X65, x60 Steel Coil Hot Continuous Rolling Process System and Grain Refinement

Taking grain refinement as the main line, the process system for producing high-strength and high-toughness pipeline steels X60 and X65 on a 1700mm hot rolling mill is studied . The heating , rolling and cooling processes in the Y recrystallization zone and Y non-recrystallization zone have been deeply studied . In addition, the refinement of kinematic recrystallization grains has also been discussed .
Key words process system grain refinement type II controlled rolling austenite

Steel for oil and gas pipelines in alpine regions should have high strength, high toughness, low brittle transition temperature, excellent welding performance and corrosion resistance. For X60 and X65 steels, see Table 1 and Table 2 for performance and composition according to the API - 5L standard and the technical regulations for pipeline steel in western China . Low-carbon, low-sulfur phosphorus, high manganese , and microalloyed steels in Table 2 are widely used in various countries , and I -type controlled rolling and controlled cooling characterized by low temperature and high pressure are carried out to obtain the desired properties.

Table 1 x60x65 performance

Stretching test
Steel grade sampling direction ratio Q E @. 5
MPa MPa % §
Bending one 20° C AKV ,J - 20° C DWTT
hardness
180 ° Wuyi ( 5 X 10 X 55 ) SA ,%
Hv one 10 ) Qin
d = 2a " average individual average individual

> 415 515 ~ 700 > 21 <0. 90 intact
$450 550 ~ 700 $21 W0.90 intact
<240 > 35 > 26 > 90 > 85>

Table 2 x60x65 chemical composition ( mass fraction )
C Si Mn P S Ni 、 Cr 、 Cu Nb 、 V 、 Ti Ceg ① Pcm ②
0.1 <0.4 < 1.4 < 0.02 <0.01 <0.5 Add one or more <0.4 <0.2

1. Rolling conditions and pre-rolling process

The rolling mill has three walking furnaces, one set is 1.7 meters ( 3/4 ) Continuous hot rolling mill ( including four rough rolling stands, seven finish rolling stands and laminar cooling device ) , three downcoilers and five hot-rolled coil finishing lines. There is an intermediate roller table with a length of H2m between the rough rolling and finishing rolling units . The main parameters of the rolling mill are shown in Table 3 .
Desulfurization of molten iron before hot rolling - top-bottom compound blowing of converter

Table 3 Main parameters of rolling mill

frame	rolling Work roll D W Backup roll D B		roll			Motor ( DC)		Torque kN . m	pressure kN
			Speedm / min _	speed ratio	Capacity kW	Number of armatures	Speed r / m in
	mm	mm
F 1	800	1570	104 ． 4/256 _ _	3 . 61	2 X 3800	2	150／365 _ _	494	25,000
F2 _	800	1570	169 ／ 411 ． 2	2 ． 23	2 X 3800	2	150／365 _ _	494	25,000
F3 _	800	1570	259 ． 7／632 ． _ _ 3	1 ． 45	2 X 3800	2	150／365 _ _	494	25000
F4 _	760	1570	358／871 _ _	1	2 X 3800	2	150／365 _ _	494	25000
F5 _	760	1570	441．5／1073．9	1	3X2450	3	185／450	387．4	25000
F6	760	1570	513．1／1252．9	1	3X2450	3	215／525	333 ． 3	25000
F 7	760	1570	572 . 7/1396 _ _	1	2 X 2500	2	240/585 _ _	203 . 1	25000

- Argon blowing ( or vacuum degassing treatment ) - Continuous casting. Continuous casting adopts long nozzle blowing Ar protection casting to prevent secondary oxidation of molten steel and reduce oxide inclusions and gas content in steel.

1. Craft system

We know that only fine ferrite grains can create the excellent performance of X60 and X65 .
The DF size of the microalloyed steel after hot rolling mainly depends on the grain boundary surface area S v in the unit volume of Y before the transformation and the formation temperature Ar3 of F. The number of available F nucleation sites in Y depends on the composition of the steel, The deformation and temperature during rolling are controlled. Ar3 is also controlled by these factors and the cooling rate in the transition temperature range. Therefore, the research on the X60 and X65 steel coil technology system , That is, the study of grain refinement under each specific process. To obtain fine ferrite grains, Dy of austenite grains must be controlled first . 3 . 1 heating system
The heating temperature Tx mainly affects the dissolution of Nb (C , N) and the growth of Dy ( see Table 4 and Figure 1) .

Table 4 The amount of solid solution of Nb in steel of x60 steel

y temperature, c	1000	1050	1100	1150	1200	1250	1300
Solid solution amount of Nb , Wt	0 . 001	0 . 007	0 . 012	0 . 020	0 . 028	0 . 028

When the temperature is low, Nb is dispersed in the steel as fine Nb (C , N) particles, pinning the Y grain boundary, and the inhibitory particles suddenly grow to 106 blood; when heated to 1200°C, Nb has All solid solution and lose the pinning effect, so D y grows up sharply

Made D y grow up. But this effect increases with Y temperature up to 235 blood. According to this rule, the raw material after heating

and weaken to disappear. Heating to 1050C, Nb (C , N) begins to decompose into solid solution , Dy grows up accordingly ; Heating to 1150C 'N b 's solid solution accounts for 72 % of the total , and the grain size of D y is limited to 1 . 0 Hn , T x should be at 1150C . It is inappropriate to control Tx at 1050 ~ 1150C , because heating in this range will cause Dy to grow unevenly, and the processed strips are prone to mixed crystals.

Table 5 shows the temperature regime and properties measured during the B and C industrial trial rolling, and Table 6 lists the average properties of various steel types during the trial rolling. From Table 6 , we know that the intensity of C is higher than
B, but the toughness is lower than B. From the temperature system in Table 5 , it can be considered that the results in Table 6 are related to the heating temperature. B times measured 16 Coil Steel Tx At 1130 ~ 1212°C, average
However, the Tx of 23 coils of steel C is 1215 ~ 1306C , with an average of 1270C. At such a high temperature, all carbonitrides are dissolved, D y grows rapidly, and precipitation strengthens during the cooling process. Therefore, high temperature heating reduces toughness and increases strength. Therefore, the heating temperature should be controlled according to the performance requirements. When it is required to improve low-temperature toughness, it is not necessary to make more Nb solid-dissolve in Y , but to use undissolved Nb (N , C) to prevent the growth of D y , so a lower heating temperature of 1150C is adopted, In order to avoid coarsening of the original Dy and solid solution precipitation of Nb (C , N) before rolling to increase precipitation strengthening. However, low-temperature heating prevents the growth of D y due to the lack of solid solution of carbonitrides, but reduces the prevention of Y recrystallization after rolling , and reduces its precipitation strengthening effect. To implement low temperature rolling, it is necessary to further reduce carbon to 0 . 03 %~ 0 . 05 % to reduce the solubility product, but this must add alloying elements to improve hardenability. When it is required to improve the strength and slightly reduce the low temperature toughness, at this time, enough Nb must be dissolved in y , so a higher heating temperature > 1250C is adopted . However, heating at high temperature not only makes D y grow sharply, but also increases the critical temperature of complete recrystallization of y [1] o The critical temperature of complete recrystallization during hot rolling is determined by the content of microalloying elements in solid solution and the original grain size of y As mentioned above, the content of microalloying elements in solid solution increases with the increase of heating temperature, and D y also grows with the increase of temperature. Therefore, the higher the heating temperature, the higher the critical temperature of y recrystallization, which is unfavorable for the refinement of y grains.
In actual production, the heating temperature can be adjusted according to the composition and performance

Table 6 The average performance of each steel type

test serial number	steel	§ s MPa	§b _ MPa	- 20AKV , J
B	X60	488	568	73
C	X60	509	583	64
B	X65	501	599	69
C	X65	534	602	67

Requirements are determined, and the load of the equipment should be considered. In the case of X60 and X65 with a large margin of toughness and strength, it is feasible to control the temperature at 1150 ~ 1220C .
3.2 Rough rolling degree ( rolling in y recrystallization zone ) Tissue after thermal deformation , According to whether recrystallization occurs , It can be divided into three regions, namely recrystallization region, partial recrystallization region and non-recrystallization region. The rough rolling zone should be a completely recrystallized zone, where D y is refined pass by pass through deformation and recrystallization . The D y dimension produced in the rough rolling zone is important because it is these grains that are flattened during finish rolling , The y intergrain boundary distance used to determine the final Df size was adjusted.
The simulation test shows that [ 2] , whether it is a single pass or multiple passes, the effect of the total deformation on the grain refinement in the y recrystallization zone is very obvious, but the influence of the deformation temperature is not very obvious. For example , when the multi-pass deformation is 58 % and 66 %, the grain size can be refined from 280mm to 60Rn and 40mm .
If the reduction ratio of y recrystallization zone is > 60%, D y can be converged to 20 ~ 40 mm, so as long as the cumulative reduction ratio of y non-recrystallization zone is small , D f can be refined to 10 ~ 11 grades. If the y recrystallization area is only refined to 3 ~ 5 grades, then the total reduction ratio of the non-recrystallization area must be increased to 70 %~ 80 % to obtain the same effect.
In actual production, the heating temperature is often too high, resulting in the coarseness of the original grains of y , but as long as it is in the complete recrystallization zone, the influence of the original grains will gradually weaken or even disappear after multiple passes of deformation. For example, the original grain size is 53 ~ 325mm , after 10 rolling passes, the total reduction rate is 70 %, and the pass reduction rate is 10 %~ 12 %, and the final grain size is narrower at 43 ~ 53mm . .
It can be seen that rolling in the rough rolling area not only refines D y but also regulates the heating temperature and finish rolling deformation, but this must make the rough rolling area in the complete recrystallization area, so the rough rolling completion temperature Tpan must be controlled . The total reduction rate in the rough rolling area is 82 %~ 85 %. If the rough rolling area can be completely located in the Y recrystallization area and the deformation of each pass is greater than the critical deformation, this reduction rate is extremely beneficial to the refinement of Dy .
Due to the multi-pass cooling rolling in the rough rolling area, the critical reduction rate for recrystallization is getting larger and larger. At nooC and 1050C , the critical reduction rate is 25 %~ 30 % and 30 %~ 40 % 4] . According to the capability of the R4 rough rolling mill, TR4 must be above 1050C to ensure that the rough rolling area is a complete recrystallization area. Document 3] It is proved by experiments that X65 steel has residual strain below 1050C , which also shows that T R4 should not be < 1050C .
It is found in the measurement that although the heating temperature of C is high, it still maintains good toughness, which is related to the high temperature of TR4 ( average 1050C) , which makes it complete rough rolling under the condition of complete recrystallization. B time T4 average 995C , it seems that in the last few passes of rough rolling has been in the partial recrystallization zone, so § s and § b are positively correlated with TR4 in the regression analysis.
The simulation test also found that although the amount of deformation is the same, the grains of some samples with multiple passes are larger than that of single pass, and the grains of the samples with low final rolling temperature are coarser than those with high temperature. This may be due to the small amount of deformation in multiple passes, or the large critical deformation amount of Nb -containing steel at low temperature, and the grains are coarse due to rolling within the range of critical deformation amount.
Obviously, if TR4 < 1050C , the reduction rate of the recrystallization zone will be reduced, possibly < 60%, and Dy will not be refined to the limit size . On the other hand, the last pass should be within the critical deformation range , Or rolled in the partially recrystallized zone , produce mixed crystals.
Rolling in the partially recrystallized zone tends to produce mixed crystals. This mixed crystal is composed of recrystallized grains, original grains that are only deformed, and some grains that grow very large induced by strain. Once this organization occurs, it cannot be eliminated by further processing. F formed by this phase transition is also a mixed crystal. Therefore, partial recrystallization should be controlled to be completed on the intermediate roller table between rough rolling and finishing rolling.
32 finishing rolling degree ( y rolling in non-recrystallized area )
to meet the requirement of high toughness, the reduction rate in the recrystallization zone can make Df finer to 4Rn , and it needs to be rolled in the Y non-recrystallization zone.
in the Y non-recrystallized region, the final Sv includes two items , one is the area of the grain boundary per unit volume ; the other is the surface area of the deformation zone and twinning zone contained in the elongated grain. As the reduction rate increases , the nucleation site increases , and this relationship remains unchanged until the reduction rate in the non-recrystallized region reaches 90% . In addition, the refined Y is not only elongated but also flattened , and the thickness direction The distance h between them decreases with the increase of the reduction rate , and the polygonal ferrite grains generated by the phase transformation of the flattened grains have a diameter of up to 1 h terminates due to phase collision. Therefore, the smaller the distance between Y grain boundaries in the thickness direction , the more potential ferrite nuclei density increases. Literature [ 1] believes that the most important factors affecting the final ferrite grain size are the yield stress of the Y recrystallization zone and the total reduction rate of the Y non-recrystallization zone. Therefore, starting from the generation of fine Dy , the higher the reduction rate of the Y non-recrystallized region, the better.
The typical regulations of the finishing mill are shown in Table 7 , and the total reduction rate is 81 %. A and xi calculated with X65 steel residual strain rate formula 3] are listed in Table 7 . A and xi are the residual strain rate and softening rate before entering channel i , xi = 1 - A . See Figure 2 for the xi of each stand at different rolling speeds V. From Figure 2 , it can be judged that the strip steel enters the fourth rack and is in a non-recrystallized state . That is, the recrystallization temperature of X65 steel is about 900C , which is consistent with the recrystallization temperature of Nb-containing steel reported in the data as 900-950C . Based on this calculation, the reduction rate in the Y non-recrystallized area is 50%, which meets the basic requirements of Type II controlled rolling, but it is not enough from the perspective of grain refinement. In order to increase the reduction rate in the unrecrystallized region of Y , It is true that the entire finishing area can be in the Y non-recrystallized area by delaying the rolled piece in the middle roller table , but this will reduce productivity , It cannot be done in actual production ; it is also difficult to increase the pressure on the continuous rolling mill due to the limitation of the continuous rolling characteristics. This needs to be compensated by increasing the pressure of the Y recrystallization zone, and adjusted by the controlled cooling described below.

Table 7 1 Finish rolling regulations

rack t	t , C	H 4 mm	hi mm	H — h h %	V m ／ the s	£ 1 s _	ln : a	£ i	A i	@ s P x i M	re-crystallize situation	the y situation
1	993	38 . 09	28 . 23	25 . 9	1 . 26	6.01 _ _	0.300 _ _	0.300 _ _	0 . 000	1.000141.7118512 _ _ _ _	part	work hardening
2	962	28 . 23	19 . 06	32 . 5	1.86 _ _	12 . 28	0 . 393	0 . 590	0 . 659	0 . 341172 . 1517677	part	work hardening
3	930	19 . 06	14 . 20	25 . 5	2.51 _ _	16 . 85	0 . 294	0 . 710	0 . 705	0 . 295188 . 9615617	part	work hardening
4	899	14 . 20	11 . 29	20 . 5	3.19 _ _	22 . 59	0 . 229	0 . 832	0 . 849	0 . 151204 . 4015339	not yet	dynamic reply
5	867	11 . 29	9 . 71	14 . 0	3.69 _ _	23 . 01	0 . 151	0 . 926	0 . 931	0 . 069216 . 4815597	not yet	dynamic reply
6	836	9 . 71	8 . 28	14 . 7	4 . 32	30 . 67	0 . 159	1.060 _ _	0 . 973	0 . 027229 . 4612617	not yet	kinetic recrystallization
7	805	8 . 28	7 . 16	13 . 5	5.00 _ _	35 . 45	0 . 145	1.195 _ _	0 . 990	0 . 010239 . 4811369	not yet	kinetic recrystallization

And make Ar3 increase, when the Nb- containing steel is finished rolling at 860-800C , Ar3 will increase by about 20 ~ 40C, which should be paid attention to when controlling the finish rolling temperature. Contains Nb . - 028 % steel, the dynamic Ar3 is about 770-810C, so it is appropriate to control the finishing temperature at about 810C .

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