DE2653847A1 - STEEL WITH A MECHANICAL ANISOTROPIC STRUCTURE AND PROCESS FOR ITS PRODUCTION - Google Patents

Description

OENTRO SPERIMENTALE METALIuRGIGO S.p.A. Rome, Italy

Steel with mechanically anisotropic structure and "process to." its manufacture

The invention relates to a method for improving the mechanical Properties of steel sheets in the transverse direction (in relation to the direction of cutting) that provide a solution for the Problem of improving the mechanical behavior of tubular components by forming and welding hot-rolled steel sheets are produced. In particular, the invention is concerned with a thermomechanical Treatment which essentially consists in using a steel of special composition with a high degree of reduction and with a rate and temperature at which austenite recrystallization occurs during the periods between every stitch is prevented.

It is known that flat steel products, which are produced in conventional rolling technology, are isotropic to the extent that

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hold as their mechanical strength "is concerned; i.e. their mechanical strength is more or less the same in "any direction in the rolling plane. These products have at most a slight mechanical anisotropy, which is usually considered an undesirable one Property is to be considered in cases, cannot be ignored.

However, in many practical applications, working conditions can create stresses that affect the Stahlbioch act along a preferred direction. For example, a steel sheet, which is intended for the production of vessels with large diameters and is intended to contain fluids under pressure, stressed predominantly along the direction perpendicular to the direction of rolling.

Sheet steel pipelines are a typical example of this situation.

Every opportunity to increase their mechanical strength, especially in the transverse direction, is great Interest in view of the current tendency to increase the pipe diameter and the gas operating pressure. Every Improvement in the toughness of the pipes (expressed in ductility-fracture transition) and ductility or deformability (expressed as maximum energy, which is absorbed during Charpy V-notch tests) in The transverse direction makes a significant contribution to the reliable durability of the pipeline, even if these two properties are present should sink lengthways.

In this context it should be pointed out that the [controlled rolling techniques

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which were introduced to the elastic transition temperature lower as much as possible, as responsible for the occurrence of an "separation" morphology apply on the fracture surfaces of the Gharpy V-notch specimens. Gas pipeline manufacturers consider a steel prone to "separations" to be extreme unreliable as these defects have been found to reduce the ability of the material to propagate ductile fractures shut down, hinder.

The invention is based on the object to produce hot-rolled steel sheets in which the mechanical Strength in the transverse direction is high and "separations" are not expected on the fracture surface of Gharpy V specimens are.

According to the invention, this result can surprisingly be achieved by using a steel with the following composition in percent by weight:

G 0.01-0.06; 1 ^ 0.012; Mn 1.5-2.5; Cr 1-3.5; Si t °> ⁶ »Hb <0, T; Al 0.005-0.05; S v0.02; P <0.02 j O <Ό, 01,

is exposed to the following treatment:

a) The steel is preheated ^{to a temperature between 1100 °} C. and 1300 ^{° C.}

b) the hot rolling is started at a temperature which is selected in the range from 900 ⁰ C to HOO ⁰ C.

c) A first hot rolling operation is conducted at a temperature between 950 ° C and 1100 ⁰ C at a relative Mederwalzrate durchgeführtc to the original thickness of 30 $ to 60 $ in not less than 3 and not more than 10 rolling passes

d) Leave the steel in air for no less than Seconds and no more than 10 minutes to cool; the minimum permissible temperature range must be sufficient to ensure complete recrystallization of the

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claimed to complete austenite.

e) A second hot rolling process is at an oil temperature between TOO ⁰ C and 90O ⁰ G with a reduction of the sheet to a final thickness between 6 mm to 50 mm (ie a total reduction of 75 to 95 $ of the original thickness) in no less than 4 and no more than 12 stitches performed.

f) The steel is cooled from 80O ⁰ G to 50O ⁰ G at a controlled average speed between 0 * 1 ° C to 10 ⁰ G per second.

The hot rolling step mentioned under e) includes suitable interruptions between each pass to ensure that the rolling temperature of the steel is within the programmed Area remains.

It is therefore completely surprising that when the steel is treated according to the method according to the invention, a very high mechanical strength of the hot-rolled sheet in the transverse direction is achieved at the same time with a complete absence of "separations" on similar fracture surfaces of the Oharpy V-notch specimens.

Aside from these two main benefits, there are additional positive results obtained by this method: breaking strength values up to 850 MN / m, a limit with regard to these properties can never be taken into account when the pipe is used in the usual way Steel is made;

Removal of stresses to failure (i.e., reduction in the area of the specimen before fracture) which is greater than $ 70;

Achievement of values in the range of 150 J for the during the Charpy V-Trials maximum energy absorbed on samples taken in the transverse direction;

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Achieving significantly lower transition temperatures (T ₂ QJ <-5O ⁰ C for specimens taken in the transverse direction: ^, compared with those that are typical for other types of steel with the same mechanical strength.

The simultaneous improvement of mechanical properties Over such a wide range is presumably due to the particular type of microstructure achieved (Needle structure or needle crystals), which were achieved in the material due to the method according to the invention. If the steel is treated according to the hot rolling program according to the invention, the austenite is heavily stressed or deformed and this, together with the very high hardening capacity that the steel has due to its composition, ensures for the complete transformation of austenite into bainite.

The process also leads to a general improvement in the overall mechanical behavior of the steel compared to that which can be obtained from the same material when recrystallized from a moderately fine austenitic grain is assumed "

The significant stress deformation or elongation ,, the exposed to austenite appears to be responsible for the high degree of anisotropy of the mechanical Strength of steel. With good reason it can be accepted that this characteristic is due to the development of "transformation textures" which can be derived from the corresponding deformation textures of the austenite due to the T / O ^ orientation, according to the the phase transition takes place.

In addition, it is not yet clear whether and what dependency between the texture of the hot-rolled according to the invention Materials and the disappearance of "separations" from the fracture surfaces of the Charpy V-notch specimens

exist.

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The measure according to the invention is intended in terms of purpose, usefulness and practicality in the. individual to be discussed. The following example explains the invention without limiting it.

Tests are carried out on four types of steel with two different carbon contents (about $ 0.025 and $ 0.045) with and without Addition of mob carried out. The composition of the four steels can be found in Table 1.

Table 1 - chemical composition of the steels examined

^x element
stole Mn Cr σ Si Nb S. 0 Έ Al ¥
X
U
V 1.95
1.97
2.08
2.05 3.05
3.04
3.05
3.12 0.030
0.047
0.028
0.048 0.33
0.38
0.29
0.31 0.052
0.050 0.007
0.007
0.007
0.008 0.007
0.007
0.008
0.007 0.007
0.006
0.005
0.006 0.007
0.008
0.004
0.004 ^;

All four steels were subjected to the hot rolling program according to Table 2 subjectο

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Table 2 -

Preheating: 115O ° C / 2 h

Temperature at the start of the hot rolling stepί

1Q5O ° 0 Initial thickness of the steel: 110 mm

Thickness After Each Stitch (mm); 96

84 72

■ ■. 62

53 45 56

• - Interruption to set the temperature to 900 ⁰ C

31 26

- Interruption due to temperature setting to 850 ° G

22 19

- Interruption due to temperature setting to 815 ° G

16

■ 1.3.5

- linter interruption due to temperature setting to 750 ° C

11 actual final steel thickness; 12 mm

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! Bafel

mechanical properties of the steels examined

stole ¥ I. J * Im '/ vfi) {M / m) HY 15.5
14th * GY
^v max
(»Toule) (Joules) ( ⁰ O) -50
-45 L.
0} 700
790 940
1000 320 15th
13.5 75.5
74 200
190 200
185 0
-20 1 1
VJl VJl
O VJ! •1
; υ T 790
890 1080
1150 360 16.5
16 70.5
69.5 200
150 190
, 140 «40
• o -35
-35 Y L. 655
710 915
955 290 15th
12th 74.5
73.5 235
200 230
190 0
0 -30
-10 700
840 1075
1145 345 70
67 200
140 200
120 20th
20th

* 0 * 012 = 5 0.2 yield strength Pm »breaking strength HV = Vickers hardness

A fo κ percent leaching Z $ s percent constriction at Eruch

SS maximum energy absorbed in the Oharpy Y-üJest

This is the energy that is absorbed at the minimum temperature for $ 100 deforming pressure 2J2)! P w mismatch temperature for 100 ^ Y deformation br ^ oh

fttr 5Qfi Deformation fracture (Ductile

cn co oo

After the completion of the hot rolling treatment, the ^{steels marked "W 11} and" Z "were quenched in oil, while the steels marked ¹¹ U" and "Y" were freely cooled in air.

The mechanical properties of the four steels after treatment are given in Table 3 (direction from the Test specimen was taken: L = longitudinal; T = transversal),

The noticeable mechanical strength anisotropy found in occurred in all four steels was particularly evident in

¹¹ V "steel, with values up to $ 20 (lengthways, χ 2 2

700 ffl / m; Transverse direction q

gg, _no =

2 '

= ⁸ 40 MN / m). have been achieved.

The voltage transition curves for standardized Charpy V inheritance test specimens are shown in FIGS. 1 and 2; the curves in "Fig. 1 refer to the longitudinal direction, those in 3? ig." 2 in the transverse direction;?. 3 ig 3 shows the typical appearance of the microstructure (acicular) shows the steel Y, obsd _a chtet in the electron microscope at (transmission on thin films;. Pig 4 the appearance of the fracture surfaces of Oharpy shows Y-notched test specimens, consisting of the steels "U" and "V ^M "; Jig. 5 shows the appearance of the fracture surfaces of Oharpy Y inheritance specimens made of the materials t! A "and ¹¹ B", as they are currently used for industrial purposes The latter steels are given in Table 4 «,

Table 4 - chemical compositions of steels "A" and "B"

Element f>
Steel3> - ^ 0 Mn Si Nb N 0 Al S. P. A.
B. 0.06
0.03 1.35
3.5 0.6
0.3 0.1
0.1 0.005
0.005 0.005
0.007 0.030
0.040 0.007
0.006 0.011
0.011

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. A.

The material A is a steel with a yield strength -O'q 2 ⁼ 530 MN / m (in the transverse direction after the rough rolling), usually used for pipes. Material B can be considered a suitable reference steel because its

mechanical strength (<y _no = 650 MN / m in the transverse direction after rolling) is comparable to that of steels obtained by the method of the invention. The fracture surfaces shown in Fig. 5 belong to test specimens taken from products which have been hot-rolled by the method according to the invention.

An analysis of the above experimental data reveals the following facts:

Fig. 3 shows very precisely that the substructure of the bainite, which resulted from your, highly stressed or highly deformed steered austenitic grain, considerably compared to the classic morphology of the "lamella pack" differs between those typical of upper bainites and martensites of non-alloyed or medium-alloyed low-carbon Steels is. The substructure is instead made up of "cells" or "subgrains" similar to those which develop in severely cold-worked pure iron.

FIG. 4 shows that the defects previously referred to as "separations" are absent on the fracture surfaces of the Charpy V specimens taken from the U and V steels in the transverse direction. In this context it is noticeable that if these defects are absent, the values of CV ₁₀₀ and GV _{m are} almost the same

for the materials in question (see Table 3); it is known that when these defects are present, the values of the absorbed energy at two opposite ends of the temperature range within which the ductile fractures or deformation fractures can only take place differ considerably from one another.

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In contrast, Figure 5 clearly shows the presence of "separations" on the fractured surfaces of the Gharpy V samples taken from Steels A and B in the transverse direction.

Claims

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Claims (6)

PATENT CLAIMS 1. Process for generating a mechanically anisotropic Steel strength, characterized in that a Steel with the following composition (in percent by weight): "■ - * = higher
0 0.01-0.06; Ή £ 0.012; Mn 1.5-2.5; Cr 1-3.5; Si £ 0.6 Fb <0.1; Al 0.005-0.05; S ^ 0.02; P £ 0.02; 0 £ 0.01, is exposed to the following treatment: -Vorwärmen zv to a temperature / een ⁰ 1100 G and 1300 G; - start of the hot rolling treatments at a temperature in the range between 900 ⁰ G and 1100 ° C; - 30 to 60? £ reduction of the initial thickness by hot rolling at a temperature between 95O ⁰ G and 1100 ⁰ G; - Cooling in air for a period not less than 30 seconds and no more than 10 minutes; - Reduction of the sheet steel thickness to 6-50 mm, that is a total reduction of 75 to 957 »of the original thickness at a temperature of between 700 G and 900 ^° G; and - controlled cooling of 800 ⁰ C to 500 ⁰ C at an average cooling rate in the range of 0.1 to 10 ⁰ C ⁰ G per second. 2. The method according to claim 1, characterized in that that the reduction from $ 30 to $ 60 of the original thickness is achieved with no less than 3 and no more than 10 stitches. 3. The method according to claim 2, characterized in that the second reduction in thickness up to a total reduction of 75 to 95 $ of the original thickness with not less than 4 and not more than 12 stitches is ₀ 09823/0723 26b3847 4. The method according to claim 3, characterized in, that the Warsiwalzverfahren, "in which an overall reduction from $ 75 to $ 95 of the original thickness is achieved, suitable breaks between each stitch for setting which includes ¥ arm rolling temperature. 5. High-strength steels, · produced according to the method of one of claims 1 "to 4, characterized in that that they are mechanically anisotropic, that their mechanical strength in the transverse direction is up to 20 $ higher than that lies in the longitudinal direction. 6. High-strength steels according to the method of one of claims 1 Ms 4-, characterized in that the Fracture surfaces of the test specimens taken in the transverse direction free sound "Irennungen" (separations) are. 7098 2 3/0723