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DNV-OS-B101 Metallic Materials
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Ch.2 Sec.1
A. General
Ch.2 Sec.1
A 100 Scope
Ch.2 Sec.1 A
101 This section specifies the requirements for weldable normal
strength, high strength and extra high strength hot rolled structural
steel plates and sections. These requirements are also applicable
to seamless steel tubes and pipes intended for structural application.
Ch.2 Sec.1 A
102 The requirements are applicable to steel products with a thickness
not exceeding 150 mm. For thickness greater than 150 mm, deviations
from these requirements may be applied as necessary after special
consideration and agreement in each case.
Ch.2 Sec.1 A
103 Steels differing from these requirements in chemical composition,
deoxidation practice, condition of supply and mechanical properties
may be acceptable, provided that they are specially considered and
demonstrated to be suitable.Ch.2 Sec.1
A 200 Designation of steel grades
Ch.2 Sec.1 A
201 The steel grades referred to in this section are divided into
three strength groups: | — | normal strength steels (NS) |
| — | high strength steels (HS) |
| — | extra high strength steels (EHS). |
Ch.2 Sec.1 A
202 Each group consists of two parallel series of steel grades:| — | steels of normal weldability |
| — | steels of improved weldability. |
The two series are intended for the same applications. However,
in addition to leaner chemistry and better weldability the improved
weldability grades have extra margins to account for reduced toughness
after welding. These grades are also limited to a specified minimum
yield stress of 500 N/mm2.
Ch.2 Sec.1 A
203 The alphanumeric designation of the steel grade is:
| — | NV xy for steels of normal weldability |
| — | NV xWy for steels of improved weldability. |
| NV | = | designation of a steel grade according to the
DNV offshore standards |
| x | = | a capital letter corresponding to a specified
impact toughness test temperature, see Table A1 |
| W | = | letter included to designate a steel grade
of improved weldability |
| y | = | a figure designating the strength group according
to the specified minimum yield stress. The figure y is omitted for
NS steels. |
Ch.2 Sec.1 A
204 Additional symbols following the alphanumeric designation
given in 203 may be:| Z | = | steel grade of improved through-thickness properties.
This symbol is omitted for steels of improved weldability although
improved through-thickness properties are required. |
Ch.2 Sec.1 A
| Table A1 Definitions
of steel grades |
| Strength group | Impact testing | Tensile properties | Symbolx | Test temperature?°C) | Symbol
y | Minimum yield stress1) (N/mm2) | Normal weldability | Improved weldability | NS | A
B2)
D
E | -
BW
DW
EW | -
0
-20
-40 | Omitted | 235 | | HS | A
D
E
F | AW
DW
EW
- | 0
-20
-40
-60 | 27
32
36
40 | 265
315
355
390 | | EHS | A
D
E
F
| -
DW
EW
-
| 0
-20
-40
-60 | 420
460
500
550
620
690 | 420
460
500
550
620
690 | - For steels of improved weldability the required minimum yield
stress is reduced for increasing material thickness.
- Charpy V-notch tests are required for thickness above
25 mm but is subject to agreement for thickness of 25 mm or less.
| | | |
Ch.2 Sec.1
A 300 Manufacture
Ch.2 Sec.1 A
301 Steel shall be manufactured
by the open hearth, an electric or one of the basic oxygen processes
or any other process involving secondary refining accepted by the
purchaser.
Ch.2 Sec.1 A
302 Steel shall be cast in
metal ingot moulds or by continuous casting. Sufficient discard
shall be made to ensure soundness in the finished product. The size
of the ingot, billet or slab shall be proportional to the dimensions
of the final product such that the cross section reduction ratio
or, in the case of slab to plate, thickness reduction ratio shall
normally be at least 3 to 1.
Ch.2 Sec.1 A
303 Conditions of supply shall be in accordance with 500.
Ch.2 Sec.1 A
304 It is the manufacturer's responsibility to ensure
that effective manufacture and process controls are implemented
in production. Where deviation from the controls occurs and this could
produce products of inferior quality, the manufacturer shall investigate
to determine the cause and establish countermeasures to prevent
its recurrence. Investigation reports to this effect shall be made
available to the purchaser on request.Ch.2 Sec.1
A 400 Chemical composition
Ch.2 Sec.1 A
401 The chemical composition
of each heat shall be determined on a sample taken preferably during
the pouring of the heat and shall be within the specified limits
in B to E. When multiple heats are tapped into a common ladle, the
ladle analysis shall apply and be within the specified limits. Variations from
the chemical compositions given may be allowed for grades supplied
in the thermo-mechanical rolled condition or when thicknesses exceed
50 mm provided that these variations are approved.
Ch.2 Sec.1 A
402 The composition shall
be determined after all alloying additions have been made and sufficient
time allowed for such an addition to homogenize.
Ch.2 Sec.1 A
403 Elements designated as residual elements in the individual
specifications shall not be intentionally added to the steel. The
content of such elements shall be reported.
Ch.2 Sec.1 A
404 When recycled scrap is
used in steelmaking, adequate controls shall be in place to prevent
accumulation of harmful elements in the final product. The content
of impurity elements such as tin, antimony and arsenic may be required
determined.
Ch.2 Sec.1 A
405 When required, the carbon equivalent value shall be calculated
from the heat analysis using the formula:
Subject to agreement, the weldability may alternatively be evaluated
by calculating the cold cracking susceptibility using the formula:
Ch.2 Sec.1
A 500 Condition of supply and heat treatment
Ch.2 Sec.1 A
501 Conditions of supply
shall be in accordance with requirements given in B to D and as
defined in 502 to 506. Where alternative conditions are permitted,
the manufacturer shall supply materials only in the conditions agreed.
Ch.2 Sec.1 A
502 As-rolled (AR) refers
to conventional rolling at high temperature followed by air cooling.
The rolling temperature and reduction may not be accurately controlled
resulting in variable grain sizes and, hence, variable mechanical
properties.
Ch.2 Sec.1 A
503 Normalising rolling (NR)
is a rolling procedure in which the final rolling temperature is
controlled within a certain range above the Ar3 temperature so that
the austenite completely re-crystallises. After the final pass,
air cooling produces a fine grained ferrite-pearlite microstructure
comparable to that obtained after normalising heat treatment.
Ch.2 Sec.1 A
504 Thermo-mechanical rolling (TM) is a rolling procedure in which
both the rolling temperatures and reductions and, when used, accelerated
cooling conditions are controlled. Generally, a high proportion
of the rolling reduction is carried out close to the Ar3 temperature
and may involve the rolling in the austenite-ferrite dual phase
temperature region. After the final pass, either air cooling or
accelerated cooling, excluding quenching, is used. Final rolling
in the same temperature range as used for NR followed by accelerated
cooling is considered to be a TM procedure. Unlike NR the properties
conferred by TM cannot be reproduced by subsequent normalising heat treatment.
Ch.2 Sec.1 A
505 Normalising (N) is a
separate heat treatment after rolling involving austenitising and
air cooling to produce a fine grained ferrite-pearlite microstructure.
Ch.2 Sec.1 A
506 Quenching and Tempering (QT) is a separate heat treatment
after rolling involving austenitising, rapid cooling for hardening
and subsequent reheating to produce a tempered martensite microstructure.
Ch.2 Sec.1 A
507 It is the manufacturer's
responsibility to ensure that the programmed rolling schedules for
NR and TM are adhered to. Production records to this effect shall
be made available to the surveyor on request. Where deviation from
the programmed rolling schedules occurs, the manufacturer must ensure
that each affected rolled piece is tested and that an investigation
is carried out according to 304.Ch.2 Sec.1
A 600 Inspection and tolerances
Ch.2 Sec.1 A
601 Surface inspection and
verification of dimensions are the responsibility of the manufacturer.
Acceptance by the purchaser of material later found to be defective
shall not absolve the manufacturer from this responsibility.
Ch.2 Sec.1 A
602 Products shall have a
workmanlike finish consistent with the method of manufacture and
shall be free from cracks, shells and seams. Acceptance criteria
for other imperfections such as rolled-in scale, indentations and
roll marks, which may occur under normal manufacturing conditions,
shall be EN 10163 Class A for plates and Class D for sections or
equivalent standard.
Ch.2 Sec.1 A
603 For plates and wide flats,
the minus tolerance on ordered nominal thickness shall not exceed
0.3 mm. The plus tolerance on nominal thickness and other dimensional
tolerances shall comply with the requirements of a recognised standard.
The tolerances on nominal thickness are not applicable to areas repaired
by grinding.
Ch.2 Sec.1 A
604 For sections and bars,
the dimensional tolerances shall comply with the requirements of
a recognised standard.
Ch.2 Sec.1 A
605 The thickness of plates
and wide flats shall be measured at locations whose distance from
a longitudinal or transverse edge of the product shall be at least
10 mm. Measurements shall be made by on-line automated methods or
off-line manual methods. The number of products to be measured,
number of measurement readings to be recorded, and spacing between any
two consecutive measured readings shall be decided and implemented
by the manufacturer and shall be generally based on sound statistical
analysis requirements.
Ch.2 Sec.1 A
606 Thickness measurement
data for plates and wide flats shall be analysed to assess that
the readings are within permissible tolerance limits and the computed
mean value shall be equal to or greater than ordered nominal thickness.
Ch.2 Sec.1 A
607 The manufacturer shall
maintain records of inspections and dimensional measurements. The
records shall be presented to the purchaser on request.Ch.2 Sec.1
A 700 Repair
Ch.2 Sec.1 A
701 Surface defects may be removed by local grinding provided
that: | — | the thickness is in no place
reduced by more than 7% of the nominal thickness, but in
no case by more than 3 mm |
| — | each single ground area does not exceed 0.25 m2 |
| — | the total area of local grinding does not exceed 2% of
the total surface area |
| — | the ground areas have smooth transitions to the surrounding
surface. |
Ground areas lying in a distance less than their average width to
each other shall be regarded as one single area.
Ch.2 Sec.1 A
702 Surface defects which
cannot be dealt with as in 701 may be repaired by chipping or grinding
followed by welding, subject to the purchaser's consent
and under his supervision, provided that:
| — | after removal of defects and
before welding, the thickness of the product is in no place reduced
by more than 20% of the nominal thickness |
| — | welding is carried out by qualified welders using qualified procedures |
| — | for plates supplied in NR or TM condition, qualification
of procedures is based on satisfactory mechanical testing |
| — | each single weld does not exceed 0.125 m2 |
| — | the total area of welding does not exceed 2% of
the surface area of the side involved |
| — | the distance between any two welds is not less than
their average width |
| — | the welds are made with an excess layer of beads and
then ground flush with the product surface |
| — | when deemed necessary, the repaired product is normalised
or otherwise suitably post-weld heat treated |
| — | the weld repairs are subjected to suitable non-destructive testing. |
Ch.2 Sec.1 A
703 The manufacturer shall
maintain records of repairs and subsequent inspections traceable
to each product repaired. The records shall be presented to the
purchaser on request.Ch.2 Sec.1
A 800 Identification
801 Every finished product
shall be clearly marked by the manufacturer in at least one place
with the following particulars:
| — | manufacturer's name
or trade mark |
| — | steel grade, e.g. NV E36. When products comply with
the requirements of E, the grade shall include the suffix Z25 or
Z35, e.g. NV E36Z25 |
| — | identification number, heat number or other marking which
will enable the full history of the product to be traced |
| — | if required by the purchaser, his order number or other identification
mark. |
802 The particulars in
801, but excluding the manufacturer's name or trade mark
where this is embossed on finished products, shall be encircled
with paint or otherwise marked so as to be easily recognisable.
803 Where a number of
products are securely fastened together in bundles, the manufacturer
may brand only the top product of each bundle or, alternatively,
a firmly fastened durable label containing the identification may
be attached to each bundle.
Ch.2 Sec.1
A 900 Certification
901 The manufacturer
shall provide the type of inspection certificate required in the
relevant construction rules giving the following particulars for
each test unit which has been accepted:
| — | purchaser's name, order
number and, if known, the vessel identification |
| — | manufacturer's name |
| — | description of products and steel grade |
| — | identification marking of products |
| — | steel making process, heat number and chemical composition |
| — | condition of supply |
| — | results of mechanical tests |
| — | when products comply with the requirements of E, the results
of through thickness tensile tests and ultrasonic tests |
| — | results of any supplementary and additional test requirements
specified. |
902 When steel is not
produced at the works at which it is rolled, a certificate shall
be supplied by the steelmaker stating the process of manufacture,
the heat number and the chemical composition.
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