JP3879232B2 - Low-carbon martensitic stainless hot-rolled steel strip and method for producing the same - Google Patents
Low-carbon martensitic stainless hot-rolled steel strip and method for producing the same Download PDFInfo
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- JP3879232B2 JP3879232B2 JP05842498A JP5842498A JP3879232B2 JP 3879232 B2 JP3879232 B2 JP 3879232B2 JP 05842498 A JP05842498 A JP 05842498A JP 5842498 A JP5842498 A JP 5842498A JP 3879232 B2 JP3879232 B2 JP 3879232B2
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Description
【0001】
【発明の属する技術分野】
この発明は耐食性および溶接性に優れた低炭素マルテンサイト系ステンレス熱延鋼帯およびその製造方法に関するものである。
【0002】
【従来の技術】
低炭素マルテンサイト系ステンレス鋼は、良好な機械的性質および耐食性を有する。更に、低炭素であることは、Cr炭化物の析出量を減少させ、且つ、溶接したときの硬度上昇も低下させるので、一般のマルテンサイト系ステンレス鋼に比べて耐食性および溶接性の向上をもたらす。そのため、低炭素マルテンサイト系ステンレス鋼は、ラインパイプ、油井管および化工機用等の材料に多く用いられている。
【0003】
【発明が解決しようとする課題】
しかしながら、従来は、低炭素マルテンサイト系ステンレス鋼の熱延鋼帯の製造条件が明かにされていない。熱延鋼帯は溶接管などに加工されるため、成形が容易でなければならない。また、強度、特に降伏応力を適度に低下させる必要がある。しかしながら、このための条件が従来開発されていない。
【0004】
また、マルテンサイト系ステンレス鋼は、熱間圧延後の冷却中に焼きが入り硬くなるので、焼き戻して硬さを適当に下げ、靱性、加工性を確保しなければならないが、焼き戻し工程にコストが係る問題がある。生産性の面からはできるだけ焼き戻し工程を簡略に行いたい。。
【0005】
【発明が解決しようとする課題】
従って、この発明の目的は、上述の問題を解決し、焼き戻し工程をできる限り簡略化し、溶接管等に加工される熱延鋼帯の強度を適度に低下させ、成形が容易となる耐食性および溶接性に優れた低炭素マルテンサイト系ステンレス熱延鋼帯およびその製造方法を提供することにある。
【0006】
【課題を解決するための手段】
請求項1記載の低炭素マルテンサイト系ステンレス熱延鋼帯は、Cr:12.0〜13.5wt.%、C+N:0.030wt.%以下、Ni:1.5〜2.5wt.%を含有し、残部がFeおよび不可避的不純物からなるステンレス鋼スラブを1100℃以上1200℃未満の温度域で加熱し、次いで、前記スラブを900℃以上の熱間圧延終了温度で熱延ミルにより熱間圧延して巻き取り、熱延鋼帯とし、前記熱延鋼帯を一旦150℃以下にまで冷却し、次いで、A 1 −50℃以上A 1 未満の温度域で焼き戻しすることにより製造されてなることに特徴を有するものである。
【0007】
請求項2記載の低炭素マルテンサイト系ステンレス熱延鋼帯の製造方法は、Cr:12.0〜13.5wt.%、C+N:0.030wt.%以下、Ni:1.5〜2.5wt.%を含有し、残部がFeおよび不可避的不純物からなるステンレス鋼スラブを1100℃以上1200℃未満の温度域で加熱し、次いで、前記スラブを900℃以上の熱間圧延終了温度で熱延ミルにより熱間圧延して巻き取り、熱延鋼帯とし、前記熱延鋼帯を一旦150℃以下にまで冷却し、次いで、A 1 −50℃以上A 1 未満の温度域で焼き戻しすることに特徴を有するものである。
【0008】
Ni含有量を熱間加工性、焼き戻し軟化特性、溶接性のすべてを満足する範囲に規定し、更に、熱延温度を規定してものである。
【0009】
【発明の実施の形態】
次に、この発明の限定理由を述べる。
▲1▼ Cr(クロム):
Crは、12.0〜13.5wt.%の範囲で含有させる。
【0010】
Crは耐食性を確保するために12.0wt.%以上必要である。しかし、Cr含有量が13.5wt.%を超えると溶接性および熱間加工性を確保するためにNiを多く含有させなければならなくなる。Niは強度を上げる作用を有するため、強度を下げるための焼き戻し工程が増え、また、焼き戻し時間が長くかかる。従って、Cr含有量は、12.0〜13.5wt.%の範囲内に限定すべきである。
【0011】
▲2▼ Ni(ニッケル):
Niは、1.5〜2.5wt.%の範囲で含有させる。
Niはオーステナイト域を広げ熱間加工性、溶接性を確保するため1.5wt.%以上必要である。しかし、Ni添加量が増えるに従い強度が増す。このため2.5wt.%以下とした。従って、Ni含有量は、1.5〜2.5wt.%の範囲内に限定すべきである。
【0012】
▲3▼ C+N(炭素、窒素):
CおよびNは、合計で0.030wt.%以下含有させる。
CおよびNは、マルテンサイトを硬くする作用を有する。C+Nが0.030wt.%を超えると強度を下げるための焼き戻し工程が増え、また、焼き戻し時間が長くかかる。また、溶接HAZ部の硬さが増して割れやすくなる。従って、C+N含有量は、0.030wt.%以下に限定すべきである。
【0013】
4 熱延加熱温度を1100℃以上1200℃未満とした理由:
加熱温度は、オーステナイト温度域でなければならない。Ni添加量によって異なるが、800〜1200℃がオーステナイト温度域である。オーステナイト温度域を超えて加熱するとフェライトが生成する。フェライトはオーステナイトと比較して柔らかく応力が集中するため、フェライトが生成した状態で圧延すると疵ができる。従って、加熱温度の上限は1200℃未満とする。加熱温度の下限を1100℃としたのは、圧延中の冷却を見込んで、圧延終了温度を900℃以上にするためである。従って、加熱温度は、1100℃以上1200℃未満とすべきである。
【0014】
▲5▼ 圧延終了温度を900℃以上とした理由:
圧延終了温度が900℃未満では、再結晶が進まず、900℃未満で圧延すると圧延により導入された内部の歪みや転位が解消されず蓄積していく。このため、焼き戻した後にも軟化し難い。900℃以上で圧延すれば、圧延中および圧延終了から巻き取りまでの間に再結晶し歪みや転位は解放される。従って、圧延終了温度は900℃以上とすべきである。
【0015】
6 焼き戻し温度をA 1 −50℃以上A 1 未満とした理由:
焼き戻しはマルテンサイト組織を加熱して炭化物を析出させ靱性を改善させるために行う。焼き戻し温度はフェライト温度域で出来るだけ高い温度が望ましい。A1変態点はNi添加量によって異なるが、600〜670℃である。温度コントロールの幅を考慮し、焼き戻し温度の下限をA1−50℃と規定した。従って、焼き戻し温度はA 1 −50℃以上A 1 未満とすべきである。
【0016】
【実施例】
次に、この発明の実施例を説明する。
表1に示す、本発明範囲内および範囲外の化学成分組成からなる鋼種A〜Jからなる鋼塊を溶解し、厚み100mmのスラブを作成した。次いで、スラブを1150℃に加熱し、更に、圧延終了温度900℃となるように5mmまで熱間圧延した。次いで、圧延後の材料を一旦室温まで冷却し、更に、550〜670℃の温度で熱処理(焼き戻し)した。このようにして調製した供試体に対して、TIGの共金溶接を行なった。そして、溶接ままの溶接金属およびHAZの低温靱性によって溶接性を評価した。供試体の機械的特性は引張試験:JIS Z 2201、衝撃試験:JIS Z 2203に規定する試験によって調べた。
【0017】
図1は、鋼種Dの、各焼き戻し温度(℃)による焼き戻し後の硬さ(ビッカース硬さ・Hv)を示すグラフである。図1に示すように、本発明範囲内の550〜670℃の焼き戻し温度で、硬さが240Hv以下となっており、靱性が改善されていることがわかる。
【0018】
図2は、鋼種A〜Hの、Ni添加量による焼き戻し後の硬さ(Hv)を示すグラフである。なお、焼き戻し温度は630℃、焼き戻し時間は1時間であった。図2に示すように、Ni添加量が本発明範囲内の2.5wt.%以下で、硬さが240Hv以下となっており十分に軟化していることがわかる。
【0019】
図3は、鋼種A〜Hの、Ni添加量による溶接HAZの靱性を示すグラフである。Ni添加量が本発明範囲内の1.5wt.%以上で、吸収エネルギー(J)が60ジュール以上であり、靱性が改善されていることがわかる。
【0020】
更に、本発明範囲内の鋼種I、Jによる焼き戻し後の硬さ(Hv)およびシャルピー衝撃試験(−20℃1/2サイズ)による吸収エネルギー(J)を調べた。その結果を表2に示す。焼き戻し温度は650℃、焼き戻し時間は1時間であった。表2に示すように、良好な試験結果により十分に軟化され加工性が確保され靱性が改善されていることがわかる。
【0021】
【表1】
【0022】
【表2】
【0023】
【発明の効果】
以上説明したように、この発明によれば、焼き戻し工程をできる限り簡略化し、溶接管等に加工される熱延鋼帯の強度を適度に低下させ、成形が容易となる耐食性および溶接性に優れた低炭素マルテンサイト系ステンレス熱延鋼帯およびその製造方法が得られ、かくして、有用な効果がもたらされる。
【図面の簡単な説明】
【図1】鋼種Dの、各焼き戻し温度(℃)による焼き戻し後の硬さ(ビッカース硬さ・Hv)を示すグラフである。
【図2】鋼種A〜Hの、Ni添加量による焼き戻し後の硬さ(ビッカース硬さ・Hv)を示すグラフである。
【図3】鋼種A〜Hの、Ni添加量による溶接HAZの靱性を示すグラフである。[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a low carbon martensitic stainless hot-rolled steel strip excellent in corrosion resistance and weldability and a method for producing the same.
[0002]
[Prior art]
Low carbon martensitic stainless steel has good mechanical properties and corrosion resistance. Furthermore, the low carbon content reduces the amount of Cr carbide precipitates and also reduces the increase in hardness when welded, resulting in improved corrosion resistance and weldability compared to general martensitic stainless steel. For this reason, low carbon martensitic stainless steel is often used for materials for line pipes, oil well pipes, and chemical machinery.
[0003]
[Problems to be solved by the invention]
However, conventionally, the production conditions for the hot-rolled steel strip of low-carbon martensitic stainless steel have not been clarified. Since the hot-rolled steel strip is processed into a welded pipe, it must be easy to form. Moreover, it is necessary to moderately reduce the strength, particularly the yield stress. However, the conditions for this have not been developed conventionally.
[0004]
In addition, martensitic stainless steel is tempered and hardened during cooling after hot rolling, so it must be tempered to lower the hardness appropriately to ensure toughness and workability. There is a problem related to cost. We want to simplify the tempering process as much as possible from the viewpoint of productivity. .
[0005]
[Problems to be solved by the invention]
Accordingly, the object of the present invention is to solve the above-mentioned problems, simplify the tempering process as much as possible, moderately reduce the strength of the hot-rolled steel strip processed into a welded pipe, etc. An object of the present invention is to provide a low-carbon martensitic stainless hot-rolled steel strip excellent in weldability and a method for producing the same.
[0006]
[Means for Solving the Problems]
The low carbon martensitic stainless hot-rolled steel strip according to claim 1 has a Cr of 12.0 to 13.5 wt. %, C + N: 0.030 wt. % Or less, Ni: 1.5 to 2.5 wt. Is heated in a temperature range of 1100 ° C. or more and less than 1200 ° C., and then the slab is heated by a hot rolling mill at a hot rolling finish temperature of 900 ° C. or more. wound by hot rolling, produced by a hot-rolled steel strip, cooled to below once 0.99 ° C. the hot rolled steel strip, then to tempering at a temperature range of less than a 1 -50 ° C. or more a 1 It is characterized by being made.
[0007]
The method for producing a low-carbon martensitic stainless hot-rolled steel strip according to
[0008]
Ni content is prescribed | regulated to the range which satisfy | fills all of hot workability, a temper softening characteristic, and weldability, and also hot-rolling temperature is prescribed | regulated.
[0009]
DETAILED DESCRIPTION OF THE INVENTION
Next, the reasons for limiting the present invention will be described.
(1) Cr (chromium):
Cr is 12.0 to 13.5 wt. % In the range.
[0010]
Cr is 12.0 wt.% To ensure corrosion resistance. % Or more is necessary. However, the Cr content is 13.5 wt. If it exceeds%, a large amount of Ni must be contained in order to ensure weldability and hot workability. Since Ni has the effect of increasing the strength, the number of tempering steps for decreasing the strength is increased, and the tempering time is long. Therefore, the Cr content is 12.0 to 13.5 wt. % Should be limited.
[0011]
(2) Ni (nickel):
Ni is 1.5 to 2.5 wt. % In the range.
Ni expands the austenite region to ensure hot workability and weldability. % Or more is necessary. However, the strength increases as the Ni addition amount increases. For this reason, 2.5 wt. % Or less. Therefore, the Ni content is 1.5 to 2.5 wt. % Should be limited.
[0012]
(3) C + N (carbon, nitrogen):
C and N are 0.030 wt. % Or less.
C and N have an effect of hardening martensite. C + N is 0.030 wt. If it exceeds%, the number of tempering steps for reducing the strength increases, and the time for tempering takes longer. In addition, the hardness of the welded HAZ portion is increased and is easily cracked. Therefore, the C + N content is 0.030 wt. % Should be limited.
[0013]
4 Reason why the hot rolling heating temperature is 1100 ° C. or more and less than 1200 ° C .:
The heating temperature must be in the austenite temperature range. Although it changes with Ni addition amount, 800-1200 degreeC is an austenite temperature range. When heated beyond the austenite temperature range, ferrite is produced. Since ferrite is softer and stress concentrated than austenite, it can be wrinkled when rolled in a state where ferrite is formed. Therefore, the upper limit of the heating temperature is less than 1200 ° C. The reason why the lower limit of the heating temperature is set to 1100 ° C. is to allow the rolling end temperature to be 900 ° C. or higher in consideration of cooling during rolling. Therefore, the heating temperature should be 1100 ° C. or more and less than 1200 ° C.
[0014]
(5) Reason for setting the rolling end temperature to 900 ° C. or higher:
When the rolling end temperature is less than 900 ° C., recrystallization does not proceed, and when rolling is performed at less than 900 ° C., internal strains and dislocations introduced by rolling are not eliminated but accumulate. For this reason, it is difficult to soften even after tempering. If rolling is performed at 900 ° C. or higher, recrystallization occurs during rolling and from the end of rolling to winding, and strain and dislocation are released. Therefore, the rolling end temperature should be 900 ° C. or higher.
[0015]
Reason for the 6 tempering temperature was A 1 -50 ° C. or more A than 1:
Tempering is performed to improve the toughness by heating the martensite structure to precipitate carbides. The tempering temperature is preferably as high as possible in the ferrite temperature range. The A 1 transformation point is 600 to 670 ° C. although it varies depending on the amount of Ni added. Considering the width of the temperature control, the lower limit of the tempering temperature was defined as A 1 -50 ° C. Therefore, the tempering temperature should be A 1 -50 ° C. or more and less than A 1 .
[0016]
【Example】
Next, examples of the present invention will be described.
The steel ingot which consists of the steel types AJ consisting of the chemical component composition within the range of the present invention and outside the range shown in Table 1 was melted to create a slab having a thickness of 100 mm. Next, the slab was heated to 1150 ° C. and further hot-rolled to 5 mm so that the rolling end temperature was 900 ° C. Next, the rolled material was once cooled to room temperature and further heat-treated (tempered) at a temperature of 550 to 670 ° C. The specimen prepared in this way was subjected to TIG co-welding. And weldability was evaluated by the weld metal as-welded and the low temperature toughness of HAZ. The mechanical properties of the specimens were examined by tests specified in tensile test: JIS Z 2201, impact test: JIS Z 2203.
[0017]
FIG. 1 is a graph showing the hardness (Vickers hardness / Hv) of steel type D after tempering at each tempering temperature (° C.). As shown in FIG. 1, at a tempering temperature of 550 to 670 ° C. within the scope of the present invention, the hardness is 240 Hv or less, indicating that the toughness is improved.
[0018]
FIG. 2 is a graph showing the hardness (Hv) of steel types A to H after tempering depending on the amount of Ni added. The tempering temperature was 630 ° C., and the tempering time was 1 hour. As shown in FIG. 2, the Ni addition amount is 2.5 wt. %, The hardness is 240 Hv or less and it can be seen that it is sufficiently softened.
[0019]
FIG. 3 is a graph showing the toughness of welded HAZ according to the amount of Ni added for steel types A to H. The amount of Ni added is 1.5 wt. %, The absorbed energy (J) is 60 joules or more, and it can be seen that the toughness is improved.
[0020]
Further, the hardness (Hv) after tempering with steel types I and J within the scope of the present invention and the absorbed energy (J) by Charpy impact test (−20 ° C. 1/2 size) were examined. The results are shown in Table 2. The tempering temperature was 650 ° C., and the tempering time was 1 hour. As shown in Table 2, it can be seen that the test results are sufficiently softened, the workability is ensured, and the toughness is improved.
[0021]
[Table 1]
[0022]
[Table 2]
[0023]
【The invention's effect】
As described above, according to the present invention, the tempering process is simplified as much as possible, the strength of the hot-rolled steel strip processed into a welded pipe or the like is moderately reduced, and corrosion resistance and weldability that facilitates forming are achieved. An excellent low carbon martensitic stainless hot-rolled steel strip and a method for producing the same are obtained, thus providing a useful effect.
[Brief description of the drawings]
FIG. 1 is a graph showing the hardness (Vickers hardness / Hv) after tempering of steel type D at each tempering temperature (° C.).
FIG. 2 is a graph showing the hardness (Vickers hardness · Hv) of steel types A to H after tempering depending on the amount of Ni added.
FIG. 3 is a graph showing the toughness of welded HAZ depending on the amount of Ni added for steel types A to H.
Claims (2)
Priority Applications (1)
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JP05842498A JP3879232B2 (en) | 1998-03-10 | 1998-03-10 | Low-carbon martensitic stainless hot-rolled steel strip and method for producing the same |
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JP05842498A JP3879232B2 (en) | 1998-03-10 | 1998-03-10 | Low-carbon martensitic stainless hot-rolled steel strip and method for producing the same |
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JPH11256232A JPH11256232A (en) | 1999-09-21 |
JP3879232B2 true JP3879232B2 (en) | 2007-02-07 |
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