Causes of Cracking in Q345D Steel Plate Cold Bending Forming
The causes of the Q345D steel plate cracking during cold bending were analyzed using macroscopic observation, chemical composition analysis, mechanical property test, fracture analysis, metallographic examination, and hardness test. The results show that the steel plate‘s chemical composition, matrix structure, and mechanical properties meet the technical requirements; When cutting, a groove is formed on the side of the steel plate. The groove is made of Martensite, which has poor plasticity. During bending, stress concentration occurs there, which is the fundamental reason for cracking the Q345D steel plate during cold bending.
A batch of thick plates cracked during the cold bending and pressing arc-forming process, resulting in material scrap. The steel plate grade is Q345D-Z25, in a normalized state, with a thickness of 60mm and a width of 400mm. To identify the causes of Q345D steel plate cracking and eliminate safety hazards, the author analyzed the causes of Q345D steel plate cracking through physical and chemical inspections to avoid the recurrence of such accidents.
1. Physical and chemical inspection
1.1 Macroscopic observation
The length of the cracked Q345D steel plate is about 840mm, and the angle between the steel plate and the horizontal direction after bending and cracking is about 30 °, as shown in Figure 1. From Figure 1, it can be seen that there are obvious indentations on the surface of the steel plate, which are caused by the contact and extrusion between the steel plate surface and the bent-core. A crack perpendicular to the rolling direction and penetrating the width of the plate can be seen directly below the indentation. From Figure 2, it can be seen that there are grooves on both sides of the crack, which are located at the lowest point of the groove. Further observation shows that the groove extends along the thickness direction of the plate, with a length of about 40mm (basically the same as the crack length). Compared with the A side of the steel plate, the groove on the B side is narrower, and the crack length in the thickness direction is about 45mm.
Figure.1 Macromorphology of Cracked Q345D Steel Plate
Figure.2 Macro morphology of A-side groove in cracked Q345D steel plate
Figure.3 Macro morphology of fracture surface of Q345D steel plate with cracking
Figure.3 shows the cross-sectional morphology of the Q345D steel plate after fracture along the crack. From Figure 3, it can be seen that the overall cross-section is relatively flat, with clear radial stripes visible. The cross-section can be divided into three parts based on different morphologies and characteristics.
(1) Crack source:
Based on the characteristics of the radiation zone, it can be seen that the radiation is converging towards the surface of side A of the Q345D steel plate, indicating that the crack source is located near the surface of side A and cracks from the bottom of the groove; The surface of the groove is rough and striped, and there is blocky peeling near the fracture surface, forming a notch, indicating that the area is relatively brittle near the surface.
(2) Eixample (cutting lip, radiation lines, extended steps):
The Eixample is large, accounting for 80% of the fracture area, and the overall area is relatively rough, with obvious radial stripes visible; Shear lip features can be seen near the surface of the Eixample, and obvious expansion steps can be seen near the center of the steel plate.
(3) Artificial tear zone:
This area is 20% of the fracture area and is artificially fractured, with slight plastic deformation visible.
Based on the above analysis, it can be seen that there are penetrating cracks (in the width direction) on the outer surface of the Q345D steel plate during cold bending and extrusion, and the fracture surface exhibits brittle fracture characteristics. The cracks originate from the side of the steel plate, and there are grooves at the crack initiation point, which may be related to the grooves.
1.2 Chemical composition analysis
Samples were taken from cracked Q345D steel plate, and chemical composition analysis was carried out according to GB/T 4336-2016 Determination of Multi-element Content of Carbon Steel and Medium low Alloy Steel Spark Discharge Atomic emission spectroscopy. See Table 1 for the test results. From Table 1, it can be seen that the chemical composition of the fractured steel plate meets the technical requirements for Q345D-Z25 steel in GB/T 1591-2008 “Low Alloy High Strength Structural Steel” and the quality assurance certificate.
Table.1 Chemical Composition% of Cracked Q345D Steel Plate
| Element | C | Si | P | Mn | S | Cr | Ni | Mo | V | Ti | Al | Cu |
| Detection value (mass fraction) | 0.17 | 0.31 | 0.018 | 1.5 | 0.003 | 0.024 | 0.01 | 0.002 | 0.002 | 0.003 | 0.039 | 0.013 |
| Standard value (mass fraction) | ≤0.18 | ≤0.50 | ≤0.030 | ≤1.70 | ≤0.007 | ≤0.30 | ≤0.50 | ≤0.07 | ≤0.15 | ≤0.20 | ≥0.020 | ≤0.30 |
1.3 Mechanical performance test
Take tensile test specimen, impact test specimen, bending test specimen, and Z-direction tensile test specimen from cracked Q345D steel plate, and conduct mechanical property test according to Standard for Tensile Testing of Metallic Materials (GB/T 228.1-2010), Charpy Pendulum Impact Test Method of Metallic Materials (GB/T 229-2007), Metallic Materials Bending Test Method (GB/T 232-2010) and Steel Plates with Thickness Direction Properties (GB/T 5313-2010). See Table 2- Table 5 for the results. From Tables 2 to 5, it can be seen that the mechanical properties of the cracked Q345D steel plate meet the technical requirements for Q345D-Z25 steel in relevant standards.
Table.2 Tensile Testing Results
| Testing items | RP0.2/MPa | Rm/MPa | A/% |
| Horizontal detection value | 340 | 545 | 36 |
| Longitudinal (rolling direction) detection value | 350 | 550 | 36 |
| Standard value | ≥325 | 470–630 | ≥20 |
Table.3 Impact Test Results
| Testing items | Impact absorption energy (-20 ℃)/J |
| Horizontal detection value | 118157166 |
| Longitudinal (rolling direction) detection value | 174144160 |
| Standard value | ≥34 |
| Required value of warranty certificate | 166 |
Table.4 Bending Test Results
| Testing items | Bending angle/(°) | Bend diameter/mm |
| Horizontal detection value | 180 | 75 |
| Longitudinal (rolling direction) detection value | 180 | 75 |
Table.5 Z-direction tensile elongation test results
| Testing items | Single value/% | Average% |
| 70 | ||
| Detection value | 72 | 68 |
| 63 | ||
| Detection value | ≥15 | ≥25 |
1.4 Fracture analysis
After cleaning the fracture specimen, observe it under a scanning electron microscope. From Figure 4a) – b), it can be seen that the crack starts at the bottom of the groove in the thickness direction of the A-side plate, and multiple cracks parallel to the cross-section can be seen at the bottom of the groove. Further magnification observation reveals a dimple morphology near the crack source. It can be seen from Figure 4c) – d) that obvious radial stripes can be seen near the crack source in the Eixample. Further magnification observation shows that the Eixample is characterized by cleavage, dimple morphology can be seen at the shear lip of the steel plate surface far from the groove, dimple morphology can be seen at the growth step in the middle of the fracture, and cleavage characteristics can be seen on both sides of the step. As shown in Figure 4e, the morphology of dimples can be observed in the artificially torn area.
Figure.4 Microscopic Morphology of Different Regions of Cracked Q345D Steel Plate Fracture Surface
1.5 Metallographic examination
1.5.1 Inclusion inspection
Take a longitudinal section sample from the crack source of the Q345D steel plate, prepare it according to GB/T 13298-2015 “Methods for Testing the Microstructure of Metals,” and then observe it under an optical microscope. According to the actual inspection method A in GB/T 10561-2005 “Determination of non-metallic inclusion content in steel – Standard rating diagram microscopic inspection method” and the ISO rating diagram in ISO4967-1998 “Determination of non-metallic inclusion content in steel – Standard rating diagram microscopic inspection method,” the rating results of non-metallic inclusions in cracked Q345D steel plate are shown in Table 6. From Table 6, it can be seen that the purity of the Q345D steel plate is good.
Table.6 Rating Results of Nonmetallic Inclusions in Cracked Q345D Steel Plate
| Types of inclusions | A | B | C | D | DS | ||||
| Fine series | Coarse series | Fine series | Coarse series | Fine series | Coarse series | Fine series | Coarse series | ||
| Level | 0.5 | 0 | 0 | 0 | 0.5 | 0 | 1 | 0 | 0.5 |
1.5.2 Microscopic observation
Figure 5 shows the microstructure morphology of the polished and corroded Q345D steel plate at different positions, which was corroded using a 4% (mass fraction) nitric acid alcohol solution. It can be seen from Figure 5 that the microstructure near the center of the Q345D steel plate is ferrite + pearlite, and the microstructure of the crack source and surface is Martensit e+, a small amount of ferrite; The crack starts at the bottom of the groove. After measurement, the depth of the groove is about 1.2mm, and there are multiple notches below the fracture surface, indicating that the crack extends towards the inside of the steel plate; The surface of the steel plate is a hardened layer formed after cutting, and the hardened layer at the notch has fallen off. In addition, there is no obvious accumulation of non-metallic inclusions at the crack source; The microstructure of the hardened layer is Martensite + ferrite, which is obtained by cooling the steel at a rate greater than the critical cooling rate at a high temperature. The microstructure is hard and brittle, and cracks are easily generated under stress concentration.
Figure.5 Microstructure of Cracked Q345D Steel Plate in Polished and Corroded States at Different Positions
Figure.6 Macromorphology of Cracked Q345D Steel Plate after Hot Acid Corrosion
1.6 Hardness inspection
Vickers hardness test was carried out at the crack source, near the crack source, and at the center of the cracked Q345D steel plate, and the results are shown in Table 7. From Table 7, it can be seen that the surface hardness at the crack source of the steel plate is about 481HV1. The internal hardness of the steel plate is about 181HV1, indicating an abnormality in the surface structure of the crack source, which is significantly different from the center structure of the steel plate. This is consistent with the microstructure observation results.
1.7 Thermal acid corrosion test
Transverse specimens were taken from the cracked Q345D steel plate and polished. According to GB/T 226-2015, a mixed solution of industrial hydrochloric acid and water with a volume ratio of 1: 1 was used for thermal acid corrosion testing—the macroscopic morphology of the steel plate after thermal acid corrosion is shown in Figure 6. Compared with the rating chart in the GB/T 1979-2001 “Rating Chart for Macrostructural Defects in Structural Steels” standard, steel plates’ macroscopic defect detection results show a central porosity level of 1.0 and a general porosity level of 0.5.
Table.7 Hardness Test Results
| Test location | Hardness/HV1 |
| Surface of crack source | 481 |
| Near the crack source | 183 |
| Steel plate core | 181 |
2. Analysis and Discussion
The physical and chemical inspection results of the cracked Q345D steel plate indicate that its chemical composition, tensile properties, impact properties, and Z-direction tensile properties all comply with the technical requirements for Q345D-Z25 steel in the GB/T 1591-2008 standard and quality assurance certificate, and there are no abnormalities in non-metallic inclusions and matrix structure.
Macroscopic observation shows that the cracking of the steel plate originates from the side groove, and the microstructure of the crack source and its adjacent surface is Martensite + a small amount of ferrite. The fractured steel plate is a normalized steel plate with the brand Q345D-Z25, and the normal microstructure should be ferrite + pearlite. The hardness of the center of the steel plate is about 181HV1, and the surface hardness at the crack source is 481HV1, with a significant difference in hardness indicating abnormal microstructure at the groove. The Martensite of low carbon steel (carbon mass fraction less than or equal to 0.25%) is mainly in the form of lath, which is obtained by heating the steel to the austenitic state and cooling it to the temperature below the transformation point at a cooling rate greater than the critical cooling rate. Martensite is characterized by high strength, high hardness, and poor plasticity.
During cold bending forming, the outer convex surface of the steel plate first cracks, and the outer convex surface, especially the opposite side of the bending core, is subjected to tensile stress with high stress. If there is a groove, it will inevitably generate significant stress concentration at that location, resulting in uneven stress distribution on the convex surface of the steel plate. Local stress is too large, exceeding the yield strength of the material, and ultimately cracking occurs at the bottom of the groove where the stress concentration is most obvious.
3. Conclusion
- (1) When cutting the cracked Q345D steel plate, a groove is formed on its a-side. The groove is a Martensite structure with poor plasticity. During bending, stress concentration occurs here, which is the fundamental reason for cracking the Q345D steel plate during cold bending.
- (2) It is recommended that the steel rolling mill pay attention to the working status of on-site cutting tools and enhance the visual inspection of products before leaving the factory to prevent such defects from appearing on the steel surface.
Author: Zeng Qinglin
