应用稀土氧化物冶金技术改善高强钢焊接性能
Enhanced Welding Properties of High Strength Steel via Rare Earth Oxide Metallurgy Technology
通讯作者: 陈芙蓉,cfr7075@imut.edu.cn,主要从事材料焊接及其纳米化研究
责任编辑: 肖素红
收稿日期: 2020-02-18 修回日期: 2020-06-10 网络出版日期: 2020-09-11
Corresponding authors: CHEN Furong, professor, Tel: (0471)6577075, E-mail:cfr7075@imut.edu.cn
Received: 2020-02-18 Revised: 2020-06-10 Online: 2020-09-11
作者简介 About authors
陆斌,男,1977年生,高级工程师,博士生
在高强钢中加入5×10-6和23×10-6稀土Ce,研究了Ce对焊接热影响区冲击韧性、微观组织、原奥氏体晶粒以及焊接接头断口形貌的影响与机理。钢中含Ce量为5×10-6时,能在镁铝夹杂物外围生成少量CeAlO3夹杂物,但不能完全改性镁铝夹杂物,当Ce添加量达到23×10-6后,Ce能够完全改性MgO-Al2O3尖晶石,生成(CeCa)S+MgO-Al2O3+MnS稀土夹杂物。对含有Ce的高强钢板进行模拟焊接,结果表明,在4组不同焊接热输入条件下,钢中加入23×10-6Ce后,比钢中加入5×10-6Ce的钢焊接热影响区的Charpy冲击功有所提高。微观组织分析发现,23×10-6Ce含量的高强钢试样焊接热影响区断口形貌呈现韧窝状,韧性更好;当热输入从25 kJ/cm逐步提高到100 kJ/cm时,含5×10-6Ce的高强钢热影响区原奥氏体晶粒平均尺寸增加了75.6%;含23×10-6Ce的高强钢的原奥氏体晶粒平均尺寸增加了52.4%,即钢中Ce含量的增加抑制了焊接热影响区原奥氏体晶粒的长大。通过微观组织分析对比,说明稀土Ce在高强钢中起到了延迟焊接热影响区上贝氏体组织形成的作用,同时抑制焊接过程中原奥氏体晶粒的长大。利用高温共聚焦显微镜观察到了稀土夹杂物钉扎于原奥氏体晶界,抑制焊接过程中晶粒的长大,验证了稀土Ce对高强钢焊接热影响区性能改善的机理。本工作表明应用稀土氧化物冶金可以改善稀土高强钢的焊接性能。
关键词:
With the increase of the strength of steel plate, the welding performance of the steel decreases sharply and the welding crack susceptibility increases. The properties of welding heat-affected zone of high strength steel lower with increasing the welding heat input. The present work aims to improve the toughness of welding heat-affected zone by rare earth oxide metallurgy technology. The effect of 5×10-6 and 23×10-6 rare earth Ce on the impact toughness, microstructure, austenite grains of heat-affected zone and fracture morphology of welded joint were studied. When the steel contains 5×10-6 rare earth Ce, the inclusions are MgO-Al2O3 spinels surrounded with a small amount of CeAlO3 inclusions. In the case of the steel with 23×10-6Ce, Ce can completely modify MgO-Al2O3 inclusions, resulting in the formation of (CeCa)S+MgO-Al2O3+MnS complex inclusions. The simulation welding of high strength steel was performed. The results show that the Charpy impact energy of heat-affected zone of the steel with 23×10-6Ce is higher at four different heat inputs, in comparison with the steel with 5×10-6Ce. The microstructure analysis shows that the fracture morphology of heat-affected zone of the steel with 23×10-6Ce appears dimples, which is an indication of a higher toughness. With increasing the heat input from 25 kJ/cm to 100 kJ/cm, the average grain size of the original austenite in the heat-affected zones of the steels with 5×10-6Ce and 23×10-6Ce was increased by 75.6% and 52.4%, respectively. It indicates that the growth of the original austenite grain during welding is suppressed with increasing the Ce content in the steel. Comparison of the microstructure shows that rare earth Ce can delay the formation of upper bainite structure in the heat-affected zone. Through the high temperature confocal microscope, it was observed that the rare earth inclusions pinned on the original austenite grain boundary, which can effectively restrain the grain growth during the welding process. It provides an evidence showing the mechanism of improvement in the heat-affected zone in the welding of the high strength steel by rare earth Ce. The present study demonstrates the rare earth oxide metallurgy can improve the weldability of the high strength steel.
Keywords:
本文引用格式
陆斌, 陈芙蓉, 智建国, 耿如明.
LU Bin, CHEN Furong, ZHI Jianguo, GENG Ruming.
日本新日铁公司开发出通过细小的粒子得到微细组织和超高的HAZ韧性(super high HAZ toughness technology with fine microstructure imparted by fine particles,HUTFF)技术,利用在1400 ℃以上高温仍能稳定存在的碱土金属(Ca、Mg)的氧化物或者硫化物,使这些细小的夹杂物弥散分布在钢中钉扎晶界,抑制焊接过程中奥氏体晶粒的长大[5]。日本钢铁公司开发的大线能量焊接热影响区韧性改善(excellent quality in large heat input welded joints,EWEL)技术,利用氮化物和氧化物共同抑制奥氏体晶粒的粗化,使热影响区奥氏体晶粒细化[6];结合低C当量,使焊接热影响区的上贝氏体转变为铁素体+贝氏体或者铁素体+珠光体,改善焊接后韧性[7,8]。Yang等[9]利用Mg脱氧剂改善焊接热影响区的韧性,开发出利用强脱氧剂改善焊接热影响区韧性(excellent heat-affected zone toughness technology improved by use of strong deoxidizers,ETISD)的技术,在大热输入焊接之后,原奥氏体粒子尺寸细小,焊接热影响区冲击韧性优异。
本工作通过实验室和工业实验,开发了一种提高焊接性能的稀土氧化物冶金技术(enhanced welding properties via rare earth oxide metallurgy technology,REOMT),从母材入手提高材料的焊接热影响区冲击韧性,改善高C当量高强度钢板的焊接性能,加大热输入提高焊接生产效率。对工业试制的高强钢板经模拟焊接,研究其焊接性能与微观组织的关系,证明适量的稀土Ce添加能够改善高强钢的焊接性能。
1 实验方法
采用210 t顶底复吹转炉→210 t钢包炉精炼(ladle furnace,LF)→210 t真空循环脱气精炼(rührstahl heraeus,RH)→300 mm×2200 mm立弯式宽厚板铸机浇铸→3800 mm四辊可逆粗轧机→4100 mm四辊可逆精轧机→层流加速冷却(ACC)→升温至910 ℃保温10~20 min,淬火至150 ℃以下→回火至620 ℃,保温30 min,空冷至室温,制备成厚钢板。选用5×10-6Ce、23×10-6Ce含量的700 MPa级高强钢板,依据Ce含量的不同试样命名为5Ce、23Ce,具体化学成分见表1。首先,利用带有Thermo NS7能谱仪(EDS)的JSM-6701F冷场发射扫描电镜(SEM)观察和分析不同Ce添加量后的母材典型夹杂物形貌和成分;然后将钢板加工成10.5 mm×10.5 mm×75 mm试样,使用Gleeble3500热模拟机模拟高强厚板的焊接热循环过程。焊接热模拟过程为:按Rykalin-2D模型分别模拟焊接热输入为25、50、75和100 kJ/cm,峰值温度为1350 ℃;将上述热模拟试样在WDW-2000万能拉伸试验机上做拉断实验,再从热电偶焊点处横向截开标记截面,至标记面以下10 mm处截下镶嵌,对标记面预磨、抛光,利用Quanta-250 SEM观察夹杂物和断口形貌;之后将抛光试样用4% (质量分数)硝酸酒精溶液侵蚀后,用DM4M光学显微镜(OM)观察显微组织;重新打磨抛光并在80 ℃恒温下侵蚀一定时间,使用DM4M OM观察原始奥氏体晶粒,并用Image-Pro Plus软件统计原始奥氏体晶粒尺寸;将模拟热输入的试样与母材加工成10 mm×10 mm×55 mm的“V”型Charpy试样,利用ZBC2752A750J冲击试验机测定各试样常温下冲击功。再将含Ce高强钢试样加工为直径5 mm×3 mm的圆柱状试样,通过VL2000DX-SVF17SP高温共聚焦显微镜观察不同温度时夹杂物对晶界的钉扎作用。将含Ce高强钢加工成直径15 mm×90 mm试样,表面打磨干净,试样作阳极,铜片作阴极,通过小样电解收集钢中析出物。将电解提取的析出物过滤、淘洗后利用Quanta-250和带有EDS的JSM-6701F SEM进行夹杂物形貌分析。
表1 不同Ce含量高强钢的化学成分 (mass fraction / %)
Table 1
Steel | C | Si | Mn | P | S | Al | Nb | V | Ti | Ca | Mg | Cr | Mo | Ce | Fe |
---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
5Ce | 0.12 | 0.33 | 1.62 | 0.014 | 0.0020 | 0.026 | 0.046 | 0.069 | 0.017 | 0.0014 | 0.0004 | 0.260 | 0.115 | 0.0005 | Bal. |
23Ce | 0.11 | 0.31 | 1.62 | 0.015 | 0.0020 | 0.025 | 0.046 | 0.066 | 0.017 | 0.0007 | 0.0005 | 0.270 | 0.122 | 0.0023 | Bal. |
2 实验结果与讨论
2.1 稀土对高强钢夹杂物的影响
表2为不同Ce含量高强钢试样的力学性能。图1和2为不同Ce含量母材夹杂物形貌的SEM像及EDS分析结果。以图1为例,根据面扫描结果可以看出,O、Al、Mg元素在夹杂物中的分布一致,对于铝脱氧钢,当钢液中有少量Mg时,易形成镁铝尖晶石夹杂物[26],因此可认定图1中夹杂物中心处的黑色部分为MgO-Al2O3。MgO-Al2O3外围为(CaMn)S和CeAlO3,因此,稀土Ce含量为5×10-6时,钢中夹杂物为MgO-Al2O3+(CaMn)S+CeAlO3;Ce含量为23×10-6时,钢中的夹杂物类型是(CeCa)S+MgO-Al2O3+MnS。面扫描分析结果表明,当Ce添加量较低时,不能完全改性MgO-Al2O3夹杂物,只能在MgO-Al2O3外围生成少量CeAlO3夹杂物,当Ce添加量达到一定量后(23×10-6),Ce能够完全改性MgO-Al2O3尖晶石,生成稀土硫化物夹杂物。
图1
图1
5Ce试样中典型夹杂物MgO-Al2O3+(CaMn)S+CeAlO3的SEM像和EDS分析
Color online
Fig.1
SEM image and EDS analyses of typical inclusions MgO-Al2O3+(CaMn)S+CeAlO3 in 5Ce steel
表2 不同Ce含量高强钢的力学性能
Table 2
Steel | ReL / MPa | Rm / MPa | A / % | AKV / J |
---|---|---|---|---|
5Ce | 782 | 812 | 15.5 | 148 |
23Ce | 794 | 838 | 17.0 | 212 |
图2
图2
23Ce试样中典型夹杂物(CeCa)S+MgO-Al2O3+MnS的SEM像和EDS分析
Color online
Fig.2
SEM image and EDS analyses of typical inclusions (CeCa)S+MgO-Al2O3+MnS in 23Ce steel
2.2 焊接热影响区冲击韧性
表3 热模拟焊接参数
Table 3
Heat input kJ·cm-1 | Peak temp. ℃ | Holding time s | t8/5 s | Cooling rate ℃·s-1 |
---|---|---|---|---|
25 | 1350 | 1 | 18 | 16.67 |
50 | 1350 | 1 | 74 | 4.05 |
75 | 1350 | 1 | 165 | 1.82 |
100 | 1350 | 1 | 295 | 1.02 |
图3
图3
热影响区(HAZ)焊接热模拟过程
Fig.3
Thermal simulation process of heat-affacted zone (HAZ)
不同热输入条件下的焊接热影响区室温冲击功见图4。可以看出,随着焊接热输入的增加,试样的室温冲击功均呈下降趋势,而相同焊接热输入下,Ce含量为23×10-6试样的冲击性能明显好于Ce含量为5×10-6的试样。钢中加入适量的稀土,提高了相同热输入下试样热影响区的冲击功。
图4
图4
不同热输入条件下的焊接热影响区室温冲击功
Fig.4
Impact energies of HAZ under different heat inputs at room temperature
2.3 焊接热影响区微观组织
不同Ce含量试样在不同焊接热输入下HAZ显微组织的OM像如图5和6所示。可以看出,焊接热输入为25 kJ/cm时,不同Ce含量试样的HAZ显微组织均为马氏体组织;热输入提高到50 kJ/cm时,Ce含量为5×10-6的试样HAZ组织为上贝氏体和粒状贝氏体组织,Ce含量为23×10-6的试样HAZ组织为马氏体和下贝氏体组织;热输入继续增加至75 kJ/cm,Ce含量为23×10-6试样的HAZ才开始出现少量上贝氏体组织,继续增加热输入至100 kJ/cm时,不同Ce含量试样HAZ显微组织均为粗大、脆性的上贝氏体和粒状贝氏体混合组织。可见,在焊接时含有23×10-6Ce的试样,Ce可阻滞HAZ中上贝氏体组织的形成。
图5
图5
5Ce试样不同焊接热输入下热影响区显微组织的OM像
Fig.5
OM images of HAZ in 5Ce steel under heat inputs of 25 kJ/cm (a), 50 kJ/cm (b), 75 kJ/cm (c) and 100 kJ/cm (d)
2.4 焊接接头断口形貌
不同Ce含量试样在不同焊接热输入下断口形貌的SEM像如图7和8所示。可以看出,热输入为25 kJ/cm时,不同Ce含量试样断口心部主要由大韧窝和小韧窝组成;热输入增加到50 kJ/cm时,Ce含量为5×10-6试样断口心部基本没有韧窝,出现了较大的解理面,而Ce含量为23×10-6试样断口心部由小解理面、小韧窝及剪切脊组成。随着热输入继续增加,2种Ce含量试样断口心部韧窝全部消失,形成了大片状的解理面,且热输入越大,试样断口心部解理面越大,验证了随着Ce含量的增加,试样焊接热影响区的室温冲击功提高。
图6
图6
23Ce试样不同焊接热输入下热影响区显微组织的OM像
Fig.6
OM images of HAZ in 23Ce steel under heat inputs of 25 kJ/cm (a), 50 kJ/cm (b), 75 kJ/cm (c) and 100 kJ/cm (d)
图7
图7
5Ce试样不同焊接热输入下断口形貌的SEM像
Fig.7
SEM fractographs of 5Ce steel under heat inputs of 25 kJ/cm (a), 50 kJ/cm (b), 75 kJ/cm (c) and 100 kJ/cm (d)
2.5 焊接热影响区原奥氏体晶粒
图8
图8
23Ce试样不同焊接热输入下断口形貌的SEM像
Fig.8
SEM fractographs of 23Ce steel under heat inputs of 25 kJ/cm (a), 50 kJ/cm (b), 75 kJ/cm (c) and 100 kJ/cm (d)
图9
图9
5Ce试样不同焊接热输入下热影响区原奥氏体晶粒形貌的OM像
Fig.9
OM images of HAZ original austenite grain in 5Ce steel under heat inputs of 25 kJ/cm (a), 50 kJ/cm (b), 75 kJ/cm (c) and 100 kJ/cm (d)
表4 不同热输入条件下的焊接热影响区的原奥氏体晶粒尺寸 (μm)
Table 4
Steel | 25 kJ·cm-1 | 50 kJ·cm-1 | 75 kJ·cm-1 | 100 kJ·cm-1 |
---|---|---|---|---|
5Ce | 40.2 | 59.9 | 66.9 | 70.6 |
23Ce | 47.3 | 49.3 | 50.2 | 72.1 |
图10
图10
23Ce试样不同焊接热输入下热影响区原奥氏体晶粒形貌的OM像
Fig.10
OM images of HAZ original austenite grain in 23Ce steel under heat inputs of 25 kJ/cm (a), 50 kJ/cm (b), 75 kJ/cm (c) and 100 kJ/cm (d)
2.6 稀土夹杂物钉扎晶界作用
通过高温共聚焦显微镜在线观察23Ce试样不同温度时夹杂物对晶界的钉扎情况,结果如图11所示。高温下晶界处的蒸发比晶粒内部更为强烈,晶界处的原子通过表面扩散形成热蚀沟,从而逐渐显现出奥氏体晶粒的轮廓,图11中较深和较浅的热蚀沟分别为老的晶界和新晶界。新晶界需要通过原子扩散显露,老的晶界也相应地通过原子扩散而逐渐宽化、填平、最终消失,不同温度下形成的热蚀沟需要足够的时间才能填平,因而可能出现新、旧奥氏体晶界(热蚀沟)共存的现象[27]。从图11可以看出,当试样从1488.5 ℃保温,随着保温时间的延长,原奥氏体晶界(图中红色虚线标记)逐渐沿箭头方向推移,时间为1030.1 s时,图中箭头标识的2条晶界合并为一条(图11d),并且可以观察到夹杂物钉扎于晶界。继续保温37.7 s后,夹杂物钉扎的原奥氏体晶界脱钉(图11e)。这表明,细小的稀土夹杂物可以有效钉扎于晶界,抑制晶界迁移,阻止晶粒长大。
图11
图11
23Ce试样高温共聚焦观察实验结果
Color online
(a) 987.2 s, 1488.5 ℃;(b) 1012.7 s, 1484.0 ℃;(c) 1023.9 s, 1488.5 ℃
;(d) 1030.1 s, 1482.0 ℃;(e) 1067.8 s, 1475.6 ℃;(f) 1202.4 s, 1453.0 ℃
Fig.11
High temperature confocal observation results of 23Ce steel (Original grain bounaries (dashed lines) move in the direction of arrows)
表5 不同焊接热输入时23Ce试样在不同温度持续时间 (s)
Table 5
Temperature / ℃ | 25 kJ·cm-1 | 50 kJ·cm-1 | 75 kJ·cm-1 | 100 kJ·cm-1 |
---|---|---|---|---|
>930 | 8.51 | 24.93 | 52.07 | 90.93 |
>1000 | 6.97 | 19.57 | 41.11 | 70.57 |
>1100 | 5.10 | 14.08 | 28.22 | 48.08 |
>1200 | 3.46 | 9.08 | 17.72 | 30.58 |
>1300 | 1.86 | 3.58 | 8.22 | 14.08 |
2.7 稀土夹杂物形貌
图12
图12
23Ce试样电解后夹杂物SEM像及EDS分析
Color online
(a~c) MgO-Al2O3;(d) Al2O3
Fig.12
SEM images and EDS analyses of inclusions in 23Ce steel after electrolysis
图13
图13
23Ce试样电解后碳氮化物的SEM像和EDS
(a, b) Ti-carbonitride;(c) Mo-carbonitride;(d) Cr-carbonitride
Fig.13
SEM images and EDS of carbonitride in 23Ce steel after electrolysis
图14
图14
23Ce试样电解后含稀土夹杂物SEM像及EDS分析
Color online
(a) backscatter mode;(b) secondary electronic mode
Fig.14
SEM images and EDS analyses of Ce-contained inclusions in 23Ce steel after electrolysis
3 结论
(1) 随着焊接热输入的增加,焊接热影响区显微组织逐渐从马氏体、下贝氏体转变为上贝氏体和粒状贝氏体组织。在Ce含量较低时,热输入为50 kJ/cm时,热影响区就出现了上贝氏体组织,而Ce含量为23×10-6的试样,热输入为75 kJ/cm时热影响区才形成了上贝氏体组织,表明适量稀土延迟了高强钢焊接热影响区上贝氏体组织的形成。
(2) 随着焊接热输入增加,原奥氏体晶粒尺寸呈增加趋势。热输入从25 kJ/cm提高到100 kJ/cm,含5×10-6Ce高强钢的原奥氏体晶粒平均尺寸增加了75.6%;含23×10-6Ce高强钢的原奥氏体晶粒平均尺寸仅增加了52.4%;试样Ce含量越高,其焊接热影响区原奥氏体晶粒尺寸增幅越小,表明稀土能够抑制焊接过程中原奥氏体晶粒的长大。
(3) 添加稀土Ce后,高强钢中的夹杂物类型发生了改变,产生了稀土氧硫化物夹杂。试样中有稀土夹杂物和碳氮化物2类,钢中稀土夹杂物主要为球形,钢中Mo、Cr的碳化物均为尺寸较小的二次碳化物。
(4) 含有弥散稀土氧化物的高强钢母材焊接热影响区韧性更好,原因是试样中的稀土夹杂物可以钉扎原奥氏体晶界,有效抑制焊接过程中晶粒的长大。
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