests the effect of improving the secondary work embrittlement resistance by the amount of solid solution C, the hot rolled sheet is pickled, cold rolled, and continuously annealed at 800 ° C. Finally, 0.5% temper rolling was performed to investigate the secondary work embrittlement resistance. Regarding the evaluation method of the secondary processing brittleness resistance, a blank with a diameter of 105 mm was punched out from a hot rolled sheet,
It was subjected to deep drawing into a cup shape, and the sample was stored in various refrigerants (ethyl alcohol or chlorofluorocarbon), and when it was stressed to expand the end of the cup as shown in Fig. 4, it was ductile. The temperature at the boundary where transition from normal deformation to brittle fracture was measured, and this was taken as the secondary working embrittlement resistance temperature.

The results are shown in FIG. 2. The resistance to secondary working embrittlement temperature changes depending on the ENT value. From the composition of components in which ENT is less than 4, the secondary working embrittlement resistance begins to decrease.
As the NT value decreased, the effect of improving the secondary work embrittlement resistance became remarkable. This is a result of the effect of improving the secondary work embrittlement resistance by the solid solution C. Furthermore, it is recognized that the secondary processing embrittlement resistance temperature tends to decrease sharply when the ENT value is close to 1. Based on such experimental facts, investigations were carried out on the effect of improving the secondary work embrittlement resistance by further adding B.

The chemical composition of steel is C: 0.0015 to 0.01%, Si:
0.5% or less, Mn: 0.05 to 2.2%, P: 0.017% or less,
S: 0.003-0.02%, Sol.Al: 0.1% or less, N: 0.004% or less, Ti: 0.005-0.1%, B: 0.0001-0.0008% A slab with various addition amounts of each element in the range, The slab containing Nb in the range of 0.005 to 0.05% in addition to the component composition in these ranges is hot-rolled, cold-rolled into a cold-rolled sheet, and then continuously annealed at 800 ° C. And then
Finally, a 0.5% temper rolling was performed to collect a sample material, and the change in secondary working embrittlement temperature was investigated.

The results are shown in FIG. From FIG. 1 and FIG. 2 described above, from the result of FIG. 2, the secondary working embrittlement resistance temperature decreased by about 10 ° C. for ENT> 4, and this decrease is considered to be the effect of B addition, but ENT = 4. It is recognized that the lowering of the secondary processing embrittlement resistance temperature sharply increases in the region of 4 or less with the boundary. And, it is clear that the temperature decrease is remarkable especially in the range of 1 ≦ ENT ≦ 4.

From the above, it was found that the secondary work brittleness resistance was improved as much as not expected until now only in the range of 1≤ENT≤4. This is because the effects of the amount of solid solution C and the amount of B added on the secondary processing brittleness resistance were realized in a composite manner. When ENT ≧ 4, as shown in FIG. 3, since the amount of solid solution C does not exist at all, only grain boundary strengthening due to B can be expected. Therefore, the secondary work embrittlement resistance at a substantially constant temperature is improved regardless of the ENT value. Can be interpreted as

On the other hand, when B is added in the case of ENT <1, the reason why the effect of improving the secondary work embrittlement resistance is almost eliminated is that even if B is not added, a sufficient amount of solid solution C already exists, so that C is present at the grain boundaries. This is because B does not exist in the grain boundary and the effect of improving the secondary work embrittlement resistance due to B does not appear. That is, the secondary processing embrittlement resistance temperature has already been sufficiently lowered, and therefore the improvement effect due to the addition of B cannot be exhibited.

As described above, the point of the present invention is that the composition of the components is in the range of 1≤ENT≤4, that is, B is added in an appropriate amount with a slight amount of dissolved C. Only in such a case, it is possible to conclude that the effect of improving the secondary work embrittlement resistance, which is much higher than the effect of improving only the solid solution C or the addition of B, can be obtained.

Regarding the Components of Steel Regarding the above invention, the reasons for limiting the component composition of steel as described above are as follows. C: 0.0015 to 0.01% C is preferably as small as possible in order to secure the formability of the steel sheet, but its upper limit is limited to 0.01% as a range that does not impair the effects of the present invention in practice, but is preferably 0.006%.
In addition, the lower limit is 0.0015% because excessively low C will increase costs.

Si: 0.5% or less. Si contributes to the strengthening of the steel sheet as a solid solution strengthening element without deteriorating the press formability, but if it is contained in excess of 0.5%, the scale generated during heating in hot rolling becomes significant, so 0.5% is made the upper limit. did.

Mn: 0.05 to 2.2% Since Mn is fixed to S, its lower limit was set to 0.05%. Also,
If the content exceeds 2.2%, the formability of the steel sheet itself deteriorates, so the upper limit was made 2.2%.

P: 0.017% or less. P is an element that can strengthen steel at the lowest cost, but if it is contained in an amount of more than 0.017% based on IF steel, it tends to segregate at grain boundaries and the problem of secondary work embrittlement becomes apparent. It was limited to 0.017% or less.

S: 0.003 to 0.02% It is desirable to reduce S as much as possible, but if it is less than 0.003%, the peelability of the scale during pickling of the steel sheet decreases, so 0.
The lower limit was 003%. On the other hand, if the content exceeds 0.02%, the ductility of the steel sheet deteriorates, so the content was made 0.02% or less.

Sol.Al: 0.1% or less. Al is necessary for deoxidizing and fixing N, but if it is added in a large amount, the cost increases, so the content is made 0.1% or less. It is preferably 0.06% or less.

N: 0.004% or less. The N content is preferably as small as possible in order to secure the formability of the steel sheet, but the upper limit was made 0.004% in order not to impair the effects of the present invention.

Ti: 0.005 to 0.1%. Ti has the effect of fixing C and improving the formability of the steel sheet. That is, if it is less than 0.005%, its effect is poor, and if it is added in a large amount, not only the cost rises but also the cause of surface defects and the chemical conversion processability are deteriorated, so the upper limit was made 0.1%.

Nb: 0.005 to 0.05% Similar to Ti, Nb has the property of fixing C. Not only does it improve the formability of the steel sheet by the combined addition of Ti, but it also dissolves when Nb-based carbides are redissolved during high temperature annealing. It also has an effect of generating C and improving the secondary processing brittleness resistance. That is, if less than 0.005%, its effect is poor, and adding a large amount causes cost increase, so its upper limit is set to 0.
It was set to 05%.

B: 0.0001 to 0.0008%. As described above, B is an important element in the present invention,
Due to its characteristics, it has the effect of segregating to the grain boundaries and strengthening the grain boundaries, so it is useful for improving the secondary work embrittlement resistance of the cold rolled steel sheet based on IF steel. In the present invention, when B is added, the secondary work embrittlement resistance is improved in a composite manner due to the presence of solid solution C. That is, even if the content exceeds 0.0008%, the improvement effect is saturated, so 0.0008% was made the upper limit. Further, in order to obtain the effect of improving the secondary processing brittleness resistance due to B, the lower limit is 0.0001%.

Manufacturing Conditions Next, the manufacturing conditions in the present invention will be described. The steel making process may be a steelmaking method using an electric furnace in addition to the melting by a normal converter. There is no difference in the casting process between continuous casting and ingot making. Normal hot rolling and cold rolling for high temperature slabs sent directly from a continuous casting machine, high temperature slabs obtained by heating, or slabs obtained by slabbing steel ingots To give a cold rolled steel sheet.

After that, annealing is performed. This annealing may be either continuous annealing or batch type annealing, and the temperature must be 700 ° C. or higher. This is because at least 700 ° C. is required to sufficiently perform recrystallization and secure the formability of the steel sheet in any annealing. In addition, even in the steel sheet finally adjusted to the composition of the steel of the present invention by decarburization annealing, the same secondary work embrittlement resistance can be obtained.

When the cold-rolled steel sheet is required to have corrosion resistance, the cold-rolled steel sheet obtained as described above is galvanized by electrogalvanizing or is passed through a continuous hot-dip galvanizing line. Does not impair the material properties of the cold rolled steel sheet of the present invention. It is clear that the cold-rolled steel sheet having extremely excellent secondary working embrittlement resistance according to the present invention is a steel sheet useful as a material that is subjected to press forming such as automobile materials as well as home electric appliances materials. Provide a press molding material that is processed into a more complicated shape by improving the subsequent processing brittleness.

[0036]

EXAMPLES Specific examples of the present invention will be described below. Steel with the chemical composition shown in Table 1 was melted, and a slab was finished into a hot rolled steel sheet by ordinary hot rolling, followed by pickling and cold rolling to 0.8 mm, and then a continuous method. Alternatively, annealing was performed by a batch method. Next, 0.
After 5% temper rolling, a test piece was sampled to investigate the secondary work embrittlement resistance.

The secondary work resistance obtained here is also shown in Table 1. From Table 1, the cold-rolled steel sheet of the present invention and the steel sheet produced by the method for producing the same all show extremely excellent secondary work embrittlement resistance with a secondary work embrittlement resistance of −120 ° C. or less. The above is a general description, but a little supplementary description will be given below.

Steels 13 and 25 have Tc of −120 ° C. and ENT <1 regardless of whether or not B is added.
The secondary work brittleness resistance is inferior as compared with the invention steels 1 and 6 with ENT≈1 (Tc is −190 ° C.).

The Tc of the steels 17 and 21 containing B is -8.
At 0 ° C., the Tc of the steels 22 and 23 to which B is not added is −60 ° C., but since both are ENT> 4, the secondary work embrittlement resistance is poor.

Within the range of 1≤ENT≤4, the Tc of the steels 14, 15, 16, 18, 19, 20 containing no B is -70.
~ -90 ° C. On the other hand, Tc of the steels 1 to 12 to which B is added is -120 to -190 ° C, and the secondary work embrittlement resistance is extremely excellent.

[0041]

[Table 1]

[0042]

As described above, according to the present invention, it is possible to provide a cold-rolled steel sheet having excellent secondary work embrittlement resistance, which can be applied to automobile body materials, difficult-to-machine members, etc. This is a highly effective invention.

[Brief description of drawings]

FIG. 1 is a diagram showing a relationship between an ENT value, a secondary work embrittlement resistance temperature, and a decrease in the secondary work embrittlement resistance temperature due to addition of B.

FIG. 2 is a diagram showing the relationship between the ENT value and the resistance to secondary working embrittlement temperature when B is not added.

FIG. 3 is a diagram showing a relationship between an E.N.T. value and a solid solution C amount.

FIG. 4 is a view showing a method for measuring a secondary working embrittlement temperature of a steel sheet.

Claims (3)

[Claims] 1. A cold-rolled steel sheet having the following component composition (the component composition is wt%), which is excellent in secondary work embrittlement resistance. (A) C: 0.0015 to 0.01%, Si: 0.5% or less, Mn: 0.05
~ 2.2%, P: 0.017% or less, S: 0.003 to 0.02%, Sol.Al:0.1%
Below, N: 0.004% or below, B: 0.0001 to 0.0008%, Ti: 0.0
When ENT is defined by the following formula (1), ENT
Is 1 to 4. ENT = Ti * / (4 × C) …… (1) However, Ti * = Ti − {(48/14) × N + (48/32) × S} 2. A cold-rolled steel sheet having the following component composition (the component composition is wt%) excellent in secondary work embrittlement resistance. (A) C: 0.0015 to 0.01%, Si: 0.5% or less, Mn: 0.05
~ 2.2%, P: 0.017% or less, S: 0.003 ~ 0.02%, Sol.
Al: 0.1% or less, N: 0.004% or less, B: 0.0001 to 0.0008%
, Ti: 0.005-0.1%, Nb: 0.005-0.05%, the balance consisting of iron and inevitable impurities. (B) When ENT is defined by the following formula (2), ENT
Is 1 to 4. ENT = Ti * / (4 * C) + (12 * Nb) / (93 * C) (2) However, Ti * = Ti-{(48/14) * N + (48/32) * S } 3. A method for producing a cold-rolled steel sheet having excellent secondary work embrittlement resistance, which comprises the following steps. (A) A step of preparing the cold-rolled steel sheet according to claim 1 or 2, and (b) a step of annealing the cold-rolled steel sheet at a plate temperature of 700 ° C or higher.