A method for preparing a low spot weld carbon equivalent alloyed galvanized dual phase steel

Abstract

The present application relates to the technical field of cold-rolled plate strip production, and particularly relates to a preparation method of 590MPa-grade low spot welding carbon equivalent alloying galvanized dual-phase steel. The method comprises the following steps: casting 590MPa-grade low spot welding carbon equivalent alloying hot-dip galvanized dual-phase steel casting blank; obtaining hot-rolled coil after the slab is subjected to heating, descaling, rough rolling, finish rolling and laminar cooling; after the hot-rolled coil is subjected to pickling, cold rolling is performed to obtain 0.6-3.0mm cold-rolled thin steel; and hot-dip galvanizing treatment is performed on the cold-rolled thin steel strip to obtain galvanized dual-phase steel.

Description

Technical Field

[0001] This invention relates to the field of cold-rolled sheet and strip production technology, and in particular to a method for preparing 590MPa grade low spot welding carbon equivalent alloyed galvanized duplex steel. Background Technology

[0002] As people become increasingly aware of energy conservation and material safety, many automakers are choosing high-strength steel for their vehicles. The automotive industry's use of hot-dip galvanized high-strength steel sheets reduces sheet thickness while improving corrosion resistance, dent resistance, durability, impact strength during large deformations, and safety. Therefore, automotive steel sheets will inevitably develop towards higher strength, higher toughness, corrosion resistance, and ease of forming and processing. With increasing demands for corrosion resistance in car bodies, galvanized automotive steel sheets are being used more and more extensively in automobile production.

[0003] The prior art CN 101942603 A discloses a 600MPa grade ultra-low silicon cold-rolled hot-dip galvanized duplex steel and its preparation method. The chemical composition by weight percentage is: C: 0.03-0.16%, Si < 0.02%, Mn: 1.2-2.2%, Cr: 0.2-0.6%, S < 0.015%, P < 0.020%, Als: 0.01-0.08%, with the balance being Fe and trace elements. The slab heating temperature is 1150–1280℃, the finishing rolling temperature is 1150–970℃, the final finishing rolling temperature is 750–920℃, and the coiling temperature is controlled at 550–740℃. The cold rolling reduction rate is 45%–80%, the heating rate is 1–10℃ / s, the annealing holding temperature is 720–820℃, the holding time is 25–100s, the rapid cooling rate is 15–35℃ / s, the galvanizing temperature is 460℃, and the galvanizing time is 10–30s. The final cooling rate is >15℃ / s. A key feature of this patent is its extremely low Si content (<0.02%). To obtain sufficient strength, a significant amount of C or Mn alloy needs to be added, increasing both alloy cost and spot welding carbon equivalent, which is detrimental to achieving good welding and forming results. To address this technical problem, a method for preparing 590MPa grade low spot welding carbon equivalent alloyed galvanized duplex steel is proposed. Summary of the Invention

[0004] In order to solve the technical problems existing in the prior art, the present invention provides a method for preparing 590MPa grade low spot welding carbon equivalent alloyed zinc duplex steel and a duplex steel billet.

[0005] To achieve the above objectives, the embodiments of the present invention provide the following technical solutions:

[0006] In a first aspect, in one embodiment of the present invention, a duplex steel billet is provided, the chemical composition of which, by mass percentage, is C: 0.04%–0.10%, Si: 0.15%–0.50%, Mn: 1.50%–1.90%, P≤0.020%, S≤0.010%, Als: 0.010%–0.060%, N≤0.0060%, Cr: 0.30%–0.40%, with the balance being Fe.

[0007] As a further aspect of the present invention, the chemical composition range of the duplex steel billet by mass percentage is: C: 0.06%, Si: 0.25%, Mn: 1.65%, P≤0.015%, S≤0.005%, Als: 0.030%, N≤0.0045%, Cr: 0.30%, with the balance being Fe.

[0008] As a further aspect of the present invention, the microstructure of the dual-phase steel billet comprises 20-30% ferrite matrix and 70-80% agglomerated martensite.

[0009] Secondly, in one embodiment of the present invention, a method for preparing 590MPa grade low spot welding carbon equivalent alloyed zinc duplex steel is provided, the method comprising the following steps:

[0010] Casting 590MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel billet;

[0011] Hot-rolled coils are obtained by heating, descaling, rough rolling, finish rolling and laminar flow cooling of slabs;

[0012] After pickling, hot-rolled coils are cold-rolled into cold-rolled thin steel sheets of 0.6–3.0 mm.

[0013] Cold-rolled thin steel strips are hot-dip galvanized to obtain galvanized duplex steel.

[0014] As a further aspect of the present invention, step S2, obtaining a hot-rolled coil by heating, descaling, rough rolling, finish rolling, and laminar flow cooling of the slab, includes:

[0015] The heating temperature is 1210~1260℃, the initial rolling temperature of the finishing mill is 1055~1160℃, and the final rolling temperature of the finishing mill is 880~950℃; laminar flow cooling adopts the front-stage cooling method, with upper and lower surface cooling rates of 50% and 75%, respectively; the coiling temperature is 580~650℃, and the hot-rolled thickness is 2.50~6.25mm.

[0016] As a further aspect of the present invention, in step S3, the hot-rolled coil is pickled and then cold-rolled into a cold-rolled thin steel strip of 0.6–3.0 mm; comprising,

[0017] The cold rolling reduction rate is 52% to 76%.

[0018] As a further aspect of the present invention, step S4, hot-dip galvanizing the cold-rolled thin steel strip to obtain galvanized duplex steel, includes:

[0019] The cold-rolled thin steel strip is first heated in sections to 300℃, 700℃ and 780-810℃ at heating rates of 9-22℃ / s, 3.5-11℃ / s and 1.0-3.5℃ / s respectively;

[0020] The heated cold-rolled thin steel strip is subjected to uniform heat preservation.

[0021] After the cold-rolled thin steel strip is subjected to uniform heat preservation, it is then subjected to balanced heat preservation, and after the balanced heat preservation is completed, the cold-rolled thin steel strip is put into the zinc bath for galvanizing treatment.

[0022] After galvanizing, the cold-rolled thin steel strip is first cooled to 380-420°C by an air knife; then it is rapidly heated at a rate of 15-43°C / s to undergo alloying treatment to obtain galvanized duplex steel.

[0023] As a further aspect of the present invention, after heat equalization and heat preservation for 60–135 s, the temperature is slowly cooled to 680–715 ℃ and rapidly cooled to 440–470 ℃ at rates of 1.5–8.5 ℃ / s and 18–35 ℃ / s respectively, and then balanced heat preservation is performed.

[0024] As a further embodiment of the present invention, the alloying treatment temperature is 520-565°C, the holding time of the alloying treatment is 5-30 seconds, and the cold-rolled thin steel strip is cooled to room temperature by a fan after the alloying treatment.

[0025] As a further aspect of the present invention, after alloying treatment, the cold-rolled thin steel strip is cooled to room temperature using a fan, wherein the cooling rate is ≥5℃ / s.

[0026] The technical solution provided by this invention has the following beneficial effects:

[0027] The yield strength of the duplex steel billet of this invention is 365–435 MPa, the tensile strength is 605–660 MPa, and the elongation A is... 80 The value is 24.0% to 30.0%, which well meets the requirements. The alloying annealing of this invention involves heating the hot-dip galvanized sheet to a certain temperature after hot-dip galvanizing, resulting in a zinc-iron alloy layer through the interdiffusion phase transformation between the iron in the substrate and the zinc-iron in the zinc layer. This coating exhibits good weldability, paintability, and stone-chip resistance.

[0028] These or other aspects of the invention will become more apparent from the following description of embodiments. It should be understood that the foregoing general description and the following detailed description are exemplary and explanatory only, and are not intended to limit the invention. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other embodiments can be obtained based on these drawings without creative effort.

[0030] Fig. 1 Metallographic photograph of 590MPa grade low spot welding carbon equivalent alloyed zinc duplex steel according to an embodiment of the present invention.

[0031] Fig. 2 This is a scanned image of a 590MPa grade low spot welding carbon equivalent alloyed zinc duplex steel according to an embodiment of the present invention. Detailed Implementation

[0032] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0033] The flowchart shown in the attached diagram is for illustrative purposes only and does not necessarily include all content and operations / steps, nor does it necessarily have to be performed in the order described. For example, some operations / steps can be broken down, combined, or partially merged, so the actual execution order may change depending on the actual situation.

[0034] This invention provides a method for preparing 590MPa grade low spot weld carbon equivalent alloyed zinc-plated duplex steel. The method includes the following steps:

[0035] S1. Cast 590MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel billet.

[0036] The chemical composition range of the duplex steel billet by mass percentage is as follows: C: 0.04%–0.10%, Si: 0.15%–0.50%, Mn: 1.50%–1.90%, P≤0.020%, S≤0.010%, Als: 0.010%–0.060%, N≤0.0060%, Cr: 0.30%–0.40%, with the balance being Fe.

[0037] Preferably, the chemical composition range of the duplex steel billet by mass percentage is: C: 0.06%, Si: 0.25%, Mn: 1.65%, P≤0.015%, S≤0.005%, Als: 0.030%, N≤0.0045%, Cr: 0.30%, with the balance being Fe.

[0038] Preferably, the chemical composition range of the duplex steel billet by mass percentage is: C: 0.09%, Si: 0.45%, Mn: 1.85%, P≤0.015%, S≤0.005%, Als: 0.055%, N≤0.0045%, Cr: 0.30%, with the balance being Fe.

[0039] Preferably, the chemical composition range of the duplex steel billet by mass percentage is: C: 0.06%~0.09%, Si: 0.25%~0.45%, Mn: 1.65%~1.85%, P≤0.015%, S≤0.005%, Als: 0.030%~0.055%, N≤0.0045%, Cr: 0.30%~0.40%, with the balance being Fe.

[0040] Preferably, the chemical composition range of the duplex steel billet by mass percentage is: C: 0.06%, Si: 0.25%, Mn: 1.65%, P≤0.015%, S≤0.005%, Als: 0.030%, N≤0.0045%, Cr: 0.30%, with the balance being Fe.

[0041] Preferably, the chemical composition range of the duplex steel billet by mass percentage is: C: 0.09%, Si: 0.45%, Mn: 1.85%, P≤0.015%, S≤0.005%, Als: 0.055%, N≤0.0045%, Cr: 0.40%, with the balance being Fe.

[0042] The microstructure of the dual-phase steel billet consists of 20-30% ferrite matrix (average grain size of 3.5 μm) and 70-80% blocky martensite.

[0043] The slab has a yield strength of 365–435 MPa, a tensile strength of 605–660 MPa, and an elongation A. 80 The value ranges from 24.0% to 30.0%.

[0044] S2. The slab is heated, descaled, rough rolled, finish rolled and laminar flow cooled to obtain a hot-rolled coil;

[0045] S2, obtaining a hot-rolled coil by heating, descaling, rough rolling, finish rolling, and laminar flow cooling of the slab, includes:

[0046] The heating temperature is 1210~1260℃, the initial rolling temperature of the finishing mill is 1055~1160℃, and the final rolling temperature of the finishing mill is 880~950℃; laminar flow cooling adopts the front-stage cooling method, with upper and lower surface cooling rates of 50% and 75%, respectively; the coiling temperature is 580~650℃, and the hot-rolled thickness is 2.50~6.25mm.

[0047] In this embodiment of the invention, the heating temperature is 1210℃, the initial rolling temperature of the finishing mill is 1055℃, the final rolling temperature of the finishing mill is 880℃; the laminar flow cooling adopts the front-stage cooling method, the cooling rates of the upper and lower surfaces are 50% and 75% respectively, the coiling temperature is 580℃, and the hot-rolled thickness is 2.50mm.

[0048] In this embodiment of the invention, the heating temperature is 1260°C, the initial rolling temperature of the finishing mill is 1160°C, the final rolling temperature of the finishing mill is 950°C, the laminar flow cooling adopts the front-stage cooling method, the cooling rates of the upper and lower surfaces are 50% and 75% respectively, the coiling temperature is 650°C, and the hot-rolled thickness is 6.25 mm.

[0049] S3. Hot-rolled coils are pickled and then cold-rolled into cold-rolled thin steel sheets of 0.6–3.0 mm.

[0050] The S3 hot-rolled coil, after pickling, is cold-rolled into a cold-rolled thin steel strip of 0.6–3.0 mm; including,

[0051] The cold rolling reduction rate ranges from 52% to 76%, gradually decreasing as the thickness of the hot-rolled coil increases. It should be noted that for every 0.4mm increase in the thickness of the cold-rolled thin strip, the raw material thickness is adjusted accordingly, resulting in a decrease in the cold rolling reduction rate of approximately 4%.

[0052] In this embodiment of the invention, the cold rolling reduction rate is 52%, and it gradually decreases as the cold rolling thickness of the hot-rolled coil increases. It should be noted that for every 0.4 mm increase in the thickness of the cold-rolled thin strip, the raw material thickness is adjusted accordingly, and the cold rolling reduction rate decreases by approximately 4%.

[0053] In this embodiment of the invention, the cold rolling reduction rate is 76%, and it gradually decreases as the cold rolling thickness of the hot-rolled coil increases. It should be noted that for every 0.4 mm increase in the thickness of the cold-rolled thin strip, the raw material thickness is adjusted accordingly, and the cold rolling reduction rate decreases by approximately 4%.

[0054] S4. Hot-dip galvanizing is performed on cold-rolled thin steel strips to obtain galvanized duplex steel.

[0055] S4, hot-dip galvanizing the cold-rolled thin steel strip to obtain galvanized duplex steel, includes:

[0056] The cold-rolled thin steel strip is first heated in sections to 300℃, 700℃ and 780-810℃ at heating rates of 9-22℃ / s, 3.5-11℃ / s and 1.0-3.5℃ / s respectively;

[0057] The heated cold-rolled thin steel strip is subjected to uniform heat preservation.

[0058] After heat preservation for 60–135 seconds, the temperature is slowly cooled to 680–715℃ at rates of 1.5–8.5℃ / s and 18–35℃ / s respectively, and then rapidly cooled to 440–470℃ before being kept at a uniform temperature.

[0059] After the cold-rolled thin steel strip is subjected to uniform heat preservation, it is then subjected to balanced heat preservation, and after the balanced heat preservation is completed, the cold-rolled thin steel strip is put into the zinc bath for galvanizing treatment.

[0060] After galvanizing, the cold-rolled thin steel strip is first cooled to 380-420°C by an air knife; then it is rapidly heated at a rate of 15-43°C / s to undergo alloying treatment to obtain galvanized duplex steel.

[0061] In this embodiment of the invention, after galvanizing, the cold-rolled thin steel strip is first cooled to 380°C by an air knife; then it is rapidly heated at a rate of 15-43°C / s to undergo alloying treatment.

[0062] In this embodiment of the invention, after galvanizing, the cold-rolled thin steel strip is first cooled to 420°C by an air knife; then it is rapidly heated at a rate of 15-43°C / s to undergo alloying treatment.

[0063] The alloying treatment temperature is 520–565℃, and the holding time is 5–30 seconds. After alloying, the cold-rolled thin steel strip is cooled to room temperature using a fan (cooling rate ≥5℃ / s). Alloying annealing involves heating the hot-dip galvanized sheet to a certain temperature after hot-dip galvanizing, resulting in a zinc-iron alloy layer through interdiffusion phase transformation between the iron in the substrate and the zinc in the zinc layer. This coating exhibits good weldability, paintability, and stone chip resistance.

[0064] In this embodiment of the invention, the zinc plating bath contains 0.15% to 0.30% Al, with the remainder being Zn and unavoidable impurities, and the zinc layer weight per unit area is 60 to 110 g / cm³. 2 The unit speed is 100–130 m / min. The speed gradually decreases as the cold-rolled material thickness increases. For every 0.4 mm increase in the thickness of the cold-rolled thin strip steel, the unit speed is adjusted accordingly, decreasing by 5 m / min. The leveling elongation ranges from 0.40% to 0.60%, decreasing with increasing material thickness.

[0065] The role of alloying elements in galvanized duplex steel:

[0066] As one of the most important components of duplex steel, carbon primarily affects the volume fraction of austenite formed during annealing. During austenite formation, the diffusion of carbon within austenite or ferrite effectively controls austenite grain growth. With increasing carbon content or critical heating temperature, the austenite volume fraction increases, leading to an increase in the martensitic phase formed after cooling, thus increasing the material's strength. If the carbon content is too low, the stability of austenite and the hardenability of martensite decrease, resulting in lower strength; in duplex steel, the carbon content is generally not less than 0.02%. If the carbon content is too high, the plasticity and weldability of duplex steel decrease; in duplex steel, the carbon content is generally not higher than 0.15%. Therefore, the carbon content of this invention is 0.04%–0.10%, preferably 0.06%–0.09%.

[0067] Si plays a significant role in solid solution strengthening in steel and effectively inhibits carbide precipitation and delays pearlite transformation during phase transformation. However, excessive Si content significantly increases deformation resistance during thin-gauge rolling and promotes carbon segregation in manganese-rich regions. During heat treatment in the two-phase region, Si accelerates carbon diffusion into austenite, significantly purifying ferrite, improving ferrite purity in dual-phase steel, promoting ferrite formation, and expanding the ferrite formation process window, thus resulting in a lower yield strength ratio. On the other hand, excessive silicon content increases martensite brittleness, leading to decreased toughness, and the formation of high-melting-point oxides on the steel plate surface affects surface quality. Therefore, it is necessary to minimize the silicon content in the steel. Thus, the Si content in this invention is 0.15–0.50%, preferably 0.25%–0.45%.

[0068] Manganese (Mn) is a good deoxidizer and desulfurizer, and also a commonly used solid solution strengthening element in steel, generally not less than 1.20% in duplex steel. Mn can combine with carbon (C) to form various carbides, providing precipitation strengthening, and can also dissolve in the matrix to enhance solid solution strengthening. Mn readily combines with sulfur (S) to form the high-melting-point compound MnS, thereby eliminating or weakening hot brittleness caused by FeS and improving the hot working properties of steel. Mn can improve the stability of austenite, shifting the C-curve to the right, thus significantly reducing the critical cooling rate of martensite. However, when the Mn content is too high, it tends to accumulate on the surface during annealing, forming a large amount of manganese compounds, leading to a decrease in the quality of surface galvanization. Therefore, in this invention, the Mn content is 1.50%–1.90%, preferably 1.65%–1.85%.

[0069] Cr can replace Mn, improving the strength of steel and reducing segregation. It can also inhibit pearlite transformation. Furthermore, the addition of a certain amount of Cr in this invention can improve surface quality. After Cr is added, it can react with oxygen and aggregate at the interface between the iron oxide scale and the iron matrix, forming a dense (Fe,Cr)₂O₃ or (Fe,Cr)₃O₄ spinel film. The presence of this Fe-Cr-rich spinel film hinders oxygen diffusion and reduces the formation of iron oxide scale. Therefore, adding Cr can effectively reduce the thickness of the iron oxide scale and the formation of AlN, and improve the adhesion of the iron oxide scale, thereby effectively reducing pitting defects caused by the indentation of iron oxide scale. Therefore, in this invention, the Cr content is 0.20%–0.50%, preferably 0.30%–0.40%.

[0070] Al is a common deoxidizer in steel and can also form AlN pinning grain boundaries, thereby refining the grains. In addition, Al has a similar effect to Si, which can inhibit carbide precipitation, thus making the austenite sufficiently carbon-rich. Therefore, the Al content in this invention is 0.010% to 0.060%, preferably 0.030% to 0.055%.

[0071] P is an impurity element in steel, which tends to segregate at grain boundaries, weakening intergranular bonding. During rapid solidification, a high P content can easily lead to cracking of the cast billet. Therefore, in this invention, the P content is ≤0.020%, preferably ≤0.015%.

[0072] Sulfur (S) is an impurity element in steel. It tends to segregate at grain boundaries and forms low-melting-point FeS with Fe in the steel, reducing the toughness of the steel. During rapid solidification, it can easily lead to fine microcracks on the surface of the cast billet. Therefore, the S content in this invention is ≤0.010%, preferably ≤0.005%.

[0073] Nitrogen (N), an impurity element in steel, readily enters the interstitial spaces of iron elements due to its small atomic size, significantly increasing lattice distortion and thus greatly enhancing strength. However, it also noticeably deteriorates the steel's ductility and toughness. Furthermore, N readily combines with Al, B, and Ti in steel to form second phases such as AlN, BN, and TiN, which, while strengthening the steel, also worsen its ductility and toughness. Especially when the N content is too high, the combined NiN with Ti can precipitate to micrometer-sized particles, failing to provide strengthening and instead causing stress concentration and crack initiation. Therefore, the N content must be strictly controlled. Thus, in this invention, the N content is ≤0.0060%, preferably 0.0045%.

[0074] Example 1

[0075] A method for preparing 590MPa grade low spot welding carbon equivalent alloyed zinc duplex steel includes the following steps:

[0076] S1. Cast 590MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel billet.

[0077] The chemical composition range of the duplex steel billet by mass percentage is: C: 0.07%, Si: 0.4%, Mn: 1.75%, P: 0.015%, S: 0.004%, Als: 0.044%, N≤0.0060%, Cr: 0.35%, with the balance being Fe.

[0078] The microstructure of the slab consists of 20-30% ferrite matrix (average grain size of 3.5 μm) and 70-80% blocky martensite.

[0079] The slab has a yield strength of 365–435 MPa, a tensile strength of 605–660 MPa, and an elongation A. 80 The value ranges from 24.0% to 30.0%.

[0080] S2. The slab is heated, descaled, rough rolled, finish rolled and laminar flow cooled to obtain a hot-rolled coil;

[0081] S2, obtaining a hot-rolled coil by heating, descaling, rough rolling, finish rolling, and laminar flow cooling of the slab, includes:

[0082] The heating temperature is 1248℃, the initial rolling temperature of the finishing mill is 1098℃, and the final rolling temperature of the finishing mill is 918℃. The laminar flow cooling adopts the front-stage cooling method, with the upper and lower surface cooling rates being 50% and 75%, respectively. The coiling temperature is 621℃, and the hot-rolled thickness is 3.6mm.

[0083] S3. Hot-rolled coils are pickled and then cold-rolled into 1.0mm cold-rolled thin steel.

[0084] The S3 hot-rolled coil, after pickling, is cold-rolled into a cold-rolled thin steel strip of 0.6–3.0 mm; including,

[0085] The cold rolling reduction rate is 72%, and it gradually decreases as the thickness of the hot-rolled coil increases. It should be noted that for every 0.4mm increase in the thickness of the cold-rolled thin strip, the raw material thickness is adjusted accordingly, and the cold rolling reduction rate decreases by approximately 4%.

[0086] S4. Hot-dip galvanizing is performed on cold-rolled thin steel strips to obtain galvanized duplex steel.

[0087] S4, hot-dip galvanizing the cold-rolled thin steel strip to obtain galvanized duplex steel, includes:

[0088] The cold-rolled thin steel strip is first heated in sections to 300℃, 700℃ and 780-810℃ at heating rates of 9-22℃ / s, 3.5-11℃ / s and 1.0-3.5℃ / s respectively;

[0089] The heated cold-rolled thin steel strip is subjected to heat homogenization and heat preservation at a temperature of 785℃.

[0090] After heat preservation for 60–135 seconds, the temperature is slowly cooled to 685℃ and rapidly cooled to 455℃ at rates of 1.5–8.5℃ / s and 18–35℃ / s respectively, followed by heat preservation for uniform heat preservation.

[0091] After the cold-rolled thin steel strip is subjected to uniform heat preservation, it is then subjected to balanced heat preservation, and after the balanced heat preservation is completed, the cold-rolled thin steel strip is put into the zinc bath for galvanizing treatment.

[0092] After galvanizing, the cold-rolled thin steel strip is first cooled to 395°C by an air knife; then it is rapidly heated at a rate of 15–43°C / s to undergo alloying treatment.

[0093] The alloying temperature is 546℃, the alloying holding time is 5-30s, and the alloyed strip is cooled to room temperature using a fan (cooling rate ≥5℃ / s).

[0094] In this embodiment of the invention, the zinc plating bath contains 0.15% to 0.30% Al, with the remainder being Zn and unavoidable impurities, and the zinc layer weight per unit area is 60 to 110 g / cm³. 2 The unit speed is 125 m / min. The speed gradually decreases as the cold-rolled material thickness increases. For every 0.4 mm increase in the thickness of the cold-rolled thin strip steel, the unit speed is adjusted accordingly, decreasing by 5 m / min. The leveling elongation range is 0.58%, decreasing with increasing material thickness.

[0095] Example 2

[0096] A method for preparing 590MPa grade low spot welding carbon equivalent alloyed zinc duplex steel includes the following steps:

[0097] S1. Cast 590MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel billet.

[0098] The chemical composition range of the duplex steel billet by mass percentage is: C: 0.65%, Si: 0.38%, Mn: 1.78%, P: 0.012%, S: 0.004%, Als: 0.042%, N≤0.0060%, Cr: 0.38%, with the balance being Fe.

[0099] The microstructure of the slab consists of 20-30% ferrite matrix (average grain size of 3.5 μm) and 70-80% blocky martensite.

[0100] The slab has a yield strength of 365–435 MPa, a tensile strength of 605–660 MPa, and an elongation A. 80The value ranges from 24.0% to 30.0%.

[0101] S2. The slab is heated, descaled, rough rolled, finish rolled and laminar flow cooled to obtain a hot-rolled coil;

[0102] S2, obtaining a hot-rolled coil by heating, descaling, rough rolling, finish rolling, and laminar flow cooling of the slab, includes:

[0103] The heating temperature is 1244℃, the initial rolling temperature of the finishing mill is 1112℃, and the final rolling temperature of the finishing mill is 911℃. The laminar flow cooling adopts the front-stage cooling method, with the upper and lower surface cooling rates being 50% and 75%, respectively. The coiling temperature is 618℃, and the hot-rolled thickness is 4.5mm.

[0104] S3. Hot-rolled coils are pickled and then cold-rolled into 1.5mm cold-rolled thin steel.

[0105] The S3 hot-rolled coil, after pickling, is cold-rolled into a cold-rolled thin steel strip of 0.6–3.0 mm; including,

[0106] The cold rolling reduction rate is 69%, and it gradually decreases as the thickness of the hot-rolled coil increases. It should be noted that for every 0.4mm increase in the thickness of the cold-rolled thin strip, the raw material thickness is adjusted accordingly, and the cold rolling reduction rate decreases by approximately 4%.

[0107] S4. Hot-dip galvanizing is performed on cold-rolled thin steel strips to obtain galvanized duplex steel.

[0108] S4, hot-dip galvanizing the cold-rolled thin steel strip to obtain galvanized duplex steel, includes:

[0109] The cold-rolled thin steel strip is first heated in sections to 300℃, 700℃ and 780-810℃ at heating rates of 9-22℃ / s, 3.5-11℃ / s and 1.0-3.5℃ / s respectively;

[0110] The heated cold-rolled thin steel strip is subjected to heat homogenization and heat preservation at a temperature of 792℃.

[0111] After heat preservation for 60–135 seconds, the temperature was slowly cooled to 691℃ and rapidly cooled to 458℃ at rates of 1.5–8.5℃ / s and 18–35℃ / s, respectively, before being kept at a uniform temperature.

[0112] After the cold-rolled thin steel strip is subjected to uniform heat preservation, it is then subjected to balanced heat preservation, and after the balanced heat preservation is completed, the cold-rolled thin steel strip is put into the zinc bath for galvanizing treatment.

[0113] After galvanizing, the cold-rolled thin steel strip is first cooled to 397°C by an air knife; then it is rapidly heated at a rate of 15–43°C / s to undergo alloying treatment.

[0114] The alloying temperature is 542℃, the alloying holding time is 5-30s, and the alloyed strip is cooled to room temperature using a fan (cooling rate ≥5℃ / s).

[0115] In this embodiment of the invention, the zinc plating bath contains 0.15% to 0.30% Al, with the remainder being Zn and unavoidable impurities, and the zinc layer weight per unit area is 60 to 110 g / cm³. 2 The unit speed is 119 m / min. The speed gradually decreases as the cold-rolled material thickness increases. For every 0.4 mm increase in the thickness of the cold-rolled thin strip steel, the unit speed is adjusted accordingly, decreasing by 5 m / min. The leveling elongation range is 0.55%, decreasing with increasing material thickness.

[0116] Example 3

[0117] A method for preparing 590MPa grade low spot welding carbon equivalent alloyed zinc duplex steel includes the following steps:

[0118] S1. Cast 590MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel billet.

[0119] The chemical composition range of the duplex steel billet by mass percentage is: C: 0.08%, Si: 0.32%, Mn: 1.68%, P: 0.013%, S: 0.005%, Als: 0.035%, N≤0.0060%, Cr: 0.31%, with the balance being Fe.

[0120] The microstructure of the slab consists of 20-30% ferrite matrix (average grain size of 3.5 μm) and 70-80% blocky martensite.

[0121] The slab has a yield strength of 365–435 MPa, a tensile strength of 605–660 MPa, and an elongation A. 80 The value ranges from 24.0% to 30.0%.

[0122] S2. The slab is heated, descaled, rough rolled, finish rolled and laminar flow cooled to obtain a hot-rolled coil;

[0123] S2, obtaining a hot-rolled coil by heating, descaling, rough rolling, finish rolling, and laminar flow cooling of the slab, includes:

[0124] The heating temperature is 1239℃, the initial rolling temperature of the finishing mill is 1108℃, and the final rolling temperature of the finishing mill is 806℃. The laminar flow cooling adopts the front-stage cooling method, with the upper and lower surface cooling rates being 50% and 75%, respectively. The coiling temperature is 633℃, and the hot-rolled thickness is 5mm.

[0125] S3. Hot-rolled coils are pickled and then cold-rolled into 1.8mm cold-rolled thin steel.

[0126] The S3 hot-rolled coil, after pickling, is cold-rolled into a cold-rolled thin steel strip of 0.6–3.0 mm; including,

[0127] The cold rolling reduction rate is 64%, and it gradually decreases as the thickness of the hot-rolled coil increases. It should be noted that for every 0.4mm increase in the thickness of the cold-rolled thin strip, the raw material thickness is adjusted accordingly, and the cold rolling reduction rate decreases by approximately 4%.

[0128] S4. Hot-dip galvanizing is performed on cold-rolled thin steel strips to obtain galvanized duplex steel.

[0129] S4, hot-dip galvanizing the cold-rolled thin steel strip to obtain galvanized duplex steel, includes:

[0130] The cold-rolled thin steel strip is first heated in sections to 300℃, 700℃ and 780-810℃ at heating rates of 9-22℃ / s, 3.5-11℃ / s and 1.0-3.5℃ / s respectively;

[0131] The heated cold-rolled thin steel strip is subjected to heat homogenization and heat preservation at a temperature of 795℃.

[0132] After heat preservation for 60–135 seconds, the temperature is slowly cooled to 795℃ and rapidly cooled to 460℃ at rates of 1.5–8.5℃ / s and 18–35℃ / s respectively, followed by heat preservation for uniform heat preservation.

[0133] After the cold-rolled thin steel strip is subjected to uniform heat preservation, it is then subjected to balanced heat preservation, and after the balanced heat preservation is completed, the cold-rolled thin steel strip is put into the zinc bath for galvanizing treatment.

[0134] After galvanizing, the cold-rolled thin steel strip is first cooled to 402°C by an air knife; then it is rapidly heated at a rate of 15–43°C / s to undergo alloying treatment.

[0135] The alloying temperature is 539℃, the alloying holding time is 5~30s, and the alloyed strip is cooled to room temperature using a fan (cooling rate ≥5℃ / s).

[0136] In this embodiment of the invention, the zinc plating bath contains 0.15% to 0.30% Al, with the remainder being Zn and unavoidable impurities, and the zinc layer weight per unit area is 60 to 110 g / cm³. 2 The unit speed is 119 m / min. The speed gradually decreases as the cold-rolled material thickness increases. For every 0.4 mm increase in the thickness of the cold-rolled thin strip steel, the unit speed is adjusted accordingly, decreasing by 5 m / min. The leveling elongation range is 0.55%, decreasing with increasing material thickness.

[0137] Example 4

[0138] A method for preparing 590MPa grade low spot welding carbon equivalent alloyed zinc duplex steel includes the following steps:

[0139] S1. Cast 590MPa grade low spot welding carbon equivalent alloyed hot-dip galvanized duplex steel billet.

[0140] The chemical composition range of the duplex steel billet by mass percentage is: C: 0.076%, Si: 0.35%, Mn: 1.72%, P≤0.011%, S: 0.003%, Als: 0.038%, N≤0.0060%, Cr: 0.33%, with the balance being Fe.

[0141] The microstructure of the slab consists of 20-30% ferrite matrix (average grain size of 3.5 μm) and 70-80% blocky martensite.

[0142] The slab has a yield strength of 365–435 MPa, a tensile strength of 605–660 MPa, and an elongation A. 80 The value ranges from 24.0% to 30.0%.

[0143] S2. The slab is heated, descaled, rough rolled, finish rolled and laminar flow cooled to obtain a hot-rolled coil;

[0144] S2, obtaining a hot-rolled coil by heating, descaling, rough rolling, finish rolling, and laminar flow cooling of the slab, includes:

[0145] The heating temperature is 1251℃, the initial rolling temperature of the finishing mill is 1123℃, and the final rolling temperature of the finishing mill is 923℃. The laminar flow cooling adopts the front-stage cooling method, with the upper and lower surface cooling rates being 50% and 75%, respectively. The coiling temperature is 640℃, and the hot-rolled thickness is 5.5mm.

[0146] S3. Hot-rolled coils are pickled and then cold-rolled into 2.1mm cold-rolled thin steel.

[0147] The S3 hot-rolled coil, after pickling, is cold-rolled into a cold-rolled thin steel strip of 0.6–3.0 mm; including,

[0148] The cold rolling reduction rate is 61%, and it gradually decreases as the thickness of the hot-rolled coil increases. It should be noted that for every 0.4mm increase in the thickness of the cold-rolled thin strip, the raw material thickness is adjusted accordingly, and the cold rolling reduction rate decreases by approximately 4%.

[0149] S4. Hot-dip galvanizing is performed on cold-rolled thin steel strips to obtain galvanized duplex steel.

[0150] S4, hot-dip galvanizing the cold-rolled thin steel strip to obtain galvanized duplex steel, includes:

[0151] The cold-rolled thin steel strip is first heated in sections to 300℃, 700℃ and 780-810℃ at heating rates of 9-22℃ / s, 3.5-11℃ / s and 1.0-3.5℃ / s respectively;

[0152] The heated cold-rolled thin steel strip is subjected to heat homogenization and heat preservation at a temperature of 798℃.

[0153] After heat preservation for 60–135 seconds, the temperature was slowly cooled to 799℃ and rapidly cooled to 463℃ at rates of 1.5–8.5℃ / s and 18–35℃ / s, respectively, before being kept at a uniform temperature.

[0154] After the cold-rolled thin steel strip is subjected to uniform heat preservation, it is then subjected to balanced heat preservation, and after the balanced heat preservation is completed, the cold-rolled thin steel strip is put into the zinc bath for galvanizing treatment.

[0155] After galvanizing, the cold-rolled thin steel strip is first cooled to 408°C by an air knife; then it is rapidly heated at a rate of 15–43°C / s to undergo alloying treatment.

[0156] The alloying temperature is 535℃, the alloying holding time is 5~30s, and the alloyed strip is cooled to room temperature using a fan (cooling rate ≥5℃ / s).

[0157] In this embodiment of the invention, the zinc plating bath contains 0.15% to 0.30% Al, with the remainder being Zn and unavoidable impurities, and the zinc layer weight per unit area is 60 to 110 g / cm³. 2 The unit speed is 112 m / min. The speed gradually decreases as the cold-rolled material thickness increases. For every 0.4 mm increase in the thickness of the cold-rolled thin strip steel, the unit speed is adjusted accordingly, decreasing by 5 m / min. The leveling elongation range is 0.50%, decreasing with increasing material thickness.

[0158] Performance comparison:

[0159] The microstructure of the above embodiments is as follows Figs. 1-2 As shown in the figure. The mechanical properties of the above-mentioned 590MPa grade low spot weld carbon equivalent alloyed hot-dip galvanized duplex steel were tested according to GB / T228~2010 "Metallic materials - Tensile testing at room temperature", and the hole expansion performance of the above-mentioned 590MPa grade low spot weld carbon equivalent alloyed hot-dip galvanized duplex steel was tested according to GB / T 24524~2009 "Metallic materials - Plate and strip - Hole expansion test method", as shown in Table 5.

[0160] Table 5 590MPa Grade Low Spot Welding Carbon Equivalent Alloyed Hot-Dip Galvanized Duplex Steel

[0161]

[0162] The preferred embodiments of the present invention have been described in detail above. However, the present invention is not limited to the above embodiments. Within the scope of knowledge possessed by those skilled in the art, various changes can be made without departing from the spirit of the present invention.

Claims

1. A method for preparing low-spot-welding carbon equivalent alloyed zinc-plated duplex steel, characterized in that, The method includes: A duplex steel billet is cast, the chemical composition of which, by mass percentage, is: C: 0.04%–0.10%, Si: 0.15%–0.50%, Mn: 1.50%–1.90%, P≤0.020%, S≤0.010%, Als: 0.010%–0.060%, N≤0.0060%, Cr: 0.30%–0.40%, with the balance being Fe; Hot-rolled coils are obtained by heating, descaling, rough rolling, finish rolling and laminar flow cooling of slabs; After pickling, hot-rolled coils are cold-rolled into cold-rolled thin steel sheets of 0.6–3.0 mm. Hot-dip galvanizing is performed on cold-rolled thin steel strip to obtain galvanized duplex steel. This involves heating the cold-rolled thin steel strip in stages to 300℃, 700℃, and 780-810℃ at heating rates of 9-22℃ / s, 3.5-11℃ / s, and 1.0-3.5℃ / s, respectively. After heating, the cold-rolled thin steel strip is subjected to uniform heat treatment for 60-135s. Then, it is slowly cooled to 680-715℃ and rapidly cooled to 440-470℃ at rates of 1.5-8.5℃ / s and 18-35℃ / s, respectively, followed by uniform heat treatment.

2. The method for preparing low-spot-welding carbon equivalent alloyed zinc-plated duplex steel as described in claim 1, characterized in that, The process of obtaining a hot-rolled coil by heating, descaling, rough rolling, finish rolling, and laminar flow cooling of a slab includes: The heating temperature is 1210~1260℃, the initial rolling temperature of the finishing mill is 1055~1160℃, and the final rolling temperature of the finishing mill is 880~950℃; laminar flow cooling adopts the front-stage cooling method, with upper and lower surface cooling rates of 50% and 75%, respectively; the coiling temperature is 580~650℃, and the hot-rolled thickness is 2.50~6.25mm.

3. The method for preparing low-spot-welding carbon equivalent alloyed zinc-plated duplex steel as described in claim 1, characterized in that, The hot-rolled coil is pickled and then cold-rolled into a cold-rolled thin steel strip of 0.6 to 3.0 mm. include, The cold rolling reduction rate is 52% to 76%.

4. The method for preparing low-spot-welding carbon equivalent alloyed zinc-plated duplex steel as described in claim 1, characterized in that, The hot-dip galvanizing treatment of cold-rolled thin steel strip to obtain galvanized duplex steel further includes: After the cold-rolled thin steel strip has undergone uniform heat preservation, it enters a zinc bath for galvanizing. After galvanizing, the cold-rolled thin steel strip is first cooled to 380-420°C by an air knife; then it is rapidly heated at a rate of 15-43°C / s to undergo alloying treatment to obtain galvanized duplex steel.

5. The method for preparing low-spot-welding carbon equivalent alloyed zinc-plated duplex steel as described in claim 1, characterized in that, The chemical composition range of the duplex steel billet by mass percentage is: C: 0.06%, Si: 0.25%, Mn: 1.65%, P≤0.015%, S≤0.005%, Als: 0.030%, N≤0.0045%, Cr: 0.30%, with the balance being Fe. The microstructure of the duplex steel billet includes 20-30% ferrite matrix and 70-80% agglomerated martensite by volume fraction.

6. The method for preparing low-spot-welding carbon equivalent alloyed zinc-plated duplex steel as described in claim 4, characterized in that, The alloying treatment temperature is 520–565°C, the holding time for alloying treatment is 5–30 seconds, and after alloying treatment, the cold-rolled thin steel strip is cooled to room temperature using a fan.

7. The method for preparing low-spot-welding carbon equivalent alloyed zinc-plated duplex steel as described in claim 6, characterized in that, After alloying treatment, the cold-rolled thin steel strip is cooled to room temperature using a fan, wherein the cooling rate is ≥5℃ / s.

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