Ultra Low Carbon Bainitic Steels: Part Two

概要:

Although one of the most hotly debated microstructure topics, control of bainitic transformation can lead to a range of diversified gains over the mechanical properties of the finished product.
Specific studies of the relationship between cooling rate and finished cooling temperature show interesting findings related to the distribution of granular bainite, martensite-austenite constituent, bainitic ferrite, and polygonal ferrite.

Microstructures of ultra-low carbon bainitic steels are often complex, consisting of mixtures of different ferrite morphologies, and therefore, wide combinations of mechanical properties can be achieved by controlling them. The bainitic transformation is one of the most complex and disputed phase transformations in steels and all microstructures found may not exhibit the typical bainitic type of transformation.

In the work of A.B.Cota and D.B.Santos on HSLA low-carbon bainitic steel containing B was submitted to torsion tests to simulate controlled rolling, followed by interrupted accelerated cooling. The final microstructure was found to contain complex mixture of granular bainite, small islands of martensite-austenite (MA) constituent, bainitic ferrite, and polygonal ferrite.

The combined effects of cooling rate and the finished cooling temperature on the microstructure are illustrated in Figures 1 (LM photomicrographs) and 2 (SEM photomicrographs). It can be seen from Figures 1(a) and 2(a) that the microstructure associated with the highest finish cooling temperature ( TFC = 650°C) exhibits a mixture of fine polygonal ferrite (with a volume fraction < 12%), granular bainite, and small islands of the MA constituent.

The MA constituent has a granular or equiaxed morphology (volume fraction < 4.5% and with islands of average size less than 2 mm). By contrast, the microstructures associated with the next lower finished cooling temperature (TFC= 500°C) [Figs. 1(b) and 2(b)] consist of granular bainite and bainitic ferrite. A small amount of polygonal ferrite is also in evidence in these microstructures.

In the case of the two higher finish cooling temperatures (TFC=650°C and TFC=600°C), the MA islands are distributed practically uniformly throughout the bainite matrix (Figure 3). This figure, a compilation of binary images in which the MA islands appear white, shows that the increase in cooling rate or decrease in finished cooling temperature implies a decrease in the volume fraction and the average size of the MA islands.

Figures 1(c)–(d) and 2(c)–(d) show that, for the lowest finish cooling temperature, TFC = 400°C, the microstructure is essentially bainitic, with fine laths of bainitic ferrite and interlath MA constituent.



Figure 1: Light photomicrographs of samples cooled at different rates and with different finish cooling temperatures: (a) 6.3°C/s, 650°C; (b) 13.3°C/s, 500°C; (c) 6.3°C/s, 400°C; (d) 33°C/s, 400°C.



Figure 2: SEM photomicrographs of samples cooled at different rates and with different finish cooling temperatures: (a) 6.3°C/s, 650°C; (b) 13.3°C/s, 500°C; (c) 6.3°C/s, 400°C; (d) 33°C/s, 400°C.



Figure 3: Binary images of samples cooled at different rates and with different finish cooling temperatures: (a) 6.38C/s, 650°C; (b) 13.0°C/s, 650°C; (c) 33°C/s, 650°C; (d) 6.3°C/s, 500°C; (e) 13.3°C/s, 500°C; (f) 33°C/s, 500°C. LePera’setchant; MA constituent is white.

ナレッジベース検索

検索したい語句を入力:

検索方法

全文一致
キーワード

前方一致
要約

この記事は連載記事として掲載されております。詳細を見るには下記のリンクをクリックして下さい。

Total Materiaデータベースは世界の国々と広範な規格に関する多くの金属組織画像を収録しております。

メニュウバーにある組織のタブを使うと材料のリストから対象とする材料を選びその金属の組織データをみる事ができます。

組織データは標準のベーシックモジュールにて探す事ができ対象とする材料に関するサブグループで関連するデータをみることができます。

フィールドに材料名を、また規格が分かれば規格も入力し’検索’をクリックします。


すると対象材料の化学成分と共に微細構造を見る事ができます。


一連の画像があり倍率レベルごとに詳細な構造が表示されます。

'条件を選ぶ’のドロップダウンメニューからさまざまな条件を選びいろいろな工程と熱処理状況での組織画像を表示します。



Total Materiaデータベースをあなたにテスト評価を頂くために15万人以上の方が登録されている無料お試しコミュニティ-へ御招待致します。