2010

KOREA IRON & STEEL ASSOCIATION

본 과제(결과물)는 정부의 정부지원금으로 수행한교육훈련혁신센터지원사업의 연구결과입니다.

●●● http://hrd.kosa.or.kr

2010

KOREA IRON & STEEL ASSOCIATION

본 과제(결과물)는 정부의 정부지원금으로 수행한교육훈련혁신센터지원사업의 연구결과입니다.

●●● http://hrd.kosa.or.kr

1장 선재의 제조공정

제1절 선 재 ··········································································································3

제2절 선재의 제조공정 ·················································································3

1. 제강공정(Steel Making) ···········································································4

2. 연속주조(Continuous Casting) ·····························································11

3. 강편압연 ···································································································14

4. 선재압연 ···································································································16

2장 선재의 제품특성

제1절 선재2차가공 ··························································································27

제2절 선재의 종류 ··························································································27

1. 보통선재제품 ····························································································27

2. 고탄소 합금강 선재제품 ··········································································31

목차

3장 산 세

제1절 산 세 ········································································································61

1. 산화 스케일 ······························································································61

2. 스케일 제거법 ··························································································63

3. 산세법의 종류 ··························································································64

제2절 피막 전처리 ··························································································71

1. 인산염 피막 ······························································································72

2. 보락스 피막 ······························································································79

3. 석회 피막 ·································································································81

4장 열처리

제1절 탄소강 ······································································································85

1. Fe-C계 평행상태도 ·················································································85

2. 탄소강의 변태 ··························································································88

3. 탄소강의 조직 ··························································································89

4. 탄소강의 성질 ··························································································91

제2절 강의 열처리 ··························································································92

1. 풀림(소둔, Annealing) ············································································92

2. 불림(소준, Normalizing) ·········································································92

3. 담금질(소입, Quenching) ·······································································92

4. 뜨임(소려, Tempering) ··········································································92

5. 항온변태(Isothermal Transformation) ··················································93

목차

제3절 선재의 열처리 ····················································································93

1. 파텐팅 방법 ······························································································93

2. 파텐팅 조건 ······························································································96

3. 파텐팅 설비 ···························································································100

4. 기타 열처리법 ························································································103

5. 열처리 생략강 ························································································103

5장 인발가공

제1절 인발가공의 정의 및 역사 ·························································107

1. 인발가공의 정의 ·····················································································107

2. 인발가공의 역사 ·····················································································108

제2절 인발가공의 종류 ·············································································110

1. 개 요 ·······································································································110

2. 선재의 다이스 신선가공 구분 ································································112

제3절 강의 신선가공기술 ········································································114

1. 신선 원재료 ····························································································114

2. 신선가공 전처리 ·····················································································115

3. 신선가공설비 및 부자재 ········································································118

제4절 신선기술이론 ······················································································128

1. 단면감소율 ······························································································128

2. 신선가공 전후의 길이변화율 ·································································129

3. 진변형(True strain) ··············································································130

4. 다이스 내 재료의 거동 ··········································································130

5. 신선응력 ·································································································131

6. 다이스압력 ·····························································································133

7. Safety Factor(△) ·················································································134

8. 다이스비율의 설계 ·················································································136

제5절 신선선의 특성치 변화 ·································································138

1. 조직의 형상변화 ·····················································································138

2. 강도(인장강도) 변화 ···············································································139

3. 기타 특성치 변화 ···················································································141

제6절 신선가공불량 ······················································································145

제7절 결 론 ······································································································147

6장 용융아연도금

제1절 역사 및 우수성 ···············································································151

1. 아연 도금 철선의 역사 ···········································································151

2. 용융아연도금이 가장 많이 사용되는 이유 ············································151

제2절 용융아연도금 공정의 이해 ·······················································152

1. 아연도금선 주요 공정 ············································································152

2. 화학성분에 따른 도금의 영향 ································································153

3. 주요 아연도금 방식 ···············································································153

4. 도금조의 가열방식 ·················································································153

5. 용융아연도금 Wiping 종류 ···································································154

6. 센지미아 (Sendzimir) 용융아연도금 공법 ············································155

7. 아연조의 욕 온도와 부착량의 변화 ·······················································155

8. 아연조의 침적시간과 부착량의 변화 ·····················································155

목차

제3절 용융아연도금의 원리 ···································································156

1. 아연의 성질(주기율표상 제2족B에 속하는 금속) ··································156

2. 아연의 제성분과 등급 ············································································156

3. 스케일(Scale)과의 반응 ········································································157

4. 스맛트(Smut)와의 반응 ·········································································157

5. 녹(rust)과의 반응 ··················································································158

6. 용융아연도금시 철과 아연간의 반응 ·····················································158

7. 아연 합금층의 종류 ···············································································159

8. Fe(철) ····································································································160

9. 알루미늄이 용융아연도금에 미치는 영향 ··············································160

10. 용융아연도금시 강중 Si함량에 따른 합금상의 변화 ···························161

11. 후락스(Flux) ·························································································165

제4절 용융아연도금선의 수리해설 ·····················································168

1. 아연 드로스의 아연중량 비율 ································································168

2. 아연 부착량 ····························································································169

3. 도금제품의 강선 톤당 아연의 중량 ·······················································169

4. 아연부착량과 선지름 증가와의 관계 ·····················································170

[표 1] 강의 기계적 성질에 미치는 불순원소의 영향 ···································5

[표 2] 연주법의 장점 ·················································································12

[표 3] 연속가열로의 furnace 비교 ···························································17

[표 4] 피아노선재와 경강선재의 화학성분 비교 ·······································34

[표 5] steel cord의 성분 ·········································································46

[표 6] steel cord의 기계적 물성 ·····························································46

[표 7] 파단 하중에 의한 구분 ···································································49

[표 8] 베어링강의 화학성분 ·······································································56

[표 9] 산화 스케일 종류별 특성 ································································63

[표 10] 탄소량 및 온도별 스케일 발생량 ····················································63

[표 11] 선재의 탈스케일 방법 ······································································63

[표 12] 염산과 황산의 특성 비교 ································································64

[표 13] 염산의 비중과 농도와의 관계 ·························································65

[표 14] 전처리 피막제의 분류 ·····································································72

[표 15] 아연계 인산염 처리욕 평형상수 ······················································75

[표 16] 온도와 산비와의 관계 ·····································································75

[표 17] 탄소강의 조직 및 결정구조 ·····························································89

[표 18] 신선가공 구분 ···············································································113

[표 19] 강의 신선가공(다이스 신선가공) 구분 ··········································114

[표 20] 강의 De‐scaling 방법 ··································································116

[표 21] 신선기 종류 ···················································································120

[표 22] 다이스 형별 치수 및 적용가능 선경 ············································122

[표 23] 금속비누의 종류 및 특성 ·····························································126

[표 24] 무기물질 종류 및 특성 ·································································126

[표 25] Safety factor의 영향 ···································································136

[표 26] KS D 2352 아연지금 종류 ··························································157

[표 27] 아연도금층의 조성과 각 층의 물리적 성질 ··································160

표목차

제1장 선재의 제조공정 7

[그림 1] 선재 제조 공정 ···············································································4

[그림 2] 최근 널리 사용되고 있는 대표적 용선 예비처리 설비 ·················5

[그림 3] 산소제강법에서 전로내 주요 반응 ················································7

[그림 4] 전기로 개략도 ················································································8

[그림 5] 전기로에서 탄소강 및 저합금강 조업 예시 ··································9

[그림 6] 대표적인 2차정련 설비 ·······························································10

[그림 7] 연주법과 조괴법의 비교 ······························································11

[그림 8] 각종 연주기의 형태 ·····································································12

[그림 9] 연주기의 기본 설비 ·····································································13

[그림 10] 고급강을 제조하기 위한 연주기 부가 설비 ································14

[그림 11] 강편 압연공정 ···············································································15

[그림 12] 선재 압연공정 ··············································································16

[그림 13] 선재 압연공장의 압연기 배치 ······················································18

[그림 14] 열간압연방법의 변화에 따른 페라이트 결정립 생성 위치 ··········21

[그림 15] 제어압연 모식도 ···········································································22

[그림 16] 스프링강선의 분류 ·······································································31

[그림 17] 자동차 엔진 및 밸브스프링 ·························································33

[그림 18] 스프링의 형상에 따른 분류 ·························································35

[그림 19] 승용차용 현가코일스프링 ·····························································36

[그림 20] 스프링 종류 ·················································································38

[그림 21] 스프링 제조공정 ···········································································38

[그림 22] 스프링 개발방향 ··········································································39

[그림 23] 타이어 단면 ·················································································41

[그림 24] 타이어 내부구조 및 steel cord ·················································43

[그림 25] 선재 제조공정 ··············································································44

[그림 26] steel cord 제조공정 ··································································45

[그림 27] wire rope 명칭 ··········································································48

[그림 28] 냉간압조용 소재 제조공정 ··························································54

[그림 29] 냉간압조용 볼트의 강도별 grade ··············································55

[그림 30] 베어링 제조공정 ··········································································57

[그림 31] 베어링 종류 ··················································································57

그림목차

선재가공기술 교육실무8

[그림 32] 산화스케일의 구성도 ···································································61

[그림 33] Fe-O 상태도 ··············································································62

[그림 34] 열처리 온도에 따른 산화스케일 성상별 구성비 ·························63

[그림 35] 산화 스케일의 박리 모식도 ························································65

[그림 36] 산세시간과 염산의 농도와의 관계 ··············································66

[그림 37] 염산의 온도에 따른 산세시간 ·····················································67

[그림 38] 염산농도별 Fe농도에 따른 산세시간 ··········································67

[그림 39] 황산농도별 산세시간 ···································································68

[그림 40] 황산 농도, 온도별 산세시간 ·······················································69

[그림 41] 전해 산세 반응 기구도 ································································70

[그림 42] 인산염 피막의 반응기구도 ··························································75

[그림 43] 정상적인 인산 아연계 피막사진 ··················································79

[그림 44] 불균일한 아연계 인산염 피막 ·····················································79

[그림 45] Fe-C계 평행상태도 ····································································86

[그림 46] Fe-C계 평행상태도와 변태조직도 ·············································88

[그림 47] Pearlite 생성기구 ·······································································89

[그림 48] 탄소강의 현미경 사진 ·································································91

[그림 49] 항온변태도(TTT곡선)와 연욕파텐팅의 냉각상태 ·························94

[그림 50] 연속냉각상태도(CCT곡선)와 공기파텐팅의 냉각상태 ·················95

[그림 51] 열처리선 인장강도에 미치는 가열온도 영향 ·······························97

[그림 52] 스케일 생성 기구 ········································································99

[그림 53] 연욕파텐팅 설비 개략도 ····························································100

[그림 54] 선재․봉의 다이스 인발 ····························································107

[그림 55] 고대의 인발가공방법 및 인발기 ···············································108

[그림 56] 각종 인발가공제품 적용현황 ·····················································109

[그림 57] 회전 다이스 ···············································································111

[그림 58] 압력 다이스 ···············································································112

[그림 59] Wire rod 제조공정 ···································································114

[그림 60] Wire rod의 표면 Scale ···························································116

[그림 61] 파텐팅 열처리 기술 ···································································116

[그림 62] 파텐팅 열처리 조직 ···································································117

그림목차

[그림 63] 본데라이트 피막 형상 ·······························································117

[그림 64] Straight식 신선기(Dnace식) ····················································119

[그림 65] Slip식 신선기(Cone식) ·····························································120

[그림 66] 다이스 팁 구조 및 명칭 ···························································121

[그림 67] 텅스텐 카바이드 탄화물 결정형태 및 코발트 함유량에 따른

경도 변화 ···················································································123

[그림 68] PCD 원석과 PCD 다이스 ························································124

[그림 69] ND 원석과 ND 다이스 ·····························································124

[그림 70] 첨가제 종류 및 특성 ·································································127

[그림 71] 균질변형과 비균질변형 ······························································131

[그림 72] 신선응력과 다이스 반각과의 관계 ············································132

[그림 73] 다이스반각, 감면율과 신선응력 ················································133

[그림 74] 다이스반각과 다이스압력 ··························································134

[그림 75] Safety factor의 정의 ·······························································135

[그림 76] 다이스각도, 감면율 및 Safety Factor(△) ······························135

[그림 77] 신선가공에 의한 조직형상 변화 ················································138

[그림 78] Grain 신장 모델 ·······································································139

[그림 79] 전위의 이동 및 증가 모델 ························································139

[그림 80] 신선 후 인장강도 예측식 ··························································140

[그림 81] 신선감면율별 인장강도 변화 ·····················································141

[그림 82] 신선감면율과 특성치 관계 ························································143

[그림 83] 신선가공에 의한 잔류응력 분포 ···············································144

[그림 84] 가공조건별 잔류응력 분포 ························································144

[그림 85] 가공조건별 응력부식 특성 ························································145

[그림 86] 중심부 크랙 발생 모델 ·····························································146

[그림 87] 다이스각도와 Bulging 발생기구 ···············································146

[그림 88] 아연도금라인의 주요설비 ··························································152

[그림 89] Si 함유량에 따른 합금층 두께 ·················································162

[그림 90] 용융아연도금시 나타나는 다양한 합금층의 단면 ·····················163

선재의 제조공정

제1절 선 재

제2절 선재의 제조공정

KOREA IRON & STEEL ASSOCIATION

1장

제1장 선재의 제조공정 3

1장 선재의 제조공정

제1절 선 재

선재(wire rod)는 고온 상태에서 압연을 실시한 압연강재(壓延鋼材) 중에서 봉형강류에

속하는 제품으로 단면이 둥글고 코일(coil) 형상으로 감겨져 있으며, 단면의 지름은 5.5∼40.0㎜ 정도이다. 탄소 함량에 따라 보통 선재와 특수 선재로 구분되고, 그대로 사용되는

경우는 거의 없으며, 상온에서 인발 가공하여 철선이나 강선이 된다.

선재는 원래 선(wire)의 소재를 의미하고 압연강재를 형상별로 분류할 때의 명칭의 일

종이다. 선재 중에서 2차가공 공정에서 인발기에 넣어 곧은 봉 형상으로 소정의 길이로

절단해 사용되는 제품들을 선재와 구분하여 BIC(bar in coil)라 부르기도 한다.

선재의 단면 형상은 대부분 원형이지만 그 용도에 따라 육각형, 정방형, 장방형, 이형 단면

이 생산되기도 한다. 치수 범위는 직경 5.5～40.0㎜가 주로 생산되고 있으나 최근에는 50.0㎜ 전후 선재도 출현하고 있다. 중량은 1,000～2,000kg 코일이 일반적이나 2차가공 메이커의 생

산성, 실수율 향상을 목적으로 3,000～4,000kg 범위의 선재도 생산되고 있다. 철강 선재 이외

에 동, 알루미늄, 티타늄 등 다른 금속을 재료로 한 선재도 시장에 공급되고 있다.

제2절 선재의 제조공정

선재의 일반적인 제조공정은 [그림 1]과 같다. 용선(선철) 또는 철스크랩을 사용하여 전

로(converter) 또는 전기로에서 용강(molten steel)을 제조하고, 이어서 정련공정(2차정련,

secondary refining)에서 목표로 하는 온도, 성분의 용강을 정련(精鍊)한다. 정련이 종료된

용강이 연속주조(continuous casting) 공정으로 보내지고, 여기서 블룸이나 빌렛을 제조하

게 된다. 이전에는 용강이 주로 분괴공정으로 보내져 잉곳(ingot)을 제조하였으나 생산성,

에너지, cost 등의 이유로 말미암아 점차 사라지고 있는 추세이다. 만약 연속주조공정에서

선재가공기술 교육실무4

블룸을 제조한다면 이들 블룸이 강편압연공정으로 보내져 빌렛으로 압연된다. 이들 빌렛은

먼저 자분탐상 등 결함검사를 통해 결함부를 검출하게 되고, 이어서 grinding 등 검출된

결함을 제거하며, 이를 빌렛 정정 공정이라 부른다. 정정이 완료된 빌렛은 가열로, 선재압

연, 냉각 공정 등을 거쳐 선재가 제조되고, 이들은 검사, 포장 공정을 거쳐 2차가공 공정

으로 인도하게 된다.

연속주조

Bloom 강편압연

정정가열선재압연Stelmor 냉각

Garret 냉각

검사포장출하

Billet

LF

RH

전로

전기로

철광석

고철

고로

연속주조

Bloom 강편압연

정정가열선재압연Stelmor 냉각

Garret 냉각

검사포장출하

Billet

LFLF

RHRH

전로전로

전기로전기로전기로

철광석철광석

고철고철

고로고로고로

[그림 1] 선재 제조공정

1. 제강공정(Steel Making)

공업적으로 사용되는 철은 소위 5대 원소로 불리는 C, Si, Mn, P, S 등을 포함할 뿐

아니라, 정련 과정에서 대기로부터 혼입되는 H, N, O 등 가스 성분을 일부 함유하게 되

며, 그러한 불순 성분들은 표 1과 같이 철의 기계적 성질을 좌우한다. 특히, 탄소는 철의

인성을 약화시키며, 탄소가 2% 이하이면 강(鋼), 2% 이상이면 선(銑)이라고 구분한다. 제

강공정은 용선(전로)이나 철스크랩(전기로)을 주원료로 사용하여 탄소 함량이 2% 이하의

강을 제조하는 일련의 작업으로써, 1) 불순원소 제거 2) 필요한 합금원소 첨가 3) 주조작

업이 가능하도록 온도 조정(증가 또는 냉각) 등을 모두 포함하게 된다. 선재를 제조하기

위한 제강공정은 예비처리/전로, 전기로, 2차정련 등으로 이루어진다.

제1장 선재의 제조공정 5

Impurities Form Influence

[S], [O]

Non-metallicInclusion

(Sulphide &Oxide)

Lowered1) Ductility, 2) Charpy Impact Value, 3) Anisotropy, 4) Formability(Elongation, Reduction of Area and Bendability), 5) Cold Forgeability, 6) Drawability, 7) Low-temperature Toughness, 8) Fatigue Strength

[C], [N]

Solid Solution

Enhanced1) Hardenability, 2) Strain Aging

Lowered1) Ductility, 2) Toughness

Carbide & Nitride

Enhanced1) Precipitation Hardening, 2) Grain Refining, 3) Embrittlement

Lowered1) Ductility, 2) Toughness

[P] Solid SolutionEnhanced1) Hardenability, 2) Temper brittleness, 3) Embrittlement in Secondary

Forming

[표 1] 강의 기계적 성질에 미치는 불순원소의 영향

가. 예비처리, 전로

용선 예비처리 기술은 전로제강 기술과 함께 발전해 왔으며, 특히 1960년대 말부터 고급

강 수요가 증가하게 되고, 전로에서 탈황능력 한계에 따라 용건 탈황기술이 본격적으로 적

용되기 시작했다. 특히 에너지 절감 및 생산성 향상을 위한 노력의 일환으로 전로를 중심

으로 전후 공정에의 기능 분담에 의한 Total Merit 추구, 그리고 급증하는 고급강 수요에

대응하기 위하여 예비처리 도입이 불가피하게 되었다. 널리 사용되고 있는 대표적인 예비처

리 설비로는 [그림 2]와 같이 HMPS(Hot Metal Pre‐treatment Station), KR(Kanbara Reactor) 등이 있다.

HMPS KRHMPS KR[그림 2] 최근 널리 사용되고 있는 대표적 용선 예비처리 설비

이들 예비처리 설비에서는 주로 용선 탈황(Hot Metal De‐sulphurization)을 실시한다.

HMPS는 상부로부터 플럭스(Flux)를 취입할 수 있는 내화물 재질의 랜스(Lance)를 래들

선재가공기술 교육실무6

에 침지시키고, 랜스를 통해 이송가스(Carrier Gas)와 함께 탈황제를 취입함으로써, 용선

과 탈황제가 탈황반응을 일으키게 유도한다. 반면 KR은 래들 상부에 탈황제를 투입하고,

기계식 Impeller를 고속으로 회전시킴으로써 용선과 탈황제의 반응을 조장한다. 용선예비

처리가 종료되면 용선의 S 함량이 10～수십ppm 범위로 감소하게 되며, 용선의 S 성분은

최종제품에서 요구되는 S 함량을 감안하여 결정하게 된다. 최근 용선예비처리 공정에서

탈황반응 이외 실리콘(Si) 이나 인(P)을 제거하는 조업을 실시하기도 하는데 이러한 일련

의 시도들은 제강공정에서의 Total Merit을 추구하는 측면에서 전로에서 수행되어야 할

기능일부를 예비처리공정에서 실시하는 것이다.

전로는 1952년 순산소 취련법이 도입된 이후 비약적으로 발전하게 되었고, 그 이유는

산소 제강법 채택으로 인해 품질이 우수한 강의 대량생산이 가능하게 되었고, 전로가 제강

공정에서 중추적인 역할을 담당하게 되었기 때문이다. 세계적으로 주종을 이루고 있는 산

소제강법은 산소를 취입하는 방법에 따라 상취전로법, 저취전로법 및 복합취련전로법 등으

로 나누어진다. 상취전로법은 고순도의 산소를 전로내의 용철표면에 분사시킴으로써 불순

물을 산화 제거하는 방법이고, 저취전로법은 전로 바닥에 설치된 Tuyere를 통해 산소를

취입하는 방법이다. 이들은 각각 장단점이 있으며, 장점만을 이용할 수 있도록 이들 2가지

기술을 조합, 개량한 것이 복합취련 전로법이다. 이는 상부에서는 랜스를 통해 산소를 취

입하고, 로 바닥에서는 Ar, N2 등 불활성 가스를 취입하여 교반력을 높임으로써, 정련효

과를 개선한 기술이다. 최근 계측기술과 컴퓨터기술 등 주변기술의 발전으로 직접출강 기

술, 전로수명 연장 기술 등이 개발되고, 정련기능의 이론적 한계 추구를 통해 보다 경제적

이고 효율적인 요소기술이 지속적으로 개발되고 있다.

예비처리가 완료된 용선을 전로에 장입하고, 용선(溶銑) 표면에 산소를 상취 하는 전로

에서는 분사된 산소가 슬래그를 통과하여 비교적 좁은 영역에서 용선과 반응하게 되고, 용

선에 함유된 C, Si, Mn, P 등과 반응하며, 이들은 모두 산화반응으로써, 반응열이 발생하

여 용탕의 온도가 증가한다. [그림 3]은 복합 취련 전로에서 일어나는 주요 정련반응을 나

타내고 있다.

전로에서 산소를 고속으로 취입하면 용탕 표면에 cavity가 형성되면서 표면에서 C, Si,

Fe, Mn, P 등이 산소와 반응하는 산화반응이 진행된다. 그때 산화반응열이 생성되고, 산

화반응으로 생성된 SiO2, FeO, MnO, P2O5 등과 외부에서 공급된 CaO, MgO 및 첨가된

부원료가 분해되어 용탕 표면에서 슬래그를 형성하며, 이 슬래그 층에서는 슬래와 용철의

반응을 진행된다.

한편, 전로 상층부에서는 용철의 탄소와 취입된 산소의 반응으로 생성된 CO 가스가 기

체산소와 반응하여 CO2를 형성하는데, 이를 2차연소 반응이라고 부르고, 이 반응 역시 산

제1장 선재의 제조공정 7

화반응으로 다량의 산화반응열이 발생된다.

복합취련 전로법은 1)고속정련 2)고반응성 슬래브의 초기형성이 탈탄과 동시에 탈인 촉

진 3)hot spot에서 불순원소의 증발 제거 4)저탄소 영역에서는 욕의 움직임이 약해지기

때문에 강의 과산화(過酸化)를 방지 5)저질소강의 제조 가능 6)S(유황)를 50% 정도 제거

등과 같은 특징을 나타내고 있다.

오늘날 대부분의 일관제철소는 순산소 전로법을 도입함으로써 P, S, O, N, H 및 미량

금속원소 함량이 지극히 강을 제조할 수 있게 되었을 뿐 아니라, 강의 성형성, 인성, 용접

성 등의 성질은 평로나 전기로 이상의 고품질을 확보하게 되었다.

슬래그 형성CaO → (CaO) MgO → (MgO) 2(CaO)+(SiO2) → (2CaO⋅SiO2)4(CaO)+(P2O5) → (4CaO⋅ P2O5)

부원료 분해Fe2O3 → 2 Fe + 3/2 O2FeO → Fe + 1/2 O2Ca2CO3 → CaO + CO2

Cavity 표면에서 산화반응C + 1/2O2 = COSi + O = SiO2Fe + 1/2O2 = FeOMn +1/2O2 = MnO2P + 5/2O2 = P2O5

CO gas의 산화CO + 1/2 O2 = CO2

슬래그/용철간 반응2P+4(CaO)+5(FeO)↔ (4CaO⋅ P2O5)+5FeMn + (FeO) ↔ (MnO) + Fe(FeO) ↔ Fe + O

슬래그 형성CaO → (CaO) MgO → (MgO) 2(CaO)+(SiO2) → (2CaO⋅SiO2)4(CaO)+(P2O5) → (4CaO⋅ P2O5)

부원료 분해Fe2O3 → 2 Fe + 3/2 O2FeO → Fe + 1/2 O2Ca2CO3 → CaO + CO2

Cavity 표면에서 산화반응C + 1/2O2 = COSi + O = SiO2Fe + 1/2O2 = FeOMn +1/2O2 = MnO2P + 5/2O2 = P2O5

CO gas의 산화CO + 1/2 O2 = CO2

슬래그/용철간 반응2P+4(CaO)+5(FeO)↔ (4CaO⋅ P2O5)+5FeMn + (FeO) ↔ (MnO) + Fe(FeO) ↔ Fe + O

[그림 3] 산소제강법에서 전로내 주요 반응

나. 전기로

전기로는 노내에 장입된 철스크랩(Scrap), DRI(직접환원철) 등 원료와 전극 사이에 전

기 아크(Arc)를 발생시켜 그 고온을 이용하여 원료를 용해하는 것이다. 1900년초 실용화

에 성공한 전기로 제강법은 ’60년대 적응기를 시작으로 ’70년대 내화물의 발달, ’80년대

UHP용 Transformer 적용 등으로 AC 전기로의 전성기를 이루었으나, 이후 설비 대형화,

소음 발생에 따른 환경개선 요구, 공장 자동화, 컴퓨터 기술의 발전, 전기 관련 기술의 발

전 등으로 DC 전기로가 활발히 보급되고 있다.

초기 전기로의 경우 철스크랩을 용해하는데 주력했으나, 최근에는 용해는 기본이고, 산

소가스 분사나 철광석 첨가로 산화정련을 실시하여, Si, Mn, Cr, P 등 불순물의 제거하거

선재가공기술 교육실무8

나, 로 바닥에 Bubbling 장치를 도입하여 불활성 Gas 취입함으로써, 강욕의 온도와 성분

을 균질화하고, 용탕/slag 반응을 촉진시키기도 한다. [그림 4]는 전기로의 개략도를 도시

한 것이며, 전기로는 노체, 변압기, 전극, 전극 승하강 장치, 집진장치, 철스크랩 예열장치,

유도 교반 장치 등으로 구성된다. 전기로의 원료는 크게 철스크랩, 선철, 환원철 등 주원

료와 석회석, 생석회, mill scale, 기체산소, 형석, 분 코크스 등 용제로 구분할 수 있다. 용

제의 주된 목적은 용탕에서 불순물 제거, 용탕의 오염 방지 등이다.

[그림 4] 전기로 개략도

한편 전기로 조업은 장입되는 원료 상태에 따라 냉재법과 용재법이 있고, 산화정련의

정도에 따라 완전산화법, 일부산화법 및 무산화법 등으로 구분할 수 있다. 냉재법은 철스

크랩과 같은 냉재를 장입하여 용해하는 방법이며, 가장 일반적인 조업 방법이다. 용재법은

용선이나 용강 등 액체 철을 장입하여 정련하는 방법으로 이중조업(Duplex Process)이라

고 부르기도 한다.

완전산화법은 산소나 철광석을 사용, 원료중의 C, Si, Mn, P 등을 산화제거함과 동시에 강

욕 중의 수소를 제거하는 것으로 고급 전기로강 전련에서 많이 이용되는 방법이다. 일부산화

법이나 무산화법은 산화정련 과정을 거의 하지 않거나 또는 전혀 실시하지 않고, 바로 환원작

업을 하는 것이다. 최근 진공탈가스법과 조합하여 무산화에 가까운 조업을 실시하기도 한다.

전기로 조업은 시간순서에 따라 1)원료장입 2)용해기 작업 3)산화기 작업 4)슬래그 배출

작업(제재기 작업) 5)환원기 작업 6)출강 작업 등의 순으로 진행되고, [그림 5]에 전기로에

서 탄소강 및 저합금강 조업예시를 나타내었다.

전기로 제강법을 일관제철소와 비교할 때, 투자비가 저렴하고, 건설기간이 단축되며, 생

산 Flexibility(유연성)가 우수한 것 등 여러 가지 장점이 있음에도 불구하고, 철스크랩 등

원료가격 변동이 심할 뿐 아니라, 전기로 조업에서는 구리(Cu), 니켈(Ni), 크롬(Cr), 바나

듐(V) 등 Tramp 원소들을 효과적으로 제어하기 어렵다는 문제점도 동시에 안고 있다.

제1장 선재의 제조공정 9

[그림 5] 전기로에서 탄소강 및 저합금강 조업 예시

다. 2차정련

철강재의 용도가 다양화되고 고객이 요구하는 품질이 더욱 엄격해짐에 따라 전로 또는

전기로 단독 처리만으로 원하는 수준의 품질과 생산성을 확보하기 곤란해졌다. 최근 전로/

전기로의 경우 산소 취입이나 철 산화물 첨가 등으로 산화성 분위기가 유지되므로 탈황반

응 등 일부 반응이 제한을 받게 된다. 또한 높은 생산성을 얻기 위해 전로정련이 종료된

이후에 정련공정을 추가함으로써 전로의 기능을 일부 분담하고 부족한 기능을 보완해 왔

다. 이와 같이 전로 혹은 전기로에서 생산한 용강을 고객니즈 및 연속주조에 부합하도록

하기 위해 야금조작을 행하는 일련의 행위를 2차정련(Secondary Refining) 또는 래들정

련(Ladle Metallurgy)이라 부른다.

2차정련은 그 목적에 따라 1)용강의 화학 성분 조정, 탈산제 및 합금철 첨가 2)승온 및

냉각 3)용강 청정도 향상 및 비금속 개재물 형태 제어 4)강 중 불순물(P, S, C, N, H 등)

제거 5)성분 및 온도의 균일화 6)출강으로부터 주조시작까지 시간 조정(완충 효과) 등으로

분류할 수 있다. 이와 같은 목적을 달성하기 위하여 전로 슬래그의 래들 유입 억제, 래들

슬래그 조성 제어, 교반, 진공 처리, 분체 취입, 용강의 가열 및 냉각, 합금철 첨가, 재산화

방지, 적정 내화물 선택 등 많은 종류의 기술이 적용되고 있다. 오늘날 널리 사용되고 있

선재가공기술 교육실무10

는 대표적인 2차정련 설비로는 3상 Arc 가열방식의 LF(Ladle Furnace)와 진공탈가스 방

식 RH가 있으며, [그림 6]에 오늘날 널리 사용되고 있는 2차정련 설비를 나타내었다.

LF RH

BAPPI Slag skimming

LFLF RH

BAPPI Slag skimming

LF

[그림 6] 대표적인 2차정련 설비

먼저, LF(Ladle Furnace)는 온도가 낮은 저온 용강의 온도를 높이기 위해 개발된 설비

이며, 가열 설비인 전극봉과 Transformer, 분위기를 제어하기 위한 Top cover, 합금철

투입 장치 등을 가지고 있으며, 용강을 교반하기 위한 장치로 상취 랜스나 저취

plug(Tuyere)를 보유하고 있고, 경우에 따라서는 분체취입(PI) 설비를 가지기도 한다.

LF 조업은 먼저 슬래그에 탄소 전극봉을 침지시키고, 전극봉과 용강 사이에서 발생되는

3상 전기 Arc를 이용, 용강의 온도를 높이거나 각종 정련에 요구되는 열원을 공급하며,

정련 목적에 따라 각종 Flux를 투입하기도 한다.

RH는 진공조 내부 압력을 수 Torr 정도의 진공상태로 만들고 이러한 진공을 이용하

여 용강에 포함된 수소, 질소, 산소 등 가스성분을 제거하는 장치이다. 용강이 전로, LF

등 여러 정련과정을 거치면서 가스 함량이 증가하게 되고, 이러한 가스 성분을 제거하지

않는다면 최종제품에 결함이 발생하게 된다. 따라서 진공원리를 이용하여 용강 중 가스성

분을 제거하는 방법이 1950년대 이후 개발되었고, 이후 고객의 더욱 엄격한 품질 니즈로

인해 진공 탈가스법이 매우 빠른 속도로 발전되어 왔다. 최근 널리 사용되는 진공탈가스

장치로는 RH가 있으며, 그 외 DH, VD(Vacuum Degasser), VTD(Vacuum Tank

제1장 선재의 제조공정 11

Degasser), VOD(Vacuum Oxygen Degasser) 등도 사용되고 있다.

PI와 BAP는 비교적 투자비가 싸고 간편한 2차정련 설비이며, 이들은 용강에 존재하는 산

화물, 황화물 등 비금속 개재물을 부상분리시키거나 유황 성분을 수 ppm 정도까지 감소시키

기도 하나 조업 중 대기로부터 질소 유입, 용강의 재산화 등 취약점도 가지고 있다. 이러한

방법들은 교반력을 이용, 개재물 부상분리 목적으로 Ar 가스를 저취 또는 상취하기도 한다.

한편, 전로 또는 전기로에서 출탕시 슬래그가 유출되는 경우 이들 슬래그에 함유된 불순

성분이 다시 용강을 오염시키거나, 슬래그에 다량 함유된 (FeO), (MnO) 등이 탈산제, 합금

철의 수율(Recovery ratio)을 떨어뜨린다. 전로/전기로 슬래그의 2차정련 공정으로의 유출로

인해 제기되는 대표적인 문제점 중 한가지가 2차정련 공정에서 시간이 경과됨에 따라 P함량

이 증가되는 현상, 즉, 복린현상이다. 이러한 부작용을 줄이기 위해 1차적으로는 출탕에 앞

서 슬래그를 제거하거나(배재작업), 출탕시 슬래그 유출을 억제하는 방법(슬래그 cutting 작

업) 또는 [그림 6]에서 보는 것과 같이 Ladle로 유출된 슬래그를 제거하는 방법(Slag Ski㎜ing) 등이 있다. 각각의 방법에는 모두 장단점이 있고, 생산성, 비용, 작업효율 등을 감안하

여 철강업체별로 알맞은 방법들을 적용하고 있다.

2. 연속주조(Continuous Casting)

연속주조는 일련의 제강 과정을 거쳐 성분과 온도가 제어된 용강을 일정한 형상의 수냉

주형(Mold)에 연속으로 주입하고 반응고된 주편을 주형 하부에서 연속으로 빼내어 블룸,

슬래브, 빌렛 등을 얻는 주조법이다. 연주법(Continuous casting)은 [그림 7]에서 보듯이

일반 조괴법의 균열로와 분괴공정 일부를 생략한 방법이다.

[그림 7] 연주법과 조괴법의 비교

선재가공기술 교육실무12

연주법은 [표 2]에서 보듯이 조괴법에 비해 수율, 생산능률, 에너지 소비 측면에서 우수

하며, 그 외 자동화, 기계화가 용이하고, 공장면적 감소, 작업환경 개선 등 많은 장점으로

인하여 1970년대 이후 강괴법이 급속히 퇴조하고 동시에 연속주조법(연주법)이 비약적으

로 증가, 발전하였다. 최근에는 산소 제강법은 물론이고, 전기로강, 특수강에 이르기까지

모든 강종에 채용되고 있다.

최초의 연주기는 주형 형상이 수직형이었으나 이후 [그림 8]에서 보듯이 주형 형태에

따라 수직형, 수직곡형, 만곡형, 수평형 등 여러 가지 형식으로 발전되었고, 최근에는 박판

주조법(Strip casting), Thin Slab Casting 기술 등으로 발전되고 있다.

[그림 8] 각종 연주기의 형태

구 분 수 율주편

제조시간

에너지소비

전 기 가 스

조 괴 법 75~90% 10~20hr 1.5~25 5~18

연 주 법 90~99% 1~2hr 1,0(기준) 1.0(기준)

[표 2] 연주법의 장점

연주기의 기본적인 설비는 [그림 9]에서 보듯이 용강용 래들, 용강을 래들로부터 받아

각 Strand의 몰드에 분배함과 동시에 주입 유량을 조절하는 턴디시(Tundish), 주입 용강

을 받아 응고시키는 수냉 주형(몰드), 주형하부에서 나오는 미응고 주편을 냉각하는 2차냉