JP2009235532A - High strength steel sheet having excellent deep drawability, and method for producing the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel sheet having excellent deep drawability, and to provide a method for producing the high strength steel sheet. <P>SOLUTION: The high strength steel sheet has a composition containing, by mass, 0.01 to 0.035% C, 0.8 to 2.0% Si, 1.0 to 3.0% Mn, 0.005 to 0.1% P, ≤0.01% S, 0.005 to 0.1% Al, ≤0.01% N, 0.01 to 0.3% Nb, ≤0.1% Ti and ≤0.3% (inclusive of 0%) V, and in which the contents (mass%) of Nb, Ti, V and C satisfy specified formula, and the balance iron with inevitable impurities, and in which TS is ≥500 MPa and both the r value in the rolling 45° direction and the average r value are ≥1.7. In its production method, a steel slab having the similar componential composition is successively subjected to: a hot rolling stage where it is subjected to hot rolling in which finish rolling is performed at a finish rolling outlet side temperature of ≥800°, the total rolling ratio of the finish rolling at ≤950°C is controlled to ≥50%, within 0.5 s after the rolling, cooling is performed at the average cooling rate of ≥20°C/s, a cooling stopping temperature is controlled to 600 to 750°C, a coiling temperature is controlled to 550 to 750°C, and by the coil cooling, a hot rolled sheet is obtained; a cold rolling stage where the hot rolled sheet is subjected to pickling and cold rolling at a rolling ratio of 50 to 85%, so as to obtain a cold rolled sheet; and a cold rolled sheet annealing stage where the cold rolled sheet is annealed at an annealing temperature of 800 to 950°C, so as to obtain a steel sheet having a TS of ≥500 MPa and in which both the r value in the rolling 45° direction and the average r value are ≥1.7. <P>COPYRIGHT: (C)2010,JPO&INPIT

Description

  The present invention is a high-strength steel sheet that is useful for use in automobile steel sheets and the like, has a high tensile strength (TS) of 500 MPa or more, and has a high r value (r value ≧ 1.7) and excellent deep drawability, and its production We are going to propose a method.

In recent years, in order to regulate CO ₂ emissions from the viewpoint of global environmental conservation, improvement in fuel efficiency of automobiles has been demanded. In addition, in order to ensure the safety of passengers in the event of a collision, it is also required to improve safety centered on the collision characteristics of the automobile body. As described above, the weight reduction of the automobile body and the reinforcement of the automobile body are being actively promoted.
In order to satisfy the weight reduction and strengthening of automobile bodies at the same time, it is said that weight reduction by increasing the strength of the component materials and reducing the plate thickness is effective within the range where there is no problem with rigidity. Tensile steel plates are actively used in automotive parts.

Since the weight reduction effect increases as the strength of the steel sheet used increases, the automotive industry tends to use a steel sheet having a tensile strength (TS) of over 440 MPa as a panel material for inner and outer plates, for example.

On the other hand, since many automotive parts made of steel plates are formed by press working, the steel plates for automobiles are required to have excellent press formability. However, high-strength steel sheets are significantly deteriorated in formability, especially deep drawability, compared to ordinary mild steel sheets. Therefore, TS ≥ 500 MPa and a depth of 440 MPa class or less are a challenge in reducing the weight of automobiles. There is an increasing demand for steel sheets having deep drawability equivalent to drawing steel sheets, and high-strength steel sheets with an average r value ≧ 1.7 at the Rankford value (hereinafter referred to as r value), which is an evaluation index of deep drawability, are required. ing.
As a means to increase the strength while having a high r value, based on steel made by adding IF (interstitial free) to the ultra-low carbon steel sheet, adding Ti and Nb in an amount to fix solute carbon and solute nitrogen, There is a method of adding a solid solution strengthening element such as Si, Mn, and P (for example, see Patent Document 1).
JP-A-56-139654

Patent Document 1 has a composition of C: 0.002 to 0.015%, Nb: C% × 3 to C% × 8 + 0.020%, Si: 1.2%, Mn: 0.04 to 0.8%, and P: 0.03 to 0.10%. This is a technology relating to a high-tensile cold-rolled steel sheet having excellent formability and having non-aging properties of a tensile strength of 35 to 45 kg / mm2 class (340 to 440 MPa class).
However, in such a technology using ultra-low carbon steel as a raw material, when trying to produce a steel sheet with a tensile strength of ≧ 440 MPa, the amount of added alloying elements increases, resulting in surface appearance problems, plating deterioration, It has been found that problems such as the manifestation of the next processing brittleness arise. Further, when a solid solution strengthening component is added in a large amount, the r value deteriorates, so that there is a problem that the level of the r value decreases as the strength is increased.

In addition, in order to reduce the C content to an extremely low carbon range, vacuum degassing must be performed in the steelmaking process, that is, this generates a large amount of CO ₂ in the manufacturing process, It is hard to say that it is optimal from the viewpoint.

In addition to the solid solution strengthening as described above, there is a structure strengthening method as a method for increasing the strength of a steel sheet. For example, there is a DP (Dual-Phase) steel plate which is a composite structure steel plate made of soft ferrite and hard martensite. DP steel is generally good in ductility, has an excellent strength-ductility balance (TS x El), and has a low yield ratio, that is, a low yield stress relative to the tensile strength. However, the r value is low and the deep drawability is inferior. In addition to the presence of martensite that does not contribute to the r value in terms of crystal orientation, solid solution C essential for martensite formation inhibits the formation of {111} recrystallized texture effective for increasing the r value. It is said to be from.
As an attempt to improve the r value of such a composite structure steel plate, for example, there is a technique of Patent Document 2 or Patent Document 3.
Japanese Patent Publication No.55-10650 JP-A-55-100934

  The technique of Patent Document 2 discloses a technique in which box annealing is performed at a temperature from the recrystallization temperature to the Ac3 transformation point after cold rolling, followed by heating to 700 to 800 ° C. to obtain a composite structure, followed by quenching and tempering. Has been. However, in this method, since the quenching and tempering is performed during the continuous annealing, the manufacturing cost becomes a problem. In the case of box annealing, it is inferior to continuous annealing in terms of processing time and efficiency.

  The technique of Patent Document 3 first performs box annealing after cold rolling in order to obtain a high r value, sets the temperature at this time to a two-phase region of ferrite (α) -austenite (γ), and then performs continuous annealing. Is. In this technique, Mn is concentrated from the α phase to the γ phase during soaking of the box annealing. This Mn-concentrated phase preferentially becomes a γ phase during the subsequent continuous annealing, and a mixed structure can be obtained even at a cooling rate as high as that of a gas jet. However, this method requires box annealing at a relatively high temperature for a long time to concentrate Mn, and therefore, many in the manufacturing process such as frequent adhesion between steel plates, generation of temper collar, and decrease in the life of the furnace inner cover. There is a problem.

Also, C: 0.003-0.03%, Si: 0.2-1%, Mn: 0.3-1.5%, Ti: 0.02-0.2% (however, (effective Ti) / (C + N) atomic concentration ratio is 0.4-0.8) A composite structure type high-tensile cold-rolled steel sheet excellent in deep drawability and shape freezeability, characterized by subjecting the steel to be contained to hot rolling, cold rolling, and continuous annealing that is heated to a predetermined temperature and then rapidly cooled. There is a manufacturing method technology (see Patent Document 4). In this technology, steel with a composition of 0.012% C-0.32% Si-0.53% Mn-0.03% P-0.051% Ti in mass% is cold-rolled and heated to 870 ° C, which is a two-phase region of α-γ. Subsequently, a technique is disclosed in which a composite-structure cold-rolled steel sheet having an r value = 1.61 and TS = 482 MPa can be manufactured by cooling at an average cooling rate of 100 ° C./s. However, in order to obtain a high cooling rate of 100 ° C / s, water quenching equipment is required, and water-quenched steel sheets have surface treatment problems, so there are problems with manufacturing equipment and materials. is there.
Japanese Patent Publication No. 1-35900

Furthermore, Patent Document 5 discloses a technique for improving the r value of a composite structure steel sheet by optimizing the V content in relation to the C content. This is because, before recrystallization annealing, C in the steel is precipitated with V-based carbides to reduce solute C as much as possible to achieve a high r value, and then V-based carbides are heated in the α-γ two-phase region. Is dissolved to concentrate C in γ, and martensite is generated in the subsequent cooling process. However, if the precipitation of VC at the hot rolling stage is insufficient, there is a problem that the r value after annealing cannot be increased.

Japanese Patent Laid-Open No. 2002-226941

Moreover, there exists a technique of patent document 6 as a technique of the advanced steel plate excellent in deep drawability, and its manufacturing method. This technique obtains a high-strength steel sheet containing a predetermined amount of C, having an average r value of 1.3 or more, and having a total of 3% or more of one or more of bainite, martensite, and austenite in the structure. As a manufacturing method, the rolling reduction ratio of cold rolling is set to 30 to 95%, and then the annealing is performed to increase the r value by developing the texture by forming Al and N clusters and precipitates. In addition, heat treatment is performed so as to have at least 3% of at least one of bainite, martensite, and austenite. In this method, after cold rolling, annealing for obtaining a good r value and heat treatment for forming a structure are required, respectively, and the annealing process requires holding for a long time of 1 hour or more. There is a problem that productivity is poor in terms of process (time). Furthermore, the second phase fraction of the obtained structure is relatively high, and in this case, it is difficult to stably secure an excellent strength-ductility balance.
JP 2003-64444 A

In order to increase the strength of a (soft) steel sheet having excellent deep drawability, the conventional method of increasing the strength only by solid solution strengthening requires the addition of a large amount or an excess of alloy components. In terms of cost, process, and improvement of the r value, there are problems. In addition, there is a limit to increasing the r value by complex organization.
It is an object of the present invention to advantageously solve such problems of the prior art and provide a high-strength steel sheet excellent in deep drawability and a method for manufacturing the same, which is not conventional.

The present invention has been intensively studied to solve the above-mentioned problems, and is a low carbon steel sheet to which Nb, Ti, and V are appropriately added, while controlling the amount of carbon that is not fixed to Nb, Ti, and V. Balance solid solution strengthening with fine grain strengthening and precipitation strengthening. Furthermore, by securing the rolling rate in the non-re-recrystallization region in the latter stage at the time of hot rolling, a high-strength steel sheet excellent in deep drawability with both the r value and the average r value in the rolling 45 ° direction being 1.7 or more is obtained. Succeeded.
The present invention employs the following means in order to solve the above problems.
(1) By mass%
C: 0.01-0.035%
Si: 0.8-2.0%
Mn: 1.0-3.0%
P: 0.005-0.1%
S: 0.01% or less
Al: 0.005% to 0.1%
N: 0.01% or less
Nb: 0.01-0.3%
Ti: 0.1% or less,
V: 0.3% or less (provided that V is 0),
And the content (mass%) of Nb, Ti, V and C is
0.0005% ≦ C− (12/93) Nb− (12/48) Ti * − (12 / 50.9) (1/10) V ≦ 0.005%
Excellent deep drawability characterized by satisfying Ti * = Ti-3.4N, balance of iron and inevitable impurities, TS500MPa or more, and r value in rolling 45 ° direction and average r value are both 1.7 or more High strength steel plate.
(2) The high-strength steel sheet having excellent deep drawability according to (1) above, further comprising at least one of Cr: 0.1 to 1.0% and Mo: 0.02 to 0.5%.
(3) The steel structure is a high-strength steel sheet excellent in deep drawability according to (1) or (2) above, wherein the area ratio of the ferrite phase is 90% or more.
(4) The high-strength steel sheet excellent in deep drawability according to any one of (1) to (3), wherein a plating layer is formed on the steel sheet surface.
(5) By mass%
C: 0.01-0.035%
Si: 0.8-2.0%
Mn: 1.0-3.0%
P: 0.005-0.1%
S: 0.01% or less
Al: 0.005% to 0.1%
N: 0.01% or less
Nb: 0.01-0.3%
Ti: 0.1% or less
V: 0.3% or less (provided that V is 0),
And the content (mass%) of Nb, Ti, V and C is
0.0005% ≦ C− (12/93) Nb− (12/48) Ti * − (12 / 50.9) (1/10) V ≦ 0.005%
Ti * = Ti−3.4N
The steel slab with the balance of iron and unavoidable impurities is subjected to finish rolling by hot rolling to a finish rolling exit temperature of 800 ° C or higher, and the total rolling ratio of finish rolling at 950 ° C or lower is 50% As described above, the steel sheet is cooled at an average cooling rate of 20 ° C./s or more within 0.5 s after rolling, the cooling stop temperature is 600 ° C. to 750 ° C., the coiling temperature is 550 ° C. or more and 750 ° C. or less, and the coil is cooled A hot rolling step for forming a sheet, a cold rolling step for subjecting the hot rolled sheet to pickling and cold rolling at a rolling rate of 50% to 85% to form a cold rolled sheet, and an annealing temperature for the cold rolled sheet : Deep drawability, characterized by producing cold rolled sheet annealing process, in which annealing is performed at 800 ° C or higher and 950 ° C or lower, and TS value is 500 MPa or higher and both the r value and the average r value in the rolling 45 ° direction are 1.7 or higher. It is a manufacturing method of a high-strength steel plate excellent in.
(6) The production of a high-strength steel sheet excellent in deep drawability according to (5), wherein the steel slab further contains at least one of Cr: 0.1 to 1.0% and Mo: 0.02 to 0.5%. Is the method.
(7) The method for producing a high-strength steel sheet having excellent deep drawability according to (5) or (6), wherein a hot dipping process is performed after the cold-rolled sheet annealing step.

  In the present invention, in the range of C content of 0.01 to 0.035 wt%, without reducing thorough solution C that adversely affects deep drawability like conventional ultra-low carbon IF steel, Despite the state in which solute C remains, the {111} recrystallized texture is developed by optimizing the amount of solute C and by optimizing the hot rolling conditions. The value and average r value achieved 1.7 or more.

  Conventionally, in order to increase the r value, that is, to develop the {111} recrystallization texture, in mild steel sheets, the solid solution C before cold rolling and recrystallization is reduced as much as possible, and the hot rolled sheet structure is made finer. It has been regarded as an effective means. On the other hand, in the DP steel as described above, the solute C necessary for martensite formation is required, so that the recrystallized texture of the parent phase does not develop and the r value is low. However, in the present invention, due to the high r-value effect due to the refinement effect of the hot-rolled sheet by securing the rolling rate in the latter stage of Nb and hot-rolling, there is a strong correlation with the average r-value even if moderate solute C exists. It was newly found that there is a good component range that develops {111} recrystallized texture and a texture near {211} <110> that develops the r value in the 45 ° direction of rolling. That is, the amount of C is reduced compared to the conventional DP steel sheet (low carbon steel level). However, in addition to the C content of 0.01 to 0.035%, which is higher than that of extremely low carbon steel, Combined with appropriate addition of Nb, Ti, V, and further ensuring the rolling rate at the later stage of hot rolling, TS500MPa was achieved with the balance of solid solution strengthening, grain refinement effect and precipitation strengthening. As for the r value, the rolling 45 ° direction and the average r value are both 1.7 or more. About strength, it is preferable to set it as TS530MPa or more, and it is more preferable to set it as TS540MPa or more. Ensuring solute C is not enough to positively develop a DP structure and strengthen the structure, but some may contain a second phase of martensite or bainite. And there is also an object to improve secondary work brittleness, which is a problem of IF-based high tensile strength that increases strength only by solid solution strengthening.

  As conventionally known, Nb has a recrystallization delay effect, so it is possible to refine the hot-rolled sheet structure by appropriately controlling the finishing temperature at the time of hot rolling. High carbide forming ability. In the present invention, in particular, in addition to refining the hot-rolled sheet structure by setting the hot-rolling finishing temperature in an appropriate range immediately above the transformation point, the total rolling ratio of finish rolling in the non-recrystallized region of γ of 950 ° C. or lower is 50 By making it at least%, the refinement of the hot-rolled sheet and the hot-rolled texture for developing both the 45 ° direction r-value and the average r-value after the cold rolling annealing are developed. In addition, when utilizing the solid solution strengthening element Si, the r value in the 45 ° direction of rolling after annealing decreases with the addition of Si only by securing solid solution C and adding Nb. It has been found that optimization of hot rolling conditions is essential.

The present invention is described in further detail below.
Hereinafter, unless otherwise specified, the element content is expressed in mass%.
First, the reason which limited the component composition of the steel plate of this invention is demonstrated.

C: 0.01-0.035%
C is an important element in the present invention together with Nb described later. C is effective for precipitation strengthening and fine grain hardening by NbC, and further improves secondary work brittleness resistance by dissolving in an annealed plate. In the present invention, it is necessary to contain 0.01% or more from the viewpoint of precipitation strengthening, fine grain hardening, and secondary work brittleness resistance. On the other hand, in order to obtain a good r value, the upper limit is made 0.035% considering that excessive addition is not preferable. More preferably, the C content is 0.03% or less.

Si: 0.8-2.0%
Si is an element that can effectively enhance the solid solution without decreasing the r value. In order to secure TS500Mpa, Si is preferably contained in an amount of 0.8% or more, more preferably 1.0% or more. On the other hand, if Si is contained in excess of 2.0%, a red scale is generated during hot rolling, which deteriorates the surface appearance of the steel sheet. In addition, when applying hot-dip zinc, the wettability of the plating is deteriorated to cause uneven plating and the plating quality is deteriorated. Therefore, the Si content is preferably 2.0% or less.

Mn: 1.0-3.0%
Mn is effective in increasing the strength by partially including the second phase during cooling after annealing, and lowering the transformation point in order to increase the r value in the rolling 45 ° direction, Has the effect of miniaturization. Mn is also an effective element for preventing hot cracking due to S. From this point of view, Mn needs to be contained at 1.0% or more. More preferably, the content is 1.2% or more. On the other hand, excessive addition degrades the r value and weldability, so 3.0% is made the upper limit.

P: 0.005-0.1%
P is an element that can be solid-solution strengthened with little reduction in r value. However, if it is less than 0.005%, not only the effect does not appear, but also the dephosphorization cost increases in the steelmaking process. Therefore, P is contained in an amount of 0.005% or more. More preferably, it contains 0.01% or more. On the other hand, excessive addition exceeding 0.1% causes P to segregate at the grain boundaries, resulting in deterioration of secondary work embrittlement resistance and weldability. Moreover, when it is set as the hot dip galvanized steel sheet, the diffusion of Fe from the steel sheet to the plated layer at the interface between the plated layer and the steel sheet is suppressed during the alloying process after the hot dip galvanizing, and the alloying processability is deteriorated. Therefore, an alloying treatment at a high temperature is required, and the obtained plating layer is not preferable because plating peeling such as powdering and chipping is likely to occur. Therefore, the upper limit of the P content is set to 0.1%.

S: 0.01% or less
S is an impurity and causes hot cracking. It also exists as an inclusion in steel and degrades various properties of the steel sheet. Therefore, it is preferable to reduce it as much as possible, but it is acceptable up to 0.01%, so 0.01% The following.

Al: 0.005% to 0.1% or less
In addition to being useful as a deoxidizing element for steel, Al has the effect of fixing solid solution N to improve normal temperature aging resistance, so it is contained in an amount of 0.005% or more. On the other hand, addition exceeding 0.1% leads to high alloy costs and further induces surface defects.

N: 0.01% or less
If N is too much, the room temperature aging resistance is deteriorated and a large amount of Al or Ti needs to be added. Therefore, it is preferable to reduce as much as possible, and the upper limit is set to 0.01%.

Nb: 0.01 to 0.3%, 0.0005% ≦ C− (12/93) Nb− (12/48) Ti * − (12 / 50.9) (1/10) V ≦ 0.005%
Nb is the most important element in the present invention, and has an effect of refining the hot-rolled sheet structure and precipitating and fixing C as NbC in the hot-rolled sheet, and contributes to increasing the r value. From this point of view, Nb is preferably contained in an amount of 0.01% or more. Excessive Nb addition softens the annealed plate, cleans the grain boundaries, lowers the grain boundary strength, and reveals secondary work brittleness. Therefore, the upper limit is set to 0.3%.
In addition, in order to achieve the above effect of Nb addition, the balance between the Nb content (% by mass) and the C content (% by mass) must be balanced
0.0005% ≦ C− (12/93) Nb− (12/48) Ti * − (12 / 50.9) (1/10) V ≦ 0.005%
It is necessary to satisfy
If the above formula of C, Nb, Ti, V is less than 0.0005%, it is not preferable because secondary processing embrittlement resistance and low strength are concerned, and if it exceeds 0.005%, it is difficult to secure an average r value of 1.7 or more. Become.

V: 0.3% or less (including 0)
V also has the same effect as Nb, has the effect of refining the hot-rolled sheet structure, precipitates and fixes C as a carbide in the hot-rolled sheet, and is an element that contributes to increasing the r value. Add V as appropriate. However, since the ability to fix carbon is inferior to Nb, Ti, etc., it is necessary to add in consideration of that amount. For this purpose, 10 is used.

Ti: 0.1% or less
Ti also has the same effect as Nb, has the effect of making the hot-rolled sheet structure finer, and precipitating and fixing C as a carbide in the hot-rolled sheet, and is an element that contributes to increasing the r value. However, since the effect of refinement of the hot-rolled sheet is large in Nb, it is preferable to add Ti to the Nb-added steel. From such a viewpoint, Ti is preferably contained in an amount of 0.005% or more. On the other hand, excessive Ti and addition are not preferable from the viewpoint of securing strength and resistance to secondary work brittleness, and the upper limit is set to 0.1%.
Also, in order to achieve the effects of Nb, Ti and addition, the relationship between Nb, Ti (Ti *), V content (mass%) and C content (mass%)
0.0005% ≦ C− (12/93) Nb− (12/48) Ti * − (12 / 50.9) (1/10) V ≦ 0.005%
It is necessary to satisfy Ti * in this case
Ti * = Ti−3.4N
Therefore, the effective Ti amount is calculated by subtracting the amount precipitated by very stable TiN.

Cr: 0.1% to 1.0%
Cr, like Mn, has the effect of slowing the critical cooling rate at which martensite is obtained, is an element that promotes martensite formation during cooling after annealing, and is effective in improving the strength level. In order to acquire these effects, it is preferable to contain Cr 0.1% or more. However, excessive addition of Cr not only saturates these effects more than necessary, but also causes high alloy costs, so the upper limit is made 1.0%.

Mo: 0.02% to 0.5%
Mo, like Mn, has the effect of slowing the critical cooling rate at which martensite is obtained, and is an element that promotes martensite formation during cooling after annealing, and is effective in increasing the strength level. Further, Mo in ferrite is an element that can be solid-solution strengthened with little decrease in r value. In order to obtain these effects, the Mo content is preferably 0.02% or more. However, excessive addition of Mo not only saturates these effects more than necessary, but also causes high alloy costs, so the upper limit is made 0.5%.

The above is the basic component of the present invention.
In the present invention, it is preferable that the balance other than the above-described components is substantially composed of iron and inevitable impurities.

It should be noted that there is no problem even if B, Ca, REM, etc. are contained within the normal steel composition range.
For example, B is an element having an effect of improving the hardenability of steel and can be contained as required. However, if the content exceeds 0.003%, the effect is saturated, so 0.003% or less is preferable.

  Further, Ca and REM have an action of controlling the form of sulfide inclusions, thereby preventing deterioration of various properties of the steel sheet. Such an effect is saturated when the content of one or two selected from Ca and REM exceeds 0.01% in total, and is therefore preferably made less than this.

  Other inevitable impurities include, for example, Sb, Sn, Zn, Co, etc. The allowable ranges of these contents are Sb: 0.01% or less, Sn: 0.1% or less, Zn: 0.01% or less , Co: 0.1% or less.

  Regarding the steel structure of the steel sheet of the present invention, the main phase is a ferrite phase, and in addition to the ferrite phase, a structure including a martensite phase, pearlite, bainite, or residual γ phase may be used. However, since the presence of the second phase causes a decrease in the r value, the area ratio of the ferrite phase is preferably 90% or more.

The rolling 45 ° direction and the average r value are 1.7 or more The steel sheet of the present invention satisfies the above component composition and structure, and the rolling 45 ° direction and the average r value is 1.7 or more.
In the present invention, the r value (r ₄₅ ) in the direction of rolling 45 ° and the average r value of 1.7 or more could be achieved for the first time by adjusting to the above component composition and optimizing the hot rolling conditions described later.
Here, the average r value means an average plastic strain ratio determined by JIS Z 2254, and is a value determined below.
Average r value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4
r ₀ = Plastic strain ratio measured by taking a test piece parallel to the rolling direction of the plate surface r ₄₅ = Plastic strain ratio measured by taking a test piece at 45 ° to the rolling direction of the plate surface r ₉₀ = Test Plastic strain ratio obtained by measuring a piece at 90 ° to the rolling direction of the plate surface

  The steel sheet of the present invention includes so-called plated steel sheets subjected to surface treatment such as electroplating or hot dip galvanizing. In addition to pure zinc, plating is conventionally applied to the surface of steel sheets, such as zinc alloy plating with zinc as the main component and addition of alloy elements, or Al alloy plating with addition of alloy elements as the main component of Al and Al. Including a plating layer.

Next, the preferable manufacturing method of this invention steel plate is demonstrated.
Since the composition of the slab used in the production method of the present invention is the same as that of the steel plate described above, the reason for limiting the steel slab is omitted.

  The steel sheet of the present invention is made of a slab having a composition within the above-mentioned range, and is subjected to hot rolling, finish rolling at a finish rolling exit temperature of 800 ° C. or higher, and finished at 950 ° C. or lower. The total rolling rate of rolling is set to 50% or more, cooling is performed at an average cooling rate of 20 ° C / s or more within 0.5s after rolling, the cooling stop temperature is set to 600 ° C to 750 ° C, and the winding temperature is set to 550 ° C or more and 750 ° C. A hot rolling step to be a coil-cooled hot rolled sheet, and a cold rolling step to cold-roll the hot-rolled sheet by pickling and cold rolling at a rolling rate of 50% to 85%, The cold-rolled sheet can be manufactured by sequentially performing a cold-rolled sheet annealing step of annealing at an annealing temperature of 800 ° C. or higher and 950 ° C. or lower. It is preferable to cool at an average cooling rate in the temperature range from the annealing temperature to 500 ° C .: 5 ° C./s or more.

  In the present invention, the steel slab is first subjected to finish rolling by hot rolling so that the finish rolling temperature at 800 ° C. or higher and 950 ° C. or lower is 50% or higher.

The steel slab used in the production method of the present invention is preferably produced by a continuous casting method in order to prevent macro segregation of components, but may be produced by an ingot-making method or a thin slab casting method. In addition to the conventional method in which the steel slab is manufactured and then cooled to room temperature and then heated again, direct feed rolling in which the steel slab is charged without being cooled and charged in a heating furnace and hot-rolled, or a little heat retention Energy-saving processes such as direct feed rolling and direct rolling, which are hot-rolled immediately after carrying out, can be applied without any problem.
The slab heating temperature is preferably low because the precipitates are coarsened to develop a {111} recrystallized texture and improve deep drawability. However, if the heating temperature is less than 1000 ° C, the rolling load increases and the risk of trouble during hot rolling increases, so the slab heating temperature is preferably 1000 ° C or higher. Note that the upper limit of the slab heating temperature is preferably set to 1300 ° C. because of an increase in scale loss accompanying an increase in oxidized weight.

The steel slab heated under the above conditions is subjected to hot rolling for rough rolling and finish rolling. Here, the steel slab is made into a sheet bar by rough rolling. The conditions for rough rolling need not be specified, and may be performed according to a conventional method. It goes without saying that using a so-called sheet bar heater for heating the sheet bar is an effective method from the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling.
Next, the sheet bar is finish-rolled to obtain a hot-rolled sheet. The finish rolling delivery temperature (FT) is 800 ° C or higher, and the total rolling ratio of finish rolling at 950 ° C or lower is 50% or more. This is to obtain a fine hot-rolled sheet structure and hot-rolled texture capable of obtaining excellent deep drawability after cold rolling and recrystallization annealing. It is important to increase the rolling rate in the non-recrystallized region at 950 ° C. or less, and if it is less than 50%, a sufficient effect cannot be obtained. In particular, the r value in the rolling 45 ° direction cannot be increased, and as a result, the average r value is also low.

Moreover, in order to reduce the rolling load at the time of hot rolling, it is good also as lubrication rolling between some or all passes of finishing rolling. Lubricating rolling is also effective from the viewpoint of uniform steel plate shape and uniform material. The coefficient of friction during lubrication rolling is preferably in the range of 0.10 to 0.25. Furthermore, it is also preferable to use a continuous rolling process in which the adjacent sheet bars are joined and finish-rolled continuously. It is desirable to apply the continuous rolling process from the viewpoint of the operational stability of hot rolling.

  As for cooling before winding the coil, it is necessary to start cooling within 0.5 s after rolling and then cool at an average cooling rate of 20 ° C./s or more to make the cooling stop temperature 600 ° C. to 750 ° C. This is an important process for refinement of hot-rolled sheet and adjustment of Nb-based precipitate size. If it exceeds 0.5 s, austenite begins to recrystallize, and the ferrite structure after transformation becomes coarse, which is not preferable for increasing the r value. On the other hand, an average cooling rate of less than 20 ° C./s is not preferable because the ferrite transformation start temperature becomes high, the ferrite grains become coarse and the r value decreases. Further, if the cooling stop temperature is less than 600 ° C., the coiling temperature becomes low and the precipitation of NbC becomes insufficient, which is not preferable, and if it exceeds 750 ° C., the ferrite grain size becomes coarse, resulting in a decrease in strength and cold rolling. The r value after annealing will decrease. The coil winding temperature (CT) is 550 ° C or higher and 750 ° C or lower. This temperature range is a suitable temperature range for precipitating NbC in the hot-rolled sheet, and especially when the CT exceeds the upper limit, the crystal grains become coarse and cause a decrease in strength and increase the r value after cold-rolling annealing. Will interfere.

  By adjusting the component composition and hot rolling conditions as described above, the average grain size including the small-angle grain boundaries can be reduced to 8 μm or less in the structure of the hot rolled sheet, which is advantageous for increasing the r value. .

  Subsequently, the hot-rolled sheet is subjected to pickling and cold rolling to perform a cold rolling process to obtain a cold-rolled sheet. Pickling may be performed under normal conditions. The cold rolling conditions are not particularly limited as long as the cold rolled sheet having a desired size and shape can be obtained, but the rolling reduction during cold rolling is preferably at least 50% or more. A high cold rolling reduction ratio is generally effective for increasing the r value. If the reduction ratio is less than 50%, the {111} recrystallization texture does not develop, and it becomes difficult to obtain excellent deep drawability. On the other hand, in the present invention, the r value increases as the cold rolling reduction is increased in the range of up to 85%. However, if it exceeds 85%, not only the effect is saturated but also the load on the roll during rolling increases. The upper limit is preferably 85%.

Next, the said cold-rolled sheet is subjected to an annealing step of annealing at an annealing temperature of 800 ° C. or higher and 950 ° C. or lower.
In the annealing process of the present invention, since the annealing needs to promote recrystallization before transformation, annealing at 800 ° C. or higher is the minimum required. On the other hand, a high temperature exceeding 950 ° C. is not preferable because it becomes a γ single phase region and the texture at the time of reverse transformation to ferrite becomes weak.

Moreover, after the cold-rolled sheet annealing step, a plating process such as an electroplating process or a hot dipping process may be performed to form a plating layer on the steel sheet surface.
For example, when performing hot dip galvanizing treatment often used for automotive steel plates as plating treatment, the annealing is performed in a continuous hot dip plating line, immersed in a hot dip galvanizing bath following cooling after annealing, A hot-dip galvanized layer may be formed on the surface, or an alloying treatment may be further performed to produce an alloyed hot-dip galvanized steel sheet.
Further, the cooling after the annealing may be performed in an annealing line, and after cooling to room temperature, hot dip galvanizing may be performed in a hot dip galvanizing line, or further alloying treatment may be performed.
Here, the plating layer is not limited to pure zinc and zinc-based alloy plating, but can of course be various plating layers conventionally applied to the steel sheet surface, such as Al or Al-based alloy plating.

  Further, the cold-rolled annealed plate and the plated steel plate may be subjected to temper rolling or leveler processing for the purpose of adjusting the shape correction, surface roughness, and the like. The total elongation of temper rolling or leveler processing is preferably in the range of 0.2 to 15%. If it is less than 0.2%, the intended purpose of shape correction and roughness adjustment cannot be achieved. On the other hand, if it exceeds 15%, the ductility is significantly reduced. In addition, although the processing form differs between temper rolling and leveler processing, it has been confirmed that there is no significant difference between the two. Temper rolling and leveler processing are effective even after plating.

Next, examples of the present invention will be described.
Molten steel having the composition shown in Table 1 was melted in a converter and made into a slab by a continuous casting method. These steel slabs were heated to 1250 ° C. and roughly rolled into sheet bars, and then hot-rolled sheets were formed by a hot rolling process in which finish rolling under the conditions shown in Table 2 was performed. These hot-rolled sheets were made into cold-rolled sheets by pickling and a cold rolling process with a rolling reduction of 70 and 80%. Subsequently, these cold-rolled sheets were subjected to continuous annealing in the continuous annealing line under the conditions shown in Table 2. Further, the obtained cold-rolled annealed sheet was subjected to temper rolling with an elongation of 0.5%.
In addition, the No. 7 steel sheet is subjected to a cold-rolled sheet annealing process in a continuous hot-dip galvanizing line, and subsequently hot-dip galvanized (plating bath temperature: 480 ° C) in-line to obtain a hot-dip galvanized steel sheet. Various characteristics were evaluated.

The resulting cold-rolled annealed plate was examined for microstructure, tensile properties, and r-value measurement. The survey method is as follows.

(1) Tensile properties JIS No. 5 tensile test specimens were sampled from each of the obtained cold-rolled annealed plates in the 90 ° direction (C direction) with respect to the rolling direction, and the crosshead speed was 10 mm / in accordance with the provisions of JIS Z 2241. A tensile test was performed at min, and tensile strength (TS) and elongation (El) were obtained.

(2) r value measurement Rolling direction (L direction) of each obtained cold rolled annealed sheet, 45 ° direction (D direction) with respect to rolling direction, 90 ° direction (C direction) with respect to rolling direction, JIS No. 5 tensile test Pieces were collected. Obtain the width strain and thickness strain of each specimen when 10% uniaxial tensile strain is applied to these specimens, and obtain the average r value (average plastic strain ratio) in accordance with the provisions of JIS Z 2254. This is the r value.

(3) Investigation of fine structure After corroding with nital, the structure was photographed 1000 times or 3000 times with SEM, and the area ratio of each phase was evaluated by the point counting method.

As is clear from Table 1 and Table 2, it contains C, Si, Mn, P, S, Al, N, Nb, Ti, V, (Cr, Mo) in a predetermined amount, 0.0005% ≦ C * ≦ 0.005% , C * = C− (12/93) Nb− (12/48) Ti * − (12 / 50.9) (1/10) V, symbols 1, 5, 6, satisfying Ti * = Ti−3.4N Examples of the present invention (Nos. 1-7, 9-12, No.15, No.16, No.20, No. 1) manufactured using steels of 9, 11, 14-16 (Table 1) under predetermined conditions. Steel sheets of No. 21, No. 27, No. 29 and No. 32 to 34 (Table 2) are all rolled with TS500MPa or higher (excluding No.15, No.20, No.21 and No32 steel plates and TS540MPa or higher). It has a high r value of 45 ° direction and an average r value of 1.7 or more, and has high ductility. In the steel sheet of the present invention, the area ratio of the ferrite phase was 90% or more. On the other hand, in the steel plate of the comparative example manufactured on the conditions which remove | deviate from the scope of the present invention, the strength is insufficient or the r value is lowered. That is, in steel plates (No. 17-19, No. 22-26, No. 28) using steels of symbols 2-4, 7, 8, 10 with C * exceeding 0.005%, r in the rolling 45 ° direction Although the value is 1.7 or more, the average r value cannot achieve 1.7 or more. For steel plates (No. 30, No. 31) using steels of symbols 12 and 13 with a Si content of less than 0.8% and C * greatly exceeding 0.005%, the r value and the average r value in the rolling 45 ° direction Both are less than 1.7. Even when steel of symbol 6 containing a predetermined amount of chemical components is used, a steel plate manufactured under conditions that do not satisfy the predetermined manufacturing conditions, for example, a steel plate manufactured under conditions where the cooling stop temperature exceeds 750 ° C. (No. 8 ) In steel plates (No. 13, No. 14) manufactured under conditions where the total rolling ratio of finish rolling at 950 ° C. or less is less than 50%, both the r value and the average r value in the 45 ° direction of rolling are less than 1.7. It is.
As described above, in the present invention, by limiting the steel component composition and production conditions, TS500 MPa or higher, rolling at 45 ° direction, and an average r value of 1.7 or higher can be achieved.

  According to the present invention, a high-strength steel sheet having a high r value of TS500 MPa or more can be produced at low cost and stably, and a remarkable industrial effect can be achieved. For example, when the high-strength steel sheet of the present invention is applied to automobile parts, it is possible to increase the strength of parts that have been difficult to press-form so far, and it is possible to sufficiently contribute to collision safety and weight reduction of the automobile body. . Moreover, it is applicable not only to automobile parts but also to household appliance parts and pipe materials.

Claims (7)

In mass%
C: 0.01-0.035%
Si: 0.8-2.0%
Mn: 1.0-3.0%
P: 0.005-0.1%
S: 0.01% or less
Al: 0.005% to 0.1%
N: 0.01% or less
Nb: 0.01-0.3%
Ti: 0.1% or less
V: 0.3% or less (provided that V is 0),
And the content (mass%) of Nb, Ti, V and C is
0.0005% ≦ C− (12/93) Nb− (12/48) Ti * − (12 / 50.9) (1/10) V ≦ 0.005%
Excellent deep drawability characterized by satisfying Ti * = Ti-3.4N, balance of iron and inevitable impurities, TS500MPa or more, and r value in rolling 45 ° direction and average r value are both 1.7 or more High strength steel plate.   Furthermore, Cr: 0.1-1.0%, Mo: 0.02-0.5% is contained 1 or more types, The high strength steel plate excellent in the deep drawability of Claim 1 characterized by the above-mentioned.   The high-strength steel sheet excellent in deep drawability according to claim 1 or 2, wherein the steel structure has a ferrite phase area ratio of 90% or more.   The high strength steel plate excellent in deep drawability as described in any one of Claims 1-3 in which the plating layer is formed in the steel plate surface. In mass%
C: 0.01-0.035%
Si: 0.8-2.0%
Mn: 1.0-3.0%
P: 0.005-0.1%
S: 0.01% or less
Al: 0.005% to 0.1%
N: 0.01% or less
Nb: 0.01-0.3%
Ti: 0.1% or less
V: 0.3% or less (provided that V is 0),
And the content (mass%) of Nb, Ti, V and C is
0.0005% ≦ C− (12/93) Nb− (12/48) Ti * − (12 / 50.9) (1/10) V ≦ 0.005%
Ti * = Ti−3.4N
The steel slab, which consists of iron and inevitable impurities in the balance, is subjected to finish rolling at a finish rolling temperature of 800 ° C or higher by hot rolling, and the total rolling ratio of finish rolling at 950 ° C or lower is 50%. As described above, the steel sheet is cooled at an average cooling rate of 20 ° C./s or more within 0.5 s after rolling, the cooling stop temperature is set to 600 ° C. to 750 ° C., the coiling temperature is set to 550 ° C. or more and 750 ° C. or less, and the coil is cooled. A hot rolling step for forming a sheet, a cold rolling step for subjecting the hot rolled sheet to pickling and cold rolling at a rolling rate of 50% to 85% to form a cold rolled sheet, and an annealing temperature for the cold rolled sheet : Deep drawability characterized by producing cold rolled sheet annealing process that anneals at 800 ° C or more and 950 ° C or less in order to produce steel plate with TS value of 500MPa or more and both rolling r value and average r value of 1.7 or more For producing high-strength steel sheets with excellent resistance.   The method for producing a high-strength steel sheet with excellent deep drawability according to claim 5, wherein the steel slab further contains at least one of Cr: 0.1 to 1.0% and Mo: 0.02 to 0.5%.   The method for producing a high-strength steel sheet excellent in deep drawability according to claim 5 or 6, wherein a hot dipping process is performed after the cold-rolled sheet annealing step.