JP4214330B2 - Steel sheet excellent in formability and hardenability and manufacturing method thereof - Google Patents

Description

【００３６】
得られた熱延板は、表面研削して冷延板と同じ1.2mm厚さにして、試験に供した。出力200ｋＷ、周波数150ｋＨzの高周波焼入れ装置により、各鋼板から切り出した幅90mm、長さ240mmの試験片にて、加熱温度950℃、２秒保持後水冷の条件にて焼入れ処理をおこなった。引張り試験はＪＩＳ５号試験片とし、焼入れ後硬さは板厚中心部にて測定した。
【００３７】
熱延鋼板による結果を表２に、冷延鋼板による結果を表３にそれぞれ示す。熱延鋼板と、冷延鋼板とでは、結果のレベルに差はあるが、鋼組成のおよぼす効果は相対的にほぼ同じであった。試番1〜4および25〜28の結果から、Ｍｎは焼入れ前の素材の状態では含有量増加とともに伸びが低下し加工性が悪くなるが、焼入れ後の硬さや強度は上昇する。本発明範囲内とすれば、加工前の伸びは十分大きく、焼入れ後の強度も大きいことがわかる。Ｔｉ含有量の、熱処理前の鋼板の伸びにおよぼす影響については、試番5〜9および29〜33の結果からわかるように、Ｔｉ量が増すにつれて伸びが低下して加工性が悪くなっている。Ｍｎ量とＮ量がほぼ同一である他の基準組成の試片による結果もあわせて、横軸にＴｉ（％）／Ｎ（％）をとり、熱処理前鋼板の伸びを整理してみると図１のようになる。Ｔｉ（％）／Ｎ（％）が3.4以上、すなわちＴｉがすべてＴｉＮになってしまう量より過剰のＴｉが含有されると伸びが悪くなることがわかる。Ｂの影響は試番10〜12および34〜36にて見られるが、0.0003％を下回ると十分な強度が得られず、焼入れ性は不足となる。Ａｌ量とＮ量を変えた試番13〜20、および37〜44の結果と、Ｍｎ、Ｔｉ、Ｂなどの量がほぼ同一である試片による結果とを合わせ、Ａｌ（％）／Ｎ（％）に対する焼入れ後の引張強さを見ると図２のようになる。ＡｌとＮの原子量比は1.93であるが、この原子量比の５倍、すなわちＡｌ（％）／Ｎ（％）にて9.6を下回る場合、引張強さが低く、焼入れ性不十分となっている。
【００３８】
【表２】

【００３９】
【表３】

【００４０】
試番21〜24の熱延鋼板は、鋼の化学組成は本発明の範囲内であるが、他の鋼板の熱間圧延の巻取り温度650℃であるのに対し、550℃と低い。同じ組成の鋼同志を比較してみると、図３左側に示すように、この550℃で巻取った熱延鋼板は、650℃の巻取りに比較し、焼入れ後の強度が不十分である。しかし、冷間圧延後焼鈍する試番45〜48の冷延鋼板では、熱延の巻取り温度がやや低めであっても、焼入れ後の強度は十分大きいことがわかる。
【００４１】
【発明の効果】
本発明によれば、プレス成形などの加工に適用できる十分な成形性を有し、かつ成形後の焼入れによって、容易に高強度化できる鋼板を得ることができる。この鋼板を例えば自動車の車体構造に活用することにより、車室構造をより強度が高く、しかも軽量にすることが可能となる。これは、自動車の衝突事故の際の搭乗者の安全性を高め、かつ自動車の軽量化による省エネルギーなど、社会的要求に合致するものである。
【図面の簡単な説明】
【図１】鋼のＴｉ含有量とＮ含有量との比と、鋼板の伸びとの関係を示す図である。
【図２】Ｂを含む鋼のＡｌおよびＮ含有量と、高周波焼入れ後のその鋼板の引張強さとの関係を示す図である。
【図３】熱間圧延の巻取り温度と、得られた鋼板を高周波焼入れした場合の引張強さとの関係を示す図である。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a steel sheet having formability and hardenability used for a material such as a vehicle body structural member of an automobile and a method for manufacturing the same.
[0002]
[Prior art]
The need for securing passenger safety has been increasing year by year, such as data on car body destruction in the event of a car crash, which controls the sales of cars, and legal regulations tend to be stricter. . The car body has a so-called crushable body structure that is partly deformed and absorbs shocks in the event of a collision, while the passenger compartment is designed to protect the passengers in the room with a high-strength structure. The That is, high strength is required depending on the part of the vehicle body. For this reason, a molded product with a large plate thickness is used for a necessary portion, a reinforcing material is attached, or a high strength steel plate is used.
[0003]
However, increasing the thickness leads to an increase in weight, which contradicts the demand for improved fuel consumption or energy saving. For reinforcement, parts for spot welding of rib material and shock absorption are incorporated, but this also increases the weight of the vehicle body. In reality, it is widely used to incorporate reinforcements made of steel pipes and hardened and hardened inside the doors of vehicles. In addition, high-strength steel sheets are difficult to press complexly due to poor press formability, and the residual stress due to processing increases as the strength increases, increasing the risk of delayed fracture. There is a limit to increasing the strength of steel sheets.
[0004]
For reinforcement of this body structure, the thickness of a steel sheet molded part applied to a specific part of the vehicle body that requires strength is not increased, or high strength steel sheet is not used, and the desired part is locally heated at high frequency. An invention of a method for improving the strength by quenching is disclosed in JP-A-61-163030. If the steel is hardened and strengthened after press forming, problems such as difficulty in forming a complicated shape and large residual stress after forming as in the case of forming a high-strength steel sheet can be eliminated. However, although the above-mentioned publication shows a method of induction hardening, there is no description about the type of steel sheet to be applied. Some steel sheets do not show an improvement in strength by quenching, and in particular, steel sheets with good press formability cannot be expected to be hardened.
[0005]
Japanese Patent Laid-Open No. 10-17933 discloses an invention of a method for realizing a hardness distribution corresponding to a strength distribution required for a press-formed part of a vehicle body by local induction hardening. As an application example thereof, a column called a center pillar, which is disposed between the front seat and the rear seat on the side of the vehicle body, is taken up. The load distribution expected at the time of a column collision is at the center of the length, and in order to cope with the collision load, the conventional method requires increasing the thickness of the entire column or welding with a reinforcing material over the center. It was. On the other hand, by strengthening the central portion of the length by induction hardening, the plate thickness is not increased to an unnecessary portion, a reinforcing material is not required, and the weight can be reduced. In this case, the material steel plate for column forming can be applied with a C content of 0.05 to 0.25%, and steel plates such as SAPH370 to SAPH440 or SPFC370 to SPFC440 of JIS standards are good. However, although the steel plate according to these standards guarantees the mechanical properties of the steel plate as a material, such as strength and elongation, the hardenability and strength after quenching are usually completely unknown.
[0006]
Although it is not a vehicle body structural member, a part obtained by processing a steel pipe that has been hardened and has a required strength is widely used as a reinforcing material incorporated in the interior of the door for protecting the interior of the vehicle interior against a side collision. In many cases, the end of an ERW steel pipe made of steel plate is processed into a state suitable for attachment to the inside of the door, and is induction-hardened. For example, in JP-A-4-52249, C: 0.10 to 0.20, Mn: 0.2 to 1.50%, Si: 0.05 to 0.50, Al: 0.01 to 0.10%, Ti: 0.01 to 0.10% and B: 0.0005 to 0.010% There has been proposed an invention for an automobile door reinforcing material having a tensile strength of 130 kgf / mm ² or more obtained by induction-quenching an ERW steel pipe made of a steel plate contained therein. If steel plates like this are used for vehicle body structural members, it seems possible to improve the local strength by induction heating and quenching as described above, but even if simple bending work such as ERW steel pipes is possible Therefore, it is considered difficult to apply to complicated press forming such as a vehicle body structure.
[0007]
An object of the present invention has sufficient formability that can be applied to a component body structure, and to provide a steel plate and a manufacturing method capable of easily increasing the strength by quenching after molding.
[0008]
[Means for Solving the Problems]
The present inventors have, after molding, by performing quenching, without significantly changing the strength of the shape and application materials member, conducted a survey study of the steel plate that can enhance the structure of the automobile body It was. The steel plate used in quenching and tempering, is polished steel strip of JIS-G3311. In this case, even the steel with the lowest carbon content excluding alloy steel is C: 0.27 to 0.33%, Si: 0.15 to 0.35%, Mn: 0.60 to 0.90%, and the contents of all of C, Si and Mn are high, In this case, even if sufficient strength is obtained by quenching, the workability as an annealed material steel plate is poor, and the required press workability cannot be obtained.
[0009]
Here, the strength required for strengthening the vehicle body structure is about 80 to 180 kgf / mm ² in consideration of the performance and economy as steel, and the carbon content is 0.05 to 0.20% if sufficient quenching is performed. realizable. By reducing the carbon content, it becomes easy to ensure the workability of the steel sheet, and when the carbon content is reduced to this level, it has a self-tempering effect and is expected to have sufficient toughness even when quenched. it can. Therefore, investigation was made on alloy components to realize sufficient quenching for this carbon content.
[0010]
In general, an alloying element that improves hardenability hardens steel and decreases workability as the amount added increases. However, since the steel plate used in materials such as body structure member of an automobile is subject, can also increase the cooling rate to the center of the steel, that amount can lower the content of the hardenability improving element It is. On the other hand, the steel plate for automobiles, because of corrosion, must have good chemical conversion treatability of the plating properties and paint undercoat, the use of hardenability improving elements such as Cr and Si inhibit these preferably Absent. Moreover, since cost is strictly required, expensive additive elements should be avoided.
[0011]
As a result of various studies from such a viewpoint, it was determined that it is best to effectively use Mn and B. Mn is inevitably added to suppress hot brittleness of the steel, but it has the effect of greatly improving the hardenability, and at a low cost, has little influence on the plating properties and chemical conversion properties. However, if added in a large amount, the workability is deteriorated and the corrosion resistance as steel tends to be deteriorated, so the addition amount is limited. In order to compensate for this, B was added together. Since the effect of B is exhibited by addition of a trace amount, the hardenability of the steel can be greatly improved without substantially affecting other general properties of the steel. However, in order to improve the hardenability by adding B, it is necessary to keep a small amount of B added in a solid solution state, and since it is easily oxidized, steel is sufficiently deoxidized, and N that is easy to bond with B is Must be fixed. For this reason, deoxidation is generally performed until acid-soluble Al (sol. Al) is detected in the steel, and Ti is added to fix the solid solution N as Ti nitride.
[0012]
At this Mn and B and the added steel and, when fabricated steel plate proceeded application study to actual parts, that there is a lack in workability as a material has a problem. As a result of further investigations to improve this workability, it was found that Ti added for nitrogen fixation had an adverse effect. That is, in order to sufficiently fix N, Ti more than that in which all N in the steel becomes TiN was contained, but this excessive Ti was precipitated as TiC in the steel, which significantly deteriorated the elongation. It was. Therefore, the addition of Ti for fixing N is the minimum necessary, and N that could not be fixed with Ti can be improved in workability and hardenability can be secured by fixing it as AlN by sufficiently containing Al. It was revealed.
[0013]
Further in the process of advancing the investigation, when thus limiting the Ti content, the coiling temperature of hot rolling of the steel plate manufacturing process, became also clear that a significant effect on hardenability. When quenching, sufficient quenching hardness is often not obtained, and as a result of investigating the cause, this phenomenon appears when the hot rolling coiling temperature is low. Therefore, the winding temperature may be sufficiently increased to improve the hardenability. However, the lower limit temperature, and when used as hot-rolled steel plate, unlike in the case of using as a cold rolled steel sheet, may lower in the cold-rolled steel sheet.
[0014]
The reason for this is considered that the amount of the solid solution B effective for improving the hardenability varies depending on the coiling temperature. When the temperature of N in steel decreases, if Al or B is present, stable precipitates such as AlN and BN are formed. However, when these two elements coexist, since B has a diffusion rate in steel much higher than that of Al, BN is first formed in the cooling process after rolling. However, since AlN is more stable, when sufficient Al is present, when the coiling temperature is high and it is cooled slowly, this BN decomposes and all N precipitates as AlN, so that solid solution B is formed. Remains. Therefore, if the coiling temperature is high, the cooling is slow, so that N in the steel is fixed as AlN. When the coiling temperature is lowered, the cooling is accelerated and N is cooled in the state of BN before sufficient AlN is formed. As a result, the solid solution B decreases.
[0015]
In a hot-rolled steel sheet, when the coiling temperature is lowered in this way, the hardenability tends to decrease. In cold-rolled steel sheets, even when the coiling temperature is low during hot rolling and N is BN, it is reheated in the annealing process after cold rolling, so if Al is sufficiently contained, the formation of AlN Advances and B enters a solid solution state. For this reason, sufficient hardenability can be ensured even if the coiling temperature is lower.
[0016]
Based on the above examination results, the limit of each condition was further clarified and the present invention was completed. The gist of the present invention is as follows.
[0017]
(1) When the composition of steel is mass %, C: 0.10 to 0.20%, Si: 0.1% or less, Mn: 0.8 to 2.0%, P: 0.02% or less, S: 0.02% or less, N: 0.005% or less, B: 0.0003-0.004%, sol.Al: 0.01-0.10%, and
sol.Al (%) ≧ 9.6 × N (%) (1)
Because
Ti is Ti (%) ≦ 3.4 × N (%) (2)
Contained in a range of, balance moldability and hardenability in good steel plate, characterized in that it consists of Fe and unavoidable impurities.
[0018]
(2) By mass%, C: 0.05 to 0.20 %, Si: 0.1 % or less, Mn: 0.8 to 2.0 %, P: 0.02 % or less, S: 0.02 % or less, N: 0.005 % or less, B: 0.0003 to 0.004% sol Al:. in from 0.01 to 0.10%, and
sol. Al ( % ) ≥ 9.6 × N ( % ) ... (1)
Because
Ti
Ti ( % ) ≤ 3.4 × N ( % ) (2)
Contained in a range of, balance hot rolled steel slab consisting of Fe and unavoidable impurities, the steel plate excellent in formability and hardenability, characterized in that the hot-rolled coil at coiling temperature 600 ° C. or higher Manufacturing method.
[0019]
(3) In mass%, C: 0.10 to 0.20 %, Si: 0.1 % or less, Mn: 0.8 to 2.0 %, P: 0.02 % or less, S: 0.02 % or less, N: 0.005 % or less, B: 0.0003 to 0.004% sol Al:. in from 0.01 to 0.10%, and
sol. Al ( % ) ≥ 9.6 × N ( % ) ... (1)
Because
Ti
Ti ( % ) ≤ 3.4 × N ( % ) (2)
Contained in a range of, balance hot rolling a steel slab consisting of Fe and unavoidable impurities, after the hot-rolled coil as coiling temperature 480 ° C. or higher, and cold rolling, characterized by annealing the molded method for producing a steel plate excellent in sex and hardenability.
[0020]
DETAILED DESCRIPTION OF THE INVENTION
The reason why the steel composition or production conditions are limited in the present invention will be described.
[0021]
The C content is 0.05 to 0.20%. C is an important element for determining the strength of steel after quenching. If the C content is less than 0.05%, the strength required for strengthening the vehicle body strength cannot be obtained even if sufficient quenching is performed. On the other hand, if the content exceeds 0.20%, the press formability becomes insufficient. Therefore, the content of C is set to 0.05 to 0.20%, but it is preferable to set the content of C to 0.10 to 0.15% in order to easily achieve both good workability and sufficient strength after quenching.
[0022]
Si is 0.1% or less. Si is generally used for deoxidation of steel and has the effect of increasing strength and improving hardenability. However, when B is added, deoxidation with Si is not sufficient, and since Al having a stronger deoxidation action is used, it is not particularly necessary. For steel plate used in the automobile body than, but plating or painting is carried out, since the presence of Si deteriorating the chemical conversion of the plating properties and painting underlying steel surface, its content is as small as possible is preferable. Therefore, the range that hardly affects is 0.1% or less, preferably 0.04% or less, and more preferably 0.01% or less.
[0023]
The Mn content is 0.8 to 2.0%. It is an important element for ensuring the hardenability of steel. If it is less than 0.8%, the hardenability is insufficient, and sufficient strength may not be obtained. On the other hand, if the content exceeds 2.0%, the workability of the steel such as a decrease in elongation is deteriorated. Moreover, the steel itself is easily rusted and the corrosion resistance is deteriorated. Preferred is 0.8 to 1.5%.
[0024]
P is one of the inevitable impurities of steel and deteriorates the toughness of the steel after quenching. Therefore, the smaller the content, the better. The range that does not have a significant adverse effect is limited to 0.02% or less, but preferably 0.005% or less.
[0025]
S is one of the inevitable impurities, and deteriorates the workability and toughness of steel, so the smaller the content, the better. As in the case of P, the range that does not exert a significant adverse effect is limited to 0.02% or less, but is preferably 0.005% or less.
[0026]
N is 0.005% or less. N combines with B and inhibits the effect of improving the hardenability of B. In order to suppress this adverse effect of N, it is made harmless by combining with Ti or Al. However, the resulting fine precipitates such as TiN and AlN deteriorate the workability of steel, so the content of N is small if it is small. The more preferable. The N content is limited to 0.005% or less as a range that does not show a noticeable influence.
[0027]
The B content is 0.0003 to 0.004%. B significantly improves the hardenability, and the effect is particularly great as the content of C is lower as in the present invention. The effect of improving hardenability is almost constant without depending on the content, but when it is less than 0.0003%, the effect is not sufficiently exhibited. Also, if the content increases, the steel becomes brittle, so at most 0.004%.
[0028]
The content of sol.Al (acid-soluble Al) is 0.01 to 0.10%. Al is added mainly for the purpose of deoxidizing molten steel during steelmaking. If the sol.Al remaining in the steel by the deoxidation treatment is less than 0.01%, the deoxidation is insufficient and there is a possibility that the solid solution B does not remain due to oxidation. In the case of the present invention, it is necessary to contain at least 0.01% of sol.Al in order to sufficiently fix N. However, even if it is contained in a large amount, the effect is saturated and only the cost is increased by meaningless addition, so the upper limit is made 0.10%.
[0029]
In order to suppress inhibition by N of the effect of adding B, Al is sufficiently contained. For this reason,
sol.Al (%) ≧ 9.6 × N (%) (1)
It is necessary to contain sol.Al that satisfies the above. This means that sol.Al is contained in an amount of 5 times or more with respect to the chemical equivalent of Al necessary to make all N in the steel AlN. Thus, by containing a sufficient amount of Al with respect to N, N can be completely rendered harmless.
[0030]
Ti does not have to be added,
Ti (%) ≤ 3.4 x N (%) (2)
Is within a range that satisfies the above. Ti is extremely effective for fixing N. In general, when B is added, it is added for the purpose of fixing N. In the case of the present invention, even if Ti is not added, N is fixed as long as the Al content satisfies the above formula (1) . However, addition of a small amount of Ti is preferable in order to reduce the influence of manufacturing conditions such as hot rolling, to securely fix N, and to fully exhibit the effect of B addition. However, if a small amount is added, the workability deteriorates, and it is considered that this workability deterioration is caused by the formation of TiC. Therefore, the addition of Ti is limited to the range in which TiC is not formed, that is, the above formula (2) is satisfied. Within range. Further, since TiN increases in terms of workability deterioration, the Ti content is preferably 0.01% or less, even within the range of the formula (2) . In addition, when it contains for the purpose of fixing N, it is desirable to make it contain at least 0.002% or more.
[0031]
This steel is melted to form a slab, and the process and conditions for hot rolling the steel may be as usual, and are not particularly restricted. However, the coiling temperature of hot rolling, when subjected to press molding as a steel plate by hot rolling, the coiling temperature of hot rolling 600 ° C. or higher, preferably to 650 ° C. or higher. In the wound at a temperature below 600 ° C., not only poor press workability such as poor high elongation yield point as hot-rolled steel plate, because curing by quenching after molding, or the hardenability becomes insufficient is there. This is because B in the hot-rolled steel sheet remains as BN and does not effectively work for improving the hardenability. Note that the winding temperature in this case is desirably at most about 700 ° C. If winding is performed at a temperature higher than this, the shape deteriorates due to deformation of the winding coil, and the scale increases. Problems such as deterioration of the surface properties due to the occurrence of.
[0032]
In addition, when hot-rolling and further cold-rolling to be used for press forming as a cold-rolled steel sheet, the hot rolling-up temperature is 480 ° C. or higher, preferably 600 ° C. or higher. This is because winding at less than 480 ° C. may be quenched by cooling to the winding temperature, making cold rolling difficult. In the case of a cold-rolled steel sheet, even if the coiling temperature is somewhat low and B is BN, N becomes AlN and becomes solid solution B by reheating in the annealing process. The impact is small.
[0033]
Cold rolling and annealing may be performed in accordance with the production of a normal cold rolled steel sheet, the rolling reduction of cold rolling is 50% or more, and annealing is performed in a temperature range from the recrystallization temperature to less than Ac ₃ points. Even a conventional box annealing method may be performed by a continuous annealing method.
[0034]
【Example】
Steels having the compositions shown in Table 1 were melted using a laboratory vacuum melting furnace. As the steel composition, C: 0.12%, Mn: 1.5%, sol.Al:0.045%, N: 0.003%, Ti: 0.005%, B: 0.0006% are set as target reference compositions, and the effect of the amount of components is observed. For each of Mn, Ti, B, Al and N, the other components were changed to be constant. These ingots are forged into 20mm thick hot rolling slabs, heated to 1200 ° C, hot rolled in the temperature range from 1150 ° C to 900 ° C, finished to 4mm thickness, forced air cooling or water spray Cooled to 500-650 ° C under the cooling conditions assuming a hot strip mill by, and immediately put the obtained steel plate into a furnace maintained at these temperatures, and cooled under the conditions assuming the coil after winding did. Some of these hot-rolled steel sheets were cold-rolled after descaling the surface, finished to a thickness of 1.2 mm, and annealed at 700 ° C. for 20 hours of box annealing.
[0035]
[Table 1]

[0036]
The obtained hot rolled sheet was subjected to surface grinding to the same 1.2 mm thickness as the cold rolled sheet and used for the test. A test piece having a width of 90 mm and a length of 240 mm cut out from each steel plate was subjected to a quenching treatment under a water cooling condition after holding at a heating temperature of 950 ° C. for 2 seconds by an induction hardening apparatus having an output of 200 kW and a frequency of 150 kHz. The tensile test was a JIS No. 5 test piece, and the hardness after quenching was measured at the center of the plate thickness.
[0037]
Table 2 shows the results of the hot-rolled steel sheet, and Table 3 shows the results of the cold-rolled steel sheet. Although there was a difference in the level of results between the hot-rolled steel sheet and the cold-rolled steel sheet, the effect exerted by the steel composition was relatively similar. From the results of trial Nos. 1 to 4 and 25 to 28, in the state of the material before quenching, the elongation decreases as the content increases and the workability deteriorates, but the hardness and strength after quenching increase. If it is within the range of the present invention, it can be seen that the elongation before processing is sufficiently large and the strength after quenching is also large. Regarding the effect of the Ti content on the elongation of the steel sheet before heat treatment, as can be seen from the results of trial numbers 5-9 and 29-33, the elongation decreases as the Ti content increases, and the workability deteriorates. . Combined with the results of other reference composition specimens with almost the same amount of Mn and N, the horizontal axis is Ti (%) / N (%), and the elongation of the steel sheet before heat treatment is organized. It becomes like 1. It can be seen that when Ti (%) / N (%) is 3.4 or more, that is, when excessive Ti is contained in an amount exceeding the amount that Ti becomes TiN, the elongation becomes worse. Although the influence of B is seen in the trial numbers 10-12 and 34-36, if it is less than 0.0003%, sufficient strength cannot be obtained, and the hardenability becomes insufficient. The results of trial numbers 13-20 and 37-44 with different amounts of Al and N were combined with the results of specimens with almost the same amount of Mn, Ti, B, etc., and Al (%) / N ( %), The tensile strength after quenching is as shown in FIG. The atomic weight ratio of Al and N is 1.93, but if this atomic weight ratio is 5 times, that is, less than 9.6 at Al (%) / N (%), the tensile strength is low and the hardenability is insufficient. .
[0038]
[Table 2]

[0039]
[Table 3]

[0040]
The hot rolled steel sheets of trial Nos. 21 to 24 have a chemical composition of steel within the scope of the present invention, but are as low as 550 ° C. compared to the coiling temperature of hot rolling of other steel sheets of 650 ° C. Comparing steels of the same composition, as shown on the left side of FIG. 3, the hot-rolled steel sheet wound at 550 ° C. has insufficient strength after quenching compared to 650 ° C. winding. . However, it can be seen that the cold-rolled steel sheets of Nos. 45 to 48, which are annealed after cold rolling, have sufficiently high strength after quenching even if the hot rolling coiling temperature is slightly lower.
[0041]
【The invention's effect】
According to the present invention, it is possible to obtain a steel sheet that has sufficient formability that can be applied to processing such as press forming and that can be easily increased in strength by quenching after forming. By utilizing this steel plate, for example, in a vehicle body structure, it is possible to make the passenger compartment structure stronger and lighter. This enhances the safety of passengers in the event of a car collision and meets social demands such as energy saving by reducing the weight of the car.
[Brief description of the drawings]
FIG. 1 is a graph showing the relationship between the ratio of Ti content and N content of steel and the elongation of a steel sheet.
FIG. 2 is a graph showing the relationship between the Al and N contents of steel containing B and the tensile strength of the steel sheet after induction hardening.
FIG. 3 is a diagram showing the relationship between the hot rolling coiling temperature and the tensile strength when the obtained steel sheet is induction hardened.

Claims (3)

When the steel composition is mass %, C: 0.10 to 0.20%, Si: 0.1% or less, Mn: 0.8 to 2.0%, P: 0.02% or less, S: 0.02% or less, N: 0.005% or less, B: 0.0003 -0.004%, sol.Al:0.01-0.10%, and
sol.Al (%) ≧ 9.6 × N (%) (1)
Because
Ti is Ti (%) ≦ 3.4 × N (%) (2)
Contained in a range of, balance moldability and hardenability in good steel plate, characterized in that it consists of Fe and unavoidable impurities. In mass%, C: 0.05 to 0.20 %, Si: 0.1 % or less, Mn: 0.8 to 2.0 %, P: 0.02 % or less, S: 0.02 % or less, N: 0.005 % or less, B: 0.0003 to 0.004 %, sol. Al: 0.01 to 0.10 %, and
sol. Al ( % ) ≥ 9.6 × N ( % ) ... (1)
Because
Ti
Ti ( % ) ≤ 3.4 × N ( % ) (2)
Contained in a range of, balance hot rolled steel slab consisting of Fe and unavoidable impurities, the steel plate excellent in formability and hardenability, characterized in that the hot-rolled coil at coiling temperature 600 ° C. or higher Manufacturing method. In mass%, C: 0.10 to 0.20 %, Si: 0.1 % or less, Mn: 0.8 to 2.0 %, P: 0.02 % or less, S: 0.02 % or less, N: 0.005 % or less, B: 0.0003 to 0.004 %, sol. Al: 0.01 to 0.10 %, and
sol. Al ( % ) ≥ 9.6 × N ( % ) ... (1)
Because
Ti
Ti ( % ) ≤ 3.4 × N ( % ) (2)
Contained in a range of, balance hot rolling a steel slab consisting of Fe and unavoidable impurities, after the hot-rolled coil as coiling temperature 480 ° C. or higher, and cold rolling, characterized by annealing the molded method for producing a steel plate excellent in sex and hardenability.

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