scholarly journals Microstructures and Properties of Auto-Tempering Ultra-High Strength Automotive Steel under Different Thermal-Processing Conditions

Metals ◽  
2021 ◽  
Vol 11 (7) ◽  
pp. 1121
Author(s):  
Han Jiang ◽  
Yanlin He ◽  
Li Lin ◽  
Rendong Liu ◽  
Yu Zhang ◽  
...  
Keyword(s):  
Great Influence ◽  
High Strength ◽  
Strengthening Mechanism ◽  
Expansion Ratio ◽  
Air Cooling ◽  
Tempered Martensite ◽  
Solid Solution Strengthening ◽  
Heating And Cooling ◽  
Calculation Results ◽  
Hole Expansion Ratio

Automotive steels with ultra-high strength and low alloy content under different heating and cooling processes were investigated. It was shown that those processes exhibited a great influence on the performance of the investigated steels due to the different auto-tempering effects. Compared with the steels under water quenching, there was approximately a 70% increase in the strength and elongation of steels under air cooling, in which the martensite was well-tempered. Although the elongation of the steel with a microstructure composed of ferrite, well-tempered martensite and less-tempered martensite could exceed 15%, the hole expansion ratio was still lower because of the undesirable hardness distribution between the hard phases and the soft phases. It followed from the calculation results based on SEM, TEM and XRD analyses, that for the steel under air cooling, the strengthening mechanism was dominated by the solid solution strengthening and the elongation was determined by the auto-tempering of martensite. Experiments and analyses aimed to explore the strengthening and plasticity mechanisms of auto-tempering steels under the special process of flash heating.

scholarly journals Effects of Graphene Nanoplates on the Mechanical Behavior and Strengthening Mechanism of 7075Al Alloy

Materials ◽  
2020 ◽  
Vol 13 (24) ◽  
pp. 5808
Author(s):  
Jinfeng Leng ◽  
Yunfan Dong ◽  
Binghui Ren ◽  
Ran Wang ◽  
Xinying Teng
Keyword(s):  
Uniform Elongation ◽  
High Strength ◽  
Strengthening Mechanism ◽  
Solid Solution Strengthening ◽  
High Toughness ◽  
Solution Strengthening ◽  
Dislocation Strengthening ◽  
Increased Strength ◽  
7075Al Alloy ◽  
Graphene Nanoplates

7075Al alloy is the preferred material for lightweight automotive applications, but the existing problem is that it is difficult to combine high strength and high toughness. This paper presents our research aimed at obtaining high strength and high toughness materials by adding a nano-phase to realize microstructure refinement. Graphene nanoplates (GNP)/7075Al composites and 7075Al alloy were prepared by a stirring casting method in the present study. In comparison to 7075Al, the tensile strength of GNP/7075Al composites was increased from 572 MPa to 632 MPa while maintaining good uniform elongation of 8% to 10%. The increased strength behavior of GNP/7075Al composites while maintaining the plasticity is discussed in terms of grain refinement and dislocation evolution by analyzing the composite microstructure and quantitatively analyzing the contributions of grain boundary strengthening, solid solution strengthening, precipitation strengthening and dislocation strengthening. GNP’s strengthening of GNP/7075Al composites is mainly attributed to the refinement of grain size and the increase of dislocation density.

The Study in Improvement of Hole Expansibility for High-Strength Steel of 980MPa Grade

2018 ◽  
Vol 941 ◽  
pp. 492-497
Author(s):  
Kuo Cheng Yang ◽  
J.F. Tu ◽  
L.J. Chiang ◽  
W.J. Cheng ◽  
C.Y. Huang
Keyword(s):  
Yield Strength ◽  
High Strength Steel ◽  
Dual Phase Steel ◽  
Single Phase ◽  
High Strength ◽  
Expansion Ratio ◽  
High Yield ◽  
Dual Phase ◽  
Hole Expansion ◽  
Hole Expansion Ratio

Recently, due to the requirements of lightweight and safety, the grade of 980MPa high-strength steel has the demand of high hole expansibility and high yield strength. Due to the large difference of hardness between the soft ferrite and hard martensite, the traditional DP980Y dual phase steel has poor hole expansibility. In order to improve the hole expansibility of DP980Y dual phase steel, the best way is to modify the microstructure into a single-phase to eliminate the large difference of hardness. In this paper, the steel of nearly full bainite microstructure with small amount of ferrite and M/A constituents was studied. Compared to the DP980Y dual phase steel, it was found that this modified steel with a single-phase microstructure has the same grade of 980MPa of tensile strength, but can achieve the demand of higher yield strength and hole-expansion ratio. This study shows reducing the amount of ferrite can increase the homogeneity of matrix with the single phase to improve the hole expansibility. In addition, the use of lower bainite transformation temperature and lower carbon content has the higher hole-expansion ratio due to the less amount of M/A constituents.

A Study on Mechanical Properties and Strengthening Mechanisms of AA5052/ZrB2 In Situ Composites

2016 ◽  
Vol 139 (1) ◽  
Author(s):  
Narendra Kumar ◽  
Gaurav Gautam ◽  
Rakesh Kumar Gautam ◽  
Anita Mohan ◽  
Sunil Mohan
Keyword(s):  
Electron Microscopy ◽  
Mechanical Properties ◽  
High Strength ◽  
Strengthening Mechanism ◽  
Strengthening Mechanisms ◽  
In Situ Composites ◽  
Solid Solution Strengthening ◽  
Theoretical Yield ◽  
The Matrix

In the present study, in situ reaction technique has been employed to prepare AA5052 matrix composites reinforced with different vol. % of ZrB2 particles (i.e., 0, 4.5, and 9 vol. %). Composites have been characterized by X-ray diffraction (XRD) to confirm the in situ formation of ZrB2 particles in the matrix. Optical Microscopy (OM) studies reveal the refinement of aluminum-rich phase due to the presence of ZrB2 particles. Scanning electron microscopy (SEM) studies reveal size and distribution of ZrB2 particles while transmission electron microscopy (TEM) reveals the presence of dislocations in the matrix around ZrB2 particles. Hardness and tensile testing of composites have been carried out at room temperature to evaluate the mechanical properties. The results reveal the improvement in hardness and strength with increased amount of ZrB2 particles. Strength of AA5052/ZrB2 in situ composites has been analyzed by various strengthening mechanism models. The analysis revealed that Orowan and Solid solution strengthening mechanisms are the predominant mechanism for high strength composites. Theoretical yield strength is about 6–10% higher than the experimental values due to clustering tendency of ZrB2 particles.

scholarly journals Influence of Ti and Nb on the Strength–Ductility–Hole Expansion Ratio Balance of Hot-rolled Low-carbon High-strength Steel Sheets

2012 ◽  
Vol 52 (1) ◽  
pp. 151-157 ◽  
Author(s):  
Kyohei Kamibayashi ◽  
Yutaka Tanabe ◽  
Yoshito Takemoto ◽  
Ichirou Shimizu ◽  
Takehide Senuma
Keyword(s):  
High Strength Steel ◽  
High Strength ◽  
Expansion Ratio ◽  
Low Carbon ◽  
Hole Expansion ◽  
Steel Sheets ◽  
Hole Expansion Ratio ◽  
Hot Rolled

scholarly journals Hole Expansion Ratio and Fatigue Properties in Piercing of High-Strength Steel Sheets by PW Punch

2011 ◽  
Vol 52 (606) ◽  
pp. 795-800 ◽  
Author(s):  
Takashi MATSUNO ◽  
Yukihisa KURIYAMA ◽  
Hiroya MURAKAMI ◽  
Shota YONEZAWA ◽  
Naonobu KANAMARU
Keyword(s):  
High Strength Steel ◽  
High Strength ◽  
Expansion Ratio ◽  
Fatigue Properties ◽  
Hole Expansion ◽  
Steel Sheets ◽  
Hole Expansion Ratio

Forming Performance of 800MPa Grade Advanced High Strength Steels

2013 ◽  
Vol 455 ◽  
pp. 173-178 ◽  
Author(s):  
Mei Zhang ◽  
Yu Xiang Ning ◽  
Jun Zhang ◽  
Zi Wan ◽  
Tao Wang
Keyword(s):  
High Strength Steels ◽  
High Strength ◽  
Expansion Ratio ◽  
Hole Expansion ◽  
Advanced High Strength Steels ◽  
Hsla Steels ◽  
Blank Holding Force ◽  
Hole Expansion Ratio ◽  
Test Hole ◽  
Springback Behavior

800MPa grade Advanced High Strength Steels (AHSS), including Complex Phase steel CP800 and Ferrite-Bainite steel FB800, were chosen to test the forming performance in different test conditions and compared with the reference traditional high strength low alloy (HSLA) steels HR700LA. Tensile test, hole expansion (HE) test, and HAT shape stamping test were taken to investigate the forming performance of the materials. Test results indicated that the studied 800MPa grade AHSS showed a better strength ductility balance compared with the reference steel. Among all the steels researched, FB800 showed the best hole expansion ratio (HER), and CP800 the worst. Springback angles of AHSS after HAT shape stamping tests were markedly smaller than those of HR700LA steels, though the springback angles of HR700LA decreased continuously with blank holding force (BHF) increasing. Steel FB800, CP800S and CP800B had much better shape stability compared with steels HR700LA. AHSS showed much smaller springback behavior under the same stamping condition, especially for steels CP800-B, FB800-2 and FB800-1. When increasing the BHF to 100KN, AHSS showed the largest springback deformation. Among the three kinds of CP800 steels researched, steel CP800-B indicated outstanding springback restrain trend in BHF further increasing attempt. So, springback behavior could be restricted obviously by using a larger BHF in AHSS CP800B forming operations.

scholarly journals Characterization of the Stretch Flangeabitity of High-Strength Bainitic Steel: The Significance of Variant Pairs

Materials ◽  
2021 ◽  
Vol 15 (1) ◽  
pp. 276
Author(s):  
Zhiquanquan Wang ◽  
Zifeng Guo ◽  
Chengjia Shang ◽  
Bin Chen ◽  
Yajun Hui
Keyword(s):  
High Strength ◽  
Expansion Ratio ◽  
Influential Factors ◽  
Low Carbon ◽  
High Angle ◽  
Governing Factor ◽  
Stretch Flanging ◽  
Hole Expansion Ratio ◽  
Micro Alloyed Steel

Variant pairs have an indispensable function on mechanical properties such as low impact toughness. Therefore, it was assumed that they would also affect the HER (Hole Expansion Ratio, an indicator to evaluate stretch flanging performance). To clarify this, a comprehensive analysis of the common influential factors in an 800 MPa grade low carbon micro-alloyed steel, i.e., the retained austenite, the M/A (Martensite/Austenite) island, the titanium precipitations, the grain diameter, the density of high angle grain boundaries and the textures, was first conducted. It was found that they did not match well with the HER, suggesting that they were not the governing factor for HER in this steel. However, the dominating crystallography groups and the variant pairing results indicated that they fitted well with the HER. In the samples with high HERs, the CP (Close Packed) groups dominated the transformation, wherein one individual CP group consisted of two or more Bain groups, whereas it evolved into the domination of joint CP groups and Bain groups for the low HER sample. Further analysis on the variant pairing features indicated that a correlation occurred between the HER and the high angle variant pairs. In the steels with high HERs, high-angle variant pairs of V1/V2, V1/V3 that transformed from the same CP group, particularly of V1/V2 pair, were mostly generated. They turned to V1/V9, V1/V10, V1/V12, V1/V15, V1/V17, and V1/V18 pairs from differential CP groups, especially the V1/V12 and V1/V15 pair for low-HER steel. This result showed that V1/V2, V1/V12, and V1/V15 might have accounted most for the HER in this steel. The underlying reason was that the V1/V2 pair was specialized in supplying a slip passage for dislocation transmission across a grain boundary with little resistance, whereas the dislocation transmission ability for V1/V12 and V1/V15 pair was particularly poor. Thus, to efficiently enhance the HER, one should regulate the variant pairs by augmenting the V1/V2 fraction and suppressing the formation of the V1/V12 and V1/V15 pair.

Increasing the Hole Expansion Ability of High Strength Steel Sheet by Improving the Pre-Hole Shearing Process

2019 ◽  
Vol 962 ◽  
pp. 167-174
Author(s):  
Prasan Saengkhiao ◽  
Komgrit Lawanwong ◽  
Pakorn Chumrum
Keyword(s):  
High Strength Steel ◽  
Steel Sheet ◽  
High Strength ◽  
Expansion Ratio ◽  
Expansion Process ◽  
Forming Process ◽  
Research Papers ◽  
Hole Expansion ◽  
Hole Edge ◽  
Hole Expansion Ratio

The hole expansion process is a forming process that the fracture easily occurred on the hole edge. Therefore, many research papers presented various processes to make a precision pre-hole shearing to a hole expansion process such as wire cut, EDM, and laser cut. The purpose of this work is to increase a hole expansion ratio on the high strength steel sheet. The conventional piercing by piercing punch and piercing with a counter punch was used to make a pre-hole shearing process in this research paper. The high strength steel grade 590 and 980 MPa of 1.2 mm of thickness were used in the experiments. From experimental results, it was found that the pre-hole shearing by piercing with a counter punch can be increased a shear surface on cutting edge of workpiece. Moreover, the used piercing with counter punch can improve the hole expansion ratio more than the conventional piercing process because sheared surface around the hole edge was increased.

scholarly journals The Potential of Self-Tempered Martensite and Bainite in Improving the Fatigue Strength of Thermomechanically Processed Steels

2018 ◽  
Vol 165 ◽  
pp. 20006 ◽  
Author(s):  
Ulrich Krupp ◽  
Mikhail Solovev ◽  
Felix Honecker ◽  
Bernhard Adams ◽  
Jan-Christoph Florian
Keyword(s):  
Fatigue Strength ◽  
Fatigue Testing ◽  
Hot Forging ◽  
High Strength ◽  
Lower Bainite ◽  
Air Cooling ◽  
Tempered Martensite ◽  
Slow Cooling ◽  
Pronounced Increase ◽  
Ultrasonic Fatigue

In contrast to a two-stage hardening and tempering process, the definition of optimized cooling routes after hot working of low-alloy Cr steel allows the adjustments of high-strength microstructures with a sufficient degree of ductility at the same time without any additional heat-treatment. While compressed air cooling after hot forging of micro-alloyed steel grades leads to the formation of lower bainite with finedispersed cementite platelets, quenching by water spray down to the martensite start temperature results in the formation of martensite, that is self-tempered during the subsequent slow-cooling in air. The precipitation of nano-sized cementite precipitates result in superior mechanical properties with respect to impact and tensile testing. Cyclic deformation and crack propagation tests being carried out using resonance testing (100Hz) and ultrasonic fatigue testing (20kHz) systems revealed a pronounced increase in fatigue strength by about 150MPa of the self-tempered martensite condition as compared to the bainitic modification. For the latter one, a steady decrease of the fatigue strength is observed rather than the existence of a real fatigue limit.

The Influences of Aging on the Two Relaxation Peaks in the Air-Cooled Fe71Al29 Alloy

2011 ◽  
Vol 80-81 ◽  
pp. 457-463
Author(s):  
S.Y. Gu ◽  
Z. C. Zhou ◽  
Y.J. Yan ◽  
H. Yang ◽  
J. Du
Keyword(s):  
Internal Friction ◽  
Heating Temperature ◽  
Great Influence ◽  
Vacuum Induction Furnace ◽  
Al Alloy ◽  
Air Cooling ◽  
Heating And Cooling ◽  
Free Decay ◽  
Temperature Spectra ◽  
Temperature Ageing

Binary Fe-Al alloy ingots with 29 at. %Al were prepared with commercially pure Fe and Al in a vacuum induction furnace. The specimens used in the internal friction measurement were cut using an electric sparkle machine from the ingots into a dimension 68×1.7×0.9 mm3. The heat treatment was performed in such regimes as heating temperature 900°C and holding time 1 hour in an argon atmosphere followed by furnace cooling or air cooling. The air-cooled samples were then aged at 350°C for different time to achieve the different degree of disorder. The internal fiction-temperature spectra of the air-cooled Fe71Al29 alloy were measured during both heating and cooling using a computer-controlled automatic inverted torsion pendulum through free-decay and forced vibration methods. It has been found that three internal friction peaks appear during heating at 180°C termed as P1, 340°C termed as P2 and 510°C termed as P3 in air-cooled specimens, respectively. P1 and P2 peaks disappear and P3 is retained during cooling. XRD results show that the air-cooled Fe71Al29 alloy is disordered and the furnace-cooled specimen possesses ordered structures. The air-cooled specimen presents similar diffraction peaks to the furnace-cooled specimen after it is heated to 650°C and then in-situ cooled to room temperature. Ageing has great influence on the P1 and P2 peaks for the air-cooled Fe71Al29 alloy. The height of P1 and P2 peaks is decreased with increasing aging time, which is due to the reduction in the disorder degree of the air-cooled specimen.

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