JPH0573802B2 - - Google Patents
Info
- Publication number
- JPH0573802B2 JPH0573802B2 JP60233896A JP23389685A JPH0573802B2 JP H0573802 B2 JPH0573802 B2 JP H0573802B2 JP 60233896 A JP60233896 A JP 60233896A JP 23389685 A JP23389685 A JP 23389685A JP H0573802 B2 JPH0573802 B2 JP H0573802B2
- Authority
- JP
- Japan
- Prior art keywords
- temperature
- hot
- slab
- strength
- rolled steel
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Lifetime
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Classifications
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B1/00—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations
- B21B1/46—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling metal immediately subsequent to continuous casting
- B21B1/466—Metal-rolling methods or mills for making semi-finished products of solid or profiled cross-section; Sequence of operations in milling trains; Layout of rolling-mill plant, e.g. grouping of stands; Succession of passes or of sectional pass alternations for rolling metal immediately subsequent to continuous casting in a non-continuous process, i.e. the cast being cut before rolling
-
- B—PERFORMING OPERATIONS; TRANSPORTING
- B21—MECHANICAL METAL-WORKING WITHOUT ESSENTIALLY REMOVING MATERIAL; PUNCHING METAL
- B21B—ROLLING OF METAL
- B21B3/00—Rolling materials of special alloys so far as the composition of the alloy requires or permits special rolling methods or sequences ; Rolling of aluminium, copper, zinc or other non-ferrous metals
- B21B3/02—Rolling special iron alloys, e.g. stainless steel
Landscapes
- Engineering & Computer Science (AREA)
- Mechanical Engineering (AREA)
- Metal Rolling (AREA)
- Heat Treatment Of Steel (AREA)
- Heat Treatment Of Sheet Steel (AREA)
Description
［発明の利用分野］
本発明は高強度熱延鋼板の製造法に関する。
［発明の背景］
近年自動車業界においては、車体の軽量化のた
め、設計強度を変更しないで板厚を薄くし得るこ
とが要望されているが、従来の析出硬化型の高張
力鋼板では、プレス成形性が良くないこと、溶接
性にも問題があること等からかかる要望に答える
ことができない。
そこで、従来の析出硬化型高張力鋼板に代わる
鋼板として、フエライトとマルテンサイトの２相
からなる複合組織型高張力鋼板の採用が増加しつ
つある。
しかし、かかる複合組織型鋼板も加工性などの
点において必ずしの好ましいものではない。
そこで、さらに、Mnを基本成分とし、Si、Cr
を多量に添加することにより、熱間圧延工程−巻
取工程を経た後においても低降伏比、良延性とい
う特性をもつ複合組織型鋼板が開発されている。
しかし、かかる複合組織型鋼板はSi、Crを大
量に使用するためコストが高いという問題点があ
る。
ところで、従来方法においては、熱延鋼板は、
普通造塊法による鋼塊を分塊して造られたスラブ
又は連続鋳造法により造られたスラブを、一旦常
温にまで冷却して、その後加熱炉にて1200〜1300
℃の高温で長時間の加熱を行なつてから連続熱間
圧延機に噛込ませて製造している。
しかし、このように1200℃以上の高温長時間加
熱をスラブに施こすことは加熱量の莫大な損失と
なる。
そこで高強度熱延鋼板を、低コストで、高能率
で製造する方法を見出すべき、鋼材成分組成及び
圧延処理手段の両面から各種試験・調査を繰り返
しながら研究を重ねた結果本発明をなすにいたつ
た。
［発明の目的］
本発明は、加工性が良く、コストの低い熱延鋼
板を低加熱費で製造することができる高強度熱延
鋼製造法を提供することを目的とする。
［発明の概要］
上記目的は、重量％で、Ｃ：0.03〜0.2％、Si：
0.02〜1.5％、Mn：0.6〜2.5％、Cr：0.3〜1.5％、
Ｓ：0.01％以下、solAl：0.01〜0.06％、残部鉄及
び不可避的不純物からなるベイナイトを含む変態
強化型の高強度熱延鋼板を製造する方法におい
て、凝固時冷却速度を70℃／分以上で連続鋳造を
行ない高温スラブを得た後、該スラブが550℃の
温度になる前に、該スラブを1050℃以上の温度に
再加熱した後に、熱間圧延を開始し、該熱間圧延
をAr3点以上の温度で終了し、次いで所定の制御
冷却を行なうことを特徴とする高強度熱延鋼板の
製造法によつて達成される。
以下に本発明の構成を説明する。
Ｃ：0.03〜0.2％、
Ｃは、必要な強度維持及びベイナイト、マルテ
ンサイトなどの低温変態生成物を形成させるうえ
で必須な元素であるが、0.2％を越えると加工性
と溶接性を劣化することに加え、本発明の鋼板の
特徴の一つである低降伏比特性を損なうこととな
る。その下限は強化及び焼入性向上効果を発揮さ
せるために0.03％とする。
Si：0.2〜1.5％
Siは溶鋼の脱酸に必要な元素であり、また高強
度かつ高延性をうるうえでもつとも有効な置換型
固溶元素である。さらに正常なポリゴナルフエラ
イト形成を有利にする働きをもつている。このよ
うな特性を発揮させるためには0.2％を下限とし
た。また、溶接部の脆化（遷移温度の上昇）を防
止し、表面酸化スケール状態の悪化を防ぐために
1.5％を上限とした。
Mn：0.6〜2.5％
Mnは焼入性を増し、所望の組織をうるうえで
必須の元素である。その効果を発揮させるために
は0.6％以上を必要とし、2.5％を越えると、溶接
上困難になると同時に延性を劣化し、鋼板の価格
が高価格となるため上限を2.5％とする。
Cr：0.3〜1.5％
Crは他の元素と異なり、それ自体には固溶強
化能はないが、焼入性を向上させ所望の組織を得
るうえで必要な元素である。その下限はその効果
を発揮させうる量から、その上限はその効果が飽
和に達し、経済的でなくなる量から0.1〜1.0％と
する。
Ｓ：0.01％以下
Ｓは硫化物を生成し、加工性を劣化させるので
可及的に少ない方が望ましいが、その含有量が
0.01％以下であれが所望の加工性確保できること
からＳ含有量の上限を0.01％と定めた。
solAl：0.01〜0.06％
solAlは鋼の脱酸剤として有効なものであるが、
その含有量が0.01未満では脱酸の効果が期待でき
なくなり、他方0.06％を越えて含有させても脱酸
の効果が飽和してそれ以上の効果が期待できない
ことからsolAl含有量を0.01〜0.06％と限定した。
凝固時冷却速度を70℃／分以上とした連続鋳造
現行の連続鋳造スラブでは、凝固時冷却速度が
中心付近で３〜30℃／分であるため凝固時に
Mn、Ｓ、Ｐ等の溶質成分の、溶鋼と固体鉄分間
の分配が完全に生じるため（分配係数小）、凝固
後一次デンドライトとデンドライト樹枝間の最終
凝固部では溶質元素の濃淡が大きい。この傾向は
複合組織型鋼板のようにMn量の高い鋼種では著
しい。このような濃淡は熱間圧延後も維持され、
極端な場合にはbanded sutructureとよばれる積
層構造を呈する。このため制御冷却を行なつた後
には、Mn量の高い領域ではマルテンサイト粒が
密集し、ひどい場合には層状のマルテンサイト層
が生成する。一方、Mn量の少ない領域では全く
マルテンサイト相が存在しない。このような不均
一組織は複合組織型鋼板の特徴である延性を劣化
する。
凝固時冷却速度を70℃／分以上とすることによ
り、分配係数が大きくなり、2.5％Mn以下の鋼に
おいて複合組織を得るうえで問題にならない程度
までMn等の濃度差が少ない事が判明した。逆に
いうならば、凝固時冷却速度が70℃／分で連続鋳
造すると複合組織型鋼強度熱延鋼板の延性が大幅
に向上する。さらに凝固時冷却速度の増大はスラ
ブ中心部のマクロ偏析も軽減し、材質を向上させ
る。
凝固時冷却速度増大の実現手段については従来
厚さの連続鋳造において強冷却してもよいし、冷
却速度に見合う厚さの薄スラブに連続鋳造しても
良い。この場合、スラブ厚の減少に伴なう圧延比
の低下は、元来溶質元素の濃度差が小さいため、
材質に全く影響を及ぼさない。
スラブの溶製後該スラブを550℃以上に保持す
るのは、未変態オーステナイト量が50％以上確保
されている状態より再加熱するためである。
この状態より再加熱−熱延後、制御冷却を行な
うと、フエライト変態が高温域で確保されるた
め、冷却時生成されるマルテンサイト粒が微細
に、かつ均一に分散され、従来材に比べきめて高
延性の複合組織型高強度鋼板となる。
なお、550℃以上の温度への保温は例えば断熱
材により行なえばよい。
なお、熱間圧延はたとば1050〜1150℃の温度で
開始すればよい。
熱間圧延終了後は所定の制御冷却を行なう。
［実施例］
第１表に示す鋼を溶製した。A1、A2、B1、
B2は実施例であり、他は比較例である。
A1、A2、A3、B1、B2、B3、C1、C2につい
てはスラブ厚50mmｔで連続鋳造機で凝固させた。
さらにA1、A2、B1、B2、C1については連続鋳
造機から出てきた高温スラブに断熱材等で保熱、
さらには軽加熱によつて第２表に示すような条件
のもとで熱間圧延し、板厚2.8mmの熱延コイルと
した。A3、B3、C3については常温まで冷却後、
従来工程で再加熱し、熱間圧延した。
また、A4、A5、B4、B5は従来タイプの連続
鋳造によつて得たスラブで、A4、B4については
直接圧延、また、A5、B5については再加熱し、
熱間圧延を行なつた。
なお、本実施例においては制御冷却の一例とし
て次の冷却を行なつた。すなわち、強度・延性の
バランスからフエライト体積率を50％以上確保す
るため、フエライトノーズ付近は10℃／ｓ以下で
徐冷し、その後600℃以下の巻取温度まで20℃／
ｓ以上で急冷した。
第３表に示すように本実施例に係る熱延鋼板は
いずれも加工性、特に強度−延性バランス（TS
−El）が飛躍的に向上している上、加熱炉原単位
の低減がはかられえいるのが明らかである。従つ
て、本実施例によれば、加工性の良好な複合組織
鋼強度熱延鋼板を安価に製造することができる。
［発明の効果］
本発明によれば次のもろもろの効果が得られ
る。
高価な元素を使用することなく加工性の良好
な熱延鋼板を製造することができ、自動車の車
体用の鋼板として適用するのに好適な高強度熱
延鋼板が得られる。
加熱費の節約が可能である。
[Field of Application of the Invention] The present invention relates to a method for producing high-strength hot rolled steel sheets. [Background of the Invention] In recent years, in the automobile industry, in order to reduce the weight of car bodies, there has been a desire to reduce the plate thickness without changing the design strength. Such requests cannot be met because of poor formability and problems with weldability. Therefore, as a steel plate to replace the conventional precipitation hardening type high-strength steel plate, a composite structure type high-strength steel plate consisting of two phases of ferrite and martensite is increasingly being adopted. However, such a composite structure steel sheet is not necessarily preferable in terms of workability and the like. Therefore, we further added Mn as a basic component, Si, Cr
By adding a large amount of , a composite structure steel sheet has been developed that has the characteristics of a low yield ratio and good ductility even after the hot rolling process and the winding process. However, such a composite structure steel sheet uses a large amount of Si and Cr, so there is a problem that the cost is high. By the way, in the conventional method, hot rolled steel sheets are
Slabs made by blooming steel ingots by ordinary ingot making method or slabs made by continuous casting method are once cooled to room temperature and then heated to 1200~1300 in a heating furnace.
It is manufactured by heating at a high temperature of °C for a long time and then rolling it into a continuous hot rolling mill. However, subjecting the slab to high-temperature, long-term heating of 1,200°C or higher in this way results in a huge loss in the amount of heating. Therefore, in order to find a method for manufacturing high-strength hot-rolled steel sheets at low cost and with high efficiency, we have repeatedly conducted various tests and investigations on both the steel material composition and rolling processing methods, and as a result, we have achieved the present invention. Ivy. [Object of the Invention] An object of the present invention is to provide a high-strength hot-rolled steel manufacturing method that can produce a hot-rolled steel sheet with good workability and low cost at low heating costs. [Summary of the invention] The above object is to reduce C: 0.03 to 0.2%, Si:
0.02~1.5%, Mn: 0.6~2.5%, Cr: 0.3~1.5%,
In a method for producing a transformation-strengthened high-strength hot-rolled steel sheet containing bainite consisting of S: 0.01% or less, solAl: 0.01 to 0.06%, and the balance consisting of iron and unavoidable impurities, the cooling rate during solidification is set at 70 ° C / min or more. After obtaining a high-temperature slab through continuous casting, before the slab reaches a temperature of 550°C, after reheating the slab to a temperature of 1050°C or higher, hot rolling is started, and the hot rolling is carried out in Ar. This is achieved by a method for manufacturing high-strength hot-rolled steel sheets, which is characterized by finishing at three or more temperature points and then performing predetermined controlled cooling. The configuration of the present invention will be explained below. C: 0.03-0.2%, C is an essential element for maintaining the necessary strength and forming low-temperature transformation products such as bainite and martensite, but if it exceeds 0.2%, workability and weldability deteriorate. In addition, the low yield ratio characteristic, which is one of the characteristics of the steel sheet of the present invention, will be impaired. The lower limit is set at 0.03% in order to exhibit the effect of strengthening and improving hardenability. Si: 0.2 to 1.5% Si is an element necessary for deoxidizing molten steel, and is also an effective substitutional solid solution element for obtaining high strength and high ductility. Furthermore, it has the function of favoring normal polygonal ferrite formation. In order to exhibit such characteristics, the lower limit was set at 0.2%. In addition, to prevent embrittlement (increase in transition temperature) of the weld and to prevent deterioration of the surface oxide scale condition.
The upper limit was set at 1.5%. Mn: 0.6-2.5% Mn is an essential element for increasing hardenability and obtaining a desired structure. In order to exhibit this effect, a content of 0.6% or more is required, and if it exceeds 2.5%, it becomes difficult to weld and at the same time deteriorates ductility, making the steel plate expensive, so the upper limit is set at 2.5%. Cr: 0.3 to 1.5% Unlike other elements, Cr itself does not have solid solution strengthening ability, but it is a necessary element to improve hardenability and obtain a desired structure. The lower limit is set at an amount that can produce the effect, and the upper limit is set at 0.1 to 1.0%, at which the effect reaches saturation and becomes uneconomical. S: 0.01% or less S produces sulfides and deteriorates workability, so it is desirable to have as little as possible, but if the content is
The upper limit of the S content was set at 0.01% because the desired processability can be ensured at 0.01% or less. solAl: 0.01~0.06% solAl is effective as a deoxidizer for steel, but
If the content is less than 0.01, no deoxidizing effect can be expected, and on the other hand, if the content exceeds 0.06%, the deoxidizing effect will be saturated and no further effect can be expected, so the solAl content should be set at 0.01-0.06. %. Continuous casting with a cooling rate of 70℃/min or higher during solidification In current continuous casting slabs, the cooling rate during solidification is 3 to 30℃/min near the center, so the cooling rate during solidification is 3-30℃/min.
Because solute components such as Mn, S, and P are completely distributed between molten steel and solid iron (partition coefficient is small), the concentration of solute elements is large in the final solidification zone between the primary dendrite and the dendrite branches after solidification. This tendency is remarkable in steel types with a high Mn content, such as composite structure steel sheets. Such shading is maintained even after hot rolling,
In extreme cases, it exhibits a layered structure called banded structure. Therefore, after controlled cooling, martensite grains become dense in areas with a high Mn content, and in severe cases, a layered martensite layer is formed. On the other hand, there is no martensite phase at all in the region with a small amount of Mn. Such a non-uniform structure deteriorates the ductility, which is a characteristic of a composite structure type steel sheet. It was found that by setting the cooling rate during solidification to 70℃/min or more, the distribution coefficient increases and the concentration difference of Mn etc. is small to the extent that it does not pose a problem in obtaining a composite structure in steel with 2.5%Mn or less. . Conversely, if continuous casting is performed at a cooling rate of 70°C/min during solidification, the ductility of the composite structure steel strength hot-rolled steel sheet will be significantly improved. Furthermore, increasing the cooling rate during solidification also reduces macro-segregation in the center of the slab, improving material quality. As for the means for increasing the cooling rate during solidification, strong cooling may be performed in continuous casting of a conventional thickness, or continuous casting may be performed to form a thin slab with a thickness commensurate with the cooling rate. In this case, the decrease in rolling ratio due to the decrease in slab thickness is due to the fact that the difference in concentration of solute elements is originally small.
It does not affect the material at all. The reason why the slab is maintained at 550° C. or higher after melting is to reheat the slab in a state where the amount of untransformed austenite is secured at 50% or more. If controlled cooling is performed after reheating and hot rolling from this state, ferrite transformation will be ensured in the high temperature range, so the martensite grains generated during cooling will be finely and uniformly dispersed, resulting in a finer grain than conventional materials. This results in a high-strength steel sheet with a highly ductile composite structure. Note that heat retention to a temperature of 550° C. or higher may be carried out, for example, by using a heat insulating material. Note that hot rolling may be started at a temperature of, for example, 1050 to 1150°C. After hot rolling is completed, predetermined controlled cooling is performed. [Example] Steels shown in Table 1 were melted. A1, A2, B1,
B2 is an example, and the others are comparative examples. A1, A2, A3, B1, B2, B3, C1, and C2 were solidified using a continuous casting machine with a slab thickness of 50 mm.
Furthermore, for A1, A2, B1, B2, and C1, the high-temperature slabs that come out of the continuous casting machine are heat-retained with insulation materials, etc.
Furthermore, it was hot-rolled by light heating under the conditions shown in Table 2 to obtain a hot-rolled coil with a thickness of 2.8 mm. For A3, B3, and C3, after cooling to room temperature,
It was reheated and hot rolled using conventional processes. In addition, A4, A5, B4, and B5 are slabs obtained by conventional type continuous casting, A4 and B4 are directly rolled, and A5 and B5 are reheated.
Hot rolling was carried out. In this example, the following cooling was performed as an example of controlled cooling. In other words, in order to ensure a ferrite volume fraction of 50% or more from the balance of strength and ductility, the area around the ferrite nose is slowly cooled at a rate of 10℃/s or less, and then cooled at a rate of 20℃/s to a coiling temperature of 600℃ or less.
It was rapidly cooled at a temperature of more than 100 s. As shown in Table 3, the hot rolled steel sheets according to this example all have good workability, especially strength-ductility balance (TS).
-El) has been dramatically improved, and it is clear that the heating furnace consumption rate can be reduced. Therefore, according to this example, a composite structure steel strength hot-rolled steel sheet with good workability can be manufactured at low cost. [Effects of the Invention] According to the present invention, the following various effects can be obtained. A hot-rolled steel sheet with good workability can be produced without using expensive elements, and a high-strength hot-rolled steel sheet suitable for use as a steel sheet for automobile bodies can be obtained. It is possible to save on heating costs.
【表】
○印は実施例
[Table] ○ marks are examples
【表】
○印は実施例
[Table] ○ marks are examples
【表】【table】
Claims (1)
％、Mn：0.6〜2.5％、Cr：0.3〜1.5％、Ｓ：0.01
％以下、solAl：0.01〜0.06％、残部鉄及び不可避
的不純物からなるベイナイトを含む変態強化型の
高強度熱延鋼板を製造する方法において、凝固時
冷却速度を70℃／分以上で連続鋳造を行ない高温
スラブを得た後、該スラブが550℃の温度になる
前に、該スラブを1050℃以上の温度に再加熱した
後に、熱間圧延を開始し、該熱間圧延をAr3点以
上の温度で終了し、次いで所定の制御冷却を行な
うことを特徴とする高強度熱延鋼板の製造法。1% by weight, C: 0.03-0.2%, Si: 0.02-1.5
%, Mn: 0.6-2.5%, Cr: 0.3-1.5%, S: 0.01
% or less, solAl: 0.01 to 0.06%, the balance is iron and unavoidable impurities in a method for manufacturing high-strength hot-rolled steel sheets containing bainite, which consists of continuous casting at a cooling rate of 70°C/min or higher during solidification. After obtaining a high-temperature slab, before the slab reaches a temperature of 550°C, hot rolling is started after reheating the slab to a temperature of 1050°C or higher, and the hot rolling is carried out at Ar 3 or more points. 1. A method for producing a high-strength hot-rolled steel sheet, the method comprising: finishing at a temperature of
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP23389685A JPS6293003A (en) | 1985-10-18 | 1985-10-18 | Production of high strength hot rolled steel sheet |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| JP23389685A JPS6293003A (en) | 1985-10-18 | 1985-10-18 | Production of high strength hot rolled steel sheet |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| JPS6293003A JPS6293003A (en) | 1987-04-28 |
| JPH0573802B2 true JPH0573802B2 (en) | 1993-10-15 |
Family
ID=16962266
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| JP23389685A Granted JPS6293003A (en) | 1985-10-18 | 1985-10-18 | Production of high strength hot rolled steel sheet |
Country Status (1)
| Country | Link |
|---|---|
| JP (1) | JPS6293003A (en) |
Families Citing this family (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JP3464611B2 (en) * | 1998-10-08 | 2003-11-10 | 新日本製鐵株式会社 | High-strength hot-dip galvanized steel sheet excellent in formability and corrosion resistance and method for producing the same |
Family Cites Families (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| JPS59208018A (en) * | 1983-05-12 | 1984-11-26 | Kawasaki Steel Corp | Improvement of toughness of steel plate |
| JPS6075518A (en) * | 1983-09-29 | 1985-04-27 | Nippon Steel Corp | Hot rolling method which is effective in improving toughness at low temperature |
-
1985
- 1985-10-18 JP JP23389685A patent/JPS6293003A/en active Granted
Also Published As
| Publication number | Publication date |
|---|---|
| JPS6293003A (en) | 1987-04-28 |
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