Fire resistance of low alloy Q420d high-strength steel column under high-temperature creep

Abstract

The high-temperature mechanical and creep characteristics of Q420D steel are investigated in this essay. To determine the steel's high-temperature yield strength, the high-temperature tensile test of Q420D steel was first performed. In the temperature range of 400°C–800°C, the high-temperature creep test under various pressures was conducted, and the creep strain curve over time was produced. Finite element analysis and comparison were done to examine the impact of creep strain on the Q420D steel column's bearing capacity under high-temperature conditions. The findings demonstrated that: Using Abaqus, a finite element fire resistance analysis of a Q420D steel column, was conducted while taking initial geometrical flaws, residual stress, and creep effect into account. As a result, the critical temperature of a Q420D steel column under various load ratios was determined. The largest deviation from the critical temperature in the standard GB51249–2017 was 2.9% when the creep effect was taken into account under the load ratio R  =  0.3. The highest reduction in fire resistance limit time under low load ratio conditions, taking into account the creeping impact of Q420D steel columns, is 35%. The findings demonstrate that the high-temperature creep energy greatly lowers the steel column's fire resistance.

Introduction

Based on carbon structure steel with a carbon content of 0.16%–0.20%, low-alloy Q420D high-strength structural steel is fused by adding a tiny quantity of alloying materials. Its toughness is greater than that of regular carbon structural steel, and it performs well under cold and hot pressure processing as well as welding and has superior plasticity and corrosion resistance. Low brittle transition temperatures are also present in several low-alloy, high-strength structural steels. ¹ Massive, high-pressure vessels, overweight machinery, oil platforms, and other large welded structural elements are the major applications for Q420D structural steel. The deformation of steel structures over time may be nearly completely disregarded under typical temperature loads. However, under high-temperature stress, the deformation of steel structures alters substantially over time.

The influence of high-temperature creep on the bearing capacity of steel structures and the fire resistance of steel structures have been paid more and more attention by researchers.^2,8 At present, many scholars have studied the influence of high-temperature creep on the stability and fire resistance of steel structures. For example, Li et al. ⁹ conducted high-temperature creep tensile tests on domestic high-strength steel Q960, obtained the creep strain–time relationship under different temperatures and loads, and analyzed the effect of creep effect on the fire resistance of Q960 steel column through finite element analysis. Wang et al. ¹⁰ established a model of the Q460 steel column by using finite element software under the condition of considering creep and residual stress and analyzed the fire-resistance analysis under different slant-length ratios, load ratios, initial bending, and heating rates. Zhou et al. ¹¹ obtained the high-temperature mechanical parameters of Q355 steel by high-temperature stretching. The creep strain–time curve was obtained by the high-temperature creep test. Fire resistance of CFS was simulated considering high-temperature creep. Maria et al. ¹² conducted a high-temperature creep experiment on ASTM A992 steel and used the hyperbolic sinusoidal model to fit the parameters of the experimental data. Finally, the creep buckling of ASTM A992 steel column at high temperatures was studied. The results show that the creep has an obvious effect on the stability of the ASTM A992 steel column under high-temperature conditions. Mohammed et al. ¹³ developed the concept of transient cutting modulus to simulate the buckling behavior of steel columns at high-temperature with time. Under the condition of obtaining creep data of ASTM A992 M steel, creep buckling was calculated by using Abaqus. The results showed that ignoring the creep effect would lead to an unsafe prediction of the strength of steel columns subjected to fire. Venkatesh et al. ⁴ conducted high-temperature creep tensile tests on low alloy ASTM A572 high-strength steel under different stresses in the range of 400–800°C, and the test results showed that with the increase in temperature, the creep deformation under low stress was also significant.

At present, domestic and foreign studies show that high-temperature creep has a significant impact on the stability and fire resistance of steel under a fire environment, so it is necessary to study the high-temperature creep of steel. Because of low alloy steel columns Q420D yet do not exist, considering the high-temperature creep under the effect of fire resistance research. Therefore, this study used TSC electronic high-temperature creep instrument to carry out a high-temperature creep test on low alloy Q420D steel and obtained creep data under different temperatures and different stress levels. Based on the creep data of Abaqus software, finite element fire resistance analysis was carried out on the Q420D steel column.

Mechanical behavior under high temperature

Test set-up and specimens

In this experiment, E45–305 microcomputer-controlled electronic universal testing machine was used for a steady-state high-temperature tensile test. The maximum test force of the experimental equipment was 300KN, and the test loading rate ranged from 0.001 mm/min to 254 mm/min. The test loading rate of this high-temperature tensile test was 1.5 mm/min. The maximum deformation range of the equipment is 30 mm. The temperature required for the test is provided by the resistance wire in the high-temperature furnace. The maximum temperature of the equipment can reach 1300°C, but due to the influence of the fixture, the maximum temperature of the test can only be heated to 900°C. The temperature monitoring is carried out by three electric couples to monitor the temperature of the specimens inside the heating furnace. High-temperature drawing equipment is shown in Figure 1(a). The specimens were processed from hot-rolled Q420D steel, the chemical composition of Q420D steel is shown in Table 1, and the specimens are shown in Figure 1(b).

Figure 1.

High-temperature tensile equipment and specimens.

Table 1.

Chemical composition of Q420D steel.

C	Mn	Si	P	S	V	Nb	Ti	Al	Cr	Ni
0.2	1.00–1.70	0.55	0.03	0.03	0.02–0.20	0.015–0.060	0.02–0.20	0.015	0.4	0.7

Test results

The mechanical properties of Q420D steel at room temperature conform to Chinese specification GB/T 1591–2018, ¹ and the values are shown in Table 2. Figure 2 shows the stress–strain curves of Q420D steel at various temperatures. It can be seen from Figure 2(a) that the yield strength and tensile strength of the material at 300°C are higher than those at 200°C–400°C, which indicates that the metal has a blue embrittlement phenomenon. Under high-temperature conditions, Q420D steel has no obvious yield plateau. According to Figure 3, it can be seen that the reduction coefficient of elastic modulus, yield strength, and tensile strength of Q420D steel at each temperature, and the reduction coefficient is the ratio of the mechanical parameters of each group of test temperature to the mechanical parameters of room temperature. According to Figure 3, it can be seen that the reduction coefficients of elastic modulus, yield strength, and tensile strength of Q420D steel at various temperatures. The reduction factor is the ratio of the mechanical parameters at each set of test temperatures to those at room temperature. The reduction coefficient at 300°C is slightly higher than 1, which is in line with the phenomenon of blue enrichment of the metal at 300°C. The technical code for fire prevention of building steel structures (CECS200) stipulates that the stress corresponding to 1% strain is taken as the yield strength. In this paper, for the yield strength of Q420D steel, the stress corresponding to 1% strain is taken as the yield strength. ¹⁴

Figure 2.

High-temperature tensile stress–strain curve of Q420D steel.

Figure 3.

Reduction coefficient.

Table 2.

Chemical composition of low alloy Q420D high strength steel.

Test number	Yield strength /Mpa	Tensile strength/MPa	Elongation/%
Q1	456	608	23
Q2	471	621	24
Q3	463	598	23
Average value	463	609	23

Creep tests

Test set-up and test specimen

The equipment used in this high-temperature creep test is the TSC304B high-temperature creep endurance testing machine, as shown in Figure 5. The maximum test force of the test equipment is 50KN, and the maximum test temperature is 1200°C. The heating furnace of the instrument is a pair of open atmosphere furnaces, the furnace shell is made of stainless steel, and the heat insulation material is made of ultrafine ceramic cotton. Using the three sections of the heating wire heating, each section of the heating wire adopts SSR modules and PWM control. The test temperature and set test parameters can be read through the display screen, the accuracy of the test instrument is less than or equal to ±0.5%, and a timing error is ±0.1%, so the instrument has a very high confirmation. The creep test method at high temperatures is following the Chinese standard “Creep Test Method for Uniaxial Tension of Metallic Materials” (GB/T2039–2012).¹⁵ The specimen is processed by a hot-rolled low-alloy Q420D high-strength steel column with a diameter of 18 mm. The physical object and size of the specimen of high-temperature creep are shown in Figure 4. There are two bumps at both ends of the specimen, which are used to install the extender.

Figure 4.

Specimen size and physical drawing. (a) Physical drawing;(b) Specimen size (UNITS:mm).

Figure 5.

Testing device for creep test at elevated temperature.

Test procedure

In the high-temperature creep test of this study, the test specimens corresponding to the test conditions were first installed on the high-temperature fixture, and then the extender and three groups of electric couples were loaded on the high-temperature fixture, the high-temperature furnace was closed, and the glass fiber was added at both ends of the high-temperature furnace for heat insulation. Secondly, in the high-temperature creep equipment, control panel set up experiment temperature, stress, and related material parameters. When the temperature of the metal material is greater than or equal to 0.3Tm (Tm is the melting temperature of the material), the creep is obvious, and the melting point temperature of steel is around 1500°C, so the value of the temperature in this test is between 400°C and 800°C. The test conditions of the high-temperature creep test are shown in Table 3, where stress ratio D is the ratio of test stress to yield stress under test temperature. When the test reaches the specified time or the specimen breaks, the test will stop.

Table 3.

Test conditions for high-temperature creep.

Test temperature t/°C	yield strength f0.01/MPa	Test stress σ/MPa	Stress ratio D
400	377.2	320.62, 282.9, 245.18	0.85, 0.75, 0.65
500	295.8	251.43, 221.85, 162.69, 73.95	0.85, 0.75, 0.55, 025
550	274.8	247.32, 206.1, 123.66, 96.18	0.90, 0.75, 0.45, 0.35
600	219.5	186.58, 164.63, 76.83, 54.88	0.85, 0.75, 0.35, 0.25
650	164.1	139.49, 123.08, 106.66, 73.85	0.85, 0.75, 0.35, 0.25
700	105.6	89.76, 68.64, 47.52	0.85, 0.65, 0.45
750	71.3	46.35, 32.09, 28.52	0.65, 0.45, 0.4
800	58.2	37.83, 26.19, 17.46	0.65, 0.45, 0.3

Test results

Figure 6 shows the specimens after the creep tests. Because the range of the extensor of the experimental equipment is only 12 mm, only a relatively short specimen has a tightening failure, and the rest are seriously deformed or moderately deformed, or the test time is more than 10 h. Due to the high temperature and the direct contact between the specimen and the air, the specimen is oxidized, and a shell-like oxide layer is formed on the surface of the specimen. When the temperature is higher than 700°C, the oxide layer becomes more brittle than the deep component. Creep curves of specimens under different test temperatures and different test stresses are shown in Figure 7. In Figure 7, “Q-400–0.85” means: Q stands for Q420D steel, 400 stands for 400°C test temperature, and 0.85 stands for the ratio of test stress to the yield strength of the material under the test temperature. Under the action of the test load, the stress produced by the load is less than yield strength.

Figure 6.

Specimens after test.

Figure 7.

Test results at different temperatures.

Then, the deformation of the specimen is calculated by formula (1). ¹² Where ε is the total strain, ${\epsilon }_e$ is the elastic deformation, and ${\epsilon }_c$ is the creep deformation, so the corresponding elastic deformation is deducted from the total creep deformation:

$$\epsilon ={\epsilon }_e+{\epsilon }_c$$

(1)

The following conclusions can be drawn from the creep curve in Figure 7:

1. According to Figure 7(a), it can be seen that the first stage and the second stage can be clearly seen in the creep curve under various stress conditions at 400°C. This is because the steel at temperature below 0.3 T_m, creep deformation is not particularly evident. According to Figure 7(b)–(h), when the temperature is higher than 0.3T_m, the second and third stages under most stress conditions dominate the whole high-temperature creep test. In the second stage, the growth rate approximately tends to be constant. In the third stage, the growth rate gradually increases. In addition, under the same stress ratio, the higher the temperature, the greater the growth rate in the second stage. According to Figure 7(f)–(h), when the steel temperature exceeds 700°C, the creep deformation is significantly enhanced.

2. The boundary between the first stage and the second stage is also obvious at higher temperature but lower stress. According to Figure 7(d), at 600°C, when stress ratio D  =  0.35, and at the same temperature, the greater the stress applied in the test, the greater the creep will be. For example, when the temperature is 500°C, the greater the creep will be as the load on the specimen increases. When the stress ratio is D  =  0.85, the creep process enters the third stage at about 1.5 h. When the stress ratio is D  =  0.75, the creep process enters the third stage in about 3.5 h, which is 2 h longer than the former.

Creep model

Introduction of creep model

Creep models include: Time-Hardening model, Norton model, ¹⁶ Dorn model, ¹⁷ Burgers model, ¹⁸ Harmathy model, ¹⁹ and Fields & Fields model. ²⁰ When the creep process of steel enters the third stage, it means that steel is about to lose its bearing capacity. Therefore, the research on high-temperature creep of steel mainly studies the first and second stages of creep. Therefore, this study mainly studies the first and second stages of high-temperature creep of Q420D steel. The time-hardening creep model has a simple structure and can well reflect the first and second stages of creep. It has been applied in finite element software such as Abaqus and ANSYS.

Time-hardening creep model is shown in Formula (2), and the calculation formula of creep rate is shown in Formula (3):

$${\epsilon }_{cr}=a{\sigma }^b{t}^c$$

(2)

$${\epsilon }_{cr}^{\mathrm{\prime }}=ca{\sigma }^b{t}^{(c-1)}$$

(3)

where ${\epsilon }_{cr}$ and ${\epsilon }_{cr}^{\mathrm{\prime }}$ represent creep strain and creep strain rate, respectively, a, $b$ _, and c are material constants; $\sigma$ is the test stress; t is the time of creep test.

Fitting of creep model parameters

The Time-Hardening creep model is used to conduct nonlinear fitting of the creep curves of Q420D steel at various temperatures. The creep fitting curves are shown in Figure 8, and the fitting parameters a, b, and c at each temperature are shown in Table 4. According to Figure 8, it can be seen that the time-hardening creep model can fit the creep curves of various temperatures at Q420D well, especially for the first and second stages. As can be seen from Table 4, when the temperature is greater than 600°C, the fitting degree of creep is generally greater than 0.97. And we can see that the size of parameter a is gradually increasing as the temperature increases. The variation trend of the parameter b is identical to the finding of literature, ⁹ and it is completely contrary to that of the parameter b.

Figure 8.

Creep strain test results and fitting values at different temperatures.

Table 4.

Temperature fitting parameters of Q420D steel time-hardening model.

Temperature	a	b	c	Correlation coefficient of ${\epsilon }_{cr}$
400°C	2.6e–11	3.93	0.55	0.968
500°C	2.204e–10	2.68	0.93	0.973
550°C	3.46e–7	2.17	1.04	0.953
600°C	1.5e–10	3.65	0.92	0.971
650°C	2.16e–6	1.55	1.44	0.975
700°C	1.89e–6	1.91	1.28	0.995
750°C	1.4e–6	2.28	1.364	0.976
800°C	4.797e–5	1.96	1.099	0.998

Fire resistance analysis of Q420D steel column

Finite element model verification

The Q420D steel column model was established by Abaqus software, and the mesh type of the finite element model was the S4R element. To verify the correctness of the model, a nonlinear buckling analysis of steel columns is carried out. Initial geometric defects and residual stresses are considered. The first mode obtained by the eigenvalue buckling of the steel column is taken as the initial geometric defect, as shown in Figure 9(a). The residual stress distribution of steel columns adopts the residual stress distribution model in literature, ²¹ and the distribution in the finite element model is shown in Figure 9(b). The model dimensions of the Q420D steel column are shown in Table 5. Finite element software is used to calculate the ultimate bearing capacity of steel columns, and the test results published by Shi Gang et al. ²² are compared and verified. The comparison between the test results and the finite element results is shown in Table 6. According to Table 6, the error range between the test results and the finite element results is within 10%. The correctness of the finite element model established in this paper is proved.

Figure 9.

Finite element model of round steel string (a) first-order mode; (b) residual stress distribution diagram.

Table 5.

Q420D steel column model size.

Specimen number	Outside diameter D/mm	Inner diameter d/mm	Wall thickness t/mm	Length l0/mm
D420–20	275	263	6	1890
D420–30	275	263	6	2833
D420–40	275	263	6	3776
D420–50	275	263	6	4720
D420–60	275	263	6	5664

Table 6.

Comparison of experimental results and finite element results for verification.

Specimen number	Test results Fu,test/KN	Finite element results Fu，FE/KN	Error/%
D420–20	2147	2098.3	2.3%
D420–30	2081	2120.5	−1.9%
D420–40	2026.5	1934.8	4.5%
D420–50	2029	1907.3	6%
D420–60	1958	1881.6	3.9%

Based on the above finite element model, the high-temperature mechanical parameters of Q420D steel and the corresponding Time-Hardening creep model are introduced. The steel column with test number D420–20 was subjected to constant load heating analysis, and the steel column model with finite element analysis was carried out. The steel column model is fixed at one end and hinged at the other. The critical temperature and lateral displacement in the middle of the Q420D steel column were calculated. The temperature of the steel column is loaded according to the Technical Code for Fire Protection of Building Steel Structures (GB 51249–2017). ²³ The air temperature was loaded according to the ISO-834 standard heating curve.

Critical temperature at which creep effects are considered

The critical temperature values of Q420D steel column under different load ratios were calculated by finite element software. In Figure 10 compares the critical temperature with and without the creep effect. According to the results in the figure, when the load ratio is greater than 0.75, there is little difference between the results of the standard critical temperature, the critical temperature with and without the creep effect. When the load ratio R  =  0.3, the maximum difference between the critical temperature with and without the creep effect is 14.5%. Because the creep effect aggravates the deformation of the steel column, the temperature of the steel column when it fails is finally reduced. The maximum difference between the critical temperature specified in the code and the finite element critical temperature considering the creep effect is 2.9%, indicating that the critical temperature specified in the code fully conforms to the critical temperature of the steel column considering the creep effect.

Figure 10.

Critical temperature.

Considering the effect of creep lateral displacement

In order to study the creep effect on the fire resistance limit of Q420D steel column, finite element software is used to calculate the change of the central lateral displacement of Q420D steel column with time under a certain load ratio, and the calculation results are shown in Figure 11. It can be seen from Figure 11(a) that when the load ratio R  =  0.3, the fire limit of steel column without considering the creep effect is 44.7 min, and the fire limit time with the creep effect considered is 29.1 min. The difference between the fire limit time with the creep effect considered and the fire limit time without considering the creep effect is 15.6 min. With the increase in the load ratio, the fire limit time decreases. When the creep effect is not considered, the fire limit time of the load ratio R  =  0.5 is reduced by 13.8 min compared to the fire limit time of the load ratio R  =  0.3. High-temperature creep aggravates the second-order effect of steel column, reduces the stiffness, and increases the deformation of steel column, which leads to the reduction of the fire resistance limit time of steel column and makes steel column more prone to instability and failure. Therefore, the influence of high-temperature creep on steel column should be considered in the stability of steel column under high temperature.

Figure 11.

Lateral displacement under different load ratios.

Conclusion

According to the experimental research and finite element analysis in this paper, the conclusions are as follows:

Q420D steel at 300°C steel will appear blue brittle phenomenon, steel yield strength, and tensile strength is higher than 200°C and 400°C, and the steel surface is blue. With the increase in temperature, the yield strength and tensile strength of steel decrease. According to the stress–strain curve of steel at high temperatures in this paper, it can be seen that Q420D steel has no yield platform at high temperatures.

By observing the high-temperature creep-time curve of Q420D steel, it can be understood that the second and third stages of high-temperature creep account for the main part of the creep curve. At the same temperature, with the increase of stress, the creep time also decreases, and the acceleration phase becomes more obvious. The total creep deformation increases with the test temperature.

According to the finite element analysis in this study, the creep effect reduces the critical temperature of the Q420D steel column. Under the same load, the creep effect also increases the lateral displacement of the Q420D steel column and reduces the fire resistance limit of the steel column.

The comparison between the calculation results of the critical temperature and the finite element results of the current code “Technical Code for Fire Prevention of Building Steel Structures” (GB 51249–2017) shows that the calculation results of the code are suitable for Q420D steel columns. The comparison between the calculation results of the critical temperature and the finite element results of the current code shows that considering the creep effect, it can well reflect the fire resistance of the Q420D steel column.