# Patent

## Title
Method for transforming steel blanks

## Abstract

The invention relates to a method for transforming steel blanks. The invention in particular relates to a method for transforming a steel blank comprising kneading in order to obtain very good mechanical properties. The obtained products may notably be used for forming a pressure device component.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This is the U.S. National Stage of International Application No. PCT/EP2007/058037, filed Aug. 2, 2007, which was published in English under PCT Article 21(2), which in turn claims the benefit of French Patent Application No. 0653273, filed Aug. 3, 2006. Both applications are incorporated herein in their entirety.
The invention relates to a method for transforming steel blanks, in particular a blank for forming at least one pressure device component.
STATE OF THE ART
Very high performance steels have been developed for many years, for manufacturing components of pressure devices which may withstand 4,000 to 10,000 bars, notably including breech plugs or sleeves or tubes for forming components of a pressure device. These steels should meet qualities of compositions which are very strictly defined and with them very good mechanical properties should be obtained, and notably a very high yield point and a good yield point/toughness ratio, notably at low temperature.
Obtaining very low silicon and manganese contents but relatively high chromium, molybdenum and nickel contents is notably required.
Different compositions have been proposed in the prior art for obtaining steels meeting these mechanical properties, however the mechanical characteristics of these steels should be further improved. Such steels are notably described in DE 195 31 260 C2. Thus, the steels should be improved as for their composition and mechanical properties, and notably as for the yield point and the yield point/toughness ratio, in particular at low temperature.
With the usual transformation methods for this type of steel, it is not possible to obtain optimum mechanical properties when it is desired to used this steel as a tube with a very high yield point and/or a good low temperature yield point/toughness ratio, notably in the field of pressure devices which in particular withstand 4,000 to 10,000 bars.
On the other hand, methods customarily known have a duration which is not compatible with significant industrial activity. This is notably the case of a method described in DE 19531260, the method of which comprises an austenitization step followed by a pearlitic annealing step for 100-200 hours.
OBJECTS OF THE INVENTION
The main object of the invention is to solve the technical problems stated above and notably to provide a steel composition with which mechanical properties may be obtained, notably in terms of yield point and of compromise between the optimized yield point/toughness notably at low temperature, suitable for forming a pressure device component.
The main object of the invention is to solve the technical problems mentioned above and notably the technical problem consisting of providing a transformation method with which a steel tube of the aforementioned composition may be obtained, having very good mechanical properties, notably including a very high yield point combined with a high level of ductility.
The object of the invention is notably to solve this technical problem within the scope of manufacturing components for pressure devices, notably by an industrially performing method in terms of cost-effectiveness and manufacturing time.


DESCRIPTION OF THE INVENTION
In particular, the present invention relates to a steel composition essentially comprising:
Carbon: 0.35-0.43,
Manganese: <0.20,
Silicon: <0.20,
Nickel: 3.00-400
Chromium: 1.30-1.80,
Molybdenum: 0.70-1.00
Vanadium: 0.20-0.35,
Iron: balance
in weight percentages of the total composition, as well as the inevitable impurities, kept at a lower level, notably as copper (preferably <0.100); aluminium (preferably <0.015); sulphur (preferably <0.002); phosphorus (preferably <0.010); tin (preferably <0.008); arsenic (preferably <0.010); antimony (preferably <0.0015); in general essentially introduced by the raw materials; and calcium (preferably <0.004), dioxygen (preferably <0.004); dihydrogen (preferably <0.0002); and dinitrogen (preferably <0.007) generally due essentially to the manufacturing process. With this steel, it is possible to meet the requirements of the mechanical properties required for forming a component of a pressure device withstanding 4,000 to 10,000 bars, such as notably a breech plug or sleeve or a tube of a pressure device, such as a cannon tube.
Surprisingly, it was discovered that it was possible to solve the aforementioned technical problems and notably obtain a very high yield point and a good low temperature yield point/toughness ratio for an aforementioned steel composition, the kneading rate is less than or equal to 5 and preferably of about 4.5, on the largest cross-section of the steel component, notably in tubular or cylindrical form.
Thus, the present invention describes a method for transforming a steel blank with a substantially tubular or cylindrical shape essentially comprising the following composition:
Carbon: 0.35-0.43,
Manganese: <0.20,
Silicon: <0.20,
Nickel: 3.00-4.00,
Chromium: 1.30-1.80,
Molybdenum: 0.70-1.00
Vanadium: 0.20-0.35,
Iron: balance
in weight percentages of the total composition, as well as the inevitable impurities including dinitrogen (preferably N2<70 ppm), dioxygen (preferably O2<30 ppm) and dihydrogen (preferably H2<2 ppm),
said method comprising a step for transforming the blank by kneading in order to obtain a kneading rate of the thickest cross-section of the substantially tubular or cylindrical form, less than or equal to 5, and preferably less than or equal to 4.5.
It is of interest to carry out a transformation of the aforementioned steel by forging comprising a rise in temperature and for a sufficient time in order to reduce segregations within the steel. Maintaining the temperature of the ingot before forging provides chemical homogenization and may participate in improving the mechanical characteristics.
It is possible to perform at least one heating operation in order to draw the tube at a temperature at which cracks may be avoided, and a kneading rate less than or equal to 5 and preferably less than or equal to 4.5 may be obtained.
By a substantially cylindrical blank is meant for example a blank with the shape of a polygonal or smooth cylinder. A tube may advantageously be obtained by drilling after kneading.
Thus, tubes having an inner diameter of at least 80 mm may be manufactured. For example, tubes of 105 mm, 120 mm, 140 mm, and 155 mm may be manufactured with very good mechanical properties for cannon tubes. The thicknesses are generally larger than 100 mm, and this up to outer diameters of 400 mm.
Advantageously, after the kneading, the method comprises annealing in order to improve the structure of the steel.
Preferably, the annealing operation comprises a normalization step in order to improve the structure of the steel, notably by maintaining it at a temperature of at least 900° C., for example for at least 1 h for a thickness of 50 mm of the tube and cooling with air down to about 400° C.
Controlling the cooling rates after forging and/or normalization advantageously participates in improving the mechanical characteristics of the material.
Preferably, the annealing comprises an anti-flaking annealing step comprising maintaining a temperature of about 650° C., when the dihydrogen content requires such a treatment.
Advantageously, the method comprises at least oven-cooling in order to avoid risks of cracks upon cooling, notably during the normalization or the anti-flaking annealing.
Preferably, heat treatment is carried out on the obtained steel cylinder or tube at the end of kneading in order to obtain a steel cylinder or tube having essentially entirely a martensitic structure, and preferably an entirely martensitic structure. The heat treatment advantageously comprises quenching in a fluid with suitable cooling power (for example: oil) in order to lead to an essentially entirely martensitic structure and for reducing the risk of cracking. The heat treatment advantageously comprises tempering in order to substantially lead to maximum hardness of the steel. The heat treatment advantageously comprises at least one tempering operation in order to substantially obtain the homogeneity of the mechanical characteristics along the steel cylinder or tube.
Very good mechanical characteristics (high yield point, good toughness at low temperature) are guaranteed even when using oil quenching, which is quite advantageous because the risk of cracking may thereby be limited during the quenching operation.
According to a particular embodiment, the steel blank with a substantially tubular or cylindrical shape is obtained by a method for elaborating the steel blank comprising electroconductive slag remelting (ESR) or vacuum arc remelting (VAR), in order to optimize the composition, notably by reducing the impurities, but also by obtaining a blank leading to excellent mechanical properties after transformation.
The present invention relates to a steel blank in order to form a pressure device component which may be obtained in any of the steps of the method described above.
Other objects, features and advantages of the invention will become clearly apparent to one skilled in the art after reading the explanatory description which refers to examples which are only given as an illustration and which could by no means limit the scope of the invention.
The examples are an integral part of the present invention and any feature which appears to be novel relatively to any prior state of the art, from the description taken as a whole, including the examples, is an integral part of the invention in its function and in its generality.
Thus, each example has a general scope.
On the other hand, in the examples, all the percentages are given by weight, unless specified otherwise, and the temperature is expressed in degrees Celsius unless specified otherwise, and the pressure is atmospheric pressure, unless specified otherwise.
EXAMPLES
Example 1
Transformation: Forging
One (or more) steel blank with a substantially tubular or cylindrical shape essentially comprising the following composition:
Carbon: 0.37-0.42
Manganese: <0.15,
Silicon: <0.100
Nickel: 3.50-3.80
Chromium: 1.50-1.70,
Molybdenum: 0.70-1.00
Vanadium: 0.25-0.30,
in weight percentages of the total composition, as well as the inevitable impurities including dioxygen (preferably <0.004); dihydrogen (preferably <0.0002); and dinitrogen (preferably <0.007),
is transformed in order to provide a tube which may be used in armament, such as a cannon tube having a very high yield point and a good yield point/toughness ratio at low temperature.
The gas contents of the steel (O2, N2, H2) are dosed during elaboration and upon casting the ingots, by means of gas analyzers. Oxygen activities and hydrogen partial pressures are measured during elaboration by electrochemical devices: 02 cell, Hydriss probe.

This blank underwent the following transformation steps:

1 Ingot heating before forging:

The ingot is heated in order to reduce segregations on the product (for example, for at least 10 hrs, up to about 1200° C. for an ingot of 8-10 tons);

2 Forging the obtained ingot (for example, in order to make a tube having an inner diameter of 120 mm) comprising at least one heating operation in order to avoid cracks and obtain a kneading rate less than 5 and preferably less than 4.5 on the cross-section, notably the largest cross-section.

Forging may notably comprise the following steps:

    
    
        After first heating, refiring is carried out at a temperature for example of about 1200-1230° C., for e.g. at least four hours.
        Performing a second hot drawing.

With this method, a cylindrical or tubular blank may be obtained for example according to the outer dimensions:

        Breech: Ø 350×1500 mm
        Ø 300×800 mm
        Ø 250×2500 mm
        barrel: Ø 235×1600 minimum, total length >6300 mm
    
    


Kneading rates of 4.5 or less are thereby obtained in the breech, which is quite surprising since the kneading rate normally obtained in the breech for this type of steel grade is larger than 5.
If the blank is not of a tubular shape, drilling is then performed in order to obtain the desired tube.
Preferably, annealing is carried out after forging in order to obtain an essentially entirely martensitic structure and thus a better yield point in applications as a pressure device component, such as a cannon tube.
Example 2
Transformation: Annealing after Forging
Annealing is carried out after forging, for example on the tube obtained in Example 1, in order to improve the microstructure of the steel (normalization step) to avoid risks of cracks upon cooling (oven-cooling steps) and to avoid <<flake>> or <<DDH>> type occurrences on products after cooling, with anti-flaking annealing when the blanks have been remelted by the ESR process in solid or liquid slag or by the vacuum remelting (VAR) method.
Example 3
Transformation: Quality Heat Treatment
For example, the tube or cylinder obtained according to Example 2 is advantageously trued up for the heat treatment profile comprising a quality heat treatment. This treatment has the purpose of imparting to the tubes or cylinders all the required mechanical properties while optimizing the compromise of yield point/resilience at −40° C. and K1c or J1c at −40° C.
Oil quenching or quenching with another suitable cooling fluid notably leads to a entirely martensitic structure while avoiding the risk of cracking. This quality heat treatment advantageously comprises first tempering leading to maximum hardness; two tempering operations are carried out at temperatures which may guarantee large homogeneity of the mechanical characteristics along the tube while improving the resilience level. By carrying out three tempering operations and slow cooling in the oven after the last tempering operation, it is possible to guarantee the final straightness of the tube and the absence of deformations during the final machining.
As an example, the quality heat treatment comprises:


    
    
        AUSTENITIZATION+QUENCHING:
        Introduction of the tube into the oven at a temperature less than about 450° C.;
        A rise in temperature, for example up to a temperature above 850° C. at a rate of less than about 80° C./h;
        Maintaining the temperature above 850° C. for a period longer than 4 hrs for a tube blank of 120 mm;
        Oil quenching with for example an injection of oil into the bore until a temperature less than, for example about 150° C. at any point, is obtained, and followed by air cooling down to about 80° C. for example.
        FIRST TEMPERING at a temperature above 500° C.;
        SECOND TEMPERING at a temperature above 550° C.;
        THIRD TEMPERING at a temperature above 500° C.
    
    


The tempering operations may be carried out vertically with setting of the products into rotation in order to guarantee proper straightness.
During the process, hot straightening operations may be performed in order to guarantee general proper straightness of the tubes or cylinders. Thus, the following mechanical properties may be obtained:

    
    
        1,350 MPa<Rm<1,600 Mpa;
        1,250<Rp0.2%<1,450 Mpa
        A %>12%;
        Z %>35%;

Excellent resilience and toughness at low temperature are obtained

        KV (−40° C.)>28 J
        K1c (ou KQ)(−40° C.)>110 Mpa·m1/2 

These strength and toughness values are obtained for yield points (Rp0.2%) up to 1,450 Mpa. This is notably obtained by selection and by the element contents of the steel (C, Ni, Cr, Mo, V), and by thermomechanical treatment (forging, heat treatments).

Examples of obtained mechanical properties:

    
    









TABLE 1







by elaboration with an electric arc oven (FEA) + vacuum arc degassing (VAD):















K1C Kq



Number of
Breech side
Mouth side
(Mpa · m1/2)















Cast
the tube
YS (Mpa)
UTS (Mpa)
KV-40 (J)
YS (Mpa)
UTS (Mpa)
KV-40 (J)
Moy. > 110


















A
1
1334
1452
35.2
1349
1464
41.1
155.9



2
1372
1480
29.5
1396
1493
34.8
139.2


B
1
1366
1481
30.5
1400
1498
35.2
113.7



2
1367
1484
29.8
1390
1493
33.6
139.5



3
1374
1480
29.2
1391
1481
35.4
137.4



4
1336
1462
29.5
1331
1460
36.9
123.7


C
1
1335
1457
33.6
1341
1469
37.6
120.1



2
1284
1427
46.7
1319
1453
49.8
—



3
1382
1486
29.5
1343
1452
33.7
149.2


D
1
1357
1475
29.7
1371
1482
31
135.5



2
1353
1482
31.1
1373
1507
29.9
146.8


E
1
1373
1499
28.7
1409
1533
28
145.8



2
1380
1489
24.2
1359
1478
33.2
—



3
1378
1495
20.7
1351
1477
31.6
—



4
1360
1450
29.7
1367
1464
28.8
166.7


F
1
1335
1451
29
1365
1468
29.5
154.3



2
1359
1460
37.2
1368
1480
34.9
149.4



3
1360
1464
30.3
1356
1468
29.3
163.6



4
1346
1451
33.5
1371
1457
33
159.1



5
1337
1453
38.5
1364
1473
36.4
146.1


G
1
1341
1454
35.9
1364
1472
38
134.9



2
1343
1455
30.9
1359
1462
31.3
162.3



3
1333
1447
29.1
1365
1468
35.8
110.9


H
1
1333
1452
29.4
1347
1464
36.2
134.5














Average
1352
1462
31.3
1365
1476
34.4
142.3


Min.
1284
1427
20.7
1319
1452
28
110.9


Max.
1382
1499
46.7
1409
1533
49.8
166.7
















TABLE 2







by ElectreoSlag remelting (ESR)











Number of
Breech side
Mouth side















Cast
tube
YS (MPa)
UTS (MPa)
KV-40 (J)
K1C Kq (Mpa · m1/2)
YS (MPa)
UTS (MPa)
KV-40 (J)


















A
1
1380
1520
34.4
162
1409
1550
33.1



2
1384
1501
35.3
153
1399
1541
34.4



3
1385
1522
33.6
133
1405
1545
37.7



4
1388
1532
32.0
151
1411
1551
35.8



5
1392
1527
37.1
147
1406
1548
37.3



6
1386
1521
36.0
157
1404
1540
35.4



7
1337
1480
41.2
164
1357
1499
42.5



8
1342
1470
38.1
161
1366
1499
39.8



9
1327
1458
35.4
144
1372
1508
41.8



10
1352
1474
38.4
146
1377
1515
41.2



11
1329
1464
38.7
141
1378
1518
40.3



12
1332
1465
37.7
155
1382
1518
38.3



13
1334
1487
42.0
150
1366
1522
43.1



14
1345
1481
37.3
145
1377
1515
35.9



15
1337
1488
34.9
142
1364
1519
40.8



16
1331
1475
37.5
135
1349
1509
40.6



17
1340
1469
35.3
157
1390
1529
34.4



18
1349
1494
31.6
149
1346
1491
36.1



19
1348
1503
31.5
144
1359
1512
38.1


B
1
1359
1511
31.5
115
1366
1517
37.5



2
1364
1513
34.2
144
1353
1510
35.3



3
1374
1521
32.2
129
1378
1527
37.4


C
1
1366
1492
35.3
155
1395
1530
36.7



2
1369
1497
35.5
163
1398
1521
40.5



3
1406
1511
37.5
151
1391
1529
37.5



4
1378
1503
37.3
155
1400
1541
34.6



5
1379
1508
37.7
164
1395
1542
35.5



6
1383
1504
32.4
153
1383
1538
36.3



7
1363
1498
33.2
144
1374
1522
33.7


D
1
1362
1483
33.9
125
1335
1485
43.6


E
1
1339
1444
38.3
132
1376
1505
37.6



2
1330
1450
42.1
138
1369
1502
44.6



3
1354
1456
37.6
119
1371
1517
34.7














Average
1359
1492
36.0
146.0
1379
1522
37.9


Minimum.
1327
1444
31.5
115
1335
1485
33.1


Maximum.
1406
1532
42.1
164
1411
1551
44.6
















TABLE 3







by Vaccum Arc Remelting (VAR)











Number
Breech side
Mouth side














Cast
of tube
YS (MPa)
UTS (MPa)
KV-40 (J)
YS (MPa)
UTS (MPa)
KV-40 (J)

















A
1
1362
1478
32.5
1274
1423
42



2
1366
1477
38.0
1280
1420
43



3
1325
1440
27.7
1293
1423
34.5



4
1340
1458
35.2
1275
1440
39.5













Average
1348.3
1463.3
33.4
1280.5
1426.5
39.8


Min.
1325
1440
27.7
1274
1420
34.5


Ma.i
1366
1477
38
1293
1440
43














B
1
1309
1430
40
1255
1388
36



2
1328
1442
36
1266
1404
38



3
1286
1390
45
1263
1380
48



4
1290
1399
49
1258
1379
54













Average
1303.3
1415.2
42.4
1260.3
1388.0
44.0


Min.
1286
1390
36
1255
1379
36


Max.
1328
1442
49
1266
1404
54

## Claims

1. A method for transforming a steel blank with a tubular or substantially cylindrical shape essentially comprising the following composition in weight percentages of the total composition:
Carbon: 0.37-0.42,
Manganese: <0.15,
Silicon: <0.100,
Nickel: 3.50-3.80,
Chromium: 1.50-1.70,
Molybdenum: 0.70-1.00
Vanadium: 0.25-0.30,
Iron: balance
as well as inevitable impurities which are generally dinitrogen, dioxygen and dihydrogen,
said method comprising a step for transforming the blank by kneading in order to obtain a kneading rate of the thickest cross-section of the tubular or substantially cylindrical form, less than or equal to 5.

2. The method according to claim 1, wherein the method comprises after kneading, annealing for improving the structure of the steel.

3. The method according to claim 2, wherein the annealing comprises an anti-flaking annealing step comprising maintaining the temperature of about 650° C.

4. The method according to claim 1 or 2, wherein it comprises at least oven-cooling after hot kneading and/or annealing in order to avoid risks of cracks upon cooling.

5. The method according to claim 1, wherein a steel cylinder or tube is obtained and wherein a heat treatment is carried out on said steel cylinder or tube in order to obtain a steel cylinder or tube having an entirely martensitic structure.

6. The method according to claim 5, wherein the heat treatment comprises heating then oil quenching or quenching with a fluid with suitable cooling power in order to lead to an entirely martensitic structure and to reduce the risk of cracking.

7. The method according to claim 5 or 6, wherein after heat treatment a first tempering operation is carried out in order to increase hardness of the steel.

8. The method according to claim 5, wherein after heat treatment at least one tempering operation is carried out in order to obtain homogeneity of in mechanical characteristics along the steel cylinder or tube.

9. The method according to claim 1, wherein the steel blank with a tubular or substantially cylindrical shape is obtained by a method for elaborating the steel blank comprising an electroconductive slag remelting (ESR) or vacuum arc remelting (VAR).

10. The method according to claim 1, wherein after kneading the method comprises a normalizing step followed by controlled cooling rates to improve the mechanical characteristics of the steel.

11. The method according to claim 1, wherein the kneading consists in forging and wherein before forging, heating the ingot and maintaining the temperature are carried out in order to homogenize the chemical composition and to participate in improving the mechanical characteristics.

12. A steel blank for forming a pressure device component capable of being obtained by a method as defined according to any of claims 1, 2, 3, 5, 6 and 8 to 11.

13. The method according to claim 1, wherein the transformation step by kneading consists in forging which comprises a rise in temperature and for a sufficient time in order to reduce segregation within the steel.

14. The method according to claim 13, wherein after forging the method comprises a controlled cooling rate to improve the mechanical characteristics of the steel.

15. The method according to claim 13, wherein it comprises at least oven-cooling after forging in order to avoid risks of cracks upon cooling.