JP3762700B2 - High-strength steel sheet excellent in formability and chemical conversion treatment and method for producing the same - Google Patents

Description

【表２】

【００１９】
【発明の効果】
本発明によれば、Si、Al、Tsのバランスを特定範囲とし、特にAl添加量を調整することで、降伏応力の低いＤＰ鋼において、これまで以上の伸びが確保できる成形性と化成処理性に優れた高強度鋼板およびその製造方法を工業的規模で実現することができ、産業上有用な、著しい効果を奏する。
【図面の簡単な説明】
【図１】 ＡｌとＳｉの質量％と目標強度、化成処理性との関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a high-strength steel plate excellent in formability and chemical conversion treatment and a method for producing the same.
[0002]
[Prior art]
In recent years, in order to improve the fuel efficiency of automobiles, the weight reduction of the vehicle body has been further demanded.
In order to reduce the weight of the vehicle body, a steel material having a high strength may be used. However, as the strength increases, press molding becomes more difficult. This is because, generally, the higher the strength of the steel material, the higher the yield stress of the steel material, and the lower the elongation.
On the other hand, a steel plate (hereinafter referred to as TRIP steel) using work-induced transformation of retained austenite has been invented for improving the elongation, and is disclosed in, for example, Japanese Patent Application Laid-Open No. 61-157625.
However, in general TRIP steel sheets, a large amount of Si is indispensable and the chemical conversion treatment property of the steel sheet surface deteriorates, so that applicable members are limited. Furthermore, in order to secure high strength in the retained austenitic steel, a large amount of C is required, which causes welding problems such as nugget cracks.
[0003]
Regarding the chemical conversion processability of the steel sheet surface, an invention aimed at reducing Si in retained austenite TRIP steel is disclosed in JP 2000-345288A, but in this invention, improvement of chemical conversion processability and ductility can be expected, The above-mentioned improvement in weldability cannot be expected, and TRIP steel sheets with a tensile strength of 980 MPa or more have a problem that the shape freezing property at the time of pressing or the like deteriorates because of a very high yield stress.
Further, as a technique for reducing yield stress, a dual phase steel containing ferrite (hereinafter referred to as DP steel) as disclosed in JP-A-57-155329 has been conventionally known, but it is not always sufficient. It did not have moldability and chemical conversion treatment.
[0004]
[Problems to be solved by the invention]
An object of the present invention is to solve the above-described problems of the prior art and to realize a high-strength steel sheet excellent in formability and chemical conversion treatment and a manufacturing method thereof on an industrial scale.
[0005]
[Means for Solving the Problems]
First, the technical idea of the present invention will be described.
As a result of intensive studies on high-strength steel sheets with excellent formability and chemical conversion properties, the present inventors have optimized the steel components, that is, set the balance of Si, Al, and Ts within a specific range, and in particular adjusted the amount of Al added. As a result, it has been found that a high strength steel sheet capable of securing a higher elongation than before can be industrially produced in DP steel having a low yield stress.
The steel sheet of the present invention has improved ductility to the same extent as conventional retained austenitic steel, and also improves chemical conversion processability by reducing Si, and is less likely to deteriorate even if alloying plating is performed. Realized high strength steel plate.
Further, in order not to cause the problem of delayed fracture and secondary work brittleness, DP steel that allows unavoidable residual austenite of 3% or less and does not substantially contain residual austenite was obtained.
The high-strength steel sheet of the present invention can realize a tensile strength of 590 Mpa to 1500 Mpa, but has a remarkable effect with a high-strength steel sheet of 980 Mpa or higher.
The present invention is based on the technical idea as described above, and includes the following contents described in the claims.
[0006]
(1) In mass%,
C: 0.01 to 0.30%
Si: 0.005-0.2%,
Mn: 0.1-2.2%
P: 0.001 to 0.06%,
S: 0.001 to 0.01%,
N: 0.0005 to 0.01%,
Al: 0.25 to 1.43 % , consisting of the remainder Fe and inevitable impurities,
Furthermore, the mass% of Si and Al and the target strength value (TS) satisfy the following formula (A):
further,
Mo: 0.05 ~ 0.5%,
V: 0.01 to 0.1%
Ti: 0.01-0.2%
Nb: 0.005 to 0.05%,
B: Contains one or more of 0.0005 to 0.002 % ,
A high-strength steel sheet excellent in formability and chemical conversion processability, characterized in that the metal structure contains ferrite and martensite.
(0.0012 × [TS target value] -0.29- [Si]) / 1.45 <Al <1.5-3 × [Si] (A)
Here, [TS target value] is the strength design value of the steel sheet, the unit is Mpa,
[Si] is the mass% of Si
[0007]
(2) Furthermore,
Ca: 0.0005 to 0.005%,
REM: One high-strength steel sheet excellent in formability and chemical conversion treatment as described in (1), containing one or two of 0.0005 to 0.005%.
(3) A method for producing a high-strength steel sheet according to (1) or (2) , wherein the steel sheet is heated to a temperature range of Ac1 to Ac3 + 100 ° C or lower in the annealing step, and maintained at 30 seconds or longer and 30 minutes or shorter, and then 600 ° C. A method for producing a high-strength steel sheet excellent in formability and chemical conversion treatment, characterized by cooling to the following temperature range.
Here, Ac1 and Ac3 are values calculated by the Andrews formula based on the steel material components.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described in detail.
First, the reasons for limiting the components and metal structure of the high-strength steel sheet of the present invention will be described.
C is an essential component from the viewpoint of securing strength and as a basic element for stabilizing martensite.
If C is less than 0.01%, the strength is not satisfied and a martensite phase is not formed. On the other hand, if it exceeds 0.3%, the strength is too high, the ductility is insufficient, and the weldability is deteriorated, so that it cannot be used as an industrial material.
Therefore, the range of C in the present invention is 0.011 to 0.3%, preferably 0.03 to 0.15%.
[0009]
Mn is an element that delays the formation of carbides and is effective for the formation of ferrite, in addition to the need for addition from the viewpoint of securing strength.
If Mn is less than 0.1%, the strength is not satisfied, and ferrite is not sufficiently formed, resulting in deterioration of ductility.
In addition, if the amount of Mn added exceeds 2.2%, the hardenability is increased more than necessary, so that a lot of martensite is generated, resulting in an increase in strength, resulting in increased product variation and insufficient ductility. Cannot be used as a material.
Therefore, the range of Mn in the present invention is set to 0.1 to 2.2%.
Si is an element that is usually added to ensure ductility in addition to ensuring strength. However, if it exceeds 0.2%, chemical conversion properties deteriorate. Therefore, the range of Si in the present invention is 0.2% or less, and 0.1% or less is preferable when the chemical conversion treatment is emphasized.
[0010]
P is added according to the strength level required as an element for increasing the strength of the steel sheet. However, if the addition amount is large, segregation to the grain boundary causes deterioration of local ductility. In addition, the weldability is deteriorated. Therefore, the P upper limit is set to 0.06%. The reason why the lower limit is set to 0.001% is that a further reduction leads to a cost increase during refining in the steelmaking stage.
S is an element that deteriorates local ductility and weldability by generating MnS, and is preferably an element that does not exist in steel. Therefore, the upper limit is made 0.01%. The reason why the lower limit is set to 0.001% is that, as in the case of P, reducing it further leads to an increase in cost during refining at the steelmaking stage.
[0011]
Al is the most important element in this system. Al is an element that promotes the formation of ferrite by addition and effectively works to improve ductility, and does not deteriorate the chemical conversion treatment property even when added in a large amount. It also acts as a deoxidizing element.
In order to improve the ductility, Al addition of 0.25% or more is necessary. On the other hand, even if Al is added excessively, the above effect is saturated and the steel is embrittled, so the upper limit was made 1.8%.
N is an element that is inevitably included, but if it is contained in a large amount, not only deteriorates the aging property but also increases the amount of precipitated AlN and decreases the effect of Al addition, so 0.01% or less Containing is preferable. Further, unnecessarily reducing N increases the cost in the steelmaking process, so it is usually preferable to control it to about 0.0005% or more.
[0012]
In order to obtain a high-strength steel sheet, it is generally necessary to add a large amount of elements, and ferrite formation is suppressed. For this reason, the ferrite fraction of the structure is reduced and the fraction of the second phase is increased, so that the elongation is remarkably lowered particularly in DP steel of 980 MPa or more. For this improvement, Si addition and Mn reduction are often used. However, the former cannot be used in the steel sheet which is the object of the present invention because the chemical conversion processability is deteriorated and the latter is difficult to ensure the strength. Therefore, as a result of intensive studies, the inventors found out the effect of Al, and when having an Al, Si, TS balance satisfying the relationship of the formula (A), a sufficient ferrite fraction can be secured, and excellent elongation is achieved. It was found that it can be secured.
(0.0012 × [TS target value] -0.29- [Si]) / 1.45 <Al <1.5-3 × [Si] (A)
Here, [TS target value] is the strength design value of the steel sheet, and the unit is MPa. [Si] is the mass% of Si.
If the amount of Al added is less than (0.0012 × [TS target value] -0.29- [Si]) / 1.45, it is not sufficient to improve ductility, and if it exceeds 1.5-3 × [Si], chemical conversion treatment is performed. Sex worsens.
[0013]
The reason why the metal structure of the present invention is characterized by containing ferrite and martensite is that when such a structure is taken, the steel sheet has an excellent balance of strength and ductility. The ferrite here refers to polygonal ferrite and bainetic ferrite, and martensite is effective not only in martensite obtained by ordinary quenching but also in martensite tempered at a temperature of 600 ° C. or lower. does not change. In addition, if austenite remains in the structure, secondary work brittleness and delayed fracture characteristics deteriorate, so in the present invention, 3% or less of retained austenite that is unavoidably present is allowed, and substantially no retained austenite is contained.
[0014]
V, Ti, and Nb are V: 0.01 to 0.1%, Ti: 0.01 to 0.2% for the purpose of securing strength,
Nb: You may add in 0.005 to 0.05% of range.
Mo is an element effective in ensuring strength and hardenability. If the minimum addition amount is 0.05% or less, the strengthening of Mo cannot be used, the quenching performance peculiar to Mo cannot be exhibited, sufficient martensite is not formed, and the strength is insufficient. Excessive addition of Mo suppresses ferrite formation in DP, causes deterioration of ductility, and may deteriorate chemical conversion property, so the upper limit was made 0.5%.
[0015]
Ca and REM may be added in the range of Ca: 0.0005 to 0.005% and REM: 0.0005 to 0.005% for the purpose of inclusion control and improvement of hole expansion.
B may be added in a range of B: 0.0005 to 0.002% for the purpose of ensuring hardenability and increasing effective Al by BN.
Inevitable impurities include, for example, Sn, but even if these elements are contained in the range of 0.02% by mass or less, the effect of the present invention is not impaired.
[0016]
The reasons for limiting the manufacturing process of the present invention are as follows.
The material used in the present invention is a hot-rolled steel sheet manufactured through a normal hot-rolling process. These can be obtained by pickling, cold rolling or directly passing through the heat history described below.
In the continuous annealing process, first, annealing is performed at a temperature of Ac1 or higher and Ac3 + 100 ° C or lower. Below this, the organization becomes uneven. On the other hand, at a temperature higher than this, since the formation of ferrite is suppressed by the coarsening of austenite, the elongation is deteriorated. Also, the annealing temperature is desirably 900 ° C. or less from an economical point. At this time, in order to eliminate the layered organization, it is necessary to hold for 30 seconds or more. However, even if it exceeds 30 minutes, the effect is saturated and the productivity is also lowered. Therefore, it is 30 seconds or more and 30 minutes or less.
Subsequently, the cooling end temperature is set to a temperature of 600 ° C. or lower. If it exceeds 600 ° C., austenite tends to remain, and problems of secondary workability and delayed fracture tend to occur.
The effect of the present invention does not change even if a tempering treatment at 600 ° C. or lower is performed for the purpose of improving hole expansibility and brittleness after this heat treatment.
[0017]
【Example】
Steel having the composition shown in Table 1 is produced in a vacuum melting furnace, reheated to 1200 ° C after cooling and solidification, finish-rolled at 880 ° C, and held at 600 ° C for 1 hour after cooling. The hot rolling coil heat treatment was reproduced. The obtained hot-rolled sheet was removed from the scale by grinding and cold-rolled by 70%. Thereafter, using a continuous annealing simulator, annealing was performed at 770 ° C. for 60 seconds, cooled to 350 ° C., held at that temperature for 10 to 600 seconds, and further cooled to room temperature.
Tensile properties were evaluated by tensile in the L direction of a JIS No. 5 tensile test piece, and the product of TS (MPa) × EL (%) was 18000 MPa%. The metal structure was observed with an engineering microscope. Ferrite is nital etching. Martensite was observed by repeller etching.
[0018]
The chemical conversion treatment is performed by using a phosphate treatment chemical (Bt3080: manufactured by Nihon Parker Rising Co., Ltd.), which is a normal automobile chemical, and then the properties of the chemical conversion film are visually observed and a scanning electron microscope is used. In this case, “○” indicates that the steel sheet substrate is densely coated, and “×” indicates that the chemical conversion film has a partial defect.
As can be seen from the results of Tables 1 and 2, the steel sheet according to the present invention can produce a high-strength steel sheet having excellent chemical conversion properties and excellent balance between strength and ductility.
On the other hand, in the comparative example in which the component range in Table 1 deviates from the range of the present invention, and in the comparative example in which the Al range in Table 2 does not satisfy the formula (A), the value of TS × EL indicating the strength / ductility balance is 18000 MPa. % Or the chemical conversion property is x.
[Table 1]

[Table 2]

[0019]
【The invention's effect】
According to the present invention, the balance of Si, Al, and Ts is within a specific range, and in particular, by adjusting the amount of Al added, DP steel with a low yield stress can be secured with formability and chemical conversion processability that can ensure more elongation than before. It is possible to realize a high-strength steel sheet and a manufacturing method thereof excellent in the industrial scale on the industrial scale, and there are remarkable effects which are industrially useful.
[Brief description of the drawings]
FIG. 1 is a diagram showing the relationship between the mass% of Al and Si, target strength, and chemical conversion properties.

Claims (3)

% By mass
C: 0.01 to 0.30%
Si: 0.005-0.2%,
Mn: 0.1-2.2%
P: 0.001 to 0.06%,
S: 0.001 to 0.01%,
N: 0.0005 to 0.01%,
Al: 0.25 to 1.43 % , consisting of the remainder Fe and inevitable impurities,
Furthermore, the mass% of Si and Al and the target strength value (TS) satisfy the following formula (A):
further,
Mo: 0.05 ~ 0.5%,
V: 0.01 to 0.1%
Ti: 0.01-0.2%
Nb: 0.005 to 0.05%,
B: Contains one or more of 0.0005 to 0.002 % ,
A high-strength steel sheet excellent in formability and chemical conversion processability, characterized in that the metal structure contains ferrite and martensite.
(0.0012 × [TS target value] -0.29- [Si]) / 1.45 <Al <1.5-3 × [Si] (A)
Here, [TS target value] is the strength design value of the steel sheet, the unit is Mpa,
[Si] is the mass% of Si further,
Ca: 0.0005 to 0.005%,
REM: 1 type or 2 types are included among 0.0005-0.005%, The high-strength steel plate excellent in the formability and chemical conversion property of Claim 1 characterized by the above-mentioned. It is a manufacturing method of the high strength steel plate of Claim 1 or Claim 2 , Comprising: It heats to the temperature range of Ac1 or more and Ac3 +100 degrees C or less in an annealing process, After holding for 30 seconds or more and 30 minutes or less, it is temperature of 600 degrees C or less The manufacturing method of the high strength steel plate excellent in the formability and chemical conversion processability characterized by cooling to a region.

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