1)Research Institute,Baoshan Iron & Steel Co.,Ltd.,Shanghai 201999,China;2)State Key Laboratory of Development and Application Technology of Automotive Steels (Baosteel),Shanghai 201999,China

Abstract: In this paper,the effect of hot-rolling processing parameters on the post-rolling transformation behavior of C-Mn-Cr-Cu weather-resistant martensitic steel was investigated.Results indicate that the post-rolling transformation behavior,particularly the post-coiling transformation behavior,strongly depends on the coiling temperature.Under a high coiling temperature,ferrite/pearlite transformation was mainly observed with a low trans-formation speed and transformation dilation after coiling.With decreasing coiling temperature,the proportion of bainite transformation increased with an increased transformation speed and transformation dilation.The dependence of transformation behavior on the coiling temperature could be one of the most important reasons behind the coil collapse of the experimental steel.The optimization of hot-rolling and coiling processing parameters of the steel strip,including laminar flow cooling,coiling temperature,and suspension time on the winding machine,can signi-ficantly reduce the transformation dilation of the steel after hot rolling,thus effectively controlling the coil collapse of the steel.

Key words: AHSS; hot rolling; coiling; phase transformation; thermal dilation

1 Introduction

Advanced high-strength steels (AHSS) are light-weight sheet steels used in transportation applica-tions,which utilize the hardenability of hard phases,such as bainite,martensite,and austenite[1].Owing to their contribution to work hardenability resulting from hard phases,AHSS show superior strength and ductility,thus facilitating energy absorption during impact,ensuring safety,and reducing weight and cost.Recently,AHSS have grown rapidly and steadily,largely attributing to the demand for light-weight and energy-saving automobiles with enhanced safety[2-3].

Take an ultrahigh-strength weather-resistant steel for containers as an example.This is a martensitic steel with a composition of C-Si-Mn-Cr-Cu,including no expensive alloys except Cr and Cu used for weather resistance.The minimum yield strength of the low-cost and high-performance steel is 700 MPa,much higher than that of the conventional weather-resis-tant 400-MPa and 500-MPa grade steels.The steel is typically adopted for manufacturing 53-feet(1 foot=0.304 8 m) trailers and railway shipping containers,which could reduce the thickness of the container plate from 1.6 mm to 1.1-1.2 mm,thus saving the overall container weight by 2-3 t.

One of the most important problems in the pro-duction of weather-resistant steel is coil collapse or slumping,wherein a coil cannot hold up its own mass and loses its circular cross-section[4],as shown in Fig.1(a).A significantly collapsed coil cannot be cold rolled and annealed then.Even a slight collapse could result in an unstable production because of the rolling-mill vibration and rolling-force fluctuation caused by the distorted coil shape,thus negatively im-pacting the production efficiency and cost.The degree of coil collapse or slumping can be characterized by the difference between the inner bore diameters in the horizontal and vertical directions (ΔD).Generally,the coil collapse could be determined when ΔDis larger than 40 mm,as shown in Fig.1(b).

Fig.1 Measurement of coil collapse

Several factors are associated with the coil col-lapse,such as the coil weight,internal stress distri-bution of the coil,coiling tension,and strip gauge.Therefore,engineering solutions such as storing the coils vertically,lowering the coiling temperature to increase the strip strength,water cooling the coils,applying a high coiling tension,and using the coil core supporter are commonly adopted to prevent coil collapse.However,these methods only have a limited effect on the existing AHSS coil collapse pro-blems,resulting in other problems,such as over-loading of the winding machine,inhomogeneous coil property distribution,and strip running-off during coiling.The coil core supporter also causes some problems,including lowering the production efficiency and stability.Another major factor affecting the coil collapse of AHSS is the phase transformation of steel coils during the cooling stage post-coiling.In most cases,the strip is hot rolled above the austenitization temperature,and a signi-ficant austenite amount would remain in the coils until the coiling begins;hence,the coiled coils will undergo phase transformation during cooling.Con-sidering the heat transferring of coils,the temper-ature difference among different parts of the coils would be significant[5].The maximum temperature difference between the cold spot (outer ring) and the hot spot (one-third of the inner ring) of the steel coil can be up to 300 K[6-7],which can cause in-homogeneous phase transformation inside the coil,especially for bainite/martensite transformation with high speed and large dilation[8].This inhomo-geneous and fierce phase transformation would cause inhomogeneous and asynchronous dilation of material in different parts of the coils and destroy the friction between the coil layers.With the loss in coil stability,coil collapse occurs.

To determine the solutions to the coil collapse,the post-coiling transformation of ultrahigh-strength weather-resistant steel must be systematically stu-died.In the present work,the relationship between the coiling process parameters and transformation behavior,including transformation speed and dilation,has been clarified,and the coiling process has been optimized accordingly.

2 Experimental procedures

2.1 Experimental material

The material adopted in the present work was an ultrahigh-strength weather-resistant steel with a thick-ness of 1.2 mm,which was commercially produced with a process involving the following steps:melt-ing,continuous casting,hot rolling,cold rolling,and continuous annealing.The chemical composition of the produced steel is shown in Table 1.

Table 1 The chemical composition of the produced steel %

2.2 Experimental procedure

2.2.1 Calculation of continuous cooling transfor-mation (CCT) and temperature-time transformation (TTT) curves

CCT and TTT curves of the experimental steel were calculated using JMatPro V12.0 (Sente Soft-ware) with the initial conditions,including the heating temperature of 900 ℃,the chemical compo-sition depicted in Table 1,and an initial grain size of 20 μm.To achieve the volume fractions of dif-ferent phases during the simulated coiling process,the post-coiling transformation behavior was calcu-lated with a complex profile,wherein the samples were heated and held at 880 ℃,then rapidly cooled at 50 K/s down to a series of coiling temperatures,and finally slowly cooled at 10 K/min down to 300 ℃.

2.2.2 CCT measurement

The thermal dilation measurement of the present steel was conducted with a Formastor-FII thermal dil-atometer (Fujitsu Electric Industrial Co.,Ltd.) with the samples (10.0 mm×3.0 mm×1.2 mm) cut from the steel sheets.During the measurement,the samples were heated to 900 ℃ at 5 K/s and held for 300 s,then cooled down at 1 K/s,2 K/s,5 K/s,10 K/s,20 K/s,50 K/s,100 K/s,and 150 K/s,respectively,then to 100 ℃ or lower.The thermal dilation curves were obtained to determine the CCT curves of the steel.

2.2.3 Thermal dilation measurement for simu-lated post-coiling transformation

Thermal dilation measurement of the fabricated steel was conducted with a Formastor-FII thermal dila-tometer (Fujitsu Electric Industrial Co.,Ltd.) with the samples (10.0 mm×3.0 mm×1.2 mm) cut from the steel sheets.During the measurement,the samples were heated to 900 ℃ at 5 K/s and held for 300 s,followed by cooling down to 500 ℃,550 ℃,600 ℃,650 ℃,700 ℃,725 ℃,750 ℃,and 800 ℃ at 20 K/s and 50 K/s,respectively.The thermal dilation curves were collected to analyze the post-coiling transformation behavior of the steel.In the present work,the transformation time was defined as the period between the onset of dilation and 95% of the maximum dilation.The dilation was defined as the difference between the maximum and mini-mum values on the dilation curves.

2.2.4 Microstructure observation

Microstructures of the steel were examined under dif-ferent experimental conditions via a Leica DM600M optical microscope with samples of 20 mm×15 mm.For optical observation,the specimens were mechani-cally polished and etched in 4% nitric acid solution.

3 Results and discussion

The calculated and measured CCT and TTT cur-ves are shown in Fig.2.The comparison between Figs.2(a) and (b) shows that the calculated CCT curves were consistent with the measured CCT curves,indicating the applicability of the calculated curves in future research work.Based on the CCT curves,A3(onset temperature of the austenite transformation to ferrite) of the steel during cooling was 825.3 ℃.Therefore,at the current hot-rolling finishing temperature (880 ℃),no phase transfor-mation occurred in the material during hot rolling.During laminar cooling after the final rolling,the steel strip was cooled from 880 ℃ to the coiling temperature at typical cooling rates of 20-50 K/s under different laminar cooling modes.During the laminar cooling stage,with decreasing temperature,ferrite/pearlite and bainite transformation occurred in succession.Considering the post-coiling cooling roughly as an isothermal process,ferrite/pearlite transformation occurred at the coiling temper-ature of over 550 ℃,while bainite transformation occurred at below 550 ℃.No martensite transfor-mation occurred because the martensite transfor-mation temperature of the steel is below 400 ℃,con-siderably lower than the coiling temperatures adopted in industrial production.With decreasing coil-ing temperature,the time required to complete the phase transformation was also reduced.Moreover,the time required for ferrite/pearlite transformation was significantly longer than that required for bainite transformation.At different transformation temperatures,the isothermal transformation time of ferrite/pearlite ranged from 100 s to 10 000 s,while that of bainite was below 50 s.

Fig.2 The CCT and TTT diagrams for the ultrahigh-strength weather-resistant steel

Thermal dilation curves indicate that phase trans-formations occurred for all samples under simulated coiling processes,accompanied by a significant volume dilation,as shown in Fig.3.The time required to complete the phase transformation increases with the increasing simulated coiling temperature.At the coil-ing temperature of 800 ℃ (intercritical region of ferrite/pearlite transformation),the phase transfor-mation process became considerably slow,and the dilation changed gently,according to the thermal dilation curves of the sample.With the decrease in the coiling temperature,the phase transformation was significantly expedited,and the thermal dilation curves became steep.

Fig.3 Thermal dilation of the samples under various coiling temperatures

The relationship between the phase transformation time and the volume dilation is shown in Fig.4.When the coiling temperature dropped below 650 ℃,a rela-tively sharp transformation occurred,exhibiting a steep dilation curve for most samples.Although ferrite/pear-lite transformation dominated over 550 ℃,the trans-formation time was still significantly decreased with the coiling temperature.With the increasing coiling temperature,the phase transformation dilation first increased and then decreased,with the maximum dilation observed at about 650 ℃.At coiling tem-peratures below 650 ℃,the phase transformation partially occurred in the cooling process before coil-ing,which reduced the volume fraction of residual austenite after coiling and decreased the post-coiling phase transformation dilation.At coiling temper-atures over 650 ℃,the transformation was signi-ficantly prolonged up to 200-1 000 s;therefore,the dilation of transformation was partially neutralized by the volume contraction of the material during the slow cooling process of simulated coiling,resulting in a decrease in the observed dilation.

Fig.4 The transformation time and dilation vs.the coiling temperature

At the coiling temperature of 700 ℃,the phase transformation time was about 200 s with a final microstructure of ferrite and pearlite.At 600 ℃,the phase transformation time was about 70 s with a final microstructure of ferrite and pearlite,and the volume fraction of pearlite was larger than that at 700 ℃.When coiling at 500 ℃,the phase trans-formation time was below 20 s,and the final micro-structure is bainite with a small amount of ferrite.The comparison of the phase transformation beha-vior of post-coiling with the pre-coiling cooling rates of 50 K/s and 20 K/s indicated that the pre-coiling cooling rate had no significant influence on the post-coiling phase transformation speed.However,at a low pre-coiling cooling rate,the phase transfor-mation dilation with the coiling temperature below 650 ℃ was significantly reduced.This might be due to the low pre-coiling cooling rate resulting in obvious phase transformation before coiling,which reduced the volume fraction of residual austenite after coiling.The calculated transformation process and the metallographic observation of the final micro-structure at the coiling temperatures of 700 ℃,600 ℃,and 500 ℃ are shown in Fig.5,respec-tively.

Fig.5 Calculated phase fraction and observed microstructures of the samples with various coiling temperatures

According to the results above,it can be deduced that the coil collapse of the steel is mainly due to the rapid pre-coiling cooling inhibiting the ferrite transformation of the material to subject the post-coiling coils to a sharp transformation of austenite-ferrite or austenite-bainite.The coiling temperature of the ultrahigh-strength weathering steel is usually 550-650 ℃.In this temperature region,the transfor-mation of ferrite or bainite was very fast,with most transformations completed within 100 s.Since the overall temperature distribution and the phase trans-formation of the steel coil are not uniform,the coil fails to coordinate the dimensional changes caused by dilation owing to the rapid phase transformation.The transformation dilation occurs from the outer coil layer to the inner layer in succession,causing the coil to lose friction and stability and finally to collapse by its own weight.Meanwhile,the dilation of ferrite transfor-mation at a high temperature is much smoother,with the entire steel coil changing slowly.Therefore,the successive expansion in the coil can be eased through the coordination of the deformation of different parts,thus preventing the coil collapse.

4 Solution to coil collapse and its implement-ation

Based on the above experimental results and analysis,two improvement solutions were pro-posed.

Solution 1 involved the reduction of the post-coiling phase transformation.According to the above results,applying a low coiling temperature to the steel could promote the pre-coiling transfor-mation,thus reducing the post-coiling phase trans-formation.Less transformation causes less dilation in the material,thus mitigating the risk of coil collapse.The coiling temperature was determined as 550 ℃,since the relationship between the coiling temperature and the transformation dilation (Fig.4) indicates the minimum dilation at 550 ℃.More-over,after coiling,the steel coil was suspended on the winding machine for a certain duration (0,10 s,15 s,30 s,40 s) to subsequently complete the phase transformation.Hence,most of the phase transfor-mation process can be completed before unloading the coil from the winding machine,thereby signi-ficantly eliminating coil collapse.

Solution 2 involved the application of a high coiling temperature (over 700 ℃) to conduct a smooth and slow phase transformation of austenite-ferrite after coiling,thereby reducing the risk of coil collapse.

Solution 3 was a routine process:650 ℃ coiling and no suspension on the winding machine.

A total of 93 coils were trial-produced in mass production,wherein 79 coils were produced by solution 1,and 3 coils by solution 2.The rest 11 coils were produced by solution 3.Among all the coils,83 coils were 3.0-mm thick and ten were 2.8-mm thick.The results are shown in Table 2.

Table 2 Verification of the collapse-control solutions

Among the 11 coils of solution 3,nearly 90% (9 coils) collapsed.In contrast,no coil collapse occurred among the coils of solution 2,indicating that high-temperature coiling indeed effectively pre-vented the coil collapse.Among the coils of solution 1,only seven (below 10%) collapsed,including 3 coils of 3.0-mm and 4 coils of 2.8-mm thickness,and the collapsed three 3.0-mm coils did not implement post-coiling suspension.Among the 4 coils of 2.8-mm thickness,one did not implement post-coiling suspension,and the rest 3 coils were suspended for below 30 s.Compared with solution 3,wherein nearly 90% of coils collapsed,the proposed solutions were proved to effectively reduce the collapse.The suspension time has also been proved a key factor for maintaining the coil dimensions.The trial production results demonstrated that the steel strip thickness also had a significant influence on the coil collapse,wherein increasing the strip thickness could increase the steel strip rigidity,thus improving the coil resistance to collapse or slump-ing.All of the 11 coils of 3.0-mm thickness which collapsed were not handled by the post-coiling suspension.For the 2.8-mm coils,the required suspension time was significantly longer than that of the 3.0-mm coils for collapse resistance.Among the 10 coils of 2.8-mm,5 coils with a suspension time of 0-25 s exhibited an inner diameter difference of over 40 mm;5 coils with a suspension time of 30-40 s exhibited an inner diameter difference of below 10 mm.The effect of the thickness and suspension time on the inner diameter difference of the coils is shown in Fig.6.

The thermal dilation measurement results indicate that within the bainite transformation range,most transformation dilation of bainite transformation is com-pleted within 30 s.Moreover,the trial production also proves that for 2.8-mm coils,30 s is the critical suspension time for the occurrence of coil collapse,well in agreement with the thermal dilation measurement results.It confirms that the coil collapse of the ultrahigh-strength weathering steel is caused by the phase transformation dilation during coiling in the rapid transformation zone.This solution can also be applied to other AHSS with similar material design and phase transformation behavior,such as dual-phase steel and martensitic steel.

5 Conclusions

A martensitic ultrahigh-strength weathering steel was studied to analyze the influence of different coiling temperatures on the post-coiling phase trans-formation behavior.Based on the results,solutions to address the coil collapse problem have been proposed and verified via a large-scale trial pro-duction.The major conclusions of this study are listed below:

(1) The post-coiling transformation of the hot-rolled ultrahigh-strength weathering steel using the typical coiling process mainly involves the rapid ferrite/pearlite transformation or bainite phase transformation,resulting in the significant dilation of the material and a high risk of coil collapse.

(2) The coiling temperature is a key factor in the post-coiling phase transformation behavior.With decreasing coiling temperature,the post-coiling trans-formation speed of the material increases signifi-cantly,and the transformation type changes from high-temperature pearlite transformation to medium-temperature bainite transformation.With decreasing coiling temperature,the observed dilation resulting from post-coiling phase transformation first increa-ses,then decreases,and the maximum dilation appears at the coiling temperature of ～650 ℃.

(3) The pre-coiling cooling rate has no signi-ficant influence on the post-coil phase transfor-mation speed.However,a low pre-coiling cooling rate reduces the dilation of post-coiling phase trans-formation at a coiling temperature of below 700 ℃,possibly because,at low cooling rates,phase trans-formation has already partially occurred during the pre-coiling cooling process.

(4) The coil collapse would be suppressed at a coil-ing temperature of below 600 ℃ or over 700 ℃.For the studied ultrahigh-strength weathering steel,coil collapse is well controlled by using a coiling temperature of below 600 ℃ with a coil suspension time of 30-40 s.