JP3370436B2 - Automotive steel sheet excellent in impact resistance and method of manufacturing the same - Google Patents

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION This invention is mainly used for automobile parts.
Do not use after processing such as press molding as a product etc.
Steel plates, especially in the event of a car crash
Suitable for use as a material for parts requiring excellent impact resistance
Related to automobile steel sheet and manufacturing method
is there. Recently, the momentum for global environmental conservation has increased
CO from a car as background ₂Reduction of emissions, ie
There is a demand for weight reduction of automobile bodies. Such weight reduction
One of the methods is to reduce the plate thickness by increasing the strength of the steel plate.
It is effective. Therefore, as a steel sheet for automobiles, this high strength
It has excellent properties in both press moldability and
Is desirable. Furthermore, based on the design concept of the car body
For example, in addition to simply strengthening the strength of the steel sheet, it may also impact during running.
Steel plate with excellent impact resistance when hit, that is, high strain rate
The development of steel sheets with high deformation resistance when deformed by
Lightening the car body while improving the safety of the car
It can be said that it will effectively contribute to the realization of the realization.

[0002]

2. Description of the Related Art Conventionally, in the strengthening of steel sheet for automobiles, solid solution strengthening by mainly adding substitutional elements such as Si, Mn and P in the ferrite single phase structure, or martensite phase and bainite phase in the ferrite phase. Alternatively, a method of strengthening the structure by precipitating an austenite phase is common. For example, in Japanese Unexamined Patent Publication No. 56-139654, ultra-low carbon steel (C ≦ 0.015 wt%) contains Nb to improve workability and aging property, and a strengthening component such as P is added to improve workability. We have proposed a steel sheet that is contained in an amount that does not harm the material to enhance its strength. In addition, for example, Japanese Patent Application Laid-Open No. 59-193221 proposes a method of further strengthening by adding a combination of B, Ti, and Nb to ultra-low carbon steel (C ≦ 0.005 wt%). Further, JP-A-60-52528 discloses a method for producing a high-strength steel excellent in ductility by annealing a low carbon steel at a high temperature and precipitating a martensite phase after cooling.

[0003]

However, even if the plate thickness of the automobile body can be reduced to some extent by increasing the strength of the steel plate by such a method, it has not been essentially improved. In other words, these proposals use yield strength or tensile strength, which is an index of steel plate strength, at a strain rate of 10 ^-3.
It is calculated based on only the so-called static evaluation method, which is extremely slow at ~ 10 ^-2 (s ^-1 ). However, in the actual design of the automobile body, rather than such static strength, with consideration of safety at the time of collision, the strain rate is 10 to 10 ⁴ (s ^-1 ) It is accompanied by shocking deformation, The strength based on the so-called dynamic evaluation method becomes more important. Therefore, the above-mentioned conventional proposals developed by focusing on only the static strength have a problem that they cannot be a fundamental index for weight reduction of an automobile body.

In the present invention, a steel sheet which has not been studied at all in the past and which has an excellent impact strength at a high strain rate, specifically, at a static-dynamic ratio = (strain rate 10 ² (s ^-1 )) Yield stress) / (strain rate
The yield stress at 10 ^-3 (s ^-1 )) is defined as the static-dynamic ratio, and the purpose is to develop a steel sheet that exhibits a predetermined value or more. According to the research conducted by the present inventors, this static-dynamic ratio is about 1.6 to 2.0 for mild steel plates, and the static-dynamic ratio decreases as the strength of the steel plate increases. Therefore, this static-dynamic ratio is 1.6
With the high-strength steel sheets described above, the strain rate dependence on strength is equal to or higher than that of mild steel sheets, and by using such high-strength steel sheets, it is easy to reduce the weight while improving the safety of the automobile body. Can be realized. Then, the main object of the present invention is to obtain a steel sheet for automobiles which is excellent in press formability and impact strength. Another object of the present invention is to
An object of the present invention is to obtain a steel sheet for automobiles that can achieve weight reduction accompanied by improvement in safety of an automobile body.

[0005]

Means for Solving the Problems As a result of intensive studies aimed at achieving the above-mentioned objects, the inventors have found that, with respect to the above static / dynamic ratio, the chemical composition, steel structure, hot rolling conditions and cold rolling It was found that the above-mentioned problems can be solved by appropriately adjusting the subsequent finish annealing conditions. That is, the present invention is: (1) C: 0.010 to 0.10 wt%, Si: 0.05 to 2.0 wt%, Mn:
0.50 to 3.00 wt%, P: 0.01 to 0.15 wt%, S: 0.01 wt%
Hereinafter, B: 0.001 to 0.01 wt% is contained, the balance is Fe and inevitable impurities, and the structure thereof is 95 to 60% by volume of ferrite phase and 5 to 5 by volume.
A steel sheet for automobiles, which is composed of 40% martensite phase and whose martensite phase has an average crystal grain size of 3 μm or less and is excellent in impact resistance. (2) In the above invention (1), the ratio of the ferrite phase and the martensite phase is 10 to 20 in the case of the martensite phase.
% Is the preferable range, and the average grain size of the martensite phase is 1.0 to 2.5 μm. (3) C: 0.010 to 0.10 wt%, Si: 0.05 to 2.0 wt%, Mn:
0.50 to 3.00 wt%, P: 0.01 to 0.15 wt%, S: 0.01 wt%
Hereinafter, a steel material containing B: 0.001 to 0.01 wt% and the balance of Fe and unavoidable impurities was hot-rolled, cold-rolled, and then finish-annealed at a temperature of 780 to 900 ° C. In the cooling process, cooling up to 650 ° C for 5-20
A method for manufacturing a steel sheet for automobiles having excellent impact resistance, which comprises cooling at a rate of ℃ / sec and further cooling from 650 ° C to 100 ° C at a cooling rate of 10 to 30 ° C / sec. (4) In the above invention (3), the finish annealing is 800 to 840 ° C.
More preferably, the upper cooling rate is 10 to 15 ° C./sec and the lower cooling rate is 17 to 25 ° C./sec in the temperature range.

[0006]

In order to improve the above-mentioned static-dynamic ratio of the steel sheet, the inventors have studied the metallurgical factors affecting the static-dynamic ratio based on Mn-containing low carbon steel, and have found that the chemical composition, microstructure and cold rolling. It was found that the post-finish annealing conditions had a strong effect. In particular, in the study of the steel structure, if a predetermined amount of martensite phase is precipitated to secure the static strength and the dislocation existing in the ferrite phase precipitated around this martensite phase is controlled, the static-dynamic ratio It was found that the improvement of This will be described in more detail below. Generally, in a two-phase structure steel composed of a ferrite phase and a martensite phase, when the martensite phase precipitates in a low temperature region in a manufacturing process, that is, a cooling process after hot rolling and winding or a cooling process after cold rolling and finish annealing. At the same time, it is known that dislocations are introduced into the ferrite phase around the martensite phase. Therefore, the inventors studied the relationship between the dislocation density of Dual Phase steel and the deformation behavior when the steel sheet is subjected to impact deformation. As a result, it was found that increasing the dislocation density in the ferrite phase increases the strength of the steel sheet when impact deformed. However, the dislocation density of about 10 ⁹ (cm ^-2 ) as obtained with the conventional Dual Phase steel is insufficient for effectively increasing the dynamic strength of the steel sheet,
Furthermore, it was found that dislocations had to be forcibly introduced. Based on such knowledge, the inventors have variously studied the method of introducing dislocations into the ferrite phase, and as a result, it is necessary to finely disperse and precipitate the martensite phase. As a result, the conclusion was reached that the addition of B was indispensable, and the heat cycle of finish annealing was required to be controlled, and the present invention was conceived.

In the present invention, the static-dynamic ratio used as an index of impact resistance characteristics has a critical point of 1.6, but as described above, this corresponds to the level of mild steel sheet having an excellent static-dynamic ratio. This is due to the fact.

(1) The reason why the chemical composition of steel is limited as described above in the present invention will be described below. C: 0.010 to 0.10 wt% When the content of C is less than 0.010 wt%, precipitation of martensite phase is small and sufficient strength cannot be obtained. Weldability deteriorates. Therefore, the C content is limited to the range of 0.010 to 0.10 wt%. More preferably, it is recommended to be 0.06 to 0.09 wt%. Si: 0.05 to 2.0 wt% If Si content exceeds 2.0 wt%, the static-dynamic ratio deteriorates. Therefore, the upper limit of the Si content is set to 2.0 wt%. If it is less than 0.05 wt%, sufficient strength of the ferrite phase cannot be obtained. Therefore, the lower limit of Si content is 0.05.
It was set to wt%. More preferable Si content is 0.5 to 0.9 wt%
Is recommended. Mn: 0.50 to 3.00 wt% When the content of Mn is less than 0.50 wt%, the precipitation of martensite phase is small and sufficient strength cannot be obtained. Since the dynamic ratio and the spot weldability are deteriorated, the Mn content is limited to the range of 0.50 to 3.00 wt%, more preferably 1.5 to 2.0 wt% is recommended. P: 0.01 to 0.15 wt% When the content of P is less than 0.01 wt%, precipitation of martensite phase is small and sufficient strength cannot be obtained, and when it exceeds 0.15 wt%, The static-dynamic ratio and spot weldability deteriorate, so the range was limited to 0.01 to 0.15 wt%. It is preferably 0.05 to 0.12 wt%. S: 0.010 wt% or less By reducing the content of S, the precipitates in the steel are reduced and the workability is improved. Such an effect can be obtained by setting the amount of S to be 0.010 wt% or less, and more preferably 0.0010 wt% or less.

B: 0.001 to 0.01 wt% B is a component that plays the most important role as an element that affects the refinement of the martensite phase in the present invention.
If this B content is less than 0.001 wt%, the effect of refining the martensite phase cannot be obtained. On the other hand, even if this amount exceeds 0.01 wt%, the martensite refining action is saturated and it is economically disadvantageous, so it is limited to 0.001 to 0.01 wt%. A more preferred range is 0.003 to 0.006 wt%.

(2) Next, in the automobile steel sheet according to the present invention, this steel structure is composed of a ferrite phase of 95 to 60% by volume and a martensite phase of 5 to 40% by volume. It must be a two-phase organization. It is particularly important to set the amount of martensite to 5-40%. That is, the reason for precipitating the martensite phase in a volume ratio of 5% or more is that if it is less than that, sufficient static strength and dynamic strength as an automobile material, particularly a required static-dynamic ratio cannot be obtained. Further, if the martensite phase exceeds 40%, the press formability is remarkably deteriorated, so the upper limit was limited to 40%. This range is more preferably 10 to 20%.

The steel sheet according to the present invention is characterized in that the martensite phase is refined (≦ 3 μm) to introduce dislocations into the ferrite phase. The introduction of dislocation density in this ferrite phase is 10 ¹⁰ (cm ^-2 ) to 10 ¹⁵
It should be about (cm ^-2 ). This is because if it is less than 10 ¹⁰ (cm ^-2 ), a sufficient effect of improving the dynamic strength cannot be obtained, while if the dislocation density exceeds 10 ¹⁵ (cm ^-2 ), the ductility of the steel sheet decreases. This is because the press formability deteriorates.

(3) Next, in the production of the steel sheet for automobiles according to the present invention, the finish annealing after cold rolling is heated to 780 to 900 ° C., and thereafter, the cooling in the upper temperature range up to 650 ° C. is performed to 5 to 5.
It is necessary to perform the cooling at a rate of 20 ° C / sec and then the cooling in the lower temperature range of 650 to 100 ° C at a rate of 10 to 30 ° C / sec. The reason for this will be described below. a. First, the heating temperature during finish annealing after cold rolling is limited to 780 ° C. or higher. If the heating temperature is lower than that, precipitation of austenite phase during annealing is insufficient and precipitation of martensite phase during cooling is insufficient. On the other hand, if the temperature exceeds 950 ° C, the crystal grains become coarse and the press formability deteriorates. The range of 800 to 840 ° C / sec is preferable. b. Also, during the cooling process in the upper temperature range during annealing,
Cooling up to 650 ℃ in the range of 5 to 20 ℃ / sec,
If it is less than 5 ° C / sec, the transformation of the austenite phase to the martensite phase is insufficient and sufficient strength and static-dynamic ratio of the steel sheet cannot be obtained. On the other hand, if it exceeds 20 ° C / sec, C, Mn, etc. to the austenite phase are obtained. In the same way, the concentration of the element in the steel sheet becomes insufficient, and the transformation to the martensite phase is also insufficient to obtain sufficient strength and static-dynamic ratio of the steel sheet. It was limited to the range of 20 ℃ / sec. Preferably 10 to
The range of 15 ℃ / sec is good. c. Furthermore, in the cooling process in the lower temperature range during annealing, cooling from 650 ° C to 100 ° C at a rate of 10 to 30 ° C / sec is necessary to reduce the precipitation of martensite phase below 10 ° C / sec. Static strength decreases, and if it exceeds 30 ℃ / sec, the concentration of solute C in the ferrite phase increases and the static-dynamic ratio decreases, so cooling in the temperature range from 650 ℃ to 100 ℃ is 10 to 30 ℃.
Limited to a speed of ° C / sec. Preferably 17-25 ° C / sec
The range is good.

The present invention can also impart the effect of improving the static-dynamic ratio to the surface-treated steel sheet obtained from the raw material having the above-mentioned composition and structure. The steel and method of the present invention are intended for automobile steel plates,
Needless to say, it is also effective for applications requiring strength at high strain rates.

[0014]

Example Steels having various chemical compositions were melted in a converter.
3mm ^t hot-rolled steel sheet was manufactured by hot rolling using these steels, and cold rolled 0.7 mm ^t cold-rolled steel sheet (No. 1 to 17) was manufactured by cold rolling and strained by tensile test. Speed 10 ^-3
The yield strengths at 10 and 10 ² were measured to determine the static-dynamic ratio, and the results are summarized in Table 1. The martensite volume ratio in the table was determined from the area ratio obtained by image-analyzing the microscopic structure of the steel plate cross section. Further, the average crystal grain size (R) of the martensite phase is obtained by measuring the area ratio and the number based on the above-mentioned image analysis and obtaining the average area by the following formula: R = 2 (A / π) ^1/2 I calculated. As is clear from the results shown in Table 1, according to the steels (Nos. 1 to 4) suitable for the present invention, it is possible to manufacture a steel sheet having an excellent static-dynamic ratio. On the other hand, in the comparative examples, when the composition is incompatible (No. 11 to 17), the steel structure is incompatible (No. 6, 8, 9, 10 to 11), the finish annealing conditions are incompatible (No. In both cases (5 to 10), the dynamic strength (YS: × 10 ² s ^-1 ) was low, and in many cases the static-dynamic ratio remained small.

[0015]

[Table 1]

[0016]

As described above, according to the present invention, by optimizing the component composition and steel structure of the steel sheet,
It is possible to manufacture steel sheets with a static-dynamic ratio that is far superior to conventional ones, and by using these steel sheets for automobiles,
The weight and safety of the car body can be improved. Further, such a steel sheet can be reliably manufactured only by appropriately controlling the annealing conditions after cold rolling.

Continuation of the front page (72) Inventor Toshiyuki Kato 1 Kawasaki-cho, Chuo-ku, Chiba, Chiba Prefecture Inside the Technical Research Division, Kawasaki Steel Co., Ltd. (58) Fields investigated (Int.Cl. ⁷ , DB name) C22C 38/00 C21D 8/02 C21D 9/46

Claims (2)

(57) [Claims] 1. C: 0.010 to 0.10 wt%, Si: 0.05 to 2.0 wt
%, Mn: 0.50 to 3.00 wt%, P: 0.01 to 0.15 wt%, S: 0.01 wt% or less, B: 0.001 to 0.01 wt%, and has a composition of the balance Fe and unavoidable impurities. Moreover, its structure is composed of a ferrite phase of 95 to 60% by volume and a martensite phase of 5 to 40% by volume, and this martensite phase has an average grain size of 3
Steel sheet for automobiles with excellent impact resistance of less than μm. 2. C: 0.010 to 0.10 wt%, Si: 0.05 to 2.0 wt
%, Mn: 0.50 to 3.00 wt%, P: 0.01 to 0.15 wt%, S: 0.01 wt% or less, B: 0.001 to 0.01 wt%, and the remaining Fe and inevitable impurities are hot-rolled. Then, after cold rolling, finish annealing is performed at a temperature of 780 to 900 ℃, and in the cooling process, cooling to 650 ℃ is performed at a speed of 5 to 20 ℃ / sec.
A method for producing a steel sheet for automobiles having excellent impact resistance, which comprises cooling to 100 ° C at a cooling rate of 10 to 30 ° C / sec.