JP2008174825A - High strength steel sheet, and method for manufacturing high strength plated steel sheet - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a high strength steel sheet which is excellent in deep drawability and useful for a steel sheet for automobile or the like, the tensile strength (TS) of which is high strength of ≥540 MPa, which has a high r value of ≥1.2 and the in-plane anisotropy of r value of which is small to be ¾Δγ¾≤0.20. <P>SOLUTION: A steel slab, which is prepared specially so that C and Nb satisfy 0.035-0.05% C, 0.05-0.12% Nb and (Nb/93)/(C/12)=0.15-0.45, is heated at the temperature of ≥1,000 to ≤1,200°C and at the temperature satisfying SRT (slab heating temperature)≤25,000[%C][%Nb]+1,050, is subjected to a finish-rolling at ≥800°C (finish rolling outlet side temperature), is coiled at 600-720°C and successively, is pickled and cold-rolled, is heated at the average temperature-raising rate of ≥15°C/s up to 700°C, is heated and annealed at the average temperature-raising rate of 0.1-2°C/s from 700°C up to the annealing temperature of ≥800 to ≤950°C, and the is cooled to 500°C at the average cooling rate of ≥5°C/s. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

INDUSTRIAL APPLICABILITY The present invention is useful for use in automobile steel sheets and the like, has a high strength with a tensile strength (TS) of 540 MPa or more, a high r value (r value ≧ 1.2), and an in-plane anisotropic of r value. The present invention relates to a method for producing a high-strength steel sheet excellent in deep drawability, having a small property (Δr) of 0.20 or less in absolute value.
Moreover, this invention relates to the manufacturing method of the high strength plated steel plate excellent in the deep drawability which gave plating with respect to said high strength steel plate.

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, improvement in safety centered on the collision characteristics of the automobile body is also required. 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 it is effective to reduce the thickness by increasing the strength of the component materials and reducing the plate thickness within the range where there is no problem with rigidity. High-strength steel plates (also called high-tensile steel plates) are actively used in automobile parts.
This lightening effect increases as the strength of the steel sheet used increases, so the automotive industry, for example, is a trend to use steel sheets with a tensile strength (TS) of 440 MPa or higher as a panel material for inner and outer panels. It is in.

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, a high-strength steel sheet is greatly deteriorated in formability, particularly deep drawability, as compared with a normal mild steel sheet. Therefore, TS ≧ 440 MPa, more preferably TS ≧ 540, is an issue in reducing the weight of an automobile. There is a growing demand for steel plates having MPa and also good deep drawability, and rank rford value (hereinafter referred to as r value), which is an evaluation index of deep drawability, has an average r value ≧ 1.2. High strength steel sheets are required.
Further, depending on the applied part, since the in-plane anisotropy is small and contributes to the improvement of the formability even with the same average r value, it is also required to reduce the in-plane anisotropy.

As a means to increase strength while maintaining a high r value, use ultra-low carbon steel, add Ti and Nb in amounts to fix carbon and nitrogen dissolved in the steel, IF (Interstitial atom free) There is a technique of adding a solid solution strengthening element such as Si, Mn, P or the like to the base of the converted steel (for example, Patent Document 1).
JP-A-56-139654

This patent document 1 has a composition of C: 0.002 to 0.015%, Nb: C% x 3 to C% x 8 + 0.020%, Si: 1.2% or less, Mn: 0.04 to 0.8%, 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 35 to 45 kgf / mm ² class (340 to 440 MPa class).

However, with such ultra-low carbon steel technology, if an attempt is made to produce a steel sheet with a tensile strength of 440 MPa or higher, the amount of alloying elements added will increase, resulting in surface appearance problems and poor plating properties. It has been found that problems such as the manifestation of secondary 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 increases.
In addition, in order to reduce the amount of C to an extremely low carbon range, it is necessary to perform vacuum degassing in the steelmaking process, but this will generate a large amount of CO ₂ in the manufacturing process, which contributes to global environmental conservation. It is hard to say that it is optimal from the viewpoint.

  As a method for increasing the strength of a steel sheet, there is a structure strengthening method other than the solid solution strengthening method described above. For example, there is a DP (Dual-Phase) steel sheet, which is a composite structure steel sheet composed of a soft ferrite phase and a hard martensite phase. This DP steel sheet is generally good in ductility, has an excellent strength-ductility balance (TS x El), and has a low yield ratio, that is, yield stress is low for tensile strength. The shape freezing property during press molding is excellent, but the r value is low and the deep drawability is poor. This is because, in addition to the presence of martensite that does not contribute to the r value in terms of crystal orientation, solid solution C, which is indispensable for martensite formation, inhibits the formation of {111} recrystallized texture effective for increasing the r value. It is said to do.

As an attempt to improve the r value of such a composite structure steel plate, for example, there is a technique disclosed in Patent Document 2 or Patent Document 3.
Japanese Patent Publication No.55-10650 JP-A-55-100934

The technique of Patent Document 2 is a method in which after cold rolling, box annealing is performed at a temperature of the recrystallization temperature to the Ac ₃ transformation point, and after that, heating to 700 to 800 ° C. is performed to obtain a composite structure, followed by quenching and tempering. . However, in this method, since the quenching and tempering is performed during the continuous annealing, the manufacturing cost becomes a problem. Further, box annealing is inferior in terms of processing time and efficiency as compared to continuous annealing.

  In the technique of Patent Document 3, in order to obtain a high r value, first, box annealing is performed after cold rolling, the temperature at this time is set to a two-phase region of ferrite (α) -austenite (γ), and then continuous annealing is performed. Is what you do. In this technique, Mn is concentrated from α phase to γ phase during soaking of 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 of the order of a gas jet. However, this method requires a long annealing at a relatively high temperature for a long period of time to concentrate Mn. Therefore, in the manufacturing process, such as frequent adhesion between steel plates, generation of temper collar, and reduction in the life of the furnace inner cover. There are many problems.

Patent Document 4 discloses that C: 0.003 to 0.03%, Si: 0.2 to 1%, Mn: 0.3 to 1.5%, Ti: 0.02 to 0.2% (however, (effective Ti) / (C + N) atomic concentration ratio) 0.4 to 0.8), a hot rolled, cold rolled, heated to a predetermined temperature, and then subjected to a rapid annealing and a composite structure excellent in deep drawability and shape freezeability A method for manufacturing a type high-tensile cold-rolled steel sheet is disclosed.
Japanese Patent Publication No. 1-35900

  In this Patent Document 4, a steel having a composition of 0.012% C-0.32% Si-0.53% Mn-0.03% P-0.051% Ti in cold mass, (α-γ) two-phase region after cold rolling. It is disclosed that a composite cold-rolled steel sheet having an r value = 1.61 and TS = 482 MPa can be manufactured by heating to 870 ° C. and cooling at an average cooling rate of 100 ° C./s. However, in order to obtain a high cooling rate of 100 ° C./s, a water quenching facility is required, and the problem of surface treatment properties of the steel plate that has been subjected to water quenching becomes obvious, and thus there remains a problem in terms of manufacturing facilities and materials.

Moreover, there exists a technique disclosed in Patent Document 5 as a high-strength steel sheet excellent in deep drawability and its manufacturing technique. 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. The manufacturing method includes a cold rolling reduction ratio of 30 to 95%, followed by annealing to increase the r value by developing the texture by forming Al and N clusters and precipitates, and subsequently the structure. A 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, and in the annealing process, the holding time is 1 hour or more. There is a problem that productivity is poor in terms of process (time). Furthermore, since the second phase fraction of the obtained structure is relatively high, there is a problem that it is difficult to stably secure an excellent strength-ductility balance. Regarding the in-plane anisotropy of the r value, in claim 3, r _{L is} 1.1 or more, r _{D is} 0.9 or more, and r _{C is} 1.2 or more. It is not a technology to control.
JP 2003-64444 A

Furthermore, as a composite structure steel plate excellent in deep drawability, in Patent Document 6, C: 0.010 to 0.050%, Si: 1.0% or less, Mn: 1.0 to 3.0%, P: 0.005 to 0.10%, S: 0.01% or less , Al: 0.005 to 0.5%, N: 0.01% or less, Nb: 0.01 to 0.3%, and the ratio of Nb to C in the steel is (Nb / 93) / (C / 12) = 0.2 to 0.7 It has a steel composition that satisfies the relationship, has a steel structure containing a ferrite phase at an area ratio of 50% or more and martensite at an area ratio of 1% or more, and an average r value is 1.2 or more. .
This technique is a technique for providing a high-strength deep drawing steel sheet without requiring special manufacturing conditions as in Patent Document 5. However, according to studies by the present inventors, in this technique, when the Nb content and the carbon content are high, although the average r value is good, the in-plane anisotropy of the r value tends to increase. In-plane anisotropy of the r value in the high C component region is a problem.
JP 2005-120467 A

In order to increase the strength of a (soft) steel sheet having excellent deep drawability, the conventional method of increasing strength 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.
Moreover, in the method using the structure strengthening, there is a problem in the manufacturing process because heating for a long time is required by twice annealing (heating) or a high-speed cooling facility is required.
Furthermore, there are few reports on the method of controlling the in-plane anisotropy of the r value of high-strength steel sheets having a high r value of 540 MPa or more, and the application is limited because the anisotropy is large even if the r value is high. There was a problem of being.

An object of the present invention is to propose a stable manufacturing method of a high-strength steel sheet having favorable deep drawability and small r-plane in-plane anisotropy, which advantageously solves the problems of the prior art. To do.
In addition, the present invention proposes a method for producing a high-strength plated steel sheet that is excellent in deep drawability by applying plating to the high-strength steel sheet obtained by the above-described production method and that has a small r-plane in-plane anisotropy. For the purpose.

Now, the inventors have intensively studied to solve the above-mentioned problems. As a result, the slab heating temperature is 1200 ° C. or less and no steel is used without using special or excessive alloy components or special equipment. It was found that by adjusting according to the amount of C and the amount of Nb, the technique as in Patent Document 6 effectively contributes to improvement of in-plane anisotropy in a region where the amount of C is high. In addition, by making the coiling temperature higher than 600 ° C., and in the annealing process, by controlling the heating rate particularly gradually in the vicinity of the annealing temperature, the average r value is 1.2 or more and the deep drawability is excellent. The in-plane anisotropy of the r value was small, and the knowledge that a high-strength steel sheet made of a steel structure containing ferrite and martensite was obtained was obtained.
The present invention is based on the above findings.

That is, the gist configuration of the present invention is as follows.
1. % By mass
C: 0.035 to 0.05%,
Si: 0.01 to 1.0%
Mn: 1.5-3.0%
P: 0.005-0.1%
Al: 0.005% to 0.1%
S: 0.01% or less,
N: 0.01% or less and
Nb: 0.05-0.12%
And the content of Nb and C in the steel is
(Nb / 93) / (C / 12) = 0.15 to 0.45 (where Nb and C are the contents of each element (mass%))
A steel slab containing the composition of Fe and inevitable impurities in the balance satisfying the following relationship: a slab heating temperature (SRT) of 1000 ° C. or higher and 1200 ° C. or lower, and an amount of C: [% C] and Nb Quantity: Depending on [% Nb] (mass%)
SRT ≦ 25000 [% C] [% Nb] +1050
Is heated to a temperature range satisfying the above relationship, and then finish rolling is performed at a finish rolling temperature of 800 ° C. or higher by hot rolling, and a coiling temperature of 600 to 720 ° C. is wound to obtain a hot rolled sheet A hot rolling process, a cold rolling process in which the hot-rolled sheet is pickled and cold-rolled to form a cold-rolled sheet, and an average heating rate up to 700 ° C. of the cold-rolled sheet: 15 ° C./s or more , Annealing temperature: 700 ° C to annealing temperature: 800 ° C to 950 ° C average heating rate: 0.1-2 ° C / s, annealing from annealing temperature to 500 ° C, average cooling rate: 5 ° C / s or more A method for producing a high-strength steel sheet, comprising sequentially performing a cold-rolled sheet annealing process.

2. In addition to the composition, the steel slab is
It contains one or two selected from Ti: 0.1% or less and V: 0.1% or less, and instead of the relationship (Nb / 93) / (C / 12) = 0.15 to 0.45, the following formula
In the case of Ti ^* > 0, Ti ^* = Ti-1.5S-3.4N (Ti, S, N in the formula is the content (% by mass) of each element), and when Ti ^* ≦ 0, Ti ^* = 0
The content of Ti ^* , V, Nb and C represented by
{(Nb / 93) + (Ti ^* / 48) + (V / 51)} / (C / 12) = 0.15 to 0.45 (where V, Nb and C are the contents of each element (mass%) )
2. The method for producing a high-strength steel sheet according to 1 above, wherein the above relationship is satisfied.

3. In addition to the composition, the steel slab is
3. The method for producing a high-strength steel sheet according to 1 or 2 above, comprising a total of one or more selected from Mo, Cu and Ni: 0.5% or less.

4). A method for producing a high-strength plated steel sheet, further comprising subjecting the high-strength steel sheet produced by the method according to any one of 1 to 3 above to plating.

  According to the present invention, when the C content is in the range of 0.035 to 0.05 mass%, martensite is not completely reduced without reducing the solid solution C that adversely affects the deep drawability as in the conventional ultra-low carbon IF steel. The effect of {111} recrystallization texture is achieved by lowering the slab heating temperature and gradually heating in the high temperature range after recrystallization after cold rolling, with the solid solution C necessary for formation remaining. As a result, it is possible to obtain a high-strength steel sheet having an average r value ≧ 1.2, a small in-plane anisotropy (| Δr | ≦ 0.20), and TS ≧ 540 MPa.

The present invention will be specifically described below.
First, the elucidation process of the present invention will be described. The unit of element content is “mass%”, but hereinafter, it is simply indicated by “%” unless otherwise specified.
In order to increase the r value, that is, to develop the {111} recrystallization texture, in the conventional mild steel sheet, the solute C before cold rolling and recrystallization is reduced as much as possible, and the hot rolled sheet structure is refined. This has been an effective means. On the other hand, in the DP steel sheet as described above, since the solute C necessary for martensite formation is required, the recrystallized texture of the parent phase does not develop and the r value is low. However, in the present invention, it has been newly found that there exists an exquisite preferred component range that enables the development of {111} recrystallization texture and the formation of martensite.
That is, although the C content is reduced as compared with the conventional DP steel sheet (low carbon steel level), the C content is 0.035 to 0.05% in the low carbon region where the C content is larger than that of the ultra low carbon steel. In addition, it was newly found that {111} recrystallization texture development and martensite formation can be achieved by adding an appropriate amount of Nb or further adding Ti or V.

As conventionally known, since Nb has a recrystallization delay effect, it is possible to refine the hot rolled sheet structure by appropriately controlling the finishing temperature during hot rolling. Nb has a high carbide forming ability in steel. In the present invention, in particular, the hot rolling finish temperature is set to an appropriate range immediately above the transformation point, and the hot rolled sheet structure is refined, and at the same time, the coil winding temperature after hot rolling is also set appropriately so that NbC is contained in the hot rolled sheet. Precipitation is performed to reduce solid solution C before cold rolling and before recrystallization. In this case, Ti and V are not as effective as Nb, but they can be added in a complex manner. Here, in the case of adding Nb alone, the amount of Nb is set to 0.15 to 0.4 as an atomic ratio with respect to the amount of C. That is,
(Nb / 93) / (C / 12) = 0.15 to 0.45 (Nb and C in the formula are the contents (mass%) of each element. The same applies hereinafter)
When adding Ti and V together,
{(Nb / 93) + (Ti ^* / 48) + (V / 51)} / (C / 12) = 0.15-0.45
Here, Ti ^* = Ti−1.5S−3.4N, where Ti ^* > 0
Therefore, C that does not precipitate as carbides of Nb, Ti, and V is intentionally present.
Conventionally, the presence of C that does not precipitate as carbides of Nb, Ti, and V has been considered to inhibit the development of the {111} recrystallized texture. In the present invention, a part of the total C is converted to Nb, Ti. , V can be achieved simply by precipitating as carbides of V, the in-plane anisotropy of the r value can be reduced, and solid solution C necessary for martensite formation can be secured.

  For this purpose, high-precision control of not only the components but also the manufacturing conditions is important. The important requirements as manufacturing conditions are the slab heating temperature and winding temperature, and the heating pattern during annealing. In the HSLA steel (High Strength Low Alloyed Steel) system, the slab heating temperature was high, but in the present invention, the in-plane anisotropy can be improved by lowering the temperature. . The reason is considered that there is an effect of preventing Δr from becoming large as a negative value due to coarsening of precipitates on the hot rolled sheet. From the viewpoint of coarsening the precipitates on the hot-rolled sheet, the coiling temperature needs to be 600 ° C. or higher.

Steel composition is 0.04% C-0.5% Si-2.0% Mn-0.03% P-0.005% S-0.035% Al-0.0020% N-0.06% Nb {(Nb / 93) / (C / 12) = 0.19} The steel slab was used to change the slab heating temperature (SRT) and the coiling temperature (CT), to produce a hot-rolled sheet at a finish rolling exit temperature of 870 ° C, and then to cold rolling and annealing. The results of examining Δr are shown in FIG.
The annealing temperature during annealing is set to 860 ° C, and the temperature up to this annealing temperature is increased uniformly at an average rate of increase of 15 ° C / s and up to 700 ° C at 15 ° C / s. The temperature was raised from 700 ° C. to the annealing temperature in two cases where the temperature was raised at 1.5 ° C./s. Further, the average cooling rate from the annealing temperature to 500 ° C. was constant at 15 ° C./s.

As shown in the figure, when the heating temperature is 1250 ° C., the coiling temperature is 550 ° C., and the average temperature rise rate up to the annealing temperature is 15 ° C./s, the Δr is −0.90. there were.
On the other hand, when the slab heating temperature was lowered and the coiling temperature was raised, Δr could be reduced to −0.30. Furthermore, when the temperature increase rate control was performed during annealing, the absolute value of Δr could be reduced to 0.20 or less.

The reason for this is not clear yet, but considering that the effect of slow heating appears at 700 ° C or higher, competition between recrystallization and transformation occurs in the two-phase structure, and if the transformation effect is too strong, It is considered that the vicinity of {100} <110> to {221} <110>, which is the main orientation, is inherited even after annealing, and Δr increases with a negative value. Therefore, in order to suppress the transformation to some extent, the average heating rate up to 700 ° C: 15 ° C / s or more on the cold-rolled sheet, and the average heating rate from 700 ° C to annealing temperature: 800 ° C to 950 ° C is 0.1 It has been found that it is necessary to perform annealing at ˜2 ° C./s.
Further, as a result of various investigations on the component composition and the production conditions, it has been found that the Nb amount and the C amount are related to the slab heating temperature at which Δr can be reduced, as will be described later.

  The following can be considered as other technical elements for increasing the r value. The fact that a high r value can be obtained without using an IF system as a component is not only a negative factor for the formation of {111} recrystallized texture due to the presence of solute C. This is thought to be due to the large positive factor of precipitating fine NbC and accumulating strain near the grain boundary during cold rolling and promoting the generation of {111} recrystallized grains from the grain boundary. In particular, the effect of precipitating NbC in the matrix is not effective at the C level of conventional ultra-low carbon steel, and is presumed to be effective for the first time at the C level of the present invention. This is one of the gist of the technical idea of the present invention. C other than NbC and its existence form are presumed to be cementite-based carbides or solute C, but the presence of C that is not fixed as NbC makes it possible to form martensite during cooling in the annealing process. It is considered that high strength has been achieved.

Next, the reason why the component composition of the steel slab used in the present invention, that is, the high-strength steel sheet to be obtained in the present invention is limited to the above range will be described.
C: 0.035-0.05%
C is an important element in the present invention together with Nb described later. This C is effective in increasing the strength and has the effect of promoting the formation of a composite structure having a second phase containing ferrite as a main phase and containing martensite. Further, in the present invention, in order to control the SRT to control the in-plane anisotropy of the r value, C is effective, and in order to ensure the strength of 540 MPa or more, 0.035% or more of C is used. It was supposed to be included. However, in order to obtain a good r value, excessive addition of C is not preferable, and the upper limit was made 0.05% in consideration of controllability of in-plane anisotropy. A more preferable amount of C is 0.038% or more and 0.045% or less.

Si: 0.01-1.0%
Si promotes ferrite transformation and increases the amount of C in untransformed austenite to facilitate the formation of a composite structure of ferrite and martensite, and also has an effect of solid solution strengthening. In order to obtain these effects, Si must be contained in an amount of 0.01% or more, and more preferably 0.05% or more.
On the other hand, if the Si content exceeds 1.0%, a red scale is generated during hot rolling, so the surface appearance of the product plate is deteriorated. In addition, when applying hot dip galvanizing (including alloying hot dip galvanizing), the wettability of the plating deteriorates, causing uneven plating, and the plating quality deteriorates. Therefore, the Si content must be 1.0% or less. is there. More preferably, it is 0.7% or less.

Mn: 1.5-3.0%
Mn is not only effective for increasing strength, but also has the effect of lowering the critical cooling rate at which a martensite phase is obtained, and the required strength level and annealing to promote the formation of martensite during cooling after annealing. It is preferable to contain it according to the subsequent cooling rate. Mn is an element that is also effective in preventing hot cracking due to S. From such a viewpoint, Mn needs to be contained by 1.5% or more. More preferably, it is 1.8% or more. On the other hand, excessive addition exceeding 3.0% deteriorates the r value and weldability, so the upper limit of the Mn content is 3.0%.

P: 0.005-0.1%
P is an element effective for solid solution strengthening. However, if the P content is less than 0.005%, not only the effect does not appear, but also the dephosphorization cost increases in the steel making process. Therefore, P is contained in an amount of 0.005% or more. Preferably it is 0.01% or more. On the other hand, when P is excessively added in excess of 0.1%, P segregates at the grain boundaries and deteriorates the secondary work brittleness resistance and weldability. In addition, when the hot dip galvanized steel sheet is used, 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 deteriorates. For this reason, an alloying treatment at a high temperature is required, and the obtained plating layer is likely to undergo plating peeling such as powdering and chipping. Therefore, the upper limit of the P amount is 0.1%.

Al: 0.005% to 0.1%
In addition to being useful as a deoxidizing element for steel, Al has the effect of fixing solid solution N and improving the normal temperature aging resistance, so it is contained in an amount of 0.005% or more. However, addition exceeding 0.1% not only causes an increase in alloy cost, but also induces surface defects, so 0.1% was made the upper limit.

S: 0.01% or less S is an impurity and not only causes hot cracking but also exists as an inclusion in the steel and degrades various properties of the steel sheet. Therefore, it is preferable to reduce it as much as possible, but 0.01% The upper limit is made 0.01% or less.

N: 0.01% or less N is excessively deteriorated at normal temperature aging resistance, and a large amount of Al or Ti needs to be added. Therefore, it is preferable to reduce it as much as possible, but up to 0.01% is acceptable, so the upper limit is 0.01 %.

Nb: 0.05 to 0.12% and (Nb / 93) / (C / 12) = 0.15 to 0.45
Nb is the most important element in the present invention, and has the 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. Furthermore, from the viewpoint of exerting the effect of controlling the in-plane anisotropy of the r value by adjusting SRT: Nb needs to be contained in an amount of 0.05% or more. On the other hand, excessive addition of Nb makes it difficult to control the in-plane anisotropy of the r value. Further, in the present invention, solid solution C for forming martensite is required in the cooling process after annealing, but excessive Nb addition tends to prevent this, so the upper limit is made 0.12%.

In order to achieve the effect of Nb addition, Nb and C are contained so that the Nb content and the C content satisfy the range of (Nb / 93) / (C / 12) = 0.15 to 0.45. It is important to let Here, Nb and C represent the content (mass%) of each element, and (Nb / 93) / (C / 12) represents the atomic concentration ratio of Nb and C.
In the present invention, if (Nb / 93) / (C / 12) is less than 0.15, the amount of solid solution C is large and the formation of {111} recrystallized texture necessary for high r-value is inhibited. become. On the other hand, if (Nb / 93) / (C / 12) exceeds 0.45, not only is it difficult to control the in-plane anisotropy of the r value, but martensite is formed to ensure a strength of 540 MPa or more. This prevents the amount of C necessary to do so from being present in the steel. Accordingly, Nb is included in the range of 0.05 to 0.12% and (Nb / 93) / (C / 12) = 0.15 to 0.45 is satisfied.

The basic components have been described above. However, in the present invention, other elements described below can be appropriately contained.
One or two selected from Ti: 0.1% or less and V: 0.1% or less
Ti and V, like Nb, are elements that have the effect of refining the hot-rolled sheet structure and precipitating and fixing C as carbides in the hot-rolled sheet and contributing to higher r values. However, since Nb has a greater effect of refinement of the hot-rolled sheet, it is preferable to add Ti and V appropriately to the Nb-added steel. From such a viewpoint, it is preferable to contain 0.005% or more of Ti and V. On the other hand, in the present invention, solid solution C for forming martensite is required in the cooling process after annealing. However, if excessive Ti and V are added to the Nb-added steel, it becomes a hindrance. And 0.1% or less, respectively.

{(Nb / 93) + (Ti ^* / 48) + (V / 51)} / (C / 12) = 0.15 to 0.45. However, when Ti ^* > 0, Ti ^* = Ti-1.5S-3.4N, and when Ti ^* ≦ 0, Ti ^* = 0
In order to achieve the effect of addition of Nb, Ti, and V, it is necessary to satisfy the ratio of Nb, Ti (Ti ^* ), V content and C content in the range of 0.15 to 0.45. When {(Nb / 93) + (Ti ^* / 48) + (V / 51)} / (C / 12) is less than 0.15, there is a large amount of solute C, which is effective for increasing the r value {111} The formation of recrystallized texture will be inhibited. On the other hand, if it exceeds 0.4, martensite is formed and the amount of C necessary to ensure the strength of 540 MPa or more is prevented from being present in the steel. A structure having a phase cannot be obtained. Therefore, the ratio of Nb, Ti (Ti ^* ), V content and C content (% by mass) satisfies the range of 0.15 to 0.45.
Ti is an element that is effective for precipitation fixation of solute S and N, and Ti in steel is likely to combine with S and N before C. Therefore, these S and N are determined from the Ti content in the steel. The effective amount of Ti (Ti ^* ) obtained by removing the amount of Ti that is considered to be consumed for fixing is used for calculating the ratio with C. In addition, when Ti ^* ≦ 0, Ti is consumed to fix S and N, and it is considered that precipitation and fixing of C due to Ti does not occur. Therefore, only when Ti ^* > 0, the ratio with C In the case of Ti ^* ≦ 0, Ti ^* is not used in the calculation of the ratio with C, and Ti ^* = 0.

Total of one or more selected from Mo, Cu and Ni: 0.5% or less
Mo, like Mn, has the effect of slowing the critical cooling rate at which martensite can be obtained, is an element that promotes martensite formation during cooling after annealing, and is effective in improving the strength level. It is also an element that has the effect of precipitating and fixing C and contributes to an increase in r value. In order to acquire these effects, it is preferable to contain Mo 0.05% or more.
Cu and Ni, like Mn, have the effect of slowing the critical cooling rate at which martensite can be obtained, are elements that promote martensite formation during cooling after annealing, and are effective in improving the strength level. In addition, the influence on the plating property is small. In order to obtain these effects, it is preferable to contain 0.05% or more of Cu and Ni, respectively.
However, excessive addition of Mo, Cu, and Ni not only saturates these effects, but also increases the alloy cost. In particular, Cu adversely affects the surface properties, so these elements can be used alone or in combination. In any case, the total content is preferably 0.5% or less.

In the present invention, the balance other than the above components is Fe and inevitable impurities.
It should be noted that there is no problem even if B, Ca, REM and the like are further 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 necessary. However, when the B content exceeds 0.003%, the effect is saturated, so 0.003% or less is preferable. Moreover, Ca and REM have the effect | action which controls the form of a sulfide type inclusion, and, thereby, prevents the deterioration of the various characteristics of a steel plate. 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%, respectively. Hereinafter, Co: 0.1% or less.

Next, the manufacturing method of the high strength steel plate according to this invention is demonstrated.
In the present invention, a steel slab adjusted to the above-described preferred component composition is used as a raw material, and after the raw material is heated, hot rolling is performed to form a hot rolled sheet, and the hot rolled sheet is pickled and cooled. A high-strength steel sheet is manufactured by sequentially performing a cold rolling process to form a cold-rolled sheet by cold rolling and a cold-rolled sheet annealing process to achieve recrystallization and composite structure on the cold-rolled sheet. Can do.
Hereinafter, each processing condition will be described.

  In producing the steel slab, it is desirable to produce the steel slab by a continuous casting method in order to prevent macro segregation of components, but it 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, it is directly fed into a heating furnace without being cooled and placed in a heating furnace for hot rolling, or a little heat retention Energy saving processes such as direct feed rolling and direct rolling that are immediately subjected to hot rolling after performing the above can be applied without any problem.

Slab heating temperature (SRT): 1000 ° C or more and 1200 ° C or less and SRT ≤ 25000 [% C] [% Nb] +1050
The slab heating temperature is desirably low in order to coarsen the precipitate and reduce the in-plane anisotropy. In this sense, it must be 1200 ° C or lower. However, if the heating temperature is less than 1000 ° C, the rolling load increases and the risk of trouble occurring during hot rolling increases, so the slab heating temperature is preferably 1000 ° C or higher.

Further, the precipitate is mainly NbC, and its coarsening is affected not only by SRT but also by the amount of C and Nb.
Fig. 2 shows the amount of C, Nb and SRT changed, cold rolled at CT: 700 ° C, rolling reduction: 65%, then heated up to 700 ° C at an average heating rate: 20 ° C / s. From 700 ° C to annealing temperature: 860 ° C, average heating rate: 1.5 ° C / s, heated at 860 ° C for 80 seconds, and then cooled from annealing temperature to 500 ° C at average cooling temperature: 1.5 ° C / s. It is the result of having measured the Δr value of the cold-rolled annealing plate.
Considering that the precipitation amount corresponds to the solubility product, the horizontal axis is [% C] [% Nb], which is the product of C content ([% C]) and Nb content ([% Nb]). , C content is 0.035% or more, Nb content is 0.05% or more (hence 1000 [% C] [% Nb] ≧ 1.75) and SRT ≦ 25000 [% C] [% Nb] +1050 Can satisfy the absolute value of Δr of 0.20 or less.
Therefore, in the present invention, the slab heating temperature (SRT) is limited to the range satisfying the relationship of SRT ≦ 25000 [% C] [% Nb] +1050 and not less than 1000 ° C. and not more than 1200 ° C.

  The slab heated under the above conditions is subjected to hot rolling consisting of rough rolling and finish rolling. Here, the 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. In addition, it goes without saying that it is an effective method to use a so-called sheet bar heater for heating the sheet bar from the viewpoint of lowering the slab heating temperature and preventing troubles during hot rolling.

Finishing rolling exit temperature in hot rolling: 800 ° C or higher Next, finish rolling the sheet bar to make a hot rolled sheet. At this time, the finish rolling outlet temperature (FT) needs to be 800 ° C. or higher. This is to obtain a fine hot-rolled sheet structure that provides excellent deep drawability after cold rolling and recrystallization annealing.
If FT is less than 800 ° C., the structure has a processed structure, and not only {111} texture develops after cold rolling annealing, but also the rolling load during hot rolling becomes high. On the other hand, when the FT exceeds 980 ° C., the structure becomes coarse, which also prevents the formation and development of {111} recrystallized texture after the cold rolling annealing, thereby inhibiting the achievement of the high r value. It is preferable to do.

  In addition, in order to reduce the rolling load at the time of hot rolling, a part or all pass of finish rolling can also be lubricated rolling. This lubrication rolling is effective from the viewpoint of uniforming the shape of the steel sheet and homogenizing the material. In addition, it is preferable to make the friction coefficient in the case of lubrication rolling into the range of 0.10-0.25. Furthermore, it is also preferable to use a continuous rolling process in which successive sheet bars are joined together and continuously subjected to finish rolling. It is desirable to apply the continuous rolling process from the viewpoint of the operational stability of hot rolling.

Winding temperature: 600 ~ 720 ℃
The coil winding temperature (CT) shall be 600 ° C or higher and 720 ° C or lower. This is because this temperature range is not only suitable for precipitating and growing carbides such as Nb, Ti, V, etc. that could not be precipitated during slab heating. This is because the grains are coarsened and the strength is lowered, and an increase in r value after cold rolling annealing is hindered. Preferably it is the temperature range of 620-700 degreeC.

Next, the hot-rolled sheet is pickled and then cold-rolled to obtain a cold-rolled sheet. Pickling may be performed under normal conditions. The cold rolling condition is not particularly limited as long as it can be a cold-rolled sheet having a desired dimension and shape, but the rolling reduction during cold rolling is preferably 40% or more. More desirably, it is 50% or more.
Generally, cold rolling at a high pressure ratio is effective for increasing the r value. If the reduction ratio is less than 40%, the {111} recrystallized texture does not develop, and it is difficult to obtain excellent deep drawability. Become. In the present invention, the r value increases as the cold rolling reduction is increased up to 90%, but when it exceeds 90%, not only the effect is saturated but also the load on the roll during rolling increases. Therefore, the upper limit is preferably 90%.

Average heating rate up to 700 ° C: 15 ° C / s or more, Average heating rate from 700 ° C to annealing temperature: 800 ° C or more and 950 ° C or less: 0.1-2 ° C / s
In annealing after cold rolling, it is necessary to at least recrystallize. For this purpose, the annealing temperature, which is the highest temperature achieved during annealing, must be at least 800 ° C. On the other hand, since the recrystallized grains become extremely coarse at a high temperature exceeding 950 ° C. and the characteristics are significantly deteriorated, the annealing temperature is limited to 800 ° C. or more and 950 ° C. or less. The holding time at the annealing temperature, so-called annealing time, is not particularly specified, but is preferably about 10 to 120 seconds from the viewpoint of performing annealing stably.

The heating rate up to the annealing temperature described above is an important requirement in the present invention.
That is, it is necessary to raise the temperature up to 700 ° C. at a somewhat high heating rate, to suppress the recovery to some extent, and to promote recrystallization. Otherwise, the transformation preferentially proceeds in a temperature range higher than that, and the in-plane anisotropy cannot be reduced. Therefore, in the present invention, the temperature is increased up to 700 ° C. at an average rate of temperature increase of 15 ° C./s or more. On the other hand, in the temperature range of 700 ° C. or higher, it is necessary to promote recrystallization as a gradual heating, to develop the {111} orientation preferable for the r value, and to improve the in-plane anisotropy. For this reason, the average temperature increase rate in this temperature range is set to 2 ° C./s or less. However, gradual heating at an ultra-low speed with a heating rate of less than 0.1 ° C./s is extremely difficult considering the temperature condition of 700 ° C. or higher and the current annealing equipment. Therefore, in the present invention, the average from 700 ° C. to the annealing temperature is used. The temperature rising rate was limited to a range of 0.1 to 2 ° C./s.

Average cooling rate from annealing temperature to 500 ° C .: 5 ° C./s or more For the cooling treatment after annealing, it is necessary to set the average cooling rate from annealing temperature to 500 ° C. to 5 ° C./s or more from the viewpoint of martensite formation. There is. This is because if the average cooling rate in the temperature range is less than 5 ° C./s, martensite is difficult to form and a ferrite single-phase structure is formed, resulting in insufficient strengthening of the structure. In the present invention, since the presence of the second phase containing martensite is indispensable, the average cooling rate up to 500 ° C. is required to be equal to or higher than the critical cooling rate. This is achieved by setting it to s or more. However, if this cooling rate exceeds 15 ° C / s, although it becomes a composite structure, the fraction of the second phase becomes high and distribution becomes unfavorable for ductility. Therefore, the average cooling rate in this temperature range is It is preferable to set it as 15 degrees C / s or less.
The cooling in the temperature range below 500 ° C. is not particularly limited because the γ phase is stabilized to some extent by the cooling so far. However, preferably, the average cooling rate of 5 ° C./s or more is desirably increased to 300 ° C. It is preferable to cool. Moreover, when performing an overaging treatment, it is preferable to cool to an overaging treatment temperature at an average cooling rate of 5 ° C./s or more.

Moreover, after the cold-rolled sheet annealing, 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. In addition to pure zinc, for example, zinc-based plating in which zinc is the main component and alloying elements are added, or pure Al, Al-based plating in which Al is the main component and alloying elements can be applied. .
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 galvanizing line, and immersed in a hot dip galvanizing bath following cooling after annealing, A hot dip galvanized layer may be formed on the surface. Further, alloying treatment may be performed to obtain an alloyed hot-dip galvanized steel sheet. In that case, it is preferable that the average cooling rate up to 300 ° C. is 5 ° C./s or more also in cooling after leaving the pot of hot dipping and further alloying.

  The cold-rolled annealed plate obtained as described above, and further, the plated steel plate subjected to the plating treatment can be subjected to temper rolling and leveler processing for the purpose of shape correction, surface roughness adjustment and the like. The temper rolling and leveler processing are preferably performed in the range of 0.2 to 15% in terms of elongation. 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. These temper rolling and leveler processing are effective even after the plating treatment.

Next, the steel structure of the steel plate obtained by the manufacturing method of the present invention described above will be described.
In order to obtain a steel sheet with good deep drawability and a tensile strength of ≧ 540 MPa, a ferrite phase with an area ratio of 50% or more and a martensite phase with an area ratio of 1% or more, preferably 3% or more. It is necessary to make the steel structure including. When the ferrite phase is reduced and the area ratio is less than 50%, it becomes difficult to ensure good deep drawability, and the press formability tends to decrease. A more preferable ratio of the ferrite phase is 70% or more in terms of area ratio. Further, in order to take advantage of the composite structure, the ferrite phase is preferably 99% or less. Here, the ferrite phase means a polygonal ferrite phase or a painitic ferrite phase having a high dislocation density transformed from an austenite phase.

As described above, the present invention requires the presence of a martensite phase, and the phase ratio is 1% or more, preferably 3% or more, and more preferably 5% or more in terms of area ratio. If the martensite phase is less than 1%, it is difficult to obtain a good strength-ductility balance.
In addition to the ferrite phase and martensite phase described above, a structure including pearlite, bainite, or residual γ phase may be used. In this case, the total ratio of these phases is preferably 30% or less in terms of area ratio.

In the present invention, an average r value of 1.2 or more can be achieved by using the above-described component composition and steel structure.
Here, the average r value means an average plastic strain ratio obtained by JIS Z 2254, and is a value obtained by the following formula.
Average r value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4
Here, r ₀ , r ₄₅ and r ₉₀ are plastic strain ratios obtained by measuring test pieces in the 0 °, 45 ° and 90 ° directions with respect to the rolling direction of the plate surface, respectively.

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. After heating these steel slabs to 1250 ° C., they were made into sheet bars by rough rolling, and then subjected to finish rolling under the conditions shown in Table 2 to obtain hot rolled sheets. These hot-rolled sheets were pickled and then cold-rolled with a reduction ratio of 65% to obtain cold-rolled sheets with a sheet thickness of 1.2 mm. Subsequently, these cold-rolled sheets were subjected to continuous annealing in the continuous annealing line under the conditions shown in Table 2. The annealing time was 60 to 80 seconds. Subsequently, the obtained cold-rolled annealed sheet was subjected to temper rolling with an elongation of 0.5% to obtain a product sheet.
The No. 16 steel plate was a product plate made of a hot dip galvanized steel plate that had been subjected to continuous annealing in a continuous hot dip galvanizing line and subsequently subjected to hot dip galvanizing (plating bath temperature: 480 ° C.) in-line.
Table 2 shows the results of investigation on the microstructure, tensile properties, r value and plating processability of each product plate thus obtained.

In addition, the investigation method of the microscopic structure and each characteristic is as follows.
(1) Microstructure of cold-rolled annealed plates Specimens were collected from each cold-rolled annealed plate, and the thickness cross section (L cross section) parallel to the rolling direction was measured using an optical microscope or scanning electron microscope. The microscopic tissue was imaged at a magnification, and the area ratio of ferrite as the main phase and the type and area ratio of the second phase were determined by an image analysis apparatus.

(2) Tensile properties JIS No. 5 tensile test specimens were sampled from each of the 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 conducted at / min to determine yield stress (YS), tensile strength (TS), and elongation (El).

(3) r value JIS No. 5 tensile specimen from the rolling direction (L direction), 45 ° direction (D direction) with respect to the rolling direction, and 90 ° direction (C direction) with respect to the rolling direction. Were collected. Measure the width strain and plate thickness strain of each specimen when 10% uniaxial tensile strain was applied to these specimens, and use these measurements to determine the average r value in accordance with JIS Z 2254 regulations. The (average plastic strain ratio) was determined from the following formula, and this was used as the average r value.
Average r value = (r ₀ + 2r ₄₅ + r ₉₀ ) / 4
Here, r ₀ , r ₄₅ and r ₉₀ are plastic strain ratios obtained by measuring test pieces in the 0 °, 45 ° and 90 ° directions with respect to the rolling direction of the plate surface, respectively.
Further, the in-plane anisotropy (Δr) of the r value was determined from the following equation.
Δr = (r ₀ + r ₉₀ −2r ₄₅ ) / 2

(4) Plating treatment property The surface of the obtained hot-dip galvanized steel sheet was visually observed, and the presence or absence of non-plating defects was determined to evaluate the plating property. In addition, the evaluation is ○ when there is no unplating defect (good plating property), △ when some unplating defect occurs (slightly good plating property), and many unplating defects occur (bad plating property) ) Was marked with x.

As is apparent from Table 2, all the inventive examples obtained according to the present invention have a TS of 540 MPa or more, an average r value of 1.2 or more, and an absolute value of Δr of 0.20 or less. And r value in-plane anisotropy were also excellent. Moreover, the plating processability was also good.
On the other hand, in the comparative example manufactured on the conditions out of the range of the present invention, the steel plate was insufficient in strength or poor in r value and in-plane anisotropy.

According to the present invention, a high-strength steel sheet excellent in deep drawability with a TS of 540 MPa or higher, an average r value of 1.2 or higher, and a small in-plane anisotropy is inexpensive and stable. It becomes possible to manufacture and has an industrially significant effect.
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 an automobile body. There is. Moreover, it is applicable not only to automobile parts but also to household appliance parts and pipe materials.

It is the figure which showed transition of (DELTA) r when changing the average rate of temperature increase to slab heating temperature (SRT), coiling temperature (CT), and annealing temperature in the case of manufacture of a steel plate. It is the figure which showed the result of having investigated about (DELTA) r of the steel plate manufactured by changing C, Nb amount and SRT variously by the relationship between SRT and 1000 [% C] [% Nb].

Claims (4)

% By mass
C: 0.035 to 0.05%,
Si: 0.01 to 1.0%
Mn: 1.5-3.0%
P: 0.005-0.1%
Al: 0.005% to 0.1%
S: 0.01% or less,
N: 0.01% or less and
Nb: 0.05-0.12%
And the content of Nb and C in the steel is
(Nb / 93) / (C / 12) = 0.15 to 0.45 (where Nb and C are the contents of each element (mass%))
A steel slab containing the composition of Fe and inevitable impurities in the balance satisfying the following relationship: a slab heating temperature (SRT) of 1000 ° C. or higher and 1200 ° C. or lower, and an amount of C: [% C] and Nb Quantity: Depending on [% Nb] (mass%)
SRT ≦ 25000 [% C] [% Nb] +1050
Is heated to a temperature range satisfying the above relationship, and then finish rolling is performed at a finish rolling temperature of 800 ° C. or higher by hot rolling, and a coiling temperature of 600 to 720 ° C. is wound to obtain a hot rolled sheet A hot rolling process, a cold rolling process in which the hot-rolled sheet is pickled and cold-rolled to form a cold-rolled sheet, and an average heating rate up to 700 ° C. of the cold-rolled sheet: 15 ° C./s or more , Annealing temperature: 700 ° C to annealing temperature: 800 ° C to 950 ° C average heating rate: 0.1-2 ° C / s, annealing from annealing temperature to 500 ° C, average cooling rate: 5 ° C / s or more A method for producing a high-strength steel sheet, comprising sequentially performing a cold-rolled sheet annealing process. In addition to the composition, the steel slab is
It contains one or two selected from Ti: 0.1% or less and V: 0.1% or less, and instead of the relationship (Nb / 93) / (C / 12) = 0.15 to 0.45, the following formula
In the case of Ti ^* > 0, Ti ^* = Ti-1.5S-3.4N (Ti, S, N in the formula is the content (% by mass) of each element), and when Ti ^* ≦ 0, Ti ^* = 0
The content of Ti ^* , V, Nb and C represented by
{(Nb / 93) + (Ti ^* / 48) + (V / 51)} / (C / 12) = 0.15 to 0.45 (where V, Nb and C are the contents of each element (mass%) )
The method for producing a high-strength steel sheet according to claim 1, wherein the relationship is satisfied. In addition to the composition, the steel slab is
The method for producing a high-strength steel sheet according to claim 1 or 2, comprising a total of 0.5% or less of one or more selected from Mo, Cu and Ni.   A method for producing a high-strength plated steel sheet, further comprising subjecting the high-strength steel sheet produced by the method according to any one of claims 1 to 3 to plating.

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