JP2012012670A - Cold-rolled steel sheet - Google Patents

Abstract

A cold-rolled steel sheet having excellent slidability is provided.
On the surfaces of both surfaces of a steel plate, the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in terms of a random intensity ratio. And the half width of the {100} plane X-ray diffraction peak on the surface is 0.15 ° or more. Since the steel sheet having such a texture distribution has a high hardness distribution on the surface extreme surface layer, the sliding resistance in a state where a surface pressure is applied is small. And when press-molding the outer panel of an automobile, deep drawability is improved by reducing the frictional resistance when sliding with the die part of the mold. Further, in order to obtain such a texture distribution, solute Ti is involved, Ti: 0.01 to 0.1%, and Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S %) − 4 × (C%), it is preferably contained in a range satisfying Ti *> 0.007.
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Description

  The present invention relates to a cold-rolled steel sheet excellent in slidability, which is used for automobiles, particularly pressed parts that require drawability.

In recent years, steel sheets used for automobiles and the like have been subjected to complicated and deep drawing from the viewpoint of design and the like. In particular, panel members such as doors and hoods are required to have excellent slidability because a material needs to flow into a mold in order to avoid cracking during pressing.
In order to solve this, there is a method of increasing the high ductility (El) and the high Rankford value (r value) as the mechanical properties of the steel sheet. Alternatively, as disclosed in Patent Document 1 and Patent Document 2, there is a method of geometrically controlling the uneven shape of the surface. Alternatively, as shown in Patent Document 3, there is a method of forming a hard layer on the surface.
However, there is a limit to improving the mechanical properties. The surface shape control described in Patent Documents 1 and 2 reduces productivity in terms of control and management of the temper rolling roll. The formation of the hard layer on the surface described in Patent Document 3 is mainly applied to hot-dip galvanized steel sheets, and an additional manufacturing process is required for cold-rolled steel sheets, which is not realistic.

Japanese Examined Patent Publication No. 3-54006 JP-A-2-280902 Japanese Patent Publication No. 03-055544

  The present invention has been made in view of such circumstances, and an object of the present invention is to propose a cold-rolled steel sheet which is advantageous in terms of cost and has excellent sliding properties, without adding a manufacturing process or inhibiting productivity.

The present inventors have conducted research in order to solve the above problems. As a result, the following knowledge was obtained.
In order to improve the slidability, it is effective to control the surface layer structure by forming non-recrystallized grains having a specific crystal orientation in the surface layer portion.
More preferably, the formation of the non-recrystallized grains largely involves the strengthening of precipitates on the surface layer, and the control of the component composition centering on Ti is important.

The present invention has been made based on the above findings, and the gist thereof is as follows.
[1] On both surfaces of the steel plate, the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in random intensity ratio, and the half width of the {100} plane X-ray diffraction peak on the surface A cold-rolled steel sheet characterized by an angle of 0.15 ° or more.
[2] In the above [1], in mass%, C: 0.0005 to 0.01%, Si: 0.2% or less, Mn: 0.3% or less, P: 0.03% or less, S: 0.003 to 0.03%, Ti: 0.01 to 0.1 %, Al: 0.01 to 0.10%, N: 0.005% or less, and Ti * = (Ti%)-3.4 x (N%)-1.5 x (S%)-4 x (C%) The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet is contained in a range satisfying Ti *> 0.007, and the balance has a composition composed of Fe and inevitable impurities.
However, (Ti%), (N%), (S%), and (C%) indicate the contents (mass%) of Ti, N, S, and C, respectively.

  In the present specification, “%” indicating the component of steel is “% by mass”.

  According to the present invention, a cold-rolled steel sheet having excellent slidability can be obtained. And since the said characteristic is acquired without the addition of a manufacturing process or the hindrance of productivity, the cold-rolled steel plate of this invention becomes a suitable material as a panel member for motor vehicles.

It is a figure which shows the measurement and evaluation method of slidability.

  The cold-rolled steel sheet of the present invention has a {100} plane X-ray intensity in a direction parallel to the plate surface on the both surfaces of the steel sheet of 2.5 or more in a random intensity ratio, and a {100} plane X-ray diffraction on the surface The peak half-value width is 0.15 ° or more. This is the most important requirement in the present invention. In addition, the component composition at that time is% by mass, C: 0.0005 to 0.01%, Si: 0.2% or less, Mn: 0.3% or less, P: 0.03% or less, S: 0.003 to 0.03%, Ti: 0.01 to 0.1%, When Al: 0.01 to 0.10%, N: 0.005% or less and Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S%) − 4 × (C%) *> It is preferably contained in a range satisfying 0.007, and the balance is composed of Fe and inevitable impurities. However, (Ti%), (N%), (S%), and (C%) indicate the contents (mass%) of Ti, N, S, and C, respectively. Thus, the cold-rolled steel plate excellent in slidability can be obtained by prescribing | regulating the state of the steel plate surface.

  In the present invention, the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in terms of random intensity ratio, and the half width of the {100} plane X-ray diffraction peak on the surface is 0.15 ° or more. Is that only the recovery phenomenon has passed without undergoing α → γ transformation or recrystallization. Compared with recrystallized grains, unrecrystallized grains in the form of dislocation density and extended in the rolling direction are parallel to the plate surface. It shows that the state is accumulated on the {100} plane of the direction. Therefore, it is different from a structure in which recrystallized grains generated through α → γ → α transformation obtained by annealing performed at a temperature equal to or higher than the transformation point are accumulated.

Thus, in the present invention, in the accumulation of {100} planes in the direction parallel to the plate surface, there is a feature in the morphology and dislocation density of the crystal grains accumulated in the {100} plane orientation, and the recrystallized grains that are usually obtained The composition is different from that of a structure composed of {100} plane orientations formed through the γ → α transformation. Therefore, in the present invention, in order to clarify the difference between unrecrystallized grains and recrystallized grains, in addition to the random intensity ratio, the half-value width of the {100} plane X-ray diffraction peak at the surface is measured, and the random intensity ratio is measured. The structure of the cold-rolled steel sheet is shown using the half width.
If the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in terms of the random intensity ratio, the area ratio of the {100} plane in the direction parallel to the plate surface on the surface is sufficiently high. Furthermore, if the half-value width of the surface {100} plane X-ray diffraction peak is 0.15 ° or more, the dislocation density is sufficiently high, so that it has a high hardness distribution and improves slidability. Is possible.

  Hereinafter, the present invention will be described in detail.

Conventionally, it is known that the texture of an automobile panel outer plate has many {111} surfaces formed in a direction parallel to the plate surface. When the present inventors repeatedly conducted experiments under various production conditions, it was found that a steel sheet having many {111} faces but many {100} faces in the surface layer was obtained. It was. When the {100} plane accumulation is not the normal {100} plane obtained by annealing above the transformation point, but the {100} plane of unrecrystallized grains with a high dislocation density, the slidability Has also found that it is far superior.
The {100} plane of the non-recrystallized grains has a higher hardness than the {111} plane that is the recrystallized grains because it contains a lot of strain in the crystal plane. Therefore, a steel sheet having a non-recrystallized grain texture distribution on the surface layer has a high hardness distribution on the surface extreme surface layer, so that the sliding resistance in a state where a surface pressure is applied is reduced. And when such a characteristic press-molds the outer panel of a motor vehicle, when it slides with the die | dye part of a metal mold | die, deep drawing formability is improved by reducing a frictional resistance. On the other hand, since the hard layer on the surface is limited to the extreme surface layer portion, the mechanical properties (El, r value) as a steel plate are not affected, and the formability is not lowered.

Based on the above examination results, in the present invention, in order to obtain excellent slidability, the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in a random intensity ratio on both surfaces of the steel plate, In addition, the half width of the {100} plane X-ray diffraction peak on the surface is set to 0.15 ° or more.
The {100} plane X-ray intensity in the direction parallel to the plate surface can be measured by the inverse pole figure method. The half-value width of the {100} plane X-ray diffraction peak on the surface can be determined by measuring the {100} plane X-ray diffraction peak on the surface using Mo as the X-ray source and the θ-2θ method. it can. Detailed conditions of the measurement method will be described in the examples described later.
Moreover, the thickness from the outermost layer to the plate thickness center direction in the region where many non-recrystallized grains exist can be measured by observing a cross section in the rolling direction of the steel plate with an optical microscope. The form of the cross section in the rolling direction of the non-recrystallized grains can be easily distinguished because it has a smaller thickness than the recrystallized grains and extends in the rolling direction. In order to obtain the effect of improving the slidability, it is generally preferable that there are many {100} planes of non-recrystallized grains from the outermost layer to the region of 5 μm in the plate thickness center direction. More preferably, it is from the outermost layer to the region of 10 μm in the plate thickness center direction.
In addition, the {100} plane X-ray intensity in the direction parallel to the plate surface on the steel sheet surface as described above has a random intensity ratio of 2.5 or more and the half width of the {100} plane X-ray diffraction peak on the surface is 0.15 ° or more. A certain steel plate is obtained by controlling the manufacturing conditions in the hot rolling and annealing processes. Specifically, for example, the coiling temperature in the hot rolling is set to 630 ° C. or lower, the atmosphere in the annealing heating process, particularly the hydrogen concentration in the atmosphere (mixed gas of nitrogen and hydrogen) is 5 vol% or more, and the dew point is By setting the temperature to −40 ° C. or lower, the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in random intensity ratio, and the half width of the {100} plane X-ray diffraction peak on the surface is 0.15 More than ゜.

Furthermore, as a result of further investigation, in the annealing process of the steel sheet, Ti does not contribute to precipitation by forming a Ti compound with C, N, and S as Ti, that is, there is Ti dissolved in the steel. It has also been found that it is important to make a steel component as described above to accumulate a large amount of {100} plane orientation of unrecrystallized grains in the surface layer as described above.
Although the exact mechanism by which the presence of solute Ti accumulates many {100} orientations of unrecrystallized grains on the surface layer is not clear, during the annealing after cold rolling, Ti reacts with N present in the atmosphere. It is presumed that the nitride formed in the vicinity of the steel sheet surface layer influences recrystallization and inhibits the formation of the originally formed {111} plane.
From the above, as the component composition for making solute Ti present in this way, when Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S%) − 4 × (C%), It is preferable to contain in the range which satisfies Ti *> 0.007. However, (Ti%), (N%), (S%), and (C%) indicate the contents (mass%) of Ti, N, S, and C, respectively. Detailed description will be given later.

Next, the reasons for limiting the component elements will be described.
C: 0.0005-0.01%
C is a solid solution strengthening element, contributes to an increase in yield strength, and is advantageous for improving slidability. However, when it adds excessively, workability and aging deterioration may be caused. In addition, if C is contained in a large amount, the amount of Ti carbide in the steel increases, the amount of solute Ti in the steel decreases, and the formation of the {100} plane parallel to the plate surface in the surface layer is obstructed. May be. From the above, 0.01% or less is preferable. On the other hand, if it is less than 0.0005%, the crystal grain size becomes extremely coarse and the yield strength is greatly reduced, so that the slidability may be lowered. Moreover, the decarburization cost may increase. Therefore, 0.0005% or more and 0.01% or less is preferable.

Si: 0.2% or less
In addition to acting as a deoxidizer, Si is a useful element that strengthens steel by solid solution strengthening. On the other hand, it has the effect of suppressing the formation of carbides and the effect of promoting the formation of Ti carbides. Moreover, when it contains excessively, workability may be inhibited. Therefore, 0.2% or less is preferable.

Mn: 0.3% or less
In addition to acting as a deoxidizer, Mn strengthens steel by solid solution strengthening, increases yield strength, and is effective for slidability. However, the sulfide of Mn acts as a precipitation site for Ti precipitates, reducing the amount of solid solution Ti, and excessive addition may impair workability. Therefore, 0.3% or less is preferable.

P: 0.03% or less
P is a solid solution strengthening element and is effective for strengthening and yield strength of steel. It is also advantageous for slidability. On the other hand, it is an element that easily segregates at grain boundaries, causing hot and cold cracking, and secondary workability may be significantly impaired. Therefore, 0.03% or less is preferable.

S: 0.003-0.03%
S is present as an inevitable impurity in steel, but if it exceeds 0.03%, hot cracking is likely to occur during the production of the steel sheet, and inclusions may be formed in the steel to significantly reduce workability. Excessive addition also promotes the formation of Ti sulfide and leads to a decrease in solid solution Ti. Therefore, 0.03% or less is preferable. On the other hand, it is preferable that the amount of S is small, but if it is less than 0.003%, the desulfurization cost increases, so 0.003% or more is preferable.

Al: 0.01-0.10%
Al is an element added as a deoxidizer. Further, Al forms nitrides with N, but if the content is small, excess N may form Ti and nitrides and the amount of dissolved Ti may decrease. Therefore, the Al content is preferably 0.01% or more. However, even if added in a large amount, a further deoxidizing effect cannot be obtained. Therefore, 0.10% or less is preferable.

N: 0.005% or less
The smaller N, the better the workability, so the smaller N is desirable. Moreover, when it exceeds 0.005% and it adds excessively, it may lead to the remarkable fall of a moldability and the fall of solid solution Ti amount. Therefore, 0.005% or less is preferable.

Ti: 0.01-0.1%
Ti is one of the most important elements in the present invention. Ti has an effect of improving workability by fixing C, N, and S in steel as precipitates. Further, in the present invention, by adding Ti in excess of the amount necessary to form the precipitate, a nitride with N in the atmosphere is formed at the time of manufacturing, and the unrecrystallized grains of the surface layer { 100} Increase the plane orientation. If it is less than 0.01%, such an effect may not be obtained. On the other hand, when Ti is added in excess of 0.1%, not only a further effect cannot be expected, but the formation of an abnormal structure inside the plate is promoted, and the workability may be lowered. From the above, 0.01% or more and 0.1% or less is preferable.
Furthermore, as described above, Ti in steel forms precipitates with C, N, and S in steel. Therefore, Ti is added in excess of the equivalent to these C, N, and S components. In the present invention, it is important in the present invention to accumulate {100} planes of non-recrystallized grains on the surface layer by causing excessive solid solution Ti to exist. Therefore, it is preferable that the following relational expression is satisfied in addition to the above definition of 0.01% to 0.1%.
Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S%) − 4 × (C%), Ti *> 0.007
However, (Ti%), (N%), (S%), and (C%) indicate the contents (mass%) of Ti, N, S, and C, respectively.
When Ti * exceeds 0.007, nitrogen in the atmosphere that penetrates into the steel during annealing and solute Ti form very fine nitrides, preventing the movement of grain boundaries and suppressing recrystallization. As a result, non-recrystallized grains having a high hardness distribution are likely to remain, and the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in a random intensity ratio on the surface of both surfaces of the steel plate, and The half-value width of the {100} plane X-ray diffraction peak is likely to be 0.15 ° or more. Therefore, Ti *> 0.007 is preferable.

  The remainder other than the above consists of Fe and inevitable impurities. As an unavoidable impurity, for example, O forms non-metallic inclusions and adversely affects quality, so it is preferably reduced to 0.003% or less. In the present invention, Cu, Cr, Ni, W, V, Zr, Sn, and Sb may be contained in a range of 0.1% or less as trace elements that do not impair the effects of the present invention.

Next, the manufacturing method of the cold rolled steel sheet of this invention is demonstrated.
The cold-rolled steel sheet of the present invention is preferably obtained by roughly rolling a steel adjusted to the above chemical composition range, finish rolling at a desired finishing temperature, then cooling under desired cooling conditions, winding, pickling Thereafter, it is obtained by cold rolling and continuous annealing. Among them, on the surface of the steel sheet, which is a feature of the present invention, the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in random intensity ratio, and the half value of the {100} plane X-ray diffraction peak on the surface In order to make the width 0.15 ° or more, the winding temperature is preferably 630 ° C. or less. By setting the temperature to 630 ° C or less, the precipitate containing Ti becomes finer, and re-dissolves during subsequent annealing to increase the solid solution Ti. On both surfaces of the steel sheet, unrecrystallized in the direction parallel to the plate surface The {100} faces of the grains can be accumulated.
In addition, the atmosphere in the heating process during annealing is a mixed gas of hydrogen and nitrogen containing 5 vol% or more of hydrogen, and the dew point is −40 ° C. or less, so that the dislocation is more effectively applied to both surfaces of the steel sheet. It is possible to accumulate {100} planes of non-recrystallized grains in a direction parallel to the dense plate surface. The reason for this is not always clear, but the higher the hydrogen concentration and the lower the dew point, the more nitrogen can penetrate into the steel and form more fine nitrides with Ti in the steel. This is presumed to be because the recrystallization suppression effect is enhanced. More preferably, the hydrogen concentration is 8 vol% or more and the dew point is −45 ° C. or less.

Molten steel consisting of the components shown in Table 1 is vacuum degassed, converted into a slab by continuous casting, reheated to 1180 ° C, hot-rolled to a thickness of 3.5mm at a finishing temperature of 900 ° C, and water after hot rolling. After cooling, the steel sheet was wound around a coil at a winding temperature of 600 ° C.
Next, the steel sheet after winding was pickled, cold-rolled to a thickness of 0.65 mm, and annealed at 820 ° C. for 30 seconds in a continuous annealing line. For steel A, the hydrogen concentration is 1 vol% or 10 vol% in a mixed atmosphere of hydrogen and nitrogen, and the dew point is -20 ° C or -50 ° C. The dew point was −50 ° C. in a mixed atmosphere of hydrogen and nitrogen with a hydrogen concentration of 10 vol%. Subsequently, temper rolling with an elongation of 0.8% was performed.

  For the cold-rolled steel sheet obtained as described above, the {100} plane X-ray random strength ratio, half-value width, mechanical properties, frictional resistance by sliding test, limit drawing ratio (LDR) are as follows. Measurement and evaluation. The results obtained are shown in Table 2.

The {100} plane X-ray random intensity ratio and the {100} plane X-ray intensity in the direction parallel to the half width plate surface were measured by the inverse pole figure method. The {100} plane X-ray intensity on the surface is measured after the test piece is cleaned and dried, while the {100} plane X-ray intensity in the direction parallel to the plate surface at the center of the thickness is measured on one side of the test piece. Each was measured after being chemically polished with oxalic acid to expose the center of the plate thickness on the surface. White X-rays were used as the X-ray source, and a Ge semiconductor detector was used to detect {100} plane X-rays. At the same time, the {100} plane X-ray intensity (random intensity) of a random sample with no selective orientation and an irregular distribution of crystal orientation was measured. The random intensity ratio was calculated by the ratio of the {100} plane X-ray intensity of the actual test piece to the {100} plane X-ray intensity of the random sample.
The half-width of {100} plane X-ray peak in the direction parallel to the plate surface at the surface is measured by measuring the {100} plane X-ray diffraction peak at the surface using the θ-2θ method with Mo as the X-ray source. I asked.

Mechanical properties Formability was evaluated by tensile properties and r-value mechanical properties. Tensile properties were measured according to the test method described in JISZ 2241 after being processed into a No. 5 test piece described in JISZ 2201. The average r value was measured by a three-point method after applying a tensile pre-strain of 15%, and the average r value in the 90 ° direction, 45 ° direction, and 0 ° direction with respect to one direction of the steel sheet = It was determined as (r (0 °) + 2 × r (45 °) + r (90 °)) / 4. An average r value of 1.6 or higher was judged to be good.

The sliding property was evaluated by a plane sliding test. Tester tool both sides of the material 1 as shown in FIG. 1 2 (Material: SKD11) pulling the material in a state of pressing at a constant surface pressure 20 kgf / mm ^2, the friction coefficient μ from drawing force D at that time μ = D / (2 · P). An experiment was conducted by applying a commonly used lubricating oil (viscosity: 15 cST / 40 ° C.) on both surfaces of the steel plate surface. A coefficient of friction of less than 0.15 was judged to have good slidability.

Limit drawing ratio (LDR)
The limit drawing ratio (LDR) is a circular blank plate coated with lubricating oil (viscosity: 15cST / 40 ° C) that is usually used on the steel plate surface, with a wrinkle restraining force of 1tf, a punch diameter of 33mmφ, and a punch R of A cylindrical squeeze test of 4.5 mm and a die R of 2.5 mm was performed, and the ratio was determined by the ratio of the maximum blank diameter to punch diameter that could be squeezed out without breaking. An LDR of 2.4 or higher was judged good.

In the example of the present invention, many {100} planes parallel to the plate surface are accumulated on the surface layer, the sliding is small, and the drawability is excellent.
Further, as a result of observing the cross section in the rolling direction of the steel sheet with an optical microscope, it was confirmed that the thickness of the region where many non-recrystallized grains existed from the outermost layer to the thickness center direction was 5 μm or more.
On the other hand, in the comparative example, the {100} plane in the direction parallel to the plate surface cannot be sufficiently accumulated on the surface layer, so that the sliding resistance is large and the drawability is poor.

  Since the cold-rolled steel sheet of the present invention is excellent in slidability, it can be used in various applications such as automotive steel sheets, especially for pressed parts that require particularly drawability.

1 Material 2 Tool

Claims (2)

  On the surfaces of both surfaces of the steel plate, the {100} plane X-ray intensity in the direction parallel to the plate surface is 2.5 or more in random intensity ratio, and the half width of the {100} plane X-ray diffraction peak on the surface is 0.15 ° A cold-rolled steel sheet characterized by the above. In mass%, C: 0.0005 to 0.01%, Si: 0.2% or less, Mn: 0.3% or less, P: 0.03% or less, S: 0.003 to 0.03%, Ti: 0.01 to 0.1%, Al: 0.01 to 0.10%, N: Within 0.005% or less and Ti * = (Ti%) − 3.4 × (N%) − 1.5 × (S%) − 4 × (C%) The cold-rolled steel sheet according to claim 1, wherein the cold-rolled steel sheet has a component composition comprising Fe and inevitable impurities.
However, (Ti%), (N%), (S%), and (C%) indicate the contents (mass%) of Ti, N, S, and C, respectively.

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