Students' difficulties in scientific argumentation have been widely reported in the literature. Researchers argue that these difficulties result mainly from students' lack of understanding of the goals and norms of argumentation. Therefore, designing and implementing appropriate instructional scaffolds to facilitate such essential knowledge of argumentation holds pedagogical significance. In this qualitative case study, two kinds of argument-focused metacognitive scaffolds (AMS) -- questioning and prompting, and modelling of thinking -- were designed and integrated into an elementary science classroom. One science teacher and her 19 students participated in this case study. To explore the pedagogical contributions of AMS, data were collected from multiple sources including classroom observation, interviews with students, and students' works. AMS in this study supported students to engage in argumentation reflectively, as these scaffolds facilitated the development of students' understanding of the goals and evidence-related norms of argumentation and abilities of metacognitive monitoring during argumentation. These influences were also recognised and appreciated by students. When AMS gradually reduced, students' knowledge of argumentation and abilities of metacognitive monitoring were retained and affected how they performed argumentation in new contexts. Pedagogical implications of these findings are discussed.