DD Cen TR 10347-2006 [PDF]

Zitiervorschau

DD CEN/TR 10347:2006

DRAFT FOR DEVELOPMENT

Guidance for forming of structural steels in processing

ICS 77. 1 40.1 0

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DD CEN/TR 10347:2006

National foreword This Draft for Development was published by BSI. It is the UK implementation of CEN/TR 1 0347:2006. The UK participation in its preparation was entrusted to Technical Committee ISE/1 2, Structural steels. A list of organizations represented on ISE/1 2 can be obtained on request to its secretary. This publication does not purport to include all the necessary provisions of a contract. Users are responsible for its correct application.

This Draft for Development was published under the authority of the Standards Policy and Strategy Committee on 29 September 2006

© BSI 2006

ISBN 0 580 49285 0

Amendments issued since publication Amd. No.

Date

Comments

TECHNICAL REPORT RAPPORT TECHNIQUE TECHNISCHER BERICHT

CEN/TR 10347 April 2006

ICS 77.1 40.1 0

English Version

Guidance for forming of structural steels in processing Guide pour le formage des aciers de construction lors de leur mise en oeuvre

Hinweise für das Umformen von Baustählen bei der Verarbeitung

This Technical Report was approved by CEN on 1 3 March 2006. It has been drawn up by the Technical Committee ECISS/TC 1 0. CEN members are the national standards bodies of Austria, Belgium, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, Switzerland and United Kingdom.

EUROPEAN COMMITTEE FOR STANDARDIZATION C OMITÉ EURO PÉEN DE NO RMALIS ATIO N E U R O P Ä I S C H E S K O M I T E E FÜ R N O R M U N G

Management Centre: rue de Stassart, 36 B-1 050 Brussels © 2006 CEN

All rights of exploitation in any form and by any means reserved worldwide for CEN national Members.

Ref. No. CEN/TR 1 0347:2006: E

CEN/TR 10347:2006

Contents

Page

Foreword ..........................................................................................................................................................3 1

Scope ...................................................................................................................................................4

2

Terms and definitions .........................................................................................................................4

3

General principles ...............................................................................................................................4

4

Hot forming..........................................................................................................................................5

5

Cold forming........................................................................................................................................7

6

Flame-straightening ............................................................................................................................8

Bibliography...................................................................................................................................................1 1

2

CEN/TR 10347:2006

Foreword This Technical Report (CEN/TR 1 0347:2006) has been prepared by Technical Committee ECISS/TC 1 0 "Structural steels – Grades and qualities", the secretariat of which is held by NEN. In the ECISS/TC 1 0 meeting of 8 and 9 December 1 998 it was decided with Resolution ECISS/TC 1 0 no 2/1 998 to publish ECSC IC 2 as a CEN report. The part on welding in ECSC IC 2 has been revised by CEN/TC 1 21 and has resulted in EN 1 01 1 -2. The part on formability has been revised in this CEN Technical Report.

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CEN/TR 10347:2006 1

Scope

This CEN Technical Report provides guidance for forming during processing of structural steel products conforming to EN 1 0025 Parts 2 to 6 and EN 1 01 49 Parts 2 and 3. This Technical Report covers hot and cold forming processes and local hot forming used in flame-straightening operations. This Technical Report does not cover the special measures necessary for the fabrication of structural components that are subjected predominantly to alternating stresses or that come into contact with aggressive media. 2

Terms and definitions

For the purposes of this Technical Report, the following terms and definitions apply. 2.1 hot forming

forming at temperatures in the austenite range, generally around 900 °C depending on the chemical composition of the steel grade 2.2 cold forming

forming up to the highest temperature permissible for stress relieving, generally in the range of 530 °C to 580 °C NOTE

See the relevant part of EN 1 0025 (e.g. 7.3.1 .1 of EN 1 0025-1 :2004 and 7.4.1 .1 of EN 1 01 49-1 :1 995).

2.3 flame-straightening

local quick heating of a component with a short holding time (generally less than one minute) at the flamestraightening temperature 2.4 flame-straightening temperature

highest temperature arising in the component during flame-straightening 3

General principles

With rising minimum yield strength values for structural steels and with an increasing wall thickness of structural steel products, extra care needs to be taken during subsequent processing of steel products. Furthermore, the technical delivery conditions of steel products complying with EN 1 0025 and EN 1 01 49 will depend on the actual process route used by the steel product manufacturer. In general, suitable steel grades for the special forming situation should be selected. EN 1 0025 also defines special steel grades for cold forming and additional options for ensuring forming properties which should be preferably used. In cases which are not covered by these order options the steel manufacturer should be consulted. All steel grades of EN 1 01 49 are suitable for cold forming. In individual cases outside of the specific guidance given here, especially when using structural steels for the first time, forming should be based on prior experience. This may be based on pre-production procedure trials undertaken by the fabricator or on documented trials undertaken by the steel product manufacturer. If a manufacturer wants to ensure that the hot forming, cold forming or flame-straightening process intended to be applied will not detrimentally influence the mechanical properties of the structure, a process verification should be performed.

4

CEN/TR 10347:2006 NOTE Temperatures referred to in this Technical Report are measured at the steel product surface and not somewhere in the furnace. Allowance should be made for the fact that the temperatures in the product may not be uniform.

4

Hot forming

4.1

General

The following products can generally be subjected to hot forming:

4.1 .1 ?

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non alloy structural steels supplied as-rolled for normalizing by the purchaser (see 7.3.1 .3 of EN 1 0025-2:2004) or supplied in the normalized delivery condition conforming to EN 1 0025-2; normalized fine-grain structural steels conforming to EN 1 0025-3 and EN 1 01 49-3; structural steels with improved atmospheric corrosion resistance supplied as-rolled for normalizing by the purchaser (see 7.3.1 .3 of EN 1 0025-5:2004) or supplied in the normalized delivery condition conforming to EN 1 0025-5.

It is not envisaged that thermomechanical rolled steel grades conforming to EN 1 0025-2, EN 1 0025-4 and EN 1 01 49-2 or quenched and tempered steels conforming to EN 1 0025-6 should require further processing by hot forming. The strength properties of thermomechanical rolled steel grades, the material condition of which is not achievable or reproducible by a heat treatment alone, are impaired by hot forming during further processing. For quenched and tempered steels the necessary heat treatment after hot forming is very difficult to reproduce. Where special hot forming operations are performed, e.g. hole flanging or inductive bending, the steel product manufacturer should be consulted. Process verification for induction bending should ensure that both the steel and the process are matched.

4.1 .2

Where only local heating to hot forming temperature is involved, other than flame-straightening, attention should be paid to the fact that, apart from the region heated in regular fashion to normalizing temperature, areas appear with temperatures between the lower limiting temperature for normalizing and the upper limiting temperature for stress relieving. In these areas, the material properties can be impaired, depending on the heating technique, temperature, and duration involved. The area heated into the two phase regime ferrite-austenite is experienced to be particularly critical for both strength and toughness.

4.1 .3

4.2

Temperature during hot forming

To carry out hot forming, the work piece should be heated above 900 °C, but not above 1 050 °C, preferably not above 1 020 °C. After reaching the planned temperature for the work piece, holding at that temperature to ensure a uniform temperature in the work piece is only necessary if stipulated in the relevant steel product standard. NOTE 1

The temperature of 900 °C relates to the lower temperatures for normalizing.

NOTE 2

For quenched and tempered steels see 4.1 .1 .

Heating rates should be as fast as possible and holding times should be as short as possible. For this reason, work pieces should not be heated in a pile.

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CEN/TR 10347:2006 During hot forming, the temperature should not drop below the lower limit of admissible temperatures as given in Table 1 . In particular, when the product thickness is small (less than 1 5 mm) the heat flow from the work piece to the forming devices should be taken into account during hot forming. After hot forming, the work piece should generally be cooled in air, in order to ensure a minimum of deformation due to internal stresses. The cooling rate should be kept as similar as possible to the one applied to the original product. In general, the cooling rate after hot forming depends on the shape and wall thickness of the component. In particular, the cooling rate of products with a small thickness is significantly higher when the work piece is cooled in contact with the forming devices than in contact with air. For this reason, retarded cooling or tempering may be necessary in special cases for normalized steel grades with minimum yield strength values equal to or greater than 420 MPa and small wall thickness (less than 1 5 mm). Details should be obtained from the steel product manufacturer. The temperature should be monitored in all hot forming operations in order to ensure that, during the forming operation, the highest allowable temperature is not exceeded (see 4.3.2). This is of particular importance where the heat treatment of the work piece is not intended to be repeated after forming. 4.3

Heat treatment after hot forming

Heating to a temperature greater than about 50 K above the transformation temperature Ac3, at which austenitization is completed, in particular if combined with a long holding time, leads to grain growth and thereby impairs the toughness properties and the yield strength. For this reason, after hot forming the finished component should be heat treated generally in accordance with the advice of the steel product manufacturer or the relevant steel product standard.

4.3.1

It is essential that normalized steel grades are renormalized. This is of particular importance where fabrication processes require the normalizing of the component after hot forming. Under these circumstances, a normalized steel product could be ordered in the 'as-rolled' delivery condition. The steel product manufacturer should be consulted for advice. 4.3.2 With normalized steel grades, normalizing after hot forming may be omitted in the following cases: a) After single-step hot forming, normalizing of the finished component may be omitted if the process has been undertaken within the limits given in Table 1 .

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CEN/TR 10347:2006 Table 1 — Criteria for omitting renewed heat treatment after single-step hot forming Test temperature for verification of Charpy impact energy

Steel grade

Maximum temperature before hot forming

°C ≥ - 20

< - 20 a

°C S235-S355

980

S420-S460

940

S235-S355

940

S420-S460

925

Lowest temperature at the end of hot forming with the degree of hot forming a

>5%

≤5%

°C

°C

750

700

Degree of hot forming is related to the highest local strength.

b) After multi-step hot forming, renewed heat treatment of the component may be omitted provided the work piece has been cooled, prior to the last step, to a temperature below 500 °C for steel grades S235 to S355, or below 300 °C for steel grades S420 to S460, and that in the last step, the process has been undertaken within the limits given in Table 1 .

4.3.3

In the course of selecting the steel product, the purchaser should ensure that the required heat treatment can be applied to the finished component, taking into account the influence of the shape and the thickness of the component on the heat flow.

4.4 Practical hints regarding hot forming The envisaged effects of the fabricating process on the mechanical properties of the components should be taken into account, particularly where special toughness requirements are involved. If, after hot forming, heat treatment of the component is to be undertaken, the cooling rate of the component when cooled from the normalizing temperature, should be planned such that it is similar to that applied to the original normalized product. For air cooling, a component should be taken out of the furnace after heating for normalizing and cooled in free air. Some forced air circulation might be advantageous in order to avoid heat accumulation as a result of local stack heat flow.

5 Cold forming 5.1 General In general, the steel grades conforming to EN 1 0025, Parts 2–6 can be subjected to cold forming. All steel grades of EN 1 01 49 Parts 2 and 3 can be subjected to cold forming. When cold forming above room temperature is planned, it should be checked whether the material is suitable for the envisaged temperatures. Forming in the range 200 °C to 380 °C should be avoided. Preferably steel grades suitable for cold forming should be used. EN 1 0025 defines special steel grades and/or order options for which recommended bending radii are given in the standard.

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CEN/TR 10347:2006 As-rolled steels, thermomechanical rolled steels and quenched and tempered steels should not be used in cold forming applications for which a heat treatment above the stress-relief temperature is required after forming.

5.2

Change in the material properties due to cold forming

Cold forming changes the material properties of steel and impairs ductility as well as toughness and enhances strength. These changes may also limit the ability to weld in cold deformed areas. The extent to which the properties are changed by cold forming depends upon the type of steel, the forming temperature and the degree of deformation.

5.3

Heat treatment after cold forming

If, after extensive cold forming, heat treatment is necessary to remove strain hardening or to improve the toughness properties impaired by cold forming, a heat treatment at stress relieving temperature may be sufficient, provided that the technical delivery conditions of the standards in question do not explicitly require renormalizing after cold forming. If the improvement is insufficient, renormalizing would be necessary to restore the initial properties.

5.4

Practical hints regarding cold forming

If cold forming is carried out, it should be recognized that the yield strength of the steels increases with the degree of deformation to the effect that the forming force required and the springback are increased. The effects of the envisaged fabricating process on the mechanical properties of the components should be taken into account, particularly where special toughness requirements are involved. Steels of high yield strength generally require a larger ratio of cold bending radius to material thickness. Thermally-cut edges and sheared edges may have reduced ductility. In order to prevent fracture initiation, the edges should be ground in the area to be cold formed. For the same reason where irregularities from deep drag lines and other defects occur in the flame cut surface, these should be removed and smoothed by local grinding to avoid localized strains. NOTE

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EN 1 993-1 -1 0 includes ways to link the degree of cold forming with specified toughness requirements.

Flame-straightening

6.1

General

6.1 .1 The purpose of flame-straightening is to give the component the desired shape, or to remove deviations from the desired shape. Depending on the intended straightening effect, the heating may be superficial only or more penetrating.

When flame-straightening is performed, flame straightening patterns are applied. Such patterns are hot points, hot lines, or hot wedges, as well as derivations of such patterns, e.g. two to five hot lines running parallel which can be called a hot track. Flame-straightening patterns are applied to remove or to produce curvatures, to remove bumps and waves, to smooth the distortion by welds, to adapt weld preparations, and to perform other types of forming. 6.1 .2 In general, all steel grades conforming to EN 1 0025-2 to –6 and EN 1 01 49-2 and -3 may be subjected to flame-straightening, but the flame-straightening temperature should not exceed defined maximum values (see Table 2). 6.1 .3

8

The straightening effect may be increased by appropriate restraint, e.g. clamping.

CEN/TR 10347:2006 6.2

Effect of flame-straightening on the material properties

When flame-straightening is performed, the heating should be quick and locally restricted. The expansion of the heated zone is restrained by the adjacent cold zones. The restraint from these cold zones, possibly supplemented by appropriate externally-applied restraint, results in a plastic compression of the heated zone as soon as the yield strength (which decreases with increasing temperatures) is exceeded. Compression stresses arise and produce a change in shape, generally an upsetting of the thickness. 6.2.1

In relation to the heat input applied for flame-straightening, there are two typical cases. In the first case only a superficial region of the component is heated. The heat input related to the product thickness at the location to be heated is small. The cooling rate of the area heated is large. In the second case, the full cross-section of the component at the location concerned is briefly heated. The heat input related to the product thickness at the location to be heated is large. The cooling rate of the area heated is smaller than that resulting in the first case.

6.2.2

6.2.3 The effect of the flame-straightening operation on the material properties depends on the flamestraightening temperature and on the cooling rate, as explained in the following paragraphs:

When the flame-straightening temperatures are lower than or equal to 700 °C, austenitizing of the material does not occur. However between about 650 °C and 700 °C, with increasing holding time spheroidizing of pearlite takes place with the effect that strength and toughness properties will decrease. Thus no significant impairment of the material properties is to be expected if the temperature is kept below 700 °C and the holding time on the flame-straightening temperature is short even if the heating is penetrative. In general this applies to plate thicknesses up to about 20 mm.

6.2.3.1

The larger the plate thickness, the longer the period of time during which the temperature of the heated area is between 650 °C and 700 °C. The flame-straightening temperature should therefore be limited to a maximum value of 650 °C when the holding time is extended, as given in Table 2. Due to the different manufacturing processes used by the various steel product manufacturers, a reduction of the flame-straightening temperature might be advisable to a maximum value of 650 °C for high strength, thermomechanical rolled plates with minimum yield strength values ( Reh-min ) > 460 MPa. In such cases, it is very advisable that the steel product manufacturer is consulted for advice. In addition to the elevated temperature effect on the microstructure, metallurgical precipitation processes play an additional role with regard to water-quenched and tempered steel grades. For this reason the flamestraightening temperature should be kept strictly at least 20 K lower than the tempering temperature. 6.2.3.2 When the flame-straightening temperatures are higher than 700 °C, it should be assumed that at least partial austenitizing of the material occurs. Where microstructure constituents richer in carbon and other alloy elements transform into austenite diffusion of additional carbon takes place from the regions still ferritic into those already austenitic. During subsequent rapid cooling, the austenitic regions transform into martensite, which might also be enriched in carbon. Increased hardness and lower toughness can result from that process.

When the cooling rate is comparatively slower, a mixed microstructure is produced, with the risk that the yield strength decreases locally and falls short of the specified minimum value. At the same time, local enrichment in carbon can lead to an impairment of toughness properties. The change in toughness is, however, less marked than in the case of rapid cooling from temperatures where austenite is present. The possible impairment of toughness properties by flame-straightening is greater with increased carbon content of the steels. Heating of the full cross-section of the component to temperatures above 700 °C will result in an impairment of the strength and toughness properties. Flame-straightening temperatures above 950 °C should be avoided even if the heating of the component is only superficial.

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CEN/TR 10347:2006 6.3

Maximum values of the flame-straightening temperature

Recommended maximum values of the flame-straightening temperature as a function of the steel type and of the flame-straightening technique are given in Table 2. Table 2 — Recommended maximum values of the flame-straightening temperature Delivery condition

Recommended maximum values of the flame-straightening temperature

Short superficial heating

Short full section heating

Full section heating with longer holding time

°C

°C

°C

normalized

≤ 900

≤ 700

≤ 650

thermomechanical rolled up to S460

≤ 900

≤ 700

≤ 650

thermomechanical rolled S500 to S700

≤ 900

≤ 600

≤ 550

quenched and tempered

10

≤ tempering temperature applied to the original product – 20 K (generally below 550 ° C)

CEN/TR 10347:2006 Bibliography EN 1 993-1 -1 0, Eurocode 3: Design of steel structures - Part 1-10: Material toughness and through-thickness

properties

EN 1 0025-1 , Hot rolled products of structural steels — Part 1: General technical delivery conditions EN 1 0025-2, Hot rolled products of structural steels — Part 2: Technical delivery conditions for non-alloy

structural steels.

EN 1 0025-3, Hot rolled products of structural steels — Part 3: Technical delivery conditions for

normalized/normalized rolled weldable fine grain structural steels.

EN 1 0025-4, Hot rolled products of structural steels — Part 4: Technical delivery conditions for

thermomechanical rolled weldable fine grain structural steels.

EN 1 0025-5, Hot rolled products of structural steels — Part 5: Technical delivery conditions for structural steels with improved atmospheric corrosion resistance. EN 1 0025-6, Hot rolled products of structural steels — Part 6: Technical delivery conditions for flat products of high yield strength structural steels in the quenched and tempered condition. EN 1 01 49-1 , Hot-rolled flat products made of high yield strength steels for cold forming — Part 1: General

delivery conditions

EN 1 01 49-2, Hot-rolled flat products made of high yield strength steels for cold forming — Part 2: Delivery

conditions for thermomechanically rolled steels

EN 1 01 49-3, Hot-rolled flat products made of high yield strength steels for cold forming — Part 3: Delivery

conditions for normalized or normalized rolled steels

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DD CEN/TR 10347:2006

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