JP2013072107A - Bake-hardenable, cold-rolled steel sheet excellent in surface quality after molding and method for manufacturing the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a bake-hardenable, cold-rolled steel sheet which is excellent in surface quality after molding and is extremely useful as an automotive exterior and interior panels.SOLUTION: The bake-hardenable, cold-rolled steel sheet has a composition comprising, by mass%, 0.0005-0.0050% C, ≤0.30% Si, ≤1.50% Mn, ≤0.100% P, ≤0.020% S, ≤0.080% sol. Al, ≤0.0070% N, 0.003-0.100% Nb, provided that C and Nb satisfy the following relation: 0.50≤([%Nb]/93)/([%C]/12)≤1.50 (wherein [%M] represents element M content in steel (mass%)), and the balance being Fe and unavoidable impurities.

Description

  The present invention relates to a bake-hardened cold-rolled steel sheet excellent in surface quality after forming and suitable for application to an outer plate of an automobile and a method for producing the same.

  In recent years, the surface quality required for automobile outer panels has become increasingly severe. The surface defects that influence the surface quality are roughly classified into those that are recognized on the surface in the manufacturing stage of the steel sheet and those that appear after being formed in the press line of an automobile.

The former surface defects are relatively easy to find, so the impact on automobile production is small. In addition, as disclosed in Patent Document 1, for example, measures at the material stage are also known.
On the other hand, since the latter surface defect may be found for the first time in the final inspection process after being molded into a part or further incorporated into a vehicle body, the influence on automobile production is extremely large.
Moreover, no effective countermeasure has been clarified so far for the means for suppressing the latter surface defects.

JP-A-9-296222

  The present invention has been developed in view of the above situation, and an object of the present invention is to propose a bake-hardened cold-rolled steel sheet that is particularly excellent in surface quality after forming together with its advantageous manufacturing method.

In order to solve the above-mentioned problems, the present inventors have intensively studied the generation mechanism of defects that appear as surface defects after molding and countermeasures for suppressing them.
As a result, in steel sheets where such surface defects occur, local uneven deformation due to yield point elongation occurs during the annealing process of the steel sheet, and it is clarified that this is the cause of surface defects after forming. It was done.

In other words, if non-uniform deformation occurs in the steel sheet during the annealing process, the hardness of the non-uniform deformation portion is larger than that of the non-deformation portion, and the amount of deformation is small. A deformation | transformation part floats up as a convex part, and becomes an external appearance defect. In addition, it becomes a sharp linear defect in appearance, and exhibits a form extending obliquely at 45 ° with respect to the longitudinal direction of the steel sheet.
Among automotive outer plates, bake-hardened steel plates are often used for doors, hoods, and the like for the purpose of improving dent resistance. In such a steel sheet, since the solid solution C is intentionally left, the elongation at the yield point appears in the state after recrystallization, and in particular, the above-described non-uniform deformation tends to occur.

  In order to suppress the occurrence of surface defects as described above, it is sufficient that a strain amount exceeding the yield point is not given during annealing. In general, in a continuous annealing furnace, facility design and sheet passing conditions are set under conditions that do not cause strain exceeding the yield point of the steel sheet. However, in practice, it has become clear that due to thermal strain due to heating / cooling, locally uneven strain occurs, and under certain conditions, the yield point of the steel sheet may be exceeded.

Therefore, as a result of further study on the generation factors of surface defects caused by the above-described non-uniform deformation during annealing, the present inventors have obtained a constant cooling rate in a specific temperature range in the cooling process after completion of recrystallization. When it exceeded, the thermal strain which generate | occur | produces in a steel plate will become large, the distortion exceeding the yield point of a steel plate will generate | occur | produce, and the knowledge that the surface defect expressed after shaping | molding was acquired.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. In mass%, C: 0.0005 to 0.0050%, Si: 0.30% or less, Mn: 1.50% or less, P: 0.100% or less, S: 0.020% or less, sol.Al: 0.080% or less, N: 0.0070% or less, and Nb : 0.003 to 0.100%, C and Nb satisfy the relationship of the following formula, the balance is composed of Fe and unavoidable impurities, and 1 to 5% of the strip-shaped specimens taken in the rolling direction. A bake-hardened cold-rolled steel sheet excellent in surface quality after forming, characterized in that a linear pattern does not occur when the surface is scratched after applying tensile strain in the direction.
Record
0.50 ≦ ([% Nb] / 93) / ([% C] / 12) ≦ 1.50
Here, [% M] represents the content (mass%) of M element in steel.

2. The bake-hardened cold-rolled steel sheet having excellent surface quality after forming according to 1 above, further comprising at least one selected from Ti: 0.005% or less and B: 0.0003-0.0030% by mass% .

3. The bake hardened cold rolled steel sheet having excellent surface quality after forming according to 1 or 2 above, wherein the steel sheet has a zinc-based plating film on the surface thereof.

4). The steel slab having the composition described in 1 or 2 is hot-rolled, pickled, then cold-rolled and then subjected to continuous annealing, and a temperature range of 400 to 200 ° C. is applied in the cooling process of the continuous annealing. A method for producing a bake-hardened cold-rolled steel sheet having excellent surface quality after forming, characterized by cooling at a cooling rate not exceeding 30 ° C / s.

5). 5. The method for producing a bake-hardened cold-rolled steel sheet having excellent surface quality after forming as described in 4 above, comprising a plating treatment step of forming a zinc-based plating film on the steel sheet surface.

  ADVANTAGE OF THE INVENTION According to this invention, the bake hardening type cold-rolled steel plate excellent in the surface quality after shaping | molding extremely useful as a motor vehicle outer plate | board or an inner board | plate can be manufactured and supplied stably, and industrial value is very high.

Hereinafter, the present invention will be specifically described.
First, the reason why the component composition of the steel sheet is limited to the above range in the present invention will be described. In addition,% showing the following component composition shall mean the mass% unless there is particular notice.
C: 0.0005-0.0050%
When the content of C increases, deep drawability and ductility deteriorate, and it becomes difficult to impart formability for automobile outer plates and inner plates. For this reason, the upper limit of the amount of C is specified as 0.0050%, preferably 0.0040%. On the other hand, if the content is less than 0.0005%, the crystal grains become coarse and the surface of the steel sheet is liable to be roughened when formed, so the lower limit of the C content is specified to 0.0005%.

Si: 0.30% or less
Si is an element having a high solid solution strengthening ability and is an effective element for obtaining high strength. On the other hand, when the content is increased, surface defects due to scale are likely to occur. For this reason, the upper limit of Si content is specified to be 0.30%, preferably 0.20%.

Mn: 1.50% or less
Mn is an element that can increase the strength of the steel sheet by adding Mn, but on the other hand, when it is added excessively, deep drawability is lowered. For this reason, the upper limit of the amount of Mn is specified as 1.50%.

P: 0.100% or less P is an effective element that can increase the strength of a steel sheet when the content increases even in a small amount, but if contained excessively, ductility and weldability deteriorate. For this reason, the upper limit of the amount of P is specified as 0.100%.

S: 0.020% or less If the content of S is high, the toughness of the welded portion deteriorates as in the case of P. For this reason, the upper limit of the amount of S is suppressed to 0.020%, preferably 0.015%.

sol.Al: 0.080% or less, N: 0.0070% or less
Since sol.Al and N do not impair the effects of the present invention as long as they are contained in ordinary steel, they are defined as sol.Al: 0.080% or less and N: 0.0070% or less, respectively.

Nb: 0.003 to 0.100%
Nb is a particularly important element in the present invention. By appropriately controlling the amount of Nb added relative to the amount of C, a part of C can be fixed as NbC or NbCN, and solid solution C can remain in the final product. Moreover, even when it is added excessively with respect to the amount of C, a desired bake hardening amount (BH amount) can be obtained by re-dissolving a part of NbC or NbCN by controlling the annealing temperature high.
The bake hardening amount (BH amount) is the amount of increase in the yield strength of parts in the paint baking process, which utilizes the strain aging phenomenon due to the residual solid solution C. Generally, after applying a tensile deformation strain of 2%, a tensile test is performed again after heat treatment at 170 ° C. for 20 minutes, which simulates a paint baking process, and yield strength at the time of re-tension (2% tensile deformation and 170% It is the amount of increase in stress obtained by subtracting the stress before heat treatment (stress after 2% tensile deformation) from the stress after heat treatment at 20 ° C. for 20 minutes.
In addition, there is Ti as an element to fix C as carbonitride, but Ti-based carbonitride has a high remelting temperature, so it is difficult to correct the steel component adjustment error with the annealing temperature. There is.
Here, if the Nb amount is less than 0.003%, not only the control of the solute C amount becomes difficult, but also the hot-rolled plate crystal grains become coarse and the deep drawability deteriorates. On the other hand, if it exceeds 0.100%, precipitates are formed. Since it increases and causes deterioration of ductility, the Nb content is specified in the range of 0.003 to 0.100%.

Moreover, in order to control the amount of solute C, the amounts of C and Nb must satisfy the following relational expression.
0.50 ≦ ([% Nb] / 93) / ([% C] / 12) ≦ 1.50
The reason why the lower limit is defined as 0.50 in the above relational expression is that if it is less than this, a large amount of solid solution C remains and material deterioration due to aging tends to occur. On the other hand, the reason why the upper limit is defined as 1.50 is that when the upper limit is exceeded, it is difficult for solid solution C to remain, and the desired bake hardenability cannot be imparted.

Although the basic components have been described above, in the present invention, other elements described below can be appropriately contained as necessary.
Ti: 0.005% or less As described above, when Ti content increases, the bake hardenability controllability deteriorates, so an upper limit of the content is required. For this reason, the upper limit of Ti amount is specified as 0.005%.

B: 0.0003-0.0030%
B is added in order to improve the secondary work brittleness resistance of the deep-drawn part. However, if the amount of B is less than 0.0003%, the desired effect cannot be obtained. On the other hand, if it exceeds 0.0030%, it becomes hard and the formability deteriorates. Therefore, the amount of B is specified in the range of 0.0003 to 0.0030%.

In addition, V, W, Cu, Ni, Sn, Cr, Mo, Sb, and the like can be added for the purpose of improving the surface quality by suppressing moldability such as deep drawability and surface element concentration in the manufacturing process. . The effects of the present invention are not impaired unless these addition amounts are larger than 0.5%.
In addition, when adding Ca for the purpose of controlling the form of inclusions, or when increasing the upper limit of the O content for the purpose of expanding the allowable range of deoxygenation for improving the efficiency during refining, 30 ppm and 50 ppm respectively. If the addition is not excessive, the effect of the present invention is not impaired.
The balance other than the above components is Fe and inevitable impurities.

Next, a method for evaluating the surface quality after molding will be described.
As described above, surface defects that appear after molding may be found for the first time in the final inspection process after being molded into a part or after being incorporated into a car body, compared to surface defects that appear as manufactured. Therefore, the impact on automobile production is extremely large. As a result of earnestly examining the method for detecting streak defects due to local plastic deformation during the manufacturing process, the inventors imparted an appropriate amount of strain to the steel sheet and rubbed the surface, which is simple and effective. It was clarified that it can be detected.
If the amount of strain is too small or too large, the difference in deformation behavior due to the difference in hardness between the portion where plastic deformation has occurred and the portion where it does not is small, so about 1 to 5% is optimal. The test piece may be a strip-shaped test piece with the rolling direction as the longitudinal direction. Since it is necessary to check the entire width of the product, it is efficient to make the area as wide as possible within the specification range of the tensile test piece.
Further, by using a test piece whose longitudinal direction is the test piece, a linear pattern (striped defect) can be appropriately evaluated. In addition, the streak defect that appears after the tensile deformation that is the subject of the present invention is not a stretcher strain caused by so-called yield point elongation, but is only caused by local plastic deformation introduced into the steel sheet during the manufacturing process. This is due to the presence of a portion that is very small but harder than the surroundings. Stretcher strain has a strip shape with a width of 10 mm or more when pulled with a strip-shaped test piece, but the defect targeted by the present invention has a width within 5 mm and a sharp linear shape. It is a feature.

Next, the manufacturing process of the present invention will be described.
In the present invention, the steel slab adjusted to the above component composition is cast, hot-rolled, pickled, then cold-rolled, and then subjected to continuous annealing to obtain a cold-rolled steel sheet. In the present invention, during the above-described continuous annealing, it is important to cool the temperature range of 400 to 200 ° C. at a cooling rate not exceeding 30 ° C./s, particularly in the cooling process.

According to the study by the inventors, in the temperature range of 400 to 200 ° C., the yield strength is relatively low and the yield point elongation is clearly expressed. This temperature range is likely to cause uniform deformation. In this respect, in a temperature range exceeding 400 ° C., the yield strength is sufficiently low and the growth of dislocations is easy, so that non-uniform deformation hardly occurs. On the other hand, in the temperature range below 200 ° C., the yield strength is sufficiently high, and even if distortion occurs, the yield strength is not exceeded.
In addition, the cooling rate is set to 30 ° C./s or less because when the cooling rate exceeds this, the thermal strain generated by the shrinkage increases, resulting in uneven deformation locally exceeding the yield strength of the steel sheet. It is. On the other hand, the smaller the cooling rate, the smaller the distortion during cooling. However, if the cooling rate is extremely small, the line length becomes too long.

  In addition, the manufacturing process other than changing the cooling in the temperature range of 400 to 200 ° C. in the cooling process of the above-described continuous annealing to the above-described controlled cooling may be performed according to a conventional method and is not particularly limited. For example, it is possible to apply a slab manufacturing method by ingot forming or continuous casting, or continuous hot rolling by rough hot rolling bar connection in hot rolling. Further, a temperature rise within 200 ° C. using an induction heater in the hot rolling process does not adversely affect the effects of the present invention.

Regarding other preferable production conditions, the steel slab heating temperature in hot rolling: 1150 to 1300 ° C, the finish rolling finishing temperature is 850 to 950 ° C, the coiling temperature is 500 to 700 ° C, and the rolling reduction of cold rolling is 60 The soaking temperature in continuous annealing (or continuous hot dip galvanizing) is preferably 800 to 900 ° C.
In this invention, it can also be set as the manufacturing method which has the plating process process which forms a zinc-type plating film on the steel plate surface. A zinc-based plating film of pure zinc or zinc alloy (zinc-iron, zinc-Ni, zinc-aluminum, etc.) can be formed on the steel plate surface by electroplating or hot dipping. In the case of galvanizing treatment, annealing and plating treatment can be made as separate steps, and annealing and plating treatment can be made into a series of steps (for example, continuous galvanizing). .

  Furthermore, the present invention is applicable to a treatment or coating that imparts lubricity to the surface even in the case of a hot-dip galvanized steel sheet, even if it is a plated steel sheet or a precoated steel sheet that has been electroplated on the surface of a cold-rolled steel sheet. Even if the coating treatment is performed, the effect of the present invention is not impaired.

Examples of the present invention will be specifically described below.
The steel adjusted to the component composition shown in Table 1 was made into a slab by continuous casting after melting, and it was made into a hot-rolled sheet under the conditions of heating temperature: 1200 ° C, finish rolling finishing temperature: 900 ° C, coiling temperature: 600 ° C. . Then, after pickling, cold rolling with a reduction ratio of 75% was made into a cold rolled sheet with a sheet thickness of 0.75 mm. Subsequently, continuous annealing or continuous hot dip galvanizing was performed under the conditions shown in Table 2 to obtain cold-rolled steel sheets or hot-dip galvanized steel sheets. Then, temper rolling with a reduction ratio of 1.2% was performed.
The conditions for hot dip galvanizing are as follows: plating bath temperature: 460 ° C, Al concentration in the plating bath (with alloying treatment: 0.13%, without alloying treatment: 0.2%), plating coverage per side (45g / m ² ) (Double-sided plating), alloying treatment temperature: 480-580 ° C., alloying degree (Fe mass%): 10%.

Next, strip test pieces with a length of 150 mm and a width of 30 mm were collected from these coils (steel strips) in the longitudinal direction, and the specimens were taken in full width and placed in a tensile testing machine (crosshead speed: 10 mm / min). 1, 3, 5% strain was applied (the tensile direction is the longitudinal direction). Table 2 shows the result of placing a pre-strained test piece on a flat desk, grinding the surface, and investigating the presence or absence of linear patterns (streaks). , O: no defect, x: defect present. The presence / absence of defects was visually observed, and x was indicated when a streak defect was observed even at one location.
As for mechanical properties, a tensile test (crosshead speed: 10 mm / min) was performed using a JIS No. 5 test piece, and a tensile strength TS and a total elongation EL were measured. The tensile test was evaluated with test pieces taken along the rolling direction.
Furthermore, the bake-hardening amount (BH) is JIS No. 5 test piece, using a tensile tester, with a crosshead speed of 10 mm / min, applying 2% strain, and then heating at 170 ° C. for 20 minutes. A tensile test was performed again, and the increase in stress (falling yield point) was evaluated. The amount of BH is desirably 25 MPa or more in order to sufficiently obtain the effect of improving the dent resistance of the part.On the other hand, if the amount of BH exceeding 60 MPa is developed, the amount of solute C remains unnecessarily, causing distortion. The deterioration of the material characteristics over time due to aging becomes large and becomes a problem.
The obtained results are also shown in Table 2.

As is apparent from Table 2, according to the present invention, by controlling the cooling rate in the temperature range of 400 to 200 ° C. after continuous annealing to 30 ° C./s or less, excellent bake hardenability and streak even after molding. A bake-hardened cold-rolled steel sheet and hot-dip galvanized steel sheet free from defects were obtained.
In addition, when the steel plate temperature is less than 200 ° C., the yield strength of the steel plate is sufficiently increased, and it can be seen that no streak defect occurs even when the steel plate is cooled at a rate exceeding 30 ° C./s.
On the other hand, Comparative Example No. 20 has a low BH amount because (Nb / 93) / (C / 12) exceeds the range of the present invention. Further, Comparative Example No. 21 has a low EL because (Nb / 93) / (C / 12) is lower than the range of the present invention.

  ADVANTAGE OF THE INVENTION According to this invention, the bake hardening type cold-rolled steel plate excellent in the surface quality after shaping | molding extremely useful as a motor vehicle outer plate | board or an inner board | plate can be manufactured and supplied stably, and industrial value is very high.

Claims (5)

In mass%, C: 0.0005 to 0.0050%, Si: 0.30% or less, Mn: 1.50% or less, P: 0.100% or less, S: 0.020% or less, sol.Al: 0.080% or less, N: 0.0070% or less, and Nb : 0.003 to 0.100%, C and Nb satisfy the relationship of the following formula, the balance is composed of Fe and unavoidable impurities, and 1 to 5% of the strip-shaped specimens taken in the rolling direction. A bake-hardened cold-rolled steel sheet excellent in surface quality after forming, characterized in that a linear pattern does not occur when the surface is scratched after applying tensile strain in the direction.
Record
0.50 ≦ ([% Nb] / 93) / ([% C] / 12) ≦ 1.50
Here, [% M] represents the content (mass%) of M element in steel.   The bake hardening type cold rolling excellent in surface quality after molding according to claim 1, further comprising at least one selected from Ti: 0.005% or less and B: 0.0003 to 0.0030% by mass%. steel sheet.   The bake-hardened cold-rolled steel sheet having excellent surface quality after forming according to claim 1 or 2, further comprising a zinc-based plating film on the surface of the steel sheet.   A steel slab comprising the component composition according to claim 1 or 2 is hot-rolled, pickled, then cold-rolled and then subjected to continuous annealing, and a temperature range of 400 to 200 ° C in the cooling process of the continuous annealing. Is produced at a cooling rate not exceeding 30 ° C./s, and a method for producing a bake-hardened cold-rolled steel sheet having excellent surface quality after forming.   The method for producing a bake-hardened cold-rolled steel sheet having excellent surface quality after forming according to claim 4, further comprising a plating treatment step of forming a zinc-based plating film on the steel sheet surface.